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      Cellular Signaling Mechanisms for Stimulation of Growth Hormone Secretion and Growth Hormone Primary Transcripts by Immunosuppressant Agents, FK506 and Cyclosporin A, in Cultured Rat Pituitary Cells

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          Although an immunosuppressant, FK506, has been known to stimulate growth hormone (GH) release from rat somatotropes, the cellular signaling mechanism is unknown. In the present study, intracellular signaling pathways were investigated for FK506- and cyclosporin A (CsA)-induced GH release in cultured rat anterior pituitary cells. Northern and Western blot analysis revealed that the FK506-binding protein (FKBP12) and the CsA-binding protein (cyclophilin A) exist at the mRNA and protein level in the rat anterior pituitary tissue. FK506 and CsA increased GH release in a dose-dependent manner and inhibited calcineurin (CaN) activity in the cultured pituitary cells. The third immunosuppressant, rapamycin (RP), inhibited the FK506-induced GH release, although RP alone had no effect. Protein kinase A (PKA) inhibitors, H-89 and HA-1004 and EGTA blocked FK506- and CsA-induced GH release. TGF-β did not alter basal GH release, but inhibited FK506-induced GH release. GH primary transcripts were increased by FK506, and the effects were blocked by H-89 and HA-1004. These results suggest that the immunosuppressants, FK506 and CsA, stimulate GH release by inhibiting CaN activity which results in the activation of the PKA system in the rat somatotropes. TGF-β receptors might be involved in FK506-induced GH release as a separate pathway. FK506 also stimulates GH primary transcripts via a PKA-dependent mechanism in a manner similar to its effects on GH release.

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          Most cited references 9

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          Single-step method of RNA isolation by acid guanidinium thiocyanate-phenol-chloroform extraction.

          A new method of total RNA isolation by a single extraction with an acid guanidinium thiocyanate-phenol-chloroform mixture is described. The method provides a pure preparation of undegraded RNA in high yield and can be completed within 4 h. It is particularly useful for processing large numbers of samples and for isolation of RNA from minute quantities of cells or tissue samples.
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            A tissue-specific transcription factor containing a homeodomain specifies a pituitary phenotype.

            Multiple related cis-active elements required for cell-specific activation of the rat prolactin gene appear to bind a pituitary-specific positive transcription factor(s), referred to as Pit-1. DNA complementary to Pit-1 mRNA, cloned on the basis of specific binding to AT-rich cell-specific elements in the rat prolactin and growth hormone genes, encodes a 33 kd protein with significant similarity at its carboxyl terminus to the homeodomains encoded by Drosophila developmental genes. Pit-1 mRNA is expressed exclusively in the anterior pituitary gland in both somatotroph and lactotroph cell types, which produce growth hormone and prolactin, respectively. Pit-1 expression in heterologous cells (HeLa) selectively activates prolactin and growth hormone fusion gene expression, suggesting that Pit-1 is sufficient to confer a characteristic pituitary phenotype. The structure of Pit-1 and its recognition elements suggests that metazoan tissue phenotype is controlled by a family of transcription factors that bind to related cis-active elements and contain several highly conserved domains.
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              High brain densities of the immunophilin FKBP colocalized with calcineurin.

              The immunophilins cyclophilin and FK506 binding protein (FKBP) are small, predominantly soluble proteins that bind the immunosuppressant drugs cyclosporin A and FK506, respectively, with high affinity, and which seem to mediate their pharmacological actions. The Ca(2+)-dependent protein phosphatase, calcineurin, binds the cyclophilin-cyclosporin A and FKBP-FK506 complexes, indicating that calcineurin might mediate the actions of these drugs. A physiological role for the immunophilins in the nervous system is implied by a close homology between the structure of NINA A, a protein in the neural retina of Drosophila, and cyclophilin, as well as by the high density of FKBP messenger RNA in brain tissue. Here we report that the levels of FKBP and mRNA in rat brain are extraordinarily high and that their regional localization is virtually identical to that of calcineurin, indicating that there may be a physiological link between calcineurin and the immunophilins. We also show that at low concentrations FK506 and cyclosporin A enhance the phosphorylation of endogenous protein substrates in brain tissue and in intact PC12 cells, indicating that these drugs may inhibit phosphatase activity by interacting with the immunophilin-calcineurin complexes.

                Author and article information

                S. Karger AG
                December 1998
                16 October 1998
                : 5
                : 6
                : 309-317
                a First Department of Internal Medicine and b First Department of Physiology, Kagawa Medical University, Ikenobe, Japan
                26350 Neuroimmunomodulation 1998;5:309–317
                © 1998 S. Karger AG, Basel

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                Page count
                Figures: 10, References: 51, Pages: 9
                Original Paper


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