MODULATION OF GH(4) CELL-CYCLE VIA A(1) ADENOSINE RECEPTORS

Identification and characterization of A(1) adenosine receptors (A(1)R s) in a tumor cell line derived from rat pituitary (GH(4) cells) was p erformed by ligand binding and immunocytochemistry. Subsequently, the involvement of A, Rs in the regulation of cell proliferation was studi ed in these cells. The agonist N-6-(R)-phenylisopropyladenosine (R-PIA ) did not modify the number of cultured cells, but it regulated the ki netics of the cell cycle. By means of experiments of pulse and of puls e and chase with bromodeoxyuridine and further labeling with Hoechst 3 3258, propidium iodide, and/or fluorescein-conjugated antibodies again st bromodeoxyuridine, it was demonstrated that R-PIA, via A(1)Rs, acce lerated progression from G(0)/G(1) to S phase and from S to G(2)/M pha se of the cell cycle, whereas the initiation of a new cycle occurred a t the same time in treated and untreated cells. As a consequence, R-PI A did not change the total length of the cycle. This is the first desc ription of cell cycle regulation without modification of cell prolifer ation. Although pertussis toxin blocked the R-PIA-induced inhibition o f cyclic AMP production in these cells, it did not affect the R-PIA ac tion on the cell cycle, In contrast, cholera toxin mimicked the action of R-PIA, Thus, it is likely that regulation of the cell cycle via A( 1)Rs is mediated by heterotrimeric G proteins different from those tha t mediate inhibition of adenylate cyclase. Due to the fact that cells in G(0)/G(1) phase were less susceptible to secretory signals, adenosi ne, in an autocrine manner and by regulating the cell cycle kinetics, may contribute to the modulation of the secretory capacity of pituitar y cells.