Biphasic Effects of Prostaglandin E2on Bone Formation in Cultured Fetal Rat Calvariae: Interaction with Cortisol*

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Abstract
Previous studies have shown that prostaglandin E2 (PGE2) has both inhibitory and stimulatory effects on the incorporation of proline into collagenase-digestible protein (CDP) in cultured fetal rat calvaria. The present studies were undertaken to analyze further these biphasic effects of PGE2. PGE2 increased [3H]thymidine incorporation at 24 h, and this effect were enhanced in the presence of cortisol (10-8 and 10-7 M). An inhibitory effect on CDP labeling was observed at 96 h with PGE2 (10-6 M) in the absence and presence of indomethacin (10-6 M), but not in the presence of cortisol (10-8 or 10-7 M). When the central osteoblast-rich bone and periosteum were analyzed separately, the inhibitory effect of PGE2, with or without indomethancin, was confined to the central bone. Addition of aphidcolin (30 .mu.M), an inhibitor of cell replication, did not prevent the inhibitory effect of PGE2 on CDP labeling. Analysis of labeled collagen by polyacrylamide gel electrophoresis showed a decrease in labeling of type I collagen in central bone. Moreover, mRNA for .alpha.1(I)procollagen was decreased, as measured by dot blot hybridization and Northern blot analysis. Cortisol (10-8-10-6)decreased the labeling of CDP as well as noncollagen protein (NCP) at 96 h. In the presence of cortisol, PGE2(10-8-10-5 M) consistently stimulated labeling of CDP and NCP, with a greater increase in CDP, resulting in an increase in the percentage of collagen synthesized. In the presence of low concentrations of cortisol (10-8 or 3 .times. 10-8 M), PGE2 (10-7 M) increased CDP labeling by 260-480%, and the absolute value was 145-160% of that in control cultures without any hormone addition. The stimulatory effect was seen in both central bone and periosteum, although absolute values for CDP and percentage of collagen synthesized were higher in central bone. PGE2 (10-7 M) had similar effects on CDP at 24 and 96 h in the presence of cortisol, and the stimulation at 10-7 M was the same in the presence and absence of ephidicolin, suggesting that it was not dependent and absence of aphidicolin, suggesting that it was not dependent on cell replication. Cortisol decreased labeling of type I collagen, determined by polyacylamide gel electrophoresis, and .alpha.1(I)procollagen mRNA levels, determined by both Northern and dot blot analysis. PGE2 reversed these effects, increasing both radiolabeled collagen type I chains and .alpha.1(I)procollagen mRNA levels. These results indicate that PGE2 can regulate bone collagen synthesis at a pretranslational site. Thus, the major effect are normally present in the circulation was to increase bone formation. This observation is consistent with a role for PGE2 as a local stimulator of bone formation which could mediate responses to local stress. The inhibitory effect might be more important under pathological conditions when PGE2 is present in high concentrations and for long periods.