The ribosomal S6 kinases, cAMP-responsive element-binding, and STAT3 proteins are regulated by different leukemia inhibitory factor signaling pathways in mouse embryonic stem cells

Mouse embryonic stem (ES) cells remain "pluripotent" in vitro in the contin uous presence of leukemia inhibitory factor (LIF). In the absence of LIF, E S cells are irreversibly committed to differentiate into various lineages. In this study we have set up an in vitro assay based on the anti-apoptotic activity of LIF to distinguish pluripotent from "differentiation-committed" ES cells. We have examined the phosphorylation profiles of known (STAT3 an d ERKs) and identified new (ribosomal S6 kinases (RSKs) and cAMP-responsive element-binding protein (CREB)) LIF-regulated targets in ES and in ES-deri ved neuronal cells. We have demonstrated that although STAT3, a crucial pla yer in the maintenance of ES cell pluripotency, is induced by LIF in all ce ll types tested, the LIF-dependent activation of RSKs is restricted to ES c ells. We have shown that LIF-induced phosphorylation of RSKs in ES cells is dependent on ERKs, whereas STAT3 phosphorylation is not mediated by any kn own MAPK activities. Our results also demonstrate that the LIF-dependent ph osphorylation of CREB is partially under the control of the RSK2 kinase.