Characterization of a complement-fragment-C5a-stimulated calcium-influx mechanism in U937 monocytic cells

Abstract

The mechanism by which complement fragment C5a elevates intracellular Ca2+ ([Ca2+]i) levels in two cell types, a monocytic cell line, U937, and neutrophils, has been investigated by the use of fluorometric and radiometric techniques. In U937 cells the influx of extracellular Ca2+ can be distinguished from the release of intracellular Ca2+ stores in terms of dose-responsiveness to C5a and sensitivity to pertussis-toxin poisoning. This suggests that the mechanism of Ca2+ influx in these cells is at least partially independent of both the production of inositol phosphates and elevation of internal Ca2+ concentration. The C5a-stimulated influx of 45Ca2+ into U937 cells is inhibited by a series of metal ions (Zn2+ > Co2+ > Mn2+ > Sr2+ approximately equal to Ni2+ > La3+). The stimulated influx of Ca2+ into neutrophils is inhibited differently (Ni2 >> Co2+ > Zn2+ approximately equal to La3+ > Mn2+ approximately equal to Sr2+), is less sensitive to C5a and both the influx of extracellular Ca2+ and the release of intracellular stores are equally sensitive to pertussis toxin treatment. Taken together these results indicate that [Ca2+]i is controlled in U937 monocytes by mechanisms distinct from those which appear to operate in other myeloid cells, such as neutrophils, stimulated with C5a and formylpeptide.

Differential activation of dibutyryl cAMP-differentiated HL-60 human leukemia cells by chemoattractants

“…4). In human neutrophils, C5a-induced Ca2+-influx is PTX-sensitive as well [31]. Taken together, these findings show that the coupling of C5a receptors to Gi-proteins and PTX-insensitive G-proteins in myeloid cells shows substantial cell type specificity.…”

“…Moreover, C5a activates phospholipase C through a partially PTX-insensitive mechanism in the monocytic cell line, THP-1, which expresses G~I 6 at high concentrations [30]. Furthermore, C5a mediates Ca 2÷ influx in BtacAMP-differentiated U937 cells through a PTX-insensitive mechanism [31]. Evidently, activation by C5a of phospholipase C via God6 or another PTX-insensitive G-protein of the Gq-family is not of relevance in Bt2cAMP-differentiated HL-60 cells, as C5a-induced Ca 2÷ mobilization was abolished by PTX (see Fig.…”

Histamine H1-receptors in HL-60 monocytes are coupled to Gi-proteins and pertussis toxin-insensitive G-proteins and mediate activation of Ca2+ influx without concomitant Ca2+ mobilization from intracellular stores

“…Moreover, complement component C3a, unlike complement component C5a, induces only Ca z+ influx but not Ca 2+ mobilization in human neutrophils (Norgauer et al 1993). Furthermore, complement C5a-induced Ca 2+ influx in dibutyryl cAMP-differentiated U937-cells apparently does not depend on prior emptying of intracellular Ca 2+ stores (Monk and Partridge 1993). Dissociations between receptor agonist-mediated Ca 2+ mobilization and Ca 2+ influx were also reported for rat pancreatic acinar cells (Dawra et al 1993) and rat thyroid cells (Aloj et al 1993).…”

Phagocytosing neutrophils down-regulate the expression of chemokine receptors CXCR1 and CXCR2

“…Nonopsonized S cerevisiae did not affect Ca ϩϩ signaling (not shown). In spite of transitory elevation of intracellular Ca ϩϩ by phagocytosis itself (Figure 2A-C), the responsiveness of OSC-treated PMNs to complement fragment C5a 19 at 100 ng/mL ( Figure 2C-D) and Ca-ionophore 20 at 1 M (not shown) was not suppressed, indicating that the capacity of phagocytosing neutrophils to increase cytoplasmic calcium was not impaired by phagocytosis itself. Interestingly, the expression of C5a receptors (CD88) 10 was reduced in phagocytosing PMNs by 38% Ϯ 17% (not shown, data of 7 experiments).…”