Extracellular calcium and platelet-derived growth factor promote receptor-mediated chemotaxis in osteoblasts through different signaling pathways.
The discovery of a calcium receptor has stimulated interest in the signaling events underlying extracellular calcium ([Ca2+]o)-induced cell-specific responses. In osteoblasts, elevated levels of extracellular calcium mediate both mitogenesis and chemotaxis. Here we provide evidence that [Ca2+]o-stimulated chemotaxis of MC3T3-E1 osteoblast-like cells involves a G-protein-linked calcium-sensing receptor. [Ca2+]o promotes chemotaxis in a concentration-dependent manner. Pertussis toxin blocked almost all of [Ca2+]o-stimulated chemotaxis but had only a small effect on platelet-derived growth factor (PDGF)-stimulated chemotaxis. Consistent with the signaling model for PDGF-mediated chemotaxis, activation of phospholipase C played a critical role in [Ca2+]o-initiated chemotaxis: U-73122, an inhibitor of the activation of phospholipase C, blocked approximately 50% of PDGF-stimulated chemotaxis but blocked nearly all of the [Ca2+]o-stimulated chemotaxis. Down-regulation of protein kinase C also blocked about 50% of PDGF-stimulated chemotaxis but did not block [Ca2+]o-stimulated chemotaxis. Thus, unlike PDGF-mediated chemotaxis, chemotaxis stimulated by [Ca2+]o does not appear to require protein kinase C activation. This finding suggests events downstream of inositol 1,4,5-trisphosphate production rather than diacylglycerol production are critical to [Ca2+]o-promoted chemotaxis of MC3T3-E1 cells. The signal transduction mechanism underlying PDGF-induced chemotaxis involves the activation of phosphoinositide 3-kinase, as judged by the in vivo production of phosphatidylinositol 3,4-diphosphate and 3,4,5-trisphosphate and the partial sensitivity of chemotaxis to wortmannin, an inhibitor of phosphoinositide 3-kinase. In contrast, [Ca2+]o-stimulated chemotaxis was not blocked by wortmannin and elevations in [Ca2+]o did not increase the production of lipid products of phosphoinositide 3-kinase. Overall, [Ca2+]o-promoted chemotaxis of osteoblasts appears to utilize a unique signaling mechanism via a calcium-sensing receptor.
Godwin
S L
SL
Division of Signal Transduction, Beth Israel Deaconness Medical Center, Department of Medicine, Harvard Institutes of Medicine, Boston, Massachusetts 02115, USA.
Soltoff
S P
SP
D000818:Animals; D002118:Calcium / Q000494:pharmacology*; D002633:Chemotaxis / Q000187:drug effects*; D002999:Clone Cells; D004305:Dose-Response Relationship, Drug; D004789:Enzyme Activation; D051379:Mice; D010006:Osteoblasts / Q000502:physiology*; D019869:Phosphatidylinositol 3-Kinases; D017853:Phosphotransferases (Alcohol Group Acceptor) / Q000378:metabolism; D010982:Platelet-Derived Growth Factor / Q000494:pharmacology*; D020794:Receptor Protein-Tyrosine Kinases / Q000378:metabolism; D017479:Receptors, Platelet-Derived Growth Factor / Q000378:metabolism*; D015398:Signal Transduction; D010738:Type C Phospholipases / Q000378:metabolism
D010982:Platelet-Derived Growth Factor; D019869:Phosphatidylinositol 3-Kinases; D017853:Phosphotransferases (Alcohol Group Acceptor); D020794:Receptor Protein-Tyrosine Kinases; D017479:Receptors, Platelet-Derived Growth Factor; D010738:Type C Phospholipases; D002118:Calcium