Questions: Insulin Signaling and Blood Glucose Regulation

Question 1

A patient with type 2 diabetes has chronically elevated blood insulin levels but persistently high blood glucose. What is the most likely molecular explanation?

• A. Pancreatic beta cells are not producing enough insulin
• B. GLUT4 transporters are permanently fused to the cell membrane, blocking glucose entry
• C. Inflammatory signals have caused serine phosphorylation of IRS-1, disrupting the insulin signaling cascade before GLUT4 translocation can occur
• D. Glucagon levels are too low to counteract insulin's effects

Answer: (C)

Elevated insulin with high glucose is the hallmark of insulin resistance. The mechanism in obesity-related type 2 diabetes is serine phosphorylation of IRS-1 by inflammatory kinases (JNK, IKK), which blocks IRS-1 from docking with PI3K, breaking the cascade before GLUT4 translocation occurs. The pancreas compensates by secreting more insulin (hyperinsulinemia) — which is why insulin is high — but this cannot overcome the broken signaling relay. Option A describes a different failure mode (later-stage beta-cell burnout), not the primary defect.

Question 2

When insulin binds its receptor on a muscle cell, glucose enters the cell because:

• A. Insulin directly opens glucose ion channels in the membrane
• B. The insulin receptor phosphorylates GLUT4 directly, activating it
• C. Akt activation triggers vesicles containing GLUT4 to fuse with the plasma membrane, increasing surface transporter density
• D. Insulin inhibits glucagon, which normally prevents GLUT4 from functioning

Answer: (C)

GLUT4 is sequestered in intracellular vesicles at rest. The insulin signaling cascade (IR → IRS-1 → PI3K → PIP₃ → PDK1 → Akt) triggers vesicle fusion with the plasma membrane, increasing GLUT4 surface density roughly 10-fold. Insulin does not directly open channels (A) or phosphorylate GLUT4 (B) — the mechanism is vesicle translocation driven by Akt. Understanding this step is what makes insulin resistance comprehensible: the transporter exists, but it never makes it to the cell surface.

Question 3

Insulin reduces blood glucose both by promoting glucose uptake in muscle and fat tissue AND by suppressing hepatic glucose production.

• T. True
• F. False

Answer: True

Akt, the central effector of insulin signaling, coordinates the metabolic response at multiple sites simultaneously. In muscle and adipose tissue, it triggers GLUT4 translocation (glucose uptake). In the liver, it phosphorylates FOXO transcription factors, suppressing gluconeogenic gene expression (reducing hepatic glucose output). Blood glucose is clamped from both the demand side and the supply side — this is why a single signaling cascade has such large whole-body metabolic effects.

Question 4

Individuals with insulin resistance should minimize exercise, since their impaired insulin signaling means exercise cannot effectively lower blood glucose.

• T. True
• F. False

Answer: False

Exercise activates AMPK, which triggers GLUT4 translocation via an insulin-independent pathway. This means exercise lowers blood glucose and improves glucose disposal even when the insulin signaling cascade is impaired. This is precisely why exercise is a first-line treatment for insulin resistance and type 2 diabetes — it bypasses the broken IRS-1 → PI3K → Akt relay entirely. Telling insulin-resistant individuals to avoid exercise would be both incorrect and harmful.

Question 5

Why does insulin resistance cause hyperinsulinemia (elevated blood insulin), and why is this compensatory response ultimately harmful?

Model answer: When insulin signaling is impaired, blood glucose rises because GLUT4 translocation is blunted. The pancreatic beta cells detect elevated glucose and increase insulin secretion to compensate, producing hyperinsulinemia. This initially maintains glucose levels but chronically elevated demand accelerates beta-cell burnout, eventually leading to insulin insufficiency and overt type 2 diabetes.

This is the central paradox of insulin resistance: high insulin AND high glucose coexist. It seems contradictory until you understand that the problem is not at the hormone level but downstream in the signaling cascade. More insulin cannot overcome a broken relay — it only stresses the beta cells further. The progression from insulin resistance → hyperinsulinemia → beta-cell failure → type 2 diabetes follows directly from this molecular bottleneck.