Stimulation of Collagen Synthesis by Insulin and Proteoglycan Accumulation by Ascorbate in Bovine Keratocytes In Vitro

Ascorbate is required for the hydroxylation of collagen that is present in the corneal stroma. The keratan sulfate proteoglycans (KSPGs) lumican and keratocan are also present, and they interact with collagen and modulate its assembly into fibrils. In this study, ascorbate was added to a defined medium containing insulin, and its effects on the synthesis of collagen and KSPGs by keratocytes were determined. Collagenase-isolated keratocytes were cultured with or without insulin with or without ascorbate. Collagen and glycosaminoglycan synthesis was determined by collagenase digestion of incorporated 3H-glycine and by chondroitinase ABC or endo-beta-galactosidase digestion of incorporated 35SO4. KSPGs were detected by Western blot. Collagen stability was determined by pepsin digestion. Ethyl-3,4-dihydroxybenzoate (EDB) was used to inhibit collagen hydroxylation. Insulin stimulated the synthesis of collagen but did not affect the accumulation of lumican and keratocan. Insulin plus ascorbate, however, stimulated the synthesis of collagen and increased the accumulation of these proteoglycans. The accumulation of PGDS, a KSPG that does not interact with collagen, was not affected by ascorbate. Only the collagen synthesized in the presence of ascorbate was pepsin resistant. EDB overrode the effects of ascorbate on pepsin resistance and proteoglycan accumulation. The results of this study indicate that the accumulation of lumican and keratocan depends in part on the level of collagen synthesis and its hydroxylation. The interaction of lumican and keratocan with the stably folded triple helix provided by hydroxylation may also serve to stabilize these proteoglycans.