The in vitro effect of platelet-derived growth factor isoforms on the proliferation of bovine corneal stromal fibroblasts depends on cell density.

PURPOSE: Recently, it has been shown that corneal stromal fibroblasts express the mRNA for PDGF-beta-type receptors, while corneal epithelial cells express the mRNA for the PDGF B-chain, suggesting a role of PDGF isoforms in the regulation of corneal homeostasis and wound healing via an unidirectional epithelial to stromal paracrine interaction. The purpose of this study was to characterize the proliferative response of cultured bovine corneal stromal fibroblasts to PDGF isoforms.
METHODS: Bovine corneal stromal fibroblasts were seeded at a cell density of 60 cells/mm2 (low density) and 120 cells/mm2 (high density) and were cultured under serum-free conditions. Except for corresponding controls, PDGF AA, BB and AB (obtained by separate expression of cloned genes in E. coli) were added in concentrations ranging from 3.125 to 100 ng/ml. Cell numbers were determined after an incubation period of 6 days using a cell counter.
RESULTS: Stromal fibroblasts, when cultured at a high density, revealed constant cell numbers during the whole incubation period. Under these culture conditions, stimulation with PDGF AA, BB and AB led to a significant dose-dependent increase in cell proliferation. When cultured at a low cell density, stromal fibroblasts revealed a significant reduction of cell numbers after 6 days of incubation. This reduction was prevented by PDGF AA and AB isoforms in a dose-dependent manner. In contrast, PDGF BB was not effective.
CONCLUSION: The results of the "high-density" assays suggest that PDGF isoforms act as mitogens for stromal fibroblasts during wound healing, when density of fibroblasts is high. The results of the "low-density" assays support the idea that PDGF AA and AB can prevent cell loss during corneal homeostasis when density of keratocytes is low.