Hypothesis

Age-dependent decline in thyroid hormone receptor beta (THRβ) expression creates a zone 3-specific vulnerability through impaired PGC-1α-mediated PARKIN upregulation and defective mitophagy. This mechanism, we argue, drives the transition from zone 3-restricted steatosis to pan-acinar disease and advanced fibrosis.

Mechanistic Foundation

Three converging findings support this framework. First, aging impairs mitophagy through insufficient PARKIN expression. Second, THRβ expression inversely correlates with MASH severity and declines with age. Third, zone 3 represents the critical initiation site for fibrosis, with 1.8-2.7× greater odds of advanced fibrosis as steatosis increases. What remains unknown is the spatial distribution of THRβ across hepatic zones and its direct relationship to PARKIN expression.

Novel Mechanistic Reasoning

We suspect zone 3 hepatocytes have inherently lower baseline THRβ expression compared to zone 1—a "zone-specific therapeutic blind spot." This zonal heterogeneity would explain several observations: why zone 3 initiates fibrosis despite similar metabolic stress across zones, why resmetirom efficacy in MAESTRO-NASH correlates with fibrosis improvement but zone-specific drug partitioning remains unexplored, and how age-related THRβ decline exacerbates an already-vulnerable zone.

The mechanistic cascade works like this: THRβ activation directly induces PGC-1α expression, which transcriptionally upregulates PARKIN and TFEM. In zone 3, age-related THRβ decline creates a "double hit"—baseline lower receptor density compounded by age-dependent reduction—resulting in insufficient PARKIN-mediated mitophagy. Accumulating dysfunctional mitochondria trigger mitochondrial DNA release, cGAS-STING activation, and pro-fibrogenic hepatic stellate cell signaling.

Testable Predictions

1. Spatial transcriptomics from aged human NASH biopsies will reveal: THRβ (THRB) and PARKIN (PRKN) mRNA expression will be significantly lower in zone 3 compared to zone 1, correlating inversely with fibrosis stage.

2. In aged mouse primary hepatocytes: CRISPR-mediated THRB knockdown will reproduce the PARKIN deficiency phenotype, while THRβ agonist treatment (resmetirom or tirzepatide) will restore PARKIN expression only in cells with sufficient THRB.

3. Pharmacokinetic zoning: Resmetirom distribution will show zone 3 enrichment in preclinical models, and zone 3-specific mitophagy markers (LC3-II/LC3-I ratio, phospho-ubiquitin chains) will normalize following treatment—preceding histological improvement.

Falsifiability

This hypothesis would be falsified if: (a) zone 3 THRβ expression equals or exceeds zone 1 in aged NASH; (b) resmetirom fails to enhance PARKIN expression in aged hepatocytes; or (c) zone 3 mitophagy markers do not correlate with THRβ expression across fibrosis stages.

Therapeutic Implications

If validated, this framework suggests: (1) THRβ agonists may require zone-specific delivery optimization; (2) combination therapy restoring both THRβ signaling and autophagy may demonstrate synergy; and (3) baseline zone 3 THRβ/PARKIN expression could serve as a predictive biomarker for resmetirom response in aged patients.