It is unclear how proliferating cells elicit suppression on cell proliferation and how cancer cells evade this growth suppression. Using a loss‐of‐function screening of the human kinome and phosphatome to identify genes suppressing tumor initiation in human hepatocellular carcinoma (HCC), we identified 19 genes and characterized one of the top‐scoring tumor suppressor candidates, protein tyrosine phosphatase receptor type F (PTPRF). We found that PTPRF was induced during cell proliferation by cell‐cell contact. Ectopic expression of wild‐type PTPRF, but not the phosphatase‐inactive mutant, suppressed cell proliferation and colony formation in soft‐agar assays. In contrast, PTPRF silencing led to cell hyperproliferation, enhanced tumor colony formation in soft agar, and increased xenograft tumor growth in nude mice. Mechanistically, PTPRF silencing showed aberrant ERK‐dependent signaling including the phosphorylation/stabilization of v‐myc avian myelocytomatosis viral oncogene homolog (MYC) through the direct activation of v‐src avian sarcoma viral oncogene homolog (SRC) and suppression of PP2A. This PTPRF‐mediated growth suppression during cell proliferation functioned independently of the Hippo‐Yap pathway. Clinically, PTPRF was down‐regulated in 42% HCC (37/89), 67% gastric cancer (27/40), and 100% colorectal cancer (40/40). PTPRF up‐regulation was found in 24% HCC (21/89) and associated with better clinical outcomes. Conclusion: A novel PTPRF‐mediated growth suppression pathway was identified by way of a functional genomics screening in human hepatoma cells. Induction of PTPRF by cell‐cell contact during cell proliferation quenched the activated ERK‐dependent proliferation signaling to prevent cell hyperproliferation and tumor initiation. PTPRF down‐regulation in HCC facilitated tumor development. Our findings shed light on how cancer cells can evade growth suppression and open a new avenue for future development of anticancer therapies. (Hepatology 2014;59:2238–2250)

No abstract

No abstract

Membrane protrusion and adhesion to the extracellular matrix, which involves the extension of actin filaments and formation of adhesion complexes, are the fundamental processes for cell migration, tumor invasion, and metastasis. How cancer cells efficiently coordinate these processes remains unclear. Here, we showed that membrane-targeted CLIC1 spatiotemporally regulates the formation of cell-matrix adhesions and membrane protrusions through the recruitment of PIP5Ks to the plasma membrane. Comparative proteomics identified CLIC1 upregulated in human hepatocellular carcinoma (HCC) and associated with tumor invasiveness, metastasis, and poor prognosis. In response to migration-related stimuli, CLIC1 recruited PIP5K1A and PIP5K1C from the cytoplasm to the leading edge of the plasma membrane, where PIP5Ks generate a PIP2-rich microdomain to induce the formation of integrin-mediated cell-matrix adhesions and the signaling for cytoskeleon extension. CLIC1 silencing inhibited the attachment of tumor cells to culture plates and the adherence and extravasation in the lung alveoli resulting in suppressed lung metastasis in mice. This study reveals an unrecognized mechanism that spatiotemporally coordinates the formation of both lamellipodium/invadopodia and nascent cell-matrix adhesions for directional migration and tumor invasion/metastasis. The unique traits of upregulation and membrane targeting of CLIC1 in cancer cells make it an excellent therapeutic target for tumor metastasis.

Overexpression of HER-2/neu (also known as c-erbB-2) proto-oncogene frequently occurs in many di erent types of human cancers, including ovarian carcinoma, and is known to enhance tumor metastasis and chemoresistance. Previous studies showed that inhibition of HER-2/neu expression by various agents, such as adenovirus E1A and simian virus 40 large T, can lead to suppression of tumorigenicity of HER-2/neu-overexpressing cancer cells. Here we report that T/t-common, which contains the N-terminal common domain of simian virus 40 large T and small t antigens, could speci®cally repress the HER-2/neu promoter. When the coding sequence of T/tcommon was stably transfected into the HER-2/neuoverexpressing human ovarian carcinoma SK-OV-3 cells, the expression of HER-2/neu was dramatically reduced by the expression of T/t-common. Accordingly the tumorigenic potential of these T/t-common-expressing clones, including the ability to grow anchorage-independently and the ability to induce tumor in nu/nu mice, was also drastically suppressed. Furthermore, when T/tcommon was transiently cotransfected with the activated genomic neu into NIH3T3 cells, the transforming activity of the latter was suppressed by T/t-common in soft-agarose microcolony formation assays. Taken together, these data suggest that T/t-common may act as a transformation suppressor of the HER-2/neu oncogene.