VASOACTIVE DIADENOSINE POLYPHOSPHATES PROMOTE GROWTH OF CULTURED RENAL MESANGIAL CELLS

Diadenosine polyphosphates (diadenosine triphosphate, Ap3A; diadenosine tetraphosphate, Ap4A; diadenosine pentaphosphate, Ap5A; diadenosine hexaphosphate, Ap6A) are potent vasoactive molecules stored and released by platelets. We examined whether these dinucleotides might contribute to the glomerular inflammatory response by stimulating the proliferation of mesangial cells. In cultured rat mesangial cells all four tested dinucleotides (10 to 100 mu mol/L) significantly stimulated DNA synthesis as measured by [H-3]thymidine uptake at 48 hours (x-fold increase compared with unstimulated control cells: Ap3A, 1.5; Ap4A, 1.8; Ap5A, 1.6; Ap6A, 1.6). In combination with the platelet products platelet-derived growth factor, epidermal growth factor, and serotonin, the dinucleotides synergistically increased DNA synthesis. Dinucleotides by themselves increased cell counts by 23% to 43% at day 2 and augmented mesangial cell growth induced by platelet-derived growth factor, epidermal growth factor, and serotonin. Furthermore, dinucleotides (100 mu mol/L) rapidly induced a modest increase in expression of the early growth response gene Egr-1 at 30 minutes (x-fold increase over baseline control: Ap3A, 1.9; Ap4A, 2.8; Ap5A, 2.2; Ap6A, 2.1). We found that extracellular Ap4A was metabolized by mesangial cell ectoenzymes to mononucleotides and adenosine, which also have been shown to be mitogenic for mesangial cells. The combination of Ap4A with mononucleotides or adenosine failed to cause additive stimulation of DNA synthesis in mesangial cells. We conclude that diadenosine polyphosphates stimulate proliferation of cultured mesangial cells and augment mesangial cell growth induced by other mitogens released from platelets. Different molecular mechanisms may be involved in dinucleotide-induced mitogenesis of mesangial cells. Direct effects of dinucleotides on cultured mesangial cells appear to play a role because dinucleotides rapidly caused activation of Egr-1. On the other hand, independent or additional effects of the mitogenic metabolites ATP, ADP, AMP, and adenosine may contribute to dinucleotide-induced mesangial cell replication.