抄録
Vitamin K is an essential cofactor for the activation (γ-glutamylcarboxylation) of vitamin K-dependent proteins including blood coagulation factors and bone matrix proteins. On the other hand, several novel functions of vitamin K other than γ-glutamylcarboxylation have been reported: inhibition of tumor cell growth, anti-inflammatory effect in macrophage, and the ligand activity of nuclear receptor PXR/SXR. Vitamin K is found not only in liver and bone, but also pancreas, brain and gonadal tissues. However, the function of vitamin K in testis has not been fully elucidated. Therefore, we explored a novel function of vitamin K in rat testis and found that testosterone production in testis is positively correlated with dietary level of vitamin K. Menaquinone-4, one of vitamin K_2, not vitamin K_1, enhanced testosterone production in mouse testis-derived tumor cells (I-10). Protein kinase A (PKA) is a crucial factor for steroidogenesis in testis Leydig cells. PKA was activated (or phosphorylated) by vitamin K treatment in I-10 cells. A PKA inhibitor H-89 abolished the enhancement of testosterone production by menaquinone-4. These results indicated that vitamin K_2 enhances testosterone production in testis via activation of PKA.
