Hormones And Second Messengers

There are more than 250 proteins that function as hormones, that range in length from 3 to 192 amino acids, for example the growth hormone.

There are 4 main modes of action for hormones:

1. Endocrine- secreted by specific glands, travel through blood stream to reach remote target site. Only few select glands capable. Example, Thyroid gland.

2. Paracrine- act in vicinity of releasing cell, normally on neighboring cells, can work on either same cell or different cell type, hormone acts via diffusion through gap junctions or intercellular fluid channels. Many cells capable.

3. Autocrine- a cell releases hormone that then effects the releasing cell itself. Nearly all known tissues are capable

4. Neuroendocrine- hormones within neurological pathways and also functioning as neurotransmitters.

Peptide hormone evolution

Primitive forms of life contain hormones, i.e. insulin. Specific segments of peptide hormone synthesis has been preserved throughout evolution.

DNA is translated into RNA

Transcription starts Poly-adenylation

5’-end____________|_____________________________________|________3’-end

Regulatory region Structural region Transcription

Termination

Structural region is made up of exons interdispersed with introns, a cap site, and a polyadenylation signal (AATAAA)

Regulatory Region has 3 parts

1.) Variable region contains

a.)MRE-metabolic response

-response element to cAMP

-steroid receptors

-thyroid

b.)TSS/E- tissue specific silencers/enhancers

2.)TATA Box

Sequence of TATAA present 30 nucleotides upstream to make sure that transcription is initiated properly. Binds to a complex of several proteins, including RNA polymerase II and at least 6 other proteins (IIA, IIb, IIC ,IID, IIE, IIF, IIH ),referred to as TATA box transcription factors These proteins form the machinery required for initiation of RNA synthesis

Among TATA box transcription factors, TFIID is a key factor. It consists of the TATA box binding protein (TBP) and at least 12 associated proteins (TBP –associated factors- TAFs).

The binding of TBP alone to TATA supports basal transcription.

However, upregulation or downregulation of transcription requires the presence (binding) of TAFs. TAFs serve as coactivators or corepressors for enhancer or silencer binding proteins.

3.) Upstream Promoter Region

Alteration in expression

In the basal state, the cAMP response element (CRE) binds a CRE binding protein (CREB).When CREB is phosphorylated by cAMP- dependent protein kinase A, the phosphoCREB recruits the coactivator designated as CREB- binding protein CBP.

CBP couples with TBP-associated factors, leading to marked increase in transcription rate

Steps in expression of protein:

Evolutional processes- rearrangement and transposition of DNA segments.

1.)Transcription: formation of RNA copies from the two alleles; catalyzed by

RNA- polymerase II- associated transcription factors

2.)Post-transcriptional processing: specific modifications of RNA such as the

formation of mRNA from precursor RNA via the excision of introns and rejoining of exons; modification of the 3’ end of RNA by polyadenylation (poly A tail- stability?) and of the 5’ end by addition of7-methylguanine “caps”.

Receptors for GHR, growth hormone differ between species.

Growth Hormone Binding Protein, GHBP, circulate within blood and bind Growth Hormone, this serves as a resevior for Growth Hormone. GHBP result from alternate splicing of the RNA that codes for Growth Hormone Receptor, by splicing the end that inserts into the cell membrane.

This is an example how a single gene can give rise to different hormones or receptors depending on the splicing

Pre-hormone

Hormones that still need to be cleaved and processed after transcription in order to become an active hormone. They contain a signal.

Pre-Prohormone

Signal| Cryptic| Bioactive| spacer| Bioactive|

Signal- NH2 terminus sequence. Upon emergence from the large ribosomal subunit, the signal sequence associates with a signal recognition complex containing 6 different proteins + a 7S RNA located at the endoplasmic reticulum and cytoplasm. This results in translational arrest. It remains in translational arrest until it binds to a high – affinity binding protein on the endoplasmic reticulum termed the signal recognition particle receptor or docking protein. Block is released, protein synthesis resumes, and the peptide prehormone is then transferred across the membrane of the endoplasmic reticulum. The NH2 terminal signal sequence is cleaved by SIGNAL HYDROLASE. The removal of the hydrophobic signal sequence frees the prohormone so it may now assume its secondary structure during further transport in the ER and Golgi apparatus.

Spacer functions as a pusher for hormone to leave ribosome

Conversion of Prohormone

-Occurs in the Golgi Appratus. The translocation of prohormone from rough ER to Golgi requires microtubules. This is known because we can block the translocation with vinblastin.

-Requires prohormone convertases endopeptidases, proteases that cleave specific sites.

Examples:

PCI- converts proinsulin to insulin, is associated with a specific disease where proinsulin is high.

PCII- converts proglucagon to glucagons.

Proopimelanocortin- responsible with formation of ACTH and MSH, difiency disease results in growth retardation, obesity.

Post translational processing

Conversion-prohormones to intermediate or final protein.

Derivatization-glycoslation, phosphorulation, or acetylation.

Folding- to achieve native conformation

Amidation- enhances stability when active site is close to C-terminal

Secretory Granules and Secretion

-Formation begins at the tran-Golgi

-Hormone concentration/ aggregation is facilitated by changes in pH,

calcium concentration and proteins such as chromogranins, sulfated proteoglycans.

-contain one or more hormones

-Secretory granules release their contents by cytoskeletal protein-

mediated movement of the granule to the cell surface followed by exocytosis of stored hormones.

Additional sites of Hormone Regulation

-elongation or termination of transcription

-variation of RNA splicing

-hormonal mRNA stability (prolactin effect on breast casein mRNA stability, thyroid hormone effect on TSH beta subunit mRNA stability)

-effect on translocation

Angiotensin II-example of proteolysis after release from granule to change effect of hormone.

Secretion Regulation- extracellular stimuli

-regulated by other hormones (inhibited or stimulated)

-changes in extracellular environment (electrolytes, minerals, or Osmolarity)

- extracellular glucose, amino acids, or nutrients

Hormone Receptors- General Principle

Responsible for the recogination and binding of hormone

1. Provides specificity- recoginition domain provides for the affinity of binding, i.e. the better the fir of the hormone at the binding site the better the affinity of the receptor for the hormone. Experiments have shown the more you alter a hormone from its native conformation the more of a decrease in its receptor affinity will be observed.

2. Contains the coupling machinery- which allows signal transduction, i.e. is what allows the stimulation of cell from hormone, effect of hormone on cell.

2 major ways of hormone coupling

1.) binding of hormone to receptor on cell membrane generates signals that regulate cell systems

2.)Nuclear receptos superfamily- hormone enters cells and interacts with receptors within the cell, and travel to nucleus, i.e. steroid, retinoic acid, and thyroid.

Signaling pathways regulating genomic actions of nuclear receptors. Ligand binds to receptor. It modifies receptors and allows binding to other peptides. Which intiates transcription.

Surface receptors

-utilize second messengers to interact (directly or indirectly) with DNA elements within the gene.

Surface receptors that use cAMP as second messenger

α2 adrenergic catecholamines(inhibitory) glucagon

β2 adrenergic catecholamines TSH

ACTH PTH

Angiotensin II(inhibitory) somatostatin

ADH (inhibitory)

Calcitonin

chorionic gonasdotropin

CRH

FSH

LH (inhibitory= inhibits adenylate cyclase)

MSH

Generation of cAMP

ATP react with Adenylate cyclase and Mg²⁺ leading to cAMP, which then reacts with phosophodiesterase to form 5’ AMP.

Cell surface receptors with cGMP as second meesenger

atrial naturetic peptides

nitric oxide

G-protein signaling

Ligand binds to extracellular receptor, GDP is released from alpha subunit, GTP binds to alpha subunit, alpha subunit releases from receptor and attaches to effector, E_A, and can initiate cascade. The α -subunit has GTPase activity, and bound GTP is hydrolyzed to GDP and Pi. Hydrolysis of GTP, which is typically slow, causes dissociation of the effector- alpha-subunit complex and terminates the regulatory signal.

• GTPase-activating proteins (GAPs) may regulate the rate at which GTP is hydrolyzed. In this way GAPs may serve as regulators of G protein signaling (i.e., RGS proteins).

Calmodulin

• 17 kDa protein, homologous to muscle troponin C

• major calcium dependent regulatory protein

• has 4 calcium binding sites

• Full calcium occupancy leads to conformational changes which capacitate

calmodulin to associate with and activate other proteins.

• Many kinases are acivated by calmodulin, some are specific calmodulin – dependent protein kinases, another is a multifunctional calmodulin- dependent protein kinase.Other enzymes are regulated directly(adenyl cyclase, phosphodiesterase)or indirectly (glycogen synthase, caMg-ATPase).

• The calcium- calmodulin system also affects the activity of structural cell elements such as the actin myosin complex in VSMC, microfilament- mediated processes etc.

Two examples calmodulin mediated cell responses

Nutrients: glucose enters the ß-cell on the GLUT2 transporter and is metabolized with a consequent generation of ATP and closure of KATP channels. The decreased efflux of K+ leads to depolarization of the ß-cell with the consequent opening of VDCCs and an influx of extracellular Ca2+ down its concentration gradient. The Ca2+ binds to CaM, and the Ca2+/CaM complex binds to the inactive kinase (CaMKi), which becomes enzymatically active (CaMKa). The active CaMK phosphorylates (PO4) ß-cell substrate proteins, and these phosphorylation events stimulate the secretory process.

Nonnutrients: agonists bind to cell-surface receptors that are coupled via the heterotrimeric GTP-binding protein Gq to PLC. hydrolysis of membrane inositol phospholipids. IP3 releases stored Ca2+ from the endoplasmic reticulum, and the elevation in intracellular Ca2+ activates CaMK, as described above

Category: Physiology Notes