Research data for the paper: Expression of IL-33 in rodent testes and its role in Leydig cell steroidogenesis and aging

<b>Abstract</b><b>Background</b>: 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 IL-33 on LC function and its expression during testicular aging.<b>Methods</b>: 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 hours. Different steroidogenic stimulators or signaling molecules (LH, 8-Br-cAMP, Forskolin, pertussis toxin and MAPK activators) were compared to elucidate mechanisms. Steroidogenic pathway proteins and potential signaling molecules were explored by Western blotting.<b>Results</b>: 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 hours. 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.<b>Conclusion</b>: 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.