Insulin Resistance

Definition

Primary Target Tissues

Pathophysiology

1. Free Fatty Acid–Driven Cycle (Central Mechanism)

• FFAs impair insulin-mediated glucose uptake in muscle
• In the liver, FFAs increase endogenous glucose production and triglyceride synthesis → VLDL secretion → dyslipidemia
• Ectopic lipid deposition in muscle and liver directly impedes insulin signaling

2. Inflammation

• Pro-IR: TNF-α, IL-6, resistin, retinol-binding protein 4, visfatin, fetuin-A, asprosin, PAI-1
• Anti-IR (reduced in obesity): adiponectin (improves insulin sensitivity in adipose and muscle)

3. Leptin Resistance

4. Mitochondrial Dysfunction

5. Gut Microbiome

6. Cellular Signaling Defects

Pathophysiology Diagram

Consequences & Associated Conditions

Key Risk Factors

• Visceral obesity (waist circumference; visceral > subcutaneous fat)
• Physical inactivity
• Aging
• Medications (glucocorticoids, antipsychotics, certain antiretrovirals)
• Genetic predisposition (family history of T2D)
• Ethnic background (higher prevalence in South/East Asians, partly due to visceral fat predominance)

Clinical Progression

Measurement

• Fasting insulin / HOMA-IR (homeostatic model assessment)
• Euglycemic hyperinsulinemic clamp (gold standard)
• OGTT with insulin levels
• Surrogate markers: fasting TG, TG/HDL ratio, waist circumference

Uric Acid Renal Excretion & Hyperuricemia

1. Uric Acid Production

2. Renal Excretion of Uric Acid

Key Transporter Details

• Located at the apical membrane of the proximal tubule
• Exchanges intracellular organic anions (e.g., lactate, pyrazinoate, nicotinate) for luminal urate → drives urate back into the tubule cell
• This is why lactate (alcohol), pyrazinoate (pyrazinamide), and nicotinate are anti-uricosuric — they provide the exchange substrate that promotes urate reabsorption
• Loss-of-function mutations → familial renal hypouricemia; gain-of-function polymorphisms → hyperuricemia and gout

• Basolateral efflux transporter — moves reabsorbed urate from tubule cell into the bloodstream
• Polymorphisms in SLC2A9 are the most statistically significant genetic determinants of serum urate in GWAS studies

• ATP-driven efflux transporter at the apical membrane of the proximal tubule and in intestinal epithelial cells
• Secretes urate into the tubular lumen (renal) and into the gut (extra-renal)
• In CKD, intestinal ABCG2-mediated excretion compensates for impaired renal handling (up to 70% of excretion may shift to the gut)
• Polymorphisms → reduced intestinal secretion → cases previously labelled as "overproducers" are actually extra-renal underexcretors

• Basolateral uptake of urate from peritubular blood into tubule cell — drive the secretory limb

3. Hyperuricemia

Mechanisms — Overproduction vs. Underexcretion

Clinically important: 24-hour urinary uric acid >1000 mg on a standard Western diet traditionally defined overproduction. However, ABCG2-mediated gut underexcretion blurs this classification.

Quantitative Threshold in Gout

Genetic Contribution

• Common polymorphisms in SLC2A9 (GLUT9), ABCG2, SLC22A12 (URAT1) explain ~6% of the total variance in serum urate between gouty and non-gouty individuals
• 67% of variance is driven by non-genetic factors: serum creatinine, alcohol consumption, components of metabolic syndrome — Goldman-Cecil, p. 2879

4. Drug Effects on Renal Urate Handling

5. Hyperuricemia and the Kidney — Bidirectional Relationship

• Hyperuricemia → CKD: Uric acid decreases nitric oxide production → endothelial dysfunction, hypertension, renal fibrosis, pro-inflammatory cytokine (TNF-α, IL-6) release → T-cell activation. Independent association with contrast-induced AKI (adjusted OR 1.68; 95% CI 1.38–2.04 in meta-analysis of 13,084 patients).
• CKD → Hyperuricemia: As GFR falls, renal urate clearance declines; intestinal ABCG2 compensates but eventually insufficient.
• Urate-lowering therapy (xanthine oxidase inhibitors) in CKD with hyperuricemia: some trials show eGFR preservation (MD +6.82 mL/min/1.73 m² vs. placebo), but current evidence is insufficient to recommend uric acid lowering in CKD without clinical gout.

Etiology of Hyperuricemia

Key proportion: >90% of cases are caused by renal underexcretion; <10% by urate overproduction alone. — Goldman-Cecil Medicine; Lippincott Biochemistry 8e

I. Renal Underexcretion (>90% of cases)

A. Primary / Idiopathic

• Inherent, uncharacterised excretory defects in tubular urate handling (most common cause of hyperuricemia in the general population)

B. Reduced GFR / Intrinsic Renal Disease

C. Enhanced Tubular Reabsorption / Reduced Secretion — Clinical States

D. Drug-Induced Underexcretion

II. Urate Overproduction (<10% of cases)

Note: ABCG2 gut transporter dysfunction may reclassify many apparent "overproducers" — they are actually extra-renal underexcretors.

A. Primary Enzymatic Defects (Rare, Genetic)

B. Accelerated Cell Turnover (Secondary)

C. Metabolic/Enzymatic Causes of ↑ ATP Degradation

D. Dietary Overproduction

III. Extra-Renal Underexcretion — The Third Mechanism

• ABCG2 polymorphisms (gut transporter): Intestinal ABCG2 normally handles ~1/3 of daily urate excretion. Loss-of-function variants reduce intestinal urate secretion → systemic accumulation. Many patients previously labelled overproducers on 24-h urine testing are in fact gut underexcretors.

IV. Genetic Causes — Summary Table

Common transportasome polymorphisms explain only ~6% of serum urate variance; 67% of variance between gouty and non-gouty individuals is driven by non-genetic factors (serum creatinine, ethanol, metabolic syndrome components). — Goldman-Cecil Medicine, p. 2879

How Insulin Resistance Starts

Step 1 — Normal Insulin Signaling (What Gets Disrupted)

1. Insulin binds extracellular α-subunits → conformational change
2. Intracellular β-subunits autophosphorylate on tyrosine residues → tyrosine kinase activated
3. Receptor phosphorylates IRS proteins (IRS-1, IRS-2, IRS-3, IRS-4) on tyrosine
4. Phospho-IRS recruits and activates PI3K
5. PI3K → PIP₃ → PDK1 → Akt (PKB)
6. Akt drives:
   • GLUT4 vesicle translocation to plasma membrane (muscle, adipose) → glucose uptake
   • GSK-3β inactivation → glycogen synthesis (liver, muscle)
   • FOXO1 phosphorylation → suppression of gluconeogenesis
   • mTOR activation → protein synthesis

Insulin resistance involves blunted steps in this cascade, particularly at the IRS → PI3K → GLUT4 level. The molecular mechanisms are incompletely understood. — Goodman & Gilman's

Step 2 — The Initiating Event: Excess Lipid Accumulation

How it unfolds:

• Excess caloric intake → adipocyte hypertrophy, especially in visceral/abdominal depots
• Visceral fat releases FFAs directly into the portal circulation
• Adipocytes become dysfunctional: ↑ lipolysis, ↑ pro-inflammatory adipokine secretion, ↓ adiponectin

• FFAs flood skeletal muscle and liver beyond their oxidative capacity
• Intramyocellular and intrahepatic triglycerides accumulate
• Toxic lipid intermediates form: diacylglycerol (DAG) and ceramide

• DAG activates protein kinase C (PKCθ in muscle, PKCε in liver)
• PKC phosphorylates IRS-1/IRS-2 on serine residues (instead of tyrosine)
• Serine phosphorylation of IRS inhibits its normal function as a PI3K docking scaffold → downstream PI3K/Akt/GLUT4 signaling is blunted
• This is the core molecular lesion at the cellular level

• Expanded adipose tissue becomes infiltrated by macrophages (M1-polarised)
• TNF-α and IL-6 are released systemically
• TNF-α independently activates IRS-1 serine kinases (JNK, IKKβ) → further serine phosphorylation of IRS-1 → worsening IR
• This creates a feed-forward inflammatory loop

• Chronic hyperinsulinemia → insulin receptor downregulation (↓ receptor synthesis, ↑ receptor degradation via endocytosis)
• Fewer surface receptors → less signal initiated even before the downstream cascade is impaired
• This is part of why obesity-associated hyperinsulinemia paradoxically worsens IR — Costanzo Physiology 7e

Step 3 — The Self-Amplifying Cycle

Visceral fat ↑
    ↓ FFA release ↑
        ↓ Ectopic lipid (muscle, liver)
            ↓ DAG → PKC → IRS-1 serine-P
                ↓ ↓ PI3K/Akt/GLUT4
                    ↓ Impaired glucose uptake
                        ↓ Compensatory hyperinsulinemia
                            ↓ Receptor downregulation
                                ↓ Deeper IR

Step 4 — Tissue-Specific Onset

Summary — The Initiating Sequence