Insulin Pathway

Insulin Receptor: Core Identity

  • Belongs to the family of receptor tyrosine kinases (RTKs).

  • Functions as both receptor and enzyme: the cytoplasmic domain possesses intrinsic kinase activity.

  • Canonical activation sequence:

    • Insulin (ligand) binds the extracellular α‐subunits.

    • Receptor dimers cluster (dimerize/oligomerize).

    • Autophosphorylation occurs on multiple tyrosine residues of the intracellular β‐subunits.

    • Creates high-affinity docking sites bearing phosphorylated tyrosines that are recognized by proteins with SH2 (Src-homology-2) domains.

Docking Protein: IRS-1

  • Insulin Receptor Substrate-1 is the primary adaptor.

  • Contains multiple Tyr motifs; each can be further phosphorylated to serve as platforms for distinct downstream enzymes.

  • IRS-1 integrates and bifurcates the signal into at least two well-studied cascades:

    • The RAS–MAPK (mitogenic) pathway (discussed in previous lecture).

    • The PI3K–Akt (metabolic) pathway (focus of current discussion).

Branch Choice: Focusing on the PI3K–Akt Arm

  • Once bound to phosphorylated IRS-1, phosphatidylinositol 3-kinase (PI3K) is itself phosphorylated and activated.

PI3K Catalytic Action

  • Substrate: membrane phospholipid PIP₂ (phosphatidylinositol 4,5-bisphosphate).

  • Reaction (simplified):
    PIP<em>2+ATP→PI3KPIP</em>3+ADPPIP<em>2 + ATP \xrightarrow{PI3K} PIP</em>3 + ADP

  • Product: PIP₃ (phosphatidylinositol 3,4,5-trisphosphate).

    • Distinct from IP₃/DAG generation by phospholipase C (mentioned to prevent confusion with the IP₃ pathway).

    • Acts as a lipid second messenger, remaining within the inner leaflet of the plasma membrane.

Recruitment & Activation of Akt (Protein Kinase B)

  • PIP₃ forms docking sites for proteins with pleckstrin homology (PH) domains; chief among them is Akt.

  • Akt is phosphorylated at key residues (Thr308, Ser473 in mammals) → conformationally active.

  • Once active, Akt mediates two central metabolic effects of insulin:

1. GLUT4 Vesicle Translocation

  • Cytosolic vesicles containing the GLUT4 glucose transporter are constitutively present but sequestered.

  • Akt triggers rapid exocytosis/fusion of these vesicles with the plasma membrane.

  • Result: sharp increase in membrane GLUT4 density → accelerated facilitated diffusion of extracellular glucose into the cell (especially in adipose tissue and skeletal muscle).

2. Promotion of Glycogen Synthesis

  • Via an indirect cascade (details not required for the exam):

    • Akt inhibits GSK-3 (glycogen synthase kinase-3).

    • Inhibition of GSK-3 relieves its inhibitory phosphorylation on glycogen synthase.

  • Net outcome: active glycogen synthase catalyzes polymerization of incoming glucose units into glycogen:
    n Glucose (UDPG)→Glycogen SynthaseGlycogen(n)+UDPn\,Glucose\,(UDPG) \xrightarrow{Glycogen\,Synthase} Glycogen_{(n)} + UDP

Integrated Physiological Outcome

  • Insulin, through PI3K–Akt:

    • Drives glucose uptake (GLUT4 insertion).

    • Drives glucose storage (glycogen synthesis).

  • Complements the RAS–MAPK route, which is more concerned with growth, differentiation, and gene expression.

Connections & Context

  • Distinction from IP₃/DAG signaling: although PIP₂ is a common precursor, PI3K phosphorylates it (adds a phosphate) rather than cleaving it (as PLC does).

  • Disorders:

    • Impaired PI3K/Akt signaling → insulin resistance, hallmark of type 2 diabetes.

    • Over-activation in oncogenic settings yields enhanced growth and survival signals (Akt is frequently hyper-active in cancers).

  • Therapeutic angles:

    • Drugs enhancing GLUT4 translocation mimic or augment the Akt effect.

    • PI3K or Akt inhibitors are explored in oncology.

Key Terms Recap (Flash-Style)

  • RTK – receptor tyrosine kinase.

  • IRS-1 – adaptor with SH2 docking motifs.

  • PI3K – lipid kinase turning PIP₂ → PIP₃.

  • PIP₃ – membrane lipid second messenger.

  • Akt/PKB – serine/threonine kinase, central metabolic switch.

  • GLUT4 – insulin-responsive glucose transporter.

  • Glycogen Synthase – enzyme linking glucose → glycogen.

Numerical / Molecular Data

  • Phosphorylation states: PIP<em>2PIP<em>2 has 2 phosphate groups; PIP</em>3PIP</em>3 has 3.

  • ATP consumption: 1 ATP per PIP₂ phosphorylated by PI3K.

  • Akt key residues: Thr308,  Ser473Thr308,\; Ser473.

  • Overall glucose polymerization stoichiometry summarized above.

Ethical & Practical Implications

  • Understanding this pathway informs treatment strategies for metabolic diseases and cancer.

  • Illustrates how a single hormone can orchestrate broad cellular changes via branching signaling networks.