Platelets stimulate proliferation of bone cells: involvement of platelet-derived growth factor, microparticles and membranes

Platelets have been implicated in accelerated bone regeneration in grafting applications. The beneficial effects of platelets may involve their ability to stimulate the proliferation of osteoblasts. We therefore determined the mitogenic response of human trabecular bone-derived cells to human platelets and supernatants of thrombin-activated platelets. We can show a similar to 50-fold increase in DNA-synthesis of bone cells (BC) cultured in the presence of platelets as determined by [H-3]-thymidine incorporation. Preventing cell-to-cell contact by a membrane filter did not abrogate the stimulatory effect, indicating the release of soluble factor(s) that are mitogenic for BC. The lipid fraction of the platelets had no effect on [H-3]-thymidine uptake into the DNA of BC. Platelet-released supernatant (PRS) increased the rate of [H-3]-thymidine incorporation to similar to 20-fold and retained 56% of their activity after incubation at 56 degreesC, and 27% at 100 degreesC, respectively. Neutralizing antibodies raised against platelet-derived growth factor (PDGF) partially suppressed the mitogenic potential of PRS. Gel exclusion chromatography analysis showed that molecules ranging from 25 kDa to more than 70 kDa within the PRS can stimulate BC proliferation. The highest amount of PDGF was detected in fractions corresponding to a molecular weight of 28-37 kDa as determined by immunoassay. The mitogenic activity was not restricted to soluble growth factors because microparticles in the PRS and platelet membranes also increased BC proliferation. Our data indicate that native platelets, the respective PRS, microparticles, and platelet membranes can stimulate the mitogenic activity of BC thereby contributing to the regeneration of mineralized tissue.