Heat shock proteins (Hsp) are increasingly employed as therapeutic targets in various solid tumors and leukemias. We have recently shown that Hsp32 is expressed in leukemic cells and serves as a survival-factor and molecular target in Ph+ chronic myeloid leukemia. In the present study, we examined the expression and functional role of Hsp32 in acute lymphoblastic leukemia (ALL). Leukemic cells were obtained from patients with Ph+ ALL (n=5) and Ph− ALL (n=5). In addition, Ph+ ALL cell lines (Z-119, BV-173, TOM-1, NALM-1) and Ph− ALL cell lines (RAJI, RAMOS, REH, BL-41) were used. As assessed by immunocytochemistry and qRT-PCR, leukemic cells were found to express the Hsp32 protein as well as Hsp32 mRNA in all patients and in all cell lines examined. The Hsp32-inductor hemin was found to promote the expression of Hsp32 in leukemic cells. To determine the functional role of Hsp32 in lymphoblasts, an siRNA against Hsp32 was applied. The siRNA-induced knock down of Hsp32 in RAJI cells was found to be associated with reduced growth and with an increase in apoptotic cells compared to a control siRNA against luciferase (p<0.05). In a next step, two pharmacologic inhibitors of Hsp32, pegylated zinc protoporphyrine (PEG-ZnPP) and styrene maleic acid-micelle-encapsulated ZnPP (SMA-ZnPP), were applied. As assessed by 3H-thymidine uptake experiments, both drugs were found to inhibit proliferation in the BCR/ABL+ cell lines Z-119, BV-173, and TOM-1, and in the BCR/ABL-negative ALL cell lines RAJI, RAMOS, REH, and BL-41. The effects of PEG-ZnPP and SMA-ZnPP were dose-dependent with IC50 values ranging between 1 and 10 μM, and were found to be associated with apoptosis as determined by microscopy as well as by flow cytometry and AnnexinV-staining. In NALM-1 cells, PEG-ZnPP and SMA-ZnPP also produced apoptosis and growth arrest, but the IC50 for SMA-ZnPP was slightly higher compared to other cell lines (20 μM). Effects of Hsp32-targeting drugs were also observed in primary leukemic cells obtained from patients with Ph+ ALL and Ph− ALL, with IC50 values ranging between 1 and 10 μM. No major differences were found when comparing results in imatinib-sensitive and imatinib-resistant patients. In drug combination experiments, Hsp32-targeting drugs were found to cooperate with imatinib and with AMN107 (nilotinib) in producing growth-inhibition and apoptosis in all Ph+ ALL cell lines tested. Furthermore, we were able to demonstrate strong cooperative antileukemic effects when applying Hsp32-targeting drugs in combination with bendamustine. Overall, these results suggest that Hsp32 may be a novel molecular target in ALL.

Disclosures: Valent:Novartis: Consultancy, Research Funding; Bristol-Myers Squibb: Research Funding.

Author notes

Corresponding author