PLATELET-DERIVED GROWTH-FACTOR (PDGF) RECEPTOR ALPHA-SUBUNIT MUTANT AND RECONSTITUTED CELL-LINES DEMONSTRATE THAT TRANSFORMING GROWTH-FACTOR-BETA CAN BE MITOGENIC THROUGH PDGF A-CHAIN-DEPENDENT AND A-CHAIN-INDEPENDENT PATHWAYS

Mitogenic stimulation of connective tissue cells by transforming growt h factor-beta (TGF-beta) has two unusual properties; entry into S-phas e is delayed compared with that induced by other mitogens, and the dos e response is biphasic, with low concentrations stimulating and high c oncentrations inhibiting or having no effect. A hypothesis that provid es an explanation for both of these properties is that TGF-beta stimul ates proliferation indirectly by inducing synthesis of platelet-derive d growth factor (PDGF) A-chain, which in turn stimulates proliferation via autocrine activation of the PDGF receptor alpha-subunit (PHGFR al pha). High concentrations of TGF-beta reduce PDGFR alpha expression an d break the autocrine loop. We tested this hypothesis by determining w hether TGF-beta and interleukin-1 alpha can induce DNA synthesis in co nnective tissue (3T3) cells derived from the Patch mouse line in which the PDGFR alpha gene is deleted. We found that these cells do respond mitogenically to TGF-beta and interleukin-1 alpha, indicating that PD GF A-chain induction is not the sole mechanism of mitogenic stimulatio n. Reestablishing PDGFR alpha expression via transfection with a human PDGFR alpha construct enhanced the response to TGF-beta. Neutralizing anti-PDGF antiserum reduced TGF-beta stimulation of PDGFR alpha-expre ssing 3T3 cells by about 35%. We conclude that induction of PDGF A-cha in/ PDGFR alpha autocrine stimulation does contribute to the ability o f TGF-beta to stimulate connective tissue cells, but that there is, in addition, a PDGF-independent pathway.