Thyroid hormones induce activation of rat hepatic stellate cells through increased expression of p75 neurotrophin receptor and direct activation of Rho.

We have previously shown that hyperthyroidism is detrimental for liver fibrosis and in this study we have investigated the mechanisms regulating triiodothyronine (T3) and L-thyroxine (T4) activation of hepatic stellate cells (HSC). Expression of alpha-smooth muscle actin (alphaSMA) and p75 neurotrophin receptor (p75NTR) was determined by western blot analyses and transient transfection of the promoters. Rho activation was assayed using a pull-down assay and by ELISA. Expression of thyroid hormone receptor alpha1 decreases, whereas T4 receptor integrin alphaVbeta3 increases, with transdifferentiation of HSC to myofibroblasts. T3 and T4 enhance HSC activation, without affecting proliferation or phosphorylation of mitogen-activated protein kinase, signal transducer and activator of transcription 3 or Akt. Addition of 10(-7) M T3 or T4 to thyroid hormone-depleted serum induces a twofold increase in activation marker alphaSMA, as well as upregulation of p75NTR protein levels. Both hormones enhance transcription of alphaSMA and p75NTR. We report a novel signaling pathway for thyroid hormones, activation of Rho. T4 induces activation of Rho acting through alphavbeta3 integrin, and the activation is abolished by the T4 antagonist, tetraiodothyroacetic acid, by peptide RGD and by a function-blocking antibody to integrin beta3. T3 and T4 increase phosphorylation of non-muscle myosin light chain II, a downstream signal to Rho/Rho-kinase activation. T3 also induces expression of tumor necrosis factor-alpha. In vivo, administration of T3 or T4 together with thioacetamide (TAA) enhances fibrosis after 3 weeks, compared with the TAA-treated group, accompanied by increased alphaSMA in T3- and T4-treated groups, and of p75NTR in T4-treated rats. Thyroid hormones enhance activation of HSC through increased p75NTR and alphaSMA expression and activation of Rho, therefore accelerating development of liver fibrosis.