Monoclonal antibody defines a macrophage intracellular Ca2+-binding protein which is phosphorylated by phagocytosis

Does protein kinase C control receptor-mediated phagocytosis in human neutrophils?

It was previously demonstrated that C3bi- but not IgG-mediated phagocytosis in human neutrophils is a Ca2+-independent process [(1985) Nature 315, 509-511]. The objective of this study was to elucidate the nature of an additional messenger signal(s) that may be involved in receptor-mediated phagocytosis in these cells. The C3bi- and IgG-mediated phagocytic ability of neutrophils was inhibited by forskolin, a potent activator of adenylate cyclase. It was found that forskolin induced a 3-fold increase in cAMP levels and simultaneously reduced the uptake of both C3bi- and IgG-opsonized particles by 65%. This inhibition was reversed by exposure to either phorbol myristate acetate (PMA) or diacylglycerol (OAG), which are both activators of protein kinase C, but not by the inactive analog 4 alpha-PMA. PMA could also restore the phagocytic ability of neutrophils with an experimentally impaired calcium response. These results suggest that activation of protein kinase C might be an important transducing signal controlling receptor-mediated phagocytosis in human neutrophils.

Cytoplasmic gels from macrophages. Evidence for the involvement of four proteins in the calcium-dependent solation of actin networks

A method has been devised to study the influence of Ca2+ on the in vitro formation of actin gel networks. Under appropriate conditions low-Ca2+ cytosolic extracts (less than 1 nM) from macrophages rapidly formed a macromolecular complex composed of actin, filamin, alpha-actinin and two new proteins of 70 kDa and 55 kDa. [Pacaud, M. (1986) Eur. J. Biochem. 156, 521-530]. Increasing concentrations of free Ca2+ to 1-2 microM resulted in complete inhibition of the association of 70-kDa protein, a protein which associates actin filaments into parallel arrays. Concentrations of Ca2+ greater than 3 microM caused incorporation of two additional proteins, gelsolin and a 18-kDa polypeptide, with no change in either the actin or alpha-actinin content of the cytoskeletal structures. Use of a polyacrylamide gel overlay technique with 125I-calmodulin revealed that a high-Mr calmodulin-binding protein analogous to spectrin was also associated with these structures when micromolar Ca2+ was present. Similar assays with 45CaCl2 indicated that the 70-kDa protein binds Ca2+ with high affinity. It is thus suggested that Ca2+ might regulate the dynamic assembly of microfilaments through several target proteins, gelsolin, the 70-kDa protein and calmodulin.

Synthesis of novel calcium-dependent proteins associated with mammary epithelial cell migration and differentiation

A class of proteins from mouse mammary epithelial cells has been isolated which, like the calcium-binding protein calmodulin (CaM), binds to phenothiazine in a calcium-dependent manner. These proteins do not bind to phenothiazine through binding to CaM; we infer that they are calcium-binding proteins, and that they may be related to the similarly isolated 'calcimedins' of Moore, P D & Dedman, J, J biol chem 257 (1982) 9663 [8]. In primary cultures of mouse mammary cells on collagen gels, synthesis of certain of these proteins is associated with the spreading of cells to form monolayers; failure of cells to spread and differentiate, through omission of serum from culture medium, results in the inhibition of calcium-binding protein synthesis, with the exception of CaM and a 15 kD species. The CaM/15 kD pair are prominent during all phases of culture, and are secreted during the secretory differentiation phase of culture (floating gels). We propose that these calcium-binding proteins play a specific role in the motility of mammary epithelial cells and that they may also be involved in mammary secretory differentiation.

The increase in intracellular free calcium associated with IgG gamma 2b/gamma 1 Fc receptor-ligand interactions: role in phagocytosis

The concentration of cytosolic free calcium, [Ca2+]i, was measured in J774 mouse macrophages by use of the fluorescent indicator quin-2. Resting [Ca2+]i was 87 nM. Addition of a number of specific ligands to the immunoglobulin gamma 2b/gamma 1 Fc receptor resulted in a transient increase in [Ca2+]i, the magnitude of which depended on the extent of receptor aggregation. Monovalent ligands gave only a small Ca2+ signal but blocked cell response to subsequent addition of multivalent ligands. Incubation with antibody-coated erythrocytes raised macrophage [Ca2+]i to micromolar levels. [Ca2+]i changes were only partially inhibited by the absence of external Ca2+, suggesting the release of Ca2+ from internal stores in addition to an influx of external Ca2+. These internal stores were not limited to mitochondria. An optimal range of [Ca2+]i was required for phagocytosis. Buffering [Ca2+]i with quin-2 and treating cells with quinine in the absence of external Ca2+ resulted in inhibition of phagocytosis. Increasing [Ca2+]i to micromolar levels with the calcium ionophore A23187 also resulted in similar inhibitory effects. We suggest the involvement of localized cytosolic Ca2+ gradients in generating the signals necessary for phagocytosis.