Purpose: To quantitate the receptors for transforming growth factor (TGF)-beta 1 on trabecular cells in culture and to determine the relative affinities of TGF-beta 1 and TGF-beta 2 for these receptors.
Methods: We quantitated the receptors for TGF-beta 1 by Scatchard analysis of radioligand binding of 125I-TGF-beta 1 to cultured porcine trabecular cells. We established the relative affinities of TGF-beta 1 and TGF-beta 2 for the receptors by competitive binding of 125I-TGF-beta 1 with increasing concentrations of the unlabeled TGF-beta 1 or TGF-beta 2. We also investigated the binding of 125I-TGF-beta 1 after pre-treatment of trabecular cells with heparinase.
Results: Trabecular cells expressed approximately 4,000 high-affinity receptors per cell for TGF-beta 1, with a dissociation constant (Kd) of 15.8 +/- 7.6 pmol/l. By varying the concentrations of the unlabeled growth factors, we determined that the relative affinities of TGF-beta 1 and TGF-beta 2 for the receptors were 16 pmol/l and 50 pmol/l, respectively. Heparinase treatment of the trabecular cells did not change the binding affinity of the receptor for 125I-TGF-beta 1.
Conclusions: Our findings show that trabecular cells express heparinase-insensitive TGF-beta receptors that have an approximately threefold greater affinity for TGF-beta 1 than for TGF-beta 2. Based on the present investigation, together with our previous data on the molecular weights of the binding sites, we conclude that trabecular cells do possess types II and III receptors but not type I receptors.