Tumor suppressor deleted in liver cancer-1 (DLC-1) gene triggers apoptosis in NPC cells by perturbing mitochondria (MI)

Abstract

DLC-1 is a bona fide tumor suppressor gene (TSG) mapped to chromosome 8p22-p21, a region frequently deleted in human malignancies. DLC-1 gene encodes a GTPase activating protein (GAP), which negatively regulates the activity of Ras homolog (Rho) family. RhoGAP activity of DLC-1 plays an important role in regulating cell motility and suppressing cancer metastasis. Genetic and epigenetic inactivation of DLC1 is frequently found in human cancers, including nasopharyngeal carcinoma (NPC) and hepatocellular carcinoma (HCC), two major endemic tumors in southern China. The anti-proliferative effect of DLC-1 is evidenced with its inhibition of colony formation in NPC cells. The pro-apoptotic function of DLC-1 has also been suggested, but the underlying mechanisms remain unanswered. Since the human DLC-1 ortholog, DLC-2 was found to be localized on MI membrane; DLC-1 has been hypothesized to play a role in MI dependent apoptosis. To test this hypothesis, NPC cells were transfected with EGFP-tagged DLC-1 plasmid, and stained with tetramethylrhodamine ethyl ester (TMRE). Microscopically, DLC-1 signal was found to localize in cytoplasm and was significantly overlapped with TMRE signal. In addition, ectopic expression of DLC-1 dropped MI membrane potential (MMP) as evidenced by flow cytometry-based TMRE staining. Furthermore, we demonstrated that blocking of death receptor pathway（DR） by dominant negative FADD failed to suppress DLC-1 mediated MMP changes, suggesting that DLC-1 induced apoptosis was independent to DR pathway. We will next evaluate the effect of blocking stress-induced pathway with specific inhibitors on DLC-1 induced apoptosis. In summary, our results suggest that DLC-1 protein may target to MI membrane and induce apoptosis. The re-expression of DLC-1 is of mild pro-apoptotic activity; and thus it prompted us to test whether DLC-1 would potentiate apoptosis induced by the challenge of genotoxins and death ligands, and our study may provide insights to design of novel therapeutic modality for cancers.