Mediation of the inhibitory effect of thyroid hormone on proliferation of hepatoma cells by transforming growth factor-beta

Thyroid hormone (triiodothyronine, T₃) regulates growth, development and differentiation. To examine the influence of T₃ on hepatoma cell growth, thyroid receptor (TR)α1 or TRβ1 over-expressing HepG2 cell lines were used. Growth of the HepG2-TR stable cell line was inhibited by over 50% following treatment with T₃. However, transforming growth factor (TGF)-β neutralizing antibody, but not the control antibody can reverse the cell growth inhibition effect of T₃. Flow cytometric analysis indicated that the growth inhibition was apparent at the transition point between the G1 and S phases of the cell cycle. The expression of major cell cycle regulators was used to provide further evidence for the growth inhibition. Cyclin-dependent kinase 2 (cdk2) and cyclin E were down-regulated in HepG2-TR cells. Moreover, p21 protein or mRNA levels were up-regulated by around 5-fold or 7.3-fold respectively following T₃ treatment. Furthermore, phospho-retinoblastoma (ppRb) protein was down-regulated by T₃. The expression of TGF-β was studied to delineate the repression mechanism. TGF-β was stimulated by T₃ and its promoter activity was enhanced six- to eight-fold by T₃. Furthermore, both T₃ and TGF-β repressed the expression of cdk2, cyclin E and ppRb. On the other hand, TGF-β neutralizing but not control antibody blocked the repression of cdk2, cyclin E and ppRb by T₃. These results demonstrated that T₃ might play a key role in liver tumor cell proliferation.