The role of histone deacetylase 4 (HDAC4) in acquired cisplatin-resistant ovarian cancer

Resistance to platinum is a major problem in the treatment of ovarian cancer. Molecular profiling of isogenic ovarian cancer cell line pairs derived from tumour cells pre- and post- clinical resistance, identified that histone deacetylase 4 (HDAC4) is over-expressed in cisplatin-resistant cells relative to sensitive derivatives. HDAC4 siRNA and HDACi treatment resensitised resistant cells to cisplatin. Furthermore, up-regulation of HDAC4 in cisplatin-resistant ovarian cell lines and tumour sections identified HDAC4 as a potential biomarker of cisplatinresistance. It was observed that HDAC4 interacts with another mediator of platinum resistance, the transcription factor STAT1, and that knockdown of HDAC4 increased acetylation levels of STAT1 protein in platinum resistant cells. Consequent decrease in cisplatin mediated phosphorylation of STAT1 and nuclear translocation of STAT1 were observed. However, STAT1 was found to be acetylated and inactive in platinum sensitive cells, expressing lower levels of HDAC4. Microarray analysis of the platinum resistant cells has identified differentially regulated genes following HDAC4 knockdown. The UCHL1 promoter is methylated in both cisplatin sensitive and resistant paired cells and yet is reexpressed in resistant cells following HDAC4 knockdown. The methylation levels at the UCHL1 promoter were analysed by pyrosequencing method and do not appear to significantly change the methylation level after HDAC4 knockdown. However, ChIP analysis revealed an increase in acetylation at the UCHL1 promoter after HDAC4 knockdown. Conversely, P21 is down-regulated by HDAC4 knockdown in resistant PEO4 cells in contrast to reports of P21 overexpression as a biomarker of HDAC inhibition. Strikingly however, it is upregulated after HDAC4 knockdown in cisplatin-sensitive paired PEO1 cells suggesting that, like STAT1, a fundamental change in its control occurs on acquisition of platinum resistance. This study provides evidence that HDAC4 is required for platinum mediated STAT1 activation; a phenomenon associated with clinical platinum resistance, and identifies frequent HDAC4 over-expression in platinum resistant tumour biopsies. Pharmacological modulation of this pathway is shown to restore sensitivity to cisplatin. Microarray analysis revealed HDAC4 regulated target genes. These studies identify new and clinically relevant insights into platinum resistance which may lead to improved therapeutic options in ovarian cancer