Hypothesis

In aged liver, hepatic stellate cells (HSCs) acquire a persistent HDAC‑mediated deacetylation of Hedgehog‑responsive enhancers that prevents the normal shift from GLI3 repressor to GLI1/2 activator, thereby flattening the intracellular GLI gradient despite normal Smo activity.

Mechanistic Basis

1. Baseline gradient model – In young hepatocytes, Shh ligand binding to Patched relieves Smo inhibition, allowing Sufu‑dependent processing of GLI proteins. A steady‑state ratio of GLI activators (GLI1/2) to repressors (GLI3R) establishes a steep intracellular gradient that drives zone‑specific gene expression (2).
2. Age‑linked epigenetic lock – RNA‑seq shows Hedgehog pathway suppression in aged liver even after injury (1). Senescent HSCs, which expand with age, overexpress class I HDACs (HDAC1/2) that are recruited by GLI3 to Hedgehog enhancers, removing H3K27ac and stabilizing a closed chromatin state (3). This HDAC‑GLI3 interaction is resistant to Smo activation because the repressor complex remains bound to DNA, blocking GLI1/2 binding.
3. Feedback from mitochondrial/telomere stress – Aged HSCs exhibit mitochondrial ROS and telomere attrition, which activate p38MAPK. p38 phosphorylates HDAC2, increasing its deacetylase activity and affinity for GLI3, thereby reinforcing the repressive loop.

Predictions

• Prediction 1: In isolated aged HSCs, basal H3K27ac levels at Ptch1 and Gli1 enhancers will be significantly lower than in young HSCs, irrespective of exogenous Shh agonist (SAG) treatment.
• Prediction 2: Pharmacological inhibition of HDACs (e.g., with entinostat) in aged HSCs will restore H3K27ac, increase GLI1/2 chromatin occupancy, and re‑establish a steep GLI activator/repressor ratio measurable by isoform‑specific western blot or targeted proteomics.
• Prediction 3: Combined SAG + HDAC inhibition will rescue angiogenic and pro‑regenerative functions of aged HSCs (e.g., VEGF secretion, collagen remodeling) to levels comparable to young cells, whereas SAG alone will produce only modest improvement.
• Prediction 4: Genetic ablation of HDAC2 specifically in HSCs of aged mice will prevent the age‑related flattening of the GLI gradient and improve liver regeneration after partial hepatectomy, without increasing tumorigenic foci.

Experimental Design

1. Cell preparation – Isolate primary HSCs from young (3 mo) and aged (24 mo) mice; validate senescence via p16^INK4a^ and SA‑β‑gal staining.
2. Chromatin assays – Perform CUT&RUN for H3K27ac, GLI1, GLI2, and GLI3 at Ptch1 and Gli1 enhancers; quantify changes with and without SAG (100 nM, 24 h) and/or entinostat (1 µM).
3. Protein processing – Use isoform‑specific antibodies to assess GLI1/2 activator and GLI3 repressor levels in cytoplasmic vs nuclear fractions; calculate activator/repressor ratios.
4. Functional readouts – Measure VEGF-A secretion (ELISA), collagen‑I expression (qPCR), and proliferation (EdU incorporation).
5. In vivo validation – Generate HSC‑specific HDAC2 knockout mice (Lrat‑Cre;Hdac2^fl/fl^) aged to 24 mo; subject to CCl4‑induced liver injury and evaluate regeneration (Ki‑67, hepatocyte proliferation) and fibrosis (Sirius Red).
6. Controls – Include Smo antagonist (cyclopamine) to confirm pathway specificity, and use GLI3‑mutant rescue constructs to test repressor dominance.

If HDAC inhibition restores the GLI gradient and regenerative capacity in aged HSCs without triggering hyperplasia or tumor formation, the hypothesis will be supported. Conversely, if HDAC blockade fails to alter GLI isoform ratios or functional outcomes despite increased H3K27ac, the model will be falsified, indicating that additional mechanisms (e.g., altered Sufu dynamics or non‑canonical Shh signaling) dominate age‑related pathway suppression.