Simulated Microgravity Suppresses Proline Biosynthesis in Skin Fibroblasts to Impair Skin Structure

INTRODUCTION: Microgravity during spaceflight is increasingly associated with cutaneous impairment, yet the structural basis and underlying metabolic mechanisms remain poorly defined. Collagen, the main component of the dermal extracellular matrix (ECM), relies on de novo proline synthesis in fibroblasts. Herein, we examined whether simulated microgravity (SMG) affects collagen synthesis by altering the proline biosynthetic pathway. METHODS: SMG was established using a hindlimb unloading (HU) model in rats and a rotating flat chamber (RFC) system in skin fibroblasts. Dermal morphology and collagen deposition were assessed in skin tissue, while collagen synthesis, intracellular proline levels, and pyrroline‑5‑carboxylate reductase 1 (PYCR1) expression were examined in fibroblasts. PYCR1 was overexpressed to evaluate its contribution to proline synthesis under SMG. RESULTS: SMG caused dermal thinning and reduced collagen deposition in rat skin, and it similarly decreased collagen production in fibroblasts. SMG also lowered intracellular proline levels in fibroblasts and downregulated the PYCR1 gene. Importantly, PYCR1 overexpression partially restored intracellular proline levels and restored collagen protein expression under SMG conditions. CONCLUSION: These findings indicate that impaired de novo proline biosynthesis, associated with PYCR1 downregulation, contributes to reduced collagen production and may underlie microgravity‑induced dermal structural alterations.