P53 induction accompanying G2/M arrest upon knockdown of tumor suppressor HIC1 in U87MG glioma cells.

Hypermethylated in cancer 1 (HIC1) is a novel tumor suppressor gene (tsg) frequently silenced by epigenetic modification, predominantly by methylation in different tumors. HIC1 functionally co-operates with p53 in cultured cells as well as in transgenic animals to suppress tumors and has binding site on its promoter. Its over expression often leads to cell cycle arrests. Although HIC1 proven to have role as tsg, its regulation to cell cycle and dependency upon p53 is grossly unknown. In this study, we investigated the role of HIC1 in cell cycle and proliferation of glioma cell line U87MG which has wild type p53, in both serum-containing and serum-deprived medium. Microscopic analysis and MTT assay showed reduced cell number and rate of proliferation upon HIC1 knock down compared to control siRNA (p = 0.025) and untreated cells (p = 0.03) in serum-containing medium and serum-free medium (p = 0.014 vs control siRNA; p = 0.018 vs untreated cells). Cell cycle analysis revealed an arrest at G2/M phase of cell cycle with no demonstrable increase in apoptosis with both medium. An increased expression of p53 concomitant with HIC1 knockdown was observed. Furthermore P21, a p53 responsive gene, along with p27 was significantly increased in comparison with controls. Our results demonstrated an important role of HIC1 for the normal progression of cell cycle, and at molecular level, it could affect the homeostasis of p53 as well as number of cell cycle-related genes, which may or may not be directly linked to p53.