Adenosine stimulation of DNA synthesis in mammary epithelial cells

Adenosine increased the DNA synthesis rate and the percentage of S-phase cells 2-3-fold in mouse mammary epithelial cells (NMuMG), with an optimum concentration of 10-100 microM. This effect was not mimicked by adenosine metabolites adenine, hypoxanthine, or inosine. N-ethylcarboxamidoadenosine (NECA, a relatively nonselective adenosine receptor agonist) and 2-p-(2-carboxyethyl) phenethylamino-5'-N ethylcarbox-amidoadenosine (CGS-21680, an A2 selective agonist) also increased DNA synthesis by mammary epithelial cells. However, N6-cyclohexyladenosine (CHA, an agonist for A1 type receptors) decreased DNA synthesis. The A1 selective antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX) had no effect on basal or adenosine-induced DNA synthesis, whereas the A2 selective antagonist 3,7-dimethyl-1-propargylxanthine (DMPX) decreased adenosine-induced DNA synthesis. Similar effects were observed in another nontumorigenic mouse mammary epithelial line, HC11, as well as the nontumorigenic human lines MCF-10A and 184.A1. Binding studies indicated that NMuMG cells contained approximately 3200 A1 receptors and about 5300 A2 receptors per cell. Both CGS-21680 and CHA increased GTPase activity in isolated cell membranes, whereas only CGS-21680 increased activity of adenylyl cyclase. Adenosine and CGS-21680 increased expression of a cyclic AMP responsive reporter gene. In addition, the protein kinase A inhibitor H-89 blocked the ability of adenosine and CGS-21680 to induce DNA synthesis, but did not affect EGF-induced DNA synthesis. These results indicate that adenosine appears to be a possible growth promoting agent in mammary tissue, and this effect may be mediated by extracellular receptors of the A2 type.