CAMP Neuropeptide Agonists Induce Pituitary Suppressor of Cytokine Signaling-3: Novel Negative Feedback Mechanism for Corticotroph Cytokine Action

Abstract

The hypothalamo-pituitary-adrenal (HPA) axis maintains a homeostatic response to stress, infection, or neoplasia. Inflammatory cytokines, including leukemia inhibitory factor (LIF), stimulate the HPA axis either directly at the pituitary corticotroph, or indirectly through induction of CRH or sympathetic noradrenergic neurons, and mediate the immuno-neuroendocrine interface. Unrestrained HPA axis activation leads, however, to immunosuppression. Because suppressor of cytokine signaling-3 (SOCS-3) is a potent inhibitor of LIF-activated HPA axis, and dynamic interactions between hypothalamus-derived cAMP-inducing neuropeptides and proinflammatory cytokines occur at the corticotroph level, we investigated SOCS-3 expression in response to peptides that stimulate cAMP including CRH, pituitary adenylate cyclase-activating polypeptide, and epinephrine. (Bu)2cAMP mediates induction of SOCS-3 promoter activity (6.7-fold ± 0.5, P < 0.001) and SOCS-3 gene expression (4-fold ± 0.8, P < 0.005) in a PKA-dependent manner. LIF and cAMP-inducing agents are additive on SOCS-3 promoter activity (22-fold ± 2.6, LIF + (Bu)2cAMP vs. 7.3-fold ± 0.6, LIF alone, P < 0.05) and on SOCS-3 transcription (11.3-fold ± 2.1, LIF + (Bu)2cAMP vs. 9.3-fold ± 1, LIF alone, P < 0.05), suggesting alternate pathways for LIF and cAMP-mediated corticotroph signaling. Similarly, LIF and CRH or pituitary adenylate cyclase-activating polypeptide are additive for SOCS-3 promoter activity and transcription (P < 0.05). Whereas signal transducer and activator of transcription 3 binding to the SOCS-3 promoter mediates LIF action, several SOCS-3 promoter regions containing cAMP-responsive elements are required for cAMP-PKA effect. Thus, both classes of POMC-inducing agents, cytokines as well as cAMP-inducing central peptides, regulate SOCS-3, providing a further level of negative HPA axis control during inflammation. These results indicate a sensitive intracellular autoregulation of corticotroph function.