Hypothesis

Aging exacerbates MASLD by inducing a hypoxia‑responsive epigenetic program in zone 3 hepatocytes that elevates labile iron and suppresses cystine uptake, thereby sensitizing these cells to ferroptosis. Pharmacologic or genetic inhibition of HIF‑2α in aged hepatocytes will reduce the Aging Hepatocyte Gene Signature (AHGS), lower ferroptotic death, and attenuate fibrosis independent of direct ferroptosis inhibitors.

Mechanistic Basis

Zone 3 hepatocytes already operate under low oxygen tension, which stabilizes HIF‑2α (not HIF‑1α) in chronic steatosis [2]. HIF‑2α transcriptionally represses ferroportin (FPN1) and upregulates transferrin receptor 1 (TFR1), expanding the intracellular labile iron pool [5]. Simultaneously, HIF‑2α inhibits SLC7A11 (xCT), decreasing cystine import and glutathione synthesis, a key prerequisite for GPX4‑mediated lipid peroxide detoxification [3]. In aged hepatocytes, declining NAD⁺/SIRT1 activity further diminishes deacetylation of HIF‑2α, prolonging its stability [6]. The combined iron overload and antioxidant deficit push lipid peroxidation past the ferroptotic threshold, explaining why ferroptosis inhibitors like ferrostatin‑1 reverse AHGS [1] but do not address the upstream hypoxic signal.

Predictions and Experimental Design

1. Genetic test: Hepatocyte‑specific Hif2a knockout (Hif2a^fl/fl; Alb‑Cre) in aged (18‑month) MASH mice will show:
   • ↓ labile iron (calcein‑AM quenching) and ↑ GSH/GSSG ratio in isolated zone 3 hepatocytes.
   • Reduced AHGS score (by snRNA‑seq deconvolution) and lower ferroptosis markers (PTGS2, ACSL4, 4‑HNE).
   • Attenuated fibrosis (Sirius Red, collagen‑1α1) despite unchanged steatosis burden.
2. Pharmacologic test: Treatment with the HIF‑2α antagonist PT2385 in aged MASH mice will phenocopy the genetic rescue, whereas PT2399 (HIF‑1α inhibitor) will not.
3. Human validation: In MASLD patient biopsies, nuclear HIF‑2α immunostaining will correlate positively with AHGS score (Spearman r > 0.5) and with serum ferritin/transferrin saturation, after adjusting for BMI and diabetes.
4. Falsifiability: If Hif2a deletion or PT2385 fails to reduce AHGS or ferroptosis markers despite efficient target inhibition (confirmed by HIF‑2α target gene downregulation), the hypothesis is refuted.

Potential Implications

Confirming HIF‑2α as a upstream driver would reposition hypoxia‑iron signaling as a druggable axis distinct from direct ferroptosis scavenging. It suggests that early MASLD patients with elevated HIF‑2α activity (detectable via PET‑HIF tracers or circulating HIF‑2α‑regulated microRNAs) could benefit from HIF‑2α antagonists before fibrosis becomes entrenched. Moreover, linking epigenetic clock acceleration to HIF‑2α‑mediated repression of antioxidant genes offers a mechanistic bridge between aging, iron homeostasis, and MASLD progression, providing a composite biomarker (AHGS + HIF‑2α target expression) for risk stratification.