Inhibition of the Heat Shock Transcription Factor 1 as a Potential New Therapeutic Strategy to Target Multiple Oncogenic Heat Shock Proteins in Multiple Myeloma.

We could recently show that the heat shock proteins (HSP) HSP90 and HSP70 are frequently overexpressed in multiple myeloma (MM), stabilize as molecular chaperones various oncogenic proteins and contribute to survival of MM cells. Currently, several clinical Phase I/II studies are under way to evaluate the concept of pharmacological HSP90 blockade in human cancer. Under cellular stress conditions the heat shock transcription factor 1 (HSF1) has a key regulatory role for the up-regulation of HSP. Importantly, it has been observed that treatment with the proteasome inhibitor bortezomib, a clinically effective anti-MM agent, induces up-regulation of HSP90, HSP70 and HSP27. Furthermore, it has recently been demonstrated that HSF1 can protect cells from oncogene-driven malignant transformation. We therefore analyzed the role of HSF1 for the malignant growth of MM cells.

Western analyses were performed to determine HSF1 expression and regulation in different human MM cell lines. To examine the expression of HSF1 and different HSP like HSP90, HSP70 and HSP27 in situ, samples from 60 bone marrow biopsies obtained from MM patients were immunohistochemically stained. To analyze the role of HSF1 for the survival of MM cells, HSF1 was either selectively depleted by siRNA-mediated knockdown using a pSUPER-based siRNA expression vector or targeted by treatment with a novel pharmacological HSF1 inhibitor triptolide. In addition, pharmacological inhibition of HSF1 was combined with concomitant pharmacological inhibition of either HSP90 (with the novel inhibitor NVP-AUY922) or bortezomib. Furthermore, gene expression analyses with an Affymetrix GeneChip were performed to identify HSF1 target genes in MM cells.

Here, we show that HSF1 is frequently overexpressed in MM cell lines in vitro and in the majority of the analyzed MM biopsies in situ, but not in MGUS or in normal plasma cells. Blockade of HSF1 either by siRNA-mediated knockdown or treatment with the novel pharamacological HSF1 inhibitor triptolide led to a strong induction of apoptosis in cells of the MM cell lines INA-6 and MM.1s. Importantly, also primary MM cells showed apoptosis induction after triptolide treatment. HSF1 inhibition led to downregulation of HSP70, HSP27 and HSP90. Gene expression analysis revealed a number of additional molecular targets of HSF1 involved in apoptosis regulation. Furthermore, initial experiments indicated that the apoptotic effect of pharmacological HSF1 inhibition is enhanced by the concomitant pharmacological inhibition of either HSP90 or the proteasome.

We demonstrate that HSF1 is overexpressed in MM, contributes to the survival of MM cells and controls the activity of oncogenic HSP like HSP90, HSP70 and HSP27. Targeting HSF1 may therefore represent an attractive potential therapeutic strategy in MM, in particular in combination with HSP90 or proteasome inhibitors.

No relevant conflicts of interest to declare.