Peptide hormones: receptors, signalling and biological functions

Peptide hormones are chemical messengers made up of amino acid chains. Specialised cells produce them, often processing them from larger precursor molecules, release them in response to specific physiological signals, and then use them to coordinate the activity of other cells and tissues.

However, their action does not stem from the mere fact that they exist peptides. The biological response depends primarily on whether the cell possesses the appropriate receptor and an intracellular signalling mechanism to interpret the received signal.

Insulin, glucagon, glucagon-like peptide-1 (GLP-1), vasopressin, parathyroid hormone (PTH), gonadotrophin-releasing hormone (GnRH) and atrial natriuretic peptide (ANP) belong to this broad group. However, they participate in very different physiological systems.

The term „peptide hormone” therefore describes a chemical category and a mode of biological communication, rather than a single shared effect.

For example, GLP-1 studies in humans concerned the communication between the gastrointestinal tract and insulin secretion, whereas studies on vasopressin-responsive water channels focused on the regulation of water balance by the kidneys. These are distinct physiological systems utilising different receptors, target tissues, and response mechanisms. [1] [2]

The action of a peptide hormone is most easily understood by following the entire process from beginning to end:

precursor synthesis → molecule processing → storage → release → receptor recognition → intracellular signalling → cellular response → feedback and signal termination.

Each of these stages can influence the final biological response.

This approach also helps to avoid a major misunderstanding. The detection of a peptide, the observation of its binding to a receptor, the measurement of a second messenger, and the demonstration of a whole-organism physiological response are different levels of evidence. A full interpretation requires establishing which of these stages has actually been investigated.

What are peptide hormones?

amino acid chain transmitting a biological signal

Peptide hormones consist of amino acid residues linked peptide bonds.

Some are relatively short chains, whereas others are much larger molecules and may also be referred to as protein hormones.

In the biology of the endocrine system, these categories partially overlap. The terminology concerning peptide hormones encompasses signalling molecules made up of amino acids and does not require the existence of a single universal chain length boundary separating all peptides from all proteins.

The word „hormone” describes primarily the signalling role of a molecule, rather than exclusively its chemical structure.

In the classical endocrine system, a cell or tissue releases a hormone into the circulation so that it can affect responsive cells located in other parts of the body.

Biological communication is not, however, limited exclusively to this mechanism. Related molecules may also participate in local communication between neighbouring cells and in other forms of cell signalling.

Therefore, chemical structure alone is not enough to consider a given peptide a hormone.

Many peptides include, among others:

• intermediary metabolites;
• fragments of larger proteins;
• structural components;
• intracellular regulatory molecules;
• protein degradation products;
• molecules fulfilling other functions unrelated to the endocrine system.

To confirm the hormonal role, researchers typically require evidence linking a given molecule to its production, regulated release, target cells, and physiological action.

Early human GLP-1 trials well illustrate this approach.

Researchers detected GLP-1-related material in the gut and observed an increase in its blood concentration following oral glucose administration or a meal. In a small infusion experiment, a specific form of GLP-1 influenced the insulin and glucose response. [1]

Taken together, these observations supported the concept of a physiological signalling role for this molecule.

However, this study should not be treated as proof confirming every mechanism, clinical application or subsequent claim regarding GLP-1 biology.

Target cells are determined by their ability to respond

A hormone can flow past many cells without eliciting the same response in every one of them.

For a cell to be able to respond, it usually needs to possess an appropriate receptor as well as intracellular molecular mechanisms to process the signal generated after its activation.

Receptors recognise specific characteristics of molecules.

Recognition may depend, among other things, on:

• amino acid sequence;
• spatial structure;
• cargo stowage;
• chemical modifications;
• receptor conformation;
• availability of suitable binding surfaces.

The mere fact that a given molecule is a peptide does not mean that it can activate the receptor of another peptide hormone.

The number of receptors is also important.

A cell with a large number of functional receptors may respond differently than a cell that has fewer of them. The response can also be influenced by receptor localisation, their modifications, interacting proteins and prior stimulation.

Even two tissues possessing the same receptor can react differently because they differ in their intracellular signalling machinery.

Receptor activation can affect, amongst other things,:

• enzyme activity;
• ion transport;
• membrane channels;
• bubble migration;
• secretion;
• metabolism;
• protein phosphorylation;
• gene transcription;
• a combination of a few of these processes.

Receptor binding is therefore the beginning of the explanation of the mechanism of action, rather than the complete answer to the question of the biological effect.

Peptide hormones participate in many different systems

These examples illustrate differing physiological functions, rather than common benefits resulting from belonging to the peptide group.

Research on insulin receptor kinase activity, GLP-1 physiology, urinary aquaporin-2, the calcium-PTH response, GnRH pulse frequency and ANP receptors analyses various elements of distinct hormonal systems. [1] [2] [3] [4] [5] [6]

For this reason, results concerning one peptide hormone should not be automatically extrapolated to another just because both molecules belong to the same broad chemical category.

How are peptide hormones synthesized and released?

Many hormones start as larger precursor molecules

Many peptide hormones are initially synthesised as larger molecules called preprohormones.

The prefix „pre-” usually refers to a signal sequence that helps direct the nascent molecule to the cellular secretory pathway.

Once this sequence has been removed, the remaining precursor is usually referred to as a prohormone.

Further processing can lead to the formation of the mature hormone, and in some cases several biologically relevant products.

In the classical regulated secretory pathway, the nascent precursor enters the endoplasmic reticulum, then passes through the Golgi apparatus and is directed to the appropriate secretory compartments.

Depending on the molecule, subsequent stages may include:

• folding;
• formation of disulphide bonds;
• enzymatic cleavage;
• chain end modifications;
• other chemical modifications;
• sorting;
• packaging into secretory granules.

These processes do not occur identically for every peptide hormone.

A single precursor can contain several potential peptide products, and different cell types can process the same precursor in different ways.

Therefore, the amino acid sequence encoded by the gene does not necessarily correspond directly to a single mature hormone present in the circulation.

The processing of the precursor affects which hormone is produced

Proglucagon is a particularly good example, because pancreatic and intestinal tissue can produce different molecular products from the same precursor.

Immunochemical and chromatographic methods were used in the study of human tissues to identify various patterns of glucagon-related molecules in the pancreas and small intestine. [7]

In later cell experiments, the involvement of enzymes processing PC2 and PC3 was analysed, with PC3 also frequently referred to as PC1/3.

The results showed that the enzymatic environment strongly influenced whether processing resembled the pattern characteristic of the pancreas or the intestine. The studies also revealed limitations in replicating all stages of cleavage using an isolated enzymatic system. [8]

The general rule is important:

The precursor sequence determines the possible products, whereas the processing machinery present in a given cell helps determine which molecular forms will actually be produced.

This also has implications for laboratory measurements.

A test that recognises several forms associated with a single precursor may yield different results to a method designed specifically to detect a single mature hormone.

Therefore, the reported concentration must be interpreted together with information on the exact molecular forms detected by the analytical method used.

Secretory granules enable controlled release

Many endocrine cells store processed peptide hormones in secretory granules.

Upon the appearance of the appropriate stimulus, regulated cellular mechanisms lead to the fusion of granules with the cell membrane.

Their contents are then released outside the cell via exocytosis.

Storage enables a rapid increase in secretion without the need to start the synthesis of every released molecule from scratch precisely at the moment the stimulus appears.

However, this does not mean that all granularities are equally accessible for immediate release.

Cells can contain functionally distinct pools of granules, and prolonged secretion requires further production, processing and replenishment of supplies.

Hormone release is therefore an actively controlled cellular process, rather than a passive leak of the entire cell contents.

Studies on human pancreatic islets and beta cells have shown that reducing the amount of the exocytotic protein Munc18c impaired insulin secretion and altered its release from different granule pools. [9]

These results directly support the role of specialised membrane fusion mechanisms.

However, they do not provide a complete explanation of all the molecular processes regulating insulin secretion.

Different hormone systems respond to different stimuli

Individual endocrine cells respond to various physiological signals.

These may include:

• nutrient concentrations;
• electrical activity;
• neurotransmitters;
• other hormones;
• ions currently outside the cell;
• osmotic conditions;
• mechanical changes;
• combinations of various regulatory stimuli.

Intracellular calcium is frequently involved in the fusion of secretory vesicles, but the processes leading to the generation of the calcium signal vary between cell types.

In pancreatic beta cells, for example, glucose metabolism is closely linked to electrical activity, calcium influx and the release of insulin-containing granules.

In turn, hypothalamic neuroendocrine cells function within complex networks of neural, hormonal and physiological signals.

It would therefore not be correct to present a single detailed secretory pathway as a universal mechanism for all peptide hormones.

From an experimental point of view, it is also important to distinguish between hormone synthesis and its secretion.

The study of rat corticotropin-releasing hormone-producing neurones revealed that gene activation and secretion-related responses had different response thresholds during sustained stress. [10]

Increased hormone release does not necessarily mean that the transcription of the gene encoding its precursor changes to the same extent and at the exact same time.

Time and rhythm of secretion can carry information

Exposure to a hormone is not solely described by its mean concentration.

The time pattern may include:

• separate pulses;
• pulse frequency changes;
• pulse amplitude changes;
• sustained elevation in concentration;
• circadian rhythm;
• responses related to meals, sleep, stress or other physiological events.

Different temporal patterns can lead to distinct cellular responses.

Experiments carried out on GnRH-deficient male rats showed that different GnRH pulse frequencies resulted in distinct transcription patterns of gonadotrophin subunits. The response involved both direct GnRH signalling and changes in local pituitary regulatory mechanisms. [5]

These studies help to explain why the timing and rhythm of a hormonal signal can convey biological information.

However, the results of such animal experiments should not be translated into human dosing schedules.

Therefore, the hormone concentration measured in the blood is merely a snapshot of a dynamically changing system at a specific moment.

Interpretation may depend on:

• time of sample collection;
• pulsatile pattern of secretion;
• measured molecular form;
• recent physiological events;
• elimination rates;
• specifics of the test used.

Receptors and cell signalling

What happens when a peptide hormone reaches its target cell?

Most peptide hormones initiate their action by binding to receptors located on the cell surface or directly associated with it.

The binding of the hormone to the receptor alters its activity or conformation, enabling the transmission of information into the interior of the cell.

The signal therefore crosses the membrane via the receptor system.

A peptide hormone usually does not need to cross the lipid cell membrane on its own and freely enter the cytoplasm to initiate signal transduction.

This makes it possible to understand how a relatively water-soluble extracellular molecule can influence processes taking place inside the cell.

The subsequent course of action depends on the type of receptor.

Peptide hormone receptors belong to several mechanistically distinct classes.

Some transmit the signal via:

• heterotrimeric G proteins;
• receptor-associated enzymes;
• of its own kinase activity;
• guanylyl cyclase activities;
• of other signalling proteins.

General layout:

hormone → receptor → intracellular signal → cellular response

It is very useful.

However, there is no single intracellular pathway common to all peptide hormones.

G protein-coupled receptors and second messengers

Many peptide hormones transmit signals via G-protein-coupled receptors, or GPCRs.

These are membrane proteins that communicate with intracellular G proteins, which can affect enzymes, ion channels and second messenger systems.

One of the important secondary messengers is cyclic AMP (cAMP).

In a typical cAMP pathway, receptor activation affects adenylate cyclase, which is the enzyme responsible for cAMP production.

Changes in cAMP concentration can subsequently affect the activity of subsequent signalling proteins.

Other GPCR receptors may utilise different mechanisms.

Importantly, not every GPCR increases the concentration of cAMP.

Some combinations of receptors and G proteins can decrease its production, while others primarily regulate pathways associated with calcium or other intracellular processes.

To state that a hormone „uses a secondary messenger” therefore describes a general signalling strategy rather than a single universal molecular mechanism.

An early study in a rat pancreatic islet cell line found that the processed form of GLP-1 increased cAMP, the level of insulin-related mRNA, and insulin release. [11]

This experiment linked an extracellular peptide signal with biochemical changes and intracellular gene expression.

However, the conclusions remain valid for the applied model and experimental conditions.

Calcium signalling and signal amplification

Another common GPCR pathway involves phospholipase C.

Its activation can lead to the generation of intracellular messengers such as inositol trisphosphate (IP3) and diacylglycerol (DAG).

IP3 can promote the release of calcium from intracellular stores, whereas DAG can participate in the activation of protein kinase C.

These processes help to translate receptor activation into a change in cellular activity.

However, this is not a pathway utilised by every peptide hormone receptor.

Signalling cascades can also amplify the signal.

One activated receptor can interact with many molecules further down the pathway, and one activated enzyme can produce many second messenger molecules.

This makes it possible to explain why a relatively small extracellular signal can sometimes trigger a significant cellular response.

However, signal amplification is not unlimited.

Its strength and duration are influenced, among other things, by:

• number of receptors;
• enzyme availability;
• substrate availability;
• feedback mechanisms;
• phosphatases;
• degradation of secondary relays;
• receptor desensitisation;
• functional capabilities of the target cell.

The secondary messenger is therefore an intermediary element of the pathway, rather than the final physiological effect.

Its type, concentration, location and duration of action are important, as well as the cell's ability to translate this signal into a specific response.

The insulin receptor works differently

The insulin receptor is a receptor tyrosine kinase.

Its initial signalling mechanism is therefore fundamentally different from the classical GPCR mechanism.

The binding of insulin leads to the activation of the intracellular kinase portion of the receptor.

The activated receptor phosphorylates specific sites on proteins and triggers subsequent signalling networks.

Biochemical studies using purified insulin receptors have linked tyrosine autophosphorylation to an increase in kinase activity. [3]

These experiments made it possible to determine important properties of the receptor.

However, the purified receptor does not reproduce all the processes occurring in whole cells of muscle, liver, adipose tissue or other insulin-responsive tissues.

The most important conclusion is simple:

Peptide hormones can activate different classes of receptors.

Insulin is an important example showing why peptide hormone signalling should not be summarised by the statement:

„peptide hormone binds to a GPCR and increases cAMP.

This mechanism applies to certain systems, but not the entire group.

Natriuretic peptide receptors can generate cGMP

ANP is an example of yet another receptor mechanism.

Its guanylyl cyclase A receptor is involved in the production of cyclic GMP (cGMP).

cGMP and cAMP are cyclic nucleotides, but they are not the same molecule and are produced by different enzyme systems.

Mutation experiments involving natriuretic peptide receptors showed that the kinase-like regulatory region played a vital role in controlling receptor activity. Changes in this region could lead to a loss of proper regulation or activity occurring without typical hormonal stimulation. [6]

In this case, precise terminology is important.

The presence of a kinase-like domain does not mean that the receptor operates via the same tyrosine kinase mechanism as the insulin receptor.

Receptor architecture, its catalytic activity, and the type of biochemical product measured must be taken into account.

Surface binding does not mean that the receptor always remains there

Activated receptors can undergo endocytosis, meaning they move from the cell membrane to internal membrane-bound compartments.

This process differs from the free diffusion of a peptide hormone through the lipid membrane into the cytoplasm.

In modified human kidney-derived cells, researchers tracked the movement of ANP receptors to intracellular compartments while cGMP production was maintained. [12]

These observations show that receptor trafficking and signalling can occur simultaneously.

However, they do not prove that every receptor signals in an identical manner following internalisation, nor that all the measured cGMP was generated within one specific cellular compartment.

The statement that peptide hormones typically initiate signalling via surface receptors is therefore a useful generalisation.

However, to state that peptide hormones and their receptors can never be found in intracellular compartments would be too categorical.

Cellular response can be rapid, delayed, or involve both stages

Receptor signalling can alter the activity of proteins already present in the cell.

Such mechanisms can lead to relatively rapid responses.

Signalling pathways can also influence transcription and protein synthesis, leading to effects that develop over a longer period.

Both mechanisms can occur simultaneously.

The same signalling pathway can cause rapid biochemical changes and subsequent transcriptional changes.

In the GLP-1 cellular study, both signalling-related changes and insulin gene expression were measured, demonstrating why cell-surface-initiated signalling should not be regarded as incapable of influencing nuclear processes. [11]

However, the increase in the concentration of the secondary messenger represents only one level of evidence.

Demonstrating the full physiological effect requires, depending on the research question, the combination of successive levels:

receptor activation → intracellular signalling → cell behaviour → tissue response → whole-organism physiology.

Peptide hormones vs steroid hormones

Other chemical bases

Peptide hormones are made up of amino acid chains.

Steroid hormones are derived from cholesterol and possess a characteristic ring system of steroid rings.

Chemical differences affect how both groups of hormones are synthesised, stored, transported, recognised and how they transmit signals.

Peptide hormones generally function well in an aqueous environment, but on the whole they do not pass through the hydrophobic interior of the cell membrane via simple diffusion.

Steroid hormones are more compatible with the lipid environment and can reach intracellular receptor systems.

Many steroid hormones circulate partly bound to transport proteins.

The transport properties of peptide hormones vary between individual molecules.

These are general physiological patterns rather than absolute rules for every representative of both categories.

Molecular modifications, binding proteins, receptor type and tissue context can alter the details.

Classic receptor comparison

A classical comparison is useful in the study of hormonal biology.

However, the two categories should not be presented as completely separate signalling worlds.

A publication discussing peptide and steroid signalling described evidence for rapid steroid action that does not require de novo protein synthesis. [13]

This source is a review based, amongst other things, on scientific discussions, rather than a single experiment confirming one universal mechanism for all steroid hormones.

Why is the statement „peptides work quickly and steroids slowly” too simplistic?

Some peptide hormone responses occur rapidly because signalling pathways modify proteins already present in the cell.

However, peptide signalling can also influence transcription and the synthesis of new proteins, resulting in slower-developing responses.

On the other hand, steroid hormones are not exclusively limited to slow, transcription-dependent effects.

Scientifically more useful questions are:

• Which hormone is being analysed?
• Which receptor is activated?
• Which intracellular pathway is subsequently activated?
• What biological effect is being measured?
• How long after stimulation was the measurement taken?

Comparing the early response of a secondary messenger with the later transcriptional response without specifying the measured effect can lead to a misleading division into „fast” and „slow” hormones. [11] [13]

Both classes of hormones can mutually regulate each other

Peptide and steroid hormones often participate in the same endocrine networks.

A good example is the stress response axis.

Peptide signals from the hypothalamus and pituitary gland are involved in the regulation of steroid production by the adrenal glands, whereas glucocorticoids participate in feedback acting on the earlier elements of this system.

Feedback, however, should not be thought of as an instant on–off switch.

A study of CRH neurones in rats showed that secretion, gene activation, and glucocorticoid-dependent feedback depended on the type of stimulus and the timing of measurement. [10]

This supports a more detailed model of hormonal regulation, in which individual elements of the system can respond on different time scales.

Feedback and signal termination

Negative feedback helps to stabilise physiological parameters

Negative feedback occurs when the consequences of a physiological response diminish the stimulus leading to further signalling.

The adjustable parameter may include, among other things:

• nutrient availability;
• ion concentration;
• water management;
• blood volume;
• endocrine gland activity;
• another physiological parameter.

A good example is the calcium–PTH system.

A lower extracellular calcium concentration usually promotes PTH secretion, whereas an increase in it can inhibit this secretion.

In a study involving healthy young men, a decrease in blood PTH concentration was observed following the administration of a calcium stimulus. [4]

This demonstrates the physiological relationship present under the conditions of this study.

However, it does not form the basis for supplementation recommendations, nor does it allow for the prediction of every response occurring in the disease.

Feedback can operate on many levels, among other things through:

• hormone secretion;
• the production of the precursor;
• gene transcription;
• receptor sensitivity;
• subsequent stages of signalling;
• interactions between endocrine tissues.

It may also work with a certain delay.

Therefore, short-term response and long-term adaptation should not automatically be treated as the same process.

Hormone degradation limits the exposure time

Peptide hormones can be cleaved by enzymes, which alters their molecular structure and biological activity.

Depending on the specific molecule, tissue uptake, kidneys, liver and other elimination processes may also participate in its removal.

GLP-1 studies demonstrated the formation of an N-terminally truncated metabolite in human plasma and indicated the involvement of dipeptidyl peptidase IV (DPP-IV) in this reaction. The researchers also differentiated the intact peptide from its metabolites in samples collected from humans. [14]

This distinction is important because the disappearance of the intact hormone and the appearance of related peptide fragments are not the same measurement.

Similarly, the peptide stability measured in an isolated plasma experiment should not automatically be equated with its full duration of action in a living organism.

The observed time course may be influenced by secretion, distribution, blood flow, tissue uptake, enzymatic cleavage and elimination.

The cell also turns off its own response

Signal termination does not depend solely on the removal of the hormone from the circulation.

Cells possess many mechanisms that decrease intracellular signalling.

For example:

• G proteins can return to an inactive state;
• cyclic nucleotides can be degraded;
• protein phosphatases can remove phosphate groups;
• the concentration and distribution of intracellular calcium can return towards resting values;
• signal proteins can be inhibited;
• receptors can undergo desensitisation;
• Receptors can be relocated to other cellular compartments.

These mechanisms provide multiple control points.

The hormone may still be present while the cell becomes less sensitive to its action.

On the other hand, some intracellular consequences may persist for a time after the hormone concentration outside the cell has decreased.

Measuring only a single route element therefore does not allow the condition of the entire signalling network to be determined.

Desensitisation changes receptor sensitivity

Desensitisation means a reduction in the system's response during continuous or repeated stimulation.

Depending on the type of receptor, the mechanisms may include:

• receptor phosphorylation;
• dephosphorylation;
• change of coupling with signalling partners;
• receptor internalisation;
• change in the number of receptors;
• changes in proteins involved in downstream signalling pathways.

The study of the guanylyl cyclase A receptor for ANP described a complex pattern of phosphorylation and dephosphorylation associated with a decreased response. [15]

The results did not support a simple model in which every phosphorylation activates the receptor and every dephosphorylation deactivates it.

Receptor internalisation should also not be automatically equated with the complete termination of the signal.

The consequences depend on the type of receptor, its destination inside the cell, the potential for reuse, degradation, and whether signalling can continue from intracellular compartments.

What does a hormone measurement show, and what does it not show?

The measured hormone concentration value may be the result of several processes occurring simultaneously:

production + processing + secretion + distribution − degradation − uptake − elimination.

The analytical method can also detect:

• intact hormone;
• precursor molecules;
• metabolites;
• fission fragments;
• several related molecular forms.

Which forms are detected depends on the test design.

In the GLP-1 degradation study, this issue was analysed by combining chromatographic separation with immunoassays and structural identification of the metabolite. [14]

The distinction was important because the result representing total immunoreactivity did not necessarily correspond to the measurement solely of the intact, biologically relevant peptide.

Hormone concentration, receptor binding, secondary messenger production, gene expression and physiological function are therefore separate types of measurement.

Combining these levels makes it possible to create a much stronger mechanistic explanation than treating a single result as a representation of the entire hormonal system.

Frequently asked questions

1. Do peptide hormones enter cells?

Most peptide hormones initiate signalling by binding to receptors located on the cell surface rather than freely diffusing through the cell membrane into the cytoplasm. Subsequent receptor-mediated internalisation may, however, lead to the translocation of hormone–receptor complexes or associated receptor systems into membrane-bound intracellular compartments. In studies of the ANP receptor, receptor translocation has been observed while cGMP production was maintained in a specific cell model. This does not imply free diffusion of the hormone through the membrane, nor does it prove that every peptide hormone utilises the same mechanism. Initial cell-surface recognition and subsequent receptor-mediated internalisation are distinct stages of signalling. [12]

2. Are all peptides hormones?

No. A peptide is defined primarily by its molecular structure based on amino acids, whereas a hormone is defined by its role in physiological communication. Confirming a hormonal function requires data regarding production, regulated release, responsive tissues, receptor action, and biological response. Early human studies of GLP-1 show how different types of evidence can jointly confirm a hormonal role. Many other peptides are protein fragments, metabolic molecules, structural components, or perform functions unrelated to the endocrine system. Therefore, the mere presence of peptide bonds is not enough to consider a molecule a hormone. [1]

3. What happens first when a peptide hormone reaches its target cell?

Typically, a hormone first interacts with a corresponding receptor located on the cell surface or directly associated with it. The binding alters the activity or conformation of the receptor and triggers intracellular signalling. The subsequent course depends on the class of receptor. In the case of the insulin receptor, studies have linked its activation to tyrosine phosphorylation and kinase activity. These results apply to one specific receptor system and not to all peptide hormones. Therefore, the identification of the correct receptor is one of the first steps needed to understand a given cell's response. [3]

4. How do peptide hormones affect target cells?

Peptide hormones affect cells through signalling pathways triggered by receptors. They can alter enzyme activity, membrane transport, ion homeostasis, secretion, metabolism, protein phosphorylation, and gene expression. In a rat pancreatic islet cell line, GLP-1 affected cAMP concentration, insulin-related mRNA, and insulin release. This experiment demonstrates several levels of action of a single peptide signal on a cell, but it does not mean that every tissue reacts in the same way. The final effect depends on the specific hormone, receptor, intracellular mechanisms, cell state, and the experimental or physiological context. [11]

5. Why do some cells respond to a hormone and others do not?

A cell usually needs to have an appropriate functional receptor and the signalling mechanisms required to process the received signal. The mere presence of a hormone does not therefore guarantee a response. The number of receptors, their structure, localisation, coupling proteins and prior stimulation of the cell are all important. Mutation studies of natriuretic peptide receptors have shown that changes in the regulatory region can disrupt proper hormone-dependent activation. Although these were experimental studies, they clearly demonstrate why, in addition to the presence of the hormone, receptor structure and cellular context are also crucial. [6]

6. What is the difference between a preprohormone and a prohormone?

The preprohormone contains a signal sequence, which typically helps direct the newly synthesised molecule to the secretory pathway. Removal of this sequence usually leads to the formation of a prohormone, which can then undergo subsequent stages of cleavage and modification, leading to the formation of one or more mature products. Research on proglucagon shows that the final products can strongly depend on the tissue and the processing enzymes present within it. A preprohormone, prohormone, intermediate form and mature hormone therefore represent different molecular stages, rather than interchangeable names for the same substance. [7] [8]

7. Can a single precursor lead to the formation of different hormones?

Yes. Different cells can process the same precursor into different products because they possess different enzymatic machinery. Studies of human proglucagon revealed distinct patterns of products in the pancreas and intestine, and subsequent experiments analysed the involvement of PC2 and PC3 in their formation. The results do not mean that identical processing occurs in every species, tissue or physiological state. Instead, they demonstrate a general principle: both the precursor sequence and the cellular enzymatic environment influence which mature peptide products are ultimately formed. [7] [8]

8. Why are many peptide hormones stored in granules?

Secretory granules provide a ready store of hormone that can be released upon the arrival of an appropriate physiological signal. This allows secretion to increase rapidly without the need to synthesise every molecule from scratch at that exact moment. The granules are actively regulated, and their different pools may vary in their readiness for release. Experiments on human beta cells have indicated the involvement of Munc18c in insulin release from various granule pools. This mechanism does not explain the functioning of all endocrine cells, but it shows how specialised exocytotic machinery links storage with controlled secretion. [9]

9. Do all peptide hormones use cAMP?

No. cAMP is an important secondary messenger for some peptide hormone receptors, but the whole group uses many different receptor systems. Insulin acts via a receptor tyrosine kinase, whereas ANP activates a receptor associated with cGMP production. Other peptide hormone receptors may use different GPCR pathways, including systems affecting intracellular calcium. Research on insulin and ANP receptors shows why peptide hormone signalling cannot be reduced to a single cAMP pathway. Before describing the mechanism, one must therefore specify the particular hormone and its receptor. [3] [6]

10. Can peptide hormones alter gene expression?

Yes. A signal starting at the cell surface can be transmitted through successive intracellular pathways and ultimately affect transcription. In GLP-1 studies in cultured rat cells, changes in insulin mRNA were observed, whereas experiments with GnRH pulses in rats showed changes in gonadotropin subunit transcription. These results relate to specific models and do not mean that every peptide hormone response requires new gene expression. However, they show that signalling starting from a membrane receptor can influence both rapid biochemical processes and transcription regulation occurring over a longer timeframe. [5] [11]

11. Do peptide hormones always act faster than steroid hormones?

No. The speed of response depends on the receptor, the signalling pathway, the biological effect under investigation and the timing of the measurement. Peptide hormones can rapidly change the activity of existing proteins, but they can also affect gene expression and cause later responses. In contrast, endocrinological literature describes rapid actions of steroids that do not require new protein synthesis. The source concerning steroids used in this article is a review rather than a single decisive experiment. A more reliable distinction relates to the chemistry of the molecules and receptor mechanisms rather than a simple division into „fast peptides” and „slow steroids”. [11] [13]

12. Why does the frequency of hormone pulses matter?

Cells can respond differently to repeated pulses, different pulse frequencies and continuous exposure, even when it is the same hormone every time. In GnRH-deficient rats, altering the pulse frequency affected transcription patterns of gonadotropin subunits in the pituitary gland and local regulatory responses. These experiments confirm the importance of timing in this biological model, but do not determine the hormone administration schedule for humans. Hormonal information can therefore be encoded not only by the concentration of the molecule, but also by the temporal pattern in which target cells are exposed to its action. [5]

13. What is negative feedback in the regulation of peptide hormones?

Negative feedback occurs when the consequences of a given pathway's action reduce the stimulus leading to further hormone secretion or the activity of earlier elements of the endocrine system. An example is calcium regulation: in a controlled study involving healthy men, an increase in calcium availability was associated with a decrease in PTH concentration. This experiment does not reflect every disease state or long-term adaptation, but it shows how a regulated parameter can provide feedback to the endocrine system. Feedback can involve secretion, gene expression, receptor sensitivity, the influence of other hormones and many other levels of regulation. [4]

14. How is the peptide hormone signal turned off?

Several mechanisms can operate simultaneously. The hormone located outside the cell can be enzymatically degraded, captured by tissues, or removed from the bloodstream. Receptors can become less sensitive or move between different cell compartments, while secondary signaling can be degraded and protein phosphorylation reversed. In GLP-1 studies, an enzymatically produced metabolite was identified, while ANP receptor studies linked desensitization to changes in its phosphorylation. None of these mechanisms is universal for all hormones. Therefore, the termination of signaling is best understood as coordinated control by both the extracellular transmitter and the cell that responds to it.[14][15]

15. What does receptor desensitization mean?

Desensitization means that the receptor system begins to generate a weaker response during continuous or repeated stimulation. Depending on the receptor, this may result from changes in phosphorylation, the way it interacts with signaling proteins, the movement of the receptor, changes in its number, or adaptation of further elements of the pathway. In experiments involving the ANP receptor, a reduction in response has been associated with a specific pattern of phosphorylation changes. This does not mean that every receptor undergoes desensitization in the same way, nor that a reduced response indicates receptor damage. Desensitization is a functional change whose mechanism must be determined separately for each system.[15]

16. Does a higher concentration of the hormone always mean stronger effects?

No. The hormone concentration is only one of the factors influencing the biological response. The importance also has to do with the number of receptors, the degree of their occupancy, the intracellular signaling system, the feedback mechanism, previous exposure, desensitization, and the current physiological state of the target cell. Experiments on the desensitization of the ANP receptor show that sensitivity can be reduced despite continued exposure to the activating hormone. Such experimental data do not allow predicting the individual hormonal response based on a single measurement of the concentration. Hormone concentration and the biological response are related, but they are not the same parameter.[15]

17. Can a hormonal test detect inactive or altered fragments?

Yes, depending on the method used, which fragment of the molecule is recognized. GLP-1 studies have shown why distinguishing the intact hormone from the metabolites shortened at the N-terminus is important when interpreting the results of immunological tests. A test that detects both forms may answer a different question than a method designed solely to detect the intact peptide. The limitation therefore depends on the specific analytical method. Before interpreting the result as the concentration of the active hormone, it is necessary to determine which molecular forms are actually detected. [14]

18. Are peptide hormones and protein hormones completely different categories?

Not necessarily. The terminology is partially overlapping because both groups are composed of amino acid chains, and there is no universally accepted threshold length separating all peptide hormones from all protein hormones. The word „hormone” refers to the signaling role, while the terms „peptide” and „protein” refer to the molecular structure and size aspects. In practice, larger hormones with an amino acid structure are more often referred to as proteins, and shorter ones as peptides. However, the exact name of the category is less informative than identifying a specific molecule, its mature structure, receptor, and experimentally confirmed function.

19. Can the same peptide hormone act differently in different tissues?

Yes. The action of a hormone depends not only on the recognition by the receptor but also on the molecular mechanisms available within the target cell. Individual tissues may differ in the number of receptors, signaling proteins, enzymes, transporters, transcription factors, and regulatory proteins. As a result, the activation of the same receptor may not lead to identical physiological effects in every cell environment. Therefore, hormonal signaling is best understood as an interaction between a specific extracellular signal and a specific responding cell, rather than as a fixed effect encoded solely in the molecule itself.

20. Is the measurement of the secondary transmitter sufficient to confirm the physiological effect?

No. A change in the concentration of cAMP, cGMP, calcium, the degree of phosphorylation, or another intracellular signal constitutes evidence that a particular element of the pathway has reacted, but does not automatically confirm the full effect at the tissue or whole-organism level. In the GLP-1 study, changes in cAMP in cells were associated with additional endpoints, whereas studies of insulin and ANP receptors analyzed other specific biochemical steps. [3] [6] [11] A strong mechanistic explanation should distinguish receptor activation, intracellular signaling, cell function, tissue response, and the physiology of the entire organism, rather than treating a single measurement as evidence of all these levels.

Limitations of the interpretation of studies on peptide hormones

When moving from basic mechanisms of hormonal signaling to broader biological conclusions, several limitations should be kept in mind.

First, peptide hormones do not constitute a functionally homogeneous group. Insulin, GLP-1, vasopressin, PTH, GnRH, and ANP belong to a broad category of amino acid-based hormones, but they act through different receptors and participate in different physiological systems.[1][2][3][4][5][6]

Second, detecting a precursor does not necessarily mean detecting a mature hormone. Tissue-dependent processing can lead to the formation of different products from the same precursor, as exemplified by studies of proglucagon.[7][8]

Thirdly, synthesis and secretion are separate processes. An increased release of the hormone does not necessarily mean an identical and simultaneous increase in the transcription of the gene encoding its precursor. [10]

Fourthly, binding to the receptor is not synonymous with a full physiological response. Signaling involves several molecular levels, and different cells can convert the activation of the same receptor into different responses.

Fifthly, studies on cell cultures and purified receptors do not recapitulate the entire organism. They may provide important information about molecular mechanisms, but conclusions should remain linked to the experimental system used. [3] [6] [11] [12]

Sixthly, time matters. Pulsatile and continuous exposure to the hormone can lead to different responses, and a single measurement in the blood may not reflect the full secretion rhythm. [5]

Seventhly, hormonal tests differ in molecular specificity. Some methods can detect metabolites, precursor-related molecules, or multiple forms simultaneously, in addition to the intact hormone.[14]

After eighth, receptor internalization does not necessarily mean the immediate termination of signaling, because receptor transport and signal transduction can occur simultaneously under certain conditions.[12]

Ninthly, desensitization is dependent on the specific receptor. Mechanisms identified for one receptor should not be automatically applied to all receptors of peptide hormones. [15]

Finally, the chemical classification does not confirm the medicinal action, benefits, or therapeutic utility. Evidence regarding a single specific hormone and a single experimental system should not be generalized to other unrelated peptides simply because they also have an amino acid structure.

Summary

Peptide hormones are signaling molecules composed of amino acids that enable communication between cells and tissues in many physiological systems.

Their biology is much more complex than the simple statement that a peptide binds to a receptor.

Many peptide hormones initially form as larger precursor molecules, then undergo specific tissue-specific processing steps, are packaged into secretory granules and released in response to regulated physiological signals. Studies of the processing of proglucagon and exocytosis of insulin granules show how the processing mechanisms of precursors and secretion influence which hormone enters the extracellular environment and when it is released. [7] [8] [9]

After being released, peptide hormones usually initiate signaling through receptors located on the surface of cells, but these receptors belong to several different molecular classes.

Some hormones utilize GPCRs and secondary messengers such as cAMP or calcium-related pathways.

Insulin uses the receptor tyrosine kinase.

ANP acts through a receptor associated with the production of cGMP.

These examples show that there is no single universal intracellular pathway common to all peptide hormones.[3][6][11]

Cellular response can also develop over different time scales.

Signaling can rapidly modify existing proteins, influence secretion or transport, and then lead to changes in gene transcription. Temporal patterns, such as pulsatile hormone secretion, can themselves convey biologically important information.[5][11]

Hormonal signaling is also actively restricted.

Degradation of the hormone, elimination by tissues, reversal of intracellular signals, displacement of receptors, desensitization, and negative feedback all help to control the strength and duration of the response.[4][12][14][15]

The most important principle can therefore be summarized as follows:

The actions of a biological peptide hormone cannot be predicted solely on the basis of the fact that it is a peptide. Its function results from the entire signaling system: from what molecular form it forms, how it is processed and released, which receptor recognizes it, how the target cell interprets the receptor signal, how exposure changes over time, and how the system limits and ultimately terminates the response.

Disclaimer

The article is intended for educational and scientific informational purposes only. It explains the biology of peptide hormones, the processing of precursors, secretion, receptor signaling, feedback mechanisms, degradation, and examples derived from published studies. It does not constitute medical advice, diagnostic guidance, treatment recommendations, dosage instructions, administration instructions, or recommendations regarding the use of peptide hormones or related substances. The results of biochemical, cellular, animal, and human studies should be interpreted in the context of the project, the conditions, the studied population, and the limitations of the individual studies.

References

1. Kreymann, B., Williams, G., Ghatei, M. A., & Bloom, S. R. (1987). Glucagon-like peptide-1 7-36: A physiological incretin in man. The Lancet, 2(8571), 1300–1304. https://doi.org/10.1016/s0140-6736(87)91194-9
2. Elliot, S., Goldsmith, P., Knepper, M., Haughey, M., & Olson, B. (1996). Urinary excretion of aquaporin-2 in humans: A potential marker of collecting duct responsiveness to vasopressin. Journal of the American Society of Nephrology, 7(3), 403–409. https://doi.org/10.1681/ASN.V73403
3. Kwok, Y. C., Nemenoff, R. A., Powers, A. C., & Avruch, J. (1986). Kinetic properties of the insulin receptor tyrosine protein kinase: Activation through an insulin-stimulated tyrosine-specific, intramolecular autophosphorylation. Archives of Biochemistry and Biophysics, 244(1), 102–113. https://doi.org/10.1016/0003-9861(86)90098-6
4. Ortolani, S., Scotti, A., & Cherubini, R. (2003). Rapid suppression of bone resorption and parathyroid hormone secretion by acute oral administration of calcium in healthy adult men. Journal of Endocrinological Investigation, 26(4), 353–358. https://doi.org/10.1007/BF03345184
5. Burger, L. L., Dalkin, A. C., Aylor, K. W., Haisenleder, D. J., & Marshall, J. C. (2002). GnRH pulse frequency modulation of gonadotropin subunit gene transcription in normal gonadotropes—assessment by primary transcript assay provides evidence for roles of GnRH and follistatin. Endocrinology, 143(9), 3243–3249. https://doi.org/10.1210/en.2002-220216
6. Koller, K. J., de Sauvage, F. J., Lowe, D. G., & Goeddel, D. V. (1992). Conservation of the kinaselike regulatory domain is essential for activation of the natriuretic peptide receptor guanylyl cyclases. Molecular and Cellular Biology, 12(6), 2581–2590. https://doi.org/10.1128/mcb.12.6.2581-2590.1992
7. Orskov, C., Holst, J. J., Poulsen, S. S., & Kirkegaard, P. (1987). Pancreatic and intestinal processing of proglucagon in man. Diabetology, 30(11), 874–881. https://doi.org/10.1007/BF00274797
8. Rouillé, Y., Martin, S., & Steiner, D. F. (1995). Differential processing of proglucagon by the subtilisin-like prohormone convertases PC2 and PC3 to generate either glucagon or glucagon-like peptide. Journal of Biological Chemistry, 270(44), 26488–26496. https://doi.org/10.1074/jbc.270.44.26488
9. Zhu, D., Xie, L., Karimian, N., Liang, T., Kang, Y., Huang, Y. C., & Gaisano, H. Y. (2015). Munc18c mediates exocytosis of pre-docked and newcomer insulin granules underlying biphasic glucose-stimulated insulin secretion in human pancreatic beta-cells. Molecular Metabolism, 4(5), 418–426. https://doi.org/10.1016/j.molmet.2015.02.004
10. Tanimura, S. M., Sanchez-Watts, G., & Watts, A. G. (1998). Peptide gene activation, secretion, and steroid feedback during stimulation of rat neuroendocrine corticotropin-releasing hormone neurons. Endocrinology, 139(9), 3822–3829. https://doi.org/10.1210/endo.139.9.6191
11. Drucker, D. J., Philippe, J., Mojsov, S., Chick, W. L., & Habener, J. F. (1987). Glucagon-like peptide I stimulates insulin gene expression and increases cyclic AMP levels in a rat islet cell line. Proceedings of the National Academy of Sciences of the United States of America, 84(10), 3434–3438. https://doi.org/10.1073/pnas.84.10.3434
12. Mani, I., Garg, R., Tripathi, S., & Pandey, K. N. (2015). Subcellular trafficking of guanylyl cyclase/natriuretic peptide receptor-A with concurrent generation of intracellular cGMP. Bioscience Reports, 35(5), Article e00260. https://doi.org/10.1042/BSR20150136
13. Watson, C. S. (1999). Signaling themes shared between peptide and steroid hormones at the plasma membrane. STKE of Science, 1999(12), PE1. https://doi.org/10.1126/stke.1999.12.pe1
14. Deacon, C. F., Johnsen, A. H., & Holst, J. J. (1995). Degradation of glucagon-like peptide-1 by human plasma in vitro yields an N-terminally truncated peptide that is a major endogenous metabolite in vivo. The Journal of Clinical Endocrinology & Metabolism, 80(3), 952–957. https://doi.org/10.1210/jcem.80.3.7883856
15. Schröter, J., Zahedi, R. P., Hartmann, M., Gassner, B., Gazinski, A., Waschke, J., Sickmann, A., & Kuhn, M. (2010). Homologous desensitization of guanylyl cyclase A, the receptor for atrial natriuretic peptide, is associated with a complex phosphorylation pattern. The FEBS Journal, 277(11), 2440–2453. https://doi.org/10.1111/j.1742-4658.2010.07658.x