Insulin determines the effects of TGF-β on HNF4α transcription and epithelial-to-mesenchymal transition in hepatocytes

Abstract

AbstractTo date, epithelial-to-mesenchymal transition (EMT) has been observed in cultured hepatocytes, but notin vivo. TGF-β is supposed to initiate EMT in hepatocytes by inhibiting HNF4αthrough the SMAD2/3 complex. We report that TGF-βdoes not directly inhibit HNF4α, but contributes to its transcriptional regulation by SMAD2/3 recruiting acetyltransferase CBP/p300 to the HNF4αpromoter. The recruitment of CBP/p300 is indispensable for C/EBPabinding, another essential requirement for constitutive HNF4αexpression in hepatocytes. In contrast to the observed induction of HNF4α, SMAD2/3 inhibits C/EBPαtranscription. Therefore, long-term TGF-βincubation results in C/EBPαdepletion, which abrogates HNF4αexpression. Intriguingly, SMAD2/3 inhibitory binding to the C/EBPαpromoter is abolished by insulin. Thus, maintaining a high insulin concentration in culture medium ensures constitutive HNF4αand 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.