Nitric oxide inhibits the synthesis of type-II collagen without altering Col2A1 mRNA abundance: prolyl hydroxylase as a possible target

Transfer of proalpha2(I) cDNA into cells of a murine model of human Osteogenesis Imperfecta restores synthesis of type I collagen comprised of alpha1(I) and alpha2(I) heterotrimers in vitro and in vivo

The oim mouse is a model of human Osteogenesis Imperfecta (OI) that has deficient synthesis of proalpha2(I) chains. Cells isolated from oim mice synthesize alpha1(I) collagen homotrimers that accumulate in tissues. To explore the feasibility of gene therapy for OI, a murine proalpha2(I) cDNA was inserted into an adenovirus vector and transferred into bone marrow stromal cells isolated from oim mice femurs. The murine cDNA under the control of the cytomegalovirus early promoter was expressed by the transduced cells. Analysis of the collagens synthesized by the transduced cells demonstrated that the cells synthesized stable type I collagen comprised of alpha1(I) and alpha2(I) heterotrimers in the correct ratio of 2:1. The collagen was efficiently secreted and also the cells retained the osteogenic potential as indicated by the expression of alkaline phosphatase activity when the transduced cells were treated with recombinant human bone morphogenetic protein 2. Injection of the virus carrying the murine proalpha2(I) cDNA into oim skin demonstrated synthesis of type I collagen comprised of alpha1 and alpha2 chains at the injection site. These preliminary data demonstrate that collagen genes can be transferred into bone marrow stromal cells as well as fibroblasts in vivo and that the genes are efficiently expressed. These data encourage further studies in gene replacement for some forms of OI and use of bone marrow stromal cells as vehicles to deliver therapeutic genes to bone.

Possible virulence factors of Staphylococcus sciuri

Staphylococcus sciuri is an opportunistic pathogen of controversial clinical significance. The factors that contribute to colonization and/or infection caused by this bacterium have not been studied intensively so far. The present research was carried out in order to study the presence of potential virulence factors in 121 human and animal isolates of this bacterium. Isolates were examined for biofilm formation, hemagglutination, presence of clumping factor, production of spreading factors and exotoxins, cytotoxicity and capacity to stimulate nitric oxide production. The results showed that S. sciuri is highly capable of biofilm production, that it displays strong proteolytic and DNase activities, produces hemolysins and stimulates nitric oxide production by rat macrophages. Although the present study showed existence of a wide spectrum of possible virulence determinants of S. sciuri, their exact contribution to virulence of this bacterium in vivo remains to be determined.

Aging-related expression of inducible nitric oxide synthase and markers of tissue damage in the rat penis

Erectile dysfunction in the aging male results in part from the loss of compliance of the corpora cavernosal smooth muscle due to the progressive replacement of smooth muscle cells by collagen fibers. We have examined the hypothesis that a spontaneous local induction of inducible nitric oxide synthase (iNOS) expression and the subsequent peroxynitrite formation occurs in the penis during aging and that this process is accompanied by a stimulation of smooth muscle apoptosis and collagen deposition. The penile shaft and crura were excised from young (3-5 mo old) and old (24-30 mo old) rats, with or without perfusion with 4% formalin. Fresh tissue was used for iNOS and proteasome 2C mRNA determinations by reverse transcription polymerase chain reaction assay, ubiquitin mRNA by Northern blot, and iNOS protein by Western blot. Penile sections from perfused animals were embedded in paraffin and immunostained with antibodies against iNOS and nitrotyrosine, submitted to the TUNEL assay for apoptosis, or stained for collagen, followed by image analysis quantitation. A 4.1-fold increase in iNOS mRNA was observed in the old versus young tissues, paralleled by a 4.9-fold increase in iNOS protein. The proteolysis marker, ubiquitin, was increased 1.9-fold, whereas a related gene, proteasome 2c, was not significantly affected. iNOS immunostaining was increased 3.6-fold in the penile smooth muscle of the old rats as compared with the young rats. The peroxynitrite indicator nitrotyrosine was increased by 1.6-fold, accompanied by a 3.6-fold increase in apoptotic cells and a 2.0-fold increase in collagen fibers in the old penis. In conclusion, aging in the penis is accompanied by an induction of iNOS and peroxynitrite formation that may lead to the observed increase in apoptosis and proteolysis and may counteract a higher rate of collagen deposition in the old penis.

Nitric oxide regulates mitochondrial respiration and functions of articular chondrocytes

OBJECTIVE

Biologic effects of nitric oxide (NO) have been shown to increase under hypoxic conditions. Because the oxygen tension in joint cavities of patients with arthritis is fairly low, biologic effects of NO would be expected to be significantly large in these compartments. This study was undertaken to investigate the effects of NO on the energy metabolism and functions of articular chondrocytes under different oxygen tension conditions.

METHODS

Articular chondrocytes from rabbits were cultured under various oxygen concentrations in the presence or absence of NO and NOC18, an NO donor. Cellular respiration was measured using a Clark-type oxygen electrode. Levels of ATP in the cells were determined according to the luciferin-luciferase method. Cellular synthesis of proteoglycans was determined by measuring the incorporation of radioactivity (derived from 35S-labeled SO4) into glycosaminoglycans. Expression of stress-related proteins was evaluated by Western blotting analysis using specific antibodies.

RESULTS

Respiration and ATP synthesis of cultured chondrocytes were inhibited by NO, particularly under low oxygen concentrations. The presence of either NO or specific inhibitors of mitochondrial electron transport suppressed the synthesis of proteoglycans without affecting cell viability. When exposed to NO, cellular levels of heat-shock protein 70 (hsp70) and heme oxygenase 1 (HO-1) increased markedly. The presence of inhibitors of mitochondrial electron transport also increased cellular levels of hsp70 and HO-1.

CONCLUSION

These results suggest that NO generated in the joint might inhibit energy metabolism and the synthesis of proteoglycans of chondrocytes, thereby modulating pathophysiologic processes occurring in patients with arthritis.

COX-2, NO, and cartilage damage and repair

The production of nitric oxide (NO) and prostaglandin E2 (PGE(2)) is increased in human osteoarthritis-affected cartilage. These and other inflammatory mediators are spontaneously released by OA cartilage explants ex vivo. The excessive production of nitric oxide inhibits matrix synthesis, and promotes its degradation. Furthermore, by reacting with oxidants such as superoxide anion, nitric oxide promotes cellular injury, and renders the chondrocyte susceptible to cytokine-induced apoptosis. PGE(2) exerts both anabolic and catabolic effects on chondrocytes, depending on the microenvironment and physiological condition. Thus, NO and PGE(2), produced by activated chondrocytes in diseased cartilage, may modulate disease rogression in osteoarthritis, and should therefore be considered potential targets for therapeutic intervention

Enhancement of fibroblast collagen synthesis by nitric oxide

Fibroblasts can be stimulated by cytokines to synthesize nitric oxide (NO, nitrogen monoxide), while wound-derived fibroblasts synthesize NO spontaneously. Since wound fibroblasts are phenotypically characterized by greater collagen synthesis when compared to fibroblasts derived from noninjured tissue, we hypothesized that there may be a correlation between wound-induced NO synthesis and enhanced collagen production. To study the role of NO on collagen metabolism, normal dermal fibroblasts were cultured in the presence or absence of the NO donor S-nitroso-N-acetyl-penicillamine (SNAP) and their collagen metabolism was studied on the transcriptional as well as translational level. Fibroblast collagen synthesis was enhanced by 74.3 +/- 18.2 and 87.5 +/- 28.2% in the presence of 100 and 400 microM SNAP, respectively. This effect was not due to increased collagen type I or type III gene transcription. Cellular proliferation measured by thymidine incorporation was significantly decreased in the presence of SNAP, indicating that the increased collagen production was due to a net increase of collagen synthesis by the cells. Investigation of the collagen breakdown pathway showed that neither collagenase gene expression nor collagenase protein expression was affected by SNAP. The results of this study demonstrate for the first time that NO enhances collagen synthesis, most likely at a posttranslational level.

Nitric oxide inhibits chondrocyte response to IGF-I: inhibition of IGF-IRbeta tyrosine phosphorylation

Chondrocytes in arthritic cartilage respond poorly to insulin-like growth factor I (IGF-I). Studies with inducible nitric oxide synthase (iNOS) knockout mice suggest that NO is responsible for part of the cartilage insensitivity to IGF-I. These studies characterize the relationship between NO and chondrocyte responses to IGF-I in vitro, and define a mechanism by which NO decreases IGF-I stimulation of chondrocyte proteoglycan synthesis. Lapine cartilage slices, chondrocytes, and cartilage from osteoarthritic (OA) human knees were exposed to NO from the donors S-nitroso-N-acetylpenicillamine (SNAP) or (Z)-1-[2-(2-aminoethyl)-N-(2-ammonioethyl)amino]diazen-1- ium-1, 2-diolate] (DETA NONOate), by transduction with adenoviral transfer of iNOS (Ad-iNOS), or by activation with interleukin-1 (IL-1). NO synthesis was estimated from medium nitrite, and proteoglycan synthesis was measured as incorporation of (35)SO(4). IGF-I receptor phosphorylation was evaluated with Western analysis. SNAP, DETA NONOate, endogenously synthesized NO in Ad-iNOS-transduced cells, or IL-1 activation decreased IGF-I-stimulated proteoglycan synthesis in cartilage and monolayer cultures of chondrocytes. OA cartilage responded poorly to IGF-I; however, the response to IGF-I was restored by culture with N(G)-monomethyl-L-arginine (L-NMA). IGF-I receptor phosphotyrosine was diminished in chondrocytes exposed to NO. These studies show that NO is responsible for part of arthritic cartilage/chondrocyte insensitivity to anabolic actions of IGF-I; inhibition of receptor autophosphorylation is potentially responsible for this effect.

Gene therapy for meniscal injury: enhanced synthesis of proteoglycan and collagen by meniscal cells transduced with a TGFbeta(1)gene

Objective To determine whether meniscal cells can express a TGFbeta(1)transgene delivered by a retroviral vector, and respond to the gene product by increasing matrix synthesis. Methods Monolayer cultures of human and canine meniscal cells were infected with retroviruses carrying either a human TGFbeta(1)cDNA or marker genes. Conditioned media were assayed for the presence of TGFbeta(1). Biosynthesis assays using radiolabeled precursors were employed to determine the effects of the transgenes on the synthesis of proteoglycan, collagen and noncollagenous proteins. Collagen phenotyping was performed by SDS-PAGE. Results Media conditioned by canine and human meniscal cells transduced with the TGFbeta(1)gene, accumulated several nanograms/10(6)cells of TGFbeta(1)during a 48 h incubation. Media conditioned by control cells contained very little TGFbeta(1). Transduction with the TGFbeta(1)gene, but not marker genes, increased the synthesis of collagen and proteoglycan by 8-15-fold. The synthesis of noncollagenous proteins was enhanced more modestly. Monolayers of meniscal cells synthesized types I, III, V and VI collagen. The TGFbeta(1)gene increased the synthesis of all types of collagen without altering the ratios between them. Conclusions Meniscal cells are readily transduced by retroviral vectors and respond to the transfer of a TGFbeta(1)cDNA by greatly increasing matrix synthesis. These findings encourage the further development of genetic approaches to the healing of meniscal lesions.

Increased matrix synthesis following adenoviral transfer of a transforming growth factor beta1 gene into articular chondrocytes

Monolayer cultures of lapine articular chondrocytes were transduced with first-generation adenoviral vectors carrying lacZ or transforming growth factor beta1 genes under the transcriptional control of the human cytomegalovirus early promoter. High concentrations of transforming growth factor beta1 were produced by chondrocytes following transfer of the transforming growth factor beta1 gene but not the lacZ gene. Transduced chondrocytes responded to the elevated endogenous production of transforming growth factor beta1 by increasing their synthesis of proteoglycan, collagen, and noncollagenous proteins in a dose-dependent fashion. The increases in collagen synthesis were not accompanied by alterations in the collagen phenotype; type-II collagen remained the predominant collagen. Transforming growth factor beta1 could not, however, rescue the collagen phenotype of cells that had undergone phenotypic modulation as a result of serial passaging. These data demonstrate that chondrocytes can be genetically manipulated to produce and respond to the potentially therapeutic cytokine transforming growth factor beta1. This technology has a number of experimental and therapeutic applications, including those related to the study and treatment of arthritis and cartilage repair.

In vivo expression of human growth hormone by genetically modified murine bone marrow stromal cells and its effect on the cells in vitro

Human growth hormone (hGH) is frequently used clinically for growth abnormalities in children and also in adults with growth hormone deficiency. The hormone is usually administered to the individuals by frequent injections. In the present study we investigated the potential of bone marrow stromal cells as vehicles to deliver the GH in vivo by infusion of cells transduced with hGH cDNA into mice femurs. The effect of the hormone on the transduced cells in vitro was also assessed. Bone marrow stromal cells established from a mouse model of human osteogenesis imperfecta mice (oim) were transduced with a retrovirus containing hGH and neomycin resistance genes. The hGH-expressing cells were selected in a medium containing G418 and were then assessed for the hGH expression in vitro. The selected cells synthesized 15 ng/10(6) cells of hGH per 24 h in vitro and exhibited alkaline phosphatase activity when they were treated with the human recombinant bone morphogenetic protein 2 (rhBMP-2). The transduced cells also proliferated faster than the LacZ transduced cells but they did not exhibit a higher rate of matrix synthesis. When 2 x 10(6) hGH+ cells were injected into the femurs of mice, hGH was detected in the serum of the recipient mice up to 10 days after injection. The highest level of growth hormone expression, 750 pg/ml, was detected in the serum of the recipient mice I day after injection of the transduced cells. hGH was also detected in the medium conditioned by cells that were flushed from the femurs of the recipient mice at 1, 3, and 6 days after cell injection. These data indicate that bone marrow stromal cells could potentially be used therapeutically for the delivery of GH or any other therapeutic proteins targeted for bone. The data also suggest that GH may exert its effects on bone marrow stromal cells by increasing their rate of proliferation.

Role of nitric oxide in wound healing

Nitric oxide is a short-lived free radical, that is capable of multiple effects at the molecular, cellular, and physiologic levels. Over the past several years, nitric oxide has been proved to play an important role in the healing of various types of wounds. The present review examines some of the recently defined roles of nitric oxide in normal and pathologic healing.

Does nitric oxide help explain the differential healing capacity of the anterior cruciate, posterior cruciate, and medial collateral ligaments?

This study compared the ability of rabbit medial collateral ligament, posterior cruciate ligament, and anterior cruciate ligament tissue to synthesize nitric oxide, and determined its effects on matrix synthesis, an important component of ligament repair. It is not known whether ligament cells can produce nitric oxide and, if so, whether it influences healing of ligament injuries. The anterior cruciate and posterior cruciate ligament tissue produced large amounts of nitric oxide in response to the inflammatory cytokine interleukin-1. Medial collateral ligament, in contrast, produced only modest amounts of nitric oxide. Furthermore, anterior cruciate ligament and, to some degree, posterior cruciate ligament synthesized nitric oxide spontaneously in culture, whereas medial collateral ligament never did so. When nitric oxide was supplied to these tissues, it strongly inhibited collagen synthesis by the two cruciate ligaments, but had little effect on collagen synthesis by the medial collateral ligament. Endogenously synthesized nitric oxide was also able to inhibit collagen synthesis as well as proteoglycan synthesis by the two cruciate ligaments, but had little effect on matrix synthesis by the medial collateral ligament. We propose a novel hypothesis, based on nitric oxide production and matrix synthesis, that may help explain why the two cruciate ligaments have such limited healing capacity compared with the medial collateral ligament.

Cartilage protection by nitric oxide synthase inhibitors after intraarticular injection of interleukin-1beta in rats

OBJECTIVE

To evaluate the effect of nitric oxide synthase (NOS) inhibitors on proteoglycan synthesis following intraarticular administration of interleukin-1beta (IL-1beta) in rats.

METHODS

Recombinant human IL-1beta and NOS inhibitors with different selectivity for inducible NOS (N-monomethyl-L-arginine [L-NMA], N-iminoethyl-L-ornithine [L-NIO], and S-methylisothiourea [SMT]) were simultaneously administered in rats by a single intraarticular injection in each knee. L-NMA was also infused for 72 hours using an Alzet mini osmotic pump implanted into the peritoneal cavity 24 hours before IL-1beta challenge. NO production was determined as nitrate and nitrite, either in synovial fluid or ex vivo in supernatants of synovium and patellae. Proteoglycan synthesis was measured by ex vivo incorporation of 35SO4(2-) into patellar cartilage.

RESULTS

IL-1beta induced a time-dependent increase in NO production in synovial fluid. Synovium and patellae released large amounts of nitrate and nitrite under ex vivo conditions, indicating that both tissues are effective sources of NO within the joint. This production of NO was accompanied by a delayed inhibition of proteoglycan synthesis. The intraarticular administration of L-NMA and L-NIO reduced NO release in synovial fluid and resulted in a partial recovery of proteoglycan synthesis. Under our experimental conditions, SMT failed to reduce NO synthesis and to restore proteoglycan synthesis. The protection of cartilage was improved by the systemic and sustained delivery of L-NMA. However, the complete inhibition of NO production in synovial fluid was not sufficient to fully restore cartilage anabolism.

CONCLUSION

Our findings show that in rats: 1) NO may be an early mediator of the effect of IL-1beta on cartilage, 2) NO inhibition may have therapeutic relevance, although it is not sufficient to fully reverse the deleterious effects of IL-1beta, 3) among NOS inhibitors tested, only amino acid derivatives are effective, 4) protection can be achieved by local administration of NOS inhibitors, and 5) systemic and sustained delivery of the NOS inhibitor with the highest efficacy after intraarticular injection provides the most benefit.

Niacinamide therapy for osteoarthritis--does it inhibit nitric oxide synthase induction by interleukin 1 in chondrocytes?

Fifty years ago, Kaufman reported that high-dose niacinamide was beneficial in osteoarthritis (OA) and rheumatoid arthritis. A recent double-blind study confirms the efficacy of niacinamide in OA. It may be feasible to interpret this finding in the context of evidence that synovium-generated interleukin-1 (IL-1), by inducing nitric oxide (NO) synthase and thereby inhibiting chondrocyte synthesis of aggrecan and type II collagen, is crucial to the pathogenesis of OA. Niacinamide and other inhibitors of ADP-ribosylation have been shown to suppress cytokine-mediated induction of NO synthase in a number of types of cells; it is therefore reasonable to speculate that niacinamide will have a comparable effect in IL-1-exposed chondrocytes, blunting the anti-anabolic impact of IL-1. The chondroprotective antibiotic doxycycline may have a similar mechanism of action. Other nutrients reported to be useful in OA may likewise intervene in the activity or synthesis of IL-1. Supplemental glucosamine can be expected to stimulate synovial synthesis of hyaluronic acid; hyaluronic acid suppresses the anti-catabolic effect of IL-1 in chondrocyte cell cultures, and has documented therapeutic efficacy when injected intra-articularly. S-adenosylmethionine (SAM), another proven therapy for OA, upregulates the proteoglycan synthesis of chondrocytes, perhaps because it functions physiologically as a signal of sulfur availability. IL-1 is likely to decrease SAM levels in chondrocytes; supplemental SAM may compensate for this deficit. Adequate selenium nutrition may down-regulate cytokine signaling, and ample intakes of fish oil can be expected to decrease synovial IL-1 production; these nutrients should receive further evaluation in OA. These considerations suggest that non-toxic nutritional regimens, by intervening at multiple points in the signal transduction pathways that promote the synthesis and mediate the activity of IL-1, may provide a substantially superior alternative to NSAIDs (merely palliative and often dangerously toxic) in the treatment and perhaps prevention of OA.

Nitric oxide inhibits wounds collagen synthesis

Nitric oxide (NO) is a messenger molecule which regulates many physiological functions like immunity, vascular tone and serves as a neurotransmitter. Although it is known to participate in healing process, its role in collagen synthesis is not clear. Therefore, the present investigation was done to study the role of NO in wound collagen synthesis. Rats received full thickness, circular (8 mm), transdermal wounds which were treated with NO releaser, sodium nitroprusside (SNP, 0.001 100 microM) topically for 5 days. Wound collagen content estimated in terms of hydroxyproline (HP) and confirmed histochemically was decreased significantly by all SNP doses. L-Arginine, a substrate for nitric oxide synthase (NOS) when applied topically decreased collagen content of the wounded tissues. N-Nitro-L-arginine methyl ester (L-NAME), a competitive inhibitor of NOS, increased wound collagen content significantly as compared to untreated and SNP treated animal wounds when administered intraperitoneally at the doses 3, 10 and 30 mg/kg. Furthermore, histological findings also demonstrated laying down of thick collagen bundles and proliferation of fibroblasts together with prominent angiogenesis in L-NAME treated wound tissues as compared to untreated and SNP treated tissues. N-nitro-D-arginine methyl ester, an inactive isomer, was found to have no effect on wound collagen levels. When L-arginine was administered in L-NAME pretreated rats, it significantly elevated wound HP content. The results indicate that NO plays an important role in regulating the collagen biosynthesis in skin model of a healing wound.

Nitric oxide production and apoptosis in cells of the meniscus during experimental osteoarthritis

OBJECTIVE

To examine the pathologic changes in meniscus tissue during experimental osteoarthritis (OA) and to determine the relationship between nitric oxide (NO) synthesis, apoptosis, and meniscus degradation.

METHODS

OA was induced in rabbits by anterior cruciate ligament (ACL) transection. Knees were harvested after 9 weeks and assessed for OA severity. Menisci were subjected to histologic, immunohistochemical, and electron microscopic analyses for the presence of nitrotyrosine and apoptosis. Menisci were also cultured for analysis of NO production.

RESULTS

All menisci from joints with ACL transection demonstrated degenerative changes. A high number of apoptotic cells was present in the medial part of menisci, which contains chondrocytic cells. Menisci from nonoperated contralateral knees contained only small numbers of cells in apoptosis. Conditioned media from meniscus cultures contained similarly elevated levels of nitrite as cartilage cultures from the same arthritic knees. Nitrotyrosine immunoreactivity, an indicator of in vivo NO production, was prominent in menisci from knees with ACL transection. In addition, menisci from normal knees produced high levels of NO in response to in vitro stimulation with interleukin-1beta or lipopolysaccharide.

CONCLUSION

These observations suggest that pathologic changes in menisci are a regular feature of experimentally induced OA and are associated with NO production and meniscus cell apoptosis.

The role of nitric oxide in articular cartilage damage

The production of large amounts of NO in vitro by cytokine-activated chondrocytes has been established. In vitro studies suggest that NO compromises chondrocyte survival. The role of NO in regulating matrix biosynthesis and degradation has received much attention. Most studies indicate that NO is at least partly responsible for IL-1-induced suppression of glycosaminoglycan and collagen synthesis. NO also may be involved as a mediator of IL-1-induced expression of MMP, mRNA, and protein and may contribute as an activator of the latent forms of the enzymes. Although the interaction of NO and prostaglandins is of considerable interest, current data are inconclusive with respect to the role of NO in the regulation of prostaglandin synthesis, although it seems clear that prostaglandin is not involved in NO synthesis. It is important to note that NO does have protective effects in cartilage and other tissues. Under certain conditions, NO may have anabolic and anticatabolic effects in cartilage. In other tissues, notably in skin and muscle, NO has been found to have a stimulatory role in extracellular matrix repair. In antimicrobial defense, in general, and in bacterial arthritis specifically, NO is an important protective molecule. Production of NO in arthritis-affected cartilage and synovium is a consistent feature of human and experimentally induced arthritis. The production of NO is associated with matrix degradation and chondrocyte apoptosis. The administration of NO synthase inhibitors in experimentally induced arthritis has resulted in reduction of synovial inflammation and destruction of cartilage and bone.

Molecular and metabolic evidence for the restricted expression of inducible nitric oxide synthase in healing wounds

Tissue injury initiates a temporally ordered sequence of local cellular and metabolic responses presumably necessary for successful repair. Previous investigations demonstrated that metabolic evidence for nitric oxide synthase (NOS) activity is detectable in wounds only during the initial 48 to 72 hours of the repair process. Present results identify the cell types contributing inducible NOS (iNOS) to experimental wounds in rats. iNOS antigen was expressed in most macrophages present in wounds 6 to 24 hours after injury, and these cells exhibited NAPDH diaphorase and NOS activity. Polymorphonuclear leukocytes contained little iNOS antigen and no NADPH diaphorase activity and were minimally able to convert L-arginine to L-citrulline. The frequency of iNOS-positive macrophages declined on days 3 and 5 after wounding. By day 10, most macrophages in the wound were negative for iNOS. These cells, however, acquired iNOS antigen and activity in culture. Wound fluids, but not normal rat serum, suppressed the induction of iNOS during culture. Findings indicate that the expression of iNOS in healing wounds is restricted to macrophages present during the early phases of repair and that components of wound fluid suppress the induction of iNOS in macrophages in late wounds. Polymorphonuclear leukocytes contribute little iNOS activity to the healing wound.

Inhibition of transforming growth factor beta production by nitric oxide-treated chondrocytes: implications for matrix synthesis

OBJECTIVE

Nitric oxide (NO) is generated copiously by articular chondrocytes activated by interleukin-1beta (IL-1beta). If NO production is blocked, much of the IL-1beta inhibition of proteoglycan synthesis is prevented. We tested the hypothesis that this inhibitory effect of NO on proteoglycan synthesis is secondary to changes in chondrocyte transforming growth factor beta (TGFbeta).

METHODS

Monolayer, primary cultures of lapine articular chondrocytes and cartilage slices were studied. NO production was determined as nitrite accumulation in the medium. TGFbeta bioactivity in chondrocyte- and cartilage-conditioned medium (CM) was measured with the mink lung epithelial cell bioassay. Proteoglycan synthesis was measured as the incorporation of 35S-sodium sulfate into macromolecules separated from unincorporated label by gel filtration on PD-10 columns.

RESULTS

IL-1beta increased active TGFbeta in chondrocyte CM by 12 hours; by 24 hours, significant increases in both active and latent TGFbeta were detectable. NG-monomethyl-L-arginine (L-NMA) potentiated the increase in total TGFbeta without affecting the early TGFbeta activation. IL-1beta stimulated a NO-independent, transient increase in TGFbeta3 at 24 hours; however, TGFbeta1 was not changed. When NO synthesis was inhibited with L-NMA, IL-1beta increased CM concentrations of TGFbeta1 from 24-72 hours of culture. L-arginine (10 mM) reversed the inhibitory effect of L-NMA on NO production and blocked the increases in TGFbeta1. Anti-TGFbeta1 antibody prevented the restoration of proteoglycan synthesis by chondrocytes exposed to IL-1beta + L-NMA, confirming that NO inhibition of TGFbeta1 in IL-1beta-treated chondrocytes effected, in part, the decreased proteoglycan synthesis. Furthermore, the increase in TGFbeta and proteoglycan synthesis seen with L-NMA was reversed by the NO donor S-nitroso-N-acetylpenicillamide. Similar results were seen with cartilage slices in organ culture. The autocrine increase in CM TGFbeta1 levels following prior exposure to TGFbeta1 was also blocked by NO.

CONCLUSION

NO can modulate proteoglycan synthesis indirectly by decreasing the production of TGFbeta1 by chondrocytes exposed to IL-1beta. It prevents autocrine-stimulated increases in TGFbeta1, thus potentially diminishing the anabolic effects of this cytokine in chondrocytes.

Nitric oxide--cyclic GMP pathway regulates vascular smooth muscle cell phenotypic modulation: implications in vascular diseases

The role of cGMP-dependent protein kinase (PKG) in the regulation of rat aortic vascular smooth muscle cells (VSMC) phenotype was examined using a transfected cell culture system. Repetitively passaged VSMC do not express PKG and exist in the synthetic phenotype. Transfection of PKG-l alpha cDNA, or the active catalytic domain of PKG-l alpha, resulted in the appearance of VSMC having a morphology consistent with the contractile phenotype. PKG-expressing cells also contained markers for the contractile phenotype (for example, smooth muscle specific myosin heavy chain, calponin, alpha-actin) and reduced levels of synthetic phenotype markers (osteopontin, thrombospondin). PKG-transfected VSMC have also reduced the levels of fibroblast growth factor receptors 1 and 2, consistent with the establishment of a more contractile phenotype. The regulation of PKG expression in VSMC is largely undefined; however, continuous exposure of cultured bovine aortic smooth muscle cells with nitric oxide (NO)-donor drugs or cyclic nucleotide analogues reduced the expression of PKG. These results suggest that PKG occupies a critical role in VSMC phenotype and that suppression of PKG expression during inflammation or injury promotes a more synthetic state of the VSMC.

Nitric oxide in arthritis

Nitric oxide's (NO) involvement in arthritis was first demonstrated when levels of nitrite, a stable endproduct of NO metabolism, were shown to be elevated in serum and synovial fluid samples of rheumatoid and osteoarthritis patients. NO production by chondrocytes, its involvement in various biochemical events of cartilage metabolism, and the in vivo suppression of experimental arthritis by NO synthase inhibitors further implicated NO in arthritis. However, a conclusive role for NO in the pathogenesis of arthritis remains to be defined, in contrast to the NO-cGMP signal transduction pathway of endothelium-mediated vasodilation. It appears that NO has limited modulating effects in cartilage metabolism, with evidence for both protective and deleterious effects. Recent developments that contribute to our understanding of NO's role in arthritis are discussed.

Complex alterations in gene expression occur in the knee ligaments of the skeletally mature multiparous rabbit during pregnancy

Pregnancy is known to influence the function of ligaments of the knee in both humans and rabbits. During pregnancy, ligament laxity increases. The mechanism by which these changes in ligament function occur is unknown. The present study was undertaken to assess changes in the pattern of gene expression for a number of molecules which could be involved in the alterations in tissue function. Using RNA isolated from pregnant and age-matched nonpregnant rabbits, levels of mRNA for matrix molecules, proteinases and inhibitors, cytokines and growth factors and inducible nitric oxide synthase were assessed by semi-quantitative RT-PCR. The results indicate that the pregnancy-induced changes in the pattern of gene expression is very complex and differences in the changes observed for the medial collateral ligament and the anterior cruciate ligament may also indicate that ligament-specific effects of pregnancy occur. mRNA levels of some molecules were significantly elevated or suppressed, while others were unchanged, indicating that the changes observed were specific rather than general. While the extent of the molecules assessed was designed to be representative rather than exhaustive, the results indicate that there is likely no simple cause and effect relationship between the observed mechanical alterations during pregnancy and alterations in gene expression in the affected ligaments.

Differences in Nitric Oxide Production by Superficial and Deep Human Articular Chondrocytes: Implications for Proteoglycan Turnover in Inflammatory Joint Diseases

During inflammatory joint diseases, chondrocytes are exposed to cytokines such as IL-1 that induce the synthesis of nitric oxide (NO). Chondrocytes from different zones of the articular cartilage are known to have different metabolic properties. In the present study, we have demonstrated that chondrocytes recovered from the superficial zone of normal, human, articular cartilage synthesize approximately 2 to 3 times as much NO in response to IL-1 as chondrocytes recovered from the deep zone of the same cartilage. Production of NO by normal cartilage in response to IL-1 was also found to decrease with age. Addition of the NO synthase inhibitor NG-monomethyl-l-arginine (l-NMA, 1 mM) blocked NO production by cells of both zones. l-NMA completely reversed the suppression of proteoglycan synthesis imposed by IL-1 in deep chondrocytes, but produced only partial reversal in superficial cells. As noted previously, IL-1 failed to elicit a strong catabolic response in cultures of human cartilage. In the presence of l-NMA, however, IL-1 reduced the metabolic t1/2 of proteoglycans by approximately 50% in both the superficial and deep zones. This suggests that NO has, directly or indirectly, an anticatabolic effect in human cartilage. These data confirm the metabolic heterogeneity of human chondrocytes, and suggest that NO may be involved to different degrees as an endogenous modulator of the turnover of the cartilaginous matrix in different zones of articular cartilage.

Generation of nitric oxide by lapine meniscal cells and its effect on matrix metabolism: stimulation of collagen production by arginine

Slices of lapine meniscus produced large amounts of nitric oxide after stimulation with interleukin-1, tumor necrosis factor alpha, or a mixture of lapine synovial cytokines known as chondrocyte-activating factors. Monolayer cultures of meniscal cells produced from the proteolysis of meniscal tissue contained a mixed population of chondrocytic and fibroblastic cells. These cultures also produced large amounts of nitric oxide in response to cytokines. Monolayer cultures of meniscal cells produced by the explant method, in contrast, were uniformly fibroblastic and did not produce nitric oxide in response to cytokines. We conclude that menisci contain two populations of cells, one fibroblastic and the other chondrocytic. The chondrocytic cells are responsible for generating most of the nitric oxide in response to cytokines. Endogenously generated nitric oxide suppressed the synthesis of collagen and proteoglycan by menisci but protected proteoglycan from the catabolic effects of interleukin-1. The inhibitory effect of nitric oxide on collagen synthesis occurred without greatly altering the abundance of mRNAs encoding the various collagen alpha chains. During further investigation, arginine was unexpectedly found to stimulate the synthesis of collagen and, to a lesser degree, of noncollagenous proteins but not of proteoglycans. Fragments of meniscus, but not meniscal cells in monolayer culture, increased their production of matrix metalloproteinases, lactate, and, especially, prostaglandin E2 in response to interleukin-1. Inhibition of nitric oxide production with NG-monomethyl-L-arginine enhanced production of matrix metalloproteinases but had little effect on the synthesis of lactate or prostaglandin E2.