Leukemia inhibitory factor as a mediator of JAK/STAT activation in murine osteoblasts

A number of cytokines have been shown to exert their effects via a recently discovered signaling cascade. One step in this pathway is mediated by a family of nonreceptor protein tyrosine kinases, the Janus kinases or JAK kinases, which become phosphorylated upon ligand-receptor binding and receptor phosphorylation. This in turn is followed by phosphorylation of certain members of a family of latent transcription factors, called signal transducers and activators of transcription (STATs), which subsequently enter the nucleus, bind to DNA in a sequence-specific fashion, and modulate transcription. In view of the apparent role of leukemia inhibitory factor (LIF) in bone remodeling, we sought to determine which, if any, of the JAK/STAT family members are involved in mediating the actions of LIF using the MC3T3-E1 cell line (a spontaneously immortalized osteoblast) and normal murine calvarial osteoblasts. We report here rapid and transient phosphorylation of the LIF receptor, and similarly, we detect phosphorylation of predominantly JAK1 and to a minor extent JAK2 in response to LIF treatment in MC3T3-E1 cells. In these experiments we also detect phosphorylation of STAT1 and to a much lesser degree STAT3 upon addition of LIF. Phosphorylation of the STAT1 proteins correlates directly with their ability to bind DNA in a gel mobility shift assay in MC3T3-E1 and in normal calvarial osteoblasts. These studies suggest that LIF action in these cells, as in other cell types, is mediated in part via specific members of the JAK/STAT pathway.