Rac1 mediates STAT3 activation by autocrine IL-6.

The activity of the small GTPase, Rac1, plays a role in various cellular processes including cytoskeletal rearrangement, gene transcription, and malignant transformation. In this report constitutively active Rac1 (Rac V12) is shown to stimulate the activation of STAT3, a member of the family of signal transducers and activators of transcription (STATs). The activity of Rac1 leads to STAT3 translocation to the nucleus coincident with STAT3-dependent gene expression. The expression of Vav (Delta1-187), a constitutively active guanine nucleotide exchange factor for the Rho GTPases, or activated forms of Ras or Rho family members, leads to STAT3-specific activation. The activation of STAT3 requires tyrosine phosphorylation at residue 705, but is not dependent on phosphorylation of Ser-727. Our studies indicate that Rac1 induces STAT3 activation through an indirect mechanism that involves the autocrine production and action of IL-6, a known mediator of STAT3 response. Rac V12 expression results in the induction of the IL-6 and IL-6 receptor genes and neutralizing antibodies directed against the IL-6 receptor block Rac1-induced STAT3 activation. Furthermore, inhibition of the nuclear factor-kappaB activation or disruption of IL-6-mediated signaling through the expression of IkappaBalpha S32AS36A and suppressor of cytokine signaling 3, respectively, blocks Rac1-induced STAT3 activation. These findings elucidate a mechanism dependent on the induction of an autocrine IL-6 activation loop through which Rac1 mediates STAT3 activation establishing a link between oncogenic GTPase activity and Janus kinase/STAT signaling.

Protease-activated receptors 1 and 4 are shut off with distinct kinetics after activation by thrombin.

Protease-activated receptors 1 and 4 (PAR1 and PAR4) mediate thrombin signaling in human platelets. Whether these receptors are redundant, interact, or serve only partially overlapping functions is unknown. We report that PAR1 and PAR4 signal with distinct tempos. In transfected fibroblasts, PAR4 triggered substantially more phosphoinositide hydrolysis per activated receptor than PAR1 and was shut off more slowly than PAR1. Shutoff and internalization of PAR1 depends upon phosphorylation of its carboxyl tail upon receptor activation. In contrast to PAR1, phosphorylation of PAR4 was undetectable, and activation-dependent internalization of PAR4 was much slower than that seen for PAR1. Mutation of potential phosphorylation sites in the carboxyl tail of PAR1 enhanced PAR1 signaling, whereas analogous mutations in PAR4 had no effect. Thus PAR4 signaling is shut off less rapidly than PAR1, probably due to differences in receptor phosphorylation. PAR1 and PAR4 also signaled with distinct tempos in platelets. PAR1 triggered a rapid and transient increase in intracellular calcium, whereas PAR4 triggered a more prolonged response. Together, the tempo of these responses accounted for that triggered by thrombin. Thus differences in the rates at which PAR1 and PAR4 are shut off allow thrombin to trigger intracellular signaling with distinct temporal characteristics.

Structure-function analysis of protease-activated receptor 4 tethered ligand peptides. Determinants of specificity and utility in assays of receptor function.

Thrombin activates protease-activated receptors (PARs) by specific cleavage of their amino-terminal exodomains to unmask a tethered ligand that binds intramolecularly to the body of the receptor to effect transmembrane signaling. Peptides that mimic such ligands are valuable as agonists for probing PAR function, but the tethered ligand peptide for PAR4, GYPGKF, lacks potency and is of limited utility. In a structure-activity analysis of PAR4 peptides, AYPGKF was approximately 10-fold more potent than GYPGKF and, unlike GYPGKF, elicited PAR4-mediated responses comparable in magnitude to those elicited by thrombin. AYPGKF was relatively specific for PAR4 in part due to the tyrosine at position 2; substitution of phenylalanine or p-fluorophenylalanine at this position produced peptides that activated both PAR1 and PAR4. Because human platelets express both PAR1 and PAR4, it might be desirable to inhibit both receptors. Identifying a single agonist for both receptors raises the possibility that a single antagonist for both receptors might be developed. The AYPGKF peptide is a useful new tool for probing PAR4 function. For example, AYPGKF activated and desensitized PAR4 in platelets and, like thrombin, triggered phosphoinositide hydrolysis but not inhibition of adenylyl cyclase in PAR4-expressing cells. The latter shows that, unlike PAR1, PAR4 couples to G(q) and not G(i).

Role of nitric oxide in poly(I-C)-induced endothelial cell expression of leukocyte adhesion molecules.

Polyinosinic-polycytidylic acid [poly(I-C)] is a synthetic double-stranded RNA (dsRNA) that simulates a viral-infected state in cells. It has been shown that viral infection, as well as poly(I-C), stimulates leukocyte adhesion to endothelial cell (EC) monolayers and that this is mediated through the surface expression of the adhesion molecules E-selectin, vascular cell adhesion molecule 1 (VCAM-1), and intercellular adhesion molecule 1. We have tested the involvement of nitric oxide (NO) in poly(I-C)-induced monocytic cell adhesion to human vascular EC. Using primary cultured EC for these studies, we confirmed the results from previous reports that these cells have higher basal levels of NO production than passaged cells. Poly(I-C)-induced monocytic cell adhesion to primary EC was concentration-dependently inhibited by 40-74% by the nitric oxide synthase (NOS) inhibitor NG-methyl-L-arginine (L-NMA), as well as three other NOS inhibitors, without significantly affecting interleukin-1 beta-induced adhesion. L-NMA inhibited poly(I-C)-induced surface expression of E-selectin and VCAM-1 by 25 and 45%, respectively, and mRNA levels of E-selectin and VCAM-1 by 62 and 74%, respectively. Primary EC transiently transfected with a plasmid containing an E-selectin promoter-driven luciferase reporter gene showed that L-NMA treatment reduced poly(I-C)-induced E-selectin promoter activity to basal levels. Electrophoretic mobility shift analysis indicated that poly(I-C)-induced nuclear factor-kappa B (NF-kappa B) binding to a radiolabeled oligonucleotide corresponding to the consensus NF-kappa B binding domain of the E-selectin promoter was decreased by L-NMA pretreatment. Hence, NO appears to augment E-selectin gene expression in response to poly(I-C) at the transcriptional level in vascular EC. Collectively, these data support the hypothesis that NO augments poly(I-C)-induced EC activation. These data suggest a novel role for NO as a response mediator in dsRNA-induced leukocyte adhesion to EC.

IFN-gamma inhibits double-stranded RNA-induced E-selectin expression in human endothelial cells.

IFN-gamma plays a role in immune regulatory functions as well as in viral defense. We show in this study that IFN-gamma treatment down-regulates the induction by a viral mimetic, polyinosinic-polycytidylic acid (poly(I:C)), of the endothelial cell-specific leukocyte adhesion protein, E-selectin. The inhibitory effect of IFN-gamma on poly(I:C)-induced E-selectin was concentration and time dependent and was specific for dsRNA, in that the induction of E-selectin by TNF-alpha, IL-1 beta, thrombin, or LPS was not inhibited significantly by this pretreatment. IFN-gamma pretreatment reduced poly(I:C)-induced E-selectin mRNA in a protein synthesis-independent manner. Poly(I:C)-induced E-selectin mRNA t1/2 was reduced slightly by IFN-gamma treatment, while the message for VCAM-1 was stabilized. Transient transfection of endothelial cells with an E-selectin promoter-driven reporter gene construct revealed that poly(I:C) stimulation of E-selectin promoter activity was decreased significantly by IFN-gamma pretreatment. Poly(I:C)-induced nuclear factor-kappa B activation following IFN-gamma pretreatment was unaffected, as shown by electrophoretic mobility shift analysis. These results indicate a novel role for IFN-gamma in the regulation of E-selectin gene expression in response to dsRNA by a transcriptional mechanism independent of nuclear factor-kappa B, as well as by a minor decrease in message stability.

Distinct mechanisms for N-acetylcysteine inhibition of cytokine-induced E-selectin and VCAM-1 expression.

We have examined the effects of N-acetyl-L-cysteine (NAC), a well-characterized, thiol-containing antioxidant, on agonist-induced monocytic cell adhesion to endothelial cells (EC). NAC inhibited interleukin-1 (IL-1 beta)-induced, but not basal, adhesion with 50% inhibition at approximately 20 mM. Monocytic cell adhesion to EC in response to tumor necrosis factor-alpha (TNF-alpha), lipopolysaccharide (LPS), alpha-thrombin, or phorbol 12-myristate 13-acetate (PMA) was similarly inhibited by NAC. Unlike published studies with pyrrolidinedithiocarbamate, which specifically inhibited vascular cell adhesion molecule 1 (VCAM-1), NAC inhibited IL-1 beta-induced mRNA and cell surface expression of both E-selectin and VCAM-1. NAC had no effect on the half-life of E-selectin or VCAM-1 mRNA. Although NAC reduced nuclear factor-kappa B (NF-kappa B) activation in EC as measured by gel-shift assays using an oligonucleotide probe corresponding to the consensus NF-kappa B binding sites of the VCAM-1 gene (VCAM-NF-kappa B), the antioxidant had no appreciable effect when an oligomer corresponding to the consensus NF-kappa B binding site of the E-selectin gene (E-selectin-NF-kappa B) was used. Because NF-kappa B has been reported to be redox sensitive, we studied the effects of NAC on the EC redox environment. NAC caused an expected dramatic increase in the reduced glutathione (GSH) levels in EC. In vitro studies demonstrated that whereas the binding affinity of NF-kappa B to the VCAM-NF-kappa B oligomer peaked at a GSH-to-oxidized glutathione (GSSG) ratio of approximately 200 and decreased at higher ratios, the binding to the E-selectin-NF-kappa B oligomer appeared relatively unaffected even at ratios > 400, i.e., those achieved in EC treated with 40 mM NAC. These results suggest that NF-kappa B binding to its consensus sequences in the VCAM-1 and E-selectin gene exhibits marked differences in redox sensitivity, allowing for differential gene expression regulated by the same transcription factor. Our data also demonstrate that NAC increases the GSH-to-GSSG ratio within the EC suggesting one possible mechanism through which this antioxidant inhibits agonist-induced monocyte adhesion to EC.