MKK7 mediates miR‐493‐dependent suppression of liver metastasis of colon cancer cells

The prognosis of advanced colon cancer patients is profoundly affected by the presence or absence of liver metastasis. miR‐493 functions as a potent suppressor of liver metastasis, and low‐level miR‐493 expression in human primary colon cancer is associated with an elevated incidence of liver metastasis. We previously showed that IGF1R is a target gene of miR‐493, and that the inhibition of IGF1R partly explains how miR‐493 suppresses liver metastasis. However, major functional targets that mediate the antimetastatic activity of miR‐493 remain elusive. Here, we extended our search for target genes and identified MKK7, a mitogen‐activated protein kinase kinase, as a novel target of miR‐493. miR‐493 inhibits MKK7 expression by targeting the binding site at the 3′‐UTR of the mkk7 gene. MKK7 was expressed in six out of seven colon cancer cell lines examined but not in non‐transformed colon epithelial cells, and its expression was required for the activating phosphorylation of JNK. RNA interference‐mediated inhibition of MKK7 resulted in marked suppression of liver metastasis of colon cancer cells. A significant decrease of metastasized cells by the MKK7 knockdown was observed, even at early stages of the metastatic settlement, in accordance with a time course of the miR‐493‐mediated inhibition of the metastasis. Immunohistochemical examination in human primary colon tumors revealed that the occurrence of liver metastasis is associated with elevated levels of MKK7. Thus, MKK7 is a major functional target of miR‐493, and its suppression thwarts liver metastasis of colon cancer cells.