Hormonal Signaling

Peptide and steroid hormones signal via distinct mechanisms. Peptide hormones primarily act as ligands that bind to G-protein coupled receptors (GPCRs) at the plasma membrane, transmitting their signal through a cascading set of interactions. In contrast, steroid hormones primarily act within the cytoplasm as ligands that bind to ligand-activated transcription factors. They subsequently translocate to the nucleus, mediating their effects through direct or indirect transcriptional regulation of target genes.

Peptide hormones

Growth hormone signaling

Growth hormone plays a critical role in growth, with defects in signaling resulting in short stature. It also plays a role in metabolism and body composition and controls physiological processes related to cardiovascular, renal, hepatobiliary, gastrointestinal and reproductive systems. Secreted by the anterior pituitary, growth hormone binds to growth hormone receptor (GHR) (1). When activated, GHR binds and activates JAK2 (Janus kinase 2), which initiates multiple signaling pathways influencing various cellular processes.

One outcome of JAK2 activation is to recruit and phosphorylate STAT (signal-transducer-and-activator-of-transcription) transcription factors including STAT1, STAT3, STAT5A and STAT5B). This leads to accumulation of the activated protein in the nucleus and increased synthesis of GH-sensitive genes including insulin-like growth factors 1 and 2 (IGF-1, IGF-2), IGFBP3 and ALS.

JAK2 additionally phosphorylates and activates insulin response substrate proteins IRS1 and IRS2, which recruit PI3K, which converts PIP2 to PIP3 in turn recruiting Akt. Activated Akt inhibits GSK3 activity, which in turn increases C/EBPβ binding to regulated promoters.

JAK2-GHR complexes also bind Shc, which in turn associates with guanine nucleotide exchange factor SOS, activating the Ras, Raf, MEK and Erk1/Erk2 cascade and leading to increased c-Fos synthesis.

Leptin signaling

Produced and secreted by adipose tissue, leptin predominately targets the brain to control appetite and maintain energy homeostasis, suppressing appetite and decreasing food intake. The leptin signaling pathways are subject to tight regulation with many negative feedback loops, with imbalance being one of the causes of obesity.

Leptin receptor, the mediator of leptin signaling, is alternatively spliced into six different isoforms (LepRa-f) with LepRb being the isoform primarily responsible for leptin’s effects on energy homeostasis (2). When leptin binds LepRb it recruits and activates JAK2, which in turn phosphorylates three tyrosine residues on LepRb.

Phosphorylation of residue Y985 activates mitogen-activated-protein-kinase (MAPK) signaling via src-homology-2 domain protein (SHP-2) to mediate energy homeostasis as well as a negative feedback loop.

Phosphorylation of residue Y1077 activates STAT5 signaling, which is believed to influence reproduction.

Phosphorylation of residue Y1138 activates STAT3 signaling, mediating energy homeostasis via increased transcription of POMC (pro-opiomelanocortin) and decreased transcription of NPY (Neuropeptide Y)/AgRP (Agouti-related protein). It also activates a powerful negative feedback loop mediated by increased expression of suppressor-of-cytokine-signaling-3 (SOCS-3) and phosphotyrosine phosphatase 1B (PTP1B), which block leptin signaling.

Leptin additionally acts via JAK2 and IRS1/2 to activate PI3K and Akt, which phosphorylate FOXO1, leading it to be sequestered in the cytoplasm making it unavailable to restrict expression of POMC and promote expression of NPY/AgRP (3). At the same time, phosphorylated FOXO1 inhibits leptin signaling via interfering with STAT3. Leptin resistance is an important contributor to obesity and FOXO1 appears to be a key mediator of that resistance. PI3K/Akt signaling also activates mTOR signaling and inhibits AMPK signaling.

GNRH signaling

Secreted from hypothalamic neurons and acting on the anterior pituitary via its receptor GnRHR, GNRH regulates the hypothalamic-pituitary-gonadal axis essential for normal reproductive function and fertility.

GnRHR activates at least three different G proteins: Gα_q-11, G_s and G_i (4). Activation of Gα_q-11 initiates phospholipase C (PLC) signaling, which generates IP₃ and DAG. IP₃ increases Ca²⁺ levels, stimulating calmodulin-mediated calcineurin/NFAT and CaMKII signaling leading to CREB activation while DAG activates PKC, initiating MAPK cascades including activation of ERK1/2, jun N-terminal kinase (JNK) and p38. Activation of G_s initiates the cAMP/PKA cascade.

Among the downstream effectors of GNRH are Follicle stimulating hormone (FSH) and Luteinizing hormone (LH), which share a common subunit and whose regulation is differentiated by pulsatile frequency of GNRH in a manner that is not yet clear.

As part of its role in regulation of cell proliferation, GNRH and its receptors are involved in multiple cancers and have applications as potential targets.

Corticotrophin releasing hormone signaling

Corticotrophin releasing hormone (CRH), also known as corticotrophin releasing factor (CRF), is expressed in the hypothalamus, amygdala and other regions of the central nervous system, and throughout the body. It plays a role in regulating the hypothalamic-pituitary-adrenal-axis and coordinating stress response, disruption of which can cause anxiety and other physiological effects.

Following a path similar to other peptide hormones, CRH binds to and activates GPCRs, in this case CRH receptors 1 (CRHR1) and 2 (CRHR2), kicking off a cAMP cascade that leads to CREB phosphorylation and binding to cyclic AMP response elements (CREs) within the promoters of targeted genes (6).

CRH also activates PKC, leading to c-Raf phosphorylation and activation, which in turn activates MEK1/2 and ERK1/2 leading to transcription of target genes including POMC. This leads to release of ACTH (adrenocorticotropic hormone) from the pituitary gland and consequently synthesis and release of glucocorticoids from the adrenal gland. It is believed that CRHR1 is mainly involved in the initial phase of the stress response while CHRH2 is mainly involved in the later phase.

Adrenomedullin signaling

Produced by the adrenal medulla and many tissues throughout the body, adrenomedullin (AM), along with other calcitonin family members such as calcitonin gene-related peptide (CGRP), regulates numerous physiological processes including gastric emptying, angiogenesis, vasodilation and regulation body fluid volume.

Adrenomedullin acts primarily through calcitonin-receptor like receptor (CLR), which forms a heterodimer with one of three receptor activity-modifying proteins (RAMPs) (7). The CLR/RAMP1 heterodimer is preferentially bound by GCRP, while the CLR/RAMP2 heterodimer (AM₁ receptor) and to a lesser degree the CLR/RAMP3 heterodimer (AM₂ receptor) is preferentially bound by adrenomedullin. Binding results in signaling via the cAMP/PKA pathway through a variety of Gα proteins. In endothelial cells this leads to activation of endothelial nitrous oxide synthase (eNOS) and production of nitrous oxide (NO).

The cAMP/PKA signaling simultaneously drives recruitment of β-arrestin-1 and β-arrestin-2, which, as part of a negative feedback loop, are recruited to ligand bound CLR receptor complexes. The β-arrestins mediate internalization and endosomal sorting of the adrenomedullin-bound receptors, eventually eliminating them. During the process of internalization and endosomal sorting, the CLR receptor complexes continue to signal through alternate MAPK and AKT pathways.

Too much or too little adrenomedullin can have pathological consequences, requiring tight regulation. The balance is maintained in part by the cAMP/PKA negative feedback loop, but also by atypical chemokine receptor 3 (ACKR3). When activated by ligands including chemokine x ligand 11 (CXCL11/I-TAC), CXCL12 (SDF-1) and others, including adrenomedullin itself, ACKR3 similarly recruits β-arrestins, which mediate the elimination of adrenomedullin-bound complexes as described.

Blockade of adrenomedullin signaling is a focus of therapeutic development for diverse applications ranging from treatment of migraine pain to many types of cancer.

Steroid hormones

Glucocorticoid receptor signaling

Glucocorticoids are a family of hormones produced by the adrenal gland, with cortisol being the most abundant. Glucocorticoids mediate their effects via glucocorticoid receptor (GR). Processes regulated by GR include stress response, metabolism, inflammation and processes in the cardiovascular system. It is GR’s effect on inflammation that has made synthetic exogenous glucocorticoids important tools in treating inflammatory and autoimmune disorders as well as some cancers.

In the absence of glucocorticoids, GR is found in the cytoplasm complexed with chaperone and immunophilin proteins that keep it in a transcriptionally inactive state (8). Cortisol binding causes a conformational change that results in GR translocation to the nucleus where it’s free to bind glucocorticoid response elements (GREs) in the genes it regulates, recruiting coregulators and chromatin-remodeling complexes. GR also regulates target genes via interaction with other transcription factors. Some interactions, such as those with STAT proteins, enhance transcription of responsive genes while others, such as those with AP1 and NF-κB, disrupt their transcriptional activity and lead to suppressive effects.

GR itself exists as a variety of alternatively spliced and alternatively translated isoforms with unique expression profiles, explaining in part the diverse physiological responses mediated by glucocorticoids. Post-translational modification of the isoforms, combined with differences in accessibility among GREs, further contributes to the diversity of responses.

Like other steroid hormones, glucocorticoids appear to also be able to exert their effects independently of direct transcriptional regulation by acting through pathways involving kinases such as Akt, PI3K and MAPKs (9). There is evidence that inhibition of MAPKs is an important component of the anti-inflammatory effects of endogenous and exogenous glucocorticoids as excessive MAPK activation is a common mechanism of glucocorticoid resistance.

Estrogen signaling

Estrogen is a family of steroid hormones including estrone, estradiol, estriol and estretrol, with estradiol being the predominant circulating form. Estrogen plays a role in many physiological processes ranging from development of secondary sexual characteristics and regulation of the menstrual cycle and reproduction to bone density, inflammation and more. It is estrogen’s role in cell proliferation that links it to breast and uterine cancer.

Estrogen’s physiological effects are primarily mediated via the ligand-activated transcription factors estrogen receptor alpha (ERα encoded by ERS1) and beta (Erβ, encoded by ERS2) and their multiple isoforms (10). Through the classical method of direct signaling, estrogen enters the cytoplasm and binds ERα/Erβ, which undergo conformational changes and translocation to the nucleus ultimately binding to estrogen response elements (ERE) in promoters of regulated genes. Through indirect mechanisms, estrogen can also regulate expression of genes without EREs by interacting with other transcriptional modulators including stimulating protein-1 (Sp-1) and c-Fos/c-Jun.

In estrogen receptor-positive (ER+) cancer cells, estrogen mediates its stimulating effect on G1/S transition and resultant cell proliferation via increased expression of cyclins, predominately Cyclin D1, which in turn activates cyclin-dependent kinase CDK4/6 (11). Active Cyclin D1-CDK4 complexes phosphorylate Rb, freeing E2F transcription factors to increase expression of S-phase genes.

References

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