mTORC1 Upregulates GP73 to Promote Proliferation and Migration of Hepatocellular Carcinoma Cells and Growth of Xenograft Tumors in Mice.

Levels of the golgi protein 73 (GP73) increase during development of hepatocellular carcinoma (HCC); GP73 is now used as a serum marker for HCC. However, little is known about the mechanisms or effects of GP73 during hepatic carcinogenesis.GP73 was overexpressed from a retroviral vector in HepG2 cells, which were analyzed in proliferation and migration assays. Xenograft tumors were grown from these cells in nude mice. The effects of monoclonal antibodies against GP73 were studied in mice and cell lines. GP73-/-, GP73+/-, and GP73+/+ mice were given injections of diethylnitrosamine to induce liver injury. Levels of GP73 were reduced in MHCC97H, HCCLM3, and HepG2.215 cell lines using small hairpin (sh)RNAs; xenograft tumors were grown in mice from MHCC97H-shGP73 or MHCC97H-vector cells. We used microarray analysis to compare expression patterns between GP73-knockdown and control MHCC97H cells. We studied the effects of the mTOR inhibitor rapamycin on GP73 expression in different cancer cell lines and on growth of tumors in mice. Levels of GP73 and activated mTOR were quantified in human HCC tissues.Xenograft tumors grown from HepG2 cells that expressed GP73 formed more rapidly and more metastases than control HepG2 cells in mice. A monoclonal antibody against GP73 reduced proliferation of HepG2 cells and growth of xenograft tumors in mice. GP73-/- mice had less liver damage following administration of diethylnitrosamine than GP73+/- or GP73+/+ mice. In PTEN-null mouse embryonic fibroblasts (MEFs) with constitutively activated mTOR, GP73 was upregulated compared with control MEFs; this increase was reversed following incubation with rapamycin. Expression of GP73 was also reduced in HCC and other cancer cell lines incubated with rapamycin. mTORC1 appeared to regulate expression of GP73 in cell lines. Activated mTOR correlated with level of GP73 in human HCC tissues. Injection of rapamycin slowed growth of xenograft tumors from MHCC97H-vector cells, compared with MHCC97H-shGP73 cells.Increased expression of GP73 promotes proliferation and migration of HCC cell lines and growth of xenograft tumors in mice. mTORC1 regulates expression of GP73, so GP73 upregulation can be blocked with rapamycin. mTOR inhibitors or other reagents that reduce the level or activity of GP73 might be developed for treatment of HCC.