Molecular cloning and characterization of the human and mouse UDP-glucose dehydrogenase genes

Aging fibroblasts resist phenotypic maturation because of impaired hyaluronan-dependent CD44/epidermal growth factor receptor signaling

Fibroblast differentiation into myofibroblasts is a key event during normal wound repair. We have previously demonstrated an age-related defect in this process associated with impaired synthesis of hyaluronan (HA) synthase (HAS) 2 but failed to prescribe its role in a mechanistic sense. Here we demonstrate that in addition to HAS2, there is loss of EGF receptor (EGF-R) in aged cells, and both are required for normal fibroblast functionality. Analysis of molecular events revealed that in young cells, transforming growth factor (TGF)-beta1-dependent phenotypic activation uses two distinct but cooperating pathways that involve TGF-beta receptor/Smad2 activation and EGF-mediated EGF-R/extracellular signal-regulated kinase (ERK) 1/2 signaling, and the latter is compromised with in vitro aging. Pharmacological inhibition of any of the five intermediates (TGF-beta receptor, Smad2, EGF, EGF-R, and ERK1/2) attenuated TGF-beta1 induction of alpha-smooth muscle actin. We present evidence that the HA receptor CD44 co-immunoprecipitates with EGF-R after activation by TGF-beta1. This interaction is HA-dependent because disruption of HA synthesis abrogates this association and inhibits subsequent ERK1/2 signaling. In aged fibroblasts, this association is lost with resultant suppression of ERK1/2 activation. Forced overexpression of EGF-R and HAS2 in aged cells restored TGF-beta1-mediated HA-CD44/EGF-R association and alpha-smooth muscle actin induction. Taken together, these results demonstrate that HA can serve as a signal integrator by facilitating TGF-beta1-mediated CD44-EGF-R-ERK interactions and ultimately fibroblast phenotype. We propose a model to explain this novel mechanism and the functional consequence of age-dependent dysregulation.

HpSulf, a heparan sulfate 6-O-endosulfatase, is involved in the regulation of VEGF signaling during sea urchin development

Cell surface heparan sulfate proteoglycans (HSPGs) play significant roles in the regulation of developmental signaling, including vascular endothelial growth factor (VEGF), fibroblast growth factor, Wnt and bone morphogenetic protein signaling, through modification of their sulfation patterns. Recent studies have revealed that one of the functions of heparan sulfate 6-O-endosulfatase (Sulf) is to remove the sulfate from the 6-O position of HSPGs at the cell surface, thereby regulating the binding activities of heparan sulfate (HS) chains to numerous ligands and receptors in animal species. In this study, we focused on the sea urchin Hemicentrotus pulcherrimus homolog of Sulf (HpSulf), and analyzed its expression pattern and functions during development. HpSulf protein was present throughout development and localized at cell surface of all blastomeres. In addition, the HS-specific epitope 10E4 was detected at the cell surface and partially colocalized with HpSulf. Knockdown of HpSulf using morpholino antisense oligonucleotides (MO) caused abnormal morphogenesis, and the development of MO-injected embryos was arrested before the hatched blastula stage, indicating that HpSulf is necessary for the early developmental process of sea urchin embryos. Furthermore, we found that injection of HpSulf mRNA suppressed the abnormal skeleton induced by overexpression of HpVEGF mRNA, whereas injection of an inactive form of HpSulf mRNA, containing mutated cysteines in the sulfatase domain, did not have this effect. Taken together, these results suggest that HpSulf is involved in the regulation of various signal transductions, including VEGF signaling, during sea urchin development.

Age-related changes in pericellular hyaluronan organization leads to impaired dermal fibroblast to myofibroblast differentiation

We have previously demonstrated that transforming growth factor-beta1 (TGF-beta1)-mediated fibroblast-myofibroblast differentiation is associated with accumulation of a hyaluronan (HA) pericellular coat. The current study demonstrates failure of fibroblast-myofibroblast differentiation associated with in vitro aging. This is associated with attenuation of numerous TGF-beta1-dependent responses, including HA synthesis and induction of the HA synthase enzyme HAS2 and the hyaladherin tumor necrosis factor-alpha-stimulated gene 6 (TSG-6), which led to an age-related defect in pericellular HA coat assembly. Inhibition of HAS2-dependent HA synthesis by gene silencing, removal of the HA coat by hyaluronidase digestion, or gene silencing of TSG-6 or cell surface receptor CD44 led to abrogation of TGF-beta1-dependent induction of alpha-smooth muscle actin in "young" cells. This result supports the importance of HAS2-dependent HA synthesis and the HA coat during phenotypic activation. Interleukin-1beta stimulation, however, failed to promote phenotypic conversion despite coat formation. A return to basal levels of HA synthesis in aged cells by HAS2 overexpression restored TGF-beta1-dependent induction of TSG-6 and pericellular HA coat assembly. However, this did not lead to the acquisition of a myofibroblast phenotype. Coordinated induction of HAS2 and TSG-6 facilitation of pericellular HA coat assembly is necessary for TGF-beta1-dependent activation of fibroblasts, and both components of this response are impaired with in vitro aging. In conclusion, the HA pericellular coat is integral but not sufficient to correct for the age-dependent defect in phenotypic conversion.

Modulation of TGFbeta1-dependent myofibroblast differentiation by hyaluronan

Myofibroblasts are contractile cells that are characterized by the expression of alpha-smooth muscle actin and mediate the closure of wounds and the formation of collagen-rich scars. Their presence in organs such as lungs, liver, and kidney has long been established as a marker of progressive fibrosis. The transforming growth factor beta(1)-driven differentiation of fibroblasts is a major source of myofibroblasts, and recent data have shown that hyaluronan is a major modulator of this process. This study examines this differentiation mechanism in more detail. Transforming growth factor beta(1)-dependent differentiation to the myofibroblastic phenotype was antagonized by the inhibition of hyaluronan synthesis, confirming that hyaluronan was necessary for differentiation. This response, however, was not reproduced by simply adding hyaluronan to fibroblasts, as the results implicated hyaladherins, as well as the macromolecular assembly of de novo hyaluronan, as essential in this process. We previously suggested that there is a relocalization of lipid-raft components during myofibroblastic differentiation. The present study demonstrates that the hyaluronan receptor CD44, the hyaluronidase HYAL 2, and the transforming growth factor beta(1)-receptor ALK5 all relocalized from raft to non-raft locations, which was reversed by the addition of exogenous hyaluronan. These data highlight a role for endogenous hyaluronan in the mediation of myofibroblastic differentiation. While hyaluronan synthesis was both essential and necessary for differentiation, exogenously provided hyaluronan antagonized differentiation, underscoring a pathological role for hyaluronan in such cell fate processes.

Hyaluronan molecular weight is controlled by UDP-N-acetylglucosamine concentration in Streptococcus zooepidemicus

The molecular weight of hyaluronan is important for its rheological and biological function. The molecular mechanisms underlying chain termination and hence molecular weight control remain poorly understood, not only for hyaluronan synthases but also for other beta-polysaccharide synthases, e.g. cellulose, chitin, and 1,3-betaglucan synthases. In this work, we manipulated metabolite concentrations in the hyaluronan pathway by overexpressing the five genes of the hyaluronan synthesis operon in Streptococcus equi subsp. zooepidemicus. Overexpression of genes involved in UDP-glucuronic acid biosynthesis decreased molecular weight, whereas overexpression of genes involved in UDP-N-acetylglucosamine biosynthesis increased molecular weight. The highest molecular mass observed was at 3.4 +/- 0.1 MDa twice that observed in the wild-type strain, 1.8 +/- 0.1 MDa. The data indicate that (a) high molecular weight is achieved when an appropriate balance of UDP-N-acetylglucosamine and UDP-glucuronic acid is achieved, (b) UDP-N-acetylglucosamine exerts the dominant effect on molecular weight, and (c) the wild-type strain has suboptimal levels of UDP-N-acetylglucosamine. Consistent herewith molecular weight correlated strongly (rho = 0.84, p = 3 x 10(-5)) with the concentration of UDP-N-acetylglucosamine. Data presented in this paper represent the first model for hyaluronan molecular weight control based on the concentration of activated sugar precursors. These results can be used to engineer strains producing high molecular weight hyaluronan and may provide insight into similar polymerization mechanisms in other polysaccharides.

Hyaluronan orchestrates transforming growth factor-beta1-dependent maintenance of myofibroblast phenotype

The differentiation of resident fibroblasts to myofibroblasts is central to wound healing. In the context of organ fibrosis, however, persistence of these myofibroblasts is associated with progressive disease. This study examines mechanisms controlling the maintenance of the myofibroblast phenotype. Myofibroblasts were induced by adding transforming growth factor-beta1 (TGF-beta1) (10 ng/ml) to fibroblasts for 72 h. The phenotype was maintained for up to 120 h following removal of TGF-beta1. Western blot for pSmad2 and -3 demonstrated persistent phosphorylation despite removal of exogenous TGF-beta1. This persistence was because of autocrine synthesis of TGF-beta1, which was inhibited by both anti-TGF-beta1 antibody and the ALK5 inhibitor SB431542. Persistence of phenotype was also associated with increased hyaluronan (HA) generation, synthesis of the hyaladherin TSG6, and HA pericellular coat formation. These were all inhibited by TGF-beta receptor blockade. To further investigate the importance of HA synthesis, 4-methylumbelliferone was used to deplete the cytoplasmic pool of UDP-glucuronic acid, essential for HA chain elongation. This prevented formation of the pericellular HA matrix and decreased expression of alpha-SMA. 4-Methylumbelliferone had no effect, however, on Smad2 and -3 phosphorylation. Similarly inhibition of HAS2 by short interfering RNA prevented phenotypic activation without altering TGF-beta1-dependent Smad phosphorylation, thus suggesting that HA-dependent regulation of cell phenotype was independent of Smad activation. These data suggest that myofibroblasts in areas of fibrosis maintain their own phenotype through autocrine TGF-beta1 action and that extracellular HA matrices are an essential mediator of this. We propose a model in which the formation of the pericellular HA matrix regulates the outcome of Smad-dependent autocrine TGF-beta1-activated signaling, and therefore persistence of the myofibroblast phenotype.

Unique attributes of orbital fibroblasts and global alterations in IGF-1 receptor signaling could explain thyroid-associated ophthalmopathy

Tissue remodeling associated with thyroid-associated ophthalmopathy (TAO) involves the complex interplay between resident cells (endothelium, vascular smooth muscle, extraocular muscle, and fibroblasts) and those recruited to the orbit, including members of the "professional" immune system. Inflammation early in the disease can later culminate in fibrosis and diminished extraocular muscle motility. TAO remains a poorly understood process, in large part because access to tissues early in the disease is limited and because no robust and complete animal models of Graves' disease have yet been devised. Remaining uncertainty as to the identity of a pathogenic autoantigen(s) that underlies lymphocyte trafficking to the orbit complicates matters. These limitations in our understanding of extrathyroidal Graves' disease have resulted in poorly served patients with severe TAO. Therapies have targeted symptoms rather than the underlying disease processes. Our laboratory group has focused over the last several years on defining the peculiarities of the human orbital fibroblasts as a strategy for shedding more light on the pathologies occurring in TAO. We have reasoned that unique properties of these cells might ultimately prove the basis for why the manifestations of Graves' disease occur in an anatomically selective manner. In this brief review we attempt to survey our findings. We believe that they might provide a "roadmap" for further discovery into the pathogenesis of TAO. Clearly, more questions remain than those thus far answered.

Alteration of the quaternary structure of human UDP-glucose dehydrogenase by a double mutation

There are conflicting views for the polymerization process of human UDP-glucose dehydrogenase (UGDH) and no clear evidence has been reported yet. Based on crystal coordinates for Streptococcus pyogenes UGDH, we made double mutant A222Q/S233G. The double mutagenesis had no effects on expression, stability, and secondary structure. Interestingly, A222Q/S233G was a dimeric form and showed an UGDH activity, although it showed increased Km values for substrates. These results suggest that Ala222 and Ser233 play an important role in maintaining the hexameric structure and the reduced binding affinities for substrates are attributable to its altered subunit communication although quaternary structure may not be critical for catalysis.

Hyaluronan--regulator and initiator of peritoneal inflammation and remodeling

Although previously described as an inert space filler, there is now compelling evidence to underscore the importance of hyaluronan in physiologic and pathologic processes. Despite its simple structure, hyaluronan plays essential roles in embryonic development, phenotypic changes, proliferation, wound healing, inflammation and angiogenesis. Hyaluronan is a major component of the glycocalyx that forms a protective barrier around mesothelial cells, and bestows upon the peritoneal membrane a slippery non-adhesive surface preventing abrasion, infection and tumor dissemination. Hyaluronan is associated with mesothelial-to-mesenchymal transdifferentiation, recruitment of leukocytes to sites of inflammation, and mediates the reparative process after tissue injury by initiating increased synthesis of growth factors. Serum and dialysate levels of hyaluronan are increased in patients maintained on peritoneal dialysis (PD), of which the levels are further increased during episodes of peritonitis. The level of hyaluronan in PD effluents is often used as a surrogate marker for peritoneal inflammation and can predict patient survival. This review will describe the multifaceted roles of hyaluronan in the peritoneum and how these roles are modulated during PD.

Molecular signatures of soy-derived phytochemicals in androgen-responsive prostate cancer cells: a comparison study using DNA microarray

The present study utilized microarray technology as a tool to elucidate the molecular signatures of soy-derived phytochemicals in the human androgen-responsive prostate cancer cell line LNCaP. Global gene expression pattern analysis of LNCaP cells exposed to 0, 1, 5, or 25 microM of the soy-derived phytochemicals equol and daidzein were conducted and compared. The data were further compared with previously generated data from exposure of LNCaP cells to the same doses of genistein, a soy isoflavone. Multidimensional scaling (MDS) analyses of the expression patterns suggest that these compounds exerted differential effects on gene expression in LNCaP cells. Further examination of specific gene changes revealed that these compounds differentially modulated genes in multiple cellular pathways, including the cell-cycle pathway genes. However, the three compounds also exerted similar effect on genes belonging to several other important cellular pathways. A universal effect of the three compounds on androgen-responsive genes, IGF-1 pathway gene, and MAP kinase-related pathway gene was observed. These results provide the foundation for establishing molecular signatures for equol, daidzein, and genistein. Moreover, these results also allow for the identification of candidate mechanism(s) by which soy phytochemicals and soy may act in prostate cancer cells.

Control of capsular polysaccharide chain length by UDP-sugar substrate concentrations in Streptococcus pneumoniae

Regulation of chain length is essential to the proper functioning of prokaryotic and eukaryotic polysaccharides. Modulation of polymer size by substrate concentration is an attractive but unexplored control mechanism that has been suggested for many polysaccharides. The Streptococcus pneumoniae capsular polysaccharide is essential for virulence, and regulation of its size is critical for survival in different host environments. Synthesis of the type 3 capsule [-4)-beta-d-Glc-(1-3)-beta-d-GlcUA-(1-] from UDP-glucose (UDP-Glc) and UDP-glucuronic acid (UDP-GlcUA) is catalysed by the type 3 synthase, a processive beta-glycosyltransferase, and requires a UDP-Glc dehydrogenase for conversion of UDP-Glc to UDP-GlcUA. Strains containing mutant UDP-Glc dehydrogenases exhibited reduced levels of UDP-GlcUA, along with reductions in total capsule amount and polymer chain length. In both the parent and mutant strains, UDP-Glc levels far exceeded UDP-GlcUA levels, which were very low to undetectable in the absence of blocking synthase activity. The in vivo observations were consistent with in vitro conditions that effect chain termination and ejection of the polysaccharide from the synthase when one substrate is limiting. These data are the first to demonstrate modulation of polysaccharide chain length by substrate concentration and to enable a model for the underlying mechanism. Further, they may have implications for the control of chain length in both prokaryotic and eukaryotic polymers synthesized by similar mechanisms.

Recombinant human hyaluronan synthase 3 is phosphorylated in mammalian cells

Hyaluronan is a ubiquitous component of vertebrate extracellular and cell-associated matrices that serves as a key structural component of skin, cartilage, eyes and joints, and plays important roles in dynamic cellular processes, including embryogenesis, inflammation, wound healing and metastasis. Hyaluronan is synthesized by three homologous hyaluronan synthases designated HAS1, HAS2 and HAS3 that differ in their tissue distribution, regulation and enzymatic characteristics. Some progress has been made in characterizing regulation of HAS transcripts and in distinguishing the enzymatic properties of the various HAS isoforms, but essentially nothing is known about their possible regulation by posttranslational modification. Using [32P]P(i) radiolabelling of a recombinant FLAG (DYKDDDDK) epitope-tagged version of human HAS3 expressed in COS-7 cells, we show that HAS3 is serine-phosphorylated and that this phosphorylation can be enhanced by a number of effectors--most significantly by a membrane-permeable analogue of cAMP. By employing a novel FLAG-tagged phosphorylated reference protein derived from EGFP (enhanced green fluorescent protein), we were able to estimate the stoichiometry of FLAG-HAS3 phosphorylation. It was approx. 0.11 in unstimulated cells and increased to as much as 0.32 in cells stimulated with 8-(4-chlorophenylthio)-cAMP.

Overexpression of hyaluronan synthase 2 alters hyaluronan distribution and function in proximal tubular epithelial cells

The functional consequences of increased renal cortical hyaluronan that is associated with both acute injury and progressive scarring are unclear. The aim of this study was to characterize hyaluronan synthase-2 (HAS2)-driven HA synthesis and determine its effect on renal proximal tubular epithelial cell (PTC) function, because this is known to be the inducible form of HA synthase in this cell type. Overexpression of HAS2 mRNA increased HA generation, which in the supernatant predominantly was HA of large molecular weight, whereas there was an increase in low molecular weight HA in cell-associated fractions. This was associated with increased expression of hyaluronidases, inhibition of HA cable formation concurrent with reduction in HA-dependent monocyte binding, and increased pericellular HA matrix. Overexpression of HAS2 led to enhanced cell migration. HA can be modified by the covalent attachment of heavy chains that are derived from the serum protein inter-alpha-inhibitor (IalphaI), a process that is known to be catalyzed by TNF-alpha-stimulated gene 6 (TSG-6; an inflammation-associated protein). Enhanced migration was abrogated by blocking antibodies to either IalphaI or TSG-6. Addition of recombinant full-length TSG-6 (TSG-6Q) or TSG-6Q_Y94F, a mutant variant with impaired HA binding, increased cell migration. Both of these proteins were able to mediate the covalent transfer of heavy chains, from IalphaI and pre-alpha-inhibitor, onto HA. Addition of the isolated TSG-6-Link module (Link_TSG-6), which binds HA but is unable to form covalent complexes with IalphaI/pre-alpha-inhibitor, had no effect on migration, suggesting that TSG-6-mediated formation of heavy chain-HA complexes is critical in the formation of a pericellular HA matrix.

Characterization and expression of Arabidopsis UDP-sugar pyrophosphorylase

At5g52560, a homolog of pea (Pisum sativum) UDP-sugar pyrophosphorylase (PsUSP) was functionally annotated by expression in Escherichia coli and subsequent characterization of substrate specificity and kinetic properties. Arabidopsis contains a single USP gene (AtUSP) and evaluation of gene databases suggests that USP is unique to plants. The 69 kDa AtUSP gene product exhibited high activity with Glc-1-P, GlcA-1-P and Gal-1-P, but low activity with GlcNAc-1-P, Fuc-1-P, Man-1-P, inositol-1-P or Glc-6-P. AtUSP was activated by magnesium and preferred UTP as co-substrate. Apparent K(m) values for GlcA-1-P, Glc-1-P and UTP were 0.13 mM, 0.42 mM and 0.14 mM, respectively. In the reverse direction (pyrophosphorolysis), the apparent K(m) values for UDP-GlcA, UDP-Glc and pyrophosphate were 0.56 mM, 0.72 mM and 0.15 mM, respectively. USP enzyme activity (UDP-GlcA --> GlcA-1-P) was detected in Arabidopsis tissues with highest activity found in the inflorescence. As determined by semi-quantitative RT-PCR, AtUSP transcript is widely expressed with high levels detected in the inflorescence. To evaluate tissue-specific expression of AtUSP, histochemical GUS staining of plants transformed with AtUSPprom:GUS constructs was performed. In 7-day-old seedlings, GUS staining was detected in cotyledons, trichomes and vascular tissues of the primary root. In the inflorescence of older plants, high levels of GUS staining were detected in cauline leaves, the epidermis of the stem and in pollen. In silico analysis of AtUSP expression in developing pollen indicates that transcript levels increase as development proceeds from the uninucleate to the tricellular stage. The results suggest that AtUSP plays an important role in pollen development in Arabidopsis.

Effects of xenobiotics and peroxisome proliferator-activated receptor-alpha on the human UDPglucose dehydrogenase gene expression

During drug metabolism, UDPglucuronate, a product of the reaction catalyzed by the enzyme UDPglucose dehydrogenase (UGDH), is conjugated with the metabolites to facilitate their elimination. So far, it is not known whether xenobiotics can modulate the UGDH gene expression. This question was tested by treating the human hepatoma cells HepG2 with several medicinal compounds and the UGDH gene expression analyzed by using real-time PCR. Both eugenol and rifampicin showed activation of the gene expression. Piperine showed slight down-regulation of the UGDH gene expression, whereas no effect was observed with acetaminophen treatment. Through promoter-reporter gene assays, we found that rifampicin showed multiple-folds activation of a 1.23-kb UGDH promoter construct, the region likely to respond to rifampicin treatment is located within the range -632 to -1,050. A bioinformatics search for xenobiotic response element in this region has predicted a binding motif for the peroxisome proliferator-activated receptor-alpha(PPARalpha) at position -1,003. A mutation at the predicted PPAR recognizing motif eliminated normal suppression as well as the rifampicin activation effect on the UGDH promoter activity. Cotransfection with the PPARalpha and retinoid X receptor-alpha expression vectors and subsequent treatment with the PPARalpha agonist led to the suppression of the UGDH promoter activity either in the presence or absence of rifampicin. Our study, for the first time, shows the UGDH gene to be under xenobiotic regulation and delineates a motif responsible for rifampicin response and transcriptional repression of the UGDH gene.

Molecular cloning and characterization of UDP-glucose dehydrogenase from the amphibian Xenopus laevis and its involvement in hyaluronan synthesis

UDP-glucose dehydrogenase (UGDH) supplies the cell with UDP-glucuronic acid (UDP-GlcUA), a precursor of glycosaminoglycan and proteoglycan synthesis. Here we reported the cloning and the characterization of the UGDH from the amphibian Xenopus laevis that is one of the model organisms for developmental biology. We found that X. laevis UGDH (xUGDH) maintained a very high degree of similarity with other known UGDH sequences both at the genomic and the protein levels. Also its kinetic parameters are similar to those of UGDH from other species. During X. laevis development, UDGH is always expressed but clearly increases its mRNA levels at the tail bud stage (i.e. 30 h post-fertilization). This result fits well with our previous observation that hyaluronan, a glycosaminoglycan that is synthesized using UDP-GlcUA and UDP-N-acetylglucosamine, is abundantly detected at this developmental stage. The expression of UGDH was found to be related to hyaluronan synthesis. In human smooth muscle cells the overexpression of xUGDH or endogenous abrogation of UGDH modulated hyaluronan synthesis specifically. Our findings were confirmed by in vivo experiments where the silencing of xUGDH in X. laevis embryos decreased glycosaminoglycan synthesis causing severe embryonic malformations because of a defective gastrulation process.

Gene expression profiling of mouse postnatal cerebellar development using oligonucleotide microarrays designed to detect differences in glycoconjugate expression

Differences in gene expression patterns between adult and postnatal day 7 (P7) mouse cerebellum, at the peak of granule neuron migration, were analyzed by hybridization to the GLYCOv2 glycogene array. This custom designed oligonucleotide array focuses on glycosyl transferases, carbohydrate-binding proteins, proteoglycans and related genes, and 173 genes were identified as being differentially expressed with statistical confidence. Expression levels for 11 of these genes were compared by RT-PCR, and their differential expression between P7 and adult cerebellum confirmed. Within the group of genes showing differential expression, the sialyltransferases (SiaTs) and GalNAc-Ts that were elevated at P7 prefer glycoprotein substrates, whilst the SiaTs and GalNAc-Ts that were elevated in the adult preferentially modify glycolipids, consistent with a role for gangliosides in maintaining neuronal function in the adult. Also within this group, three proteoglycans--versican, bamacan and glypican-2--were elevated at P7, along with growth factor midkine, which is known to bind to multiple types of proteoglycans, and fibroblast growth factor receptor 1, whose activity is known to be influenced by heparan sulfate proteoglycans. Two sulfotransferases that can modify the extent of proteoglycan sulfation were also differentially regulated, and may modify the interaction of a subset of proteoglycans with their binding partners during cerebellar development. Bamacan, glypican-2 and midkine were shown to be expressed in different cell types, and their roles in cerebellar development during granule neuron migration and maturation are discussed.

Insights into the role of fibroblasts in human autoimmune diseases

Traditional wisdom has considered fibroblasts as contributing to the structural integrity of tissues rather than playing a dynamic role in physiological or pathological processes. It is only recently that they have been recognized as comprising diverse populations of cells exhibiting complex patterns of biosynthetic activity. They represent determinants that react to stimuli and help define tissue remodelling through the expression of molecules imposing constraints on their cellular neighbourhood. Moreover, fibroblasts can initiate the earliest molecular events leading to inflammatory responses. Thus they must now be viewed as active participants in tissue reactivity. In this short review, I will provide an overview of contemporary thought about the contribution of fibroblasts to the pathogenesis of autoimmune processes through their expression of, and responses to, mediators of inflammation and tissue remodelling.

Analysis of human UDP-glucose dehydrogenase gene promoter: identification of an Sp1 binding site crucial for the expression of the large transcript

UDP-glucose dehydrogenase (UGDH) catalyzes the conversion of UDP-glucose to UDP-glucuronic acid, which is required in liver for the excretion of toxic compounds, and for the biosynthesis of complex carbohydrates, such as hyaluronan, in many cell types. Analysis of a human EST database, as well as the results of a 5'-RACE experiment, have revealed the presence of two transcription start sites approximately 160 bp apart in the human UGDH gene confirming previous Northern hybridization results. To delineate the regions in the UGDH promoter required for regulating the expression of the gene, in particular the synthesis of the large transcript, serial deletions of the 2.1-kb UGDH promoter region were constructed and their activities determined by the firefly luciferase reporter gene assay. Our results indicate that the region from nucleotide position -486 to -632 relative to the start of the small transcript contains positive regulatory elements that contribute to gene expression. Mithramycin A, an inhibitor of transcription factor Sp1, abrogates the promoter activity, suggesting the involvement of this specific protein in UGDH expression. By using site-directed mutagenesis, we analyzed the functional contribution of three putative Sp1 binding elements within this region. A mutation at position -564 demonstrated that this site serves as an enhancing element in both HepG2 and HeLa cells. The complex formation pattern revealed by an electrophoretic mobility shift assay as well as an anti-Sp1 antibody-mediated supershift assay confirmed the identity of this GC box as an Sp1 binding motif. Our results thus identify an alternative transcription start site on the UGDH promoter, and locate the cis-element that greatly enhances the basal transcriptional activity of UGDH gene.

c-Krox down-regulates the expression of UDP–glucose dehydrogenase in chondrocytes

Chondrocyte glycosaminoglycan (GAG) synthesis is regulated by the availability of UDP-glucuronate, the substrate of glucuronosyl transferases which form the GAG chains in proteoglycans and hyaluronan. UDP-glucose dehydrogenase (UDPGD) is therefore a key enzyme in the synthesis of UDP-glucuronate from glucose. However, the mechanisms regulating its expression in chondrocytes are not fully understood. We investigated the effect of c-Krox, a zinc-finger transcription factor previously shown to modulate several matrix genes, on the synthesis of GAG and transcriptional activity of several UDPGD gene promoter constructs, using transient transfection and decoy experiments in rabbit articular chondrocytes (RACs). We show that overexpression of c-Krox inhibits radiosulfate incorporation into neosynthesized GAG and that the effect was mediated by a cis-sequence located between +18 and +39bp of the UDPGD gene. Since that sequence can also bind Sp1/Sp3 factors, it is likely that c-Krox acts in concert with these proteins to modulate the UDPGD gene expression in articular chondrocytes.

UGD1, encoding the Cryptococcus neoformans UDP-glucose dehydrogenase, is essential for growth at 37 degrees C and for capsule biosynthesis

We report the identification and disruption of the Cryptococcus neoformans var. grubii UGD1 gene encoding the UDP-glucose dehydrogenase, which catalyzes the conversion of UDP-glucose into UDP-glucuronic acid. Deletion of UGD1 led to modifications in the cell wall, as revealed by changes in the sensitivity of ugd1Delta cells to sodium dodecyl sulfate, NaCl, and sorbitol. Moreover, two of the yeast's major virulence factors-capsule biosynthesis and the ability to grow at 37 degrees C-were impaired in ugd1Delta strains. These results suggest that the UDP-dehydrogenase represents the major, and maybe only, biosynthetic pathway for UDP-glucuronic acid in C. neoformans. Consequently, deletion of UGD1 blocked not only the synthesis of UDP-glucuronic acid but also that of UDP-xylose. To differentiate the phenotype(s) associated with the UDP-glucuronic acid defect alone from those linked to the UDP-xylose defect, ugd1Delta mutants were phenotypically compared to strains from which the gene encoding UDP-xylose synthase (i.e., that required for synthesis of UDP-xylose) had been deleted. Finally, studies of strains from which one of the four CAP genes (CAP10, CAP59, CAP60, or CAP64) had been deleted revealed common cell wall phenotypes associated with the acapsular state.

Induction by IL-1β of Tissue Inhibitor of Metalloproteinase-1 in Human Orbital Fibroblasts: Modulation of Gene Promoter Activity by IL-4 and IFN-γ

Thyroid-associated ophthalmopathy (TAO), an autoimmune component of Graves' disease, is associated with profound connective tissue remodeling and fibrosis that appear to involve the selective activation of orbital fibroblasts. Accumulation of extracellular matrix molecules is a hallmark of this process. Here we report that orbital fibroblasts treated with IL-1beta express high levels of tissue inhibitor of metalloproteinase-1 (TIMP-1), an important modulator of matrix metalloproteinase activity. These high levels are associated with increased TIMP-1 activity. The induction is mediated at the pretranslational level and involves activating the TIMP-1 gene promoter. IL-1beta activates the ERK 1/2 pathway in these fibroblasts and interrupting this signaling either with PD98059, a chemical inhibitor of MEK, or by transfecting cells with a dominant negative ERK 1 plasmid results in the attenuation of TIMP-1 induction. Surprisingly, treatment with IL-4 or IFN-gamma could also block the IL-1beta induction by attenuating TIMP-1 gene promoter activity. These findings suggest that TIMP-1 expression in orbital fibroblasts following activation with IL-1beta could represent an important therapeutic target for modifying the proteolytic environment. This might alter the natural course of tissue remodeling in TAO.

Selective activation of the MEK-ERK pathway is regulated by mechanical stimuli in forming joints and promotes pericellular matrix formation

It is well established that local modification of extracellular matrix (ECM) hyaluronan composition is vital in the regulation of cell behavior. Indeed, the formation of articulating chick joint cavities, which requires mechanical stimuli derived from skeletal movement, is dependent upon the accumulation of an ECM rich in hyaluronan (HA). However, the mechanisms responsible for such precise mechano-dependent regulation of cell behavior and the formation of a HA-rich ECM remain undefined. Here we show that extracellular-regulated kinase 1/2 (ERK1/2) is selectively activated in cells at sites of cavity formation and activity diminished by in ovo immobilization that induces cartilaginous fusion across presumptive joint interzones. In vitro analyses offer mechanistic support for the role of mechanical stimuli in promoting a MEK-dependent activation of ERK1/2. In addition, our direct regulation of ERK1/2 phosphorylation status via modulation of its up-stream "classical cascade" activator either pharmacologically or by transfection with dominant negative or constitutively active Mek confirms the essential role for ERK1/2 activation in the elaboration of HA-rich pericellular matrices. Together, our findings demonstrate that the MEK-ERK pathway, regulated by mechanical stimuli, controls HA-rich matrix assembly. The precision of ERK1/2 activation selectively distinguishing cells at the joint line suggests that it directly contributes to the loss of tissue cohesion essential for generating HA-rich cavities between joint elements during their development.

Biosynthesis of UDP-GlcA, a key metabolite for capsular polysaccharide synthesis in the pathogenic fungus Cryptococcus neoformans

UDP-glucose dehydrogenase catalyses the conversion of UDP-glucose into UDP-GlcA, a critical precursor for glycan synthesis across evolution. We have cloned the gene encoding this important enzyme from the opportunistic pathogen Cryptococcus neoformans. In this fungus, UDP-GlcA is required for the synthesis of capsule polysaccharides, which in turn are essential for virulence. The gene was expressed in Escherichia coli and the 51.3-kDa recombinant protein from wild-type and five mutants was purified for analysis. The cryptococcal enzyme is strongly inhibited by UDP-xylose and NADH, has highest activity at pH 7.5 and demonstrates Km (app) values of 0.1 and 1.5 mM for NAD+ and UDP-glucose respectively. Its activity was significantly decreased by mutations in the putative sites of NAD+ and UDP-glucose binding. Unlike previously reported eukaryotic UDP-glucose dehydrogenases, which are hexamers, the cryptococcal enzyme is a dimer.

Synchronized Chemoenzymatic Synthesis of Monodisperse Hyaluronan Polymers

The length of the hyaluronan (HA) polysaccharide chain dictates its biological effects in many cellular and tissue systems. Long and short HA polymers often appear to have antagonistic or inverse effects. However, no source of very defined, uniform HA polymers with sizes greater than 10 kDa is currently available. We present a method to produce synthetic HA with very narrow size distributions in the range of approximately 16 kDa to approximately 2 MDa. The Pasteurella HA synthase enzyme, pmHAS, catalyzes the synthesis of HA polymer utilizing monosaccharides from UDP-sugar precursors. Recombinant pmHAS will also elongate exogenously supplied HA oligosaccharide acceptors in vitro in a nonprocessive fashion. As a result of bypassing the slow initiation step in vitro, the elongation process is synchronized in the presence of acceptor; thus all of polymer products are very similar in length. In contrast, without the use of an acceptor, the final polymer size range is difficult to predict and the products are more polydisperse. HA polymers of a desired size are constructed by controlling the reaction stoichiometry (i.e. molar ratio of precursors and acceptor molecules). The use of modified acceptors allows the synthesis of HA polymers containing tags (e.g. fluorescent, radioactive). In this scheme, each molecule has a single foreign moiety at the reducing terminus. Alternatively, the use of radioactive UDP-sugar precursors allows the synthesis of uniformly labeled native HA polymers. Overall, synthetic HA reagents with monodisperse size distributions and defined structures should assist in the elucidation of the numerous roles of HA in health and disease.

Importance of Gly-13 for the coenzyme binding of human UDP-glucose dehydrogenase

UDP-glucose dehydrogenase (UGDH) is the unique pathway enzyme furnishing in vertebrates UDP-glucuronate for numerous transferases. In this report, we have identified an NAD(+)-binding site within human UGDH by photoaffinity labeling with a specific probe, [(32)P]nicotinamide 2-azidoadenosine dinucleotide (2N(3) NAD(+)), and cassette mutagenesis. For this work, we have chemically synthesized a 1509-base pair gene encoding human UGDH and expressed it in Escherichia coli as a soluble protein. Photolabel-containing peptides were generated by photolysis followed by tryptic digestion and isolated using the phosphopeptide isolation kit. Photolabeling of these peptides was effectively prevented by the presence of NAD(+) during photolysis, demonstrating a selectivity of the photoprobe for the NAD(+)-binding site. Amino acid sequencing and compositional analysis identified the NAD(+)-binding site of UGDH as the region containing the sequence ICCIGAXYVGGPT, corresponding to Ile-7 through Thr-19 of the amino acid sequence of human UGDH. The unidentified residue, X, can be designated as a photolabeled Gly-13 because the sequences including the glycine residue in question have a complete identity with those of other UGDH species known. The importance of Gly-13 residue in the binding of NAD(+) was further examined with a G13E mutant by cassette mutagenesis. The mutagenesis at Gly-13 had no effects on the expression or stability of the mutant. Enzyme activity of the G13E point mutant was not measurable under normal assay conditions, suggesting an important role for the Gly-13 residue. No incorporation of [(32)P]2N(3)NAD(+) was observed for the G13E mutant. These results indicate that Gly-13 plays an important role for efficient binding of NAD(+) to human UGDH.

The putative role of fibroblasts in the pathogenesis of Graves' disease: evidence for the involvement of the insulin-like growth factor-1 receptor in fibroblast activation

Graves' disease when fully expressed affects the thyroid gland and connective tissues of the orbit and pretibium. While the glandular disease is relatively well-characterized, the pathogenesis of the orbital and dermal components remains enigmatic. In the following article, we review some of the evidence suggesting that fibroblast activation in Graves' disease might play an integral role in the tissue remodeling associated with ophthalmopathy. The thyrotropin receptor (TSHR) is expressed at low levels in several connective tissue depots and by their derivative fibroblasts, including those from the orbit. Little direct evidence currently links extra-thyroidal TSHR expression with Graves' disease. Very recent observations now implicate the insulin-like growth factor-1 receptor (IGF-1R) as a fibroblast activating antigen. When immunoglobulins from patients with the disease, with or without clinical ophthalmopathy, bind IGF-1R on the surface of fibroblasts, the receptor becomes activated and upregulates the expression of two T lymphocyte chemoattractants, IL-16 and RANTES. Thus, IGF-1R may represent a second self-antigen with a pathogenic role in extra-thyroidal Graves' disease.

In Vivo Hyaluronan Synthesis upon Expression of the Mammalian Hyaluronan Synthase Gene in Drosophila

Hyaluronan (HA) is a large linear polymer of repeating disaccharides of glucuronic acid and GlcNAc. Although HA is widely distributed in vertebrate animals, it has not been found in invertebrates, including insect species. Insects utilize chitin, a repeating beta-1,4-linked homopolymer of GlcNAc, as a major component of their exoskeleton. Recent studies illustrate the similarities in the biosynthetic mechanisms of HA and chitin and suggest that HA synthase (HAS) and chitin synthase have evolved from a common ancestral molecule. Although the biochemical properties and in vivo functions of HAS proteins have been extensively studied, the molecular basis for HA biosynthesis is not completely understood. For example, it is currently not clear if proper chain elongation and secretion of HA require other components in addition to HAS. Here, we demonstrate that a non-HA-synthesizing animal, the fruit fly Drosophila melanogaster, can produce HA in vivo when a single HAS protein is introduced. Expression of the mouse HAS2 gene in Drosophila tissues by the Gal4/UAS (upstream activating sequence) system resulted in massive HA accumulation in the extracellular space and caused various morphological defects. These morphological abnormalities were ascribed to disordered cell-cell communications due to accumulation of HA rather than disruption of heparan sulfate synthesis. We also show that adult wings with HA can hold a high level of water. These findings demonstrate that organisms synthesizing chitin (but not HA) are capable of producing HA that is structurally and functionally relevant to that in mammals. The ability of insect cells to produce HA supports the idea that in vivo HA biosynthesis does not require molecules other than the HAS protein. An alternative model is that Drosophila cells use endogenous components of the chitin biosynthetic machinery to produce and secrete HA.

Characterization of human UDP-glucose dehydrogenase. CYS-276 is required for the second of two successive oxidations

UDP-glucose dehydrogenase (UGDH) catalyzes two oxidations of UDP-glucose to yield UDP-glucuronic acid. Pathological overproduction of extracellular matrix components may be linked to the availability of UDP-glucuronic acid; therefore UGDH is an intriguing therapeutic target. Specific inhibition of human UGDH requires detailed knowledge of its catalytic mechanism, which has not been characterized. In this report, we have cloned, expressed, and affinity-purified the human enzyme and determined its steady state kinetic parameters. The human enzyme is active as a hexamer with values for Km and Vmax that agree well with those reported for a bovine homolog. We used crystal coordinates for Streptococcus pyogenes UGDH in complex with NAD+ cofactor and UDP-glucose substrate to generate a model of the enzyme active site. Based on this model, we selected Cys-276 and Lys-279 as likely catalytic residues and converted them to serine and alanine, respectively. Enzymatic activity of C276S and K279A point mutants was not measurable under normal assay conditions. Rate constants measured over several hours demonstrated that K279A continued to turn over, although 250-fold more slowly than wild type enzyme. C276S, however, performed only a single round of oxidation, indicating that it is essential for the second oxidation. This result is consistent with the postulated role of Cys-276 as a catalytic residue and supports its position in the reaction mechanism for the human enzyme. Lys-279 is likely to have a role in positioning active site residues and in maintaining the hexameric quaternary structure.

Novel aspects of orbital fibroblast pathology

Orbital fibroblasts exhibit a unique phenotype including exaggerated responses to proinflammatory cytokines. We hypothesize that the unusual susceptability of these fibroblasts to molecular cues underlies the involvement of the orbit in Graves' ophthalmopathy. A number of attributes of orbital fibroblasts are reviewed in this article. In addition, we have found IgG circulating in patients with Graves' disease that binds and activates the insulin-like growth factor-1 receptor displayed on fibroblasts from many anatomic regions. Activation of this receptor leads to the expression of T-cell chemoattractants. Thus, fibroblast activation, and the resulting T-cell trafficking to connective tissue in Graves' disease may be systemic. The consequences of lymphocyte-derived cytokine action may differ vastly in the orbit and other tissues manifesting clinically obvious disease.

The deafness genedfna5is crucial forugdhexpression and HA production in the developing ear in zebrafish

Over 30 genes responsible for human hereditary hearing loss have been identified during the last 10 years. The proteins encoded by these genes play roles in a diverse set of cellular functions ranging from transcriptional regulation to K+ recycling. In a few cases, the genes are novel and do not give much insight into the cellular or molecular cause for the hearing loss. Among these poorly understood deafness genes is DFNA5. How the truncation of the encoded protein DFNA5 leads to an autosomal dominant form of hearing loss is not clear. In order to understand the biological role of Dfna5, we took a reversegenetic approach in zebrafish. Here we show that morpholino antisense nucleotide knock-down of dfna5 function in zebrafish leads to disorganization of the developing semicircular canals and reduction of pharyngeal cartilage. This phenotype closely resembles previously isolated zebrafish craniofacial mutants including the mutant jekyll. jekyll encodes Ugdh [uridine 5′-diphosphate (UDP)-glucose dehydrogenase], an enzyme that is crucial for production of the extracellular matrix component hyaluronic acid (HA). In dfna5 morphants, expression of ugdh is absent in the developing ear and pharyngeal arches, and HA levels are strongly reduced in the outgrowing protrusions of the developing semicircular canals. Previous studies suggest that HA is essential for differentiating cartilage and directed outgrowth of the epithelial protrusions in the developing ear. We hypothesize that the reduction of HA production leads to uncoordinated outgrowth of the canal columns and impaired facial cartilage differentiation.

Current perspective on the pathogenesis of Graves' disease and ophthalmopathy

Graves' disease (GD) is a very common autoimmune disorder of the thyroid in which stimulatory antibodies bind to the thyrotropin receptor and activate glandular function, resulting in hyperthyroidism. In addition, some patients with GD develop localized manifestations including ophthalmopathy (GO) and dermopathy. Since the cloning of the receptor cDNA, significant progress has been made in understanding the structure-function relationship of the receptor, which has been discussed in a number of earlier reviews. In this paper, we have focused our discussion on studies related to the molecular mechanisms of the disease pathogenesis and the development of animal models for GD. It has become apparent that multiple factors contribute to the etiology of GD, including host genetic as well as environmental factors. Studies in experimental animals indicate that GD is a slowly progressing disease that involves activation and recruitment of thyrotropin receptor-specific T and B cells. This activation eventually results in the production of stimulatory antibodies that can cause hyperthyroidism. Similarly, significant new insights have been gained in our understanding of GO that occurs in a subset of patients with GD. As in GD, both environmental and genetic factors play important roles in the development of GO. Although a number of putative ocular autoantigens have been identified, their role in the pathogenesis of GO awaits confirmation. Extensive analyses of orbital tissues obtained from patients with GO have provided a clearer understanding of the roles of T and B cells, cytokines and chemokines, and various ocular tissues including ocular muscles and fibroblasts. Equally impressive is the progress made in understanding why connective tissues of the orbit and the skin in GO are singled out for activation and undergo extensive remodeling. Results to date indicate that fibroblasts can act as sentinel cells and initiate lymphocyte recruitment and tissue remodeling. Moreover, these fibroblasts can be readily activated by Ig in the sera of patients with GD, suggesting a central role for them in the pathogenesis. Collectively, recent studies have led to a better understanding of the pathogenesis of GD and GO and have opened up potential new avenues for developing novel treatments for GD and GO.

Essential role of glycosaminoglycans in Fgf signaling during mouse gastrulation

In vitro studies have suggested that proteoglycans facilitate signaling by mammalian growth factors, but genetic evidence supporting this role has been lacking. Here, we characterize the ENU-induced mutation lazy mesoderm (lzme), which disrupts the single mouse gene encoding UDP-glucose dehydrogenase (Ugdh), an enzyme required for the synthesis of the glycosaminoglycan (GAG) side chains of proteoglycans. lzme mutants arrest during gastrulation with defects in migration of mesoderm and endoderm, a phenotype similar to that of mutants in the fibroblast growth factor (Fgf) pathway. Analysis of the expression of molecular markers indicates that Fgf signaling is blocked in lzme mutant embryos. In contrast, signaling by the growth factors Nodal and Wnt3, which are also essential during mouse gastrulation, appears to be normal in lzme embryos. The results demonstrate that proteoglycans are required during mouse gastrulation specifically to promote Fgf signaling.

Overexpression of UDP-glucose dehydrogenase in Escherichia coli results in decreased biosynthesis of K5 polysaccharide

The Escherichia coli K5 capsular polysaccharide (glycosaminoglycan) chains are composed of the repeated disaccharide structure: -GlcAbeta1,4-GlcNAcalpha1,4-(where GlcA is glucuronic acid and GlcNAc is N-acetyl-D-glucosamine). The GlcA, present in most glycosaminoglycans, is donated from UDP-GlcA, which, in turn, is generated from UDP-glucose by the enzyme UDP-glucose dehydrogenase (UDPGDH). The formation of UDP-GlcA is critical for the biosynthesis of glycosaminoglycans. To investigate the role of UDPGDH in glycosaminoglycan biosynthesis, we used K5 polysaccharide biosynthesis as a model. E. coli was transformed with the complete gene cluster for K5 polysaccharide production. Additional transformation with an extra copy of UDPGDH resulted in an approx. 15-fold increase in the in vitro UDPGDH enzyme activity compared with the strain lacking extra UDPGDH. UDP-GlcA levels were increased 3-fold in overexpressing strains. However, metabolic labelling with [14C]glucose showed, unexpectedly, that overexpression of UDPGDH lead to decreased formation of K5 polysaccharide. No significant difference in the K5 polysaccharide chain length was observed between control and overexpressing strains, indicating that the decrease in K5-polysaccharide production most probably was due to synthesis of fewer chains. Our results suggest that K5-polysaccharide biosynthesis is strictly regulated such that increasing the amount of available UDP-GlcA results in diminished K5-polysaccharide production.

Specific protein-1 is a universal regulator of UDP-glucose dehydrogenase expression: its positive involvement in transforming growth factor-beta signaling and inhibition in hypoxia

UDP-glucose dehydrogenase (UGDH) is a key enzyme of the unique pathway for the synthesis of UDP-glucuronate, the substrate for the numerous glucuronosyl transferases, which act on the synthesis of glycosaminoglycans and glucuronidation reaction of xeno- and endobiotics. Using the bacterial artificial chromosome approach, we have cloned and characterized the human UGDH promoter. The core promoter of -644 nucleotides conferred reporter gene activity in transient transfection assay of a variety of cell types, including MRC5 fibroblasts and the HepG2 hepatoma cell line. The minimal promoter of -100 nucleotides contains a functional inverted TATA box. No consensus CAAT sequence was found up to -2133 nucleotides. The expression of UGDH was up- and down-regulated by transforming growth factor (TGF)-beta and hypoxia, respectively. TGF-beta enhanced the activity of all the deletion constructs, except the minimal promoter. Hypoxia slightly increased the activity of the short promoter-containing constructs but decreased that of the -374 nucleotides and core promoter constructs. The core promoter contained numerous GC-rich sequences for the binding of Sp1 transcription factor. Bisanthracycline, an anti-Sp1 compound, decreased UGDH mRNA expression and inhibited the core promoter constructs activity. Gel mobility shift and supershift assays after TGF-beta stimulation demonstrated an increased DNA binding of the nuclear extract proteins to the two Sp1 sequences located in the -374-bp promoter. By contrast, nuclear extract proteins from hypoxia-treated cells demonstrated a decreased binding of the consensus Sp1 sequence. These results indicate that numerous Sp1 cis-acting sequences of the UGDH core promoter are responsible for up- and down-regulation of the gene after TGF-beta stimulation and in hypoxic conditions, respectively.

Alteration of polysaccharide size distribution of a vertebrate hyaluronan synthase by mutation

Hyaluronan (HA) is a nonsulfated glycosaminoglycan that has long been known to play structural roles in vertebrates. Recently, it has become increasingly obvious that this linear polysaccharide has many more uses than simply scaffolding or space filler. HA has been found to be involved in development, cell signaling, cell motility, and metastasis. These roles are often dictated by the length of the HA polymer, which can vary from a few to about 10,000 sugar residues in length. Three distinct isoforms of HA synthase exist in mammals. It has been shown previously by others that each isoform produces HA that differs in size distribution, but the regulatory mechanism is not yet known. Mutations have been described that alter the size distribution of the HA produced by the streptococcal HA synthases. We show that by mutating one particular amino acid residue of a vertebrate HA synthase, depending on the introduced side chain, the size of HA produced can be either reduced or increased. We postulate that several cysteine residues and a serine residue may be involved in binding directly or indirectly to the nascent HA chain. These data support the theory that the relative strength of the interaction between the catalyst and the polymer may be a major factor in HA size control.

'Piggy-back' transport of Xenopus hyaluronan synthase (XHAS1) via the secretory pathway to the plasma membrane

Hyaluronan is the sole glycosaminoglycan whose biosynthesis takes place directly at the plasma membrane. The mechanism by which hyaluronan synthase (HAS) becomes inserted there, as well as the question of how the enzyme discriminates between particular membrane species in polarized cells, are largely unknown. In vitro translation of HAS suggested that the nascent protein becomes stabilized in the presence of microsomal membranes, but would not insert spontaneously into membranes after being translated in the absence of those. We therefore monitored the membrane attachment of enzymatically active fusion proteins consisting of Xenopus HAS1 and green fluorescent protein shortly after de novo synthesis in Vero cells. Our data strongly suggest that HAS proteins are directly translated on the ER membrane without exhibiting an N-terminal signal sequence. From there the inactive protein is transferred to the plasma membrane via the secretory pathway. For unknown reasons, HAS inserted into membranes other than the plasma membrane remains inactive.

Proinflammatory cytokines regulate the gene expression of hyaluronic acid synthetase in cultured rabbit synovial membrane cells

To elucidate the mechanism of accumulation and fragmentation of hyaluronic acid (HA) under inflammatory conditions, we investigated the effect of proinflammatory cytokines on hyaluronic acid synthetase (HAS) mRNA expression using cultured rabbit synovial membrane cells. HASs mRNA levels were determined by real-time PCR. HAS2 mRNA expression was maximally enhanced 3.3- and 2.8-fold after 3-hour stimulation with IL-1beta (1 ng/ml) and after 1-hour stimulation with TNF-alpha (10 ng/ml). HAS3 mRNA expression was increased by a maximum of 4.3 times after 3-hour stimulation with IL-1beta (10 ng/ml), whereas 1-hour stimulation with TNF-alpha (10 ng/ml) and IFN-gamma (10 ng/ml) induced around a 2.5-fold increase in HAS3 mRNA. Although IFN-gamma (1-100 ng/ml) alone showed little effect on HAS2 mRNA expression, the effect was synergized by combined with both IL-1beta and TNF-alpha, substantially increasing HAS2 mRNA expression. These results suggest that proinflammatory cytokines regulate the HAS expression, and consequently may contribute to the accumulation and fragmentation of HA.

Orbital fibroblasts exhibit a novel pattern of responses to proinflammatory cytokines: potential basis for the pathogenesis of thyroid-associated ophthalmopathy

Thyroid-associated ophthalmopathy (TAO) represents a process confined to the orbit where the connective tissue becomes inflamed and accumulates the glycosaminoglycan, hyaluronan. Ultimately, the orbital tissues become extensively remodeled. Evidence points to the recruitment and activation of T cells as critical elements initiating and driving the pathogenesis of TAO. The phenotype of orbital fibroblasts appears to be distinct from that of other types of fibroblasts. These cells exhibit particularly robust responses to a number of T-cell-derived cytokines. Notable among these are the inductions of key inflammatory genes and their products. We hypothesize that exaggerated cellular responses represent the basis for the involvement of the orbit in Graves' disease.

Fibroblast subsets in the human orbit: Thy-1+ and Thy-1- subpopulations exhibit distinct phenotypes

An emerging concept is that fibroblasts are not homogeneous, but rather consist of subsets, capable of producing regulatory mediators that control regional inflammatory responses. Fibroblasts are key effector cells in Graves' ophthalmopathy, responsible for the connective tissue remodeling, and are a rich source of inflammatory mediators. The purpose of this research was to characterize subsets of the fibroblasts in the human orbit. The strategy used was to define fibroblast subpopulations based on surface expression of the Thy-1 antigen. Fibroblast strains derived from human orbital connective tissue exhibit heterogeneous Thy-1 expression. We show, for the first time, separation of orbital fibroblasts into functionally distinct Thy-1+ and Thy-1- subsets using magnetic beading techniques. Both subsets produced the pro-inflammatory cytokine interleukin-6 (IL-6) after stimulation with IL-1beta or the CD40 pathway, whereas Thy-1+ fibroblasts produced higher levels of prostaglandin endoperoxide H synthase-2 (PGHS-2) and prostaglandin E2 (PGE(2)). Thy-1- fibroblasts produced more IL-8 than Thy-1+ fibroblasts, and when treated with interferon-gamma (IFN-gamma) up-regulated MHC class II expression more robustly. Furthermore, CD40 was expressed in a bimodal distribution within each fibroblast subset. These observations suggest that fibroblast subsets in the human orbit play distinct roles in the regulation of immune and inflammatory responses crucial in the initiation and development of thyroid-associated ophthalmopathy.

Igs from patients with Graves' disease induce the expression of T cell chemoattractants in their fibroblasts

Thyroid-associated ophthalmopathy and dermopathy are connective tissue manifestations of Graves' disease (GD). Tissue remodeling is a prominent feature of both and is apparently driven by recruited T cells. In this study, we report that IgG isolated from patients with GD (GD-IgG) up-regulates T lymphocyte chemoattractant activity in GD-derived fibroblasts from orbit, thyroid, and several regions of skin. This chemoattractant activity, absent in fibroblasts from donors without known thyroid disease, is partially susceptible to neutralization by anti-IL-16 and anti-RANTES Abs. IL-16 is a CD4(+)-specific chemoattractant and RANTES is a C-C-type chemokine. IL-16 and RANTES protein levels, as determined by specific ELISAs, are substantially increased by GD-IgG in GD fibroblasts. Addition of the macrolide, rapamycin, to fibroblast culture medium blocked the up-regulation by GD-IgG of IL-16, implicating the FRAP/mTOR/p70(s6k) pathway in the induction of IL-16 expression. These findings suggest a specific mechanism for activation of fibroblasts in GD resulting in the recruitment of T cells. They may provide insight into a missing link between the glandular and extrathyroidal manifestations of GD.

UDP-glucose pyrophosphorylase: up-regulation in hypertrophic cartilage and role in hyaluronan synthesis

Previously, we have performed subtractive hybridization to identify genes up-regulated in hypertrophic chondrocytes of the avian epiphyseal growth plate. In the present study, we report the identification of one of the clones as UDP-glucose pyrophosphorylase (UDPG-PPase) and propose a possible function for this enzyme in regulating hyaluronan (HA) synthesis in hypertrophic cartilage. We have cloned the 2.6 kb full-length cDNA for avian UDPG-PPase and confirmed its up-regulation in hypertrophic versus non-hypertrophic cartilage by Northern-blot analysis. The 6-fold increase in mRNA was paralleled by an equivalent increase in enzymic activity. The enzyme catalyses the conversion of glucose 1-phosphate into UDP-glucose, which is used to synthesize a number of cellular components, including HA. Overexpression of enzymically active UDPG-PPase in non-hypertrophic chondrocytes resulted in a 2-3-fold increase in total HA, as determined by a competitive binding assay and immunohistochemistry. In the developing growth plate, HA synthesis was elevated in the hypertrophic zone along with the up-regulation of the HA synthase (HAS)-2 gene. Our data suggest that an increase in both activities, UDPG-PPase and HAS-2, is required for non-hypertrophic chondrocytes to synthesize an amount of HA comparable with that in hypertrophic chondrocytes. Therefore we conclude that HA synthesis during chondrocyte differentiation is regulated at the level of the substrate-provider gene, UDPG-PPase, as well as the HAS genes.

Regulation of hyaluronan synthase gene expression in human periodontal ligament cells by tumour necrosis factor-alpha, interleukin-1beta and interferon-gamma

Accumulation and fragmentation of hyaluronic (HA) accompanies the inflammatory changes in the periodontium and gingival crevicular fluid are involved in periodontitis, but the mechanism for this is unknown. Recently, three human hyaluronan-synthase (HAS1, 2, and 3) genes have been cloned and characterised as synthesising hyaluronans of different molecular weights. Both HAS1 and HAS2 synthesise high molecular-weight HA, whereas HAS3 produces lower molecular weight HA. In the present study the regulation of HAS genes by cytokines in cultured human periodontal ligament (PDL) cells was investigated using a novel real-time fluorescence polymerase chain reaction detection system. Human PDL cells derived from premolars were cultured with or without tumour necrosing factor (TNF)-alpha (1-100 ng/ml), interleukin (IL)-1beta (0.1-10 ng/ml) and interferon (IFN)-gamma (1-100 ng/ml). Expression of HAS mRNA was assessed in cultured cells treated with these cytokines for 0-24 h. The expression of HAS2 mRNA was enhanced about 4.5- and 2.2-fold at maximum after 3-h stimulation with 10 ng/ml TNF-alpha and 1 ng/ml IL-1beta, respectively, whereas IFN-gamma exerted little effect on HAS2 or HAS3 mRNA expression during the experiment. Expression of HAS3 mRNA was increased by about 14- and 10-fold after 3-h stimulation with 10 ng/ml TNF-alpha and 1 ng/ml IL-1beta, respectively. These results suggest that TNF-alpha and IL-1beta regulate HAS expression, and consequently may result in an accumulation of HA and an increase in HA of a lower molecular-weight.

L-ascorbic acid biosynthesis

Biosynthesis of L-ascorbate (vitamin C) occurs by different pathways in plants and mammals. Yeast contain D-erythroascorbate, a C5 analog of ascorbate. UDP-D-glucuronic acid is the precursor in mammals. Loss of UDP forms glucuronic acid/glucuronolactone. Reduction of these at C-1 then forms L-gulonic acid/L-gulono-1,4-lactone. The lactone is oxidized by a microsomal L-gulono-1,4-lactone oxidase to ascorbate. Only the L-gulono-1,4-lactone oxidase has been purified and cloned, and very little is known about the properties of the other enzymes. Plants form ascorbate from GDP-D-mannose via GDP-L-galactose, L-galactose, and L-galactono-1,4-lactone. The final oxidation of L-galactono-1,4-lactone to ascorbate is catalyzed by a mitochondrial L-galactono-1,4-lactone dehydrogenase located on the inner membrane and using cytochrome c as electron acceptor. GDP-mannose pyrophosphorylase and L-galactono-1,4-lactone dehydrogenase have been cloned. Yeast synthesizes D-erythroascorbate from D-arabinose and D-arabinono-1,4-lactone in a pathway analogous to that in plants. The plant, mammalian, and yeast aldonolactone oxidase/dehydrogenases that catalyze the last step in each pathway have significant sequence homology. L-Gulono-1,4-lactone oxidase is mutated and not expressed in animals, such as primates, that have lost ascorbate biosynthesis capacity. Assessment of the literature reveals that little is known about many of the enzymes involved in ascorbate biosynthesis or about the factors controlling flux through the pathways. There is also a possibility that minor alternative pathways exist in plants and mammals.

Differential regulation and expression of hyaluronan synthases in human articular chondrocytes, synovial cells and osteosarcoma cells

Recently three isoforms of hyaluronan synthase (HAS), the enzyme responsible for hyaluronate/hyaluronan (HA) biosynthesis, have been cloned, allowing us to study their expression pattern. Our objective was to determine which of the HAS isoenzymes were expressed in human articular chondrocytes, synovial fibroblasts and osteosarcoma cells, whether their expression could be modulated by growth factors (insulin-like growth factor-1, basic fibroblast growth factor and transforming growth factor (TGF-beta1) and cytokines [interleukin 1beta1 (IL-1beta)], and whether changes in the rate of HA synthesis by the cells correlated with changes in mRNA levels for one or more of the HAS isoforms. All three HAS isoforms were found to be expressed in the cultured cells analysed in this study, although the relative proportions varied for each cell type. HAS2 mRNA was usually predominant in chondrocytes, whereas synovial cells contained increased amounts of HAS1. HAS3 was always the least abundant message. The rapidly growing osteosarcoma cells contained almost exclusively HAS2 message. HAS usage in uncultured cartilage and synovial tissues was similar to that in the cultured cells, with HAS2 message being the predominant species in cartilage and HAS1 usually being the predominant species in synovium. HA synthesis was stimulated by the growth factors, but the extent of the response was cell-type specific. Synovial cells responded particularly well to IL-1beta, and showed a unique synergistic response when IL-1beta was used in combination with TGF-beta1. This response was much reduced in articular chondrocytes and absent in the osteosarcoma cells. Analysis of changes in HAS message levels indicated that there was often no correlation with the changes in HA secretion following exposure to growth factors. Although HAS-1 mRNA was increased in synovial cells after exposure to TGF-beta1/IL-1beta, the magnitude of the change was far less than the effect on HA synthesis. Our data thus suggest that HAS gene usage is tissue specific, and the regulation by growth factors is unique for each HAS gene and is further modulated by cell-specific factors. In addition, regulation of HA biosynthesis appears to be multi-faceted, with control of HAS gene expression and mRNA levels being only one aspect of this process.

Roles of glucuronidation and UDP-glucuronosyltransferases in xenobiotic bioactivation reactions

Glucuronide conjugates represent one of the major types of naturally occurring phase 2 metabolites of xenobiotics and endobiotics. The process underlying their formation, glucuronidation, is normally considered detoxifying, because glucuronides usually possess less intrinsic biological or chemical activity than their parent aglycones and they are rapid excreted. However, a number of glucuronide conjugates are known that are active and may contribute to pharmacological activities or toxicities associated with their parent compounds. These include two classes of glucuronides with electrophilic chemical reactivity (N-O-glucuronides of hydroxamic acids and acyl glucuronides of carboxylic acids) and several types of glucuronides that impart biological effects through non-covalent interactions (morphine 6-O-glucuronide, retinoid glucuronides, and D-ring glucuronides of estrogens). Glucuronides may thus contribute to clinically significant effects, including environmental arylamine-induced carcinogenesis, drug hypersensitivity and other toxicities associated with carboxylic acid drugs, morphine analgesia, and cholestasis from estrogens. This review summarizes the rat and human UDP-glucuronosyltransferases that may be involved in the formation of bioactive glucuronides, including their substrate- and tissue-specificity and genetic and environmental influences on their activity. This knowledge may be useful for enhancing the therapeutic efficacy and minimizing the risk of adverse effects associated with xenobiotics that undergo bioactivating glucuronidation reactions.