Endotrophin, a Key Marker and Driver for Fibroinflammatory Disease

Our overview covers several key areas related to recent results obtained for collagen type VI and endotrophin (ETP). (1) An introduction to the history of ETP, including how it was identified, how it is released, and its function and potential receptors. (2) An introduction to the collagen family, with a focus on what differentiates collagen type VI from an evolutionary standpoint. (3) An overview of collagen type VI, the 6 individual chains (COL6A1, A2, A3, A4, A5, and A6), their differences and similarities, as well as their expression profiles and function. (4) A detailed analysis of COL6A3, including the cleaved product endotrophin, and what separates it from the other 5 collagen 6 molecules, including its suggested function based on insights gained from knockout and gain of function mouse models. (5) The pathology of ETP. What leads to its presence and release and what are the consequences thereof? (6) Functional implications of circulating ETP. Here we review the data with the functional roles of ETP in mind. (7) We propose that ETP is a mediator for fibrotic (or fibroinflammatory) disorders. Based on what we know about ETP, we have to consider it as a target for the treatment of fibrotic (or fibroinflammatory) disorders. What segment(s) of the patient population would most dramatically respond to an ETP-targeted intervention? How can we find the population that would profit most from an intervention? We aim to present a broad overview over the ETP field at large, providing an assessment of where the future research efforts need to be placed to tap into the vast potential of ETP, both as a marker and as a target in different diseases.

A highly selective humanized DDR1 mAb reverses immune exclusion by disrupting collagen fiber alignment in breast cancer

BACKGROUND

Immune exclusion (IE) where tumors deter the infiltration of immune cells into the tumor microenvironment has emerged as a key mechanism underlying immunotherapy resistance. We recently reported a novel role of discoidin domain-containing receptor 1 (DDR1) in promoting IE in breast cancer and validated its critical role in IE using neutralizing rabbit monoclonal antibodies (mAbs) in multiple mouse tumor models.

METHODS

To develop a DDR1-targeting mAb as a potential cancer therapeutic, we humanized mAb9 with a complementarity-determining region grafting strategy. The humanized antibody named PRTH-101 is currently being tested in a Phase 1 clinical trial. We determined the binding epitope of PRTH-101 from the crystal structure of the complex between DDR1 extracellular domain (ECD) and the PRTH-101 Fab fragment with 3.15 Å resolution. We revealed the underlying mechanisms of action of PRTH-101 using both cell culture assays and in vivo study in a mouse tumor model.

RESULTS

PRTH-101 has subnanomolar affinity to DDR1 and potent antitumor efficacy similar to the parental rabbit mAb after humanization. Structural information illustrated that PRTH-101 interacts with the discoidin (DS)-like domain, but not the collagen-binding DS domain of DDR1. Mechanistically, we showed that PRTH-101 inhibited DDR1 phosphorylation, decreased collagen-mediated cell attachment, and significantly blocked DDR1 shedding from the cell surface. Treatment of tumor-bearing mice with PRTH-101 in vivo disrupted collagen fiber alignment (a physical barrier) in the tumor extracellular matrix (ECM) and enhanced CD8+ T cell infiltration in tumors.

CONCLUSIONS

This study not only paves a pathway for the development of PRTH-101 as a cancer therapeutic, but also sheds light on a new therapeutic strategy to modulate collagen alignment in the tumor ECM for enhancing antitumor immunity.

Sustained plasma hepcidin suppression and iron elevation by Anticalin-derived hepcidin antagonist in cynomolgus monkey

BACKGROUND AND PURPOSE

Anaemia of chronic disease (ACD) has been linked to iron-restricted erythropoiesis imposed by high circulating levels of hepcidin, a 25 amino acid hepatocyte-derived peptide that controls systemic iron homeostasis. Here, we report the engineering of the human lipocalin-derived, small protein-based anticalin PRS-080 hepcidin antagonist with high affinity and selectivity.

EXPERIMENTAL APPROACH

Anticalin- and hepcidin-specific pharmacokinetic (PK)/pharmacodynamic modelling (PD) was used to design and select the suitable drug candidate based on t_1/2 extension and duration of hepcidin suppression. The development of a novel free hepcidin assay enabled accurate analysis of bioactive hepcidin suppression and elucidation of the observed plasma iron levels after PRS-080-PEG30 administration in vivo.

KEY RESULTS

PRS-080 had a hepcidin-binding affinity of 0.07 nM and, after coupling to 30 kD PEG (PRS-080-PEG30), a t_1/2 of 43 h in cynomolgus monkeys. Dose-dependent iron mobilization and hepcidin suppression were observed after a single i.v. dose of PRS-080-PEG30 in cynomolgus monkeys. Importantly, in these animals, suppression of free hepcidin and subsequent plasma iron elevation were sustained during repeated s.c. dosing. After repeated dosing and followed by a treatment-free interval, all iron parameters returned to pre-dose values.

CONCLUSIONS AND IMPLICATIONS

In conclusion, we developed a dose-dependent and safe approach for the direct suppression of hepcidin, resulting in prolonged iron mobilization to alleviate iron-restricted erythropoiesis that can address the root cause of ACD. PRS-080-PEG30 is currently in early clinical development.

First-in-human phase I study of PRS-050 (Angiocal), an Anticalin targeting and antagonizing VEGF-A, in patients with advanced solid tumors

Background

To report the nonrandomized first-in-human phase I trial of PRS-050, a novel, rationally engineered Anticalin based on human tear lipocalin that targets and antagonizes vascular endothelial growth factor A (VEGF-A).

Methods

Patients with advanced solid tumors received PRS-050 at 0.1 mg/kg to 10 mg/kg by IV in successive dosing cohorts according to the 3+3 escalation scheme. The primary end point was safety.

Results

Twenty-six patients were enrolled; 25 were evaluable. Two patients experienced dose-limiting toxicity, comprising grade (G) 3 hypertension and G3 pyrexia, respectively. The maximum tolerated dose was not reached. Most commonly reported treatment-emergent adverse events (AEs) included chills (52%; G3, 4%), fatigue (52%; G3, 4%), hypertension (44%; G3, 16%), and nausea (40%, all G1/2). No anti–PRS-050 antibodies following multiple administration of the drug were detected. PRS-050 showed dose-proportional pharmacokinetics (PK), with a terminal half-life of approximately 6 days. Free VEGF-A was detectable at baseline in 9/25 patients, becoming rapidly undetectable after PRS-050 infusion for up to 3 weeks. VEGF-A/PRS-050 complex was detectable for up to 3 weeks at all dose levels, including in patients without detectable baseline-free VEGF-A. We also detected a significant reduction in circulating matrix metalloproteinase 2, suggesting this end point could be a pharmacodynamic (PD) marker of the drug’s activity.

Conclusions

PRS-050, a novel Anticalin with high affinity for VEGF-A, was well-tolerated when administered at the highest dose tested, 10 mg/kg. Based on target engagement and PK/PD data, the recommended phase II dose is 5 mg/kg every 2 weeks administered as a 120-minute infusion.

Trial Registration

ClinicalTrials.gov NCT01141257 http://clinicaltrials.gov/ct2/show/NCT01141257

Abstract 3621: A monovalent Anticalin antagonist: A novel approach for inhibition of c-Met-dependent tumor growth

The c-MET receptor has important functions in embryonic development and adult organ regeneration but its tyrosine kinase activity is often aberrantly activated in human malignancies. Activation is associated with enhanced tumor cell proliferation, migration and the promotion of an invasive cell phenotype resulting in increased metastasis and diminished survival.

Both animal studies and clinical studies suggest that inhibition of this oncogene product could be beneficial and small molecules that target the enzyme activity of the kinase receptor and biologics that block the HGF/cMet pathway are under development.

Each of these has advantages and disadvantages in reducing the activity of this pathway. While the current biologics in clinical development display target specificity rarely achievable with small molecule inhibitors they are costly and often times complex to develop/manufacture, often lack the penetration of small molecules, and under some circumstances can have partial agonist activity. While small molecules are generally inexpensive to produce, specificity and toxicity, especially with kinase inhibitors, remains a problem. To address the possible limitations of these current therapeutic strategies directed at attenuation of this important oncogenic pathway, we present an alternative approach, the development of a cMet Anticalin (PRS-110).

Anticalins are targeted muteins based on the lipocalin protein scaffold and represent a new class of biologics. All lipocalins are made up of a single beta-barrel domain with four loops protruding from the opening of the calyx although they share limited sequence identity. The lipocalin family has naturally evolved to perform various binding functions. The loop residues of human tear lipocalin were randomized to generate a combinatorial library and successive rounds of panning and screening were employed to generate an Anticalin with high affinity and specificity for c-MET.

We show that this cMet Anticalin binds to cells expressing this kinase receptor with both high affinity and specificity. Importantly, we show, using a xenograft strategy, that the cMet Anticalin inhibits the growth of established tumors expressing this receptor.

Because of the small size and stable three-dimensional structure, this molecule can be produced and purified from E. coli with limited steps, high titer, and a line of sight to late stage development manufacturability and product launch.

In summary, our data suggest that the cMet Anticalin displays the specificity of anti cMet/HGF antibodies, but because of its structure, and small size it exemplifies some of the advantages of small molecules directed at this important tumor target.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 3621. doi:10.1158/1538-7445.AM2011-3621

Abstract 3632: Nonclinical pharmacokinetics and safety of Angiocal, the first VEGF-A-specific alternative scaffold protein entering clinical development for cancer

Anticalins are high affinity scaffold proteins based on human lipocalins and represent a new class of biologics. Although lipocalins share limited sequence identity among each other they are all made up of a single conserved beta-barrel. The four loops protruding from the opening of the calyx contribute to the observed various binding functions of different lipocalins. The loop residues of human tear lipocalin were randomized to generate a combinatorial library and successive rounds of panning and screening were employed to generate an Anticalin with high affinity and specificity for VEGF-A, Angiocal. Angiocal was found to antagonize VEGF-A function by preventing its interaction with cognate membrane receptors. Following the initial characterization in cell-based and in vivo models as communicated previously (AACR 2008 #4077, AACR 2009 #2318) we sought to estimate the human pharmacokinetics of Angiocal by interspecies scaling. Therefore, PK profiles in mice, rats and cynomolgus monkeys were obtained and will be presented.

Therapeutic antibodies such as bevacizumab are usually well tolerated while significant toxicities occur in a subset of patients, for example leading to an enhanced risk for thromboembolic events. It has been demonstrated that bevacizumab forms multimeric immune complexes in vivo which may be a cause for these effects. Furthermore, immune complex deposition in glomeruli of the kidney may cause glomerulosclerosis. With respect to thromboembolic complications of bevacizumab use, these were mirrored in human FcgammaIIa receptor transgenic mice where administration of complexes between heparin, Bevacizumab and heparin-binding isoforms of its VEGF-A target lead to platelet aggregation and thrombosis (Meyer, T. et al. J. Thromb. Haemost. 2009, 7: 171-181). Angiocal was tested in the same model and does not cause any of these adverse effects on platelets in vivo.

GLP toxicology studies with Angiocal were performed in two relevant species to determine the NOAEL. In addition, we did not observe effects on inflammatory cytokines or complement factors in these studies while exaggerated pharmacodynamic effects consistent with potent VEGF neutralization were seen. Angiocal is currently undergoing phase Ib clinical testing in cancer patients to establish Anticalins as novel and safe class of therapeutic molecules.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 3632. doi:10.1158/1538-7445.AM2011-3632

A novel role of prostaglandin E2 in neuropathic pain: blockade of microglial migration in the spinal cord

Neuropathic pain produced by damage to or dysfunction of the nervous system is a common and severely disabling state that affects millions of people worldwide. Recent evidence indicates that activated microglia are key cellular intermediaries in the pathogenesis of neuropathic pain and that ATP serves as the mediator. However, the in vivo mechanism underlying the retention of activated microglia in the injured region has not yet been completely elucidated. Prostaglandin E(2) (PGE(2)) is the principal proinflammatory prostanoid and plays versatile roles by acting via four PGE receptor subtypes, EP1-EP4. In the present study, we investigated the role of PGE(2) in spinal microglial activation in relation to neuropathic pain by using genetic and pharmacological methods. Mice deficient in microsomal prostaglandin E synthase-1 impaired the activation of microglia and the NMDA-nitric oxide (NO) cascade in spinal neurons in the dorsal horn and did not exhibit mechanical allodynia after peripheral nerve injury. The intrathecal injection of indomethacin, a nonsteroidal anti-inflammatory drug, ONO-8713, a selective EP1 antagonist, or 7-nitroindole, a neuronal NO synthase inhibitor, attenuated mechanical allodynia and the increase in activated microglia observed in the established neuropathic-pain state. We further demonstrated that ATP-induced microglial migration was blocked in vitro by PGE(2) via EP2 and by S-nitrosoglutathione, an NO donor. Taken together, the present study suggests that PGE(2) participated in the maintenance of neuropathic pain in vivo not only by activating spinal neurons, but also by retaining microglia in the central terminals of primary afferent fibers via EP2 subtype and via EP1-mediated NO production.

Distinct expression profiles of Notch-1 protein in human solid tumors: Implications for development of targeted therapeutic monoclonal antibodies

Biological therapies, such as monoclonal antibodies (mAbs) that target tumor-associated antigens have been considered an effective therapeutic approach in oncology. In considering Notch-1 receptor as a potential target, we performed immunohistochemistry on tissue microarrays to determine 1) whether the receptor is overexpressed in tumor cells as compared to their corresponding normal tissues and 2) the clinical significance of its expression levels in human breast, colorectal, lung and prostate cancers. We found that the expression of Notch-1 protein was overexpressed in primary colorectal adenocarcinoma and nonsmall cell lung carcinoma (NSCLC), but not in primary ductal breast carcinoma or prostate adenocarcinoma. Further analysis revealed that higher levels of Notch-1 protein expression were significantly associated with poorer differentiation of breast and prostate tumors. Strikingly, for NSCLC, the expression levels of Notch-1 protein were found to be inversely correlated with tumor differentiation and progression. For colorectal tumors, however, no correlation of Notch-1 protein expression was found with any tumor clinicopathological parameters, in spite of its overexpression in tumor cells. Our data demonstrated the complexity of Notch-1 protein expression in human solid tumors and further supported the notion that the roles of Notch-1 expression in tumorigenesis are highly context-dependent. The findings could provide the basis for development of distinct therapeutic strategies of Notch-1 mAbs for its applications in the treatment of suitable types of human cancers.

Characterization of Notch1 antibodies that inhibit signaling of both normal and mutated Notch1 receptors

Background

Notch receptors normally play a key role in guiding a variety of cell fate decisions during development and differentiation of metazoan organisms. On the other hand, dysregulation of Notch1 signaling is associated with many different types of cancer as well as tumor angiogenesis, making Notch1 a potential therapeutic target.

Principal Findings

Here we report the in vitro activities of inhibitory Notch1 monoclonal antibodies derived from cell-based and solid-phase screening of a phage display library. Two classes of antibodies were found, one directed against the EGF-repeat region that encompasses the ligand-binding domain (LBD), and the second directed against the activation switch of the receptor, the Notch negative regulatory region (NRR). The antibodies are selective for Notch1, inhibiting Jag2-dependent signaling by Notch1 but not by Notch 2 and 3 in reporter gene assays, with EC₅₀ values as low as 5±3 nM and 0.13±0.09 nM for the LBD and NRR antibodies, respectively, and fail to recognize Notch4. While more potent, NRR antibodies are incomplete antagonists of Notch1 signaling. The antagonistic activity of LBD, but not NRR, antibodies is strongly dependent on the activating ligand. Both LBD and NRR antibodies bind to Notch1 on human tumor cell lines and inhibit the expression of sentinel Notch target genes, including HES1, HES5, and DTX1. NRR antibodies also strongly inhibit ligand-independent signaling in heterologous cells transiently expressing Notch1 receptors with diverse NRR “class I” point mutations, the most common type of mutation found in human T-cell acute lymphoblastic leukemia (T-ALL). In contrast, NRR antibodies failed to antagonize Notch1 receptors bearing rare “class II” or “class III” mutations, in which amino acid insertions generate a duplicated or constitutively sensitive metalloprotease cleavage site. Signaling in T-ALL cell lines bearing class I mutations is partially refractory to inhibitory antibodies as compared to cell-penetrating gamma-secretase inhibitors.

Conclusions/Significance

Antibodies that compete with Notch1 ligand binding or that bind to the negative regulatory region can act as potent inhibitors of Notch1 signaling. These antibodies may have clinical utility for conditions in which inhibition of signaling by wild-type Notch1 is desired, but are likely to be of limited value for treatment of T-ALLs associated with aberrant Notch1 activation.

The impact of microsomal prostaglandin e synthase 1 on blood pressure is determined by genetic background

Prostaglandin (PG)E(2) has multiple actions that may affect blood pressure. It is synthesized from arachidonic acid by the sequential actions of phospholipases, cyclooxygenases, and PGE synthases. Although microsomal PGE synthase (mPGES)1 is the only genetically verified PGE synthase, results of previous studies examining the consequences of mPGES1 deficiency on blood pressure (BP) are conflicting. To determine whether genetic background modifies the impact of mPGES1 on BP, we generated mPGES1(-/-) mice on 2 distinct inbred backgrounds, DBA/1lacJ and 129/SvEv. On the DBA/1 background, baseline BP was similar between wild-type (WT) and mPGES1(-/-) mice. By contrast, on the 129 background, baseline BPs were significantly higher in mPGES1(-/-) animals than WT controls. During angiotensin II infusion, the DBA/1 mPGES1(-/-) and WT mice developed mild hypertension of similar magnitude, whereas 129-mPGES1(-/-) mice developed more severe hypertension than WT controls. DBA/1 animals developed only minimal albuminuria in response to angiotensin II infusion. By contrast, WT 129 mice had significantly higher levels of albumin excretion than WT DBA/1 and the extent of albuminuria was further augmented in 129 mPGES1(-/-) animals. In WT mice of both strains, the increase in urinary excretion of PGE(2) with angiotensin II was attenuated in mPGES1(-/-) animals. Urinary excretion of thromboxane was unaffected by angiotensin II in the DBA/1 lines but increased more than 4-fold in 129 mPGES1(-/-) mice. These data indicate that genetic background significantly modifies the BP response to mPGES1 deficiency. Exaggerated production of thromboxane may contribute to the robust hypertension and albuminuria in 129 mPGES1-deficient mice.

Evolution of anti-CD20 monoclonal antibody therapeutics in oncology

Approval of an anti-CD20 chimeric monoclonal antibody, rituximab, has revolutionized cancer treatment and also validated CD20 targeting for providing benefit and improvement of overall response rate in B cell malignancies. Although many patients have benefited from the treatment of rituximab, there are still significant numbers of patients who are refractory or develop resistance to the treatment. Here we discuss pre-clinically well-defined potential mechanisms of action for rituximab and review the ways next generation anti-CD20 monoclonal antibodies can potentially exploit them to further enhance the treatment of B cell malignancies. Although the relative importance of each of these mechanism remains to be established in the clinic, well-designed clinical trials will help to define the efficacy and understanding of which effector activity of modified next generation anti-CD20 mAb will be important in the treatment of B-cell malignancies.

Microsomal prostaglandin E synthase-2 is not essential for in vivo prostaglandin E2 biosynthesis

Prostaglandin E(2) (PGE(2)) plays an important role in the normal physiology of many organ systems. Increased levels of this lipid mediator are associated with many disease states, and it potently regulates inflammatory responses. Three enzymes capable of in vitro synthesis of PGE(2) from the cyclooxygenase metabolite PGH(2) have been described. Here, we examine the contribution of one of these enzymes to PGE(2) production, mPges-2, which encodes microsomal prostaglandin synthase-2 (mPGES-2), by generating mice homozygous for the null allele of this gene. Loss of mPges-2 expression did not result in a measurable decrease in PGE(2) levels in any tissue or cell type examined from healthy mice. Taken together, analysis of the mPGES-2 deficient mouse lines does not substantiate the contention that mPGES-2 is a PGE(2) synthase.

Predominance of cyclooxygenase 1 over cyclooxygenase 2 in the generation of proinflammatory prostaglandins in autoantibody-driven K/BxN serum-transfer arthritis

OBJECTIVE

Prostaglandins (PGs) are found in high levels in the synovial fluid of patients with rheumatoid arthritis, and nonsteroidal blockade of these bioactive lipids plays a role in patient care. The aim of this study was to explore the relative contribution of cyclooxygenase (COX) isoforms and PG species in the autoantibody-driven K/BxN serum-transfer arthritis.

METHODS

The prostanoid content of arthritic ankles was assessed in ankle homogenates, and the importance of this pathway was confirmed with pharmacologic blockade. The presence of COX isoforms was assessed by Western blotting and their functional contribution was compared using COX-1-/- and COX-2-/- mice as well as isoform-specific inhibitors. The relative importance of PGE2 and PGI2 (prostacyclin) was determined using mice deficient in microsomal PGE synthase 1 (mPGES-1) and in the receptors for PGI2.

RESULTS

High levels of PGE2 and 6-keto-PGF1alpha (a stable metabolite of PGI2) were detected in arthritic joint tissues, correlating strongly with the intensity of synovitis. Pharmacologic inhibition of PG synthesis prevented arthritis and ameliorated active disease. While both COX isoforms were found in inflamed joint tissues, only COX-1 contributed substantially to clinical disease; COX-1-/- mice were fully resistant to disease, whereas COX-2-/- mice remained susceptible. These findings were confirmed by isoform-specific pharmacologic inhibition. Mice lacking mPGES-1 (and therefore PGE2) developed arthritis normally, whereas mice incapable of responding to PGI2 exhibited a significantly attenuated arthritis course, confirming a role of PGI2 in this arthritis model.

CONCLUSION

These findings challenge previous paradigms of distinct "housekeeping" versus inflammatory functions of the COX isoforms and highlight the potential pathogenic contribution of prostanoids synthesized via COX-1, in particular PGI2, to inflammatory arthritis.

Targeting different isoforms of HMGB1 leads to different beneficial effects in preclinical models of sepsis and inflammatory arthritis (100.6)

HMGB1 is a multifunctional protein that exerts diverse proinflammatory effects when released into the extracellular milieu. Because several HMGB1 isoforms may be generated depending on the activation state of a cell, these isoforms may play distinct roles in different inflammatory disorders. To examine this, we generated a panel of fully human anti-HMGB1 monoclonal antibodies using a phage display approach. The two most potent mAbs, IA-4 and IA-6, displayed comparable binding affinity to HMGB1 (KD=30 and 39nM) and blockade of HMGB1/RAGE binding (IC50=0.9 and 1.2nM). However, only IA-4 but not IA-6 could detect HMGB1 actively secreted from LPS-treated cells. IA-4 and IA-6 were then tested in rodent models of inflammatory arthritis (rat AIA) and sepsis (mouse CLP). IA-4 reduced paw swelling in the rat AIA model (32% inhibition @ 10mg/kg, p<0.05 vs. isotype control, n=32) while IA-6 had little effect. In contrast, only IA-6 but not IA-4 prolonged survival in the mouse CLP model (@ 8mg/kg , p<0.05 vs. isotype control, n=72). IA-4 and IA-6 displayed similar pharmacokinetics and recognized recombinant rodent HMGB1 with equal affinity. In summary, these results suggest that different isoforms of HMGB1 may have distinct in vivo roles and that HMGB1 mAbs targeting these isoforms may provide tailored therapeutics.

Antagonizing HMGB1 inhibits proteinuria in a murine model of lupus-like disease (131.24)

High mobility group box 1 (HMGB1) is an abundant chromatin protein that acts like an inflammatory cytokine when it is released by dying or stimulated cells. Elevated levels are detected at sites of inflammation in a number of autoimmune disorders including SLE. In addition, HMGB1 has been shown to contribute to the activation of plasmacytoid dendritic cells as well as B cells, cell types that are implicated in the pathogenesis of SLE. To address the role of HMGB1 in experimental lupus, we investigated the protective efficacy of a fully humanized mAb (IA-4, which recognizes both mouse and human HMGB1) in an IFNα-accelerated lupus model. In this model, adenovirus IFNα-treated NZB/W F1 mice exhibit 90–100% proteinuria incidence at 4–6 weeks post-treatment. The functionality of IA-4 has been demonstrated in vitro by its ability to inhibit rHMGB1-induced IL-6 secretion in human PBMC (IC50 = 0.9 nM). Prophylactic administration of the anti-HMGB1 mAb IA-4 at 10 mg/kg significantly inhibited the proteinuria severity score at seven weeks (65% inhibition; p<0.05 compared to an isotype control). Consistent with the proteinuria data, histopathological assessment of the kidney also revealed protection with IA-4 treatment. Despite its protection in nephritis, IA-4 treatment showed no detectable effect on serum levels of IFN-α and autoantibodies against dsDNA or SSA/Ro. Taken together, these data suggest that HMGB1 is a critical effector molecule in lupus nephritis and that anti-HMGB1 treatment may provide a potential therapeutic benefit for the treatment of SLE.

Prostacyclin antagonism reduces pain and inflammation in rodent models of hyperalgesia and chronic arthritis

The inhibition of prostaglandin (PG) synthesis is at the center of current anti-inflammatory therapies. Because cyclooxygenase-2 (COX-2) inhibitors and nonsteroidal anti-inflammatory drugs (NSAIDs) inhibit the formation of multiple PGs, there is currently a strong focus on characterizing the role of the different PGs in the inflammation process and development of arthritis. Evidence to date suggests that both PGE(2) and PGI(2) act as mediators of pain and inflammation. Most of the data indicating a role for PGI(2) in this context have been generated in animal models of acute pain. Herein, we describe the role of PGI(2) in models of osteoarthritis (OA) and rheumatoid arthritis using a highly selective PGI(2) receptor (IP, Ptgir) antagonist and IP receptor-deficient mice. In the rat OA model using monoiodoacetate injection into the knee joint, the IP antagonist reduced pain with an efficacy approaching that of the NSAID diclofenac. In a chronic model of inflammatory arthritis, collagen-antibody induced arthritis model in mice, IP receptor-deficient mice displayed a 91% reduction in arthritis score. Interestingly, pretreatment with the IP [N-[4-(imidazolidin-2-ylideneamino)-benzyl]-4-methoxy-benzamide] antagonist in this model also caused a significant reduction of the symptoms, whereas administration of the compound after the initiation of arthritis had no detectable effect. Our data indicate that, in addition to its role in acute inflammation, PGI(2) is involved in the development of chronic inflammation. The results also suggest that the inhibition of PGI(2) synthesis by NSAIDs and COX-2 inhibitors, in addition to that of PGE(2), contributes to their efficacy in treating the signs of arthritis.

c-Jun N-terminal Kinase-mediated Stabilization of Microsomal Prostaglandin E2 Synthase-1 mRNA Regulates Delayed Microsomal Prostaglandin E2 Synthase-1 Expression and Prostaglandin E2 Biosynthesis by Cardiomyocytes

Microsomal prostaglandin (PG) E(2) synthase-1 (mPGES-1) catalyzes the terminal step in the biosynthesis of PGE(2), a key proinflammatory mediator. The purpose of this study was to elucidate the regulation of mPGES-1 mRNA expression in cardiomyocytes, define the role of JNK enzymes in this process, and characterize the role of mPGES-1 in cardiomyocyte PGE(2) biosynthesis. In neonatal cardiomyocytes, interleukin-1beta and lipopolysaccharide (LPS) both stimulated mPGES-1 mRNA expression and increased mPGES-1 mRNA stability and protein synthesis but failed to increase mPGES-1 mRNA transcription. Treatment with the JNK1/2 inhibitor, SP600125, abrogated the increases in mPGES-1 mRNA stability, mPGES-1 protein synthesis, and PGE(2) release induced by interleukin-1beta or LPS. mPGES-1 protein synthesis was observed in LPS-stimulated neonatal cardiomyocytes from jnk1(-/-) or jnk2(-/-) mice. In contrast, infection of jnk1(-/-) cardiomyocytes with an adenovirus encoding phosphorylation-resistant JNK2 (ad-JNK2-DN), or of jnk2(-/-) cardiomyocytes with ad-JNK1-DN, significantly decreased LPS-stimulated mPGES-1 protein synthesis. Similarly, co-infection with ad-JNK1-DN and ad-JNK2-DN attenuated LPS-stimulated mPGES-1 protein synthesis in cardiomyocytes from wild type mice. Targeted deletion of the gene encoding mPGES-1 led to a 3.2-fold decrease in LPS-stimulated PGE(2) release by cardiomyocytes in comparison with wild type cells but had no effect on COX-1, COX-2, mPGES-2, or cytosolic PGES mRNA levels. These studies provide direct evidence that mPGES-1 mRNA levels in cardiomyocytes are augmented by stabilization of mPGES-1 mRNA, that JNK1 or JNK2 can participate in the regulation of mPGES-1 protein synthesis in these cells, and that mPGES-1 catalyzes the majority of LPS-induced PGE(2) biosynthesis by cardiomyocytes.

Neutrophil-derived leukotriene B4 is required for inflammatory arthritis

Neutrophils serve as a vanguard of the acute innate immune response to invading pathogens. Neutrophils are also abundant at sites of autoimmune inflammation, such as the rheumatoid joint, although their pathophysiologic role is incompletely defined and relevant effector functions remain obscure. Using genetic and pharmacologic approaches in the K/BxN serum transfer model of arthritis, we find that autoantibody-driven erosive synovitis is critically reliant on the generation of leukotrienes, and more specifically on leukotriene B₄ (LTB₄), for disease induction as well as perpetuation. Pursuing the cellular source for this mediator, we find via reconstitution experiments that mast cells are a dispensable source of leukotrienes, whereas arthritis susceptibility can be restored to leukotriene-deficient mice by intravenous administration of wild-type neutrophils. These experiments demonstrate a nonredundant role for LTB₄ in inflammatory arthritis and define a neutrophil mediator involved in orchestrating the synovial eruption.

COX-2-derived prostacyclin protects against bleomycin-induced pulmonary fibrosis

Prostacyclin is one of a number of lipid mediators elaborated from the metabolism of arachidonic acid by the cyclooxygenase (COX) enzymes. This prostanoid is a potent inhibitor of platelet aggregation, and its production by endothelial cells and protective role in the vasculature are well established. In contrast, much less is known regarding the function of this prostanoid in other disease processes. We show here that COX-2-dependent production of prostacyclin plays an important role in the development of fibrotic lung disease, limiting both the development of fibrosis and the consequential alterations in lung mechanics. In stark contrast, loss of prostaglandin E(2) synthesis and signaling through the G(s)-coupled EP2 and EP4 receptors had no effect on the development of disease. These findings suggest that prostacyclin analogs will protect against bleomycin-induced pulmonary fibrosis in COX-2(-/-) mice. If such protection is observed, investigation of these agents as a novel therapeutic approach to pulmonary fibrosis in humans may be warranted.

Coupling of COX-1 to mPGES1 for prostaglandin E2 biosynthesis in the murine mammary gland

The mammary gland, like most tissues, produces measurable amounts of prostaglandin E2 (PGE2), a metabolite of arachidonic acid produced by sequential actions of two cyclooxygenases (COX-1 and COX-2) and three terminal PGE synthases: microsomal prostaglandin E2 synthase-1 (mPGES1), mPGES2, and cytosolic prostaglandin E2 synthase (cPGES). High PGE2 levels and COX-2 overexpression are frequently detected in mammary tumors and cell lines. However, less is known about PGE2 metabolic enzymes in the context of normal mammary development. Additionally, the primary COX partnerships of terminal PGE synthases and their contribution to normal mammary PGE2 biosynthesis are poorly understood. We demonstrate that expression of COX-1, generally considered constitutive, increases dramatically with lactogenic differentiation of the murine mammary gland. Concordantly, total PGE2 levels increase throughout mammary development, with highest levels measured in lactating tissue and breast milk. In contrast, COX-2 expression is extremely low, with only a modest increase detected during mammary involution. Expression of the G(s)-coupled PGE2 receptors, EP2 and EP4, is also temporally regulated, with highest levels detected at stages of maximal proliferation. PGE2 production is dependent on COX-1, as PGE2 levels are nearly undetectable in COX-1-deficient mammary glands. Interestingly, PGE2 levels are similarly reduced in lactating glands of mPGES1-deficient mice, indicating that PGE2 biosynthesis results from the coordinated activity of COX-1 and mPGES1. We thus provide evidence for the first time of functional coupling between COX-1 and mPGES1 in the murine mammary gland in vivo.

Prostaglandin E2 protects lower airways against bronchoconstriction

Prostaglandin E2 (PGE2), similar to beta-adrenergic receptor agonists, can protect airways from bronchoconstriction and resulting increase in airway resistance induced by a number of agents, including cholinergic receptor agonists and antigen. We examined the impact of sustained alterations in PGE2 pathways on changes in airway resistance. Genetic methods were utilized to alter PGE2 metabolism and signal transduction in the murine lung. PGE2 levels were elevated by generating mice lacking 15-hydroxyprostaglandin (Hpgd-/-), the major catabolic enzyme of PGE2, and by generating a transgenic line in which mouse PGE2 synthase (Ptges) expression is driven by a human lung-specific promoter, hSP-C. Conversely, to determine the impact of loss of PGE2 on airway reactivity, we examined mice lacking this synthase (Ptges-/-) and receptors that mediate the actions of PGE2, particularly the PGE2 EP2 receptor (Ptger2). Diminished capacity to produce and respond to PGE2 did not alter the response of mice to cholinergic stimuli. In contrast, the responsiveness to cholinergic stimulation was dramatically altered in animals with elevated PGE2 levels. The Hpgd-/- and hSP-C-Ptges transgenic lines both showed attenuated airway responsiveness to methacholine as measured by lung resistance. Thus, whereas compromise of the Ptges/PGE2/Ptger2 pathway does not alter airway responsiveness, genetic modulation that elevates PGE2 levels in the lung attenuates airway responsiveness.

Redirection of Eicosanoid Metabolism in mPGES-1-deficient Macrophages

Microsomal prostaglandin E synthase (mPGES)-1 is one of several prostaglandin E synthases involved in prostaglandin H2 (PGH2) metabolism. In the present report, we characterize the contribution of mPGES-1 to cellular PGH2 metabolism in murine macrophages by studying the synthesis of eicosanoids and expression of eicosanoid metabolism enzymes in wild type and mPGES-1-deficient macrophages. Thioglycollate-elicited macrophages isolated from mPGES-1-/- animals and genetically matched wild type controls were stimulated with diverse pro-inflammatory stimuli. Prostaglandins were released in the following order of decreasing abundance from wild type macrophages stimulated with lipopolysaccharide: prostaglandin E2 (PGE2)>thromboxane B2 (TxB2)>6-keto prostaglandin F1alpha (PGF1alpha), prostaglandin F(2alpha) (PGF2alpha), and prostaglandin D2 (PGD2). In contrast, we detected in mPGES-1-/- macrophages a >95% reduction in PGE2 production resulting in the following altered prostaglandin profile: TxB2>6-keto PGF1alpha and PGF2alpha>PGE2, despite the comparable release of total prostaglandins. No significant change in expression pattern of key prostaglandin-synthesizing enzymes was detected between the genotypes. We then further profiled genotype-related differences in the eicosanoid profile using macrophages pre-stimulated with lipopolysaccharide followed by a 10-min incubation with 10 microm [3H]arachidonic acid. Eicosanoid products were subsequently identified by reverse phase high pressure liquid chromatography. The dramatic reduction in [3H]PGE2 formation from mPGES-1-/- macrophages compared with controls resulted in TxB2 and 6-keto PGF1alpha becoming the two most abundant prostaglandins in these samples. Our results also suggest a 5-fold increase in 12-[3H]hydroxyheptadecatrienoic acid release in mPGES-1-/- samples. Our data support the hypothesis that mPGES-1 induction in response to an inflammatory stimulus is essential for PGE2 synthesis. The redirection of prostaglandin production in mPGES-1-/- cells provides novel insights into how a cell processes the unstable endoperoxide PGH2 during the inactivation of a major metabolic outlet.

Exploring type I angiotensin (AT1) receptor functions through gene targeting

The renin-angiotensin system (RAS) modulates a diverse set of physiological processes including development, blood pressure, renal function and inflammation. The principal effector molecule of this system, angiotensin II, mediates most of these actions. The classically recognized functions of the RAS are triggered via the type 1 (AT(1)) class of angiotensin receptors. Pharmacological blockade of the AT(1) receptor lowers blood pressure and slows the progression of cardiovascular and renal diseases. Gene-targeting technology provides an experimental approach for precisely dissecting the physiological functions of the RAS. Here, we review how gene-targeting experiments have elucidated AT(1) receptor functions.

Expression of microsomal prostaglandin E synthase 1 in rheumatoid arthritis synovium

OBJECTIVE

Microsomal prostaglandin E synthase 1 (mPGES-1) catalyzes the formation of PGE(2) from cyclooxygenase-derived PGH(2). Microsomal PGES-1 is induced by proinflammatory cytokines and is strongly linked to conditions that result in high PGE(2) biosynthesis. PGE(2) contributes to the pathogenesis of rheumatoid arthritis (RA), acting as a mediator of inflammation and promoting bone destruction. Induction of mPGES-1 in rheumatoid synoviocytes by proinflammatory cytokines has been demonstrated in vitro, indicating an important role in RA pathogenesis. Recent studies using mPGES-1-deficient mice demonstrated the importance of this gene in chronic inflammation. The aim of this study was to investigate the expression and localization of mPGES-1 in synovial biopsy specimens obtained from patients with RA.

METHODS

Synovial tissue samples from 24 patients with RA were obtained, and immunohistologic analysis was performed using polyclonal antibodies against mPGES-1. Double immunofluorescence staining was performed with antibodies to CD3, CD19, CD20, CD68, CD163, and prolyl 4-hydroxylase.

RESULTS

Intracellular mPGES-1 staining was observed in synovial membranes from all of the RA patients studied. Specifically, strong expression of mPGES-1 was detected in synovial lining cells. In sublining mononuclear and fibroblast-like cells, the extent of mPGES-1 staining was less than that in the synovial lining cells. In some patients, positive staining was observed in endothelial cells. With the double immunofluorescence technique, mPGES-1 production was detected in synovial macrophages and fibroblasts, while mPGES-1 expression was not observed in lymphocytes.

CONCLUSION

The demonstration of mPGES-1 expression in synovial tissues from patients with RA suggests a role for mPGES-1 in the RA disease process. Microsomal PGES-1 might be a potential new target for treatment strategies to control PGE(2) synthesis in patients with RA, without the systemic side effects associated with cyclooxygenase inhibitors.

Microsomal prostaglandin E synthase-1 is the central switch during immune-induced pyresis

We studied the febrile response in mice deficient in microsomal prostaglandin E synthase-1 (mPGES-1), an inducible terminal isomerase expressed in cytokine-sensitive brain endothelial cells. These animals showed no fever and no central prostaglandin (PG) E2 synthesis after peripheral injection of bacterial-wall lipopolysaccharide, but their pyretic capacity in response to centrally administered PGE2 was intact. Our findings identify mPGES-1 as the central switch during immune-induced pyresis and as a target for the treatment of fever and other PGE2-dependent acute phase reactions elicited by the brain.

Inhibition of IL-1beta-dependent prostaglandin E2 release by antisense microsomal prostaglandin E synthase 1 oligonucleotides in A549 cells

The metabolism of arachidonic acid through the cyclooxygenase pathway is a highly regulated cellular process that results in the formation of PGH2. This unstable intermediate can be enzymatically metabolized to PGE2 by the actions of a microsomal 17 kDa PGE synthase (mPGES1). Treatment of A549 cells with IL-1beta for 24 h resulted in a twofold increase in mPGES1 mRNA, protein expression, and PGES specific activity. To understand the relationship between expression of mPGES1 and PGE2 formation, IL-1beta treated cells were incubated with increasing concentrations of antisense oligonucleotides (ASO) and their effects compared to cells treated with reverse sense oligonucleotides (RSO) designed against the ATG translation initiation codon of mPGES1. Incubation with ASO resulted in a 44% reduction in mRNA expression level as compared to RSO-treated cells. Microsomal preparations isolated from ASO- and RSO-treated cells were analyzed for their ability to convert PGH2 to PGE2 in the presence 2.5 mM reduced glutathione. An approximate 50% reduction (ASO: 1.8 nmol/min/mg, RSO: 3.7 nmol/min/mg) in PGES activity, protein expression by immunodetection, and extracellular PGE2 release was detected in these samples. As a control in these studies, the protein levels of COX2 and secreted IL-8 were quantified; no change in these levels was observed. These results demonstrate the direct association between mPGES1 expression, its enzymatic activity, and total PGE2 production following an inflammatory stimulus.

Impaired inflammatory and pain responses in mice lacking an inducible prostaglandin E synthase

Prostaglandin (PG)E2 is a potent mediator of pain and inflammation, and high levels of this lipid mediator are observed in numerous disease states. The inhibition of PGE2 production to control pain and to treat diseases such as rheumatoid arthritis to date has depended on nonsteroidal antiinflammatory agents such as aspirin. However, these agents inhibit the synthesis of all prostanoids. To produce biologically active PGE2, PGE synthases catalyze the isomerization of PGH2 into PGE2. Recently, several PGE synthases have been identified and cloned, but their role in inflammation is not clear. To study the physiological role of the individual PGE synthases, we have generated by targeted homologous recombination a mouse line deficient in microsomal PGE synthase 1 (mPGES1) on the inbred DBA/1lacJ background. mPGES1-deficient (mPGES1-/-) mice are viable and fertile and develop normally compared with wild-type controls. However, mPGES1-/- mice displayed a marked reduction in inflammatory responses compared with mPGES1+/+ mice in multiple assays. Here, we identify mPGES1 as the PGE synthase that contributes to the pathogenesis of collagen-induced arthritis, a disease model of human rheumatoid arthritis. We also show that mPGES1 is responsible for the production of PGE2 that mediates acute pain during an inflammatory response. These findings suggest that mPGES1 provides a target for the treatment of inflammatory diseases and pain associated with inflammatory states.

Deletion of the P2X7 nucleotide receptor reveals its regulatory roles in bone formation and resorption

The P2X7 nucleotide receptor is an ATP-gated ion channel expressed widely in cells of hematopoietic origin. Our purpose was to explore the involvement of the P2X7 receptor in bone development and remodeling by characterizing the phenotype of mice genetically modified to disrupt the P2X7 receptor [knockout (KO)]. Femoral length did not differ between KO and wild-type (WT) littermates at 2 or 9 months of age, indicating that the P2X7 receptor does not regulate longitudinal bone growth. However, KO mice displayed significant reduction in total and cortical bone content and periosteal circumference in femurs, and reduced periosteal bone formation and increased trabecular bone resorption in tibias. Patch clamp recording confirmed expression of functional P2X7 receptors in osteoclasts from WT but not KO mice. Osteoclasts were present in vivo and formed in cultures of bone marrow from KO mice, indicating that this receptor is not essential for fusion of osteoclast precursors. Functional P2X7 receptors were also found in osteoblasts from WT but not KO mice, suggesting a direct role in bone formation. P2X7 receptor KO mice demonstrate a unique skeletal phenotype that involves deficient periosteal bone formation together with excessive trabecular bone resorption. Thus, the P2X7 receptor represents a novel therapeutic target for the management of skeletal disorders such as osteoporosis.

Receptors and signaling mechanisms required for prostaglandin E2-mediated regulation of mast cell degranulation and IL-6 production

Mast cells are implicated in the pathogenesis of a broad spectrum of immunological disorders. These cells release inflammatory mediators in response to a number of stimuli, including IgE-Ag complexes. The degranulation of mast cells is modified by PGs. To begin to delineate the pathway(s) used by PGs to regulate mast cell function, we examined bone marrow-derived mast cells (BMMC) cultured from mice deficient in the EP(1), EP(2), EP(3), and EP(4) receptors for PGE(2). Although BMMCs express all four of these PGE(2) receptors, potentiation of Ag-stimulated degranulation and IL-6 cytokine production by PGE(2) is dependent on the EP(3) receptor. Consistent with the coupling of this receptor to G(alphai), PGE(2) activation of the EP(3) receptor leads to both inhibition of adenylate cyclase and increased intracellular Ca(2+). The magnitude of increase in intracellular Ca(2+) induced by EP(3) activation is similar to that observed after activation of cells with IgE and Ag. Although PGE alone is not sufficient to initiate BMMC degranulation, stimulation of cells with PGE along with PMA induces degranulation. These actions are mediated by the EP(3) receptor through signals involving Ca(2+) mobilization and/or decreased cAMP levels. Accordingly, these studies identify PGE(2)/EP(3) as a proinflammatory signaling pathway that promotes mast cell activation.

The role of prostaglandin E2 receptors in the pathogenesis of rheumatoid arthritis

Rheumatoid arthritis (RA) is a chronic inflammatory disorder leading to bone and cartilage destruction. A substantial body of evidence suggests that prostaglandin E2 (PGE2) contributes to the pathogenesis of RA, and nonsteroidal anti-inflammatory drugs, inhibitors of the synthesis of PGE2 and other prostanoids, continue to be used in the treatment of this disease. To begin to understand the mechanism by which prostaglandins modulate the pathophysiology of this disease, we examined mice lacking each of the four known PGE2 (EP) receptors after generation of collagen antibody-induced arthritis, an animal model of RA. Homozygous deletion of the EP1, EP2, or EP3 receptors did not affect the development of arthritis, whereas EP4 receptor-deficient mice showed decreased incidence and severity of disease. These animals also showed reduced inflammation as assessed by circulating IL-6 and serum amyloid A levels. Joint histopathology of EP4(-/-) animals revealed reduced bone destruction, proteoglycan loss, and type II collagen breakdown in cartilage compared with EP4(+/+) mice. Furthermore, liver and macrophages isolated from EP4(-/-) animals produced significantly less IL-1 beta and IL-6 than control samples. Thus, PGE2 contributes to disease progression at least in part by binding to the EP4 receptor. Antagonists of this receptor might therefore provide novel agents for the treatment of RA.

Deficiency of COX-1 causes natriuresis and enhanced sensitivity to ACE inhibition

BACKGROUND

Prostanoid products of the cyclo-oxygenase (COX) pathway of arachidonic acid metabolism modulate blood pressure (BP) and sodium homeostasis. Conventional non-steroidal anti-inflammatory drugs (NSAIDs), which inhibit both COX isoforms (COX-1 and -2), cause sodium retention, exacerbate hypertension, and interfere with the efficacy of certain anti-hypertensive agents such as angiotensin-converting enzyme (ACE) inhibitors. While a new class of NSAIDs that specifically inhibit COX-2 is now widely used, the relative contribution of the individual COX isoforms to these untoward effects is not clear.

METHODS

To address this question, we studied mice with targeted disruption of the COX-1 (Ptgs1) gene. Blood pressure, renin mRNA expression, and aldosterone were measured while dietary sodium was varied. To study interactions with the renin-angiotensin system, ACE inhibitors were administered and mice with combined deficiency of COX-1 and the angiotensin II subtype 1A (AT1A) receptor were generated.

RESULTS

On a regular diet, BP in COX-1-/- mice was near normal. However, during low salt feeding, BP values were reduced in COX-1-/- compared to +/+ animals, and this reduction in BP was associated with abnormal natriuresis despite appropriate stimulation of renin and aldosterone. Compared to COX-1+/+ mice, the actions of ACE inhibition were markedly accentuated in COX-1-/- mice. Sodium sensitivity and BP lowering also were enhanced in mice with combined deficiency of COX-1 and AT1A receptor.

CONCLUSIONS

The absence of COX-1 is associated with sodium loss and enhanced sensitivity to ACE inhibition, suggesting that COX-1 inhibition does not cause hypertension and abnormal sodium handling associated with NSAID use.

The prostaglandin E2 EP1 receptor mediates pain perception and regulates blood pressure

The lipid mediator prostaglandin E2 (PGE2) has diverse biological activity in a variety of tissues. Four different receptor subtypes (EP1-4) mediate these wide-ranging effects. The EP-receptor subtypes differ in tissue distribution, ligand-binding affinity, and coupling to intracellular signaling pathways. To identify the physiological roles for one of these receptors, the EP1 receptor, we generated EP1-deficient (EP1-/-) mice using homologous recombination in embryonic stem cells derived from the DBA/1lacJ strain of mice. The EP1-/- mice are healthy and fertile, without any overt physical defects. However, their pain-sensitivity responses, tested in two acute prostaglandin-dependent models, were reduced by approximately 50%. This reduction in the perception of pain was virtually identical to that achieved through pharmacological inhibition of prostaglandin synthesis in wild-type mice using a cyclooxygenase inhibitor. In addition, systolic blood pressure is significantly reduced in EP1 receptor-deficient mice and accompanied by increased renin-angiotensin activity, especially in males, suggesting a role for this receptor in cardiovascular homeostasis. Thus, the EP1 receptor for PGE2 plays a direct role in mediating algesia and in regulation of blood pressure.

Role of EP(2) and EP(3) PGE(2) receptors in control of murine renal hemodynamics

The kidney plays a central role in long-term regulation of arterial blood pressure and salt and water homeostasis. This is achieved in part by the local actions of paracrine and autacoid mediators such as the arachidonic acid-prostanoid system. The present study tested the role of specific PGE(2) E-prostanoid (EP) receptors in the regulation of renal hemodynamics and vascular reactivity to PGE(2). Specifically, we determined the extent to which the EP(2) and EP(3) receptor subtypes mediate the actions of PGE(2) on renal vascular tone. Renal blood flow (RBF) was measured by ultrasonic flowmetry, whereas vasoactive agents were injected directly into the renal artery of male mice. Studies were performed on two independent mouse lines lacking either EP(2) or EP(3) (-/-) receptors and the results were compared with wild-type controls (+/+). Our results do not support a unique role of the EP(2) receptor in regulating overall renal hemodynamics. Baseline renal hemodynamics in EP(2)-/- mice [RBF EP(2)-/-: 5.3 +/- 0.8 ml. min(-1). 100 g kidney wt(-1); renal vascular resistance (RVR) 19.7 +/- 3.6 mmHg. ml(-1). min. g kidney wt] did not differ statistically from control mice (RBF +/+: 4.0 +/- 0.5 ml. min(-1). 100 g kidney wt(-1); RVR +/+: 25.4 +/- 4.9 mmHg. ml(-1). min. 100 g kidney wt(-1)). This was also the case for the peak RBF increase after local PGE(2) (500 ng) injection into the renal artery (EP(2)-/-: 116 +/- 4 vs. +/+: 112 +/- 2% baseline RBF). In contrast, we found that the absence of EP(3) receptors in EP(3)-/- mice caused a significant increase (43%) in basal RBF (7.9 +/- 0.8 ml. min(-1). g kidney wt(-1), P < 0.05 vs. +/+) and a significant decrease (41%) in resting RVR (11.6 +/- 1.4 mmHg. ml(-1). min. g kidney wt(-1), P < 0.05 vs. +/+). Local administration of 500 ng of PGE(2) into the renal artery caused more pronounced renal vasodilation in EP(3)-/- mice (128 +/- 2% of basal RBF, P < 0.05 vs. +/+). We conclude that EP(3 )receptors mediate vasoconstriction in the kidney of male mice and its actions are tonically active in the basal state. Furthermore, EP(3) receptors are capable of buffering PGE(2)-mediated renal vasodilation.

Insights into the functions of type 1 (AT1) angiotensin II receptors provided by gene targeting

The renin-angiotensin system (RAS) has a wide range of actions in biological processes ranging from development and reproduction to cardiovascular and renal functions. Most of these actions are mediated by the octapeptide hormone angiotensin II. The identified family of angiotensin II receptors is divided into two pharmacological classes: type 1 (AT1) and type 2 (AT2). The classically recognized actions of the RAS are primarily mediated by the AT1 subtype of angiotensin receptors, and these receptors are the targets of a new class of anti-hypertensive agents. In recent years, our understanding of the physiological functions of AT1 receptors has been advanced through the use of gene-targeting technology. In this review, we will summarize the emerging picture of AT1 receptor functions that has been provided by gene-targeting experiments.

Cardiovascular responses to the isoprostanes iPF(2alpha)-III and iPE(2)-III are mediated via the thromboxane A(2) receptor in vivo

BACKGROUND

Isoprostanes (iPs) are free radical-catalyzed products of arachidonic acid that reflect lipid peroxidation in vivo. Several iPs exert biological effects in vitro and may contribute to the functional consequences of oxidant stress. For example, iPF(2alpha)-III (8-iso PGF(2alpha)) and iPE(2)-III modulate platelet function and vascular tone. Although these effects are blocked by antagonists of the receptor (TP) for the cyclooxygenase product thromboxane A(2), it has been speculated that the iPs may activate a receptor related to, but distinct from, the TP.

METHODS AND RESULTS

Transgenic mice (TPOEs) were generated in which the TP-beta isoform was under the control of the preproendothelin promoter. They overexpressed TP-beta in the vasculature but not in platelets and exhibited an exaggerated pressor response to infused iPF(2alpha)-III compared with wild-type mice. This was blocked by TP antagonism. The platelet response to the iP was unaltered in TPOEs compared with wild-type mice. By contrast, both the pressor response to iPF(2alpha)-III and its effects on platelet function were abolished in mice lacking the TP gene. This was also true of the effects of infused iPE(2)-III on mean arterial pressure and platelet aggregation.

CONCLUSIONS

Both iPF(2alpha)-III and iPE(2)-III exert their effects on platelet function and vascular tone in vivo by acting as incidental ligands at membrane TPs rather than via a distinct iP receptor. Activation of TPs by iPs may be of importance in syndromes in which cyclooxygenase activation and oxidant stress coincide, such as in atherosclerosis and reperfusion after tissue ischemia.

What can knockout mice contribute to an understanding of hypertension?

The generation of knockout mice using homologous recombination in embryonic stem cells is a powerful tool for physiologic investigations. This experimental approach has provided unique insights into the study of hypertension. Studies using knockout mice have shed new light on blood pressure regulatory mechanisms, molecular mechanisms of end-organ injury, and genetic mechanisms for hypertension. With the development of more accessible approaches for carrying out sophisticated manipulation of the mouse genome, there will be continuing utility of this technique for future studies of hypertension.

Angiotensin II regulates cellular immune responses through a calcineurin-dependent pathway

The renin-angiotensin system (RAS) is a key regulator of vascular tone and blood pressure. In addition, angiotensin II also has a number of cellular effects that may contribute to disease pathogenesis. Using Agtr1a(-/-) mice, which lack AT(1A) receptors for angiotensin II, we have identified a novel function of the RAS to modulate the immune system. We find that angiotensin II, acting through type 1 (AT(1)) receptors on immune cells, triggers the proliferation of splenic lymphocytes. These actions contribute to the vigor of cellular alloimmune responses. Within lymphoid organs, sufficient components of the RAS are present to activate AT(1) receptors during an immune response, promoting cell growth. These actions require activation of calcineurin phosphatase. In an in vivo model of cardiac transplantation, the absence of AT(1) signaling accentuates the immunosuppressive effects of the calcineurin inhibitor cyclosporine. We conclude that inhibition of AT(1) receptor signaling should be useful as an anti-inflammatory and immunosuppressive therapy. Furthermore, the actions of the RAS to promote lymphocyte activation may contribute to inflammation that characterizes a number of diseases of the heart and the vascular system.

Decreased platelet aggregation, increased bleeding time and resistance to thromboembolism in P2Y1-deficient mice

Platelet activation is characterized by shape change, induction of fibrinogen receptor expression and release of granular contents, leading to aggregation and plug formation. While this response is essential for hemostasis, it is also important in the pathogenesis of a broad spectrum of diseases, including myocardial infarction, stroke and unstable angina. Adenosine 5'-diphosphate (ADP) induces platelet aggregation, but the mechanism for this has not been established, and the relative contribution of ADP in hemostasis and the development of arterial thrombosis is poorly understood. We show here that the purinoceptor P2Y1 is required for platelet shape change in response to ADP and is also a principal receptor mediating ADP-induced platelet aggregation. Activation of P2Y1 resulted in increased intracellular calcium but no alteration in cyclic adenosine monophosphate (cAMP) levels. P2Y1-deficient platelets partially aggregated at higher ADP concentrations, and the lack of P2Y1 did not alter the ability of ADP to inhibit cAMP, indicating that platelets express at least one additional ADP receptor. In vivo, the lack of P2Y1 expression increased bleeding time and protected from collagen- and ADP-induced thromboembolism. These findings support the hypothesis that the ATP receptor P2Y1 is a principal receptor mediating both physiologic and pathological ADP-induced processes in platelets.

Identification of specific EP receptors responsible for the hemodynamic effects of PGE2

To identify the E-prostanoid (EP) receptors that mediate the hemodynamic actions of PGE2, we studied acute vascular responses to infusions of PGE2 using lines of mice in which each of four EP receptors (EP1 through EP4) have been disrupted by gene targeting. In mixed groups of males and females, vasodepressor responses after infusions of PGE2 were significantly diminished in the EP2 -/- and EP4 -/- lines but not in the EP1 -/- or EP3 -/- lines. Because the actions of other hormonal systems that regulate blood pressure differ between sexes, we compared the roles of individual EP receptors in males and females. We found that the relative contribution of each EP-receptor subclass was strikingly different in males from that in females. In females, the EP2 and EP4 receptors, which signal by stimulating adenylate cyclase, mediate the major portion of the vasodepressor response to PGE2. In males, the EP2 receptor has a modest effect, but most of the vasodepressor effect is mediated by the phospholipase C-coupled EP1 receptor. Finally, in male mice, the EP3 receptor actively opposes the vasodepressor actions of PGE2. Thus the hemodynamic actions of PGE2 are mediated through complex interactions of several EP-receptor subtypes, and the role of individual EP receptors differs dramatically in males from that in females. These differences may contribute to sexual dimorphism of blood pressure regulation.

Reproductive failure and reduced blood pressure in mice lacking the EP2 prostaglandin E2 receptor

Prostaglandins (PGs) are bioactive lipids that modulate a broad spectrum of biologic processes including reproduction and circulatory homeostasis. Although reproductive functions of mammals are influenced by PGs at numerous levels, including ovulation, fertilization, implantation, and decidualization, it is not clear which PGs are involved and whether a single mechanism affects all reproductive functions. Using mice deficient in 1 of 4 prostaglandin E2 (PGE2) receptors -- specifically, the EP2 receptor -- we show that Ep2(-/-) females are infertile secondary to failure of the released ovum to become fertilized in vivo. Ep2(-/-) ova could be fertilized in vitro, suggesting that in addition to previously defined roles, PGs may contribute to the microenvironment in which fertilization takes place. In addition to its effects on reproduction, PGE2 regulates regional blood flow in various vascular beds. However, its role in systemic blood pressure homeostasis is not clear. Mice deficient in the EP2 PGE2 receptor displayed resting systolic blood pressure that was significantly lower than in wild-type controls. Blood pressure increased in these animals when they were placed on a high-salt diet, suggesting that the EP2 receptor may be involved in sodium handling by the kidney. These studies demonstrate that PGE2, acting through the EP2 receptor, exerts potent regulatory effects on two major physiologic processes: blood pressure homeostasis and in vivo fertilization of the ovum.

Prostaglandin E-prostanoid-3 receptor activation of cyclic AMP response element-mediated gene transcription

The prostaglandin E-prostanoid (EP)3 receptor signals primarily through the inhibitory G protein Gi, thereby decreasing intracellular cAMP levels. To study the signal transduction properties of the rabbit EP3 receptor, five splice variants were expressed in HEK293tsA201 cells: 72A, 74A, 77A, 80A and the novel splice variant NT, which lacks the C-terminal sequence. The ability of the EP3 receptor splice variants to modulate expression of a beta-galactosidase reporter gene under the control of a promoter containing cAMP response elements (CRE) was assessed. Each splice variant induced sulprostone-mediated increase in beta-galactosidase enzymatic activity with EC50 ranging from 0.8 nM for the NT splice variant to 3.1 nM for the 77A splice variant. Substitution of either Asp338 with Ala, or Arg329 with Ala or Glu in the 77A splice variant resulted in a loss of receptor-evoked increases in beta-galactosidase activity, whereas substitution of Lys300 with alanine had no effect on signal transduction. These phenotypes correlate with the inhibition of cAMP generation by direct cAMP measurement. Signal transduction was insensitive to pretreatment of cells with pertussis toxin, suggesting that a nonGi/Go pathway is activated by the EP3 receptor. Direct measurement of second messenger levels confirmed that there was no increase in cAMP levels mediated by the 77A splice variant, however, there was a modest increase in intracellular Ca2+. Partial blockade of the reporter activity with kinase inhibitors demonstrates that CRE activation is mediated in part by a Ca2+-dependent kinase pathway. These data suggest that the EP3 receptor signals through a novel cAMP response element binding protein/CRE pathway.

Regulation of glucose production by NEFA and gluconeogenic precursors during chronic glucagon infusion

We previously reported that simulation of the chronic hyperglucagonemia seen during infection was unable to recreate the infection-induced increase in hepatic glucose production. However, chronic hyperglucagonemia was accompanied by a fall in the arterial levels of gluconeogenic precursors as opposed to a rise as is seen during infection. Thus our aim was to determine whether an infusion of gluconeogenic precursors could increase hepatic glucose production in a setting of hyperglucagonemia. Studies were done in 11 conscious chronically catheterized dogs in which sampling (artery and portal and hepatic veins) and infusion catheters (splenic vein) were implanted 17 days before study. Forty-eight hours before infusion of gluconeogenic (GNG) precursors, a sterile fibrinogen clot was placed into the peritoneal cavity. Glucagon was infused over the subsequent 48-h period to simulate the increased glucagon levels (approximately 500 pg/ml) seen during infection. On the day of the experiment, somatostatin was infused peripherally, and basal insulin and simulated glucagon were infused intraportally. After a basal period, a two-step increase in lactate and alanine was initiated (120 min/step; n = 5). Lactate (Delta479 +/- 25 and Delta1, 780 +/- 85 microM; expressed as change from basal in periods I and II, respectively) and alanine (Delta94 +/- 13 and Delta287 +/- 44 microM) levels were increased. Despite increases in net hepatic GNG precursor uptake (Delta0.7 +/- 0.3 and Delta1.1 +/- 0.4 mg glucose . kg-1 . min-1), net hepatic glucose output did not increase. Because nonesterified fatty acid (NEFA) levels fell, in a second series of studies, the fall in NEFA was eliminated. Intralipid and heparin were infused during the two-step substrate infusion to maintain the NEFA levels constant in period I and increase NEFA availability in period II (Delta -29 +/- 29 and Delta689 +/- 186 microM; n = 6). In the presence of similar increases in net hepatic GNG precursor uptake and despite increases in arterial glucose levels (Delta17 +/- 5 and Delta38 +/- 12 mg/dl), net hepatic glucose output increased (Delta0.6 +/- 0.1 and Delta0.7 +/- 0.2 mg . kg-1 . min-1). In summary, a chronic increase in glucagon, when combined with an acute increase in gluconeogenic precursor and maintenance of NEFA supply, increases hepatic glucose output as is seen during infection.