Role of HSF1 in proteasome regulation during proteotoxic stress

The heat-shock response, a fundamental defense mechanism against proteotoxic stress, is regulated by a family of heat shock transcription factors (HSFs). In humans HSF1 is considered the central regulator of heat-induced transcriptional responses. The main targets for heat shock factor-1 (HSF1) are specific promoter elements (HSE) located upstream of heat-shock genes encoding cytoprotective heat-shock proteins (HSP) with chaperone function. In addition to its cytoprotective function, HSF1 was recently hypothesized to play a more complex role in human cells. The zinc-finger AN1-type domain 2a (ZFAND2A) gene, also known as AIRAP, was recently identified as a novel human canonical heat shock gene, strictly controlled by HSF1. Little is known about AIRAP gene regulation in human cells. In this study we demonstrate that bortezomib, a proteasome inhibitor with anticancer and antiangiogenic properties used in the clinic for treatment of multiple myeloma, potently induces AIRAP expression in human cells. Using endothelial cells as a model, we unraveled the molecular mechanism regulating AIRAP expression during proteasome inhibition. Bortezomib induces AIRAP expression at the transcriptional level early after treatment, concomitantly with polyubiquitinated proteins accumulation and HSF activation. AIRAP protein is detected at high levels for at least 48 h after bortezomib exposure, together with the accumulation of heat shock factor-2 (HSF2), a factor implicated in differentiation and development regulation. We demonstrated that, differently from heat-mediated induction, in endothelial cells AIRAP expression is regulated at the transcriptional level by a mechanism involving both HSF1 and HSF2 transcription factors, via the formation of HSF1/HSF2 heterotrimeric complexes. These complexes were recruited to a specific heat shock element in the AIRAP promoter. Furthermore we show that, while HSF1 is critical for AIRAP gene transcription, HSF2 negatively regulates AIRAP expression after bortezomib treatment, further emphasizing an important modulatory role of this transcription factor during proteotoxic stress. AIRAP function is still not defined; however, the fact that AIRAP is abundantly expressed in primary human and cancer cells at bortezomib concentrations comparable to plasma-levels in treated patients suggests that AIRAP may participate in the regulatory network controlling proteotoxic stress during bortezomib treatment.