KLHL5 Contributes to Colorectal Cancer Cell Survival by Promoting Cell Cycle Progression and Suppressing Apoptotic Cell Death

ABSTRACT Kelch‐like protein 5 (KLHL5) is highly expressed in colorectal cancer (CRC) compared to that in adjacent normal mucosa, and its expression level increases with CRC stage, showing a correlation with poor prognostic factors. However, its functional role in the malignant progression still remains unknown. To elucidate the role of KLHL5 in CRC, we characterized human CRC cell lines, including HCT116 and SW480, under KLHL5‐depleted conditions. KLHL5‐depleted HCT116 and SW480 cells suppressed their growth and migration in culture. Further duration induced cell death characterized by apoptotic cell death with down‐regulation of antiapoptotic factor Bcl‐2 and up‐regulation of proapoptotic factors Bac, Boc, Puma, Bid, Noxa, and Bik. Proteomic analyses indicated KLHL5 depletion suppressed cell cycle progression by affecting multiple pathways, including the activation of the G2/M DNA damage pathway and inhibition of the G1/S transition. Further biochemical and cell biological analyses revealed the downregulation of CDT1 and CDC6 proteins, which are essential factors for the initiation of DNA replication, and the downregulation of cyclins A and B, which are essential factors for the progression of S and G2/M phases. Arrested cells undergo apoptotic cell death. Taken together, these data strongly indicate that KLHL5 expression in CRC serves as a survival factor to strengthen the cell cycle and protect against apoptotic cell death under harsh tumor microenvironments.