Molecular mechanism of basic calcium phosphate crystal-induced mitogenesis. Role of protein kinase C

Synovial tissue hyperplasia in basic calcium phosphate deposition disease has been suggested to develop through the stimulation of cell growth by basic calcium phosphate (BCP) crystals deposited in joints. These crystals have been used in vitro to stimulate DNA synthesis in quiescent fibroblasts to experimentally study this proliferative disease. The stimulation of DNA synthesis, in density-arrested Balb/c 3T3 cells, by BCP crystals was inhibited after down-regulating protein kinase C activity with 12-O-tetradecanoyl-phorbol 13-acetate (TPA). No effect on platelet-derived growth factor (PDGF)-stimulated DNA synthesis was observed under the same conditions. The expression of c-myc and c-fos increased in response to BCP stimulation in a manner similar to the increase produced by stimulation with PDGF. The BCP stimulation of c-fos and c-myc messages was inhibited 60 and 90%, respectively, in TPA-pretreated, protein kinase C-down-regulated cells. The induction of these transcripts by PDGF was unaffected in cells pretreated with TPA. TPA was unable to stimulate c-fos and c-myc expression or DNA synthesis following protein kinase C down-regulation. Both PDGF and TPA stimulated phosphorylation of an 80-kDa protein, whereas BCP crystals had no effect on phosphorylation of this protein. The exposure of density-arrested Balb/c 3T3 cells to BCP crystals had no effect on high affinity epidermal growth factor receptor binding under conditions in which PDGF and TPA reduced epidermal growth factor binding. The data suggest that PDGF can act to stimulate c-fos and c-myc expression as well as DNA synthesis through a protein kinase C-independent pathway, whereas BCP crystals require at least endogenous levels of protein kinase C to stimulate these events.