ER Stress–Mediated Transcriptional Repression of StAR Underlies Age‑Related Leydig Cell Decline and Is Reversible by Ketone Body Signaling

Hypothesis

Chronic ER stress in aging Leydig cells suppresses StAR transcription via the PERK‑eIF2α‑ATF4 axis, which interferes with SF‑1 binding to the StAR promoter, and this repression can be relieved by β‑hydroxybutyrate‑mediated inhibition of PERK signaling.

Rationale

• Aging Leydig cells show ER dilation and increased lipofuscin 5, hallmarks of sustained ER stress.
• ER stress activates PERK, leading to eIF2α phosphorylation and selective translation of ATF4, which can recruit corepressors to steroidogenic promoters (e.g., SF‑1 sites) 5.
• Reduced SF‑1 activity diminishes StAR transcription, compounding the LH/cAMP/PKA defect described in 1.
• β‑Hydroxybutyrate (BHB) attenuates aging markers and boosts testosterone via FOXO3a 4 and also acts as an endogenous HDAC inhibitor that can blunt PERK activation (observed in other tissues).

Testable Predictions

1. In isolated old rat Leydig cells, tunicamycin‑induced ER stress will lower StAR mRNA and testosterone output; co‑treatment with BHB will rescue these defects.
2. Pharmacologic PERK inhibition (GSK2606414) will mimic BHB’s effect, increasing StAR promoter activity in a luciferase assay driven by SF‑1 sites.
3. Chromatin immunoprecipitation will show increased ATF4 occupancy and decreased SF‑1 binding at the StAR promoter in aged cells, reversible by BHB or PERK inhibitor.
4. In vivo, old mice fed a ketone‑ester diet will exhibit lower hepatic p‑PERK levels in testes, higher StAR protein, and elevated serum testosterone compared with controls.

Falsifiability If BHB fails to restore StAR expression despite PERK inhibition, or if ATF4 binding does not change with age, the hypothesis is refuted.

Broader Impact Linking ER stress to transcriptional control of StAR integrates metabolic (ketone) and organelle stress pathways, offering a dual target for treating late‑onset hypogonadism.