Adenosine-Regulated Cell Proliferation in Pituitary Folliculostellate and Endocrine Cells: Differential Roles for the A1 and A2B Adenosine Receptors

Abstract

Abstract A1 and A2 adenosine receptors have been identified in the pituitary gland, but the cell type(s) on which they are located and their effects on pituitary cell growth are not known. Therefore, we analyzed the expression of A1 and A2 receptors in primary rat anterior pituitary cells, two pituitary folliculostellate (TtT/GF and Tpit/F1) and two pituitary endocrine (GH3 and AtT20) cell lines, and compared their effects on cell proliferation. In anterior pituitary and folliculostellate cells, adenosine and adenosine receptor agonists (5′-N-ethylcarboxamidoadenosine, a universal agonist, and CGS 21680, an A2A receptor agonist) stimulated cAMP levels with a rank order of potency that indicates the presence of functional A2B receptors. This stimulation, however, was not observed in either GH3 or AtT20 cells, where adenosine and the A1 receptor agonist 2-chloro-N6-cyclopentyladenosine inhibited VIP/forskolin-stimulated cAMP production. Expression of A2B and A1 receptors in the folliculostellate cells and that of the A1 receptor in the endocrine cells were confirmed by RT-PCR, immunocytochemistry, and ligand binding. Adenosine and 5′-N-ethylcarboxamidoadenosine dose-dependently (10 nm to 10 μm) stimulated growth in the folliculostellate, but not in the endocrine, cells, whereas in the latter, 100 μm adenosine and 2-chloro-N6-cyclopentyladenosine inhibited cell proliferation by slowing cell cycle progression. These data highlight the differential expression of A1 and A2B adenosine receptors in pituitary cells and provide evidence for opposing effects of adenosine on pituitary folliculostellate and endocrine cell growth.