Rabbit articular chondrocytes (RAC) express distinct transforming growth factor-beta receptor phenotypes as a function of cell cycle phases

Abstract

We previously showed that TGF-beta 1 exerted a bifunctional effect on the proliferation of cultured rabbit articular cells (RAC), depending on the serum level present in the medium. Slowly proliferating cells (2% fetal calf serum, FCS) were growth-inhibited by TGF-beta 1, whereas actively dividing cells (10% FCS) exhibited a transient growth increase in response to the factor. Here we demonstrate that both of these cycling populations of RAC display two distinct systems of high-affinity binding sites for TGF-beta 1. However, a significant increase (60%) in the number of the highest affinity receptors was observed in the 2% FCS-treated cells compared to those cultured in 10% FCS (in 2% FCS: Kd1 = 295 +/- 78 pM, 1899 +/- 99 sites/cell; Kd2 = 1106 +/- 61 pM, 9935 +/- 940 sites/cell; in 10% FCS:Kd1 = 287 +/- 10 pM, 1054 +/- 65 sites/cell; Kd2 = 1128 +/- 101 pM, 8257 +/- 61 sites/cell). This finding was correlated with the greater number of G0/G1 cells in the population cultured in 2% FCS (70%) compared to that exposed to 10% FCS (55%). The data was further confirmed using cells synchronized in late G1/early S phase (> 80% of S phase). This cell population exhibited a single class of TGF-beta 1 high-affinity binding sites (Kd = 1140 +/- 85 pM, 6836 +/- 1787 sites/cell). In contrast, cells synchronized in G0/G1 (> 80% of cells in G0/G1) expressed one binding system of Kd = 402 +/- 59 pM (940 +/- 56 sites/cell). These results clearly demonstrate that cultured RAC express different TGF-beta 1 receptor systems as a function of the cell cycle.