1,25-DIHYDROXYVITAMIN-D-3 AND AGENTS THAT INCREASE INTRACELLULAR ADENOSINE-3',5'-MONOPHOSPHATE SYNERGISTICALLY INHIBIT FIBROBLAST PROLIFERATION

Agents that increase intracellular cAMP (cAMP elevating agents) and 1, 25(OH)(2)D-3 inhibit the proliferation of many cell types. We investig ated the combined effect of 1,25(OH)(2)D-3 and cAMP elevating agents o n exponentially growing mouse 3T3 fibroblasts. The following cAMP elev ating agents were used: theophylline and pentoxyfilline, which inhibit cAMP-dependent phosphodiesterase; prostaglandin E-2 which activates a denylate cyclase by a receptor-mediated mechanism; forskolin, which di rectly stimulates adenylate cyclase; and the cell permeable cAMP analo gs 8-bromo cAMP and N-6 benzoyl cAMP. 1,25(OH)(2)D-3 and cAMP elevatin g agents were added to exponentially growing fibroblasts cultured in 9 6-well microtiter plates and cell number was monitored 3-7 d later. 1, 25(OH)(2)D-3 and the cAMP elevating agents as single agents inhibited the growth of the 3T3 cells. The combined treatment of the fibroblasts with 1,25(OH)(2)D-3 and the cAMP elevating agents resulted in an anti proliferative effect that was more than additive. The synergistic inte raction depended on the dose of 1,25(OH)(2)D-3 and was apparent alread y at 10(-8) M of the hormone. The specificity of the effect of 1,25(OH )(2)D-3 was demonstrated by the finding that 24,25-dihydroxyvitamin D- 3, a vitamin D metabolite with low affinity for the vitamin D receptor , did not affect the antiproliferative effect of cAMP elevating agents . From the synergistic interaction between 1,25(OH)(2)D-3 and the cell permeable cAMP analogs, we infer that the site of interaction between the two signaling pathways is distal to the cAMP generating and degra ding machinery.