Bortezomib-induced heat shock response protects multiple myeloma cells and is activated by heat shock factor 1 serine 326 phosphorylation

Abstract

Proteasome inhibitors such as bortezomib are highly active in multiple myeloma by affecting signaling cascades and leading to a toxic buildup of misfolded proteins. Bortezomib-treated cells activate the cytoprotective heat shock response (HSR), including upregulation of heat shock proteins (HSPs). Here we inhibited the bortezomib-induced HSR by silencing its master regulator, Heat Shock Factor 1 (HSF1). HSF1 silencing led to bortezomib sensitization. In contrast, silencing of individual and combination HSPs, except HSP40β, did not result in significant bortezomib sensitization. However, HSP40β did not entirely account for increased bortezomib sensitivity upon HSF1 silencing. To determine the mechanism of HSF1 activation, we assessed phosphorylation and observed bortezomib-inducible phosphorylation in cell lines and patient samples. We determined that this bortezomib-inducible event is phosphorylation at serine 326. Prior clinical use of HSP inhibitors in combination with bortezomib has been disappointing in multiple myeloma therapy. Our results provide a rationale for targeting HSF1 activation in combination with bortezomib to enhance multiple myeloma treatment efficacy.

Phosphorylation alters Bim‐mediated Mcl‐1 stabilization and priming

“…1C). Importantly, when RPCI-WM1 cells were treated with Bz, the heat shock transcriptional response was induced as previously described [39], however, Bz alone had no impact on Mcl-1 mRNA levels, nor did siRNA mediated knockdown of Bim, further supporting a role for Bim in regulating the post-translational stability of Mcl-1 ( Fig. 1D).…”

TG02 inhibits proteasome inhibitor–induced HSF1 serine 326 phosphorylation and heat shock response in multiple myeloma

“…We previously determined that neither AKT, calcium/calmodulindependent protein kinase II, JAK, JNK, nor MAPK/extracellular signal-regulated kinase is the responsible kinase (Shah et al 6 ; S.P.S. and L.H.B., unpublished data, 10 July 2015).…”

“…6 Extra bone marrow was collected from patients who consented on an institutional review board-approved protocol for research purposes. Samples were deidentified before delivery to the laboratory.…”

“…3,4 The HSR consists of heat shock protein (HSP) upregulation, and heat shock factor 1 (HSF1) is the master transcription factor that regulates the bortezomib-induced HSR. 5,6 HSF1 drug screens have failed to lead to an inhibitor approved by the US Food and Drug Administration, largely because of off-target effects or lack of efficacy at therapeutically relevant concentrations. 5 We have recently shown that HSF1-mediated bortezomibinduced HSP upregulation is associated with HSF1 serine 326 (S326) phosphorylation.…”

“…5 We have recently shown that HSF1-mediated bortezomibinduced HSP upregulation is associated with HSF1 serine 326 (S326) phosphorylation. 6 Therefore, we sought to identify and inhibit the kinase(s) responsible for PI-induced S326 phosphorylation (PI-pS326) and observe PI-pS326 and apoptotic effects. We found that the multikinase inhibitor TG02 could inhibit PI-induced HSF1 phosphorylation at S326.…”