An HSP90 Inhibitor, PU-H71, Antagonizes Stroma-Induced Pro-Survival Effects in CLL through Its Inhibition of Multi-Component B-Cell Receptor Signaling Pathway

Background: Chronic lymphocytic leukemia (CLL) is characterized by the accumulation of B-cells in the hematopoietic system. The B-cell receptor (BCR) plays an essential role in the pathogenesis of CLL and many components of the BCR signaling pathway are known clients of HSP90. HSP90 is a highly conserved molecular chaperone that ensures the proper folding and stabilization of its client proteins. In this study, we investigated whether PU-H71 a novel purine-scaffold HSP90 Inhibitor, has anti-tumor activity in CLL by destabilizing BCR signaling pathway constituents.

Design: Fresh CLL cells were isolated and cultured ex vivo with or without stromal co-culture. Molecular and cellular events were studied in PU-H71-treated and control CLL cells.

Results: Immunoblotting revealed that a significantly higher amount of HSP90 is present in CLL cells than in peripheral blood mononuclear cells (PBMC), suggesting the chaperone is pathogenically relevant. We found that PU-H71 caused the death of CLL cells in a dose and time dependent manner while the viability of either PBMC or normal B lymphocytes were not affected. PU-H71 induced apoptosis resulting in CLL cell death as it caused mitochondrial cytochrome C release and a decrease in the abundance of several anti-apoptotic proteins. Interestingly, PU-H71 has the ability to counteract the pro-survival effects of the stroma and caused apoptosis in CLL cells co-cultured with stroma. To gain mechanistic insights into how PU-H71 acts, we examined the BCR signaling pathway. We found that the amounts of several key components of the pathway were reduced by PU-H71 treatment. This occurred even in the presence of stromal co-culture. The results suggest that PU-H71 antagonizes the function of HSP90 leading to the destabilization of the BCR signaling transducers. A chemical pull-down experiment revealed the co-existence of the BCR components and HSP90 in the same complex, suggesting these BCR constituents are indeed clients of HSP90 in CLL cells. Further, specific genetic knock-down of the signal transducers by siRNA confirmed their key roles in mediating the survival of CLL cells.

Conclusions: PU-H71 antagonizes stroma-induced pro-survival effects in CLL through its inhibition of the B-cell receptor signaling pathway. Our results suggest that PU-H71 may serve as a useful therapy against CLL and is worth further clinical development.