We have reported that histone deacetylase (HDAC) inhibitors can selectively repress ERBB2 transcript levels by two seemingly independent mechanisms: nuclear repression of promoter-driven synthesis and accelerated decay of mature cytoplasmic transcripts. Pan-HDAC inhibiting hydroxamates (e.g. TSA, LAQ824, SAHA) are capable of repressing both mechanisms. In contrast, the recently synthesized hydroxamate, ST71, exhibiting 20-fold greater inhibitory specificity for HDAC-6 (IC50 = 4uM) relative to HDAC-1 (IC50 = 83uM) and reduced ability to increase cellular histone H3/H4 acetylation, was shown to accelerate ERBB2 transcript decay without affecting ERBB2 promoter activity. A 5h treatment of ERBB2-positive SKBR3 breast cancer cells with 50uM ST71 reduced ERBB2 transcript levels comparable to a 10uM dose of LAQ824; and 3-5d exposure to ST71 inhibited growth of these ERBB2-positive cells but had no significant inhibitory effect on ERBB2-negative MCF7 cells. ERBB2 transcript stability in SKBR3 appears dependent on HuR binding to a conserved element in its 3’UTR, since siRNA induced reduction of HuR reduces ERBB2 transcript levels, and immunoprecipitates of HuR contain RT-PCR measured ERBB2 3’UTR sequences. These HuR immunoprecipitates were found to contain HDAC activity, measured by a fluorogenic deacetylation assay, which was largely eliminated by pretreatment of the immunorecipitate with RNase A. Cytosolic extracts of SKBR3 probed with HDAC-specific antibodies have narrowed the source of the HuR-associated deacetylation activity to HDAC-3, HDAC-4 or HDAC-6. These findings indicate that isotype-specific HDAC inhibitors, like ST71, can induce ERBB2 transcript destabilization and promote growth inhibition of ERBB2 overexpressing breast cancer cells. Thus, ERBB2 mRNA stability represents an exploitable target for the design of novel anticancer therapies.

98th AACR Annual Meeting-- Apr 14-18, 2007; Los Angeles, CA