TGF-β1 mediates cell proliferation and development of hepatocarcinogenesis by downregulating deiodinase 1 expression.

Hepatocellular carcinoma (HCC) is the most common primary liver tumor. Hexachlorobenzene (HCB) is an endocrine disruptor and a liver tumor promoter. Deregulation of thyroid hormone (TH) homeostasis may play a significant role in early neoplastic transformation. The aim of this study was to evaluate the relation between TH metabolism and the regulation of cell growth in an in vivo and in vitro model. We examined the role of transforming growth factor-β1 (TGF-β1) on TH deiodinase expression and hepatocyte proliferation. An initiation (DEN)/promotion (HCB) tumor model from rat liver and HepG2 cells were used. We evaluated PCNA, p21, p27, SMAD2/3, TGF-β1, deiodinase 1 (D1), D3, protein expression levels; D1 and D3 mRNA expression; TH and TGF-β1, D1, D3, and GST-P protein levels in focal/non-focal areas. In vivo, HCB decreased triiodothyronine (T3) and D1 mRNA levels and increased thyroxine (T4) and D3 mRNA levels in liver from DEN+HCB vs. DEN group. HCB increased protein levels from D3, TGF-β1, and PCNA and decreased D1 in focal-areas. In vitro, HCB increased PCNA, pSMAD 2/3, and TGF-β1 protein levels and mRNA expression and decreased p21 and p27 protein levels. Exogenous T3 treatment prevent HCB induced molecular alterations related to hepatocyte proliferation whereas T4 did not have any effect. These effects were prevented by using a TGF-β1 receptor II inhibitor. Results suggest that alteration of TH homeostasis, through D1 function, play a key role in hepatocyte proliferation and that TGF-β1-SMAD pathway is involved in this process confirming their role in early neoplastic transformation in HCC.