We recently showed that leukemia inhibitory factor (LIF) stimulates ⁴⁵Ca release from neonatal mouse calvariae in vitro and that it increases DNA and protein synthesis in this model. To elucidate further the actions of LIF on bone we now report the effects of this cytokine on DNA synthesis and cell proliferation in isolated fetal rat osteoblasts and in the osteogenic sarcoma cell line, UMR‐106. In both actively growing and growth‐arrested rat osteoblasts, LIF stimulated [³H]thymidine incorporation in a dose‐dependent manner. The increase in DNA synthesis was time dependent, was associated with an increase in the number of osteoblasts, and was not blocked by indomethacin. LIF‐treated cells showed reduced [³H]thymidine incorporation in comparison with control, as they approached confluence, possibly because of the increased cell density in the LIF‐treated cultures.

In UMR‐106 cells, treatment with LIF inhibited [³H]thymidine incorporation in both actively growing and growth‐arrested cultures. The effect was dose dependent and sustained with time. There was a corresponding decrease in cell numbers. It is concluded that although LIF causes an early stimulation of proliferation in isolated osteoblasts, it has opposing effects on UMR‐106 cells. It is not possible to determine which of these effects is more relevant to the actions of LIF in vivo. The demonstration of a LIF effect on both these cell types, however, provides further evidence that this cytokine acts directly on osteoblasts.