Knockdown of translocon-associated protein subunit beta (TRAPβ) stimulates cell cycle arrest and apoptosis in human colorectal cancer cells

Translocon-associated protein subunit beta (TRAPβ), also known as signal sequence receptor 2 (SSR2) serves as an auxiliary protein facilitating co-translational translocation in the endoplasmic reticulum (ER); however, its role in colorectal cancer is unknown to date. The objectives of the current study are to examine if TRAPβ/SSR2 knockdown affects the cell proliferation and to elucidate mechanisms by which TRAPβ/SSR2 regulates proliferation of human colorectal cancer. We silenced TRAPβ/SSR2 transiently and stably in human colorectal cancer cell lines and analyzed cell proliferative properties. Transient transfection of TRAPβ/SSR2 siRNA significantly repressed the viability of five different types of human colorectal cancer cells. Flow cytometry and western blot showed that TRAPβ/SSR2 knockdown led to significant increase of G2/M-phase arrest in SW480 cells and S-phase arrest in HCT116 and DLD-1 cells. Annexin V-fluorescein isothiocyanate and propidium iodide staining showed that TRAPβ/SSR2 knockdown significantly induced apoptosis in SW480, HCT116, and DLD-1 cells. Similarly, SW480 stable cells with TRAPβ/SSR2 knockdown showed a significant inhibition of anchorage-independent cell growth, an increase of G2/M-phase arrest with downregulation of cyclin B1, and increase of apoptosis. Regarding mechanisms, TRAPβ/SSR2 knockdown mitigated epidermal growth factor-stimulated activation of mitogen-activated protein kinase (MAPK) pathways and showed significantly decreased expression of inositol-requiring enzyme 1 alpha (IRE1α), while IRE1α reintroduction in TRAPβ/SSR2 knockdown cells reversed G2/M-phase arrest and promoted cell cycle progression. All taken together, our data demonstrate that TRAPβ/SSR2 in the ER could be a molecular target to control cell cycle progression and apoptosis through MAPK-mediated and IRE1α-mediated pathways in human colorectal cancer cells.