Alterations of cell adhesion molecules have increasingly been implicated in disruption of cell organization, signaling and growth control, leading to tumor development and progression. In this study, using the technique of suppression subtractive hybridization, we examined differential gene expression in human hepatocellular carcinoma (HCC) and identified a new member of the immunoglobulin superfamily, designated as CAMSIT (cell adhesion molecule suppressed in tumors). Real-time RT-PCR showed that CAMSIT lacked tissue specificity and was widely expressed in normal human tissues, such as liver, brain, skeletal muscle, colon and blood. Its expression was significantly decreased in 20/23 of HCC specimens and undetectable in 17/19 cell lines derived from various human tumors. Sequence analyses revealed that gene CAMSIT, mapped to chromosome 11q24, had no significant similarities to any known genes. The predicted protein of 416 amino acids displayed a typical structure of Ig-like adhesion molecules, including two extracellular Ig-like domains, a transmembrane segment and a cytoplasmic tail. Through transfection studies on HepG2 and MCF7 cells, we explored the biochemical characteristics and biological functions of CAMSIT. The subcellular localization of CAMSIT appeared to be cell density-dependent. In well-spread cells, CAMSIT was distributed in punctuate structures in the cytoplasm and at the cell surface protrusions. In confluent cells, CAMSIT was predominantly localized on the cytoplasmic membrane, particularly in the areas of cell-cell contacts, but absent at free cell borders. Crosslinking assay suggested that, independent of the cytoplasmic tail, CAMSIT formed cis-dimers on cell surface. Western analysis following deglycosylation and dephosphorylation indicated that CAMSIT protein, approximately 75 kDa, was glycosylated and phosphorylated. Re-expression of CAMSIT reduced cell colony formation by 10 folds (P = 0.0022), inhibited cell proliferation by 14 folds (P < 0.001), and induced senescence-like phenotype. In addition, cellular interaction assays demonstrated that CAMSIT was capable of enhancing cell-matrix adhesion and cell migration, but not cell aggregation. More intriguingly, the cytoplasmic domain of CAMSIT, a 27 kDa phophorylated fragment, was consistently detectable by western analysis, and it was translocated into cell nuclei and triggered cell death through apoptosis when exogenously expressed in cells. In conclusion, gene CAMSIT is frequently downregulated in a variety of human tumors. The gene encodes a new Ig-like cell adhesion molecule which modulates cell-matrix adhesion and cell motility. The activated CAMSIT, possibly through phosphorylation, may undergo proteolytic cleavage and consequently be involved in cell signaling and growth control.
[Proc Amer Assoc Cancer Res, Volume 46, 2005]