Leukemia inhibitory factor (LIF) promotes differentiated cell function in several systems. We recently reported LIF and LIF receptor expression in human fetal pituitary corticotrophs in vivo and demonstrated LIF stimulation of adrenocorticotrophin (ACTH) transcription in vitro, suggesting a role for LIF in corticotroph development. We therefore assessed the action of LIF on proliferating murine corticotroph cells (AtT20). LIF impairs proliferation of AtT20 cells (25% reduction versus control, P < 0.03), while simultaneously enhancing ACTH secretion (2-fold, P < 0.001) and augmenting ACTH responsiveness to corticotrophin-releasing hormone (CRH) action (4-fold, P < 0.001). This attenuation of cell growth is due to a block of cell cycle progression from G, into S phase, as measured by flow cytometric analysis (24 + 0.8 versus 11.57 + 1.5, P < 0.001). Using bromodeoxyuridine incorporation assays, loss of cells in S phase was confirmed (25 + 0.08 to 9.4 ± 1.4, P < 0.008). In contrast, CRH induced the G2/M phase (3.6 ± 0.2 to 15.4 + 39 P < 0.001). This effect was blunted by LIF (P < 0.001 versus CRH alone). Cyclin A mRNA levels, which decline in S phase, were stimulated 3.5-fold by LIF and markedly suppressed by CRH. These results indicate a LIF-induced cell cycle block occurring at G1/S in corticotroph cells. Thus, LIF reduces proliferation, enhances ACTH secretion, and potentiates effects of CRH on ACTH secretion while blocking effects of CRH on the cell cycle. Responses of these three markers of differentiated corticotroph function indicate LIF to be a differentiation factor for pituitary corticotroph cells by preferential phenotypic switching from proliferative to synthetic.Several cytokines are important modulators of neuroendocrine functions and putative mediators of the immune and neuroendocrine system interaction (1-3). One of these intercellular messengers, leukemia inhibitory factor (LIF), was originally isolated as an inhibitor of mouse Ml myeloid leukemia cells (4) and subsequently isolated from bovine pituitary conditioned medium (5). LIF exerts pleiotropic effects on diverse tissues, either inhibiting differentiation and maintaining the developmental potential of embryonic stem cells (6) or stimulating proliferation of human erythroleukemia TF-1 cells (7). In the nervous system, LIF induces cholinergic switching of sympathetic neurons both in vivo and in vitro (8). LIF acts through a specific receptor subunit forming a heterodimeric complex with gpl3O (9-10), a signal transduction molecule shared with oncostatin M (OSM) and interleukin 6.Corticotrophin-releasing hormone (CRH) is an important regulator of the hypothalamic-pituitary-adrenal axis during development and also in response to stress (11)(12) Culture Collection were grown as described (18). 3T3 F442A fibroblasts were cultivated as described (19). Mouse pituitary glands were obtained from B6SJ1 mice within minutes of decapitation (17). Materials and reagents were obtained from Sigma; LIF and OSM were purchased from R&D; CRH was obtained f...

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