Stimulation of interleukin-1 alpha and interleukin-1 beta production in human monocytes by protein phosphatase 1 and 2A inhibitors

Unconventional Pathways of Secretion Contribute to Inflammation

In the conventional pathway of protein secretion, leader sequence-containing proteins leave the cell following processing through the endoplasmic reticulum (ER) and Golgi body. However, leaderless proteins also enter the extracellular space through mechanisms collectively known as unconventional secretion. Unconventionally secreted proteins often have vital roles in cell and organism function such as inflammation. Amongst the best-studied inflammatory unconventionally secreted proteins are interleukin (IL)-1β, IL-1α, IL-33 and high-mobility group box 1 (HMGB1). In this review we discuss the current understanding of the unconventional secretion of these proteins and highlight future areas of research such as the role of nuclear localisation.

Mechanism of PP2A-mediated IKK beta dephosphorylation: a systems biological approach

Background

Biological effects of nuclear factor-κB (NFκB) can differ tremendously depending on the cellular context. For example, NFκB induced by interleukin-1 (IL-1) is converted from an inhibitor of death receptor induced apoptosis into a promoter of ultraviolet-B radiation (UVB)-induced apoptosis. This conversion requires prolonged NFκB activation and is facilitated by IL-1 + UVB-induced abrogation of the negative feedback loop for NFκB, involving a lack of inhibitor of κB (IκBα) protein reappearance. Permanent activation of the upstream kinase IKKβ results from UVB-induced inhibition of the catalytic subunit of Ser-Thr phosphatase PP2A (PP2Ac), leading to immediate phosphorylation and degradation of newly synthesized IκBα.

Results

To investigate the mechanism underlying the general PP2A-mediated tuning of IKKβ phosphorylation upon IL-1 stimulation, we have developed a strictly reduced mathematical model based on ordinary differential equations which includes the essential processes concerning the IL-1 receptor, IKKβ and PP2A. Combining experimental and modelling approaches we demonstrate that constitutively active, but not post-stimulation activated PP2A, tunes out IKKβ phosphorylation thus allowing for IκBα resynthesis in response to IL-1. Identifiability analysis and determination of confidence intervals reveal that the model allows reliable predictions regarding the dynamics of PP2A deactivation and IKKβ phosphorylation. Additionally, scenario analysis is used to scrutinize several hypotheses regarding the mode of UVB-induced PP2Ac inhibition. The model suggests that down regulation of PP2Ac activity, which results in prevention of IκBα reappearance, is not a direct UVB action but requires instrumentality.

Conclusion

The model developed here can be used as a reliable building block of larger NFκB models and offers comprehensive simplification potential for future modeling of NFκB signaling. It gives more insight into the newly discovered mechanisms for IKK deactivation and allows for substantiated predictions and investigation of different hypotheses. The evidence of constitutive activity of PP2Ac at the IKK complex provides new insights into the feedback regulation of NFκB, which is crucial for the development of new anti-cancer strategies.

Lysophosphatidic acid inhibits bacterial endotoxin-induced pro-inflammatory response: potential anti-inflammatory signaling pathways

Previous studies have demonstrated that heterotrimeric guanine nucleotide-binding regulatory (Gi) protein-deficient mice exhibit augmented inflammatory responses to lipopolysaccharide (LPS). These findings suggest that Gi protein agonists will suppress LPS-induced inflammatory gene expression. Lysophosphatidic acid (LPA) activates G protein-coupled receptors leading to Gi protein activation. We hypothesized that LPA will inhibit LPS-induced inflammatory responses through activation of Gi-coupled anti-inflammatory signaling pathways. We examined the anti-inflammatory effect of LPA on LPS responses both in vivo and in vitro in CD-1 mice. The mice were injected intravenously with LPA (10 mg/kg) followed by intraperitoneal injection of LPS (75 mg/kg for survival and 25 mg/kg for other studies). LPA significantly increased the mice survival to endotoxemia (P < 0.05). LPA injection reduced LPS-induced plasma TNF-alpha production (69 +/- 6%, P < 0.05) and myeloperoxidase (MPO) activity in lung (33 +/- 9%, P < 0.05) as compared to vehicle injection. LPS-induced plasma IL-6 was unchanged by LPA. In vitro studies with peritoneal macrophages paralleled results from in vivo studies. LPA (1 and 10 microM) significantly inhibited LPS-induced TNFalpha production (61 +/- 9% and 72 +/- 9%, respectively, P < 0.05) but not IL-6. We further demonstrated that the anti-inflammatory effect of LPA was reversed by ERK 1/2 and phosphatase inhibitors, suggesting that ERK 1/2 pathway and serine/threonine phosphatases are involved. Inhibition of phosphatidylinositol 3 (PI3) kinase signaling pathways also partially reversed the LPA anti-inflammatory response. However, LPA did not alter NFkappaB and peroxisome proliferator-activated receptor gamma (PPARgamma) activation. Inhibitors of PPARgamma did not alter LPA-induced inhibition of LPS signaling. These studies demonstrate that LPA has significant anti-inflammatory activities involving activation of ERK 1/2, serine/threonine phosphatases, and PI3 kinase signaling pathways.

Identification of PP2A as a crucial regulator of the NF-kappaB feedback loop: its inhibition by UVB turns NF-kappaB into a pro-apoptotic factor

Although nuclear factor-kappaB (NF-kappaB) usually exerts anti-apoptotic activity, upon activation by interleukin-1 (IL-1) it enhances ultraviolet-B radiation (UVB)-induced apoptosis. This paradoxical effect is associated with NF-kappaB-dependent pronounced secretion of tumour necrosis factor-alpha (TNF) which activates TNF-R1 in an autocrine fashion to enhance UVB-induced apoptosis. We demonstrate that sustained TNF transcription in UVB+IL-1-treated cells involves complete abrogation of the negative feedback loop of NF-kappaB preventing IkappaBalpha resynthesis, hence allowing uncontrolled NF-kappaB activity. We show that IkappaBalpha is not transcriptionally inhibited but resynthesized protein is immediately marked for degradation due to persistent inhibitor of kappaB kinasebeta (IKKbeta) activity. Continuous IKKbeta phosphorylation and activation is caused by UVB-mediated inhibition of the phosphatase PP2A. This study demonstrates that the cellular response to different NF-kappaB activators may be converted to the opposite reaction when both stimuli act in concert. Our data shed new light on the significance of negative feedback regulation of NF-kappaB and identifies PP2A as the key regulator of this process.

Light-induced photoreceptor degeneration may involve the NF kappa B/caspase-1 pathway in vivo

PURPOSE

To determine the role of nuclear factor-kappaB (NFkappaB) in light-induced photoreceptor degeneration.

METHODS

Dark-adapted BALB/cJ mice, 4-8 weeks, were exposed to an intense green light (3.1-3.5 klux) for 1, 3, 6, 9, 12, or 24 h and killed immediately after exposure. The photoreceptor apoptosis was detected by TUNEL. Co-localization of NFkappaB p65 immunoreactivity and TUNEL in photoreceptor cells was detected by double immunolabeling. The protein levels of X-linked inhibitor of apoptosis protein (XIAP), Bcl-xL, caspase-1, and opsin after light exposure were analyzed by Western blot analysis. In addition, the initiation of NFkappaB activation was assessed by measuring the increase in phosphorylated IkappaBalpha (pIkappaBalpha). Immunohistochemical localization of caspase-1 was also performed on the mouse retinas.

RESULTS

Co-localization of NFkappaB p65 immunoreactivity with TUNEL was observed in scattered photoreceptor cells after 24 h of light exposure. The amount of pIkappaBalpha was increased after 1 h of light exposure, and in parallel, the amounts of XIAP and Bcl-xL were increased at 1 h. In contrast, caspase-1 did not increase until after 6 h of light exposure. Caspase-1-immunolabeling was observed in scattered photoreceptor cells after 3 h of light exposure but was markedly increased in many more cells at 6 h.

CONCLUSIONS

These findings suggest that NFkappaB may play an anti-apoptotic role in the early response to light stress and that photoreceptor apoptosis induced by light stress may be mediated through an NFkappaB/caspase-1 pathway.

Stimulation of hepatocyte growth factor production in human fibroblasts by the protein phosphatase inhibitor okadaic acid

In this study, we examined whether the production of hepatocyte growth factor (HGF) in fibroblasts is regulated by protein phosphatase(s). Inhibitors of the enzymes okadaic acid and calyculin A were used for this purpose. Both inhibitors markedly stimulated HGF production in human skin fibroblasts in a dose-dependent manner. The effects of okadaic acid and calyculin A were maximal at 25-37.5 and 1.25 nM, respectively. Highly active HGF production in MRC-5 human embryonic lung fibroblasts was also promoted by both inhibitors. The effect of okadaic acid was accompanied by an up-regulation of HGF gene expression. The stimulating effect of okadaic acid on HGF production was synergistic with that of phorbol 12-myristate 13-acetate (PMA) and epidermal growth factor (EGF), whereas it was additive to the effect of cholera toxin. The protein kinase C (PKC) inhibitor GF 109203X inhibited the effect of PMA, but not of okadaic acid and EGF. The effect of okadaic acid as well as EGF was not inhibited, but rather enhanced in human skin fibroblasts pretreated for 24 hr with a high dose of PMA to deplete PKC, as compared with its effect in untreated cells. PD 98059, an inhibitor of mitogen-activated protein (MAP) kinase kinase, suppressed the effects of okadaic acid and EGF, but not those of cholera toxin and 8-bromo-adenosine 3',5'-cyclic monophosphate (cAMP). These results suggest that HGF production in human skin fibroblasts is down-regulated by protein phosphatase(s) and that HGF production stimulated by okadaic acid is, at least in part, dependent on the activation of the MAP kinase cascade.

The p38 MAP kinase inhibitor SB203580 enhances nuclear factor-kappa B transcriptional activity by a non-specific effect upon the ERK pathway

In the present study we investigated a possible role for the p38 mitogen-activated protein (MAP) kinase pathway in mediating nuclear factor-kappa B (NF-kappaB) transcriptional activity in the erythroleukaemic cell line TF-1. TF-1 cells stimulated with the phosphatase inhibitor okadaic acid (OA) demonstrated enhanced NF-kappaB and GAL4p65-regulated transcriptional activity which was associated with elevated p38 phosphorylation. However, pretreatment with the p38 MAPK specific inhibitor SB203580 (1 microM) or overexpression of kinase-deficient mutants of MKK3 or MKK6 did not affect OA-enhanced NF-kappaB transcriptional potency, as determined in transient transfection assays. In fact, 5 and 10 microM SB203580 enhanced rather than inhibited NF-kappaB-mediated promoter activity by 2 fold, which was independent of phosphorylation of the p65 subunit. The SB203580-mediated increase in NF-kappaB transcriptional activity was associated with enhanced phosphorylation of extracellular signal-regulated kinase (ERK)1/2 and c-Jun N-terminal kinase (JNK), but not p38 kinase. Overexpression of kinase-deficient mutants belonging to the ERK1/2, JNK, and p38 pathways showed that only dominant-negative Raf-1 abrogated SB203580-enhanced NF-kappaB activity. This would implicate the involvement of the ERK1/2 pathway in the enhancing effects of SB203580 on NF-kappaB-mediated gene transcription. This study demonstrates that the p38 MAP kinase pathway is not involved in the OA-induced activation of NF-kappaB. SB203580 at higher concentrations activates the ERK pathway, which subsequently enhances NF-kappaB transcriptional activity.

Extracellular-Regulated Kinase 1/2, Jun N-Terminal Kinase, and c-Jun Are Involved in NF-κB-Dependent IL-6 Expression in Human Monocytes

In the present study we investigated the possible involvement of the mitogen-activated protein kinase family members extracellular-regulated kinase 1/2 (ERK1/2) and c-Jun N-terminal kinase (JNK) in mediating IL-6 gene expression in human monocytes, in particular their role in enhancing NF-κB activity. Freshly isolated monocytes treated with the protein phosphatase inhibitor okadaic acid secreted high levels of IL-6 protein, which coincided with enhanced binding activity of NF-κB as well as with phosphorylation and activation of the ERK1/2 and JNK proteins. The ERK pathway-specific inhibitor PD98059 inhibited IL-6 secretion from monocytes. Transient overexpression of inactive mutants of either Raf-1 or JNK1 showed that both pathways were involved in κB-dependent IL-6 promoter activity. By using PD98059, we demonstrated that the Raf1/MEK1/ERK1/2 pathway did not affect the DNA binding of NF-κB but, rather, acted at the level of transcriptional activity of NF-κB. Interestingly, it was shown that NF-κB-mediated gene transcription, both in the context of the IL-6 promoter as well as on its own, was dependent on both serine kinase activity and interaction with c-Jun protein. We conclude that okadaic acid-induced IL-6 gene expression is at least partly mediated through the ERK1/2 and JNK pathway-dependent activation of NF-κB transcriptional capacity. Our results suggest that the JNK pathway may regulate NF-κB-mediated gene transcription through its phosphorylation and activation of c-Jun.

Expression and activation of lyn in macrophages treated in vitro with cisplatin: regulation by kinases, phosphatases and Ca2+/calmodulin

Cisplatin [cis-dichlorodiammine platinum (II)], a potent chemoimmunotherapeutic drug, activates macrophages to tumoricidal state which is inhibited by protein tyrosine kinase(s) inhibitor. Cisplatin induces protein tyrosine phosphorylation of a number of cellular proteins suggesting the involvement of protein tyrosine kinase(s) in the activation process of macrophages. Therefore, the effect of cisplatin treatment on the expression and activation of lyn, a protein tyrosine kinase of src family, in macrophages was investigated. The underlying mechanism of lyn expression and activation was also analyzed. Cisplatin treatment increased lyn expression and activation in macrophages within 5 min of treatment. The expression and activation of lyn were observed to be biphasic processes in cisplatin-treated macrophages with the first peak appearing at 15 min and the second peak at 2 h of treatment. The appearance of second phase of lyn activation and second phase of lyn expression were two unrelated processes. The second peak of lyn activation was produced by the autocrine action of some soluble product(s) of cisplatin-treated macrophages, whereas the second phase of lyn expression was due to some intracellular factor. It was further observed that cisplatin-induced lyn expression and activation involves serine/threonine phosphatases 1/2A, protein tyrosine phosphatases, protein tyrosine kinase and protein kinase C. It was also observed that Ca2+/calmodulin and calmodulin-dependent kinases are involved in the regulation of cisplatin-induced lyn expression and activation in macrophages.

Interleukin-1beta expression after inhibition of protein phosphatases in endotoxin-tolerant cells

Endotoxin (lipopolysaccharide [LPS]) is a potent activator of a number of inflammatory genes in blood leukocytes, including interleukin-1 (IL-1). Blood leukocytes isolated from patients with septic shock fail to produce IL-1 in response to LPS, a phenomenon known as endotoxin tolerance. To study the regulation of IL-1 expression in endotoxin-tolerant cells, the protein phosphatase inhibitor okadaic acid was used to examine the effects of protein phosphorylation on IL-1beta gene expression. We found that endotoxin-tolerant cells produced normal levels of IL-1beta when protein phosphatases were inhibited. In the human pro-monocytic cell line THP-1, okadaic acid increased mRNA accumulation and synthesis of IL-1beta protein. Normal and endotoxin-tolerant THP-1 cells accumulated IL-1beta mRNA and protein with similar delayed kinetics. Okadaic acid stabilization of IL-1beta mRNA appears to be the primary mechanism through which endotoxin-tolerant cells accumulate IL-1beta mRNA and protein. Endotoxin-tolerant cells were unable to activate transcription in response to okadaic acid. However, the transcription factor NF-kappaB, which is known to be involved in IL-1beta expression, was translocated to the nucleus in both normal and endotoxin-tolerant cells after treatment with okadaic acid. These studies revealed that protein phosphorylation can affect gene expression on at least two distinct levels, transcription factor activation and mRNA stability. Endotoxin-tolerant cells have decreased transcription activation potential, while IL-1beta mRNA stability remains responsive to protein phosphorylation.

Stimulation of interleukin-8 production by okadaic acid and vanadate in a human promyelocyte cell line, an HL-60 subline. Possible role of mitogen-activated protein kinase on the okadaic acid-induced NF-kappaB activation

Most types of cells can produce interleukin (IL)-8 in response to various inflammatory stimuli. To study the role of protein phosphatases in the signal transduction leading to IL-8 production, a subline of HL-60 (C-15) was treated with okadaic acid (OA) and sodium orthovanadate (VA), inhibitors of phosphoserine/phosphothreonine phosphatase and phosphotyrosine phosphatase, respectively. Both OA and VA dramatically increased IL-8 secretion up to 200-fold in the HL-60 cells. OA and VA stimulation was accompanied by a marked increase in IL-8 mRNA expression and also by activation of a transcription factor, NF-kappaB. In addition, an essential role of the NF-kappaB site in the IL-8 gene activation was confirmed by the chloramphenicol acetyltransferase assay. IL-8 production by OA or VA was inhibited by protein kinase inhibitors, including staurosporine, H-7, K252a, herbimycin A, and genistein. Both OA and VA induced significant tyrosine phosphorylation of p44, which was presumed to be Erk1, a member of the mitogen-activated protein kinase family, with concomitant activation of the mitogen-activated protein kinase activity. In parallel, rapid degradation of IkappaB-alpha, an inhibitory component of NF-kappaB, was observed. Since OA-activated Erk1 phosphorylated recombinant IkappaB-alpha in vitro, we assumed that Erk1 is involved in the phosphorylation and subsequent degradation of IkappaB-alpha, thus leading to the activation of IL-8 gene transcription.

Signaling pathway triggered by a short immunomodulating peptide on human monocytes

A short synthetic peptide (Pa) containing a structural motif ("2-6-11" motif) present in a number of human extracellular matrix proteins was found to stimulate the production of cytokines IL-1alpha, IL-1beta, IL-6, and TNFalpha by human peripheral blood mononuclear cells. We have now investigated the signal transduction pathway involved in the elicitation of these immunomodulating properties on isolated human monocytes. Our results show that active peptide Pa provoked phosphoinositide hydrolysis, intracellular calcium elevation, and cAMP accumulation. Herbimycin A, an inhibitor of protein tyrosine kinases (PTK), markedly reduced these effects of peptide Pa. We have also found that this peptide stimulated CREB, NF-kappaB, and AP-1 DNA-binding activity. With the help of inhibitors of PTK (herbimycin A), phospholipase C (neomycin sulfate), protein kinase C (bis-indolyl maleimide), protein kinase A (H89), and the calmodulin antagonist W-7, as well as cholera toxin, an agent that increases intracellular cAMP, we showed that cytokine (IL-1alpha, IL-1-beta, IL-6, and TNFalpha) production could be modified by the signal transduction pathway triggered by peptide Pa on monocytes.

All-trans-retinoic acid in POEMS syndrome. Therapeutic effect associated with decreased circulating levels of proinflammatory cytokines

Chronically elevated serum levels of proinflammatory cytokines is a feature of the syndrome known as POEMS (plasma cell dyscrasia with polyneuropathy, organomegaly, endocrinopathy, monoclonal [M] protein, skin changes). A patient had a POEMS syndrome with thrombocytosis and biclonal gammopathy and was treated as follows: all-trans-retinoic acid (tretinoin) at 90 mg/day for 50 days, no treatment for 70 days, readministration of tretinoin at 75 mg/day for 180 days. Focal bone lesion irradiation was performed from day 26 to day 50. Serum levels of interleukin-6 (IL-6), tumor necrosis factor alpha (TNF alpha), and IL-1 beta normalized within 7 days after the first administration of tretinoin, transiently increased at the time of radiotherapy, increased again after withdrawal of the tretinoin, and decreased again after its reintroduction. The platelet count and gammopathy paralleled the changes in the cytokine levels. This study documents in vivo the ability of all-trans-retinoic acid to down-regulate the release of IL-6, IL-1 beta, and TNF alpha, and illustrates its potential as a therapeutic agent in conditions associated with chronic overproduction of proinflammatory cytokines.

Altered cell signaling and mononuclear phagocyte deactivation during intracellular infection

Given the critical antimicrobial properties of mononuclear phagocytes, an important concern in cell biology and immunology has been to understand how intracellular microbes are able to establish states of chronic infection within these cells. Recent studies indicate that mononuclear phagocytes become functionally deactivated during intracellular infection. Here, Neil Reiner considers the experimental evidence to indicate that this is a frequent event that may be accounted for by induced defects in the signaling pathways required to bring cells to an activated state.