BAG3 induction is required to mitigate proteotoxicity via selective autophagy following inhibition of constitutive protein degradation pathways

Protein quality control systems (PQC), i.e. UPS and aggresome-autophagy pathway, have been suggested to be promising targets in cancer therapy: simultaneous pharmacological inhibition of both pathways have shown increase efficacy in various tumors such as ovarian and colon carcinoma. Here, we investigate the effect of concomitant inhibition of 26S proteasome by FDA approved inhibitor Bortezomib, and HDAC6, as key mediator of the aggresome/autophagy system, by the highly specific inhibitor ST80 in Rhabdomyosarcoma (RMS) cell lines. We demonstrated that simultaneous inhibition of 26S proteasome and selective aggresome/autophagy pathway significantly increases apoptosis in all tested RMS cell lines. Interestingly, we observed that a subpopulation of RMS cells was able to survive the co-treatment and, upon drug removal, to recover similarly to untreated cells. In this study, we identified co-chaperone BAG3 as the key mediator of this recovery: BAG3 is transcriptionally up-regulated specifically in the ST80/Bortezomib surviving cells and mediates clearance of cytotoxic protein aggregates by selective autophagy. Impairment of the autophagic pathway during the recovery phase, both by conditional knock down of ATG7 or by inhibition of lysosomal degradation by BafylomicinA1, triggers accumulation of insoluble protein aggregates, loss of cell recovery and cell death similarly to stable short harpin mRNA BAG3 knock.
Our results are the first demonstration that BAG3 mediated selective autophagy is engaged to cope with proteotoxicity induced by simultaneous inhibition of constitutive PQC systems in cancer cell lines during cell recovery. Moreover our data give new insight in regulation of constitutive and on demand PQC mechanisms pointing to BAG3 as a promising target in RMS therapy.