A GC-Rich Domain With Bifunctional Effects on mRNA and Protein Levels: Implications for Control of Transforming Growth Factor β1 Expression

Abstract

Chimeric plasmids containing selected reporter coding domains and portions of the transforming growth factor β 1 (TGF-β 1) 3' untranslated region (UTR) were prepared and used to identify potential mechanisms involved in regulating the biosynthesis of TGF-β 1. Transient transfections with core and chimeric constructs containing the chloramphenicol acetyltransferase (CAT) reporter showed that steady-state CAT mRNA levels were decreased two- to threefold in response to the TGF-β 1 3' UTR. Interestingly, CAT activity was somewhat increased in the same transfectants. Thus, production of CAT protein per unit of mRNA was stimulated by the TGF-β 1 3' UTR (approximately fourfold in three cell lines of distinct lineage). The translation-stimulatory effect of the TGF-β 1 3' UTR suggested by these studies in vivo was confirmed in vitro by cell-free translation of core and chimeric transcripts containing the growth hormone coding domain. These studies showed that production of growth hormone was stimulated threefold by the TGF-β 1 3' UTR. A deletion analysis in vivo indicated that the GC-rich domain in the TGF-β 1 3' UTR was responsible for both the decrease in mRNA levels and stimulation of CAT activity-mRNA. We conclude that this GC-rich domain can have a bifunctional effect on overall protein expression. Moreover, the notable absence of this GC-rich domain in TGF-β 2, TGF-β 3, TGF-β 4, and TGF-β 5 indicates that expression of distinct TGF-β family members can be differentially controlled in cells.