Hypothesis

In aging Leydig cells, the autophagic hierarchy that normally prioritizes lipophagy to fuel StAR‑mediated cholesterol import becomes inverted: oxidized lipid products impair LC3 recruitment to lipid droplets, suppressing lipophagy while concurrently activating TFEB‑driven mitophagy and PERK‑dependent ER‑phagy. This shift starves the steroidogenic pathway of cholesterol, amplifies ER stress, and drives a senescent phenotype that underlies late‑onset hypogonadism.

Mechanistic Rationale

1. Lipophagy relies on LC3‑LD interaction – In young cells, LC3‑II binds phosphatidylethanolamine on lipid droplet (LD) surfaces via adaptor proteins such as p62 and NBR1, enabling cholesterol mobilization for StAR (see autophagy regulates testosterone synthesis).
2. Oxidized lipids modify LC3 – Age‑associated increase in lipid peroxidation generates 4‑hydroxynonenal (4‑HNE) and malondialdehyde, which covalently modify LC3 lysine residues (predicted from mass‑spec studies of oxidized autophagy proteins). This modification reduces LC3’s affinity for LD‑anchored tethering complexes, decreasing lipophagic flux.
3. Compensatory upregulation of quality‑control autophagies – Reduced lipophagy elevates mitochondrial ROS and ER calcium leak, activating TFEB (via calcineurin) and PERK‑eIF2α‑ATF4 pathways. TFEB drives transcription of lysosomal and mitophagy genes (e.g., BNIP3, Parkin) while PERK‑ATF4 upregulates ER‑phagy receptors (FAM134B, SEC62), as shown in inflammation‑induced ER‑phagy (ER‑phagy shift).
4. Hierarchy inversion – Consequently, mitophagy and ER‑phagy consume a larger proportion of autophagosomal capacity, leaving insufficient LC3‑LD events to sustain cholesterol supply. The resulting deficit in StAR‑mediated mitochondrial import lowers pregnenolone synthesis, reduces testosterone output, and activates a senescence‑associated secretory phenotype (SASP) via chronic ER stress.

Testable Predictions

• Prediction 1: Aged Leydig cells will show increased 4‑HNE adducts on LC3-II and decreased LC3 colocalization with perilipin2‑positive LDs after LH stimulation, reversible by 4‑HNE scavengers (e.g., hydralazine) or aldehyde dehydrogenase overexpression.
• Prediction 2: Pharmacological enhancement of lipophagy (e.g., trehalose‑induced LD‑LC3 tethering) in aged cells will restore cholesterol delivery to mitochondria, elevate StAR activity, and raise testosterone levels without altering overall autophagic flux.
• Prediction 3: Genetic knockdown of TFEB or PERK in aged Leydig cells will suppress mitophagy/ER‑phagy markers (LC3‑BNIP3, LC3‑FAM134B) and preferentially rescue lipophagy, confirming the compensatory switch.
• Prediction 4: Exposing young Leydig cells to exogenous 4‑HNE‑treated LDL will recapitulate the aged autophagic hierarchy (reduced LD‑LC3, increased mitophagy/ER‑phagy) and diminish testosterone synthesis.

Experimental Approach

• Use primary rat or human iPSC‑derived Leydig cells treated with LH ± 4‑HNE scavengers or aldehyde dehydrogenase activators.
• Measure LD‑LC3 colocalization by immunofluorescence and proximity ligation assay; quantify 4‑HNE‑LC3 adducts via Western blot with anti‑4‑HNE after LC3 immunoprecipitation.
• Assess mitochondrial membrane potential (JC‑1), ER stress markers (BiP, CHOP), and steroidogenesis (pregnenolone, testosterone ELISA).
• Manipulate TFEB (siRNA or overexpressing dominant‑negative) and PERK (GSK2606414) to test hierarchy reversal.
• In vivo, administer 4‑HNE scavenger to aged male mice and monitor testicular LC3‑LD interaction, serum testosterone, and Leydig cell senescence (p16^INK4a^ staining).

Falsifiability

If aging does not increase LC3 oxidation, or if preventing LC3 modification fails to rescue lipophagy and testosterone levels despite restored mitophagy/ER‑phagy, the hypothesis would be refuted. Conversely, demonstrating that lipophagy restoration alone (without altering mitophagy/ER‑phagy) rescues steroidogenesis would support the proposed mechanistic inversion.

Implications

This work reframes autophagic decline not as a global loss of bulk degradation but as a specific rewiring of substrate preference driven by lipid oxidative stress. Targeting the LC3‑LD interface—or the oxidized lipid signal that disrupts it—could selectively reinstate the youthful autophagic hierarchy and mitigate age‑related testosterone decline.