A 28-year-old man with type 1 diabetes mellitus presents with diabetic ketoacidosis (pH 7.15, HCO₃⁻ 12 mEq/L, β-hydroxybutyrate 8 mmol/L). Despite insulin infusion and fluid resuscitation, his blood glucose remains elevated at 380 mg/dL. Serum cortisol is 45 μg/dL (normal fasting: 10–20 μg/dL). Which of the following best explains the persistent hyperglycemia despite insulin therapy?

Explanation

## Clinical Context This patient has diabetic ketoacidosis (DKA) with elevated cortisol—a stress hormone response to severe illness. The persistent hyperglycemia despite insulin therapy suggests a **counter-regulatory mechanism** is driving gluconeogenesis faster than insulin can suppress it. ## Cortisol's Role in Gluconeogenesis ```mermaid flowchart TD A[Severe stress/DKA]:::urgent --> B[↑ Cortisol secretion]:::outcome B --> C[Glucocorticoid receptor<br/>activation]:::action C --> D[↑ PEPCK transcription]:::action C --> E[↑ G6Pase transcription]:::action C --> F[↑ Proteolysis<br/>alanine release]:::action D --> G[↑ Oxaloacetate → PEP]:::outcome E --> H[↑ G6P → Glucose]:::outcome F --> I[↑ Gluconeogenic substrate]:::outcome G --> J[↑ Hepatic glucose output]:::urgent H --> J I --> J J --> K[Hyperglycemia persists]:::urgent ``` **Key Point:** Cortisol is a **potent gluconeogenic hormone** that upregulates the expression of key gluconeogenic enzymes—particularly **PEPCK** (phosphoenolpyruvate carboxykinase) and **G6Pase** (glucose-6-phosphatase)—at the **transcriptional level**. This increases hepatic glucose output independent of insulin signaling. **High-Yield:** In DKA, the stress response causes: 1. **Cortisol ↑** → upregulates PEPCK and G6Pase 2. **Glucagon ↑** → activates hormone-sensitive lipase and promotes proteolysis 3. **Epinephrine ↑** → inhibits insulin secretion, promotes lipolysis 4. **Result:** Hepatic glucose production overwhelms insulin-mediated glucose uptake **Clinical Pearl:** The **elevated cortisol (45 μg/dL)** is the diagnostic clue. In DKA, cortisol rises as part of the acute stress response. Cortisol directly **increases transcription** of PEPCK and G6Pase genes via glucocorticoid response elements (GREs) in their promoters. This means more enzyme protein is synthesized, and gluconeogenesis proceeds at a higher rate even in the presence of insulin. ## Mechanism of Insulin Resistance in DKA | Mechanism | Effect on Gluconeogenesis | |-----------|---------------------------| | **Cortisol ↑ PEPCK/G6Pase** | Increases glucose production (transcriptional) | | **Glucagon ↑ cAMP** | Activates PKA → phosphorylates PFK-2, shifts to F-2,6-BPase mode (↓ glycolysis, ↑ gluconeogenesis) | | **Epinephrine ↑** | Inhibits insulin secretion, promotes lipolysis (↑ acetyl-CoA, ↑ pyruvate carboxylase activity) | | **Insulin ↓ (absolute deficiency in T1DM)** | Cannot suppress gluconeogenesis or promote glucose uptake | | **Result** | Hepatic glucose output >> peripheral glucose uptake → **persistent hyperglycemia** | ## Why Not the Other Options? **Option 1 (Correct):** Cortisol upregulates PEPCK and G6Pase transcription, driving gluconeogenesis despite insulin therapy. This is the primary mechanism of hyperglycemia in DKA. **Option 2 (Biotin depletion):** Biotin is a cofactor for pyruvate carboxylase, but: - Biotin deficiency is rare and requires severe malnutrition or prolonged TPN without biotin supplementation - DKA does not deplete biotin - The patient's elevated cortisol is a more direct and immediate cause of hyperglycemia **Option 3 (LDH inhibition by β-hydroxybutyrate):** - β-hydroxybutyrate does not inhibit LDH; it is a substrate for LDH - LDH is not rate-limiting for gluconeogenesis - Blocking lactate → pyruvate conversion would not explain persistent hyperglycemia (gluconeogenesis uses multiple substrates) **Option 4 (Hexokinase downregulation):** - Hexokinase is not downregulated by high glucose; it is **inhibited** by its product (G6P product inhibition) - Hexokinase deficiency would impair glucose phosphorylation in peripheral tissues, not explain **hepatic glucose overproduction** - This is a peripheral uptake issue, not a gluconeogenesis issue **Mnemonic:** **"CAGES"** — Cortisol, Epinephrine, Glucagon, and Sympathetic activation all promote gluconeogenesis and hyperglycemia in stress states (DKA, sepsis, trauma). ## Clinical Management Insight In DKA, insulin therapy alone is insufficient because: - Insulin suppresses gluconeogenesis via **dephosphorylation of PEPCK** (post-translational) - But cortisol **increases PEPCK transcription** (transcriptional), overwhelming insulin's effect - Treatment requires: insulin + fluids + electrolytes + **addressing the underlying stress** (infection, etc.) - Cortisol levels normalize as DKA resolves [cite:Harrison 21e Ch 417; Stryer Biochemistry 9e Ch 30]