Prolactin (PL) is a mammotropic neuropeptide produced by the pituitary and extrapituitary cells existing as several isoforms and belongs to the growth and lactogenic hormone family, which includes growth hormone and placental lactogens. The secretion of pituitary PL is under hypothalamic control. The cytokines IL-1, IL-2, and IL-6 also stimulate production, while IFN-gamma and endothelin-3 are inhibitory. PL exerts its effects via PL receptors (PLr) which exist as three isoforms. PL regulates reproduction, osmoregulation, and behavior and has potent immunomodulatory effects. PL is structurally related to members of the cytokine/hematopoietic family such as erythropoietin, GM-CSF, growth hormone, and IL-2 to IL-7. The PLr is a member of the cytokine/hematopoietic receptor family. Interaction of PL with PLr activates the Jak kinases which phosphorylate latent STAT proteins resulting in the activation of transcription. PL counteracts the effects of corticosteroids by enhancing Th1 cellular responses. Perturbations of PL physiology have significant immunologic effects. Hypoprolactinemia impairs immune function while hyperprolactinemia enhances active systemic lupus erythematosus, Reiter's disease, juvenile and adult rheumatoid arthritis (RA), autoimmune thyroiditis, multiple sclerosis, and cardiac allograft rejection. PL gene polymorphisms have now been shown to be linked to RA. Thus, manipulation of PL may have significant clinical utility. Further study of the relationship of the PL/PLr complex, other hormones, and the immune system will provide further insights into the potential therapeutic utility of this complex in immune diseases.
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