Cytokines & Cells Online Pathfinder Encyclopaedia
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abbr. PRL. Prolactin, known also as lactogenic hormone, lactotropin, lactotropic hormone, mammotropin, mammotropic hormone, luteotropic hormone (LTH), or luteotropin is a neuroendocrine pituitary hormone of 23 kDa. It is produced in increasing amounts during pregnancy and during suckling and acts primarily on the mammary gland by initiating and maintaining lactation in the postpartal phase. A potent inhibitor of basal secretion of prolactin by the pituitary is TGF-beta. Its secretion from the pituitary is stimulated by VIP (vasoactive intestinal peptide).
Prolactin has been shown also to have cytokine-like activities and to have important immunoregulatory activities. It contributes to the development of lymphoid tissues and the maintenance of physiological immune function and also modulates a variety of T-cell immune responses (see also: Dwarf mice). Hypophysectomy has been shown to lead to a regression of the thymus and a loss of immune competence which can be restored by treatment with prolactin.
In vitro studies suggest that lymphocytes are an important target tissue for circulating prolactin. Prolactin stimulates ornithine decarboxylase and activates protein kinase C, which are pivotal enzymes in the differentiation, proliferation, and function of lymphocytes. Prolactin antibodies inhibit the proliferation of lymphocytes. Prolactin induces IL2 receptors on the surface of lymphocytes and acts as a progression factor during the proliferation of lymphocytes stimulated by IL2, probably functioning in the nucleus without binding to its cell surface receptor. Some human B-lymphoblastoid cells lines have been reported to produce prolactin constitutively.
A prolactin-like molecule is synthesized and secreted by concanavalin A- or phytohemagglutinin-stimulated human peripheral blood mononuclear cells and functions in an autocrine manner as a growth factor for lymphoproliferation. A 29 kDa prolactin-like protein is secreted also as an autocrine growth modulator by Ramos Burkitt lymphoma cells during continuous serum-free growth.
In addition to triggering resting lymphocytes to cell division, the hormone can also control the magnitude of their response to polyclonal stimuli.
Prolactin promotes the proliferation of Nb2 pre-T-cell lymphoma cells. In these cells prolactin induces the biphasic expression of a transcription factor, IRF1 (interferon regulatory factor; see: IRS, interferon response sequence) and may be involved in Cell cycle activation and S phase progression. Prolactin can also protect Nb2 cells against glucocorticoid-receptor mediated induction of apoptosis.
Elevated levels of prolactin have been observed in patients during acute cardiac allograft rejection. It has been assumed that CsA (Cyclosporin A), which was used to suppress graft rejection, can act as an antagonist to prolactin binding to the prolactin receptor on lymphocytes.
Prolactin has been reported to activate cellular proliferation in nonreproductive tissue, such as liver, spleen, and thymus. It induces significant proliferation in aortic smooth muscle cells and also enhances proliferation of these cells induced by PDGF. Prolactin also appears to be directly mitogenic for pancreatic beta-cells. Prolactin is also mitogenic for cultured astrocytes.
Prolactin has been found to enhance superoxide anion generation and hydrogen peroxide release from murine macrophages. Growth hormone augments superoxide anion secretion of human neutrophils by binding to the prolactin receptor.
16K Prolactin is a 16 kDa aminoterminal fragment of prolactin formed by enzymatic cleavage of intact 23 kDa prolactin in the pituitary gland and in target tissues for prolactin. This fragment has been shown to inhibit the growth of bovine brain capillary endothelial cells, while intact prolactin is inactive. This activity is mediated by specific, high affinity, saturable binding sites (52 kDa, 32 kDa) for the 16 kDa protein which is distinct from the normal prolactin receptor. The prolactin cleavage product is a potent inhibitor of angiogenesis and has been termed vasoinhibin (see also: vasoinhibins); it inhibits bFGF induced cell division, migration, and organization of capillary endothelial cells. Treatment of bovine capillary endothelial cells with 16K Prolactin inhibits urokinase (urokinase type plasminogen activator, uPA) activity stimulated by bFGF. The activation of urokinase is an essential step in the regulation of angiogenesis since the enzyme activates a cascade of proteases that play essential roles in migration of endothelial cells and tissue remodeling. 16K Prolactin has been shown to be a potent mitogen for mammary epithelial cells that acts via prolactin receptors. Macotela et al (2000) have reported that 16K Prolactin is expressed by chondrocytes, which generate it by cleavage of prolactin, mediated by metalloproteinases such as cartilage-derived MMP-1, MMP-2, MMP-3, MMP-8, MMP-9 and MMP-13. They have also shown that 16K Prolactin inhibits bFGF induced endothelial cell proliferation in vitro.
For other proteins, or fragments thereof, with activities different from their established and well known biochemical functions see also: Dual identity proteins MiniCOPE Dictionary.
LAST MODIFIED: March 2000
See REFERENCES for entry Prolactin
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