1,25-Dihydroxyvitamin D3 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)₂D₃ inhibit the proliferation of many cell types. We investigated the combined effect of 1,25(OH)₂D₃ and cAMP elevating agents on exponentially growing mouse 3T3 fibroblasts. The following cAMP elevating agents were used: theophylline and pentoxyfilline, which inhibit cAMP-dependent phosphodiesterase; prostaglandin E₂ which activates adenylate cyclase by a receptor-mediated mechanism; forskolin, which directly stimulates adenylate cyclase; and the cell permeable cAMP analogs 8-bromo cAMP and N⁶ benzoyl cAMP. 1,25(OH)₂D₃ and cAMP elevating agents were added to exponentially growing fibroblasts cultured in 96-well microtiter plates and cell number was monitored 3–7 d later. 1,25(OH)₂D₃ 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)₂D₃ and the cAMP elevating agents resulted in an antiproliferative effect that was more than additive. The synergistic interaction depended on the dose of 1,25(OH)₂D₃ and was apparent already at 10⁻⁸ M of the hormone. The specificity of the effect of 1,25(OH)₂D₃ was demonstrated by the finding that 24,25-dihydroxyvitamin D₃, 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)₂D₃ 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 degrading machinery. [ABSTRACT FROM AUTHOR]