Posttranscriptional Regulation of TGFβ2 by Mevalonate Metabolism

Abstract

Transforming growth factor-beta(TGF-β) as a multifunctional signaling molecule regulates either directly or indirectly numerous physiological processes, such as cell growth and differentiation, extracellular matrix formation, and immune function. The expression of TGF-β 1,2 and 3 is differentially regulated in many cultured cell types. Especially TGF-β2 was proved to be regulated by lovastatin, the HMG CoA reductase inhibitor from this study using PC3 cells, the human prostate adenocarcinoma cell line. Also the regulation of TGF-β2 mRNA stability by lovastatin was demonstrated to be a posttranscriptional event. Mevalonic acid recovered the expression of TGF-β2 which was blocked by lovastatin. Zaragozic acid, the squealene synthetase inhibitor, does not inhibit the expression of TGF-β2. So cholesterol intermediate(s) between mevalonate and farnesyl pyrophosphate was believed to have a role in the regulation of TGF-β2 mRNA stability by Lovastatin. The addition of cycloheximide reversed the lovastatin-blocked expression of TGF-β2. This suggested the presence of unknown protein which was assumed to interact with the open reading frame(ORF) of TGF-β2. Recently, SCH56582, a novel tricyclic inhibitor of farnesyltransferase, blocked the expression of TGF-β2. So the mevalonate metabolite essential for the posttranscriptional regulation of TGF-β2 was assumed to be farnesyl pyrophosphate. This metabolite would be necessary for farnesylation of unknown protein which was supposed to regulate the stability of TGF-β2. Finally, several TGF-β2 ORF expression vectors were generated to contain partial coding sequences of TGF-β2 . PC3 cells transfected stably with these TGF-β2 ORF expression were treated with lovastatin to locate the specific region(s) on ORF involved in the regulation of TGF-β2 mRNA stability by lovastatin. However, the expression of these TGF-β2 ORF was increased by lovastatin in comparison with control group. This unexpected result suggested the three dimensional conformation of TGF-β2 ORF might be critical for the regulation of TGF-β2 mRNA stability by lovastatin.