Menaquinone-4 enhances testosterone production in I-10 and R2C cells without cAMP stimulation. I-10 cells were treated with various concentrations of menaquinone-4 (MK-4) for 24 h (A) or 30 μM MK-4 for different time periods (B). Rat testis tumor-derived R2C cells were treated with various concentrations of MK-4 for 24 h (C). Testosterone levels in the culture medium were determined by ELISA. Data are represented as means ± SE (n = 3). Values indicated with different letters in A and C are significantly different at p < 0.05. Values indicated by an asterisk in B are significantly different at p < 0.05.