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.