Interleukin-1alpha and tumour necrosis factor-alpha modulate airway smooth muscle DNA synthesis by induction of cyclo-oxygenase-2: inhibition by dexamethasone and fluticasone propionate

Glucocorticoid Insensitivity in Asthma: The Unique Role for Airway Smooth Muscle Cells

Although most patients with asthma symptoms are well controlled by inhaled glucocorticoids (GCs), a subgroup of patients suffering from severe asthma respond poorly to GC therapy. Such GC insensitivity (GCI) represents a profound challenge in managing patients with asthma. Even though GCI in patients with severe asthma has been investigated by several groups using immune cells (peripheral blood mononuclear cells and alveolar macrophages), uncertainty exists regarding the underlying molecular mechanisms in non-immune cells, such as airway smooth cells (ASM) cells. In asthma, ASM cells are among the targets of GC therapy and have emerged as key contributors not only to bronchoconstriction but also to airway inflammation and remodeling, as implied by experimental and clinical evidence. We here summarize the current understanding of the actions/signaling of GCs in asthma, and specifically, GC receptor (GR) “site-specific phosphorylation” and its role in regulating GC actions. We also review some common pitfalls associated with studies investigating GCI and the inflammatory mediators linked to asthma severity. Finally, we discuss and contrast potential molecular mechanisms underlying the impairment of GC actions in immune cells versus non-immune cells such as ASM cells.

Cellular Microenvironment Stiffness Regulates Eicosanoid Production and Signaling Pathways

Pathological changes in the biomechanical environment are implicated in the progression of idiopathic pulmonary fibrosis (IPF). Stiffened matrix augments fibroblast proliferation and differentiation and activates TGF-β1 (transforming growth factor-β1). Stiffened matrix impairs the synthesis of the antifibrogenic lipid mediator prostaglandin E₂ (PGE₂) and reduces the expression of the rate-limiting prostanoid biosynthetic enzyme cyclooxygenase-2 (COX-2). We now show that prostaglandin E synthase (PTGES), the final enzyme in the PGE₂ biosynthetic pathway, is expressed at lower levels in the lungs of patients with IPF. We also show substantial induction of COX-2, PTGES, prostaglandin E receptor 4 (EP4), and cytosolic phospholipase A₂ (cPLA₂) expression in human lung fibroblasts cultured in soft collagen hydrogels or in spheroids compared with conventional culture on stiff plastic culture plates. Induction of COX-2, cPLA₂, and PTGES expression in spheroid cultures was moderately inhibited by the p38 mitogen-activated protein kinase inhibitor SB203580. The induction of prostanoid biosynthetic enzyme expression was accompanied by an increase in PGE₂ levels only in non-IPF-derived fibroblast spheroids. Our study reveals an extensive dysregulation of prostanoid biosynthesis and signaling pathways in IPF-derived fibroblasts, which are only partially abrogated by culture in soft microenvironments.

Airway smooth muscle cells are insensitive to the anti-proliferative effects of corticosteroids: The novel role of insulin growth factor binding Protein-1 in asthma

Airway remodeling in asthma manifests, in part, as enhanced airway smooth muscle (ASM) mass, due to myocyte proliferation. While the anti-proliferative effects of glucocorticoid (GC) were investigated in normal ASM cells (NASMC), little is known about such effects in ASM cells derived from asthma subjects (AASMC). We posit that GC differentially modulates mitogen-induced proliferation of AASMC and NASMC. Cells were cultured, starved, then treated with Epidermal growth factor (EGF) (10 ng/ml) and Platelet-derived growth factor (PDGF) (10 ng/ml) for 24 h and/or fluticasone propionate (FP) (100 nM) added 2 h before. Cell counts and flow cytometry analyses showed that FP failed to decrease the cell number of and DNA synthesis in AASMC irrespective of mitogens used. We also examine the ability of Insulin Growth Factor Binding Protein-1 (IGFBP-1), a steroid-inducible gene that deters cell growth in other cell types, to inhibit proliferation of AASMC where FP failed. We found that FP increased IGFBP1 mRNA and protein levels. Interestingly, the addition of IGFBP1 (1 μg/ml) to FP completely inhibited the proliferation of AASMC irrespective to the mitogens used. Further investigation of different signaling molecules involved in ASM growth and GC receptor functions (Protein kinase B (PKB/AKT), Mitogen-activated protein kinases (MAPKs), Focal Adhesion Kinase (FAK)) showed that IGFBP-1 selectively decreased mitogen-induced p38 phosphorylation in AASMC. Collectively, our results show the insensitivity of AASMC to the anti-proliferative effects of GC, and demonstrate the ability of IGFBP1 to modulate AASMC growth representing, hence, a promising strategy to control ASM growth in subjects with GC insensitive asthma.

PGE2 enhanced TNFα-mediated IL-8 induction in monocytic cell lines and PBMC

BACKGROUND & PURPOSE

Recent studies suggested a role of prostaglandin E₂ (PGE₂) in the expression of the chemokine IL-8 by monocytes. The function of EP4 receptor for TNFα-induced IL-8 expression was studied in monocytic cell lines.

EXPERIMENTAL APPROACH

IL-8 mRNA and protein induction as well as IL-8 promoter activity and transcription factor activation were assessed in monocytic cell lines, primary blood mononuclear cells (PBMC) and transgenic HEK293 cells expressing the EP4 receptor.

KEY RESULTS

In monocytic cell lines THP-1, MonoMac and U937 PGE₂ had only a marginal impact on IL-8 induction but strongly enhanced TNFα-induced IL-8 mRNA and protein synthesis. Similarly, in PBMC IL-8 mRNA induction was larger by simultaneous stimulation with TNFα and PGE₂ than by either stimulus alone. The EP4 receptor subtype was the most abundant EP receptor in all three cell lines and in PBMC. Stimulation of THP-1 cells with an EP4 specific agonist enhanced TNFα-induced IL-8 mRNA and protein formation to the same extent as PGE₂. In HEK293 cells expressing EP4, but not in wild type HEK293 cells lacking EP4, PGE₂ enhanced TNFα-induced IL-8 protein and mRNA synthesis. In THP-1 cells, the enhancement of TNFα-mediated IL-8 mRNA induction by PGE₂ was mimicked by a PKA-activator. Furthermore in these cells PGE₂ induced expression of transcription factor C/EBPß, enhanced NF-κB activation by TNFα and inhibited TNFα-mediated AP-1 activation. PGE₂ and TNFα synergistically activated transcription factor CREB, induced C/EBPß expression and enhanced the activity of an IL-8 promoter fragment containing -223 bp upstream of the transcription start site.

CONCLUSIONS AND IMPLICATIONS

These findings suggest that a combined stimulation of TNFα and PGE₂/EP4 signal chains in monocytic cells leads to maximal IL-8 promoter activity, as well as IL-8 mRNA and protein induction, by activating the PKA/CREB/C/EBPß as well as NF-κB signal chains.

Dietary omega-6, but not omega-3, polyunsaturated or saturated fatty acids increase inflammation in primary lung mesenchymal cells

Obesity is an important risk factor for developing severe asthma. Dietary fatty acids, which are increased in sera of obese individuals and after high-fat meals, activate the innate immune system and induce inflammation. This study investigated whether dietary fatty acids directly cause inflammation and/or synergize with obesity-induced cytokines in primary human pulmonary fibroblasts in vitro. Fibroblasts were challenged with BSA-conjugated fatty acids [ω-6 polyunsaturated fatty acids (PUFAs) and ω-3 PUFAs or saturated fatty acids (SFAs)], with or without TNF-α, and release of the proinflammatory cytokines, IL-6 and CXCL8, was measured. We found that the ω-6 PUFA arachidonic acid (AA), but not ω-3 PUFAs or SFAs, upregulates IL-6 and CXCL8 release. Combined AA and TNF-α challenge resulted in substantially greater cytokine release than either alone, demonstrating synergy. Synergistic upregulation of IL-6, but not CXCL8, was mainly mediated via cyclooxygenase (COX). Inhibition of p38 MAPK reduced CXCL8 release, induced by AA and TNF-α alone, but not in combination. Synergistic CXCL8 release, following AA and TNF-α challenge, was not medicated via a single signaling pathway (MEK1, JNK, phosphoinositide 3-kinase, and NF-κB) nor by hyperactivation of NF-κB or p38. To investigate if these findings occur in other airway cells, effects of AA in primary human airway smooth muscle (ASM) cells and human bronchial epithelial cells were also investigated. We found proinflammatory effects in ASM cells but not epithelial cells. This study suggests that diets rich in ω-6 PUFAs might promote airway inflammation via multiple pathways, including COX-dependent and -independent pathways, and in an obese person, may lead to more severe airway inflammation.

PPAR-α Modulates the Anti-Inflammatory Effect of Melatonin in the Secondary Events of Spinal Cord Injury

Melatonin is the principal secretory product of the pineal gland, and its role as an immunomodulator is well established. Recent evidence shows that melatonin is a scavenger of oxyradicals and peroxynitrite and reduces the development of inflammation and tissue injury events associated with spinal cord trauma. Previous results suggest that peroxisome proliferator-activated receptor α (PPAR-α), a nuclear receptor protein that functions as a transcription factor activated by fatty acids, plays a role in control of secondary inflammatory process associated with spinal cord injury (SCI).With the aim to characterize the role of PPAR-α in melatonin-mediated anti-inflammatory activity, we tested the efficacy of melatonin (30 mg/kg) in an experimental model of spinal cord trauma, induced in mice, by the application of vascular clips (force of 24 g) to the dura via a four-level T5-T8 laminectomy, and comparing mice lacking PPAR-α (PPAR-α KO) with wild-type (WT) mice.The results obtained indicate that melatonin-mediated anti-inflammatory activity is weakened in PPAR-α KO mice, as compared to WT controls. In particular, melatonin was less effective in PPAR-α KO, compared to WT mice, as evaluated by inhibition of the degree of spinal cord inflammation and tissue injury, neutrophil infiltration, pro-inflammatory cytokine expression, nuclear factor κB (NF-κB) activation, and inducible nitric oxide synthase (iNOS) expression. This study indicates that PPAR-α can contribute to the anti-inflammatory activity of melatonin in SCI.

Cyclooxygenase 2: its regulation, role and impact in airway inflammation

Cyclooxygenase 2 (COX-2: official gene symbol - PTGS2) has long been regarded as playing a pivotal role in the pathogenesis of airway inflammation in respiratory diseases including asthma. COX-2 can be rapidly and robustly expressed in response to a diverse range of pro-inflammatory cytokines and mediators. Thus, increased levels of COX-2 protein and prostanoid metabolites serve as key contributors to pathobiology in respiratory diseases typified by dysregulated inflammation. But COX-2 products may not be all bad: prostanoids can exert anti-inflammatory/bronchoprotective functions in airways in addition to their pro-inflammatory actions. Herein, we outline COX-2 regulation and review the diverse stimuli known to induce COX-2 in the context of airway inflammation. We discuss some of the positive and negative effects that COX-2/prostanoids can exert in in vitro and in vivo models of airway inflammation, and suggest that inhibiting COX-2 expression to repress airway inflammation may be too blunt an approach; because although it might reduce the unwanted effects of COX-2 activation, it may also negate the positive effects. Evidence suggests that prostanoids produced via COX-2 upregulation show diverse actions (and herein we focus on prostaglandin E2 as a key example); these can be either beneficial or deleterious and their impact on respiratory disease can be dictated by local concentration and specific interaction with individual receptors. We propose that understanding the regulation of COX-2 expression and associated receptor-mediated functional outcomes may reveal number of critical steps amenable to pharmacological intervention. These may prove invaluable in our quest towards future development of novel anti-inflammatory pharmacotherapeutic strategies for the treatment of airway diseases.

Sustained Inhibition of Proliferative Response After Transient FGF Stimulation Is Mediated by Interleukin 1 Signaling

Transient FGF stimulation of various cell types results in FGF memory--a sustained blockage of efficient proliferative response to FGF and other growth factors. FGF memory establishment requires HDAC activity, indicating its epigenetic character. FGF treatment stimulates proinflammatory NFκB signaling, which is also critical for FGF memory formation. The search for FGF-induced mediators of FGF memory revealed that FGF stimulates HDAC-dependent expression of the inflammatory cytokine IL1α. Similarly to FGF, transient cell treatment with recombinant IL1α inhibits the proliferative response to further FGF and EGF stimulation, but does not prevent FGF receptor-mediated signaling. Interestingly, like cells pretreated with FGF1, cells pretreated with IL1α exhibit enhanced restructuring of actin cytoskeleton and increased migration in response to FGF stimulation. IRAP, a specific inhibitor of IL 1 receptor, and a neutralizing anti-IL1α antibody prevent the formation of FGF memory and rescue an efficient proliferative response to FGF restimulation. A similar effect results following treatment with the anti-inflammatory agents aspirin and dexamethasone. Thus, FGF memory is mediated by proinflammatory IL1 signaling. It may play a role in the limitation of proliferative response to tissue damage and prevention of wound-induced hyperplasia.

Epigenetic regulations of inflammatory cyclooxygenase-derived prostanoids: molecular basis and pathophysiological consequences

The potential relevance of prostanoid signaling in immunity and immunological disorders, or disease susceptibility and individual variations in drug responses, is an important area for investigation. The deregulation of Cyclooxygenase- (COX-) derived prostanoids has been reported in several immunoinflammatory disorders such as asthma, rheumatoid arthritis, cancer, and autoimmune diseases. In addition to the environmental factors and the genetic background to diseases, epigenetic mechanisms involved in the fine regulation of prostanoid biosynthesis and/or receptor signaling appeared to be an additional level of complexity in the understanding of prostanoid biology and crucial in controlling the different components of the COX pathways. Epigenetic alterations targeting inflammatory components of prostanoid biosynthesis and signaling pathways may be important in the process of neoplasia, depending on the tissue microenvironment and target genes. Here, we focused on the epigenetic modifications of inflammatory prostanoids in physiological immune response and immunological disorders. We described how major prostanoids and their receptors can be functionally regulated epigenetically and consequently the impact of these processes in the pathogenesis inflammatory diseases and the development of therapeutic approaches that may have important clinical applications.

Development of the HPA axis: where and when do sex differences manifest?

Sex differences in the response to stress contribute to sex differences in somatic, neurological, and psychiatric diseases. Despite a growing literature on the mechanisms that mediate sex differences in the stress response, the ontogeny of these differences has not been comprehensively reviewed. This review focuses on the development of the hypothalamic-pituitary-adrenal (HPA) axis, a key component of the body's response to stress, and examines the critical points of divergence during development between males and females. Insight gained from animal models and clinical studies are presented to fully illustrate the current state of knowledge regarding sex differences in response to stress over development. An appreciation for the developmental timelines of the components of the HPA axis will provide a foundation for future areas of study by highlighting both what is known and calling attention to areas in which sex differences in the development of the HPA axis have been understudied.

NF-κB-dependent IL-8 induction by prostaglandin E(2) receptors EP(1) and EP(4)

BACKGROUND AND PURPOSE

Recent studies suggested a role for PGE(2) in the expression of the chemokine IL-8. PGE(2) signals via four different GPCRs, EP(1) -EP(4) . The role of EP(1) and EP(4) receptors for IL-8 induction was studied in HEK293 cells, overexpressing EP(1) (HEK-EP(1) ), EP(4) (HEK-EP(4) ) or both receptors (HEK-EP(1) + EP(4) ).

EXPERIMENTAL APPROACH

IL-8 mRNA and protein induction and IL-8 promoter and NF-κB activation were assessed in EP expressing HEK cells.

KEY RESULTS

In HEK-EP(1) and HEK-EP(1) + EP(4) but not HEK or HEK-EP(4) cells, PGE(2) activated the IL-8 promoter and induced IL-8 mRNA and protein synthesis. Stimulation of HEK-EP(1) + EP(4) cells with an EP(1) -specific agonist activated IL-8 promoter and induced IL-8 mRNA and protein, whereas a specific EP(4) agonist neither activated the IL-8 promoter nor induced IL-8 mRNA and protein synthesis. Simultaneous stimulation of HEK- EP(1) + EP(4) cells with both agonists activated IL-8 promoter and induced IL-8 mRNA to the same extent as PGE(2) . In HEK-EP(1) + EP(4) cells, PGE(2) -mediated IL-8 promoter activation and IL-8 mRNA induction were blunted by inhibition of IκB kinase. PGE(2) activated NF-κB in HEK-EP(1) , HEK-EP(4) and HEK-EP(1) + EP(4) cells. In HEK-EP(1) + EP(4) cells, simultaneous activation of both receptors was needed for maximal PGE(2) -induced NF-κB activation. PGE(2) -stimulated NF-κB activation by EP(1) was blocked by inhibitors of PLC, calcium-signalling and Src-kinase, whereas that induced by EP(4) was only blunted by Src-kinase inhibition.

CONCLUSIONS AND IMPLICATIONS

These findings suggest that PGE(2) -mediated NF-κB activation by simultaneous stimulation of EP(1) and EP(4) receptors induces maximal IL-8 promoter activation and IL-8 mRNA and protein induction.

Cytokines and growth factors promote airway smooth muscle cell proliferation

Chronic airway diseases, such as asthma or chronic obstructive pulmonary disease, are characterized by the presence in the airways of inflammation factors, growth factors and cytokines, which promote airway wall remodelling. The aim of this study was to investigate the effect of cytokines and growth factors on airway smooth muscle cell (ASMC) proliferation, phenotype and responsiveness. Incubation of serum starved human bronchial ASMCs with TNF- α , TGF, bFGF, and PDGF, but not IL-1 β , increased methyl-[(3)H]thymidine incorporation and cell number, mediated by the PI3K and MAPK signalling pathways. Regarding rabbit tracheal ASMC proliferation, TNF- α , IL-1 β , TGF, and PDGF increased methyl-[(3)H]thymidine incorporation in a PI3K- and MAPK-dependent manner. bFGF increased both methyl-[(3)H]thymidine incorporation and cell number. Moreover, incubation with TGF, bFGF and PDGF appears to drive human ASMCs towards a synthetic phenotype, as shown by the reduction of the percentage of cells expressing SM- α actin. In addition, the responsiveness of epithelium-denuded rabbit tracheal strips to carbachol was not significantly altered after 3-day treatment with bFGF. In conclusion, all the tested cytokines and growth factors increased ASMC proliferation to a different degree, depending on the specific cell type, with bronchial ASMCs being more prone to proliferation than tracheal ASMCs.

Smooth muscle in tissue remodeling and hyper-reactivity: airways and arteries

Smooth muscle comprises a key functional component of both the airways and their supporting vasculature. Dysfunction of smooth muscle contributes to and exacerbates a host of breathing-associated pathologies such as asthma, chronic obstructive pulmonary disease and pulmonary hypertension. These diseases may be marked by airway and/or vascular smooth muscle hypertrophy, proliferation and hyper-reactivity, and related conditions such as fibrosis and extracellular matrix remodeling. This review will focus on the contribution of airway or vascular smooth dysfunction to common airway diseases.

Peroxisome proliferator-activated receptor-alpha modulates the anti-inflammatory effect of glucocorticoids in a model of inflammatory bowel disease in mice

Glucocorticoids (GCs) are effective anti-inflammatory agents widely used in therapeutic approach to treatment of inflammatory bowel disease (IBD). Previous results suggest that peroxisome proliferator-activated receptor [alpha] (PPAR-[alpha]), an intracellular transcription factor activated by fatty acids, plays a role in control of inflammation. With the aim to characterize the role of PPAR-[alpha] in GC-mediated anti-inflammatory activity, we tested the efficacy of dexamethasone (DEX), a synthetic GC specific for GR, in an experimental model of IBD induced by dinitrobenzene sulfonic acid, comparing mice lacking PPAR-[alpha] (PPAR-[alpha]KO) with wild-type (WT) mice. Results indicate that DEX-mediated anti-inflammatory activity is weakened in PPAR-[alpha]KO mice as compared with WT controls. In particular, DEX was less effective in PPAR-[alpha]KO compared with WT mice, as evaluated by inhibition of proinflammatory cytokines production, cell migration, oxidative stress, apoptosis, and colon injury. These results indicate that PPAR-[alpha] can contribute to the anti-inflammatory activity of GCs in IBD.

Airway smooth muscle in asthma: phenotype plasticity and function

Clinical asthma is characterized by reversible airway obstruction which is commonly due to an exaggerated airway narrowing referred to as airway hyperresponsiveness (AHR). Although debate exists on the complex etiology of AHR, it is clear that airway smooth muscle (ASM) mediated airway narrowing is a major contributor to airway dysfunction. More importantly, it is now appreciated that smooth muscle is far from being a simple cell with only contractile ability properties. Rather, it is more versatile with the capacity to exhibit numerous cellular functions as it adapts to the microenvironment to which it is exposed. The emerging ability of individual smooth muscle cells to undergo changes in their phenotype (phenotype plasticity) and function (functional plasticity) in response to physiological and pathological cues is an important and active area of research. This article provides a brief review of the current knowledge and emerging concepts in the field of ASM phenotype and function both under healthy and asthmatic conditions.

Glucocorticoid- and protein kinase A-dependent transcriptome regulation in airway smooth muscle

Glucocorticoids (GCs) and protein kinase A (PKA)-activating agents (beta-adrenergic receptor agonists) are mainstream asthma therapies based on their ability to prevent or reverse excessive airway smooth muscle (ASM) constriction. Their abilities to regulate another important feature of asthma--excessive ASM growth--are poorly understood. Recent studies have suggested that GCs render agents of inflammation such as IL-1 beta and TNF-alpha mitogenic to ASM, via suppression of (antimitogenic) induced cyclooxygenase-2-dependent PKA activity. To further explore the mechanistic basis of these observations, we assessed the effects of epidermal growth factor and IL-1 beta stimulation, and the modulatory effects of GC treatment and PKA inhibition, on the ASM transcriptome by microarray analysis. Results demonstrate that ASM stimulated with IL-1 beta, in a manner that is often cooperative with stimulation with epidermal growth factor, exhibit a profound capacity to function as immunomodulatory cells. Moreover, results implicate an important role for induced autocrine/paracrine factors (many whose regulation was minimally affected by GCs or PKA inhibition) as regulators of both airway inflammation and ASM growth. Induction of numerous chemokines, in conjunction with regulation of proteases and agents of extracellular matrix remodeling, is suggested as an important mechanism promoting upregulated G protein-coupled receptor signaling capable of stimulating ASM growth. Additional functional assays suggest that intracellular PKA plays a critical role in suppressing the promitogenic effects of induced autocrine factors in ASM. Finally, identification and comparison of GC- and PKA-sensitive genes in ASM provide insight into the complementary effects of beta-agonist/GC combination therapies, and suggest specific genes as important targets for guiding the development of new generations of GCs and adjunct asthma therapies.

Is airway smooth muscle the "missing link" modulating airway inflammation in asthma?

Airway smooth muscle (ASM) plays a central role in regulating bronchomotor tone in patients with asthma. New evidence, however, suggests that ASM may also orchestrate and perpetuate airway inflammation by promoting the recruitment, activation, and trafficking of inflammatory cells in the airways. This review addresses the immunomodulatory function of ASM and highlights how such function may have therapeutic implications in the management of asthma.

Cytokines induce an early steroid resistance in airway smooth muscle cells: novel role of interferon regulatory factor-1

We have previously shown that long-term treatment of airway smooth muscle (ASM) cells with a combination of TNF-alpha and IFN-gamma impaired steroid anti-inflammatory action through the up-regulation of glucocorticoid receptor beta isoform (GRbeta) (Mol Pharmacol 2006;69:588-596). We here found that steroid actions could also be suppressed by short-term exposure of ASM cells to TNF-alpha and IFN-gamma (6 h) as shown by the abrogated glucocorticoid responsive element (GRE)-dependent gene transcription; surprisingly, neither GRalpha nuclear translocation nor GRbeta expression was affected by cytokine mixture. The earlier induction of CD38, a molecule recently involved in asthma, seen with TNF-alpha and IFN-gamma combination but not with cytokine alone, was also completely insensitive to steroid pretreatment. Chromatin-immunoprecipitation (IP) and siRNA strategies revealed not only increased binding of interferon regulatory factor 1 (IRF-1) transcription factor to CD38 promoter, but also its implication in regulating CD38 gene transcription. Interestingly, the capacity of fluticasone to completely inhibit TNF-alpha-induced IRF-1 expression, IRF-1 DNA binding, and transactivation activities was completely lost in cells exposed to TNF-alpha and IFN-gamma in combination. This early steroid dysfunction seen with cytokine combination could be reproduced by enhancing IRF-1 cellular levels using constitutively active IRF-1, which dose-dependently inhibited GRE-dependent gene transcription. Consistently, reducing IRF-1 cellular levels using siRNA approach significantly restored steroid transactivation activities. Collectively, our findings demonstrate for the first time that IRF-1 is a novel alternative GRbeta-independent mechanism mediating steroid dysfunction induced by pro-asthmatic cytokines, in part via the suppression of GRalpha activities.

Regulatory features of interleukin-1β-mediated prostaglandin E2 synthesis in airway smooth muscle

Exposure of airway smooth muscle (ASM) cells to the cytokine IL-1β results in an induction of PGE2 synthesis that affects numerous cell functions. Current dogma posits induction of COX-2 protein as the critical, obligatory event in cytokine-induced PGE2 production, although PGE2 induction can be inhibited without a concomitant inhibition of COX-2. To explore other putative regulatory features we examined the role of phospholipase A2 (PLA2) and PGE synthase (PGES) enzymes in IL-1β-induced PGE2 production. Treatment of human ASM cultures with IL-1β caused a time-dependent induction of both cytosolic PLA2 (cPLA2) and microsomal PGES (mPGES) similar to that observed for COX-2. Regulation of COX-2 and mPGES induction was similar, being significantly reduced by inhibition of p42/p44 or p38, whereas cPLA2 induction was only minimally reduced by inhibition of p38 or PKC. COX-2 and mPGES induction was subject to feed-forward regulation by PKA, whereas cPLA2 induction was not. SB-202474, an SB-203580 analog lacking the ability to inhibit p38 but capable of inhibiting IL-1β-induced PGE2 production, was effective in inhibiting mPGES but not COX-2 or cPLA2 induction. These data suggest that although COX-2, cPLA2, and mPGES are all induced by IL-β in human ASM cells, regulatory features of cPLA2 are dissociated, whereas those of COX-2 and mPGES are primarily associated, with regulation of PGE2 production. mPGES induction and, possibly, cPLA2 induction appear to cooperate with COX-2 to determine IL-1β-mediated PGE2 production in human ASM cells.

CD38 expression is insensitive to steroid action in cells treated with tumor necrosis factor-alpha and interferon-gamma by a mechanism involving the up-regulation of the glucocorticoid receptor beta isoform

Evidence shows that the CD38 molecule, recently involved in the two main features of asthma, bronchial hyper-responsiveness and airway inflammation, could represent a new potential therapeutic target for asthma. In this study, we investigated whether glucocorticoid (GC), the most effective treatment for lung diseases, can affect CD38 expression in human airway smooth muscle (ASM) cells treated with different pro-inflammatory cytokines, such as tumor necrosis factor-alpha (TNFalpha) and interferons (IFNs). We found that CD38 expression induced by TNFalpha alone was completely abrogated by fluticasone (100 nM), dexamethasone (1 microM), or budesonide (100 nM). In contrast, the synergistic induction of CD38 by the combination of TNFalpha with IFNgamma or IFNbeta, but not with IL-1beta or IL-13, was completely insensitive to the GC inhibitory effects. We also found that TNFalpha and IFNgamma impaired GC responsiveness by inhibiting steroid induced both 1) GRalpha-DNA binding activity and 2) GC-responsive element-(GRE)-dependent gene transcription. Although levels of the GC receptor (GR) alpha isoform remained unchanged, expression of GRbeta, the dominant-negative GR isoform, was synergistically increased by TNFalpha and IFNgamma with a GRalpha/GRbeta ratio of 1 to 3. More importantly, fluticasone failed to induce GRE-dependent gene transcription and to suppress TNFalpha-induced CD38 expression in ASM cells transfected with constitutively active GRbeta. We conclude that, upon pro-inflammatory cytokine stimulation, CD38 expression becomes insensitive to GC action by a mechanism involving the up-regulation of GRbeta isoform, thus providing a novel in vitro cellular model to dissect GC resistance in primary cells.

Differential effects of IL-10 on prostaglandin H synthase-2 expression and prostaglandin E2 biosynthesis between spleen and bone marrow macrophages

Different populations of mononuclear phagocytes (MO) show considerable diversity of cellular function including prostaglandin E2 (PGE2) biosynthesis. Certain bacterial components enhance PGE2 biosynthesis differentially in selected populations of MO. Interleukin (IL)-10 is proposed to inhibit modulation of PGE2 biosynthesis by down-regulating prostaglandin G/H synthase-2 (PGHS-2) expression. To assess whether IL-10 regulates PGE2 biosynthesis and PGHS-2 expression, splenic and bone marrow MO were isolated from IL-10-deficient (IL-10(-/-)), C57Bl/6 [wild-type (WT) control], and Balb/c (comparison control) mice and were treated with lipopolysaccharide (LPS) and/or interferon-gamma (IFN-gamma) as a model of bacterial inflammation. LPS-induced PGHS-2 expression was similar for splenic MO isolated from the three strains of mice. However, PGE2 released by LPS-treated splenic MO was significantly higher in IL-10(-/-) and Balb/c than in WT cells. In the presence of LPS and IFN-gamma, PGHS-2 expression and PGE2 release by IL-10(-/-) and Balb/c splenic MO were enhanced compared with stimulation with LPS alone or IFN-gamma alone. However, there was no significant increase in PGE2 release from WT splenic MO treated with LPS plus IFN-gamma despite increased PGHS-2 expression. In sharp contrast, PGHS-2 expression and PGE2 release by bone marrow MO were greatly enhanced in IL-10(-/-) cells compared with control cells. Our results indicate that IL-10 regulation of MO PGE2 biosynthesis and PGHS-2 expression is compartment-dependent and that PGE2 production is not linked directly to PGHS-2 levels. Furthermore, our findings emphasize strain-specific differences between C57Bl/6 and Balb/c mice, and Balb/c appears more similar to the IL-10(-/-) than to the C57Bl/6 with respect to prostanoid production.

Synthetic responses in airway smooth muscle

Human airway smooth muscle (ASM) has several properties and functions that contribute to asthma pathogenesis, and increasing attention is being paid to its synthetic capabilities. ASM can promote the formation of the interstitial extracellular matrix, and in this respect, ASM from asthmatic subjects compared with normal subjects responds differently, both qualitatively and quantitatively. Thus, ASM cells are important regulating cells that potentially contribute to the known alterations within the extracellular matrix in asthma. In addition, through integrin-directed signaling, extracellular matrix components can alter the proliferative, survival, and cytoskeletal synthetic function of ASM cells. ASM also functions as a rich source of biologically active chemokines and cytokines that are capable of perpetuating airway inflammation in asthma and chronic obstructive pulmonary disease by promoting recruitment, activation, and trafficking of inflammatory cells in the airway milieu. Emerging evidence shows that airway remodeling may also be a result of the autocrine action of secreted inflammatory mediators, including T(H)2 cytokines, growth factors, and COX-2-dependent prostanoids. Finally, ASM cells contain both beta(2)-adrenergic receptors and glucocorticoid receptors and may represent a key target for beta(2)-adrenergic receptor agonist/corticosteroid interactions. Combinations of long-acting beta(2)-agonists and corticosteroids appear to have additive and/or synergistic effects in inhibiting inflammatory mediator release and the migration and proliferation of ASM cells.

Proliferative aspects of airway smooth muscle

Increased airway smooth muscle (ASM) mass is perhaps the most important component of the airway wall remodeling process in asthma. Known mediators of ASM proliferation in cell culture models fall into 2 categories: those that activate receptors with intrinsic receptor tyrosine kinase activity and those that have their effects through receptors linked to heterotrimeric guanosine triphosphate-binding proteins. The major candidate signaling pathways activated by ASM mitogens are those dependent on extracellular signal-regulated kinase and phosphoinositide 3'-kinase. Increases in ASM mass may also involve ASM migration, and in culture, the key signaling mechanisms have been identified as the p38 mitogen-activated protein kinase and the p21-activated kinase 1 pathways. New evidence from an in vivo rat model indicates that primed CD4(+) T cells are sufficient to trigger ASM and epithelial remodeling after allergen challenge. Hyperplasia has been observed in an equine model of asthma and may account for the increase in ASM mass. Reduction in the rate of apoptosis may also play a role. beta(2)-Adrenergic receptor agonists and glucocorticoids have antiproliferative activity against a broad spectrum of mitogens, although it has become apparent that mitogens are differentially sensitive. Culture of ASM on collagen type I has been shown to enhance proliferative activity and prevent the inhibitory effect of glucocorticoids, whereas beta(2)-agonists are minimally affected. There is no evidence that long-acting beta(2)-agonists are more effective than short-acting agonists, but persistent stimulation of the beta(2)-adrenergic receptor probably helps suppress growth responses. The maximum response of fluticasone propionate against thrombin-induced proliferation is increased when it is combined with salmeterol.

Inhibition of IL-6, TNF-alpha, and cyclooxygenase-2 protein expression by prostaglandin E2-induced IL-10 in bone marrow-derived dendritic cells

Several endogenously produced mediators, including cytokines such as IL-6, IL-10, and TNF-alpha and prostanoids such as prostaglandin E(2) (PGE(2)), regulate dendritic cell (DC) function and contribute to immune homeostasis. In this study, we report that exogenous PGE(2) enhances the production of IL-10 from bone marrow-derived DC (BM-DC). IL-6, but not TNF-alpha, release is enhanced by PGE(2) in the presence of anti-IL-10, suggesting that endogenous IL-10 masks PGE(2)-induced IL-6. Furthermore, both exogenous IL-10 and PGE(2) inhibit LPS-induced IL-6 and TNF-alpha, whereas selective inhibition of cyclooxygenase-2 (COX-2) or addition of anti-IL-10 causes the reverse effects. Exogenous IL-10, but not IL-6, dose-dependently suppresses COX-2 protein expression and PGE(2) production, and TNF-alpha does not reverse this effect. In contrast, anti-IL-10 up-regulates prostanoid production by LPS-stimulated BM-DC. Taken together, our results show that in response to PGE(2), BM-DC produce IL-10, which in turn down-regulates their own production of IL-6-, TNF-alpha-, and COX-2-derived prostanoids, and plays crucial roles in determining the BM-DC pro-inflammatory phenotype.

Human neutrophil-derived elastase induces airway smooth muscle cell proliferation

Neutrophils and their derived elastase are abundant in chronic inflammatory responses of asthma. This study aimed to investigate the mitogenic effect of elastase on airway smooth muscle (ASM) cells and the implicated signal transduction pathway. Near confluent cultured human ASM cells were treated with human neutrophil elastase (HNE, 0.01 to 0.5 microg/ml) or vehicle for 24 hours with or without extracellular signal-regulated kinase (ERK) inhibitor (PD98059, 30 microM), p38 kinase inhibitor (SB203580, 10 microM) or elastase inhibitor II (100 microg/ml). The ASM cell numbers were counted by a hemocytometer and DNA synthesis was assessed by flowcytometry. Western blots analysis for the expression of ERK, p38 and cyclin D1 was determined. HNE dose-dependently increased ASM cell numbers and the percentage of cells entering S-phase of cell cycle. This response was abolished by neutrophil elastase inhibitors and attenuated by PD98059, but not SB203580. HNE increased ERK phosphorylation and cyclin D1 expression. Pretreatment with PD98059 significantly inhibited elastase-induced cyclin D1 activity. The increased ASM cellular gap and cell shape change by proteolytic activity of HNE may be contributory to ERK activation and therefore cell proliferation. Our results demonstrate that HNE is mitogenic for ASM cells by increasing cyclin D1 activity through ERK signaling pathway.

Ceramide mediates age-associated increase in macrophage cyclooxygenase-2 expression

Previously, we showed that macrophages (MØ) from old mice have significantly higher levels of lipopolysaccharide (LPS)-induced prostaglandin E(2) (PGE(2)) production than young mice, due to increased cyclooxygenase-2 (COX-2) mRNA levels. The aim of the current study was to determine the underlying mechanisms of age-associated increase in COX-2 gene expression. The results demonstrate that increased COX-2 mRNA expression in the old mice is due to a higher rate of transcription rather than increased stability of COX-2 mRNA. Furthermore, the results show that LPS-induced ceramide levels from the old mice are significantly higher than those of young mice, whereas there is no age-related difference in concentration of its down stream metabolite, sphingosine. The addition of ceramide in the presence or absence of LPS resulted in a significant increase in PGE(2) production in a dose- and time-dependent manner. Inhibition of ceramide conversion to sphingosine had no effect on this ceramide-induced effect. The ceramide-induced up-regulation in PGE(2) production was mediated through increase in COX activity and transcriptional up-regulation of COX-2 mRNA. Collectively, these data suggest that the age-associated increase in MØ COX-2 mRNA is due to transcriptional up-regulation. Furthermore, this increase in transcription is mediated by higher cellular ceramide concentration in old MØ compared with that of young MØ.

Mechanisms of cytokine effects on G protein-coupled receptor-mediated signaling in airway smooth muscle

Numerous in vitro and in vivo studies have implicated the cytokines interleukin-1 beta (IL-1 beta) and tumor necrosis factor-alpha (TNF-alpha) as mediators of airway inflammation and therefore potentially important substances in the pathogenesis of asthma. In this study, we examined the mechanisms by which IL-1 beta and TNF-alpha affect inhibition of cell growth, G protein-coupled receptor (GPCR) desensitization, and the recently reported adenylyl cyclase sensitization in human airway smooth muscle (HASM) cultures. Our findings demonstrate that adenylyl cyclase sensitization is independent of cytokine-mediated cyclooxygenase type 2 (COX-2) and prostaglandin E(2) (PGE(2)) induction, whereas COX-2 induction appears to be required for both growth inhibition and GPCR desensitization. However, GPCR desensitization was highly dependent on the presence of EGF during chronic treatment with cytokines, which could be explained by a synergistic effect of EGF on cytokine-mediated COX-2 and PGE(2) induction. Interestingly, various agents (including inhibitors of p42/p44 and p38 mitogen-activated protein kinase signaling) were significantly more effective in inhibiting cytokine-mediated PGE(2) induction, GPCR desensitization, and cell growth inhibition than in inhibiting COX-2 induction. These data demonstrate disparity in the requirement and sufficiency of COX-2 induction in promoting different functional effects of IL-1 beta and TNF-alpha in HASM.

Airway wall remodelling and hyperresponsiveness: modelling remodelling in vitro and in vivo

Airway wall remodelling contributes to the airway hyperresponsiveness that characterizes asthma. An increase in airway smooth muscle (ASM) volume is quantitatively important in the overall remodelling response and may be considered as a target for new therapeutic approaches to chronic asthma. ASM volume increases result from both hypertrophic as well as hyperplastic growth, the latter having been more extensively investigated. There are relatively few in vivo models available for analysis of the underlying mechanism(s) or their regulation by drugs. Human ASM in culture has been used to investigate potential stimuli for ASM proliferation and the signal transduction pathways that subserve these responses. The mitogen-activated protein kinase (MAPK) family members, ERK 1/2 and the phosphoinositol-3-kinase (PI3K) pathways each contribute to the signalling of G1 progression/S-phase entry in ASM. Glucocorticoids and beta(2)-adrenoceptor agonists attenuate, but do not prevent proliferative responses of ASM. Thus, there is scope for improved pharmacological control of this chronic and progressive aspect of asthma pathogenesis.

Thrombin-stimulated DNA synthesis in human cultured airway smooth muscle occurs independently of products of cyclo-oxygenase or 5-lipoxygenase

Arachidonic acid (AA) liberation and metabolism via cyclo-oxygenase or lipoxygenases may be an important regulatory pathway for mitogenic signalling in human cultured airway smooth muscle (ASM) cells. In cytokine-treated cells, thrombin markedly enhances production of the anti-mitogenic arachidonic acid metabolite, PGE(2). In this study, in the absence of cytokines, we examined the role of endogenous AA metabolism in thrombin-stimulated ASM DNA synthesis. Selective inhibitors of cyclo-oxygenase of 5-lipoxygenase metabolism had no significant effect on 0.3 U/ml thrombin-stimulated DNA synthesis. However, the non-selective, redox-active lipoxygenase inhibitors NDGA and BWA4C inhibited thrombin-stimulated DNA synthesis. Under basal conditions, and following stimulation by thrombin, the levels of the AA metabolites PGE(2), TxA(2), and LTC(4), remained below assay detection limits. Exogenous addition of AA, LTD(4), or 5-, 12-, and 15-HETE and HpETE metabolites had no consistent or substantial stimulatory effect on either basal or thrombin-stimulated DNA synthesis. These data suggest that the non-selective lipoxygenase inhibitors influence DNA synthesis via effects unrelated to lipoxygenase inhibition. The lack of detection of AA metabolites, the lack of influence of selective antagonists/inhibitors of the AA pathway, and the failure of selected AA metabolites to either enhance or directly stimulate DNA synthesis suggest that in the absence of cytokines, cyclo-oxygenase and lipoxygenase metabolism has little role in signalling of human ASM DNA synthesis by thrombin.

Il-10 is a central regulator of cyclooxygenase-2 expression and prostaglandin production

IL-10 is a potent anti-inflammatory and immune regulatory cytokine. IL-10(-/-) mice produce exaggerated amounts of inflammatory cytokines when stimulated with LPS, indicating that endogenous IL-10 is a central regulator of inflammatory cytokine production in vivo. PGs are lipid mediators that are also produced in large amounts during the inflammatory response. To study the role of IL-10 in the regulation of PG production during the acute inflammatory response, we evaluated LPS-induced cyclooxygenase (COX) expression and PG production in wild-type (wt) and IL-10(-/-) mice. LPS-induced PGE(2) production from IL-10(-/-) spleen cells was 5.6-fold greater than that from wt spleen cells. LPS stimulation resulted in the induction of COX-2 mRNA and protein in both wt and IL-10(-/-) spleen cells; however, the magnitude of increase in COX-2 mRNA was 5.5-fold greater in IL-10(-/-) mice as compared with wt mice. COX-1 protein levels were not affected by LPS stimulation in either wt or IL-10(-/-) mice. Neutralization of IFN-gamma, TNF-alpha, or IL-12 markedly decreased the induction of COX-2 in IL-10(-/-) spleen cells, suggesting that increased inflammatory cytokine production mediates much of the COX-2 induction in IL-10(-/-) mice. Treatment of IL-10(-/-) mice with low doses of LPS resulted in a marked induction of COX-2 mRNA in the spleen, whereas wt mice had minimal expression of COX-2 mRNA. These findings indicate that, in addition to IL-10's central role in the regulation of inflammatory cytokines, endogenous IL-10 is an important regulator of PG production in the response to LPS.

The importance of ERK activity in the regulation of cyclin D1 levels and DNA synthesis in human cultured airway smooth muscle

The relationship between persistent ERK (extracellular signal-regulated kinase) activity, cyclin D1 protein and mRNA levels and cell cycle progression in human cultured airway smooth muscle was examined in response to stimulation by ET-1 (endothelin-1), thrombin and bFGF (basic fibroblast growth factor). Thrombin (0.3 and 3 u ml(-1)) and bFGF (0.3 and 3 nM) increased ERK activity for more than 2 h and increased cell number, whereas ET-1 (100 nM) transiently stimulated ERK activity and was non-mitogenic. The MEK1 (mitogen-activated ERK kinase) inhibitor, PD 98059 (30 microM), inhibited both ERK phosphorylation and activity, and either prevented (thrombin 0.3 and 3 u ml(-1), bFGF 300 pM) or attenuated (bFGF 3 nM) DNA synthesis. Thrombin and bFGF increased both cyclin D1 mRNA and protein levels. PD 98059 decreased cyclin D1 protein levels stimulated by the lower but not higher thrombin concentrations. Moreover, increases in cyclin D1 mRNA levels were unaffected by PD 98059 pretreatment, irrespective of the mitogen or its concentration, suggesting that inhibition of cyclin D1 protein levels occurred by a post-transcriptional mechanism. These findings indicate that the control of cyclin D1 protein levels may occur independently of the MEK1/ERK signalling pathways. The inhibition of S phase entry by PD 98059 at higher thrombin concentrations appears to result from effects on pathways downstream or parallel to those regulating cyclin D1 protein levels. These findings suggest heterogeneity in the signalling of DNA synthesis in human cultured airway smooth muscle.

Aging increases PGHS-2-dependent vasoconstriction in rat mesenteric arteries

During aging, the vascular endothelium changes functionally and morphologically. Although previous studies have shown that endothelium-derived eicosanoids increase vessel tone in aging, the precise mechanism(s) has not been fully determined. We hypothesized that aging would increase prostaglandin H synthase (PGHS)-dependent vasoconstriction as well as decrease nitric oxide-dependent relaxation. Mesenteric arteries from 3-month-old (n=9) and 12-month-old (n=14) female Sprague-Dawley rats were studied in a myograph system. Aging significantly blunted the endothelium-dependent relaxation response to methacholine compared with young rats (EC(50)=7.77x10(-8) versus 2.68x10(-8) mol/L, P<0. 05). Nitric oxide synthase inhibition reduced methacholine-induced relaxation in the young (P<0.05) but had no effect in the aging group. Specific inhibition of the PGHS-1 isoform did not significantly affect methacholine-mediated relaxation in the young or aged groups. However, PGHS-2 inhibition greatly enhanced relaxation to methacholine (1.59x10(-8) versus 7.77x10(-8) mol/L, P<0.01) in the aged group only, restoring vessel function to that of the young. In the aged group, inhibition of the prostaglandin H(2)/thromboxane A(2) receptor enhanced methacholine-dependent relaxation similar to that of PGHS-2 inhibition. Moreover, arterial expression of PGHS-2 protein increased with age. In summary, nitric oxide-dependent modulation of vessel function decreased with age, PGHS-1 did not significantly affect vessel tone in either the young or aging group, and PGHS-2 greatly increased vasoconstriction in aging. Thus, we have identified enhanced PGHS-2-mediated vasoconstriction in aging and therefore suggest that inhibition of this isoform is potentially a new target for therapeutic intervention to improve vascular function.

Differential effects of nitric oxide on the activity of prostaglandin endoperoxide H synthase-1 and -2 in vascular endothelial cells

A number of studies have demonstrated that prostacyclin and nitric oxide (NO) regulate blood pressure, blood flow and platelet aggregation. In this paper, we have examined the possible relationship between NO and prostaglandin endoperoxide H synthase (PGHS)-1 and -2 activities in cultured bovine aortic endothelial cells. In the non-activated condition endothelial cells expressed PGHS-1 activity alone. When these cells were pretreated with aspirin to inactivate their PGHS-1 and then activated by serum and phorbol ester (TPA) for 6 h, the cells expressed PGHS-2 activity alone. The PGHS activity was assessed by the generation of 6-ketoprostaglandin F1alpha (6-ketoPGF1alpha), a stable metabolite of prostacyclin, after the treatment of these cells with arachidonic acid. The simultaneous addition of NOC-7, a NO donor, with arachidonic acid did not affect the production of 6-ketoPGF1alpha in PGHS-1 expressed cells, but attenuated it in PGHS-2-expressed cells. The inhibitory effect of NOC-7 on PGHS-2 activity was dose dependent, and the different effects of NOC-7 on the activities of PGHS isozymes were also observed in other NO donors. To confirm the different effect of NO on PGHS isozymes demonstrated in the cultured endothelial cells, we carried out an ex vivo perfusion assay in aorta isolated from normal and lipopolysaccharide (LPS)-treated rats. In the aortae isolated from normal rats, where dominant expression of PGHS-1 was expected, the NO donor did not affect the PGHS activity, while in aortae isolated from LPS-treated rats, where PGHS-2 was dominantly expressed, the NO donor dramatically inhibited the PGHS activity, suggesting that NO suppressed PGHS-2 activity alone. The inhibitory effect of NO on PGHS-2 activity was not mediated by cyclic GMP (cGMP), since (a) methylene blue, an inhibitor of soluble guanylate cyclase did not abolish the inhibitory effect of the NO donor on PGHS-2 activity, and (b) 8-Br-cGMP, a permeable cGMP analogue, failed to mimic the effect of NO donors. These data suggest that the effect of NO on prostacyclin production in endothelial cells was dependent on the expression rate of PGHS-1 and PGHS-2 in the cells.

Beta2-adrenergic receptor agonists and cAMP arrest human cultured airway smooth muscle cells in the G(1) phase of the cell cycle: role of proteasome degradation of cyclin D1

Hyperplasia of airway smooth muscle (ASM) contributes to the airway hyperresponsiveness that characterizes asthma. We have investigated the relationship between cAMP-induced growth arrest of ASM cells and thrombin-stimulated, extracellular-regulated protein kinase (ERK) activity, cyclin D1, and the restriction protein retinoblastoma. The beta(2)-adrenergic receptor agonist albuterol (100 nM) inhibited DNA synthesis after incubation with ASM for periods as brief as 1 h when these coincided with the timing of the restriction point. Inhibition of thrombin-stimulated DNA synthesis by albuterol (1-100 nM), 8-bromo-cAMP (300 microM), or prostaglandin E(2) (1 microM) was accompanied by a reduction in cyclin D1 protein levels. The ERK kinase inhibitor PD98059 (3-30 microM) attenuated thrombin-stimulated ERK phosphorylation and activity and the increase in cyclin D1 protein levels, as did albuterol (1-100 nM) or 8-bromo-cAMP (300 microM). In contrast, neither albuterol (100 nM) nor PD98059 (30 microM) reduced cyclin D1 mRNA levels between 4 and 20 h after thrombin addition, which suggests that elevation of cAMP regulates cyclin D1 by a post transcriptional mechanism. The proteasome inhibitor MG132 (30 and 100 nM) and the calpain I inhibitor N-acetyl-Leu-Leu-leucinal (10 microM) attenuated the reduction in thrombin-stimulated cyclin D1 levels in ASM exposed to albuterol (100 nM), 8-bromo-cAMP (300 microM), or the phosphodiesterase inhibitor isobutylmethylxanthine (100 microM). Thus, the cAMP-induced arrest of ASM in the G(1) phase of the cell cycle is associated with a proteasomal degradation of cyclin D1 protein and a reduced protein retinoblastoma phosphorylation that prevents passage through the restriction point.