Pathophysiology of NASH (Nonalcoholic Steatohepatitis)

Nomenclature note: Recent guidelines have renamed NASH to MASH (Metabolic dysfunction-Associated SteatoHepatitis), and NAFLD to MASLD (Metabolic dysfunction-Associated Steatotic Liver Disease). Both terms are used below.

1. Disease Spectrum

• Steatosis alone is clinically benign and may remain stable for years
• NASH is the inflammatory, injurious form that drives progression
• Up to 20% of patients with NASH develop cirrhosis over 10-15 years
• NASH-cirrhosis carries risk for hepatocellular carcinoma (HCC)
• Robbins & Kumar Basic Pathology, p. 1921

2. Core Pathogenic Mechanisms

A. Insulin Resistance - The Central Driver

• Adipose tissue lipolysis: IR impairs the antilipolytic action of insulin on adipocytes, causing unrestrained release of nonesterified (free) fatty acids (NEFAs/FFAs) into portal circulation
• Reduced adiponectin: Adipocytes in IR states produce less adiponectin, which normally promotes skeletal muscle FFA oxidation and limits hepatic FFA uptake - its loss worsens hepatic fat accumulation
• Hyperinsulinemia: Drives upregulation of SREBP-1c, activating genes for de novo lipogenesis (DNL) in hepatocytes
• Goldman-Cecil Medicine, p. 3535; Robbins & Kumar, p. 1920

B. Hepatic Steatosis - "First Hit"

• Reduced beta-oxidation of fatty acids (mitochondrial dysfunction)
• Impaired VLDL export (reduced apoB synthesis/lipid transfer)

• Harrison's Principles of Internal Medicine, 22E, p. 2745

C. Lipotoxicity - "Second Hit" Driving NASH

• Diacylglycerols (DAG) - activate PKC, further worsening IR
• Ceramides - promote apoptosis and inflammation
• Lysophosphatidylcholine (LPC) - hepatocyte membrane disruptor
• Free cholesterol - mitochondrial and ER membrane toxicity
• Saturated fatty acids - directly lipotoxic vs. unsaturated FFAs
• Harrison's 22E, p. 2745

D. Oxidative Stress

• Excess FFAs undergo beta-oxidation in mitochondria, generating reactive oxygen species (ROS)
• When overwhelmed, FFAs are shunted to microsomal omega-oxidation (CYP2E1, CYP4A), producing even more ROS
• ROS cause lipid peroxidation of hepatocyte membranes → formation of lipid peroxidation products (4-HNE, MDA) that are cytotoxic and pro-inflammatory
• ROS also cause mitochondrial DNA damage, impairing oxidative phosphorylation in a vicious cycle
• Goldman-Cecil Medicine, p. 3537

E. Endoplasmic Reticulum (ER) Stress

• Excess lipids and oxidative damage overwhelm the hepatocyte ER's protein-folding capacity
• Activates the unfolded protein response (UPR):
  • Short-term UPR: protective, restoring ER homeostasis
  • Chronic UPR: activates apoptosis pathways (CHOP/DDIT3), NF-κB (inflammation), and JNK signaling
• ER stress amplifies hepatocyte injury and death

F. Inflammasome Activation and Innate Immunity

• Lipid intermediates directly activate the NLRP3 inflammasome in hepatocytes and Kupffer cells
• This drives cleavage and release of IL-1β and IL-18 - potent proinflammatory cytokines
• TNF-α (from adipocytes and Kupffer cells) promotes hepatocyte apoptosis and necroptosis
• Damaged hepatocytes release DAMPs (danger-associated molecular patterns) that further activate Kupffer cells via TLR4 signaling
• Robbins & Kumar, p. 1920; Goldman-Cecil, p. 3537

G. Adipokine Dysregulation

H. Gut Microbiome - "Multiple Hit" Model

• Dysbiosis: Altered gut microbiota composition promotes gut permeability ("leaky gut")
• Increased bacterial endotoxin (LPS) translocates via the portal vein to the liver
• LPS activates hepatic TLR4 on Kupffer cells → NF-κB activation → TNF-α, IL-6, IL-1β production
• Short-chain fatty acids and secondary bile acids from dysbiotic flora further modulate hepatic inflammation
• Robbins & Kumar, p. 1920; Harrison's 22E, p. 892

I. Hepatic Stellate Cell Activation and Fibrosis

1. Hepatocyte death (apoptosis/necroptosis) releases DAMPs, TGF-β, and PDGF
2. Kupffer cells and recruited macrophages amplify TGF-β production
3. Hepatic stellate cells (HSCs) are activated from their quiescent, lipid-storing state into myofibroblasts
4. Activated HSCs deposit type I and III collagen in the space of Disse (perisinusoidal fibrosis - characteristic of NASH)
5. Progressive fibrosis: perisinusoidal (F1) → bridging (F3) → cirrhosis (F4)

• Goldman-Cecil, p. 3537; Harrison's 22E, p. 890

J. Genetic and Epigenetic Modifiers

• Harrison's 22E, p. 892; Goldman-Cecil, p. 3537

3. Integrated Pathogenic Model (Summary)

Obesity + Sedentary lifestyle + Dietary excess (fructose/saturated fat)
              ↓
    INSULIN RESISTANCE (adipose, muscle, liver)
              ↓
  ┌───────────────────────────────────────┐
  │  STEATOSIS ("First Hit")             │
  │  ↑ FFA from adipose lipolysis         │
  │  ↑ De novo lipogenesis (DNL)          │
  │  ↓ FFA beta-oxidation                 │
  │  ↓ VLDL export                        │
  └──────────────┬────────────────────────┘
                 │ + Multiple parallel hits
  ┌──────────────▼────────────────────────┐
  │  LIPOTOXICITY → NASH                  │
  │  • Oxidative stress (ROS, CYP2E1)     │
  │  • ER stress / UPR → apoptosis        │
  │  • NLRP3 inflammasome → IL-1β         │
  │  • Kupffer cell activation (TLR4/LPS) │
  │  • Adipokine dysregulation            │
  │  • Gut dysbiosis → ↑ endotoxin        │
  └──────────────┬────────────────────────┘
                 ↓
    Stellate cell activation → TGF-β → Collagen deposition
                 ↓
         FIBROSIS → CIRRHOSIS → HCC

4. Histological Features of NASH (Biopsy)

1. Macrovesicular steatosis (>5% of hepatocytes)
2. Hepatocyte ballooning degeneration (cytoskeletal injury from lipotoxicity)
3. Lobular inflammation (mixed inflammatory infiltrate)

• Goldman-Cecil Medicine, p. 3523

5. Key Associations and Risk Factors

• Obesity (BMI >30), central adiposity, metabolic syndrome
• Type 2 diabetes mellitus (80% of T2DM patients have NAFLD imaging evidence)
• Dyslipidemia (hypertriglyceridemia, low HDL)
• Polycystic ovary syndrome (PCOS), hypothyroidism, hypogonadism, sleep apnea
• Ethnicity: highest in Hispanic/Asian-Indian (~50%), intermediate in White (~30%), lowest in African American (~25%)

6. Recent Nomenclature Update

• NAFLD → MASLD (Metabolic dysfunction-Associated Steatotic Liver Disease)
• NASH → MASH (Metabolic dysfunction-Associated SteatoHepatitis)

• Harrison's Principles of Internal Medicine, 22E (2025) - Chapters on MASLD/MASH (Molecular and Cellular Pathogenesis section)
• Goldman-Cecil Medicine - Chapter 138: Nonalcoholic Fatty Liver Disease
• Robbins & Kumar Basic Pathology - Chapter 14: Nonalcoholic Fatty Liver Disease
• Yamada's Textbook of Gastroenterology, 7E - NAFLD/NASH chapter