Distinct Transforming Growth Factor-beta (TGF-beta) Receptor Subsets As Determinants of Cellular Responsiveness to Three TGF-beta Isoforms

Characterization of the three mammalian transforming growth factor-beta (TGF-beta) isoforms, TGF-beta 1, -beta 2, and -beta 3, indicates that TGF-beta 3 is somewhat more potent (ED50 = 0.5 pM versus 2 pM) than TGF-beta 1 and TGF-beta 2 as a growth inhibitor of the Mv1Lu mink lung epithelial cell line. In the fetal bovine heart endothelial (FBHE) cell line, however, TGF-beta 1 and -beta 3 are at least 50-fold more potent than TGF-beta 2 which is a very weak growth inhibitor (ED50 greater than or equal to 0.5 nM). Thus, as growth inhibitors, TGF-beta 1 and -beta 3 resemble each other more than TGF-beta 2. The presence of serum alpha 2-macroglobulin in the FBHE cell assays decreases the biological potency of TGF-beta s, in particular TGF-beta 2. This effect of alpha 2-macroglobulin, however, is not sufficient to explain the low responsiveness of FBHE cells to TGF-beta 2. Evaluation of the role of TGF-beta receptors as determinants of cell-specific responsiveness to TGF-beta isoforms indicates that TGF-beta 1, -beta 2, and -beta 3 have similar affinity for the membrane proteoglycan, betaglycan. They differ, however, in their ability to bind to receptor types I and II which are implicated in TGF-beta signal transduction. TGF-beta 1 is similar, albeit not identical, to TGF-beta 3 and much more potent than TGF-beta 2 as a competitor for binding to the overall population of receptors I and II in all cell lines tested. A subset of receptors I and II has been identified in Mv1Lu cells which has high affinity for TGF-beta 2 (KD approximately 10 pM) and binds this factor at concentrations that are biologically active in Mv1Lu cells. This receptor subset could not be detected in FBHE cells, suggesting that cell-specific differences in the level of high affinity of TGF-beta 2 receptors may lead to cell-specific differences in responsiveness to this isoform. Thus, despite their structural and biological similarities, TGF-beta 1, -beta 2, and -beta 3 diverge in their ability to bind to receptors in a manner that correlates with their potency as growth inhibitors.