HSF1 Signaling Inhibits Apoptosis in Hepatoblastoma

Abstract

IntroductionHepatoblastoma (HB) is the most common pediatric liver malignancy. While relatively rare, its incidence is increasing. Currently surgical resection with or without chemotherapy is the main treatment for the tumor though in severe cases children require liver transplantation. Development of precision therapy for the tumor has been hampered by gaps in fundamental knowledge about how the tumor develops and grows. Heat shock factor 1 (HSF1) is the master regulator of the heat shock response, which classically mediates cellular stress from heat but it likely has a cancer specific. Recent work has shown that HSF1 controls a suite of genes in cancer related to apoptosis, cell cycle signaling, DNA repair, energy metabolism and extracellular matrix formation. Previously, we conducted a meta‐analysis of six human HB transcriptomic data sets, which found expression levels of HSF1 was higher in tumors versus normal liver. Intriguingly, we found higher levels of HSF1 expression in tumors correlated with mortality. We wanted to investigate how inhibiting HSF1 impacted tumor growth in our lab’s mouse model of hepatoblastoma based on transfection with constitutively active forms of β‐catenin and yes‐associated protein 1 (Yap1). This model induces tumors that genetically and histologically resemble human hepatoblastoma. The objective of this study was to investigate the specific role of HSF1 signaling in the development of hepatoblastoma. Our hypothesis is inhibiting HSF1 in our mouse model of hepatoblastoma leads to increased apoptosis in tumor cells and thus decreased tumor size and number.MethodsWe inhibited HSF1 signaling in mice by co‐transfecting a dominant negative HSF1 (dnHSF1) plasmid along with β‐catenin and Yap1 plasmids. The dnHSF1 forms a truncated form of the protein that cannot initiate transcription of target genes. Mice injected with just β‐catenin and Yap1 plasmids served as controls. Eight weeks after transfection, the mice were harvested and the livers were preserved for analysis. Liver and body weights of each mouse was measured to calculate liver‐to‐body weight ratio, an index of tumor burden. At least three mice per condition was assessed.ResultsWe found transfection with the dnHSF1 lead to decreased liver‐to‐body weight ratio compared to control mice transfected with just the β‐catenin and Yap1 plasmids (p‐value<0.05)(Figure 1). What few tumor foci remained featured larger, more differentiated cells. Indeed, these foci showed positive staining via immunohistochemistry (IHC) for both activated mTORC1 and glutamine synthetase, markers seen early in tumor development. Next, we assessed apoptosis by conducting IHC for cleaved caspase 3 and by performing a TUNEL assay on liver sections from both dnHSF1/β‐catenin/Yap1 mice and from control β‐catenin/Yap1 animals. Staining for both markers was increased nearly three times in dnHSF1 treated animals versus control(p‐value<0.05)(Figure 2).ConclusionsWe conclude inhibiting HSF1 signaling in burgeoning hepatoblastoma cells leads to smaller, more differentiated tumors via increased apoptosis. More work is needed to determine if HSF1 is a viable pharmacologic target for hepatoblastoma.