Characterization of Retinoic Acid‐induced Alterations in the Proliferation and Differentiation of a Murine and a Human Melanoma Cell Line in Culture*

lease of lysosomal enzymes; and (c) the growth-inhibited retinoic acid-treated cells resume the control rate of cell proliferation 2-3 days after the removal of retinoic acid from the growth m e d i ~m . 1 ~ Recent studies using the human Hs939 melanoma cell lines have demonstrated that cellular proliferation is inhibited by retinoic acid in a fashion similar to that described above for the mouse melanoma.The human melanoma cells are, however, less sensitive to the growth-inhibitory effect of retinoic acid than the S91 melanoma cells.10It is noteworthy that a reduction in cell proliferation rate by retinoic acid is not limited to malignant cells, since a similar effect was reported on several untransformed fibroblastic cells,"* and on an intestinal epithelial cell line.9 SEMI-SOLID MEDIUM BY RETINOIC ACIDSeveral retinoids have been shown to possess a remarkable ability to reduce the formation of tumor cell colonies in semi-solid medium.5~11It was intriguing to find out whether the S91 melanoma, which is sensitive to retinoic acid in liquid medium, would also be inhibited in semi-solid medium.In initial experiments it was established that the S91 melanoma cells are capable of forming colonies in medium containing 0.5% agarose with an efficiency of 3040%.This anchorageindependent growth was markedly inhibited by retinoic acid in a concentrationdependent process.Thus, colony formation by cells plated in medium containing ' 1 2 3 4 DAYS IN CULTURE FIGURE 2. Growth curves of S91 melanoma cells in the absence or presence of various retinoic acid concentrations.Cells were seeded in 35 mm dishes in Dulbecco's modified Eagle's minimum essential medium (DMEM) supplemented with 10% fetal bovine serum and containing either 0.1% ethanol only (control) or the indicated retinoic acid concentrations.The cultures were refed on the second day and detached and counted on the indicated days.Each point represents the average of duplicate cultures that did not differ by >lo%.