Identification of Inhibitors of HSF1 Functional Activity by High-Content Target-Based Screening

Cancer cells are known to experience a high level of stress and may require constant repair for survival and proliferation. Recent studies showed that inhibition of heat shock factor 1 (HSF1), the key regulator for the stress-activated transcription of heat shock protein (HSP), can reduce the tumorigenic potential of cancer cells. Such a "nononcogene addiction" phenomenon makes HSF1 an attractive cancer drug target. Here, the authors report an image-based high-content screening (HCS) assay for HSF1 functional inhibitors. A heat shock-based methodology was used to stimulate the stress response followed by quantitative measurement of HSF1/HSP70 granules for compound-induced inhibitory effects. The authors discovered a small molecule from a compound library that inhibits HSF1 granule formation substantially in heat-shocked HeLa cells with IC(50) at 80 nM. Electorphoretic mobility shift of HSF1 by this compound suggested significant inhibition of HSF1 phosphorylation, accompanied by reduced expression levels of HSP70 and HSP90 after heat induction. Importantly, HeLa cells stably transfected with HSF1 shRNA were more resistant to the compound treatment under lethal temperature than cells containing HSF1, further validating an HSF1-dependent mechanism of action. The HCS assay the authors developed was robust with a Z' factor of 0.65 in a 384-well plate format, providing a valuable method for identifying small-molecule functional inhibitors of HSF1 for potential cancer treatment.