CKIP-1 acts as a colonic tumor suppressor by repressing oncogenic Smurf1 synthesis and promoting Smurf1 autodegradation

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Dysregulation of cellular signaling pathways can lead to colon cancer. However, research on the key signaling effectors or regulators in colon carcinogenesis is limited. Casein kinase-2 interacting protein-1 (CKIP-1; also known as PLEKHO1) is crucial during adult bone formation and is a promising drug target for osteoporosis therapy. In this study, we observed that CKIP-1 was downregulated in human colon cancer tissues and colon cancer cell lines, and this result was correlated with colon cancer progression. CKIP-1 silencing in colon cancers involved promoter methylation. In colon cancer HCT116 and SW480 cells, CKIP-1 overexpression inhibited cell growth and migration. CKIP-1 also suppressed in-vivo tumor formation. Notably, the growthsuppressive role of CKIP-1 was dependent on the downregulation of the cell cycle-regulated oncogene Smad ubiquitylation regulatory factor-1 (Smurf1). During cell cycle progression, phosphatidylinositol-3-kinase (PI3K)/Akt/mammalian target of rapamycin (mTOR) signaling increased Smurf1 production by an mTOR-dependent translational control mechanism. Rapamycin, the mTOR inhibitor, significantly reduced Smurf1 protein levels, and Smurf1 was degraded in mitosis. In colon cancer, CKIP-1 controlled Smurf1 expression by suppressing PI3K/Akt/mTOR signaling and enhancing Smurf1 autodegradation, and CKIP-1 downregulation was correlated with Smurf1 upregulation in colon carcinogenesis. These findings provide novel insight into the mechanisms of the candidate tumor suppressor CKIP-1.

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