Recent studies indicate that vitamin D metabolites exert rapid effects on growth plate chondrocytes via changes in PG production and protein kinase C (PKC) activity. This suggests that these two products of vitamin D action may be interrelated. To test this hypothesis, we examined the effect of PGE2 on rat costochondral resting zone and growth zone cartilage cells and determined whether the effects of PGE2 are mediated by changes in the level of cAMP and/or PKC activity, whether there is a relationship between cAMP production and PKC activity, and whether cell maturation-specific effects are involved. Confluent, fourth passage resting zone and growth zone cartilage cell cultures were incubated in DMEM containing 10% FBS, 50 μg/ml vitamin C, and 1% antibiotics. The PGE2 concentration was varied from 0.007-15 ng/ml. Low concentrations of PGE2 caused a dose-dependent increase in cell number and [3H]thymidine incorporation and stimulated alkaline phosphatase specific activity. These effects were comparable in resting zone and growth zone cartilage cells at the same PGE2 concentrations. At higher concentrations, PGE2 caused a general increase in the synthesis of collagenase-digestible protein and noncollagenase-digestible protein in resting zone cartilage cells and of collagenase-digestible protein in growth zone cartilage cells, resulting in a net increase in the percent collagen synthesis for both cell types. cAMP production was increased over the entire range of chondrocyte response. Prevention of cAMP metabolism with the protein kinase A inhibitors H-8 and H-89 blocked the PGE2-dependent inhibition of PKC in resting zone cartilage cells in a dose-dependent manner. H-8 alone had no effect on PKC in resting zone cartilage cells, but stimulated PKC activity in growth zone cartilage cells; H-89 alone stimulated PKC activity in resting zone cartilage cells. These results suggest that low levels of PGE2 promote differentiation, whereas high doses promote an anabolic response; PGE2 increases cAMP production and PKC activity in a cell maturation-dependent manner; PGE2 exerts its effects via cAMP production and PKC activity; and regulation of PGE2-dependent PKC is via cAMP.
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