Critical role for STAT3 in murine pituitary adrenocorticotropin hormone leukemia inhibitory factor signaling

Leukemia inhibitory factor (LIF) is a pleiotropic neuroimmune cytokine that promotes corticotroph cell differentiation and induces proopiomelanocortin (POMC) mRNA expression and adrenocorticotropin hormone (ACTH) secretion. However, molecular mechanisms for this induction remain elusive. We therefore developed ACTH-secreting AtT20 transformants for wild-type or mutated STAT3, a cytokine signaling molecule, to address whether STAT3 is a determinant of LIF-mediated ACTH regulation. We show that these mutants act in a dominant negative manner by blocking endogenous STAT3 tyrosine phosphorylation or STAT3 DNA binding. Attenuation of STAT3 activity in the dominant negative AtT20 clones prevented LIF from promoting transcriptional activation of the POMC promoter (2.1-fold), whereas this LIF action was enhanced (7.7-fold; p < 0.05) in wild-type STAT3-overexpressing clones in comparison to mock-transfected cells (4.5-fold). However, wildtype or dominant negative STAT3-overexpressing clones showed comparable (4-fold) POMC induction after treatment with cyclic adenosine monophosphate (cAMP), an alternate inducer of POMC transcription, indicating the STAT3 specificity for LIF signaling. Moreover, dominant negative inactivation of STAT3 activity resulted in abrogation of LIF-induced POMC mRNA levels and ACTH secretion, confirming the in vivo role of STAT3 in LIF-mediated corticotroph action. Chemical or molecular blockade of the mitogen-activated protein kinase pathway did not affect LIF-mediated corticotroph function. These results indicate that STAT3 is a critical intrapituitary component of the LIF-mediated neuroimmunoendocrine interface in corticotroph cells.