Rapamycin preserves adrenalzonation by mimicking nutrient stress without reproducing aging's endocrine collapse
Chronic mTOR inhibition extends lifespan by activating hormetic pathways that resemble ancestral scarcity signals, yet it remains unclear whether this pharmacological state reproduces or opposes the endocrine signature of natural adrenal aging. We hypothesize that rapamycin maintains zona reticularis DHEA production and normalizes the cortisol:DHEA ratio by simultaneously suppressing maladaptive zona glomerulosa activity and enhancing stromal support, thereby creating a compensatory adaptive endocrine state distinct from the high‑cortisol/low‑DHEA profile of adrenopause.
To test this, we will treat middle‑aged male C57BL/6 mice with rapamycin (14 mg/kg diet) or control diet for 6 months. At the end of the treatment we will measure:
- Plasma corticosterone (mouse cortisol analog) basal levels and stress‑induced response after 15 min restraint (to approximate the cortisol awakening response).
- Plasma DHEA‑S concentration.
- Aldosterone levels to assess zona glomerulosa output.
- Adrenal gland histology: zone thickness (glomerulosa, fasciculata, reticularis), collagen deposition (Sirius Red), and capillary density (CD31 immunostaining).
- Single‑cell RNA‑seq of adrenal cells to quantify zone‑specific steroidogenic enzyme expression (Cyp11b2, Cyp11b1, Cyp17a1) and stress‑response genes (Atg5, Mapk1/3).
We predict that rapamycin‑treated mice will show:
- No increase in basal corticosterone and a blunted stress‑induced rise relative to controls, indicating preserved HPA feedback.
- Elevated or unchanged DHEA‑S with a significantly lower corticosterone:DHEA‑S ratio compared to aged controls.
- Reduced aldosterone output, reflecting attenuated zona glomerulosa hyperactivity.
- Histologically, thinner zona glomerulosa, unchanged zona fasciculata thickness, and preserved zona reticularis morphology with decreased perivascular fibrosis and increased capillary density.
- Transcriptomic signature showing upregulated autophagy and ERK/MEK signaling in fasciculata/reticularis cells, alongside downregulated profibrotic Tgf‑b1 in stromal compartments.
These outcomes would falsify the notion that mTOR inhibition merely impersonates aging’s endocrine stress. Instead, they would support a model where rapamycin induces a hormetic preconditioning that selectively rescues zona reticularis function while dampening maladaptive mineralocorticoid drive, thereby uncoupling longevity‑associated stress responses from the pathological HPA dysregulation seen in natural adrenal aging. If rapamycin fails to preserve DHEA‑S or worsens the corticosterone:DHEA‑S ratio, the hypothesis would be refuted, suggesting that lifespan extension under mTOR inhibition does indeed mimic the endocrine exhaustion of aging rather than opposing it.