Exposure to platelet-derived growth factor modulates the porcine aortic smooth muscle cell response to somatomedin-C.

Abstract

Platelet-derived growth factor (PDGF) is a potent mitogen for smooth muscle cells, but to detect maximal stimulation by PDGF, the cells must be incubated with plasma. Somatomedin-C (Sm-C), a peptide growth factor that is present in plasma, has been shown to interact with PDGF synergistically to stimulate DNA synthesis in cultured fibroblasts. These studies were designed to test the hypothesis that PDGF interacted with Sm-C to stimulate smooth muscle cell replication and to compare the response of this cell type to that of fibroblasts. When PDGF or Sm-C was added individually to smooth muscle cell cultures, each growth factor induced only minimal increases in [3H]thymidine incorporation into DNA (i.e. PDGF, 3,400 +/- 1,120 to 54,900 +/- 1,550 cpm; Sm-C, 3,400 +/- 1,120 to 10,950 +/- 980 cpm). In contrast, addition of increasing concentrations of Sm-C to cultures that were continuously exposed to PDGF in the presence of Sm-C-deficient plasma resulted in a synergistic increase in [3H]thymidine incorporation (3,400 +/- 1,120 to 54,500 +/- 1,800 cpm; P less than 0.001). To determine if Sm-C was required for smooth muscle cell replication, cultures were sequentially exposed to PDGF, followed by Sm-C-deficient plasma. The rate at which the non-Sm-C-exposed cells synthesized DNA was retarded compared to that of cells exposed to Sm-C-containing plasma; however, 68% nuclear labeling was present after 44 h of incubation. To exclude the possibility that some cellular secretory product was substituting for Sm-C, the medium was changed every 2 h and replaced by fresh Sm-C-deficient medium. Using these test conditions, exposure to PDGF and Sm-C-deficient plasma induced only 11% labelling. Readdition of pure Sm-C to this medium restored nuclear labeling to 82% at 44 h. Other variables that appeared to modulate the cellular response to Sm-C were culture density and simultaneous PDGF exposure. Sm-C and PDGF both appear to be potent mitogens for porcine aortic smooth muscle cells, and when added together to quiescent cultures, their effects are synergistic. Smooth muscle cells appear to require Sm-C to initiate DNA synthesis, and in its absence produce a Sm-like factor that can partially compensate for Sm-C deficiency and allow replication.