Lisa A. Marshall

Manipulation of Distinct NFκB Proteins Alters Interleukin-1β-induced Human Rheumatoid Synovial Fibroblast Prostaglandin E2 Formation

Interleukin 1β (IL-1β) up-regulates human rheumatoid synovial fibroblast (RSF) 85-kDa phospholipase A2 (PLA2) and mitogen-inducible cyclooxygenase (COX) II. Promoter regions for these genes contain a motif that closely resembles the “classic” NFκB consensus site. Immunoblot analysis identified NFκB1 (p50), RelA (p65), and c-Rel in RSF. Upon IL-1β-stimulation, p65 and c-Rel but not p50 protein levels were reduced suggesting nuclear translocation. IL-1β-induced RSF nuclear extracts contained a p65-containing complex, which bound to the classical NFκB consensus motif. An NFκB classical oligonucleotide decoy produced a concentration-dependent decrease in IL-1-stimulated PGE2 production (IC50 = ∼2 μM), indicating a role of NFκB. Utilization of antisense technology showed that p65 but not p50 or c-Rel mediated IL-1β-stimulated PGE2 formation. Treated RSF could not transcribe COX II or 85-kDa PLA2 mRNA, which reduced their respective proteins. Interestingly, stimulated IL-8 production was not inhibited by the classical NFκB decoy but was reduced by treatment with antisense to both p65 and c-Rel supporting preferential binding of c-Rel-p65 to the “alternative” IL-8 κB motif. Taken together, these data provide the first direct evidence for a role of p65 in COX II and 85-kDa PLA2 gene induction and support the IL-1 activation and participation of distinct NFκB protein dimers in RSF prostanoid and IL-8 formation.

The human polo-like kinase, PLK, regulates cdc2/cyclin B through phosphorylation and activation of the cdc25C phosphatase

Entry into mitosis by mammalian cells is triggered by the activation of the cdc2/cyclin B holoenzyme. This is accomplished by the specific dephosphorylation of key residues by the cdc25C phosphatase. The polo-like kinases are a family of serine/threonine kinases which are also implicated in the control of mitotic events, but their exact regulatory mechanism is not known. Recently, a Xenopus homologue, PLX1, was reported to phosphorylate and activate cdc25, leading to activation of cdc2/cyclin B. Jurkat T leukemia cells were chemically arrested and used to verify that PLK protein expression and its phosphorylation state is regulated with respect to cell cycle phase (i.e., protein is undetectable at G1/S, accumulates at S phase and is modified at G2/M). Herein, we show for the first time that endogenous human PLK protein immunoprecipitated from the G2/M-arrested Jurkat cells directly phosphorylates human cdc25C. In addition, we demonstrate that recombinant human (rh) PLK also phosphorylates rhcdc25C in a time- and concentration-dependent manner. Phosphorylation of endogenous cdc25C and recombinant cdc25C by PLK resulted in the activation of the phosphatase as assessed by dephosphorylation of cdc2/cyclin B. These data are the first to demonstrate that human PLK is capable of phosphorylating and positively regulating human cdc25C activity, allowing cdc25C to dephosphorylate inactive cdc2/cyclin B. As this event is required for cell cycle progression, we define at least one key regulatory mode of action for human PLK in the initiation of mitosis.

A Potent and Selective Nonpeptide Antagonist of CXCR2 Inhibits Acute and Chronic Models of Arthritis in the Rabbit

Much evidence implicates IL-8 as a major mediator of inflammation and joint destruction in rheumatoid arthritis. The effects of IL-8 and its related ligands are mediated via two receptors, CXCR1 and CXCR2. In the present study, we demonstrate that a potent and selective nonpeptide antagonist of human CXCR2 potently inhibits 125I-labeled human IL-8 binding to, and human IL-8-induced calcium mobilization mediated by, rabbit CXCR2 (IC50 = 40.5 and 7.7 nM, respectively), but not rabbit CXCR1 (IC50 = >1000 and 2200 nM, respectively). These data suggest that the rabbit is an appropriate species in which to examine the anti-inflammatory effects of a human CXCR2-selective antagonist. In two acute models of arthritis in the rabbit induced by knee joint injection of human IL-8 or LPS, and a chronic Ag (OVA)-induced arthritis model, administration of the antagonist at 25 mg/kg by mouth twice a day significantly reduced synovial fluid neutrophils, monocytes, and lymphocytes. In addition, in the more robust LPS- and OVA-induced arthritis models, which were characterized by increased levels of proinflammatory mediators in the synovial fluid, TNF-α, IL-8, PGE2, leukotriene B4, and leukotriene C4 levels were significantly reduced, as was erythrocyte sedimentation rate, possibly as a result of the observed decreases in serum TNF-α and IL-8 levels. In vitro, the antagonist potently inhibited human IL-8-induced chemotaxis of rabbit neutrophils (IC50 = 0.75 nM), suggesting that inhibition of leukocyte migration into the knee joint is a likely mechanism by which the CXCR2 antagonist modulates disease.

Scytonemin-a marine natural product inhibitor of kinases key in hyperproliferative inflammatory diseases

Chronic inflammatory diseases are associated with the persistent production of proinflammatory mediators and tissue hyperplasia. Protein kinases play an important role in regulating the signaling events controlling mediator release and cell proliferation. For instance, in in vivo models, the tumor promoter, phorbol myristate acetate (PMA), is used as an initiator of inflammation, both acute and chronic. PMA produces these effects by activating protein kinase C (PKC), an enzyme involved in a number o f cellular activities, including growth, differentiation, and mediator formation [1]. In addition, cell cycle progression is highly regulated predominantly through kinase activity. One example is polo-like kinase 1 (PLK1), an enzyme important for the G2-M transition and mitotic spindle formation. Therefore, by targeting certain kinases involved in both processes, it may be possible to reduce the adverse nature o f chronic inflammatory disorders. Here we report the novel pharmacological properties o f scy-

Human Calcium-independent Phospholipase A2 Mediates Lymphocyte Proliferation

Activation of lymphocytes induces blastogenesis and cell division which is accompanied by membrane lipid metabolism such as increased fatty acid turnover. To date little is known about the enzymatic mechanism(s) regulating this process. Release of fatty acids such as arachidonic acid requiressn-2-deacylation catalyzed by a class of enzymes known as phospholipases A2 (PLA2, EC3.1.1.4). Herein, we confirm that human peripheral blood B or T lymphocytes (PBL) do not possess measurable levels of 85-kDa PLA2 as assessed by Western immunoblot. Low levels of 14-kDa PLA2 protein and activity were detectable in the particulate fraction of PBL and Jurkat cells. Western immunoblot analysis indicates that PBLs possess the calcium-independent PLA2 (iPLA2) protein. Calcium-independentsn-2-acylhydrolytic activity was measurable in PBL cytosols and could be inhibited by the selective iPLA2inhibitor bromoenol lactone. Mitogen activation of PBLs resulted in maintenance of activity levels which remained constant over 72 h suggesting an important role for iPLA2 in this proliferative process. Indeed, evaluation of iPLA2 activity in cell cycle-arrested Jurkat T cell fractions revealed the highest iPLA2 levels occurring at the G2/M phase. Addition of the iPLA2 inhibitors, bromoenol lactone, or arachidonyl trifluoromethyl ketone (AAOCF3), inhibited both mitogen-induced PBL as well as Jurkat T cell proliferation. Moreover, specific depletion of iPLA2 protein by antisense treatment also resulted in marked suppression of cell division. Inhibition of Jurkat cell proliferation was not associated with arrest at a particular phase of the cell cycle nor was it associated with apoptosis as assessed by flow cytometry. These findings provide the first evidence that iPLA2 plays a key role in the lymphocyte proliferative response.

Cytosolic 85-kDa Phospholipase A2-mediated Release of Arachidonic Acid Is Critical for Proliferation of Vascular Smooth Muscle Cells

Recent evidence suggests that arachidonic acid (AA) may be involved in regulating cellular proliferation. The predominant mechanism of AA release from cellular phospholipids is via phospholipase A2 (PLA2) hydrolysis. The purpose of this study was to examine the roles of the distinct 14-kDa and 85-kDa PLA2 enzymes in human coronary artery vascular smooth muscle cell (hCAVSMC) proliferation. Cultured hCAVSMCs proliferate in the presence of growth medium with a typical doubling time of 30–40 h, grow at a slower proliferative rate upon reaching confluency (day 8), and eventually undergo contact inhibition of growth (day 10). Neither Type II 14-kDa PLA2activity nor mass changed over a 10-day culture period. In contrast, 85-kDa PLA2 protein activity and mRNA decreased as time in culture progressed. This reduction in 85-kDa PLA2correlated with reductions in DNA synthesis and suggested a possible association between 85-kDa PLA2 and proliferation. To directly evaluate the role of the 85-kDa PLA2 in proliferation we examined the effects of an 85-kDa PLA2inhibitor (AACOCF3) and 85-kDa PLA2 antisense oligonucleotides on proliferation. Both reagents dose dependently inhibited proliferation, whereas a 14-kDa PLA2 inhibitor (SB203347), a calcium-independent PLA2 inhibitor (HELSS), an 85-kDa sense oligonucleotide, and a nonrelevant scrambled control oligonucleotide had no effect. The mechanism by which 85-kDa PLA2 influences cellular proliferation remains unclear. Inhibition of 85-kDa PLA2 activity produced neither phase-specific cell cycle arrest nor apoptosis (fluorescence-activated cell sorter analysis). Addition of AA (20 μm) attenuated the effects of both AACOCF3 and 85-kDa antisense oligonucleotides implicating AA as a key mediator in cellular proliferation. However, although prostaglandin E2(PGE2) was present in the culture medium, it peaked early (day 3) in culture, and indomethacin had no effect on cellular proliferation indicating that hCAVSMC proliferation was not mediated through PGE2. These data provide the first direct evidence that PLA2 is involved in control of VSMC proliferation and indicate that 85-kDa PLA2-mediated liberation of AA is critical for cellular proliferation.

Depletion of Human Monocyte 85-kDa Phospholipase A2 Does Not Alter Leukotriene Formation

Human monocytes possess several acylhydrolase activities and are capable of producing both prostanoids (PG) and leukotriene (LT) products upon acute stimulation with calcium ionophore, A23187 or phagocytosis of zymosan particles. The cytosolic 85-kDa phospholipase (PLA) A2 co-exists with the 14-kDa PLA2 in the human monocyte, but their respective roles in LT production are not well understood. Reduction in 85-kDa PLA2 cellular protein levels by initiation site-directed antisense (SK 7111) or exposure to the 85-kDa PLA2 inhibitor, arachidonyl trifluoromethyl ketone (AACOCF3), prevented A23187 or zymosan-stimulated monocyte prostanoid formation. In contrast, neither treatment altered stimulated LTC4 production. This confirmed the important role of the 85-kDa PLA2 in prostanoid formation but suggests that it has less of a role in LT biosynthesis. Alternatively, treatment of monocytes with the selective, active site-directed 14-kDa PLA2 inhibitor, SB 203347, prior to stimulation had no effect on prostanoid formation at concentrations that totally inhibited LT formation. Addition of 20 μM exogenous arachidonic acid to monocytes exposed to SK 7111 or SB 203347 did not alter A23187-induced PGE2 or LTC4 generation, respectively, indicating that these agents had no effect on downstream arachidonic acid-metabolizing enzymes in this setting. Taken together, these results provide evidence that the 85-kDa PLA2 may play a more significant role in the formation of PG than LT. Further, utilization of SB 203347 provides intriguing data to form the hypothesis that a non-85-kDa PLA2 sn-2 acyl hydrolase, possibly the 14-kDa PLA2, may provide substrate for LT formation.

The role of platelet activating factor and other lipid mediators in inflammatory angiogenesis

Chronic inflammatory diseases are often accompanied by intense angiogenesis. A model of inflammatory angiogenesis is the murine air pouch granuloma which has a hyperangiogenic component. Proinflammatory lipid mediator generation is also a hallmark of chronic inflammation and the role of endogenous production of these mediators in angiogenesis is not known. The 14 kDa phospholipase A2 (PLA2) deacylates phospholipid, liberating arachidonic acid, which is used for leukotriene production, and lysophospholipid, which can drive the production of platelet-activating factor (PAF). Therefore, SB 203347, an inhibitor of the 14 kDa PLA2, zileuton, an inhibitor of 5-lipoxygenase, and Ro 24-4736 a PAF receptor antagonist were evaluated for their effects in the murine air pouch granuloma. SB 203347 reduced both LTB4 and PAF, but not PGD2 levels measured in the day 6 granuloma. This correlated with a significant reduction in angiogenesis. Zileuton reduced LTB4 levels as expected, but did not significantly inhibit angiogenesis, whereas Ro 24-4736 potently reduced angiogenesis. These data support the hypothesis that PAF, and to a lesser extent leukotrienes contribute to the angiogenic phenotype in chronic inflammation.

Effects of scalaradial, a novel inhibitor of 14 kDa phospholipase A2, on human neutrophil function

Scalaradial, a marine natural product with anti-inflammatory activity, has been shown to be a selective inhibitor of 14 kDa type II phospholipase A2(PLA2). We have examined the inhibition by scalaradial (0.1 nM to 10 microM) of neutrophil function (degranulation) in response to receptor-mediated activation [N-formyl-L-methionyl-L-leucyl-L-phenylalanine (fMLP), 30 nM; leuokotriene B4 (LTB4), 100 nM; platelet-activating factor (PAF), 100 nM] and non-receptor-mediated stimuli [A23187 (1 microM) and thapsigargin (100 nM)]. Furthermore, we evaluated the ability of scalaradial to inhibit the increase in intracellular Ca2+ in response to fMLP, LTB4, A23187, and thapsigargin as well as its ability to prevent either fMLP- or LTB4-mediated elevation in inositol phosphate production (InsP). Scalaradial was a potent inhibitor of both receptor- (IC50 = 50-200 nM) and non-receptor- (IC50 = 40-900 nM) mediated degranulation. Although scalaradial inhibited the mobilization of Ca2+ induced by fMLP, LTB4, and PAF, it did not affect the maximal Ca2+ levels attained with A23187 or thapsigargin. Neutrophil-binding studies with [3H]fMLP and [3H]LTB4 would suggest that the effect of scalaradial on agonist-induced degranulation and increase in intracellular Ca2+ was not at the receptor level because 50-fold higher concentrations were required to have a significant effect on the binding of these agonists. To determine if scalaradial affected phosphatidylinositol selective phospholipase C (PI-PLC) activity, assays were conducted to monitor fMLP- and LTB4-induced formation of InsPs using myo-[3H]inositol-labeled U-937 cells. In these cells, 2.5 to 9-fold higher concentrations of scalaradial were required to inhibit PI-PLC activity than to inhibit agonist-induced degranulation of neutrophils, suggesting that the effects of scalaradial on Ca2+ and degranulation are not the sole result of blocking receptor activation of PI-PLC. Results obtained with receptor-mediated stimuli suggest that scalaradial may have direct effects on Ca2+ channels and InsP turnover, but inhibition of intracellular Ca2+ levels was not required for scalaradial to block degranulation since scalaradial was capable of inhibiting degranulation produced by either A23187 or thapsigargin, without changing the maximal Ca2+ levels obtained with these two stimuli. These results demonstrate that scalaradial can inhibit degranulation in the presence of micromolar intracellular Ca2+ concentration, thus supporting the hypothesis that a 14 kDa PLA2 may be important in the regulation of neutrophil degranulation.

Anti-human RP105 sera induces lymphocyte proliferation.

Cellular environment dictates whether antigen binding to the B lymphocyte receptor together with co‐stimulatory molecules will result in proliferation, anergy, or apoptosis. Murine RP105 is a member of the leucine‐rich repeat family of proteins, which is specifically expressed on mature B cells. Monoclonal antibodies to the murine RP105 induce proliferation and protect B cells from apoptosis, suggesting an important regulatory role in murine B lymphocyte function. We identified a human RP105 homolog and mapped the gene to chromosome 5q 12.3‐13.1. Tissue distribution analysis shows that the transcript is found predominately in lymphoid tissues including spleen, tonsils, appendix, and peripheral blood leukocytes. Polymerase chain reaction analysis of isolated primary human cell populations confirms that mRNA exists in spleen B lymphocytes and monocytes but not T lymphocytes. Western blot analysis demonstrates specific expression of human RP105 in human B lymphocytes. Murine anti‐human RP105 sera was generated using DNA immunization. The antisera contained antibodies that recognized and bound to human B lymphocytes from both spleen and peripheral blood as assessed by flow cytometry. Assessment of biological function showed that human peripheral blood leukocytes incubated with anti‐RP105 sera were induced to proliferate as measured by tritiated thymidine incorporation. Moreover, anti‐CD40 and interleukin‐4‐treated cells but not those exposed to anti‐RP105 sera produced soluble CD23, suggesting distinct functional roles. This is the first demonstration of both the existence of RP105 protein on human B lymphocytes and its role in the regulation of B lymphocyte activation. J. Leukoc. Biol. 65: 43–49; 1999.