PLAC8 promotes adriamycin resistance via blocking autophagy in breast cancer

Background Adriamycin (ADM) is currently one of the most effective chemotherapeutic agents in breast cancer treatment. However, growing resistance to ADM can lead to treatment failure and poor outcome. The underlying molecular mechanisms in ADM resistance in breast cancer remains unclear. PLAC8 is reported as a novel highly-conserved protein and functions as an oncogene or tumor suppressor in various tumors. Methods Here, we analyzed the expression profile of PLAC8 in breast cancer tissues and breast cancer cell lines, and explored the correlation of PLAC8 expression levels with patients’ outcomes and ADM response. One ADM resistant MCF-7 breast cancer cell (MCF-7/ADM) and its parental cell was used as in vitro models to identify the underlying mechanism of PLAC8 and ADM resistance. Breast cancer cells were transfected with PLAC8 knockdown and overexpression vectors, and MTT and colony formation assays were performed to test the cell response to ADM. Then, we tested the effect of PLAC8 on autophagy pathway by flow cytometry and immunofluorescence analysis, and the change of main autophagy-correlated factors expressions: LC3 and p62. Next, combining treatment of autophagy inhibitor/inducer and PLAC8 downregulation/upregulation revealed the participation of PLAC8 in autophagy pathway to synergistically regulate ADM resistance in breast cancer. Results Here, higher PLAC8 expression was correlated with poorer outcome and aggressive phenotype in breast cancer, and breast cancer patients with higher PLAC8 expression showed potential ADM resistance. PLAC8 expression level was also significantly elevated in ADM resistant MCF-7 breast cancer cells (MCF-7/ADM), compared to parental MCF-7 cells. In vitro experiments further confirmed that PLAC8 inhibition by siRNA or enforced overexpression by infecting pcDNA3.1(C)-PLAC8 plasmid correspondingly decreased or increased the ADM resistance. Subsequently, we demonstrated that ectopic PLAC8 expression in MCF-7/ADM cell blocked the accumulation of the autophagy-associated protein LC3II, and resulted in cellular accumulation of p62. Rapamycin-triggered autophagy significantly increased cell response to ADM, while the autophagy inhibitor 3-MA enhanced ADM resistance. Actually, 3-MA and PLAC8 could synergistically enhance ADM resistance via blocking the autophagy process. Additionally, the downregulation of p62 by siRNA attenuated the activation of autophagy and PLAC8 expression in breast cancer cells. Conclusion Our findings suggest that PLAC8, through participation of p62, inhibits autophagy and consequently results in ADM resistance in breast cancer. PLAC8/p62/autophagy pathway may act as novel therapeutic targets in breast cancer treatment and has potential clinical application in overcoming ADM resistance.