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 HNF4α through the SMAD2/3 complex. We report that TGF-β does not directly inhibit HNF4α, but contributes to its transcriptional regulation by SMAD2/3 recruiting acetyltransferase CBP/p300 to the HNF4α promoter. The recruitment of CBP/p300 is indispensable for C/EBPa binding, another essential requirement for constitutive HNF4α expression in hepatocytes. In contrast to the observed induction of HNF4α, SMAD2/3 inhibits C/EBPα transcription. Therefore, long-term TGF-β incubation results in C/EBPα depletion, which abrogates HNF4α expression. Intriguingly, SMAD2/3 inhibitory binding to the C/EBPα promoter is abolished by insulin. Thus, maintaining a high insulin concentration in culture medium ensures constitutive HNF4α 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.
Competing Interest Statement
The authors have declared no competing interest.
Footnotes
Language polishing to make it easier to follow.