Targeting HSP 90 induces apoptosis and inhibits critical survival and proliferation pathways in multiple myeloma.

The second most commonly diagnosed hematologic malignancy, multiple myeloma, affects predominantly older patients (>60s) and is characterized by paraprotein in the serum or urine. Clinical manifestations include anemia, hypercalcaemia, progressive renal impairment, and osteolytic bone destruction. Despite promising new therapies, multiple myeloma eventually relapses in almost all patients. HSP are ubiquitous and highly conserved in prokaryotes and eukaryote organisms. Exposure to a broad range of stimuli results in increased HSP protein expression. These chaperone proteins are involved in protein transportation, prevent protein aggregation, and ensure correct folding of nascent and stress-accumulated misfolded proteins. In cancer, HSP expression is dysregulated, resulting in elevated expression, which promotes cancer by preventing programmed cell death and supporting autonomous cells growth, ultimately leading to resistance to heat, chemotherapy, and other stresses. Client proteins of HSP90 such as AKT, p53, MEK, STAT3, and Bcr-Abl are vital in tumor progression, including multiple myeloma, and their maturation and stability is dependent on HSP90. Therefore, inhibition of HSP90 via a HSP90 inhibitor (such as NVP-HSP990) should interrupt multiple signaling pathways essential for oncogenesis and growth in multiple myeloma. Our study showed that NVP-HSP990 triggered apoptosis in a panel of human multiple myeloma cells, induced cell-cycle arrest, PARP cleavage, downregulation of client proteins, the inability to reactivate phospho-STAT3 following exogenous IL-6 stimulation, and it synergized with azacytidine and bortezomib in cell lines and primary multiple myeloma samples. The mechanism of HSP90 inhibition in multiple myeloma warrants further evaluation.