Menaquinone-4 stimulates the protein kinase A pathway in I-10 cells. I-10 cells were treated with menaquinone-4 (MK-4) or forskolin (Fsk) for 2 h. After the treatment, whole cell extracts were prepared, and the phosphorylation levels of protein kinase A (p-PKA) and CREB (p-CREB) were measured by Western blot analysis (A). I-10 cells were treated with MK-4 for the indicated time. After the treatment, p-PKA and p-CREB levels in the whole cell extract were measured by Western blot analysis (B). Data are represented as mean ± SE (n = 3), normalized to α-tubulin levels and expressed as a fold-increase/decrease compared to the control cell values (A: cells not treated with MK-4 or Fsk, B: cells treated with MK-4 for 0 min). Values with different letters are significantly different at p < 0.05.