Lecture Notes: Hormone Receptor Pathways

Introduction to Hormone Types

• Hormones can be classified into two types:
  • Peptide Hormones: Water-soluble, cannot penetrate cell membranes directly, require membrane receptors.
  • Steroid Hormones: Lipid-soluble, can diffuse through cell membranes and bind to intracellular receptors.

Peptide Hormones

• Characteristics:
  • Water-soluble
  • Require membrane-bound receptors
  • Use second messenger systems for signaling inside the cell.

Second Messenger Systems

• G Protein-Coupled Receptors (GPCRs):
  • Structure: Seven transmembrane domains.
  • Example Pathways:
    • G Stimulatory Pathway:
      • Hormone (e.g., epinephrine) binds to receptor → activates G protein by replacing GDP with GTP → activates adenylyl cyclase → converts ATP to cyclic AMP (cAMP) → activates protein kinase
      • Protein Kinase: Phosphorylates proteins, affecting ion channels, metabolic pathways, gene expression, etc.
    • GQ Pathway:
      • Hormone (e.g., oxytocin) binds to receptor → activates GQ protein → activates phospholipase C → cleaves PIP2 into DAG and IP3
      • DAG: Activates protein kinase C → phosphorylates proteins
      • IP3: Releases calcium ions from smooth ER or sarcoplasmic reticulum → activates calmodulin → activates kinases

Example Peptide Hormones

• Follicle-Stimulating Hormone (FSH)
• Luteinizing Hormone (LH)
• Growth Hormone
• Glucagon
• Insulin

Steroid Hormones

• Characteristics:
  • Lipid-soluble
  • Pass through cell membranes
  • Bind to intracellular receptors

Mechanism of Action

• Bind to intracellular receptors in the cytosol or nucleus → displace heat shock proteins → activate and bind to hormone response elements (HRE) on DNA
• Effects:
  • Stimulate transcription and translation, leading to protein synthesis
  • Control metabolism, cell growth, and ion permeability

Example Steroid Hormones

• Testosterone
• Estrogen
• Progesterone
• Cortisol
• Vitamin D

Regulation and Inhibition

• Phosphodiesterase (PDE): Degrades cyclic AMP to stop signaling pathways
• Feedback Mechanisms: Ensure hormone pathways do not over-activate

Summary

• Discussed the differences between peptide and steroid hormones
• Explored key receptor pathways: G stimulatory, GQ, and steroid hormone pathways
• Highlighted the importance of second messengers in peptide hormone signaling and the direct gene activation by steroid hormones

Important Concepts

• Water-soluble vs. lipid-soluble hormones
• Membrane-bound vs. intracellular receptors
• Role of G proteins and second messengers in signal transduction
• Intracellular signaling mechanisms for peptide vs. steroid hormones

Note: The lecture provided a focused overview of specific pathways and not all possible receptor pathways were covered. Further study is necessary for a complete understanding of hormone receptor interactions.