Menaquinone-4 enhances testosterone production in I-10 cells in the presence of cAMP. Mouse testis tumor-derived I-10 cells were treated with dibutyryl-cAMP (db-cAMP, 3 μM) and various concentrations of menaquinone-4 (MK-4) for 24 h. (A) Testosterone levels in cultured medium were determined by ELISA. Data are represented as means ± SE (n = 3). (B) The cell proliferation assay was performed using the WST-1 assay kit. Data are represented as means of relative absorbance values at 450 nm ± SE (n = 3). Values with different letters are significantly different at p < 0.05.