Insulin determines TGF-β effects on HNF4α transcription in liver injury and hepatocyte epithelial-to-mesenchymal transition

Abstract To date, epithelial-to-mesenchymal transition (EMT) has been observed in cultured hepatocytes, but not in vivo . TGF-β is supposed to initiate EMT in hepatocytes by inhibiting HNF4 α through the SMAD2/3 complex. We report that TGF- β does not directly inhibit HNF4 α , but contributes to its transcriptional regulation by SMAD2/3 recruiting acetyltransferase CBP/p300 to the HNF4 α promoter. The recruitment of CBP/p300 is indispensable for C/EBP a binding, another essential requirement for constitutive HNF4 α expression in hepatocytes. In contrast to the observed induction of HNF4 α , SMAD2/3 inhibits C/EBP α transcription. Therefore, long-term TGF- β incubation results in C/EBP α depletion, which abrogates HNF4 α expression. Intriguingly, SMAD2/3 inhibitory binding to the C/EBP α promoter is abolished by insulin. Thus, maintaining a high insulin concentration in culture medium ensures constitutive HNF4 α and thereby prevents TGF-β-induced hepatocyte EMT. Furthermore, insulin inhibits TGF- β -induced SMAD2/3 binding to the promoters of core EMT transcription factors e.g., SNAI1. SNAI1 transcription requires both SMAD2/3 and FOXO1 in nuclei. Insulin inhibits SNAI1 transcription through impeding SMAD2/3 binding to its promoter and inducing FOXO1 phosphorylation. Hence, insulin is the key factor that prevents TGF- β -induced EMT in hepatocytes.