Identification of mitochondrial F_oF₁-ATP synthase involved in liver metastasis of colorectal cancer

Liver metastasis is a major cause of poor survival of colorectal cancer patients. In order to identify the proteins associated with liver metastasis in colorectal cancer, we carried out two-dimensional gel electrophoresis-based comparative proteomic analysis of normal colon mucosa, primary colon cancer tissue and corresponding metastatic tumor tissue in liver. The proteins identified were further validated by immunohistochemical analysis of 67 quadruplet samples of normal colon primary colorectal cancer and normal liver-synchronous liver metastasis, and 251 colorectal cancers as well as in.vitro invasion assay of the human colon cancer cell line, SNU-81. From proteomic assessment, the mitochondrial F_oF₁-ATP synthase (ATP synthase) α-subunit was identified as a protein that is upregulated in liver metastasis compared with the primary tumor. Immunohistochemical analyses confirmed a significant increase in the expression of ATP synthase α- and d-subunits in synchronous liver metastasis compared with primary tumor and normal mucosa, respectively. ATP synthase α- and d-subunits were overexpressed in 197 (78.5%) and 190 (75.7%), respectively, of the 251 colorectal cancers. The α- and d-subunits were significantly associated with liver metastasis (P < 0.05) as well as low histological grade (P < 0.0001). The d-subunit also correlated with venous invasion (P = 0.026) and distant metastasis (P = 0.032). In stage III cancers, d-subunit expression was independently associated with poor survival (P = 0.017). Furthermore, transfection of small interfering RNA targeted to ATP synthase α- and d-subunits resulted in decreased in.vitro invasiveness of the human colon cancer cell line. Our overall findings demonstrate that increased ATP synthase is associated with liver metastasis of colorectal cancer.