Conditional Tetracycline–Regulated Expression of Tgf–β1 in Liver of Transgenic Mice Leads to Reversible Intermediary Fibrosis

Abstract

Based on the tetracycline–regulated gene expression system, a double–transgenic mouse model for liver fibrosis was established in which the expression of transforming growth factor β1 (TGF–β1) can be regulated deliberately by addition or removal of doxycycline hydrochloride to the drinking water. TGF–β1 plasma levels in induced double–transgenic mice reached values ranging from 250 to 1,200 ng/mL, being 10 to 30 times above the normal plasma levels. By applying a cyclic induction–deinduction protocol, deleterious effects of the high plasma TGF–β1 levels were overcome. By using this protocol, liver fibrosis occurred within a few cycles and progressed further to an intermediary fibrosis when cyclic induction was continued. On histochemical staining, a marked perisinusoidal deposition of extracellular matrix was detected accompanied by the activation of hepatic stellate cells as shown by alpha–smooth muscle actin (α–SMA) expression. Apoptosis of hepatocytes was prominent in TGF–β1 high producers, leading to a decreasing number of TGF–β1–expressing cells with time. No compensatory proliferation of hepatocytes could be detected. In advanced stages, fibrogenesis could be stopped by switching off TGF–β1 production and reversal of fibrosis could be shown by (immuno)histochemistry within 6 to 21 days. Determination of messenger RNA (mRNA) levels of procollagen I and III, laminin (B1), matrix metalloproteinase (MMP)–2, –9, and –13, and tissue inhibitor of matrix metalloproteinase (TIMP)–1 and –2 by real–time reverse–transcription polymerase chain reaction (RT–PCR) provided insight into some mechanistic details of the fibrogenic process and its reversal. In conclusion, this model will enable the analysis of fibrogenesis at progressive stages and help in elucidating the cellular changes during development and regression of liver fibrosis caused by elevated TGF–β1 expression.