Thyroid hormone receptors downregulation is an early event and is maintained all across the step-by-step process of hepatocarcinogenesis.

Thyroid hormone receptors (THRs) mediate the pleiotropic activities of the hormone Triiodothyronine (T3) in many biological functions. These receptors are transcription factors expressed as different isoforms encoded by THRα and THRβ genes. Recently, several studies showed altered expression of THRs at mRNA and protein levels and identified somatic mutations of THR genes in several human cancers. Hepatocellular carcinoma (HCC) is the third most common cause of cancer mortality. Despite progress in understanding the molecular mechanism leading to hepatocarcinogenesis, HCC prognosis remains very poor. To investigate the molecular basis of hepatocarcinogenesis we employed a rat model of chemically-induced HCC (R-H model) in which it is possible to generate and analyze discrete lesions at different stages of progression. These lesions can be classified according to their positivity to two markers, glutathione S-transferase (GSTP) and Cytokeratin 19 (CK-19). In contrast with what has been described in human HCC, our studies have shown the complete absence of mutations in the coding regions of THRα and THRβ genes in rat HCCs. In these tumors, instead, we demonstrated a significant downregulation of the expression of the isoforms THRα1 and THRβ1. Moreover, downregulation of THRβ1 was present in very early preneoplastic lesions detected 10 weeks after initiation, suggesting that its downregulation is a very early event in the hepatocarcinogenesis process. Remarkably, THRβ1 downregulation was observed in preneoplastic lesions positive for CK-19, a marker of stem/progenitor cells, characterized by a more aggressive behaviour. Interestingly, we found an inverse relationship between degree of differentiation and THRβ1 expression in 5 human hepatoma cell lines. In order to analyze the possible mechanism responsible for the decreased expression of THRβ1, we performed epigenetic studies. The results obtained, however, did not show a significant methylation of CpG islands in THRβ promoter. Since miRNAs are important regulators of gene expression, we are currently performing analysis aimed at assessing the role of this miRNAs in the aberrant expression of THRβ1. In particular, 5 identified microRNAs predicted by in silico analysis to bind to THRβ gene will be analyzed.