Hormone Action and Signal Transduction

What are the signals used to regulate metabolism?

Hormones — chemical messengers.

What is the main function of hormonal signals?

They integrate and coordinate metabolic activities across different tissues and ensure fuels are used efficiently.

How do hormones act on target cells?

By binding to highly specific receptors in hormone-sensitive cells.

What is autocrine hormone signaling? Example?

A cell releases a hormone that acts on the same cell that produced it.

Example: Interleukin-2 in T-cells.

What is paracrine hormone signaling? Example?

A hormone acts on nearby (adjacent) cells.

Example: Interleukin-1 released by macrophages acting on nearby T-cells.

What is endocrine hormone signaling? Example?

Hormones are released into the bloodstream and act on distant target cells.Examples: Insulin, epinephrine, glucagon.

How do peptide or amine hormones signal inside the body?

They bind to receptors on the cell surface and signal through second messengers (e.g., cAMP).They do NOT enter the cell.→ They alter the activity of pre-existing enzymes.

How do steroid or thyroid hormones signal?

They enter the cell, bind intracellular receptors, and act in the nucleus to alter gene transcription.

→ They require new protein synthesis.

What is the key difference between steroid and peptide hormone action?

Peptide hormones use cell-surface receptors + second messengers (fast).

Steroid/thyroid hormones act inside the cell on DNA (slow, require new proteins).

How do steroid hormones reach their target cells?

They travel in the blood bound to serum-binding proteins, then diffuse into the target cell because they are lipid-soluble.

What happens after a steroid hormone binds its receptor inside the cell?

The receptor changes shape (conformational change) and binds to specific DNA regions called hormone response elements (HREs).

How do steroid hormones affect gene expression?

They regulate transcription—increasing or decreasing mRNA production for specific genes.

What is the cellular response to steroid hormones?

The cell makes new proteins, which change cell function.

What is the first event after insulin binds to its receptor?

Insulin binding causes a conformational change in the receptor's β-subunits, activating the tyrosine kinase domains.

What happens to the insulin receptor's tyrosine kinase domains once activated?

They undergo autophosphorylation—the receptor phosphorylates its own tyrosine residues.

How does the insulin signal get amplified inside the cell?

The activated receptor phosphorylates other proteins on tyrosine residues, triggering a signaling cascade.

What metabolic pathways are turned on by insulin signaling?

Insulin turns on fed-state pathways, including:

Glycolysis

Fatty acid synthesis

Protein synthesis

Glucose uptake (via GLUT4 in muscle & fat)

Which pancreatic cells make insulin and which make glucagon?

β-cells → insulin

α-cells → glucagon

What causes Type I diabetes?

Destruction of β-cells, so the body cannot make insulin → requires exogenous insulin.

Why do untreated Type I diabetics enter a starvation-like metabolic state?

Without insulin, cells cannot take up glucose → body switches to

β-oxidation (fat breakdown)

Ketone production

Gluconeogenesis continuesEven though blood glucose is high, cells act as if they are starving.

What triggers insulin release from the pancreas?

High blood glucose → stimulates β-cells to release insulin (fed state).

What triggers glucagon release?

Low blood glucose → stimulates α-cells to release glucagon (fasting state).

Which organ controls blood glucose levels?

The liver is the main controller of blood glucose through phosphorylation/dephosphorylation of key enzymes.