Effect of retinoic acid on the proliferation and alkaline phosphatase activity of osteoblastic MC3T3-E1 cells by modulating the release of local regulators from monocytes.

There has been some evidence suggesting an important role of mononuclear cells at bone remodeling sites in the coupling of bone formation to bone resorption. Since cells of the monocyte-macrophage lineage produce important local regulators of bone remodeling, we examined effects of human monocytes-conditioned medium (CM) treated with retinoic acid on [3H] thymidine incorporation (TdR) and alkaline phosphatase (ALP) activity of osteoblastic MC3T3-E1 cells. Treatment of MC3T3-E1 cells with retinoic acid (10(-8) to 10(-6) M) caused an inhibition of TdR in a dose-dependent manner and an inhibition of ALP activity at 10(-6) M. Conditioned medium from monocytes untreated with retinoic acid caused a stimulation of TdR and an inhibition of ALP activity in these cells. In contrast, treatment of monocytes with retinoic acid (10(-8) or 10(-6) M) abolished both stimulation of DNA synthesis and inhibition of ALP activity induced by CM. The present study suggested that retinoic acid modulated osteoblast proliferation and ALP activity not only directly but also indirectly, presumably through modulating the release of local regulators as to bone remodeling from monocytes.