Biphasic effects of leukemia inhibitory factor on osteoblastic differentiation

Leukemia inhibitory factor (LIF) is a cytokine produced by multiple cell ty pes including osteoblasts and which is active on bone metabolism. We have p reviously shown that in a bone nodule forming in vitro model of osteogenesi s, the fetal rat calvaria (RC) cell model, LIF inhibits osteoblast differen tiation, acting on late osteoprogenitors and/or early osteoblasts. These re sults are in contrast to in vivo experiments, in which LIF has been found t o increase bone formation. To resolve this discrepancy, we have tested the effect of LIF on rat bone marrow (RBM) stromal cell cultures, an in vitro m odel encompassing earlier osteoprogenitor stages. LIF inhibited cell growth in early, proliferating RBM cultures, but increased the culture saturation density. The effect of LIF on bone nodule formation in this model was cell density dependent and biphasic. Continuous treatment with LIF reduced the number of bone nodules present in confluent, more mature cultures, and the inhibitory effect was strongest when cells were plated at higher cell densi ty than lower. in contrast, during the early stages of RBM culture, nodule numbers were higher in LIF-treated dishes than in controls, and this effect was greater in lower density cultures. Acute LIF treatment restricted to e arly time points increased the final number of bone nodules formed in matur e RBM cell cultures, but not in RC cell cultures. Our results indicate that LIF exerts complex, stage-specific effects on osteoprogenitor recruitment, differentiation, and bone formation, and that the effects are cell nonauto nomous, in the rat bone marrow stromal cell model. (C) 2001 Wiley-Liss, Inc .