Hormonal Control of Metabolism - Comprehensive Notes

Hormonal Control of Metabolism

Introduction

• Each cell has the potential to carry out various chemical reactions and pathways, many of which may conflict.
• Cells coordinate metabolic activities within themselves and integrate these activities with cells in different parts of the organism.
• This coordination allows for efficient utilization of metabolites to meet the organism's current needs.
• Cell processes need to be controlled for efficient metabolite use.

Metabolic Activity Control Mechanisms

• Substrate supply.
• Allosteric enzymes.
• Hormonal control.
• Nervous control.
• The focus is on how these mechanisms control energy production and utilization in different tissues.

Hormonal Control

• Hormones are chemical messengers that regulate and coordinate metabolic activity.
• Endocrine hormones: Produced in one tissue (gland) and travel through the circulation to reach a target cell with a receptor for that hormone.
• Paracrine hormones: Produced in one cell and travel a short distance to reach a neighboring target cell with a receptor.
• Autocrine hormones: Produced by a cell that is also the target cell for that hormone.

Specific Hormones

• Hypothalamic releasing hormones: TRH, GnRH, GRH, somatostatin (GIH), CRH, PRH, PIH.
• Pituitary stimulating hormones: GH, TSH, ACTH, Prolactin, FSH, LH; oxytocin, vasopressin.
• Parathyroid hormone (PTH).
• Thyroid hormones (T3/T4), calcitonin.
• Cortisol, aldosterone, adrenalin (epinephrine).
• Insulin, glucagon.
• Oestrogens – 17b-oestradiol.
• Androgens - testosterone.

Hormonal Cascade

• Hormones released by one gland (e.g., hypothalamus) often stimulate the release of a hormone from another gland (e.g., pituitary gland).
• The release of hormones can be linked to signals from the central nervous system (CNS), creating a cascade system.
• The hormonal cascade system involves:
  • Stimulus from the CNS.
  • Releasing hormones (ng) from the hypothalamus.
  • Stimulating hormones (mg) from the anterior pituitary.
  • Specific target endocrine gland.
  • Ultimate hormone (mg).
  • Metabolic effect.

Endocrine Hormones: Types and Characteristics

• Three types:
  • Steroid hormones
  • Peptide/protein hormones
  • Amino acid-derived hormones (catecholamines)
• Differ in:
  • Solubility characteristics
  • Mechanism of action
  • Speed of action

Steroid Hormones

• Lipid-soluble molecules with a basic steroid structure.
• Transported in the blood bound to specific transport proteins.
• The transport protein binds to a receptor on the cell surface.
• The hormone enters target cells.
• The hormone binds to specific receptors within the cell (either in the cytosol or nucleus).
• If binding occurs in the cytosol, the entire complex moves into the nucleus.
• The hormone-receptor complex binds to specific regions of DNA (response elements).
• Influences transcription (up or down).
• Alters proportions of specific proteins in a cell.
• Results in a slow response to the hormone.

Glucocorticoid Receptor Domain Structure

• The glucocorticoid receptor is made up of domains with distinct functions.
  • Amino acid numbers:
    • 1-421: Transcription activation
    • 487-532: DNA binding
    • 777: Glucocorticoid hormone

Mechanism of Transcription Activation by Glucocorticoids

1. In the absence of a hormone, a protein called hsp90 keeps the glucocorticoid receptor in the cytosol. Hsp90 is a chaperone protein that prevents inappropriate interactions.
2. Glucocorticoid hormone enters the cell by diffusion and binds to the glucocorticoid receptor in the cytosol. Binding of hormone causes hsp90 to dissociate.
3. The glucocorticoid receptor contains a nuclear localization signal, which is masked by hsp90.
4. When hsp90 dissociates, the receptor binds to the glucocorticoid response element (GRE) in genes such as those for tyrosine aminotransferase and PEP carboxykinase, and activates transcription.

Glucocorticoid Response Elements (GRE)

• The GRE is an approximate palindrome with two halves separated by an intermediate spacer (6-3-6).
• Consensus sequence: GGTACA|NNN|TGTTCT
CCATGT|NNN|ACAAGA
• The glucocorticoid receptor binds to the GRE as a dimer, with one member of the dimer binding to each half sequence.
• Steroid hormone receptors are zinc finger proteins whose function is to bind to DNA in a dimer formation.
• Zinc fingers are loops of amino acids stabilized by Zn^{2+} ions.

Differential Sensitivity to Hormones

• Response elements for different steroid hormone receptors have similar DNA sequences.
• Example: the estrogen response element (ERE) is found in genes that respond to estrogen.
• It has a right half-site consensus TGACCT, versus TGTTCT for GRE.
• The amino acid sequence of the DNA-binding zinc finger of the estrogen receptor differs from that of the glucocorticoid receptor.
• Not all cells respond to a hormone.
• One explanation: insensitive cells do not contain the hormone receptor.
• Example: liver cells do not contain the progesterone receptor and do not respond to progesterone.

Peptide and Catecholamine Hormones

• Small molecules – proteins, peptides, or amino acid derivatives.
• Transported in free form in the blood.
• Bind to specific cell surface receptors on the target cell.
• The hormone does not enter target cells.
• Binding to the cell surface receptor initiates a response in target cells through second messengers.
• The main function is to influence the activity of proteins already present in cells.
• Rapid response to the hormone.

Peptide and Catecholamine Hormone Receptors

• Transmembrane proteins with distinct domains:
  • A ligand-binding domain to interact with the hormone on the cell surface.
  • Transmembrane domains crossing the membrane.
• Binding of ligand (hormone) induces a conformational change, resulting in either:
  • Activation of intrinsic enzyme activity (receptor tyrosine kinase).
  • Interaction with other proteins in the membrane to affect enzyme activity (G Protein-linked receptor).
• The effect is to induce a second messenger to be formed within the cell.
• There are several second messengers, including cAMP, inositol triphosphate (IP3), diacylglycerol (DAG), phosphatidyl inositol triphosphate (PIP3), and Ca^{2+}.
• The second messengers activate a phosphorylation cascade initiated by protein kinases (PKs) that phosphorylate Ser or Thr residues on existing cell proteins.
• Phosphorylation of specific proteins converts them from inactive to active or vice versa, depending on the protein.

Signal Transduction

• Hormone binds to the receptor on the cell surface.
• Production of second messengers.
• Activation of protein kinases.
• Phosphorylation of proteins present in the cell.
• Metabolic effect.

G Protein-Linked Receptors

• G proteins, short for guanosine nucleotide-binding proteins, have three subunits: α, β, and γ. The α subunit binds GDP/GTP.
• When the hormone binds to its receptor, it induces a change in the shape of the intracellular part of the receptor molecule.
• The G protein interacts with the receptor, inducing a change in the conformation of the α subunit and the replacement of GDP with GTP.
• After conformational change, the α subunit interacts with its target enzyme, such as adenylate cyclase or phospholipase C.
• G proteins can be either stimulatory (Gs and Gq) or inhibitory (Gi), depending on whether the α subunit is stimulatory (αs, αq) or inhibitory (αi).

G Protein Activation of Adenylate Cyclase and the PKA Pathway

• If adenylate cyclase is the target of the Gs protein, ATP is converted to cAMP (a 2nd messenger).
• cAMP activates protein kinase A (PKA).
• PKA is normally in its inactive form, comprising two regulatory subunits and two catalytic subunits which are non-active.
• 4 cAMP molecules bind to the two regulatory subunits, causing them to dissociate from the two catalytic subunits, which then become active.
• Active PKA catalyzes the phosphorylation of specific proteins.
• Many of the proteins are enzymes whose activity is turned on or off by phosphorylation.

G Protein Activation of Phospholipase C and PKC Pathway

• Phospholipase C is present on the cytoplasmic side of the cell membrane and is the Gq protein target.
• It converts the 4,5-bisphosphate form of the phospholipid, phosphatidyl inositol (PIP2), to inositol-1,4,5-triphosphate (IP3) and diacylglycerol (DAG), both 2nd messengers.
• IP3 binds to an IP3 receptor on the endoplasmic reticulum membrane, opening an ion channel to release Ca^{2+} into the cytoplasm.
• DAG activates protein kinase C (PKC), located either in the membrane or cytoplasm.
• PKC requires Ca^{2+} as a cofactor.
• Ca^{2+} induces metabolic effects, so Ca^{2+} is also a second messenger.

Tyrosine Specific Kinases (Receptor Tyrosine Kinases - RTKs)

• Transmembrane receptor proteins.
• When their ligand binds, there is a conformational change, and it becomes a tyrosine kinase.
• Phosphorylates itself and other proteins on Tyr residues.
• Initiates cascades:
  • One via PIP3 and protein kinase B (PKB).
  • The other via Ras and MAP kinase.

PIP3 – PKB Cascade

• The first proteins activated by the receptor tyrosine kinase are insulin-responsive substrates (IRSs), which bind to and activate the enzyme PI-3K, forming PIP3 from PIP2 in the membrane.
• PIP3 attracts protein kinase B (PKB, Akt), which, when bound to PIP3, is activated by phosphorylation by the enzyme PDK1.
• Active PKB is then able to phosphorylate specific proteins, modifying their activity.
• Glycogen synthase kinase 3 (GSK3) is activated by phosphorylation.
• It phosphorylates other proteins but also initiates control of gene expression.

Summary of Second Messenger Pathways

• G Protein-linked Receptor:
  • Adenylate cyclase → cAMP → PKA
  • Phospholipase C → IP3 → Ca^{2+} and DAG → PKC
• Receptor Tyrosine Kinase:
  • PI-3K → PIP3 → PKB, Akt
• Hormone:
  • Activate/inactivate cytosolic proteins.
  • IRS

Hormonal Control of Energy Metabolism

• A mechanism is needed to control what is happening in different tissues to keep metabolism coordinated.
• This is achieved by hormones.
• The three major hormones involved in the control of energy metabolism in tissues are insulin, glucagon, and adrenalin.
• These hormones act by changing the activity of specific proteins via phosphorylation/dephosphorylation.

Key Proteins and Their Regulation by Phosphorylation

Some proteins are active when dephosphorylated (-P) and inactive when phosphorylated (+P), while for others, the opposite is true:

Insulin, Glucagon, and Adrenalin: Roles

• Insulin: Responsible for the uptake, utilization, or storage of nutrients when concentrations in the blood rise. It reflects the fed state and ensures blood glucose concentrations are not too high.
• Glucagon: Responsible for raising blood glucose concentrations if they fall too low and protecting glucose by causing other nutrients (e.g., FA) to be used as an energy source.
• Adrenalin (epinephrine): Responsible for providing sources of energy (e.g., glucose, FA) to tissues in times of stress.

Insulin Action

• A polypeptide hormone released by β cells of the pancreas.
• Particularly important in maintaining glucose homeostasis.
• Signals the fed state (particularly increased blood concentrations of glucose, amino acids).
• The insulin receptor is a tyrosine kinase.
• Consists of 4 subunits – 2 α chains on the outside of the cell membrane linked to 2 β chains that span the membrane.
• Insulin binds to the α chains and activates the tyrosine kinase activity of the β chains, which self-phosphorylate to make it an active tyrosine kinase.

Overall Effects

• Glycogen synthesis is turned on because GS is activated by dephosphorylation by PP1.
• Glycogen degradation is turned off because GP is inactivated by dephosphorylation by PP1.
• Fatty acid synthesis is turned on because ACC is activated by dephosphorylation by PP1.
• Lipolysis (breakdown of TAG) is turned off because HSL activity is reduced by (a) dephosphorylation by PP1 and (b) the action of PDE.
• Glycolysis is turned on because PFK2 is activated by dephosphorylation by PP1, and PK is activated by dephosphorylation by PP1.
• Gluconeogenesis is turned off because F-2,6-BPase is inactivated by dephosphorylation by PP1.

Insulin and Glucose Uptake

• Insulin also affects glucose uptake to cells by influencing the glucose transporter GLUT4.
• GLUT4 is only expressed in muscle and adipose tissue – not in the liver.
• GLUT4 is normally found associated with internal vesicle membranes, but in response to two proteins (Akt/PKB and TC10, both activated by the RTK by differing mechanisms), the vesicles move to and fuse with the cell membrane.
• Insulin acts to decrease blood glucose by enhancing uptake into cells, enhancing glucose metabolism, and enhancing glucose storage as glycogen.

Glucagon Action

• A polypeptide hormone released from the α cells of the pancreas.
• It signals a lack of glucose.
• The main target is the liver, but it also acts on adipose tissue (NOT muscle).
• The glucagon receptor is a G protein, resulting in cAMP as a second messenger and a PKA cascade that phosphorylates:
  • Phosphorylase kinase (activating GP).
  • Inhibitor of PP1 (IP – activating).
  • Pyruvate kinase (inactivating).
  • HSL (activating).
• It also inactivates PFK2 and activates FBP2ase.

Glucagon: Overall Effects

• Glycogen (liver) degradation is turned on because GP is activated (a) by phosphorylation via PKA and (b) prevention of dephosphorylation by activation of the PP1 inhibitor.
• Glycogen (liver) synthesis is turned off because the dephosphorylation of GS by PP1 is inhibited.
• Gluconeogenesis is turned on, and glycolysis is turned off via inactivation of PFK2 and activation of F26bPase.
• Glucose is released from the liver into the blood.
• Lipolysis (breakdown of TAG) is turned on because HSL activity is increased by phosphorylation by PKA.

Adrenalin (Epinephrine) Action

• Derived from tyrosine. Synthesized in adrenal glands. Released in response to stress.
• Targets are liver, muscle, and adipose tissue.
• The receptor is a G protein. Two types of receptors:
  • β-adrenergic receptors, which initiate the cAMP-PKA pathway.
  • α-adrenergic receptors, which initiate the phospholipase C-PKC pathway.
• PKA phosphorylates:
  • GS in muscle (inactivating).
  • Phosphorylase kinase, which phosphorylates GP (activating).
  • Inhibitor of PP1 (IP – activating).
  • HSL (activating).

Adrenalin: Overall Effects

• Glycogen (liver and muscle) degradation is turned on because GP is activated (a) by phosphorylation via PKA and (b) prevention of dephosphorylation by activation of the PP1 inhibitor.
• Glycogen synthesis is turned off because GS (muscle) is inactivated by phosphorylation by PKA and dephosphorylation of GS by PP1 is inhibited (liver, muscle).
• Glucose is released from the liver into the blood, but in muscle, it is used for glycolysis, not released into the blood.
• Lipolysis (breakdown of TAG) is turned on because HSL activity is increased by phosphorylation by PKA.