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

Fluorescence resonance energy transfer (FRET) and proximity ligation assays reveal functionally relevant homo- and heteromeric complexes among hyaluronan synthases HAS1, HAS2, and HAS3

In vertebrates, hyaluronan is produced in the plasma membrane from cytosolic UDP-sugar substrates by hyaluronan synthase 1-3 (HAS1-3) isoenzymes that transfer N-acetylglucosamine (GlcNAc) and glucuronic acid (GlcUA) in alternative positions in the growing polysaccharide chain during its simultaneous extrusion into the extracellular space. It has been shown that HAS2 immunoprecipitates contain functional HAS2 homomers and also heteromers with HAS3 (Karousou, E., Kamiryo, M., Skandalis, S. S., Ruusala, A., Asteriou, T., Passi, A., Yamashita, H., Hellman, U., Heldin, C. H., and Heldin, P. (2010) The activity of hyaluronan synthase 2 is regulated by dimerization and ubiquitination. J. Biol. Chem. 285, 23647-23654). Here we have systematically screened in live cells, potential interactions among the HAS isoenzymes using fluorescence resonance energy transfer (FRET) and flow cytometric quantification. We show that all HAS isoenzymes form homomeric and also heteromeric complexes with each other. The same complexes were detected both in Golgi apparatus and plasma membrane by using FRET microscopy and the acceptor photobleaching method. Proximity ligation assays with HAS antibodies confirmed the presence of HAS1-HAS2, HAS2-HAS2, and HAS2-HAS3 complexes between endogenously expressed HASs. C-terminal deletions revealed that the enzymes interact mainly via uncharacterized N-terminal 86-amino acid domain(s), but additional binding site(s) probably exist in their C-terminal parts. Of all the homomeric complexes HAS1 had the lowest and HAS3 the highest synthetic activity. Interestingly, HAS1 transfection reduced the synthesis of hyaluronan obtained by HAS2 and HAS3, suggesting functional cooperation between the isoenzymes. These data indicate a general tendency of HAS isoenzymes to form both homomeric and heteromeric complexes with potentially important functional consequences on hyaluronan synthesis.

Hyaluronan synthase 1: a mysterious enzyme with unexpected functions

Hyaluronan synthase 1 (HAS1) is one of three isoenzymes responsible for cellular hyaluronan synthesis. Interest in HAS1 has been limited because its role in hyaluronan production seems to be insignificant compared to the two other isoenzymes, HAS2 and HAS3, which have higher enzymatic activity. Furthermore, in most cell types studied so far, the expression of its gene is low and the enzyme requires high concentrations of sugar precursors for hyaluronan synthesis, even when overexpressed in cell cultures. Both expression and activity of HAS1 are induced by pro-inflammatory factors like interleukins and cytokines, suggesting its involvement in inflammatory conditions. Has1 is upregulated in states associated with inflammation, like atherosclerosis, osteoarthritis, and infectious lung disease. In addition, both full length and splice variants of HAS1 are expressed in malignancies like bladder and prostate cancers, multiple myeloma, and malignant mesothelioma. Interestingly, immunostainings of tissue sections have demonstrated the role of HAS1 as a poor predictor in breast cancer, and is correlated with high relapse rate and short overall survival. Utilization of fluorescently tagged proteins has revealed the intracellular distribution pattern of HAS1, distinct from other isoenzymes. In all cell types studied so far, a high proportion of HAS1 is accumulated intracellularly, with a faint signal detected on the plasma membrane and its protrusions. Furthermore, the pericellular hyaluronan coat produced by HAS1 is usually thin without induction by inflammatory agents or glycemic stress and depends on CD44–HA interactions. These specific interactions regulate the organization of hyaluronan into a leukocyte recruiting matrix during inflammatory responses. Despite the apparently minor enzymatic activity of HAS1 under normal conditions, it may be an important factor under conditions associated with glycemic stress like metabolic syndrome, inflammation, and cancer.

Tissue distribution and subcellular localization of hyaluronan synthase isoenzymes

Hyaluronan synthases (HAS) are unique plasma membrane glycosyltransferases secreting this glycosaminoglycan directly to the extracellular space. The three HAS isoenzymes (HAS1, HAS2, and HAS3) expressed in mammalian cells differ in their enzymatic properties and regulation by external stimuli, but clearly distinct functions have not been established. To overview the expression of different HAS isoenzymes during embryonic development and their subcellular localization, we immunostained mouse embryonic samples and cultured cells with HAS antibodies, correlating their distribution to hyaluronan staining. Their subcellular localization was further studied by GFP-HAS fusion proteins. Intense hyaluronan staining was observed throughout the development in the tissues of mesodermal origin, like heart and cartilages, but also for example during the maturation of kidneys and stratified epithelia. In general, staining for one or several HASs correlated with hyaluronan staining. The staining of HAS2 was most widespread, both spatially and temporally, correlating with hyaluronan staining especially in early mesenchymal tissues and heart. While epithelial cells were mostly negative for HASs, stratified epithelia became HAS positive during differentiation. All HAS isoenzymes showed cytoplasmic immunoreactivity, both in tissue sections and cultured cells, while plasma membrane staining was also detected, often in cellular extensions. HAS1 had brightest signal in Golgi, HAS3 in Golgi and microvillous protrusions, whereas most of the endogenous HAS2 immunoreactivity was localized in the ER. This differential pattern was also observed with transfected GFP-HASs. The large proportion of intracellular HASs suggests that HAS forms a reserve that is transported to the plasma membrane for rapid activation of hyaluronan synthesis.

The activity of hyaluronan synthase 2 is regulated by dimerization and ubiquitination

Hyaluronan is a component of the extracellular matrix, which affects tissue homeostasis. In this study, we investigated the regulatory mechanisms of one of the hyaluronan-synthesizing enzymes, HAS2. Ectopic expression of Flag- and 6myc-HAS2 in COS-1 cells followed by immunoprecipitation and immunoblotting revealed homodimers; after co-transfection with Flag-HAS3, also heterodimers were seen. Furthermore, the expressed HAS2 was ubiquitinated. We identified one acceptor site for ubiquitin on lysine residue 190. Mutation of this residue led to inactivation of the enzymatic activity of HAS2. Interestingly, K190R-mutated HAS2 formed dimers with wt HAS2 and quenched the activity of wt HAS2, thus demonstrating a functional role of the dimeric configuration.

Modulation of hyaluronan synthase activity in cellular membrane fractions

Hyaluronan (HA), the only non-sulfated glycosaminoglycan, is involved in morphogenesis, wound healing, inflammation, angiogenesis, and cancer. In mammals, HA is synthesized by three homologous HA synthases, HAS1, HAS2, and HAS3, that polymerize the HA chain using UDP-glucuronic acid and UDP-N-acetylglucosamine as precursors. Since the amount of HA is critical in several pathophysiological conditions, we developed a non-radioactive assay for measuring the activity of HA synthases (HASs) in eukaryotic cells and addressed the question of HAS activity during intracellular protein trafficking. We prepared three cellular fractions: plasma membrane, cytosol (containing membrane proteins mainly from the endoplasmic reticulum and Golgi), and nuclei. After incubation with UDP-sugar precursors, newly synthesized HA was quantified by polyacrylamide gel electrophoresis of fluorophore-labeled saccharides and high performance liquid chromatography. This new method measured HAS activity not only in the plasma membrane fraction but also in the cytosolic membranes. This new technique was used to evaluate the effects of 4-methylumbeliferone, phorbol 12-myristate 13-acetate, interleukin 1beta, platelet-derived growth factor BB, and tunicamycin on HAS activities. We found that HAS activity can be modulated by post-translational modification, such as phosphorylation and N-glycosylation. Interestingly, we detected a significant increase in HAS activity in the cytosolic membrane fraction after tunicamycin treatment. Since this compound is known to induce HA cable structures, this result links HAS activity alteration with the capability of the cell to promote HA cable formation.

Hyaluronan accumulation is elevated in cultures of low density lipoprotein receptor-deficient cells and is altered by manipulation of cell cholesterol content

The extracellular matrix molecule hyaluronan (HA) accumulates in human atherosclerotic lesions. Yet the reasons for this accumulation have not been adequately addressed. Because abnormalities in lipid metabolism promote atherosclerosis, we have asked whether disrupted cholesterol homeostasis alters HA accumulation in low density lipoprotein receptor-deficient cell cultures. Cultured aortic smooth muscle cells (ASMC) from Watanabe heritable hyperlipidemic (WHHL) rabbits and skin fibroblasts from homozygous patients with familial hypercholesterolemia accumulated 2-4-fold more HA than corresponding cells from age- and sex-matched normolipidemic rabbits and individuals. This occurred in both cell-associated and secreted HA fractions and was independent of cell density or medium serum concentration. WHHL ASMC cultures synthesized twice the proportion of high molecular mass HA (>2x10(6) Da) as normal rabbit ASMC but showed a lower capacity to degrade exogenous [3H]HA. Most importantly, cholesterol depletion or blocking cholesterol synthesis markedly reduced HA accumulation in WHHL ASMC cultures, whereas cholesterol replenishment or stimulation of cholesterol synthesis restored elevated HA levels. We conclude the following: 1) maintaining normal HA levels in cell cultures requires normal cell cholesterol homeostasis; 2) HA degradation may contribute to but is not the predominant mechanism to increase high molecular mass HA accumulation in low density lipoprotein receptor-deficient WHHL ASMC cultures; and 3) elevated accumulation of HA depends on cellular or membrane cholesterol content and, potentially, intact cholesterol-rich microdomains.

Hyaluronan and human endothelial cell behavior

Hyaluronan (HA) is the only nonsulphated glycosaminoglycan of extracellular matrix. In mammals HA is synthesised by three homologues HA synthases: HAS1, HAS2, and HAS3. The HA is daily catabolized by the hyaluronidase enzymes to either oligosaccharides or larger polymer. Despite its simple structure, HA is involved in a great number of biological functions, such as cell proliferation and migration, morphogenesis, wound healing, inflammation, angiogenesis, and tumor growth. Moreover, an important biological role is related to HA oligosaccharides that stimulate cytokine secretion and endothelial cell proliferation. Nevertheless no data about HA presence in endothelium are reported in literature. Several studies underline HA involvement in endothelial cell proliferation, migration, new vessels formation, and leucocytes recruitment. We review the role of HA in endothelial cell in normal condition and during vascular injury.

Heregulin-mediated ErbB2-ERK signaling activates hyaluronan synthases leading to CD44-dependent ovarian tumor cell growth and migration

Heregulin (HRG)-induced cell responses are mediated by the ErbB family of tyrosine kinase receptors. In this study we have investigated HRG activation of ErbB2, extracellular signal-regulated kinase (ERK) signaling, and their role in regulating hyaluronan synthase (HAS) activity in human ovarian tumor cells (SK-OV-3.ipl cells). Immunological and biochemical analyses indicate that ErbB2, ErbB3, and ErbB4 are all expressed in SK-OV-3.ipl cells and that ErbB4 (but not ErbB3) is physically linked to ErbB2 following HRG stimulation. Furthermore, our data indicate that the HRG-induced ErbB2.ErbB4 complexes stimulate ErbB2 tyrosine kinase, which induces both ERK phosphorylation and kinase activity. The activated ERK then increases the phosphorylation of HAS1, HAS2, and HAS3. Consequently, all three HAS isozymes are activated resulting in hyaluronan (HA) production. Because HRG-mediated HAS isozyme phosphorylation/activation can be effectively blocked by either AG825 (an ErbB2 inhibitor) or thiazolidinedione compound (an ERK blocker), we conclude that ErbB2-ERK signaling and HAS isozyme phosphorylation/HA production are functionally coupled in SK-OV-3.ipl cells. HRG also promotes HA- and CD44-dependent oncogenic events (e.g. CD44-Cdc42 association, p21-activated kinase 1 activation, and p21-activated kinase 1-filamin complex formation) and tumor cell-specific behaviors in an ErbB2-ERK signaling-dependent manner. Finally, we have found that the down-regulation of HAS isozyme expression (by transfecting cells with HAS1/HAS2/HAS3-specific small interfering RNAs) not only inhibits HRG-mediated HAS phosphorylation/activation and HA production but also impairs CD44-specific Cdc42-PAK1/filamin signaling, cytoskeleton activation and tumor cell behaviors. Taken together, these findings clearly indicate that HRG activation of ErbB2-ERK signaling modulates HAS phosphorylation/activation and HA production leading to CD44-mediated oncogenic events and ovarian cancer progression.

Hyaluronan synthesis induces microvillus-like cell surface protrusions

Hyaluronan synthases (HASs) are plasma membrane enzymes that simultaneously elongate, bind, and extrude the growing hyaluronan chain directly into extracellular space. In cells transfected with green fluorescent protein (GFP)-tagged Has3, the dorsal surface was decorated by up to 150 slender, 3-20-microm-long microvillus-type plasma membrane protrusions, which also contained filamentous actin, the hyaluronan receptor CD44, and lipid raft microdomains. Enzymatic activity of HAS was required for the growth of the microvilli, which were not present in cells transfected with other GFP proteins or inactive GFP-Has3 mutants or in cells incubated with exogenous soluble hyaluronan. The microvilli induced by HAS3 were gradually withered by introduction of an inhibitor of hyaluronan synthesis and rapidly retracted by hyaluronidase digestion, whereas they were not affected by competition with hyaluronan oligosaccharides and disruption of the CD44 gene, suggesting independence of hyaluronan receptors. The data bring out the novel concept that the glycocalyx created by dense arrays of hyaluronan chains, tethered to HAS during biosynthesis, can induce and maintain prominent microvilli.

Plasma membrane residence of hyaluronan synthase is coupled to its enzymatic activity

Hyaluronan is a multifunctional glycosaminoglycan up to 10(7) Da molecular mass produced by the integral membrane glycosyltransferase, hyaluronan synthase (HAS). When expressed in keratinocytes, N-terminally tagged green fluorescent protein-HAS2 and -HAS3 isoenzymes were found to travel through endoplasmic reticulum (ER), Golgi, plasma membrane, and endocytic vesicles. A distinct enrichment of plasma membrane HAS was found in cell protrusions. The total turnover time of HAS3 was 4-5 h as judged by the green fluorescent protein signal decay and hyaluronan synthesis inhibition in cycloheximide-treated cells. The transfer from ER to Golgi took about 1 h, and the dwell time on the plasma membrane was less than 2 h in experiments with a relief and introduction, respectively, of brefeldin A. Constructs of HAS3 with 16- and 45-amino-acid C-terminal deletions mostly stayed within the ER, whereas a D216A missense mutant was localized within the Golgi complex but not the plasma membrane. Both types of mutations were almost or completely inactive, similar to the wild type enzyme that had its entry to the plasma membrane experimentally blocked by brefeldin A. Inhibition of hyaluronan synthesis by UDP-glucuronic acid starvation using 4-methyl-umbelliferone also prevented HAS access to the plasma membrane. The results demonstrate that 1) a latent pool of HAS exists within the ER-Golgi pathway; 2) this pool can be rapidly mobilized and activated by insertion into the plasma membrane; and 3) inhibition of HAS activity through mutation or substrate starvation results in exclusion of HAS from the plasma membrane.