A Novel Function of Molecular Chaperone HSP70

Abstract

The oncogenic transcription factor FOXM1 is overexpressed in the majority of human cancers, and it is a potential target for anticancer therapy. We identified proteasome inhibitors as the first type of drugs that target FOXM1 in cancer cells. Here we found that HSP90 inhibitor PF-4942847 and heat shock also suppress FOXM1. The common effector, which was induced after treatment with proteasome and HSP90 inhibitors or heat shock, was the molecular chaperone HSP70. We show that HSP70 binds to FOXM1 following proteotoxic stress and that HSP70 inhibits FOXM1 DNA-binding ability. Inhibition of FOXM1 transcriptional autoregulation by HSP70 leads to the suppression of FOXM1 protein expression. In addition, HSP70 suppression elevates FOXM1 expression, and simultaneous inhibition of FOXM1 and HSP70 increases the sensitivity of human cancer cells to anticancer drug-induced apoptosis. Overall, we determined the unique and novel mechanism of FOXM1 suppression by proteasome inhibitors.Forkhead Box M1 (FOXM1) is an oncogenic transcription factor that is also a well recognized cell cycle regulator (1). FOXM1 expression is eliminated from quiescent or differentiated cells, but it is highly expressed in proliferating and malignant cells (1). It is also well documented that FOXM1 is overexpressed in broad types of human cancers and it contributes to several aspects of cancer development (2, 3). Consequently, it is not surprising that FOXM1 has evolved as a potential target of anticancer therapy (4).Our group identified the thiazole antibiotics Siomycin A (5) and thiostrepton (6) as the first chemical inhibitors of FOXM1. Investigation of their mechanism of action revealed that they are proteasome inhibitors (7). Bona fide proteasome inhibitors, including MG132 and bortezomib, have also been found to hinder the transcriptional activity and expression of FOXM1 (7), suggesting that FOXM1 is one of the general targets of these drugs. Proteasome inhibitors stabilize the expression of cellular proteins. Therefore, suppression of FOXM1 is counterintuitive and has not been well understood. As an explanation for this effect, we put forward the hypothesis that a negative regulator of FOXM1 (NRFM) 2 is stabilized by proteasome inhibitors (8), which directly or indirectly inhibits FOXM1 transcriptional activity (9). Here we determined that the NRFM is HSP70. Because a positive autoregulation loop is required for FOXM1 expression (9), HSP70 (NRFM) also represses FOXM1 mRNA and protein expression after treatment with proteasome inhibitors.Heat shock proteins (HSPs) are stress proteins that are mainly expressed in response to different cellular and environmental insults. HSPs are divided by their molecular weight into the subfamilies HSP110, HSP90, HSP70, HSP60, and small HSPs. Besides their major function of aiding protein folding as molecular chaperones, HSPs also play important roles in cell signaling, protein trafficking, and blocking cell death (10 -13). Elevated levels of HSPs are linked not only to cytoprotection but to neurodegenerative ...

FOXM1 in Cancer: Interactions and Vulnerabilities

“…Moreover, when we used HSP70 inhibitor 9AA or HSP70-siRNA in combination with proteasome/HSP90 inhibitors we found that these treatments reversed suppression of FOXM1 (27), suggesting that HSP70 is a specific FOXM1 inhibitor. To investigate if FOXM1 inhibition linked to HSP70/FOXM1 interaction we performed reciprocal co-immunoprecipitations and found that FOXM1 interacts with HSP70 (27). In addition, we found that HSP70 is able to interfere with the transcriptional activity of FOXM1 (unpublished data).…”

“…Later we demonstrated that HSP90 inhibitor PF-4942847 and heat-shock also suppress FOXM1 in several human cancer cell lines (27). It turned out that all these treatments have something in common: they all induce HSP70.…”

“…In addition, we found that HSP70 is able to interfere with the transcriptional activity of FOXM1 (unpublished data). Furthermore, quantitative ChIP assays revealed that HSP70 reduces up to 2-fold the ability of exogenous FOXM1 to bind to its regulatory elements, while the levels of exogenous FOXM1 was not affected (27). These data suggest that the primary effect of HSP70 on FOXM1 is the inhibition of its transactivation via binding and that the suppression of FOXM1 expression is a secondary effect.…”

YcaO-Dependent Posttranslational Amide Activation: Biosynthesis, Structure, and Function

“…529 In an example of overlapping mode of action, proteosomal inhibition by thiostrepton stabilizes a negative regulator of FOXM1 (Hsp70). 530 It appears that thiostrepton can also directly bind FOXM1 and blocks its ability to activate gene transcription. 531 These modes of action are likely most responsible for thiostrepton’s anticancer activity, but it remains possible that other means of contributing to apoptosis, such as inhibiting mitochondria protein synthesis, also play a role.…”

FOXM1 contributes to treatment failure in acute myeloid leukemia

“…We have previously shown that FOXM1 is a general target of proteasome inhibitors (PIs) ( 28 ). The mechanism underlying this effect is stabilization of HSP70, which is a negative regulator of FOXM1 ( 29 ). Ixazomib is an oral PI that is approved for the treatment of relapsed multiple myeloma and is very well tolerated ( 30 ).…”