Determinants of Apoptotic Sensitivity to HSP90 Inhibition In Acute Myeloid Leukemia

Acute myeloid leukemia (AML) is a heterogeneous and intrinsically resistant disease group of malignant hematopoietic disorders that accounts for approximately 80% of all adult leukemias. Heat shock proteins (HSPs) are often overexpressed in AML are their expression is associated with poor-prognosis and resistance to chemotherapy. Among HSPs, HSP90 is the main chaperone required for the stabilization of multiple oncogenic kinases, which contribute to AML pathogenesis, providing a rationale for the use of HSP90 inhibitors in the treatment of AML.

To identify patients with AML who will benefit from HSP90 inhibitor therapy there is a need to discover molecules and pathways in AML cells that confer sensitivity and lead to significant apoptosis upon HSP90 inhibition.

To evaluate the spectrum of sensitivities of AML cells to HSP90 inhibitors, and to investigate a possible relationship between their genetic background and apoptotic sensitivity to HSP90 inhibition, we investigated the effects of HSP90 inhibitors in a set of genetically characterized human AML cells. Addition of several HSP90 inhibitors to each of these cell lines potently inhibited cell growth, with a potency reflective of their affinity for HSP90. Normal peripheral blood leukocytes were unaffected at similar concentrations. HSP90 inhibition was associated with destabilization and subsequent degradation of Akt and c-Raf in all tested cells, as well as of several cell-specific onco-proteins such as mutant Flt3 in MOLM-13, TEL-TRKC in M0-91, AML1-ETO and mutant cKit in Kasumi-1 and SKNO-1, and mutant Jak2 in HEL cells, respectively. Notably, the proclivity for these cells to undergo apoptosis upon HSP90 inhibition varied considerably. The most sensitive cell lines were MOLM-13, MV-4-11 and M0-91 cells, and for each these cell lines we observed near 100% killing of the initial cell population after 48–72 h of HSP90 inhibitor treatment. In contrast, only 20% death was seen in HEL and HL-60 cells under these conditions. We next made use of specific inhibitors of known oncogenic signaling pathways known to be dysregulated in AML to demonstrate that apoptotic sensitivity of AML cells to HSP90 inhibition correlated with PI3K-Akt and STAT5 activation, but not with activation of the Raf-MAPK pathway. Importantly, similar results were observed in cells lines, xenograft models and isogenic cell line systems. We also found that dual activation of these two pathways, especially in the context of Bcl-xL overexpression, lowers the apoptotic threshold of AML when HSP90 is inhibited.

We found that activation of oncogenic signaling pathways and expression of leukemogenic anti-apoptotic molecules, most importantly p-Akt, predicts for AML sensitivity to HSP90 inhibitors. Importantly, 50– 70% of patients with AML display phosphorylation of both Thr308 and Ser4 Akt. This molecule contributes to proliferation, survival and drug resistance in AML, and is associated with adverse outcome. Taken together, our findings suggest that AML patients with activation of Akt and STAT5 signaling are most likely to benefit from HSP90 inhibitor therapy, and clinical trials should aim to enroll patients with specific activation of these important signaling pathways.

No relevant conflicts of interest to declare.