fMLP-induced arachidonic acid release in db-cAMP-differentiated HL-60 cells is independent of phosphatidylinositol-4, 5-bisphosphate-specific phospholipase C activation and cytosolic phospholipase A(2) activation

Functional role of phosphatidylcholine-specific phospholipase C in regulating leukotriene synthesis and degranulation in human eosinophils

Phosphatidylinositol-specific phospholipase C (PI-PLC) and cytosolic phospholipase A₂ (cPLA₂) regulate both eosinophil degranulation and leukotriene (LT) synthesis via PI-PLC-mediated calcium influx and cPLA₂ activation. Phosphatidylcholine-specific phospholipase C (PC-PLC) likely plays a key role in cellular signaling, including the eosinophilic allergic inflammatory response. This study examined the role of PC-PLC in eosinophil LT synthesis and degranulation using tricyclodecan-9-yl-xanthogenate (D609), a PC-specific PLC inhibitor. D609 inhibited N-formyl-met-leu-phe + cytochalasin B (fMLP/B)-induced arachidonic acid (AA) release and leukotriene C₄ (LTC₄) secretion. However, at concentrations that blocked both AA release and LTC₄ secretion, D609 had no significant inhibitory effect on stimulated cPLA₂ activity. D609 also partially blocked fMLP/B-induced calcium influx, indicating that inhibition of AA release and LTC₄ secretion by D609 is due to inhibition of calcium-mediated cPLA₂ translocation to intracellular membranes, not inhibition of cPLA₂ activity. In addition, D609 inhibited fMLP/B-stimulated eosinophil peroxidase release, indicating that PC-PLC regulates fMLP/B-induced eosinophil degranulation by increasing the intracellular calcium concentration ([Ca²⁺]_i). Overall, our results showed that PC-PLC is critical for fMLP/B-stimulated eosinophil LT synthesis and degranulation. In addition, degranulation requires calcium influx, while PC-PLC regulates LTC₄ synthesis through calcium-mediated cPLA₂ activation.

Receptor Ck-dependent signaling regulates hTERT gene transcription

BACKGROUND

Available evidence suggests that the regulation of telomerase activity primarily depends on the transcriptional control of the human telomerase reverse transcriptase (hTERT) gene. Although several activators and repressors of hTERT gene transcription have been identified, the exact mechanism by which hTERT transcription is repressed in normal cells and activated in cancer cells remains largely unknown. In an attempt to identify possible novel mechanisms involved in the regulation of hTERT transcription, the present study examined the role of Receptor Ck, a cell surface receptor specific for cholesterol, in the transcription of hTERT gene in normal human peripheral blood mononuclear cells.

RESULTS

Activated Receptor Ck was found to down-regulate hTERT mRNA expression by repressing the transcription of c-myc gene. Receptor Ck-dependent signaling was also found to down-regulate the mRNA expression of the gene coding for the ligand inducible transcription factor, peroxisome proliferator-activated receptor gamma (PPARgamma). The ligand activation of PPARgamma resulted in the down-regulation of c-myc and hTERT mRNA expression. By using specific activator and inhibitor of protein kinase C (PKC), it was demonstrated that Receptor Ck dependent down-regulation of hTERT gene transcription involved inhibition of PKC. In addition, 25-hydroxycholesterol was found to contribute to the transcriptional regulation of hTERT gene.

CONCLUSION

Taken together, the findings of this study present evidence for a molecular link between cholesterol-activated Receptor Ck and hTERT transcription, and provide new insights into the regulation of hTERT expression in normal human peripheral blood mononuclear cells.

Impaired NADPH oxidase activity in Rac2-deficient murine neutrophils does not result from defective translocation of p47phox and p67phox and can be rescued by exogenous arachidonic acid

Rac2 is a hematopoietic-specific Rho-GTPase that plays a stimulus-specific role in regulating reduced nicotinamide adenine dinucleotide phosphate (NADPH) oxidase activation and other functional responses in neutrophils. In this study, rac2-/- neutrophils were shown to have significantly decreased NADPH oxidase activity and actin remodeling in response to exogenous arachidonic acid (AA), as previously observed for phorbol 12-myristate 13-acetate (PMA) or formyl-Met-Leu-Phe (fMLP) as agonists. PMA-, fMLP-, or AA-induced translocation of p47phox and p67phox to the plasma membrane was not impaired in rac2-/- neutrophils. Combined stimulation of rac2-/- neutrophils with exogenous AA and PMA had a synergistic effect on NADPH oxidase activity, and superoxide production increased to a level that was at least as high as wild-type cells and had no effect on fMLP-elicited enzyme activity. Membrane translocation of p47phox and p67phox as well as Rac1 activation was not increased further by combined PMA and AA stimulation. Inhibitor studies were consistent with important roles for phorbol ester-activated protein kinase C (PKC) isoforms and an atypical isoform, PKCzeta, in superoxide production by wild-type and rac2-/- neutrophils stimulated with AA and PMA. In addition, PMA-stimulated release of AA and cytoplasmic phospholipase A2 expression in rac2-/- neutrophils were similar to wild-type, suggesting that deficient AA production by PMA-stimulated rac2-/- neutrophils does not explain the effect of exogenous AA on oxidase activity. Although not required for translocation of p47phox and p67phox, Rac2 is necessary for optimal activity of the assembled oxidase complex, an effect that can be replaced by exogenous AA, which may act directly or via an exogenous AA-induced mediator.

Hepatic arachidonic acid metabolism is disrupted after hexachlorobenzene treatment

Hexaclorobenzene (HCB), one of the most persistent environmental pollutants, can cause a wide range of toxic effects including cancer in animals, and hepatotoxicity and porphyria both in humans and animals. In the present study, liver microsomal cytochrome P450 (CYP)-dependent arachidonic acid (AA) metabolism, hepatic PGE production, and cytosolic phospholipase A2 (cPLA2) activity were investigated in an experimental model of porphyria cutanea tarda induced by HCB. Female Wistar rats were treated with a single daily dose of HCB (100 mg kg(-1) body weight) for 5 days and were sacrificed 3, 10, 17, and 52 days after the last dose. HCB treatment induced the accumulation of hepatic porphyrins from day 17 and increased the activities of liver ethoxyresorufin O-deethylase (EROD), methoxyresorufin O-demethylase (MROD), and aminopyrine N-demethylase (APND) from day 3 after the last dose. Liver microsomes from control and HCB-treated rats generated, in the presence of NADPH, hydroxyeicosatetraenoic acids (HETEs), epoxyeicosatrienoic acids (EETs), 11,12-Di HETE, and omega-OH/omega-1-OH AA. HCB treatment caused an increase in total NADPH CYP-dependent AA metabolism, with a higher response at 3 days after the last HCB dose than at the other time points studied. In addition, HCB treatment markedly enhanced PGE production and release in liver slices. This HCB effect was time dependent and reached its highest level after 10 days. At this time cPLA2 activity was shown to be increased. Unexpectedly, HCB produced a significant decrease in cPLA2 activity on the 17th and 52nd day. Our results demonstrated for the first time that HCB induces both the cyclooxygenase and CYP-dependent AA metabolism. The effects of HCB on AA metabolism were previous to the onset of a marked porphyria and might contribute to different aspects of HCB-induced liver toxicity such as alterations of membrane fluidity and membrane-bound protein function. Observations also suggested that a possible role of cPLA2 in the early increase of AA metabolism cannot be excluded. However, the existence of other pathway(s) for metabolizable AA generation different from cPLA2 activation is also proposed.

Evaluation of the effects of peptide antibiotics human beta-defensins-1/-2 and LL-37 on histamine release and prostaglandin D(2) production from mast cells

Antimicrobial peptides, human beta-defensins (hBD-1/-2), and LL-37 (a peptide of human cathelicidin CAP18) are predominately expressed at epithelial tissues, where they participate in the innate host defense by killing invading microorganisms. In this study, to investigate the interactions between epithelial cell-derived antimicrobial peptides and mast cells, we evaluated the effects of hBD-1/-2 and LL-37 on mast cell functions using rat peritoneal mast cells. hBD-2 and LL-37 but not hBD-1 induced histamine release and intracellular Ca(2+) mobilization, and hBD-2 was more potent than LL-37. Interestingly, histamine release and intracellular Ca(2+) mobilization elicited by hBD-2 and LL-37 were markedly suppressed by BAPTA-AM (an intracellular Ca(2+) chelating agent), pertussis toxin and U-73122 (a phospholipase C inhibitor). In addition, among the peptides examined, only hBD-2 significantly induced PGD(2) production, which was abolished by indomethacin (cyclooxygenase-1/-2 inhibitor) but not NS-398 (cyclooxygenase-2 inhibitor), suggesting that hBD-2-induced PGD(2) production is mediated by cyclooxygenase-1. Likewise, the PGD(2) production was suppressed by pertussis toxin and U-73122. These observations suggest that hBD-2 and LL-37 stimulate mast cells to mobilize intracellular Ca(2+) and release histamine or generate PGD(2) in a G protein-phospholipase C-dependent manner. Thus, hBD-2 and LL-37 may have modulatory effects on inflammatory reactions.