Thrombin signalling in U937 human monocytic cells is coupled to inositol phosphate formation but not to thromboxane B2 synthesis nor to inhibition of adenylate cyclase: distinct differences in thrombin signalling between U937 cells and platelets

Involvement of proteinase-activated receptors 1 and 2 in spreading and phagocytosis by murine adherent peritoneal cells: modulation by the C-terminal of S100A9 protein

Proteinase-activated receptors (PAR) are widely recognized for their modulatory properties in inflammatory and immune responses; however, their direct role on phagocyte effector functions remains unknown. S100A9, a protein secreted during inflammatory responses, deactivates activated peritoneal macrophages, and its C-terminal portion inhibits spreading and phagocytosis of adherent peritoneal cells. Herein, the effect of PAR1 and PAR2 agonists was investigated on spreading and phagocytosis by adherent peritoneal cells, as well as the ability of murine C-terminal of S100A9 peptide (mS100A9p) to modulate this effect. Adherent peritoneal cells obtained from mouse abdominal cavity were incubated with PAR1 and PAR2 agonists and spreading and phagocytosis of Candida albicans particles were evaluated. PAR1 agonists increased both the spreading and the phagocytic activity, but PAR2 agonists only increased the spreading index. mS100A9p reverted both the increased spreading and phagocytosis induced by PAR1 agonists, but no interference in the increased spreading induced by PAR2 agonists was noticed. The shorter homologue peptide to the C-terminal of mS100A9p, corresponding to the H(92)-E(97) region, also reverted the increased spreading and phagocytosis induced by PAR1 agonists. These findings show that proteinase-activated receptors have an important role for spreading and phagocytosis of adherent peritoneal cells, and that the peptide corresponding to the C-terminal of S100A9 protein is a remarkable candidate for use as a novel compound to modulate PAR1 function.

Subtypes of inositol 1,4,5-trisphosphate receptor in human hematopoietic cell lines: dynamic aspects of their cell-type specific expression

Inositol 1,4,5-trisphosphate (IP3)-mediated Ca2+ signaling plays important roles in cellular responses to extracellular stimuli. We recently succeeded in cloning human counterparts of the three subtypes derived from separate genes. Using the cDNA sequences type-specific to these subtype receptors, we here analyzed the expression profile of IP3R subtypes in stimulated and unstimulated human hematopoietic cell lines representing T cells, B cells, neutrophils, macrophages, erythrocytes and megakaryocytes. Northern and dot blot analysis showed that each IP3R subtype is expressed differently in these cells and that the expression profile in each cell is dynamically changed upon stimuli which induce differentiation. Moreover, most of these cells were found to simultaneously express at least two different subtype receptors.

The N-terminal thrombin receptor fragment SFLLRN, but not catalytically inactive thrombin-derived agonists, activate U937 human monocytic cells: evidence for receptor hydrolysis in thrombin-dependent signalling

It has previously been reported that murine macrophages can respond chemotactically and mitogenically to the serine proteinase thrombin. There is a similar response in these macrophages to catalytically inactivated thrombin or to peptide fragments of the thrombin B-chain [Bar-Shavit, Kahn, Mann and Wilner (1986) Proc. Natl. Acad. Sci. U.S.A. 83, 976-980]. However, the existence of a non-proteolytic mechanism of thrombin receptor activation in mononuclear cells was not evident in the present study using U937 human monocytic cells. The ability of thrombin to stimulate intracellular Ca2+ mobilization, actin polymerization or cell proliferation was not mimicked by N alpha-tosyl-L-lysine chloromethyl ketone (TLCK)-treated thrombin or by a synthetic 14-amino-acid peptide (single amino acid letter code YPPWNKNFTENDLL) corresponding to a part of the B-chain of thrombin which was reported to be mitogenic in murine macrophages. Evidence was obtained, however, in U937 cells for the presence of proteolytic-dependent thrombin receptor similar to the thrombin receptor expressed in platelets, which following thrombin cleavage exposes a new N-terminal tethered ligand. In support of this, a thrombin-receptor-derived hexapeptide (TRP; sequence SFLLRN), corresponding to a part of the thrombin receptor tethered ligand, mimicked all the actions of thrombin in U937 cells. Further, TRP and thrombin cross-desensitized U937 cells to subsequent stimulation with either TRP or thrombin, suggesting that TRP acted through the same U937 cell surface receptor as did thrombin. Thrombin activation of U937 monocytic cells can therefore be accounted for entirely by a proteolytic mechanism of thrombin receptor activation.

Thrombin promotes actin polymerization in U937 human monocyte-macrophage cells. Analysis of the signalling mechanisms mediating actin polymerization

The U937 human monocyte-macrophage cell line was used to examine the effect of thrombin, an ill-defined chemoattractant, on the polymerization of actin, a process essential for cell motility. In differentiated macrophage-like U937 cells, thrombin (0.5-50 units/ml) caused a rapid dose-dependent increase in the formation of filamentous (F-) actin, detected by the staining of F-actin with the fluorescent toxin, 7-nitrobenz-2-oxa-1,3-diazole-phallacidin. In contrast with other chemoattractants such as N-formylmethionyl-leucylphenylalanine or C5a, actin polymerization in response to thrombin occurred via a pertussis-toxin-insensitive G1-(inhibitory G-protein) independent signalling pathway. Further, this response was not affected by the Ca2+ chelator EGTA or by the specific protein kinase C (PKC) inhibitor RO-31-8220. The response to thrombin was not mimicked by the Ca2+ ionophore ionomycin or by the direct PKC activator phorbol 12-myristate 13-acetate. The thrombin response was, however, inhibited by the non-specific protein kinase inhibitor staurosporine. The present results suggest that in U937 cells thrombin stimulates the formation of F-actin via a signalling pathway independent of (i) the activation of PKC, (ii) the mobilization of intracellular Ca2+ and (iii) the activation of Ca(2+)-dependent protein kinases, but dependent on the activation of an undefined staurosporine-sensitive protein kinase.