HSP90 Stabilizes B-Cell Receptor Kinases in a Multi-Client Interactome: PU-H71 Induces CLL Apoptosis in a Cytoprotective Microenvironment

Chronic lymphocytic leukemia (CLL) is characterized by the accumulation of B-cells in the hematopoietic system and lymphoid tissues. Although inhibitors targeting the B-cell receptor (BCR) pathway have been successful in the treatment of the disease, the underlying mechanisms leading to BCR over-activity in CLL are not fully understood. In this study, we found that HSP90, a highly conserved molecular chaperone, is overexpressed in CLL compared to resting B-cells. HSP90 overexpression is accompanied by the over-expression of several BCR kinases including LYN, SYK, BTK and AKT. Chemical and immune-precipitation demonstrated that these BCR constituents are present in a multi-client chaperone complex with HSP90. Inhibition of HSP90 with PU-H71 destabilized the BCR kinases and caused apoptosis of CLL cells through the mitochondrial apoptotic pathway. Further, PU-H71 induced apoptosis in the presence of stromal co-culture or cytoprotective survival signals. Finally, genetic knock-down of HSP90 client AKT, but not BTK, reduced CLL viability. Overall, our study suggests that the chaperone function of HSP90 contributes to the over-activity of the BCR signaling in CLL and inhibition of HSP90 has the potential to achieve a multi-targeting effect. Thus, HSP90 inhibition may be explored to prevent or overcome drug resistance to single targeting agents.