Expression of IL-33 in rodent testes and its role in Leydig cell steroidogenesis and aging

BACKGROUND: Serum testosterone (T) concentration declines with aging in men, potentially affecting reproduction, mental and physical well-beings. A role of immune factors in Leydig cell (LC) function is well-known, but the specific factors involved, especially these playing roles in LC aging, are still unclear. This study investigated effects of interleukin 33 (IL-33) on LC function and its expression during testicular aging. METHODS: Immunohistochemistry and Western blotting were used to determine IL-33 and its receptor IL1RL1 expressions in testes of young (3-month-old) and old (19-24-month-old) Wistar rats. In vitro, the effects of IL-33 on sex steroid hormone productions were evaluated in primary and MLTC-1 LCs over 2-24 h. Different steroidogenic stimulators or signaling molecules (luteinizing hormone [LH], 8-Br-cAMP, Forskolin, pertussis toxin, and MAPK activators) were compared with elucidate mechanisms. Steroidogenic pathway proteins and potential signaling molecules were explored by Western blotting. RESULTS: IL-33 is expressed by mesenchymal cells, with the number increasing significantly with aging. IL1RL1, its receptor, is expressed by LCs and remains unchanged. In vitro, IL-33 acutely inhibited LC steroidogenesis in a dose-dependent manner (1-100 ng/mL) within 2-24 h. The effect was LH-dependent; replacing LH with either 8-Br-cAMP or Forskolin abolished the inhibition. IL-33 mainly affected STAR in the steroidogenic pathway. Signaling molecules involving STAR regulation (AKT and MAPK) were down-regulated while PKA phosphorylation was increased. P38 MAPK involvement was confirmed as increased Tyr182 phosphorylation of P38 by SB203580 partly reversed the IL-33-induced steroidogenesis inhibition. CONCLUSION: Testicular mesenchymal cells can synthesize IL-33, and LCs express the receptor IL1RL1. IL-33 inhibits LC steroidogenesis in vitro, partially via inhibiting P38 MAPK phosphorylation. As IL-33-expressing cell numbers rise significantly with aging, its role in age-related LC T production decline warrants further study.