Carmen Chak Lui Wong1, Irene Oi Lin Ng1
1 University of Hong Kong
Liver cancer (hepatocellular carcinoma, HCC) is one of the most prevalent cancers worldwide. Intrahepatic metastasis is a major cause of mortality in HCC patients, and understanding the molecular mechanisms involved in intrahepatic metastasis is of crucial significance. Deleted in liver cancer 1 (DLC1), a tumor suppressor gene and a member of RhoGTPase activating protein (RhoGAP) family, possesses suppressive activities in tumorigenicity and cancer cell invasion in HCC. DLC1 has RhoGAP activity specific for inactivating RhoA. Rho-kinase (ROCK) is an immediate down-stream effector of RhoA in regulating cellular cytoskeletal events and cell movement. However, the underlying molecular mechanisms of how DLC1 represses cell motility have not been fully understood. In this study, we examined the effects of DLC1 on Rho/ROCK signaling pathway in hepatocellular carcinoma (HCC). We demonstrated that DLC1 antagonistically regulated ROCK-dependent actomyosin contractility. Ectopic expression of DLC1 abrogated Rho/ROCK-mediated cytoskeletal reorganization including formation of stress fibers and focal adhesions. DLC1 also suppressed cortical phosphorylation of myosin light chain 2 (MLC2). We demonstrated that these inhibitory events by DLC1 were RhoGAP-dependent, as RhoGAP-deficient mutant of DLC1 (DLC1 K714E) lost the above inhibitory functions of DLC1. Furthermore, DLC1 inhibited ROCK-related myosin light chain phosphatase targeting unit 1 (MYPT1) phosphorylation at Threonine 853. In addition, ectopic expression of dominant-active ROCK released cells from DLC1-induced cytoskeletal collapse and cell shrinkage. Our data suggest that DLC1 negatively regulates Rho/ROCK/MLC2 and this implicates a ROCK-mediated pathway of DLC1 in suppressing metastasis of HCC cells.