Cloning the Promoter for Transforming Growth Factor-β Type III Receptor

Abstract

Transforming growth factor-␤ binds to three high affinity cell surface molecules that directly or indirectly regulate its biological effects. The type III receptor (TRIII) is a proteoglycan that lacks significant intracellular signaling or enzymatic motifs but may facilitate transforming growth factor-␤ binding to other receptors, stabilize multimeric receptor complexes, or segregate growth factor from activating receptors. Because various agents or events that regulate osteoblast function rapidly modulate TRIII expression, we cloned the 5 region of the rat TRIII gene to assess possible control elements. DNA fragments from this region directed high reporter gene expression in osteoblasts. Sequencing showed no consensus TATA or CCAAT boxes, whereas several nuclear factors binding sequences within the 3 region of the promoter co-mapped with multiple transcription initiation sites, DNase I footprints, gel mobility shift analysis, or loss of activity by deletion or mutation. An upstream enhancer was evident 5 proximal to nucleotide ؊979, and a silencer region occurred between nucleotides ؊2014 and ؊2194. Glucocorticoid sensitivity mapped between nucleotides ؊687 and ؊253, whereas bone morphogenetic protein 2 sensitivity comapped within the silencer region. Thus, the TRIII promoter contains cooperative basal elements and dispersed growth factor-and hormone-sensitive regulatory regions that can control TRIII expression by osteoblasts. Several cell surface receptors for transforming growth factor-␤ (TGF-␤)1 are now known. Type I and type II receptors (TRI and TRII) have intracellular kinase domains responsible for heterologous receptor activation or downstream signal transduction (1-4). The type III receptor (TRIII), a membraneanchored proteoglycan also termed betaglycan, is thought to have a biological function distinct from TRI and TRII (5-7). The rat TRIII gene encodes a 91.6-kDa protein core that is modified by approximately 10 kDa of N-linked glycosyl residues and 150 -200 kDa of heparan and chondroitin sulfate side chains.TRIII has a relatively short, 43-amino acid cytoplasmic domain that lacks commonly recognized protein docking or kinase like motifs but is enriched with serines and threonines to approximately 42% (5, 6).TRIII is prevalent on many fetal cells, where it can be the most abundant TGF-␤-binding site. All TGF-␤ isoforms bind to TRIII with comparably high affinity, although this is about 3-5-fold less than that for TRI and TRII (7). Certain cells lack TRIII but maintain TGF-␤ sensitivity. Nonetheless, TRIII may attract and enhance TGF-␤ binding to TRII and form a more stable ligand-receptor complex (6). On certain cell types this appears more pronounced or limited to the TGF-␤2 isoform (8, 9). Moreover, disproportionately high levels of TRIII may sequester and possibly limit its binding to signaling receptor complexes (10 -12).The relative amount of TRIII is thought to vary with development, with differentiation, or in a tissue-specific manner. For example, TRIII levels are prevalent on less different...

Betaglycan Inhibits TGF-β Signaling by Preventing Type I-Type II Receptor Complex Formation

“…Mesenchyme-derived cells including mesangial cells, which are generally poorly responsive to TGF-␤, express high levels of betaglycan. 3 There are examples in the literature that correlate increased TGF-␤ responsiveness with decreased betaglycan expression, or vice versa (33). These are physiologically relevant models for further studies investigating the function of betaglycan, as are cell types in which betaglycan expression changes dramatically during differentiation (34).…”

TβRIII independently binds type I and type II TGF-β receptors to inhibit TGF-β signaling

“…, 2007 ). However, there are reports of TβRIII decreasing TGF-β signaling in specific cell contexts ( Ji et al. , 1999 ), which in some cases was shown to be independent of soluble TβRIII ( Eickelberg et al.…”

Transforming Growth Factor β (TGF-β) Signaling Is Regulated by Electrical Activity in Skeletal Muscle Cells

“…In fact, using site-directed mutagenesis of reporter constructs carrying an enhancer of the rat nAChR ␦-subunit, both Sp1 and E-boxes were demonstrated to be necessary for conferring susceptibility to electrical activity to a heterologous promoter (23). Given that the rat T␤RI promoter sequence contains seven consensus Sp1 binding sites, which is also true for T␤RII and receptor type III (T␤RIII or betaglycan) (40,41,44), and two E-boxes, our results showing that myogenin can mediate the electrical activity-dependent regulation of T␤RI indicate that direct binding to E-boxes and/or a cooperative activation process with other transcription factors, such as Sp1, could underlie the observed modulation of T␤RI by myogenin. Furthermore, the mouse T␤RIII promoter has also been reported to contain two E-boxes, localized at Ϫ1500 bp, whose transcriptional activity is positively regulated by MyoD (19).…”