The molecular network of the proteasome machinery inhibition response is orchestrated by HSP70, revealing vulnerabilities in cancer cells.

Proteasome machinery is a major proteostasis control system in human cells, actively compensated upon its inhibition. To understand this compensation, we compared global protein landscapes upon the proteasome inhibition with carfilzomib, in normal fibroblasts, cells of multiple myeloma, and cancers of lung, colon, and pancreas. Molecular chaperones, autophagy, and endocytosis-related proteins are the most prominent vulnerabilities in combination with carfilzomib, while targeting of the HSP70 family chaperones HSPA1A/B most specifically sensitizes cancer cells to the proteasome inhibition. This suggests a central role of HSP70 in the suppression of the proteasome downregulation, allowing to identify pathways impinging on HSP70 upon the proteasome inhibition. HSPA1A/B indeed controls proteasome-inhibition-induced autophagy, unfolded protein response, and endocytic flux, and directly chaperones the proteasome machinery. However, it does not control the NRF1/2-driven proteasome subunit transcriptional bounce-back. Consequently, targeting of NRF1 proves effective in decreasing the viability of cancer cells with the inhibited proteasome and HSP70.