Clinicopathological significance of SPC18 in colorectal cancer: SPC18 participates in tumor progression

Colorectal cancer (CRC) is one of the leading causes of cancer-related death worldwide. In order to identify novel prognostic markers or therapeutic targets for CRC, we searched for candidate genes in our comprehensive gene expression libraries, and focused on SEC11A, which encodes the SPC18 protein. SPC18 plays a key role in the endoplasmic reticulum-Golgi secretory pathway and presumably regulates the secretion of various secretory proteins. An immunohistochemical analysis of SPC18 in 137 CRC tissue samples demonstrated that 79 (58%) CRC cases were positive for SPC18. SPC18-positive CRC cases were more advanced in terms of N classification (P = 0.0315) and tumor stage (P = 0.0240) than SPC18-negative CRC cases. Furthermore, the expression of SPC18 was an independent prognostic classifier for CRC patients. The cell growth and invasiveness of SPC18 siRNA-transfected CRC cell lines was less than that of the negative control siRNA-transfected cell lines. The levels of phosphorylated epidermal growth factor receptor, Erk and Akt were lower in SPC18 siRNA-transfected CRC cells than in control cells. The expression of SPC18 was colocalized with β-catenin nuclear localization and MMP7 at the invasive front. An immunohistochemical analysis of human colorectal polyp specimens revealed a sequential increase in the expression of SPC18 through the conventional adenoma-carcinoma pathway, while SPC18 was not expressed or was expressed to a lesser extent in serrated pathway-related tumors. These results suggest that SPC18 is involved in tumor progression, and is an independent prognostic classifier in patients with CRC.