HSF1 is a direct sensor of proteotoxic stress.

Abstract Stress responses play a pivotal role in normal physiology, a feature exploited by cancer cells to facilitate growth, counteract genomic instability, and resist therapeutic stress. Heat shock factor-1 (HSF1), a major stress-response transcription factor, exhibits a significant upregulation in activity in cancer. While HSF1's role in transcription is relatively well-known, the mechanisms governing its activation, particularly in tumors, remain unclear. To determine whether HSF1 is the primary sensor of proteotoxic stress, we studied the range of conditions that modulate HSF1 activation both in vitro and in cells. We show that purified HSF1 adopts a stable monomeric conformation in vitro. Conditions that lead to protein denaturation, such as heat stress, molecular crowding, Hsp90 inhibition, and proteasome inhibition, directly trigger the activation of HSF1 by inducing conformational changes and trimer assembly. In contrast, proteosynthesis inhibition prevents HSF1 activation by reducing denatured proteins in the cell. Our findings highlight HSF1 as a direct sensor of proteotoxic stress, independent of post-translational modifications and molecular chaperones. Abrupt environmental changes causing protein denaturation induce a conformational change in monomeric HSF1, explaining the universal cellular response to proteotoxic stress. This activation mechanism underscores the cells' ability to initiate a protective response, enhancing chaperone activities to restore homeostasis.