Partial restoration of the keratocyte phenotype to bovine keratocytes made fibroblastic by serum

PURPOSE: To determine whether keratocytes made fibroblastic in vitro by addition of fetal bovine serum to the medium regain the keratocyte phenotype after culture in serum-free medium. METHODS: Collagenase-isolated keratocytes from bovine corneas were plated in DMEM/F-12 containing 1% horse plasma, to allow cell attachment, and then cultured until day 4 in either DMEM/F-12 alone, to retain the keratocyte phenotype, or in DMEM containing 10% fetal bovine serum, to cause the keratocytes to become fibroblastic. Medium for the fibroblastic cells was replaced on day 4 with serum-free medium, and cells were cultured until day 12. Cell phenotypes were determined on days 4 to 5 and 11 to 12 of culture as follows: (1) by the morphologic appearance on phase-contrast microscopy; (2) by the levels of aldehyde dehydrogenase in the cells, determined by SDS-PAGE and Coomassie blue staining; (3) by the relative synthesis of collagen types I and V, determined by (14)C-proline radiolabeling; (4) by pepsin digestion and analysis of collagen types by SDS-PAGE autoradiography; (5) by relative synthesis of cornea-specific proteoglycan core proteins determined by analysis of chondroitinase- or endo-beta-galactosidase-generated radiolabeled core proteins by SDS-PAGE autoradiography; and (6) by the relative synthesis of keratan sulfate and chondroitin sulfate determined by (35)SO(4) radiolabeling and measuring the sensitivity to endo-beta-galactosidase and chondroitinase ABC. RESULTS: Keratocytes cultured in serum-free medium appeared dendritic and became fibroblastic in appearance when exposed to medium containing serum. Keratocytes and fibroblasts synthesized a similar proportion of collagen types I and V. However, compared with the keratocytes, the fibroblasts possessed no aldehyde dehydrogenase and synthesized significantly higher levels of decorin and significantly lower levels of prostaglandin D synthase (PGDS) and keratan sulfate. Subsequent culture of the fibroblasts in serum-free medium did not restore aldehyde dehydrogenase to keratocyte levels but did restore the cell morphology to a more dendritic appearance and returned the synthesis of decorin, PGDS, and keratan sulfate to keratocyte levels. CONCLUSIONS: The results of these studies indicate that primary cultures of keratocytes made fibroblastic by exposure to serum can return to their keratocyte phenotype in synthesizing extracellular matrix. These results also indicate that the differences in the organization of the collagenous matrix produced by keratocytes and fibroblasts may be related more to the different proteoglycan types than to the collagen types produced.