Hormones are chemical products, released in very small amounts from the cell, that exert a biologic action on a target cell.
Chemical Structures of Hormones
There types of hormones, including protein/peptide, steroid hormones, and amino acid-derived hormones. Let’s talk them below. Based on their chemical structure, hormones can be classified into peptides/proteins (i.e., insulin, glucagon, ACTH), steroids, and amino acid derivatives/amines.
Protein or peptide hormones constitute the majority of hormones. These are molecules ranging from 3 to 200 amino acid residues. They are synthesized as preproxhormones and undergo post-translational processing. They are stored in secretory granules before being released by exocytosis. Steroid hormones are derived from cholesterol and are synthesized in the adrenal cortex, gonads, and placenta. They are lipid soluble, circulate bound to binding proteins in plasma, and cross the plasma membrane to bind to intracellular cytosolic or nuclear receptors.
Vitamin D and its metabolites are also considered steroid hormones. Amino acid-derived hormones are those hormones that are synthesized from the amino acid tyrosine and include the catecholamines norepinephrine, epinephrine, and dopamine; as well as the thyroid hormones.d Hormone structure, to a great extent, dictates the location of the hormone receptor, with amines and peptide hormones binding to receptors in the cell surface and steroid hormones being able to cross plasma membranes and bind to intracellular receptors. An exception to this generalization is thyroid hormone, an amino acid-derived hormone that is transported into the cell in order to bind to its nuclear receptor.
Depending on where the biologic effect of a hormone is elicited in relation to where the hormone was released, its effects can be classified in 1 of 3 ways. 1.The effect is endocrine when a hormone is released into the circulation and then travels in the blood to produce a biologic effect on distant target cells. 2.The effect is paracrine when a hormone released from 1 cell produces a biologic effect on a neighboring cell, which is frequently a cell in the same organ or tissue. and 3.The effect is autocrine when a hormone produces a biologic effect on the same cell that released it.
Hormones released into the circulation can circulate either freely or bound to carrier proteins, also known as binding proteins. This binding proteins serves as a reservoir for the hormone and prolong the hormone’s half-life, the time during which the concentration of a hormone decreases to 50% of its initial concentration. However, the free or unbound hormone is the active form of the hormone, which binds to the specific hormone receptor. Thus, hormone binding to its carrier protein serves to regulate the activity of the hormone by determining how much hormone is free to exert a biologic action.
Most carrier proteins are globulins and are synthesized in the liver. Some of the biding protein are specific for a given protein, such as cortisol-binding proteins. However, proteins such as globulins and albumin are known to bind hormones as well. Because for the most part these proteins are synthesized in the liver, alterations in hepatic function may result in abnormalities in binding-protein levels and my indirectly affect total hormone levels (also see the thread of The Plasma Protein Concentration And The Interpretation Of TDM Report by Tom Hsiung).
In general, the majority of amines, peptides, and protein (hydrophilic) hormones circulate in their free form. However, a notable exception to this rule is the binding of the insulin-like growth factors to 1 of 6 different high-affinity binding proteins. Steroid and thyroid (lipophilic) hormones circulate bound to specific transport protein.
Once hormones are released into the circulation, they can bind to their specific receptor in a target organ, they can undergo metabolic transformation by the liver, or they can undergo urinary excretion. Also they can be degraded at their target cell through internalization of the hormone-receptor complex followed by lysosomal degradation of the hormone. Only a very small fraction of total hormone production is excreted intact in the urine and feces.
Hormone Receptors and Signal Transduction
Hormone receptors are classified depending on their cellular localization, as cell membrane or intracellular receptors. Peptides and catecholamines are unable to cross the cell membrane lipid bilayer and in general bind to cell membrane receptors, with the exception of thyroid hormones as mentioned above. Steroid hormones are lipid soluble, cross the plasma membrane, and bind to intracellular receptors.
Cell Membrane Receptors Binding of hormones to cell membrane receptors and formation of the hormone-receptor complex initiates a signaling cascade of intracellular events, resulting in a specific biologic response.
Cell membrane receptors can be divided into two subcategories of receptors, that are, ligand-gated ion channels and receptors that regulate activity of intracellular proteins. Ligand-gated ion channels/receptors are functionally coupled to ion channels, and with hormone binding they produce a conformational change that opens ion channels on the cell membrane, producing ion fluxes in the target cell. The cellular effects occur within seconds of hormone binding. Detail information about ligand-gated ion channels/receptors could be found at this thread Ligand- And Voltage-gated Channels (Receptors) by Tom Hsiung.
The other subcategory of cell membrane receptors is a little bit complex, including G protein-coupled receptors and receptor protein tyrosine kinases, both of which are transmembrane proteins that transmit signals to intracellular targets when activated. Ligand binding to the receptor on the cell surface and activation of the associated protein initiate a signaling cascade of events that activates intracellular proteins and enzymes and that can include effects on gene transcription and expression. Intracellular Receptors Receptors in this category belong to the steroid receptor superfamily. These receptors are transcription factors that have binding sites for the hormone (ligand) and for DNA and function as ligand (hormone)-regulated transcription factors.
Hormone-receptor complex formation and binding to DNA result in either activation or repression of gene transcription. Binding to intracellular hormone receptors requires that the hormone be hydrophobic and cross the plasma membrane. Steroid hormones and the steroid derivative vitamin D3 fulfill this requirement, whereas thyroid hormones must be actively transported into the cell. This distribution of the unbound intracellular hormone receptor can be cytosolic or nuclear. Hormone-receptor complex formation with cytosolic receptors produces a conformational change that allows the hormone-receptor complex to enter the nucleus and bind to specific DNA sequences to regulate gene transcription. Once in the nucleus, the receptors regulate transcription by binding, generally as dimers, to hormone response elements normally located in regulatory regions of target genes. In all cases, hormone binding leads to a nearly complete nuclear localization of the hormone-receptor complex. Unbound intracellular receptors may be located in the nucleus, as in the case of thyroid hormone receptors. The unoccupied thyroid receptor represses transcription of genes. Binding of thyroid hormone to the receptor activates gene transcription.
Hormone Receptor Regulation
Two ways exist for the receptor regulation, including desensitization and receptor downregulation. Two types of receptors are involved in the process of desensitization and receptor downregulation, G protein-coupled receptors and receptor protein tyrosine kinases, where both happen to G protein-coupled receptors and only downregulation happens to receptor protein tyrosine kinases, respectively. Note that the process of desensitization and downregulation are reversible. Hormone receptors can also undergo upregulation. Upregulation of receptors involves an increase in the number of receptors for the particular hormone and frequently occurs when the prevailing levels of the hormone have been low of some time.
Characteristics of Hormone-Receptor Binding (Also see the thread The Properties of Drugs – Receptor Rationale by Tom Hsiung)
The binding of hormones and their receptors have phenomenon of spareness, where the maximal biologic response to a hormone can be achieved without reaching 100% hormone-receptor occupancy. Frequently, the hormone-receptor occupancy needed to produce a biologic response in a given target cell is very low, for example, insulin-mediated cellular effects occurs when less than 3% of the total number of receptors in adipocytes is occupied.
Control of Hormone Release
The secretion of hormones involves synthesis or production of the hormone and its release from the cell. Plasma levels of hormones oscillate throughout the day, showing peaks and troughs that are hormone specific. This variable pattern of hormone release is determined by the interaction and integration of multiple control mechanisms, which include 1) hormonal, 2) neural, 3) nutritional, and 4) environmental factors that regulate the constitutive (basal) and stimulated (peak levels) secretion of hormones. The periodic and pulsatile release of hormones is critical in maintaining normal endocrine function and in exerting physiologic effects at the target organ. Although the mechanisms that determine the pulsatility and periodicity of hormone release are not completely understood for all the different hormones, 3 general mechanisms can be identified as common regulators of hormone release including: neural control, hormonal control, and nutrient or ion regulation.
Control and integration by the central nervous system is a key component of hormonal regulation and is mediated by direct neurotransmitter control of endocrine hormone release. Neural control plays an important role in the regulation of central (e.g., pituitary) and peripheral (e.g., adrenal glands) endocrine hormone release. Endocrine organs such as the pancreas receive sympathetic and parasympathetic input, which contributes to the regulation of insulin and glucagon release.
Hormone release from an endocrine organ is frequently controlled by another hormone. When the outcome is stimulation of hormone release, the hormone that exerts that effect is referred to as a tropic hormone (feed-forward mechanismas) is the case for most of the hormones produced and released from the anterior pituitary. Hormones can also supress another hormone's release (negative feedback regulation).
Nutrient or Ion Regualtion
Plasma levels of nutrients or ions can also regulate hormone release. In all cases, the particular hormone regulates the concentration of the nutrient or ion in plasma either directly or indirectly. Examples of nutrient and ion regulation of hormone release include the control of insulin release by plasma glucose levels and the control of parathyroid hormone release by plasma calcium and phosphate levels.
In several instances, release of 1 hormone can be influenced by more than 1 of these mechanisms. For example, insulin release is regulated by nutrients (plasma levels of glucose and amino acids), neural (sympathetic and parasympathetic stimulation), and hormonal (somatostatin) mechanisms. The ultimate function of these control mechanisms is to allow the neuroendocrine system to adapt to a changing environment, integrate signals, and maintain homeostasis.
The Clincial Art of Assessment of Endocrine Function
In general, disorders of the endocrine system result from 1) alterations in hormone secretion or 2) target cell responsiveness to hormone action. Alterations in target cell response can be caused by increased or decreased biologic responsiveness to a particular hormone. The initial approach to assessment of endocrine function is measurement of plasma hormone levels.
Interpretation of Hormone Measurements
Hormone concentrations in biologic fluids are measured using immunoassays. These assays rely on the ability of specific antibodies to recognize specific hormones. Specificity for hormone measurement depends on the ability of the antibodies to recognize antigenic sites of the hormone. Hormone levels can be measured in plasma, serum, urine, or other biologic samples. Hormone determinations in urine collected over 24 hours provide an integrated assessment of the production of a hormone or metabolite, which vary considerably throughout the day as is the case for cortisol.
Because of the variability in ciculating hormone levels resulting from pulsatile release, circadian rhythms, sleep/wake cycle, and nutritional status, interpretation of isolated plasma hormone measurements should always be done with caution and with understanding of the integral components of the hormone axis in question. Plasma hormone measurements reflect endocrine function only when interpreted in the right context. Some general aspects that should be considered when interpreting hormone measurements are: 1) Hormone levels should be evaluated with their appropraite regulatory factors; 2) stimultaneous elevation of pairs (elevation of both the hormone and the substrate that it regulates) suggests a hormone-resistance state; 3) Urinary excretion of hormone or hormone metabolites over 24 hours, in individuals with normal renal function, may be a better estimate of hormone secretion than one-time plasma-level measurement; 4) target hormone excess should be evaluated with the appropriate tropic hormone to rule out ectopic hormone production, which is usually caused by a hormone-secreting tumor.
The possible interpretations of altered hormone and regulatory factor pairs are summarized in Table 1-1. Increased tropic hormone levels with low target hormone levels indicate primary failure of the target endocrine organ. Increased tropic hormone levels with increased target gland hormone levels indicate autonomous secretion of tropic hormone or inability of target gland hormone to suppress tropic hormone release (impaired negative feedback mechanisms). Low tropic hormone levels with low target gland hormone levels indicate a tropic hormone deficiency, as seen with pituitary failure. Low tropic hormone levels with high target gland hormone levels indicate autonomous hormone secretion by the target endocrine organ.
Dynamic Measurements of Hormone Secretion
In some cases, detection of abnormally high or low hormone concentrations may not be sufficient to conclusively establish the site of endocrine dysfunction. Dynamic measures of endocrine function provide more information than that obtained from hormone-pair measurements and rely on the integrity of the feedback control mechanisms that regulate hormone release. These tests of endocrine function are based on either stimulation or suppression of the endogenous hormone production.
Stimulation tests are designed to determine the capacity of the target gland to respond to its control mechanism, either a tropic hormone or a substrate that stimulates its release. Suppression tests are used to determine whether the negative feedback mechanisms that control that hormone's release are intact.
The measurement of hormone-receptor presence, nunber, and affinity has become a useful diagnostic tool, particularly in instituting hormone therapy for the treatment of some tumors. Receptor measurements made in tissue samples obtained surgically allow determination of tissue responsiveness to hormone and prediction of tumor responsiveness to hormone therapy.
Insulin Synthesis of Insulin And The Significance of C-Peptide
Proinsulin consists of an amino-terminal beta-chain, a carboy-terminal alpha-chain, and a connecting peptide, known as the C-peptide, that links the alpha- and beta-chains. In the endoplasmic reticulum, proinsulin is processed by specific endopeptidases, which cleave the C-peptide exposing the end of the insulin chain that interacts with the insulin receptor, generating the mature form of insulin. Insulin and the free C-peptide are packaged into secretory granules in the Golgi. These secretory granules accumulate in the cytoplasm in 2 pools: a readily releasable (5%) and a reserve pool of the granules (more than 95%).
On stimulation, the beta-cell releases insulin in a biphasic pattern; initially from the readily releasable pool followed by the reserve pool of granules. Only a small proportion of the cellular stores of insulin are released even under maximal stimulatory conditions. Insulin circulates in its free form, has a half-life of 3-8 minutes, and is degraded predominantly by the liver, with more than 50% of insulin degraded during its first pass. Additional degradation of insulin occurs in the kidneys as well as at target tissues by insulin proteases following endocytosis of the receptor-bound hormone. Because exocytosis of secretory granule content results in the release of equal amounts of insulin and C-peptide into the portal circulation. The importance of C-peptide is that unlike insulin, it is not readily degraded in the liver. Thus, the relatively long half-life of the peptide (35 minutes) allows its release to be used as an index of the secretory capacity of the endocrine pancreas.
Regulation of Insulin Secretion
The pancreatic beta-cell functions as a neuroendocrine integrator that senses and responds to changes in plasma levels of energy substrates (glucose and amino acids), hormones (insulin, glucagon-like peptide I, somatostatin, and eninephrine), and neurotransmitters (norepinephrine and acetylcholine) by increasing or decreasing insulin release. A brief mechanisms of the sensing and responding of beta-cell are summarized in the figure below. The key role to regulate the secretion of insulin (stored in the vesicles) is the intracellular concentration of free calcium, which depends on the voltage-dependent Ca2+ channel crossing on the membrane of the beta-cell. So the depolarization of beta-cell’s membrane, caused by the ATP-sensitive K+ channel (target of sulfonylurea anti-diabetic drugs) enhances the influx of calcium and the resultant increased secretion of insulin.
PS: The increased ATP/ADP ratio leads to inhibition and closure of the ATP-sensitive K+ channels (the target of sulfonylurea drugs), resulting in plasma membrane depolarization and opening of the voltage-dependent Ca2+ channels
Insulin Receptor and The Signal After Binding
The insulin receptor is part of the insulin-receptor family, which includes the insulin-like growth-factor receptor. The insulin receptor is a heterotetrameric glycoprotein membrane receptor composed of 2 alpha- and 2 beta-subunits, linked by disulfide bonds, which belongs to the enzymes including receptor tyrosine kinases (detail here http://www.tomhsiung.com/wordpress/2014/11/transmembrane-enzymes-including-receptors/). The extracellular alpha-chain is the site for insulin binding. The intracellular segment of the beta-chain has intrinsic tyrosine kinase activity, which on insulin binding, undergoes autophosphorylation on tyrosine residues. The activated receptor phosphorylates tyrosine residues of several proteins known as insulin receptor substrates 1 through 4 (IRS-1-4), facilitating the interaction of the insulin receptor with intracellular substrates. Signal transduction by the insulin receptor is not limited to its activation at the cell surface.
The activated ligand-receptor complex is internalized into endosomes. Endocytosis of activated receptors is thought to enhance the insulin receptor tyrosine kinase activity on substrates that are distant from those readily accessible at the plasma membrane. Following acidification of the endosomal lumen, insulin dissociates from its receptor, ending the insulin receptor-mediated phosphorylation events, and promoting the degradation of insulin by proteases such as the acidic insulinase. The insulin receptor can then be recycled into the cell surface, where it becomes available for insulin binding again. The number of available insulin receptors is modulated by exercise, diet, insulin, and other hormones.
Chronic exposure to high insulin level, obesity, and excess growth hormone all lead to a downregulation of insulin receptors. In contrast, exercise and fasting upregulate the number of receptors, improving insulin responsiveness.
Insulin Effects At Target Organs
The effects of insulin at target organs can be divided into three categories from the perspective of time, including early effects, intermediate effects, and long-term effects. Insulin produces a wide variety of effects that range from immediate (within seconds), such as the modulation of ion (K+) and glucose transport into the cell; early (within minutes), such as the regulation of metabolic enzyme activity; moderate (within minutes to hours), such as the modulation of enzyme synthesis; to delayed (within hours to days), such as the effects on growth and cellular differentiation. Overall, the actions of insulin at target organs are anabolic and promote the synthesis of carbohydrate, fat, and protein, and these effects are mediated through binding to the insulin receptor.Early Effects (metabolic enzyme activity/in fat and muscle cells)
Although the expression of insulin receptors is widespread, the specific effects of insulin on skeletal muscle glucose utilization dominate insulin action. The movement of glucose into the cell is mediated by glucose transporters, accurately, the GLUT 4, most of which is sequestered intracellularly in the absence of insulin or other stimuli such as exercise.Insulin binding to its receptor results in increased GLUT 4 translation through targeted exocytosis and decreased rate of its endocytosis.
Intermediate Effects (enzyme synthesis/in muscle, fat, and liver)
The intermediate effects of insulin are mediated by modulation of protein phosphorylation of enzymes involved in metabolic process in muscle, fat, and, liver. In fat, insulin inhibit its lipolysis and ketogenesis by triggering the dephosphorylation of hormone sensitive lipase and stimulates lipogenesis by activating acetyl coenzyme A (acetyl-CoA) carboxylase. Dephosphorylation of hormone-sensitive lipase inhibits the breakdown of triglycerides to fatty acids and glycerol, the rate-limiting step in the release of free fatty acids mediated by lipolysis.
This process thereby reduces the amount of substrate that is available for ketogenesis. In the liver, insulin stimulates the gene expression of enzymes involved in glucose utilization (e.g., glucokinase, pyruvate kinase) and lipogenic enzymes and inhibits the gene expression of enzymes involved in glucose production (e.g., phosphoenolpyruvate carboxykinase and glucose-6-phosphatase). Insulin stimulates glycogen synthesis by increasing phosphatase activity, leading to the dephosphorylation of glycogen phosphorylase and glycogen synthase. In addition, insulin-mediated dephosphorylation of inhibitory sites on hepatic acetyl-CoA carboxylase increases the production of malonylcoenzyme A and simultaneously reduces the rate at which fatty acids can enter hepatic mitochondria for oxidation and ketone body production. In muscle, insulin stimulates glucose uptake and favors protein synthesis though phosphorylation of a serine/threonine protein kinase known as mammalian target of rapamycin (mTOR). In addition, insulin favors lipid storage in muscle as well as in adipose tissue.
Sustained insulin stimulation enhances the synthesis of lipogenic enzymes and the repression of gluconeogenci enzymes. The growth promoting and mitogenic effects of insulin are long-term response mediated through the MAPK pathway.
Insulin resistance (IR) implies there is target level resistance to the physiological actions of insulin. This resistance is seen at peripheral tissues especially muscle, adipose tissue and liver. The end results of IR are: 1) Decreased peripheral uptake of glucose; 2) inadequate supression of hepatic glucose production; and 3) increased lipolysis. Clinically, the IR is apparent when an appropriate dose of insulin fails to lower plasma glucose to the same extent as seen in controls. Biochemically, the presence of relatively higher fasting and post-secretagogue insulin values are indicative of IR. The impairment of insulin action can be measured by euglycaemic hyperinsulinaemic clamp studies as well as the minimal and homeostasis model assessment (HOMA) methods. In euglycaemic hyperinsulinaemic clamp, a fixed amount of insulin is infused intravenously and a titrated infusion of intravenous glucose is administered to maintain normoglycaemia. A low rate of exogenous glucose infusion indicates insulin resistance. Glucose disposal in clamp studies typically measures up to 7 mg/kg/min in controls while in overweight T2DM it is observed to be much lower, (around 2.5 mg/kg/min).
Mechanisms for Insulin Resistance
Cellular Mechanism of insulin resistance are summarized in Table 25.2.2.
In muscles, the sketeal muscles require insulin for optimal glucose uptake and utilization. In T2DM, available insulin fails to recruit more GLU-4 for facilitated glucose transportaiton across the cell membrane. Therefore, the entry of glucose into the myocytes is only through mass action. This results in reduction in glucose uptake from the standard 60 G to 44 G in 3 to 5 hours after a stipulated glucose load. The insulin mediated glucose utilization by the muscles is also impaired in T2DM. Both these factors combined together contribute to the elevated postprandial blood glucose levels seen so frequently in early stages of T2DM.
In the adipose tissue, the resistance of adipocytes to insulin action leads to increased lipolysis resulting in elevated FFAs in circulation and tissues. Leptin is a protein secreted by adipose tissue. In rodents it inhibits neuropeptide neurones, (they stimulate feeding) to the hypothalamus. Its defects give rise to over feeding an dobesity in rodents. The role of leptin in human T2DM is unclear. Adipose tissue also secrets the cytokine TNFalpha, which may cause insulin resistance by inhibiting tyrosine kinase activity of the insulin receptor and decreasing the expression of glucose transporter (GLUT-4). IL-6 secreted by adipose tissue and other cells also induces insulin resistance. The protein adiponectin is secreted by fat cells and ameliorates insulin resistance probably by increasing fat oxidation. Its level is low in obesity. The role of the protein resistin which is also secreted by adipocytes and implicated in IR is not very clear as yet. Moreover, in obesity there is often an increased sympathetic over activity which leads to increased lipolysis, reduced muscle blood flow and thus decreased glucose delivery. Recently it has been observed that foetal and postnatal overnutrition in the first 5 to 10 years which leads to obesity results in IR and metabolic syndrome in later life.
In the liver, both glycogenolysis and gluconeogensis are under the influence of insulin. In patients with IR, there is impaired restraining effect of insulin on gluconeogeenesis and hepatic glucose production (HGP). In T2DM, the requirement of insulin for the control of HGP is nearly double the amount required in normal subjects. Moreover, loss of first phase of insulin secretion seen even at the stage of IGT also contributes to the failure of prompt postload supression of gluconeogenesis. The end result is elevated post-absorptive (fasting) and postprandial (PP) blood sugar values.