The synergistic effect exerted by the HDAC inhibitor SAHA and the sesquiterpene lactone parthenolide on triple negative breast canc er cells

Triple-negative breast cancer (TNBC) is a subtype o f breast cancer, insensitive to endocrine therapy. Chemotherapy is the main form of treatment, but is accompanied by a high rate of recidivism. The sesquiterpene lactone Parthenolide (PN) exerts a cy totoxic effect on MDA-MB231 cells, a TNBC cell line (1), but was ineffective at low doses (2-5μM). This repr esents an obstacle for a therapeutic utilization of PN. We supposed, in line with other authors (2), that PN c auses a protective response, which at low doses pre vails on the cytotoxic effect. With the aim of inhibiting this protective effect we have shown that pre-trea tment of MDA-MB231 cells with SAHA (2-5μM), an histone deace tylates inhibitor, synergistically sensitizes the c ells to the cytotoxic effect of PN, also at low doses of th is compound. SAHA/PN combination induced hyperacetylation of his tones H3 and H4 and hypomethylation of DNA. These changes cause epigenetic effects, which can be resp onsible for the increased expression of tumour suppressors p21 and p27 and decreased levels of Bcl 2 and p65, a component of NFkB. Moreover SAHA alone induced ROS generation as well as autophagy, which favours cell survival, and apoptosis. The addition of PN (8 μ M) to SAHA reduced production of ROS and autophagy, while increased the apoptotic process. Interestingly PN activates Akt, mTOR, phospho-p70S6 kinase and ULK1/2, a factor that inhibits autophagy . In addition PN caused nuclear accumulation of Nrf2 wit h stimulates antioxidant genes. SAHA prevented thes e effects. In conclusion SAHA/PN stimulated cytotoxicity throu gh many mechanisms: (i) induces epigenetic events w ith changes in gene expression, (ii) PN prevents SAHA e ffect on autophagy and (iii) SAHA suppresses the protective response exerted by PN through inactivat ion of m-TOR. Taken together our results suggest th at combination SAHA/PN can be a candidate for TNBC the rapy.