Dual mTOR/HDAC Inhibition: Preclinical Development of a Novel Breast Cancer Therapy

HER2 (human epidermal growth factor receptor-2) and/or estrogen receptor (ER) are overexpressed in ~80% of human breast cancers. Although modern therapeutics (e.g. Trastuzumab, Tamoxifen) target the HER2 and ER receptors, clinical resistance often develops due to activation of downstream signaling pathways, including phosphoinositide 3 kinase-AKT/protein kinase Bmammalian target of rapamycin (PI3K/AKT/mTOR), despite effective upstream receptor inhibition. Activation of this pathway increases ribosome biogenesis and translation of oncogenic mRNAs, which are controlled by intracellular histone deacetylase (HDAC) activity as well. In fact, HDAC inhibitors that have been implemented into clinical practice are capable of overcoming resistance to HER2 and ER targeted therapeutics, in part by inducing the degradation of oncogenic transcripts. Given this rationale, we tested the hypothesis that mTOR and HDAC inhibitors are more effective in combination than as single agents. This project examines the effects of combining a novel investigational inhibitor of both complexes MLN0128 that targets mTORC1 and mTORC2, and Trichostatin-A (TSA), a potent inhibitor of both class I and class II HDACs, on the viability, downstream signaling, and polysome assembly of human breast cancer cell lines of various receptor subtypes (HER2-/+ and/or ER -/+), as well as on nontransformed breast epithelial cells. Our data suggest that combining MLN0128 and TSA caused synergistic growth inhibition in almost all breast cancer cell lines tested. Furthermore, this dual treatment induced greater apoptosis in comparison to the single agent treatments in HER2-positive SKBR3 cells, while the viability and apoptosis of non-transformed MCF-10A cells were less affected. Dual MLN0128/TSA treatment also decreased AKT-S473 phosphorylation significantly more than the single agent treatments in all breast cancer cells, and reduced polysome formation in SKBR3 cells, suggesting that the mechanism of action of dual MLN0128/TSA treatment involves, in part, the inhibition of ribosome function through two separate modes of attack on PI3K/AKT/mTOR signaling. In summary, the synergistic effects of this treatment combination across phenotypically diverse breast cancer cell lines warrants further study and clinical development of this promising breast cancer treatment strategy