Activation of the C3b Receptor to Mediate Phagocytosis in Human Polyphonuclear Leukocytes: Role of Protein Kinase C and Calcium Flux
Siwik, Steven Anthony
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Polymorphonuclear neutrophilic leukoctes are one class of white blood cells that function to phagocytise antigenic material. In humans, this process can be mediated by two distinct classes of proteinaceous cell surface receptors: receptors for the Fc portion of immunoglobulin and receptors for fragments of the third complement component, C3. While Fc receptors constitutively mediate phagocytosis of IgG-coated particles, C3 receptors are under regulatory control. Purtubation of neutrophil membranes with tumor-promoting phorbol ester induces a functional alteration of complement receptors rendering them capable of efficiently mediating phagocytosis. Since phagocytosis is a cytoskeletal dependent process, some mechanism of signal transmission from receptors to elements of the cytoskeleton is suggested. Recently, investigation of a group of membrane phospholipids has given considerable insight into a potential mechanism by which the type one complement receptor, termed CR1, is activated to mediate phagocytosis. This system, the polyphosphoinositide system, is believed to transduce signals for a variety of receptor systems. Turnover of the phosphoinositols releases two agents, diacylglycerol and inositol trisphosphate, both of which are now known to function as second messengers. Under physiologic conditions, diacylglycerol functions in the plane of the membrane and directly activates the enzyme protein kinase C, while inositol trisphosphate induces calcium flux from the smooth endoplasmic reticulum. The current study has been designed to elucidate, in part, the nature of a signal transducing mechanism for CR1 mediated phagocytosis as influenced by turnover of the polyphosphoinositides. Effects of the known protein kinase C activating agents sn-1,2-dioctanoylglycerol and sn-I-oleoyl-4-acetoylqlycerol were studied for their capacity to induce CR1 internalization. Additionally, since protein kinase C is a calcium dependent enzyme, the effect of calcium flux was investigated. These data suggest that complement receptors are activated to mediate phagocytosis via action of protein kinase C.