Combined histone deacetylase inhibition and tamoxifen induces apoptosis in tamoxifen-resistant breast cancer models, by reversing Bcl-2 overexpression

Abstract

IntroductionThe emergence of hormone therapy resistance, despite continued expression of the estrogen receptor (ER), is a major challenge to curing breast cancer. Recent clinical studies suggest that epigenetic modulation by histone deacetylase (HDAC) inhibitors reverses hormone therapy resistance. However, little is known about epigenetic modulation of the ER during acquired hormone resistance. Our recent phase II study demonstrated that HDAC inhibitors re-sensitize hormone therapy-resistant tumors to the anti-estrogen tamoxifen. In this study, we sought to understand the mechanism behind the efficacy of this combination.MethodsWe generated cell lines resistant to tamoxifen, named TAMRM and TAMRT, by continuous exposure of ER-positive MCF7 and T47D cells, respectively to 4-hydroxy tamoxifen for over 12 months. HDAC inhibition, along with pharmacological and genetic manipulation of key survival pathways, including ER and Bcl-2, were used to characterize these resistant models.ResultsThe TAMRM cells displayed decreased sensitivity to tamoxifen, fulvestrant and estrogen deprivation. Consistent with previous models, ER expression was retained and the gene harbored no mutations. Compared to parental MCF7 cells, ER expression in TAMRM was elevated, while progesterone receptor (PGR) was lost. Sensitivity of ER to ligands was greatly reduced and classic ER response genes were suppressed. This model conveyed tamoxifen resistance through transcriptional upregulation of Bcl-2 and c-Myc, and downregulation of the cell cycle checkpoint protein p21, manifesting in accelerated growth and reduced cell death. Similar to TAMRM cells, the TAMRT cell line exhibited substantially decreased tamoxifen sensitivity, increased ER and Bcl-2 expression and significantly reduced PGR expression. Treatment with HDAC inhibitors reversed the altered transcriptional events and reestablished the sensitivity of the ER to tamoxifen resulting in substantial Bcl-2 downregulation, growth arrest and apoptosis. Selective inhibition of Bcl-2 mirrored these effects in presence of an HDAC inhibitor.ConclusionsOur model implicates elevated ER and Bcl-2 as key drivers of anti-estrogen resistance, which can be reversed by epigenetic modulation through HDAC inhibition.Electronic supplementary materialThe online version of this article (doi:10.1186/s13058-015-0533-z) contains supplementary material, which is available to authorized users.

HDACs and HDAC Inhibitors in Cancer Development and Therapy

“…Although histone deacetylation plays a key role in estrogen receptor (ER) gene silencing, it remains unclear whether the addition of HDACi actually reactivate functional ERα expression (de Cremoux et al 2015). A recent study demonstrates that Bcl-2 downregulation and induction of pro-apoptotic proteins by combined ER and HDAC inhibition leads to apoptotic cell death of tamoxifen-resistant cells (Raha et al 2015). …”

Current medical treatment of estrogen receptor-positive breast cancer

“…To restore endocrine sensitivity of ER+ cancer cells, several other drugs have been studied, including cyclin-dependent kinase (CDK) 4/6 inhibitors (palbociclib), epigenetic modulators that inhibit histone deacetylase (HDAC), and other signal pathway inhibitors [59] . Experimentally, combined HDAC inhibition and TAM reverse ET resistance, inducing apoptosis of TAM-resistant BC cells, and in women with advanced ER+ BC and disease progression or recurrence the progression-free survival (PFS) can be reduced adding palbociclib to letrozole [60,61] . The phosphoinositide 3-kinase (PI3K)/protein kinase B (AKT) pathway and the serine/threonine kinase mammalian target of rapamycin (mTOR) are part of a signaling network regulating several cell processes, including growth, proliferation and survival of cancer cells [62] .…”

Targeting ceramide metabolic pathway induces apoptosis in human breast cancer cell lines

“…However, its significance over cell proliferation has also been reported in ER À breast cancer cells indicating its ER independent effect [4]. Eventhough, Tamoxifen has been approved to be the gold standard for Breast cancer therapy the major problem encountered with Tamoxifen is resistance after long term treatment [5,6].…”