Peptide hormones: receptors, signaling, and biological functions

Peptide hormones are chemical messengers made up of amino acid chains. Specialized 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 with peptides. The biological response depends primarily on whether a given cell possesses the appropriate receptor and intracellular signaling mechanism allowing it to interpret the received signal.

Insulin, glucagon, glucagon-like peptide-1 (GLP-1), vasopressin, parathyroid hormone (PTH), gonadotropin-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 common effect.

For example, human GLP-1 studies investigated 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 utilizing different receptors, target tissues, and response mechanisms. [1] [2]

The mechanism of action of a peptide hormone can be most easily understood by tracing the entire process from start to finish:

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

Each of these stages can influence the final biological response.

This approach also helps avoid a major misunderstanding. Detecting a peptide, observing its binding to a receptor, measuring a secondary messenger, and demonstrating a whole-organism physiological response represent different levels of evidence. A full interpretation requires establishing which of these stages was actually investigated.

What are peptide hormones?

Amino acid chain transmitting a biological signal

Peptide hormones are composed of amino acid residues linked peptide bonds.

Some are relatively short chains, while 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 for peptide hormones encompasses signaling molecules composed 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 signaling 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.

However, biological communication is not limited exclusively to this mechanism. Related molecules can also participate in local communication between neighboring cells and in other forms of cellular signaling.

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

Many peptides include, among others:

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

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

Early human GLP-1 studies well illustrate this approach.

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

Altogether, these observations supported the concept of a physiological signaling role for this molecule.

However, this study should not be treated as evidence 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 triggering the same response in every one of them.

For a cell to respond, it typically must possess an appropriate receptor and intracellular molecular mechanisms that allow it to process the signal generated after its activation.

Receptors recognize specific characteristics of molecules.

Recognition may depend, among other things, on:

• amino acid sequence;
• spatial structure;
• cargo placement;
• chemical modifications;
• receptor conformation;
• availability of appropriate 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 also matters.

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

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

Receptor activation can affect, among other things:

• enzyme activity;
• ion transport;
• membrane channels;
• bubble migration;
• secretion;
• metabolism;
• protein phosphorylation;
• gene transcription;
• a combination of several 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 about the biological effect.

Peptide hormones are involved in many different systems.

These examples represent distinct physiological functions rather than shared 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 analyzes 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 synthesized as larger molecules called preprohormones.

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

After the removal of this sequence, 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 classic regulated secretory pathway, the resulting 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 disulfide bonds;
• enzymatic cleavage;
• chain end modifications;
• other chemical modifications;
• sorting;
• packaging into secretory granules.

These processes do not occur identically for every peptide hormone.

One precursor can contain several potential peptide products, and different cell types can process the same precursor differently.

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

Processing of the precursor affects which hormone will be produced

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

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

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

The results showed that the enzymatic environment strongly influenced whether the processing resembled a pancreas- or intestine-like pattern. The studies also revealed limitations in reproducing all cleavage steps using an isolated enzymatic system. [8]

The general rule is important:

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

This is also important for laboratory measurements.

A test that recognizes multiple forms associated with a single precursor can yield different results than a method specifically designed to detect a single mature hormone.

Therefore, the reported concentration should 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 the 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 granularity, and prolonged secretion requires further production, processing, and replenishment of supplies.

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

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

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

However, they do not constitute a full explanation of all molecular processes regulating insulin secretion.

Different hormonal systems react to different stimuli

Individual endocrine cells respond to various physiological signals.

They may include:

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

Intracellular calcium is often involved in the fusion of secretory vesicles, but the processes leading to the generation of the calcium signal differ 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 neurons showed that gene activation and secretion-related responses had different reaction thresholds during sustained stress. [10]

Increased hormone release therefore 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 described solely by its average concentration.

The time pattern may include:

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

Different time patterns can lead to distinct cellular responses.

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

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

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

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

Interpretation may depend on:

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

Receptors and cellular signaling

What happens when a peptide hormone reaches a 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 a hormone to a receptor changes its activity or conformation, enabling the transmission of information into the cell.

The signal thus crosses the membrane thanks to 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 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 signaling proteins.

General scheme:

hormone → receptor → intracellular signal → cellular response

It is very useful.

However, there is no single common intracellular pathway characteristic of 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 secondary 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 downstream signaling proteins.

Other GPCR receptors may use different mechanisms.

Importantly, not every GPCR receptor increases cAMP concentration.

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

The statement that a hormone „uses a secondary messenger” therefore describes a general signaling strategy rather than a single universal molecular mechanism.

An early study conducted on a rat pancreatic islet cell line found that the processed form of GLP-1 increased cAMP, insulin-related mRNA levels, 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 signaling and signal amplification

Another commonly encountered 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 translate receptor activation into a change in cellular activity.

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

Signaling cascades can also amplify the signal.

One activated receptor can interact with multiple molecules further down the pathway, and one activated enzyme can generate multiple secondary 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 messengers;
• receptor desensitization;
• 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 signaling mechanism therefore differs fundamentally from the classical GPCR mechanism.

Insulin binding leads to the activation of the intracellular kinase portion of the receptor.

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

Biochemical studies using purified insulin receptors 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 signaling should not be summarized by the statement:

„a peptide hormone binds to a GPCR and increases cAMP.

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

Natriuretic peptide receptors can produce cGMP

ANP is an example of yet another receptor mechanism.

His guanylyl cyclase receptor A is linked to the generation 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 on natriuretic peptide receptors showed that the kinase-like regulatory region played a crucial role in controlling receptor activity. Alterations 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 by the same tyrosine kinase mechanism as the insulin receptor.

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

Surface binding does not mean the receptor always stays 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 trafficking of ANP receptors to intracellular compartments while cGMP production was sustained. [12]

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

They do not prove, however, that every receptor signals in an identical manner after internalization, nor that all measured cGMP was generated in one specific cellular compartment.

The statement that peptide hormones typically initiate signaling through surface receptors is therefore a useful generalization.

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

The cellular response can be rapid, delayed, or involve both stages

Receptor signaling can change the activity of proteins already present in the cell.

Such mechanisms can lead to relatively quick responses.

Signaling pathways can also influence transcription and protein synthesis, leading to long-term effects.

Both mechanisms can occur simultaneously.

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

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

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

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

receptor activation → intracellular signaling → cell behavior → tissue response → whole organism physiology.

Peptide hormones vs steroid hormones

Other chemical bases

Peptide hormones are composed of amino acid chains.

Steroid hormones are formed from cholesterol and have a characteristic structure of steroid rings.

The chemical differences affect the way both groups of hormones synthesize, store, transport, recognize, and transmit the signal.

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

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

Many steroid hormones circulate in part associated with transport proteins.

The transport properties of peptide hormones vary between different molecules.

These are general physiological patterns, not absolute rules for every member of both categories.

Molecular modifications, binding proteins, receptor types, and tissue context can all change the details.

Classical comparison of receptors

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

However, it should not be presented that both categories are completely separate signaling worlds.

The publication discussing peptide and steroid signaling described evidence of rapid action of steroids that does not require new protein synthesis.[13]

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

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

Some responses to peptide hormones occur quickly because the signaling pathways modify the proteins already present in the cell.

Peptide signaling can also affect the transcription and synthesis of new proteins, causing responses to develop more slowly.

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

More scientifically useful questions are:

• Which hormone is being analyzed?
• Which receptor is activated?
• Which intracellular pathway is then activated?
• What biological effect is being measured?
• After how long after stimulation was the measurement performed?

Comparing the early response of the secondary transmitter 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 regulate each other

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

A good example is the eight responses to stress.

Peptide signals from the hypothalamus and pituitary gland participate in the regulation of steroid production by the adrenal glands, while glucocorticosteroids participate in the feedback loop acting on the earlier components of this system.

However, feedback control should not be imagined as an immediate „on-off” switch.

The study of CRH neurons in rats showed that the secretion, gene activation, and glucocorticoid-dependent feedback mechanisms depended on the type of stimulus and the moment of measurement. [10]

This supports a more detailed model of hormonal regulation, in which the individual components of the system can respond at different timescales.

Feedback and signal termination

Negative feedback helps stabilize physiological parameters

Negative feedback occurs when the consequences of a physiological response reduce the stimulus leading to further signaling.

The regulated parameter can include, among other things:

• availability of nutrients;
• ion concentration;
• water economy;
• volume of blood;
• activity of the endocrine organ;
• another physiological parameter.

A good example is the calcium–PTH system.

Lower extracellular calcium levels usually favor PTH secretion, whereas its increase can inhibit this secretion.

In a study conducted with healthy young men, a reduction in PTH levels in the blood was observed after the application of a calcium stimulus.[4]

This shows the physiological dependence that exists under the conditions of this study.

However, it does not serve as a basis for recommendations regarding supplementation or predict every response that may occur in the disease.

Feedback can operate on many levels, including:

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

It can also work with some delay.

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

Degradation of hormones limits the exposure time

Peptide hormones can be broken down by enzymes, which changes 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 showed that a truncated metabolite was formed at the N-terminus in human plasma and indicated the involvement of dipeptidyl peptidase IV (DPP-IV) in this reaction. The researchers also distinguished 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 measure.

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

The observed time course can be influenced by secretion, distribution, blood flow, uptake by tissues, enzymatic breakdown, and elimination.

The cell also disables its own response

The termination of the signal does not depend solely on removing the hormone from the bloodstream.

Cells possess many mechanisms that reduce intracellular signaling.

For example:

• G proteins can return to an inactive state;
• cyclic nucleotides can be degraded;
• Protein phosphatases can remove phosphate groups;
• the intracellular calcium concentration and distribution may return towards resting levels;
• signaling proteins can be inhibited;
• the receptors may become desensitized;
• The receptors can be moved to other cell compartments.

These mechanisms provide many checkpoints.

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 some time after the concentration of the hormone outside the cell has decreased.

Measuring only one element of the route therefore does not allow determining the state of the entire signaling network.

Desensitization changes the sensitivity of the receptor

Desensitization means reducing the response of the system during continuous or repeated stimulation.

Depending on the type of receptor, mechanisms may include:

• receptor phosphorylation;
• dephosphorylation;
• change in the connection with signaling partners;
• receptor internalization;
• a change in the number of receptors;
• changes in the proteins involved in the subsequent stages of signaling.

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

The results did not confirm the simple model in which each phosphorylation activates the receptor and each dephosphorylation turns it off.

The internalization of the receptor should also not be automatically equated with the complete termination of the signal.

The consequences depend on the type of receptor, its target location within the cell, the possibility of reuse, degradation, and whether signaling can continue from intracellular compartments.

What does the hormone measurement show, and what doesn't it show?

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

production + processing + excretion + distribution − degradation − absorption − elimination.

The analytical method can also detect:

• an intact hormone;
• precursor molecules;
• metabolites;
• fragments formed as a result of splitting;
• several related molecular forms.

Which forms will be detected depends on the structure of the test.

In the GLP-1 degradation study, this problem was analyzed by combining chromatographic separation with immunological tests and structural identification of the metabolite.[14]

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

The hormone concentration, receptor binding, production of the secondary messenger, gene expression, and physiological function are therefore separate types of measurements.

Combining these levels allows for 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 penetrate into cells?

Most peptide hormones initiate signaling by binding to receptors located on the cell surface, rather than freely penetrating the cell membrane into the cytoplasm. However, subsequent receptor-mediated internalization can lead to the transfer of hormone–receptor complexes or associated receptor systems into the membrane-enclosed intracellular compartments. In studies of the ANP receptor, movement of the receptor was observed while maintaining cGMP production in a specific cell model. This does not mean free passage of the hormone through the membrane or indicate that each peptide hormone uses the same mechanism. Initial recognition on the cell surface and subsequent receptor-mediated internalization are separate stages of signaling. [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. Confirmation of a hormonal function requires data regarding production, regulated release, responding tissues, receptor activity, and biological response. Early studies of GLP-1 in humans show how various types of evidence can collectively confirm a hormonal role. Many other peptides are fragments of proteins, metabolic molecules, structural components, or have functions unrelated to the hormonal system. Therefore, the mere presence of peptide bonds is not sufficient to recognize a molecule as a hormone. [1]

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

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

4. How do peptide hormones affect target cells?

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

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

The cell usually must possess the appropriate functional receptor and signaling mechanisms necessary to process the received signal. Therefore, the mere presence of a hormone does not guarantee a response. The importance of the number of receptors, their structure, localization, binding proteins, and previous stimulation of the cell are all factors. Mutations in the receptors of natriuretic peptides have shown that changes in the regulatory region can disrupt the proper hormone-dependent activation. Although these were experimental studies, they well demonstrate why, in addition to the presence of the hormone, the receptor structure and the cellular context are also important.[6]

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

The preprohormone contains a signaling sequence that usually helps direct the newly synthesized molecule to the secretory pathway. Removal of this sequence usually results in the formation of a prohormone, which can then undergo further cleavage and modification steps leading to the formation of one or more mature products. Studies of proglucagon show that the final products may strongly depend on the tissue and the enzymes present in it for processing. Therefore, the preprohormone, prohormone, intermediate form, and mature hormone represent different molecular stages, rather than interchangeable names for the same substance.[7][8]

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

Yes. Different cells can process the same precursor into different products because they possess different enzymatic mechanisms. In human studies of proglucagon, different patterns of products were observed in the pancreas and intestine, and subsequent experiments analyzed the role of PC2 and PC3 in their formation. The results do not imply that identical processing occurs in every species, tissue, or physiological state. They do, however, indicate a general principle: both the precursor sequence and the cellular enzymatic environment influence which mature peptide products ultimately form. [7] [8]

8. Why are many peptide hormones stored in the granules?

The secretory granules provide a prepared reserve of the hormone, which can be released after the appropriate physiological signal occurs. This allows secretion to increase rapidly without the need to synthesize each molecule from scratch at that exact moment. The secretory granules are actively regulated, and different clusters of them can differ in their readiness to release. Experiments on human beta cells indicated the involvement of Munc18c in the release of insulin from different clusters of secretory granules. This mechanism does not explain the function of all endocrine cells, but it shows how a specialized exocytotic machinery combines 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 entire group uses many different receptor systems. Insulin acts through the receptor tyrosine kinase, while ANP activates a receptor associated with the production of cGMP. Other peptide hormones may use different GPCR pathways, including those affecting intracellular calcium. Studies of insulin and ANP receptors show why the signaling of peptide hormone signaling cannot be attributed to a single cAMP pathway. Therefore, before describing the mechanism, it is necessary to identify the specific hormone and its receptor.[3][6]

10. Can peptide hormones change gene expression?

Yes. The signal initiated on the cell surface can be transmitted through subsequent intracellular pathways and ultimately influence transcription. In GLP-1 studies in mouse cells, changes in insulin mRNA were observed, while experiments with GnRH pulses in mice showed changes in the transcription of gonadotropin subunits. These results apply to specific models and do not imply that every response to a peptide hormone requires new gene expression. However, they do show that signaling initiated by the membrane receptor can influence both rapid biochemical processes and long-term transcriptional regulation. [5] [11]

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

No. The response speed depends on the receptor, the signaling pathway, the biological effect being studied, and the time taken to perform the measurement. Peptide hormones can rapidly alter the activity of existing proteins, but they can also affect gene expression and cause subsequent responses. Meanwhile, the endocrinology literature describes the rapid effects of steroids that do not require new protein synthesis. The source of the steroids used in this article is a review, not a single conclusive experiment. A more reliable distinction concerns the chemistry of the molecules and receptor mechanisms than a simple division into „rapid peptides” and „free steroids”. [11] [13]

12. Why is the frequency of hormonal pulses important?

Cells may react differently to repeated pulses, different frequencies of pulses, and continuous exposure, even if it is the same hormone each time. In rats with a GnRH deficiency, the change in pulse frequency affected the transcription patterns of gonadotropin subunits in the pituitary gland and local regulatory responses. These experiments confirm the importance of time in this biological model, but they do not specify the dosing schedule for humans. Therefore, hormonal information may be encoded not only by the concentration of the molecule itself, but also by the temporal pattern in which target cells are exposed to it. [5]

13. What is negative feedback in peptide hormone regulation?

Negative feedback occurs when the consequences of a given pathway’s action reduce the stimulus leading to further secretion of a hormone or the activity of earlier elements of the hormonal 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 levels. This experiment does not reflect every disease state or long-term adaptation, but it shows how a regulated parameter can convey feedback to the endocrine system. The feedback can involve secretion, gene expression, receptor sensitivity, the interaction 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 broken down, 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 the signal should best be understood as coordinated control from both the extracellular transmitter and the responding cell. [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 number of receptors, the degree of their occupancy, the intracellular signaling system, the feedback loop, previous exposure, desensitization, and the current physiological state of the target cell also play a role. 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 an individual hormonal response based on a single concentration measurement. 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 made up 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 a signaling role, while the terms „peptide” and „protein” refer to aspects of molecular structure and size. 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 exclusively 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 individual studies.

References

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