Targeted inactivation of HDAC2 restores p^16INK4a activity and exerts antitumor effects on human gastric cancer

Aberrant regulation of histone deacetylase 2 (HDAC2) was reported for gastric cancers. However, responsive cancer genes in disease onset and progression are less understood. HDAC2 expression was studied by quantitative RT-PCR and Western blotting. The functional consequences of HDAC2 knockdown on cell-cycle regulation, programmed cell death, and gene target identification was investigated by flow cytometry, Western blotting, electron microscopy, anchorage-independent colony formation, and cell migration assay and by whole-genome microarray. Therapeutic efficacy of HDAC2 knockdown was determined in nude mice with small hairpin expressing human gastric cancer cells. Epigenetic regulation of p^16INK4awas studied by methylation-specific PCR and chromatin-IP to evidence HDAC2 or acetylated-histone-H4 binding at gene specific promoter sequences. HDAC2 gene and protein expression was significantly upregulated in different histopathologic grades of human gastric cancers and cancer cell lines. HDAC2 inactivation significantly reduced cell motility, cell invasion, clonal expansion, and tumor growth.HDAC2knockdown-inducedG ₁-S cell cycle arrest and restored activity of p^16INK4a and the proapoptotic factors. This treatment caused PARP cleavage and hypophosphorylation of the Rb-protein, repressed cyclinD1, CDK4, and Bcl-2 expression and induced autophagic phenotype, that is, LC3B-II conversion. Some gastric tumors and cancer cells displayed p^16INK4a promoter hypermethylation but treatment with 5-azadeoxycitidine restored activity. With others the methylation status was unchanged. Here, chromatin-IP evidenced HDAC2 binding. Nonetheless, expression of p^16INK4awas restored by HDAC2 knockdown with notable histone- H4-acetylation, as determined by chromatin-IP. Thus, p^16INK4ais regulated by HDAC2. HDAC2 is a bona fide target for novel molecular therapies in gastric cancers.