Expression of C5a anaphylatoxin receptor in monoblastic cells involves facilitation of an adenosine...

We have previously reported a synergistic effect of 1,25-dihydroxyvitamin D [1,25-(OH)2D] and agents that elevate intracellular cAMP to induce the expression of the C5a complement receptor in U937 cells. In this report we examine the mechanism of this synergy, considering the hypothesis that the steroid hormone works by facilitating what is ultimately a cAMP-dependent process. We show that U937 cells cultured with 1,25-(OH)2D alone before culture with prostaglandin (PGE2) alone will express C5a receptor (an average of 55 +/- 4% of the receptors expressed with continuous exposure of cells to both agents; P less than 0.05). The reverse, PGE2 followed by 1,25-(OH)2D, causes very little receptor induction. This demonstrates the ability of 1,25-(OH)2D to induce changes in the state of the cell, such that activation of cAMP-dependent protein kinase has effects that are otherwise not seen, in other words 1,25-(OH)2D can prime the cell for the subsequent action of the cAMP messenger system. Furthermore, we are able to substitute, during the priming period, the protein synthesis inhibitor cycloheximide (CHX) for 1,25-(OH)2D. Cells cultured for 24 h with CHX will express C5a receptor when cultured for a second 2-day period with PGE2 at about 77 +/- 7% of the amount obtained with simultaneous exposure to 1,25-(OH)2D and PGE2. The CHX effect is time dependent and visible after 2 h. CHX is not synergistic with 1,25-(OH)2D. Other agents that can also substitute for 1,25-(OH)2D, but not for cAMP, in facilitating C5a receptor expression include retinoic acid and ionomycin, but with less potency. The 1,25-(OH)2D and PGE2 synergy is sensitive to the presence of isobutylmethylxanthine, implicating its dependence on the maintained elevation of intracellular cAMP levels. The synergy does not appear to be sensitive to changes in extracellular or intracellular calcium. We conclude from these results that 1,25-(OH)2D may promote the expression of C5a receptor in these cells in a fashion similar to that by which CHX potentiates other genes, i.e. that 1,25-(OH)2D increases levels of the mRNA encoding the C5a receptor. The mechanism of cAMP's subsequent and necessary action in the induction of C5a receptor expression is not yet clear.