Complement C5a receptors in the pituitary gland: expression and function.

Communication between the immune and endocrine system is important for the control of inflammation that is primarily mediated through the hypothalamic-pituitary-adrenal axis. The innate immune system rapidly responds to pathogens by releasing complement proteins that include the anaphylatoxins C3a and C5a. We previously reported the existence of C3a receptors in the anterior pituitary gland and now describe the presence of C5a receptors in the gland. C5a and its less active derivative (C5adR) can bind to its own receptor and to another receptor called C5L2. Using RT-PCR and immunocytochemistry, C5a receptors and C5L2 were demonstrated in the rat anterior pituitary gland and in several rodent anterior pituitary cell lines. Western blotting analysis showed that C5a stimulated the phosphorylation of MAPK and AKT but not p38; C5adR on the other hand, had no effect on any of the signal molecules investigated. The effects of C5a and C5adR on the secretion of the inflammatory molecule, macrophage migration inhibitory factor (MIF) were investigated by ELISA. Both compounds showed a dose-dependent inhibition of MIF release, 30-40% inhibition at around 35-70 nM agonist with IC50 values of around 20 nM. C5a and C5adR also stimulated ACTH secretion (up to 25%) from AtT-20DV16 cells. These data show that functional C5a receptors (C5a and C5L2) are present in the anterior pituitary gland and they may play a role in dampening down inflammation by inhibiting the release of MIF and stimulating the release of ACTH.

Adenosine stimulates connexin 43 expression and gap junctional communication in pituitary folliculostellate cells.

Adenosine is known to stimulate interleukin (IL)-6 and vascular endothelial growth factor (VEGF) secretion from pituitary TtT/GF folliculostellate [corrected] (FS) cells indicating that it is an important paracrine regulator of anterior pituitary function. This study demonstrates that rodent anterior pituitary cell lines produce extracellular adenosine that is able to increase intercellular gap junction communication in FS cells. Ecto-5'-nucleotidase (CD73), the enzyme that generates adenosine from AMP, was demonstrated by immunocytochemistry in approximately 20% of anterior pituitary cells, and some of these cells colocalized with prolactin and growth hormone. CD73 mRNA and protein were detected in GH3 and MMQ (somatotroph-lactotroph lineages) and TtT/GF cells, and enzyme activity was demonstrated by the conversion of exogenously added fluorescent ethenoAMP to ethenoadenosine. Adenosine production, as measured by HPLC, was detected in GH3 (1 microM/h) and MMQ (3 microM/h) but not in TtT/GF cells. Adenosine (EC50: 0.5 microM) and NECA (universal adenosine receptor agonist; EC50 0.1 microM) stimulated connexin 43 (Cx43) mRNA and protein expression within 1-2 h in TtT/GF cells. Adenosine and NECA also stimulated gap junctional intercellular communication (as assessed by transmission of Alexa Fluor 488) by 6- to 8-fold in comparison with untreated TtT/GF cells. In cocultures of MMQ and TtT/GF cells, Cx43 expression in TtT/GF cells increased in proportion to the number of MMQ cells plated out. These data suggest that adenosine, formed locally in the anterior pituitary gland can stimulate gap junction communication in FS cells.

Complement C3a receptors in the pituitary gland: a novel pathway by which an innate immune molecule releases hormones involved in the control of inflammation.

Two-way communication exists between the endocrine and immune systems using molecules such as hormones and cytokines. Here we describe a new pathway by which C3a, a complement-derived cytokine, stimulates anterior pituitary hormone release and activates the hypothalamic-pituitary-adrenal axis, a reflex central to the stress response and to the control of inflammation. We show that C3a receptors are expressed in pituitary hormone secreting and non-hormone secreting (folliculostellate) cells and that both C3a and C3adesArg (a non-inflammatory metabolite) stimulate pituitary cell cultures to release prolactin, growth hormone, and adrenocorticotropin. Serum levels of these hormones, together with adrenal corticosterone, increase dose dependently with recombinant C3a and C3adesArg administration in vivo. Pertussis toxin blocks the response to C3a but not C3adesArg, which indicates the presence of two receptors, only one of which is coupled to Galphai-proteins. We propose that the complement innate immune molecules (cytokines) modulate tissue-specific and systemic inflammatory responses through communication with the endocrine pituitary gland.

Adenosine-induced IL-6 expression in pituitary folliculostellate cells is mediated via A2b adenosine receptors coupled to PKC and p38 MAPK.

Activation of adenosine receptors in folliculostellate (FS) cells of the pituitary gland leads to the secretion of IL-6 and vascular endothelial growth factor (VEGF). We investigated the action of adenosine A2 receptor agonists on IL-6 and VEGF secretion in two murine FS cell lines (TtT/GF and Tpit/F1), and demonstrated a rank order of potency, 5'-N-ethylcarboxamidoadenosine (NECA)>2-p-(2-carboxyethyl)phenethylamino-5'-N-ethylcarboxamidoadenosine>adenosine, suggesting mediation via the A2b receptor. NECA-mediated IL-6 release was inhibited by the PLC inhibitor 1-[6-((17beta-3-methoxyestra-1,3,5(10)-tiene-17-yl)amino)hexyl]-1H-pyrrole-2,5-dione, the PI3 kinase inhibitor wortmannin and the PKC inhibitors bisindolylmaleimide 1 and bisindolymaleimide X1 HCl (Ro-32-0432). NECA-mediated IL-6 release was attenuated (<50%) by the extracellular signal-regulated kinase MAPK inhibitor 2'-amino-3'-methoxyflavone, and completely (>95%) inhibited by the p38 MAPK inhibitor 4-(4-fluorophenyl)-2-(4-methylsulphinylphenyl)-5-(4-pyridyl)1H-imidazole. NECA stimulates p38 MAPK phosphorylation that is inhibited by Ro-32-0432 but not by wortmannin. Dexamethasone inhibits NECA-stimulated IL-6 and VEGF secretion. These findings indicate that adenosine can stimulate IL-6 secretion in FS cells via the A2b receptor coupled principally to PLC/PKC and p38 MAPK; such an action may be important in the modulation of inflammatory response processes in the pituitary gland.