Ligand Binding and Functional Properties of Betaglycan, a Co-receptor of the Transforming Growth Factor-β Superfamily

Abstract

In the present work we have functionally characterized these ligand binding regions. Similar to the wild type receptor, both regions bind TGF-␤2 with higher affinity than TGF-␤1. However, only the endoglin-related region increases the TGF-␤2 labeling of the TGF-␤ type II receptor, the so-called "TGF-␤ -presentation" function of the wild type receptor. Despite this preference, both regions as well as the wild type receptor mediate the TGF-␤2-dependent Smad2 phosphorylation, indicating that they can function indistinguishably as TGF-␤-enhancing coreceptors. On the other hand, we found that the recently described ability of the wild type betaglycan to bind inhibin A is a property of the core protein that resides in the uromodulin-related region. Binding competition experiments indicate that this region binds inhibin and TGF-␤ with the following relative affinities: TGF-␤2 > inhibin A > TGF-␤1. All together, the present results suggest that betaglycan ectodomain is endowed with two bona fide independent ligand binding domains that can perform specialized functions as co-receptors of distinct members of the TGF-␤ superfamily. Transforming growth factor-␤ (TGF-␤)1 is the prototype of a superfamily of growth factors involved in the regulation of cell proliferation, differentiation, and development (1, 2). TGF-␤ signals through a complex of transmembrane serine/threonine kinase receptors, the TGF-␤ type I and type II receptors. Ligand binding promotes the association between the type I and II receptors. In this complex, phosphorylation of the type I receptor kinase by the constitutively active type II receptor kinase results in its activation. Active type I receptor phosphorylates members of a novel family of transcriptional regulators, the Smads, which transduce the TGF-␤ signal into the cell nucleus (3, 4).TGF-␤ has two known co-receptors, betaglycan and endoglin, which are transmembrane glycoproteins with large extracellular regions that bind TGF-␤ and small cytoplasmatic regions without any clearly identifiable signaling motif (5-8). Betaglycan is a membrane proteoglycan containing heparan and chondroitin sulfate chains whose core protein binds all three TGF-␤ isoforms (9 -11). Betaglycan is capable of fine tuning the availability of TGF-␤ to the signaling receptors, thereby determining the outcome of the TGF-␤ stimulation (12, 13). This regulation is both positive and negative. Although the membrane-bound form of betaglycan increases the binding of TGF-␤ to the signaling complex, the soluble form of betaglycan prevents this binding and therefore blocks the actions of TGF-␤ (14). These effects are more dramatic for TGF-␤2, the isoform for which betaglycan has higher affinity (15-17). Expression of membrane betaglycan in cells that normally do not express this co-receptor increases their binding to TGF-␤2 and corrects for their low sensitivity to this TGF-␤ isoform (18,19). Presumably, this effect is mediated by a TGF-␤-induced "presentation complex" formed between membrane-bound betaglycan and the TGF-␤ type II rece...

Genomic profiling identifies alterations in TGFβ signaling through loss of TGFβ receptor expression in human renal cell carcinogenesis and progression

“…As shown in Figure 6c, reexpression of TBR2 in UMRC3 cells ( þ TBR2) results in a sevenfold increase in collagen IV type 6 mRNA levels over that of UMRC3 controls, while reintroduction of both TBR2 and TBR3 enhanced collagen IV type 6 mRNA expression 11-fold. These data are consistent with a number of published reports that indicate that expression of TBR3 is essential for full TGFb responsiveness (Blobe et al, 2001;Esparza-Lopez et al, 2001). …”

“…TBR3 is the most abundant TBR in cells and has higher affinity for TGFb2 and similar affinity for TGFb1 and TGFb3, when compared to TBR2 (Boyd and Massague, 1989). TBR3 possesses two extracellular ligand-binding domains, capable of binding TGFb independently (Esparza-Lopez et al, 2001).…”

Extracellular domain of TGFβ type III receptor inhibits angiogenesis and tumor growth in human cancer cells

“…It is a membrane bound proteoglycan that is expressed in most cell types and tissues, and has been shown to generate, upon ectodomain shedding, a soluble form of the receptor (Andres et al, 1989;Lopez-Casillas et al, 1991). While the membrane receptor enhances the TGFb e ects, the soluble form is a potent TGFb neutralizing agent (Esparza-Lopez et al, 2001;Vilchis-Landeros et al, 2001). In our previous studies, we have shown that expression of the extracellular domain of RIII in its soluble form (sRIII) in human breast cancer MDA-MB-231 cells can sequester endogenous TGFb isoforms and antagonize their activity.…”

The Shedding of Betaglycan Is Regulated by Pervanadate and Mediated by Membrane Type Matrix Metalloprotease-1

“…As a matter of fact, up to date, betaglycan stands as the only identified inhibin A receptor capable of mediating its characteristic activin antagonism (15,16). Although the glycosaminoglycan chains are not required for all of these functions (10,17), they may modulate them. It has been reported that in some cell lines the nature of the glycosaminoglycan attached to membrane betaglycan core protein may sterically prevent its association with the TGF-␤ type II receptor, turning it into an inhibitor of TGF-␤ (18).…”