Hypothesis

Zone 3 (pericentral) hepatocytes accumulate senescent cells earlier and more densely than zone 1 hepatocytes during aging and metabolic stress, and this zonated senescence burden causally links mitochondrial dysfunction, lipid overload, and ferroptosis to steer MASLD progression.

Mechanistic Rationale

1. Oxygen gradient and HIF‑1α signaling – The hepatic lobule exhibits a steep pO2 gradient, with zone 3 experiencing the lowest oxygen tension. Chronic hypoxia stabilizes HIF‑1α, which transcriptionally upregulates p16^Ink4a^ and suppresses PGC‑1α, thereby reducing mitochondrial biogenesis and fatty‑acid oxidation [1].
2. Mitochondrial fragility – Senescent zone 3 hepatocytes show depressed NAD^+ levels and impaired mitophagy, leading to accumulation of damaged mitochondria that leak ROS. ROS further activates the DNA‑damage response, reinforcing p16 expression in a feed‑forward loop.
3. Lipid‑induced ferroptosis sensitization – ROS‑driven lipid peroxidation generates toxic phospholipid‑hydroperoxides. In zone 3, low glutathione peroxidase 4 (GPX4) expression (a known hypoxic adaptation) renders these cells prone to ferroptosis [2]. Senescent SASP factors (IL‑6, IL-1β, TGF-β) suppress GPX4 and upregulate ACSL4, amplifying ferroptotic susceptibility.
4. SASP‑mediated paracrine spread – Senescent zone 3 hepatocytes release SASP cytokines that diffuse toward zone 1, inducing insulin resistance and de novo lipogenesis in neighboring cells, thereby expanding the steatotic field beyond the original senescent niche.

Testable Predictions

• Prediction 1: In human MASLD livers, the proportion of p16^Ink4a^+/SA‑β‑gal+ hepatocytes will be significantly higher in zone 3 than zone 1, and this ratio will correlate with hepatic ferroptosis markers (ACSL4, PTGS2, 4‑HNE adducts) and serum SASP cytokines (IL-6, IL-1RA).
• Prediction 2: Genetic ablation of p16^Ink4a^ specifically in pericentral hepatocytes (using a Cyp2e1‑Cre driver) will reduce zone‑specific mitochondrial ROS, lower ferroptosis incidence, and attenuate steatosis and fibrosis in mice fed a methionine‑choline‑deficient diet, without affecting zone 1 senescence.
• Prediction 3: Senolytic treatment (dasatinib + quercetin) will preferentially clear p16^Ink4a^+ cells from zone 3, leading to a greater reduction in hepatic triglyceride content and ferroptosis biomarkers compared with non‑targeted senolytic administration.

Experimental Approach

• Human validation: Obtain formalin‑fixed, paraffin‑averaged liver biopsies from MASLD patients across fibrosis stages. Perform multiplex immunofluorescence for p16^Ink4a^, CYP2E1 (zone 3 marker), and glutamine synthetase (zone 1 marker). Quantify zonated senescence index. Stain adjacent sections for ACSL4 and 4‑HNE to assess ferroptosis. Correlate histology with circulating SASP panels.
• Mouse lineage‑specific knockout: Generate Alb‑CreERT2;Cyp2e1‑Cre;p16^fl/fl^ mice. Induce pericentral p16 deletion after tamoxifen, then challenge with MCD diet. Measure mitochondrial respiration (Seahorse), lipidomics (DG, ceramide), ferroptosis (lipid ROS C11‑BODIPY), and fibrosis (Sirius Red).
• Senolytic spatial tracking: Treat MCD‑fed mice with Dasatinib + Quercetin conjugated to a zone‑3‑targeted nanoparticle (galactose‑modified lipids). Use p16‑luciferase reporter to monitor senescent cell clearance in real time via bioluminescence imaging, comparing zone‑specific signal loss to untargeted senolytics.

Implications

If confirmed, this hypothesis would redefine MASLD as a zonally initiated senescence‑ferroptosis axis, providing a mechanistic basis for zone‑targeted senolytics or HIF‑1α modulators as precision therapeutics. It also offers a biomarker strategy—zonated senescence burden detectable via imaging‑guided biopsies or circulating zonated SASP signatures—to identify patients most likely to benefit from senolytic interventions, addressing the current therapeutic‑window gap in MASLD management.

[1] https://doi.org/10.1038/ncomms15691 [2] https://pmc.ncbi.nlm.nih.gov/articles/PMC12943086/