Tunicamycin-Induced Endoplasmic Reticulum Stress Is Associated with Increased Hsp60 Expression, a Marker of the Mitochondrial Unfolded Protein Response, and Increased Apoptosis under Carboplatin Co-Treatment in Triple-Negative Breast Cancer Cells

Objective: The present study aimed to investigate the effect of tunicamycin-induced endoplasmic reticulum (ER) stress on Hsp60 gene expression as a surrogate marker of mitochondrial unfolded protein response (UPRmt), and on cell death in response to carboplatin in human triple-negative breast cancer (TNBC) cells.Materials and Methods: The human TNBC cell line MDA-MB-231 was used as the experimental model. Cells were treated with 1 mM carboplatin and/or increasing concentrations of tunicamycin (1, 2, and 5 µg/mL) for 24 h. Gene expression levels of the mitochondrial chaperone Hsp60 were measured by real-time qPCR. Cell death was quantified by flow cytometry using FITC-Annexin V apoptosis/necrosis assay.Results: Neither carboplatin nor tunicamycin alone altered Hsp60 gene expression, whereas their combined treatment was associated with increased Hsp60 levels in a tunicamycin dose-dependent manner with 1.48-, 1.51-, and 1.59-fold increases at 1, 2, and 5 µg/mL, respectively. Likewise, compared to carboplatin alone, tunicamycin co-treatment was associated with increased carboplatin-induced apoptosis in a dose-dependent trend, with 80.18%, 98.12%, and 105.63% increases at 1, 2, and 5 µg/mL tunicamycin, respectively. Conclusion: Our findings suggest that ER stress alone does not alter Hsp60 expression, but tends to increase it in the presence of additional stress imposed by carboplatin in human TNBC cells. Moreover, UPRER and Hsp60 may act in coordination to drive the cell fate toward apoptosis under tunicamycin and carboplatin co-stress conditions. Understanding the interplay between UPRER and UPRmt in response to platinum-based chemotherapy will help identify novel therapeutic targets within the integrated stress adaptation networks that underlie chemosensitivity and chemoresistance in TNBC.