PURPOSE:Endocrine therapy is the most commonly used approach for the treatment of estrogen receptor (ERα)-positive breast cancer. The cure rate of patients with ERα-positive breast cancer is, however, limited due to the occurrence of endocrine resistance. Loss of ERα is one major mechanism for the occurrence of endocrine resistance. Recent studies have shown that pan-HDAC inhibitors may be effective in reversing endocrine resistance. However, the molecular mechanism underlying this reversal has remained largely unknown. Here we aimed to unravel this mechanism. METHODS:Endocrine resistant breast cancer cell lines were established through exposure to tamoxifen. mRNA expression was assessed by qRT-PCR and protein expression by Western blotting. The effect of HDAC3 inhibition on the viability of endocrine resistant breast cancer cells was evaluated using CCK-8 and colony forming assays. Immunohistochemistry was used to detect protein expression in primary breast cancer tissues. RESULTS:We found that in endocrine resistant breast cancer cells loss of ERα led to HDAC3 stabilization via decreased ERα-mediated caspase7 expression, resulting in reduced caspase7-mediated HDAC3 cleavage. We also found that the ERα-caspase7-HDAC3 axis determined the global H3K9 and H4K16 acetylation status, which was positively correlated with ERα expression. Finally, we found that inhibition of HDAC3 significantly decreased the viability of endocrine resistant breast cancer cells exhibiting ERα deficiency. The ERα-caspase7-HDAC3 axis was subsequently verified in primary endocrine resistant breast cancer samples. CONCLUSIONS:From our data we conclude that the ERα-caspase7-HDAC3 axis may play a role in promoting the proliferation of endocrine resistant breast cancer cells. HDAC3 may serve as a therapeutic target for (a subset of) endocrine resistant breast cancers exhibiting ERα loss.