DISSECTING THE ROLE OF CLPP IN MULTIPLE MYELOMA

Despite the efficacy of targeted treatments, multiple myeloma (MM) is still incurable, urging to identify novel vulnerabilities to design more effective therapies. Mitochondria are emerging therapeutic targets in oncology for their crucial role not only as cellular powerhouses, but also in signalling, redox homeostasis, initiation of apoptosis, production of metabolites, and supply of biosynthetic precursors. Owing to intensive immunoglobulin production, myeloma cells are heavily reliant on protein homeostasis and experience significant exposure to mitochondrial stressors. We hypothesized that myeloma cells depend on the prototypical mitochondrial stress-adaptive pathway, the mitochondrial unfolded protein response (UPRmt), for their fitness and survival. We tested its activation status and its manipulation as a possible tool against myeloma. We found that while a clear upregulation of the UPRmt signature is evident in MM, its regulation is independent of the master transcription factor ATF5, so far believed to mediate the mammalian UPRmt. One of the key players of the UPRmt and gatekeeper of mitochondrial homeostasis is ClpP, a resident mitochondrial protease suggested to maintain oxidative phosphorylation efficiency. Prompted by its distinctive expression in malignant plasma cells, we investigated the role of ClpP in multiple myeloma cells and tested it as a possible anti-myeloma target. We found that ClpP downregulation leads to disappearance of MM cells from culture due to apoptosis or cell cycle arrest. Surprisingly, toxicity extends to glycolytic cell lines and we demonstrated that ClpP knockdown has no effects on mitochondrial oxygen consumption by MM cells, thus unveiling an energy-independent vulnerability. By combining RNA-seq, proteomics, and metabolomics we identified unprecedented and unexpected cellular features regulated by ClpP, including protein translation both in the cytosol and in mitochondria, impairment of fatty acid metabolism with accumulation of acyl-carnitines and long chain unsaturated fatty acids, deregulation of the polyamine pathway with depletion of spermine, spermidine and putrescine. Intriguingly, we also detected a strong impact on interferon-regulated pathways, hinting at mitochondria and ClpP as possible tools to manipulate MM immunogenicity. Our data suggest that ClpP is vital to MM cells due to a novel non-bioenergetic function, and pave the way for its further evaluation as a therapeutic target.