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Heat shock factor 1 (HSF1) is a transcriptional factor that determines the cell's efficiency of heat shock responses (HSRs). Heat shock response enables cells to cope with the deleterious effects of protein-damaging stresses, including heat, heavy metals, viral and bacterial infections. In general, the characteristics of heat shock response refer to a down-regulation of gene transcription and protein synthesis, whereas the transcription of a specific subset of genes called the heat shock genes is induced. Dysregulation of HSF1 and its target genes are associated with the diseases. In fact, cancer cells rely on the constitutively active HSF1 to promote rapid growth and malignancy. Moreover, HSF1 systematically guards proteome stability and proteostasis by regulating the expression of heat shock protein (HSP), thus rendering cancer cells addicted to HSF1. Therefore, HSF1 is widely exploited as a potential therapeutic target in a broad spectrum of cancers.
Fig. 1. Mechanisms implicated in the regulation of HSF1 activity.
Heat shock factor 1 (HSF1)-mediated proteotoxic stress response is pivotal. Four HSFs (HSF1–4) exist in vertebrates. HSF1 and HSF2 are the most studied factors because they co-expression in most tissues and cell lines. HSF3 has been found only in avian species, and HSF4 is the most recently identified member in mammals and expressed predominantly in the lens and brain. Furthermore, HSF1 function has now been correlated with worse outcomes in solid tumors, including lung, skin, colon, liver, pancreas, myeloma, prostate, cervix, as well as in hematological malignancies. Nevertheless, studies have identified the unique HSF1 transcriptional program activated in highly malignant cells, known as the HSF1 cancer signature (HSF1-CaSig). This transcriptional program has been used in retrospective studies to stratify patients according to the relative expression of HSF1-CaSig, and found that patients with higher HSF1-CaSig expression exhibit worse outcomes regardless of tissue origin. In addition to HSF1 protein levels and transcriptional activity predicting poor prognoses, the phosphorylation of HSF1 on Ser326, a modification associated with HSF1 activation, has been linked to worse outcomes in ovarian cancer, chronic lymphocytic leukemia, and myeloma.
Triptolide is a structurally unique diterpene triepoxide with potent antitumor activity, which performs its biological activities via mechanisms including induction of apoptosis, targeting pro-inflammatory cytokines, and reshaping the epigenetic landscape of target cells. HSF1-mediated heat shock response is activated in most tumors and plays an important role in regulating tumor homeostasis. HSF1 function was inhibited either by siRNA-mediated knockdown or through treatment with triptolide pharmacologically. The potential efficacy of triptolide on murine leukemia by measuring the triptolide-induced cytotoxicity in murine leukemia WEHI-3 cells in vitro was investigated. Results indicated that triptolide induced cell morphological changes and cytotoxic effects through G0/G1 phase arrest, thus induced apoptosis.