Chromatin is pivotal for regulation of the DNA damage process insofar as it influences access to DNA and serves as a DNA repair docking site. Recent works identify histone chaperones as key regulators of damaged chromatin’s transcriptional activity. However, understanding how chaperones are modulated during DNA damage response is still challenging. This study reveals that the histone chaperone SET/TAF-Iβ interacts with cytochrome c following DNA damage. Specifically, cytochrome c is shown to be translocated into cell nuclei upon induction of DNA damage, but not upon stimulation of the death receptor or stress-induced pathways. Cytochrome c was found to competitively hinder binding of SET/TAF-Iβ to core histones, thereby locking its histone-binding domains and inhibiting its nucleosome assembly activity. In addition, we have used NMR spectroscopy, calorimetry, mutagenesis, and molecular docking to provide an insight into the structural features of the formation of the complex between cytochrome c and SET/TAF-Iβ. Overall, these findings establish a framework for understanding the molecular basis of cytochrome c-mediated blocking of SET/TAF-Iβ, which subsequently may facilitate the development of new drugs to silence the oncogenic effect of SET/TAF-Iβ’s histone chaperone activity.

HER2/Neu/ERBB2 is a receptor tyrosine kinase overexpressed in approximately 20% of human breast tumors. Truncated or mutant isoforms which show increased oncogenicity compared to the wild-type receptor are found in many breast tumors. Here we report that constitutively active ERBB2 sensitizes human breast epithelial cells to agents that induce endoplasmic reticulum (ER) stress, altering the unfolded protein response (UPR) of these cells. Deregulation of the ERK, AKT and mTOR activities elicited by mutant ERBB2 were involved in mediating this differential UPR response, elevating the response to ER stress and apoptotic cell death. Mechanistic investigations revealed that the increased sensitivity of mutant ERBB2-expressing cells to ER stress relied upon a UPR effector signaling involving the PERK-ATF4-CHOP pathway, upregulation of the proapoptotic cell surface receptor TRAIL-R2 and activation of proapoptotic caspase-8. Collectively, our results offer a rationale for the therapeutic exploration of treatments inducing ER stress against mutant ERBB2-expressing breast tumor cells.

Recent evidences indicate that triple-negative breast cancer (TNBC) cells with a mesenchymal phenotype show a basal activation of the unfolded protein response (UPR) that increases their sensitivity to endoplasmic reticulum (ER) stress although the underlying cell death mechanism remains largely unexplored. Here we show that both caspase-8-dependent and -independent apoptotic mechanisms are activated in TNBC cells undergoing sustained ER stress. Activation of the extrinsic apoptotic pathway by ER stress involves ATF4-dependent upregulation of tumor necrosis factor-related apoptosis-inducing ligand receptor 2 (TRAIL-R2/DR5). In addition, accumulation of BH3-only protein Noxa at the mitochondria further contributes to apoptosis following ER stress in TNBC cells. Accordingly, simultaneous abrogation of both extrinsic and intrinsic apoptotic pathways is required to inhibit ER stress-induced apoptosis in these cells. Importantly, persistent FLICE-inhibitory protein (FLIP) expression plays an adaptive role to prevent early activation of the extrinsic pathway of apoptosis upon ER stress. Overall, our data show that ER stress induces cell death through a pleiotropic mechanism in TNBC cells and suggest that targeting FLIP expression may be an effective approach to sensitize these tumor cells to ER stress-inducing agents.