**3. Vitamin K modulates the activation of PKA in various cell types**

Vitamin K has been shown to affect several biological processes by modulating the activity of PKA in different cell lines. Thus, vitamin K has been reported to enhance nerve growth factor‐ mediated neurite outgrowth via PKA activation in PC12D pheochromocytoma cells [22]. Vitamin K2 has also been reported to inhibit growth and invasion of hepatocellular carcinoma cells through the activation of PKA, which modulates the activities of several transcriptional factors and inhibits the small GTPase Rho [23]. Furthermore, vitamin K2 has been shown to modulate gene expression in osteoblasts upon activation of PKA [24]. Taken together, these findings indicate that vitamin K plays an important role in the activation of PKA in various cell types.
