Invertebrate aspartyl/asparaginyl beta-hydroxylase: potential modification of endogenous epidermal growth factor-like modules

Active site characterization and activity of the human aspartyl (asparaginyl) β-hydroxylase

Human aspartyl/asparaginyl beta-hydroxylase (HAAH) is a member of the superfamily of nonheme Fe2+/α-ketoglutarate (αKG) dependent oxygenase enzymes with a noncanonical active site. HAAH hydroxylates epidermal growth factor (EGF) like domains to form the β-hydroxylated product from substrate asparagine or aspartic acid and has been suggested to have a negative impact in a variety of cancers. In addition to iron, HAAH also binds divalent calcium, although the role of the latter is not understood. Herein, the metal binding chemistry and influence on enzyme stability and activity have been evaluated by a combined biochemical and biophysical approach. Metal binding parameters for the HAAH active site were determined by use of isothermal titration calorimetry, demonstrating a high-affinity regulatory binding site for Ca2+ in the catalytic domain in addition to the catalytic Fe2+ cofactor. We have analyzed various active site derivatives, utilizing LC-MS and a new HPLC technique to determine the role of metal binding and the second coordination sphere in enzyme activity, discovering a previously unreported residue as vital for HAAH turnover. This analysis of the in vitro biochemical function of HAAH furthers the understanding of its importance to cellular biochemistry and metabolic pathways.

Hydroxylase Activity of ASPH Promotes Hepatocellular Carcinoma Metastasis Through Epithelial-to-Mesenchymal Transition Pathway

Over-expression of aspartyl (asparagynal)-β-hydroxylase (ASPH) contributes to hepatocellular carcinoma (HCC) invasiveness, but the role of ASPH hydroxylase activity in this process remains to be defined. As such, the current study investigated the role of ASPH hydroxylase activity in downstream signalling of HCC tumorgenesis and, specifically, metastasis development. Over-expression of wild-type ASPH, but not a hydroxylase mutant, promoted HCC cell migration in vitro, as well as intrahepatic and distant metastases in vivo. The enhanced migration and epithelial to mesenchymal transition (EMT) activation was notably absent in response to hydroxylase activity blockade. Vimentin, a regulator of EMT, interacted with ASPH and likely mediated the effect of ASPH hydroxylase activity with cell migration. The enhanced hydroxylase activity in tumor tissues predicted worse prognoses of HCC patients. Collectively, the hydroxylase activity of ASPH affected HCC metastasis through interacting with vimentin and regulating EMT. As such, ASPH might be a promising therapeutic target of HCC.

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Over-expression of ASPH promoted HCC intrahepatic and distant metastases in vivo.

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ASPH interacts with vimentin to promote HCC cell migration.

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Enhanced hydroxylase activity in tumor predicted worse prognoses of HCC patients.

Hepatocellular carcinoma has aggressive invasiveness and high metastatic rate. The reason for metastasis is largely unknown and the effective treatment is still lacking. Although over-expression of ASPH has been demonstrated to enhance hepatocellular carcinoma invasiveness, whether its hydroxylase activity is necessary remains uncharacterized. Here, we found the hydroxylase activity was critical to promote hepatocellular carcinoma invasiveness in vitro and metastasis in vivo, and associated with post-surgery survival. ASPH hydroxylase activity play an important role in epithelial-to-mesenchymal transition through interacting with vimentin. Our findings imply that ASPH antagonists might be promising in developing novel therapy.

Structural Divergence in O-GlcNAc Glycans Displayed on Epidermal Growth Factor-like Repeats of Mammalian Notch1

Extracellular O-GlcNAc is a novel class of modification catalyzed by epidermal growth factor-like (EGF)-domain specific O-GlcNAc transferase (EOGT). In mammals, EOGT is required for ligand-mediated Notch signaling for vascular development. Previous studies have revealed that O-GlcNAc in mammalian cultured cells is subject to subsequent glycosylation, which may impose additional layers of regulation. This study aimed to analyze the O-GlcNAc glycans of Drosophila EGF20 as model substrates and mouse Notch1 EGF repeats by mass-spectrometry. The analysis of Drosophila EGF20 expressed in HEK293T cells revealed that the majority of the proteins are modified with an elongated form of O-GlcNAc glycan comprising terminal galactose or sialic acid residues. In contrast, recombinant Notch1 EGF repeats isolated from HEK293T cells revealed structural divergence of O-GlcNAc glycans among the different EGF domains. Although the majority of Notch1 EGF2 and EGF20 domains contained the extended forms of the glycan, the O-GlcNAc in many other domains mostly existed as a monosaccharide irrespective of the exogenous EOGT expression. Our results raised a hypothesis that an array of O-GlcNAc monosaccharides may impact the structure and function of Notch receptors.

Aspartate β-Hydroxylase expression promotes a malignant pancreatic cellular phenotype

Pancreatic cancer (PC) is one of the leading causes of cancer related deaths due to aggressive progression and metastatic spread. Aspartate β-hydroxylase (ASPH), a cell surface protein that catalyzes the hydroxylation of epidermal growth factor (EGF)-like repeats in Notch receptors and ligands, is highly overexpressed in PC. ASPH upregulation confers a malignant phenotype characterized by enhanced cell proliferation, migration, invasion and colony formation in vitro as well as PC tumor growth in vivo. The transforming properties of ASPH depend on enzymatic activity. ASPH links PC growth factor signaling cascades to Notch activation. A small molecule inhibitor of β-hydroxylase activity was developed and found to reduce PC growth by downregulating the Notch signaling pathway. These findings demonstrate the critical involvement of ASPH in PC growth and progression, provide new insight into the molecular mechanisms leading to tumor development and growth and have important therapeutic implications.

Glial vascular degeneration in CADASIL

CADASIL is a genetic vascular dementia caused by mutations in the Notch 3 gene on Chromosome 19. However, little is known about the mechanisms of vascular degeneration. We characterized upstream components of Notch signaling pathways that may be disrupted in CADASIL, by measuring expression of insulin, IGF-1, and IGF-2 receptors, Notch 1, Notch 3, and aspartyl-(asparaginyl)-β-hydroxylase (AAH) in cortex and white matter from 3 CADASIL and 6 control brains. We assessed CADASIL-associated cell loss by measuring mRNA corresponding to neurons, oligodendroglia, and astrocytes, and indices of vascular degeneration by measuring smooth muscle actin (SMA) and endothelin-1 expression in isolated vessels. Immunohistochemical staining was used to assess SMA degeneration. Significant abnormalities, including reduced cerebral white matter mRNA levels of Notch 1, Notch 3, AAH, SMA, IGF receptors, myelin-associated glycoproteins, and glial fibrillary acidic protein, and reduced vascular expression of SMA, IGF receptors, Notch 1, and Notch 3 were detected in CADASIL-lesioned brains. In addition, we found CADASIL-associated reductions in SMA, and increases in ubiquitin immunoreactivity in the media of white matter and meningeal vessels. No abnormalities in gene expression or immunoreactivity were observed in CADASIL cerebral cortex. In conclusion, molecular abnormalities in CADASIL are largely restricted to white matter and white matter vessels, corresponding to the distribution of neuropathological lesions. These preliminary findings suggest that CADASIL is mediated by both glial and vascular degeneration with reduced expression of IGF receptors and AAH, which regulate Notch expression and function.

siRNA inhibition of aspartyl-asparaginyl β-hydroxylase expression impairs cell motility, Notch signaling, and fetal growth

Aspartyl-asparaginyl-β-hydroxylase (AAH) regulates cell motility and invasiveness by enhancing Notch signaling. Invasive trophoblastic cells, which mediate placentation, normally express high levels of AAH. Previously, we showed that ethanol-impaired placentation is associated with reduced AAH expression. The present study determines the degree to which inhibition of AAH expression is sufficient to impair functions required for placentation. Immortalized, first trimester-derived, human trophoblastic cells (HTR-8/SVneo) were transfected with siRNA targeting AAH (siRNA-AAH) or no specific sequences (siRNA-Scr) using the Amaxa electroporation system. Directional motility was measured using an ATP luminescence-based assay. For in vivo studies, we microinjected siRNA-AAH or siRNA-Scr directly into the implantation sites (mesometrial triangle) of gestation-day-17, Long Evans pregnant rats, and harvested placentas 24 h later for histologic and molecular studies. siRNA-AAH transfection reduced AAH expression and directional motility in HTR-8/SVneo cells. In vivo delivery of siRNA-AAH reduced AAH expression and mean number of invasive trophoblastic cells at the implantation site. These adverse effects of siRNA-AAH were associated with impaired fetal growth and significantly reduced expression of Notch-signaling network genes. AAH is an important, positive regulator of trophoblastic cell motility, and inhibition of AAH in vivo leads to impaired implantation and fetal growth, and alters Notch-signaling mechanisms, similar to the effects of chronic ethanol exposure.

Role of aspartyl-(asparaginyl)-β-hydroxylase mediated notch signaling in cerebellar development and function

BACKGROUND

Aspartyl-(Asparaginyl)-β-Hydroxylase (AAH) is a hydroxylating enzyme that promotes cell motility by enhancing Notch-Jagged-HES-1 signaling. Ethanol impaired cerebellar neuron migration during development is associated with reduced expression of AAH.

METHODS

To further characterize the role of AAH in relation to cerebellar development, structure, and function, we utilized an in vivo model of early postnatal (P2) intracerebro-ventricular gene delivery to silence AAH with small interfering RNA (siAAH), or over-express it with recombinant plasmid DNA (pAAH). On P20, we assessed cerebellar motor function by rotarod testing. Cerebella harvested on P21 were used to measure AAH, genes/proteins that mediate AAH's downstream signaling, i.e. Notch-1, Jagged-1, and HES-1, and immunoreactivity corresponding to neuronal and glial elements.

RESULTS

The findings demonstrated that: 1) siAAH transfection impaired motor performance and blunted cerebellar foliation, and decreased expression of neuronal and glial specific genes; 2) pAAH transfection enhanced motor performance and increased expression of neuronal and glial cytoskeletal proteins; and 3) alterations in AAH expression produced similar shifts in Notch-1, Jagged-1, and HES-1 protein or gene expression.

CONCLUSIONS

The results support our hypothesis that AAH is an important mediator of cerebellar development and function, and link AAH expression to Notch signaling pathways in the developing brain.

Aspartyl-(asparaginyl) beta-hydroxylase, hypoxia-inducible factor-alpha and Notch cross-talk in regulating neuronal motility

Aspartyl-(Asparaginyl)-β-Hydroxylase (AAH ) promotes cell motility by hydroxylating Notch. Insulin and insulin-like growth factor, type 1 (IGF-I) stimulate AAH through Erk MAP K and phosphoinositol-3-kinase-Akt (PI3K-Akt). However, hypoxia/oxidative stress may also regulate AAH . Hypoxia-inducible factor-1alpha (HIF-1α) regulates cell migration, signals through Notch, and is regulated by hypoxia/oxidative stress, insulin/IGF signaling and factor inhibiting HIF-1α (FIH) hydroxylation. To examine cross-talk between HIF-1α and AAH , we measured AAH , Notch-1, Jagged-1, FIH, HIF-1α, HIF-1β and the hairy and enhancer of split 1 (HE S-1) transcription factor expression and directional motility in primitive neuroectodermal tumor 2 (PNET2) human neuronal cells that were exposed to H₂O₂ or transfected with short interfering RNA duplexes (siRNA) targeting AAH , Notch-1 or HIF-1α. We found that: (1) AAH , HIF-1α and neuronal migration were stimulated by H2O2; (2) si-HIF-1α reduced AAH expression and cell motility; (3) si-AAH inhibited Notch and cell migration, but not HIF-1α and (4) si-Notch-1 increased FIH and inhibited HIF-1α. These findings suggest that AAH and HIF-1α crosstalk within a hydroxylation-regulated signaling pathway that may be transiently driven by oxidative stress and chronically regulated by insulin/IGF signaling.

Intermodule cooperativity in the structure and dynamics of consecutive complement control modules in human C1r: structural biology

The modular C1r protein is the first protease activated in the classical complement pathway, a key component of innate immunity. Activation of the heteropentameric C1 complex, possibly accompanied by major intersubunit re-arrangements besides proteolytic cleavage, requires targeted regulation of flexibility within the context of the intramolecular and intermolecular interaction networks of the complex. In this study, we prepared the two complement control protein (CCP) modules, CCP1 and CCP2, of C1r in their free form, as well as their tandem-linked construct, CCP1CCP2, to characterize their solution structure, conformational dynamics and cooperativity. The structures derived from NMR signal dispersion and secondary chemical shifts were in good agreement with those obtained by X-ray crystallography. However, successful heterologus expression of both the single CCP1 module and the CCP1CCP2 constructs required the attachment of the preceding N-terminal module, CUB2, which could then be removed to obtain the properly folded proteins. Internal mobility of the modules, especially that of CCP1, exhibited considerable changes accompanied by interfacial chemical shift alterations upon the attachment of the C-terminal CCP2 domain. Our NMR data suggest that in terms of folding, stability and dynamics, CCP1 is heavily dependent on the presence of its neighboring modules in intact C1r. Therefore, CCP1 could be a focal interaction point, capable of transmitting information towards its neighboring modules.

Calcium-dependent conformational flexibility of a CUB domain controls activation of the complement serine protease C1r

C1, the first component of the complement system, is a Ca(2+)-dependent heteropentamer complex of C1q and two modular serine proteases, C1r and C1s. Current functional models assume significant flexibility of the subcomponents. Noncatalytic modules in C1r have been proposed to provide the flexibility required for function. Using a recombinant CUB2-CCP1 domain pair and the individual CCP1 module, we showed that binding of Ca(2+) induces the folding of the CUB2 domain and stabilizes its structure. In the presence of Ca(2+), CUB2 shows a compact, folded structure, whereas in the absence of Ca(2+), it has a flexible, disordered conformation. CCP1 module is Ca(2+)-insensitive. Isothermal titration calorimetry revealed that CUB2 binds a single Ca(2+) with a relatively high K(D) (430 mum). In blood, the CUB2 domain of C1r is only partially (74%) saturated by Ca(2+), therefore the disordered, Ca(2+)-free form could provide the flexibility required for C1 activation. In accordance with this assumption, the effect of Ca(2+) on the autoactivation of native, isolated C1r zymogen was proved. In the case of infection-inflammation when the local Ca(2+) concentration decreases, this property of CUB2 domain could serve as subtle means to trigger the activation of the classical pathway of complement. The CUB2 domain of C1r is a novel example for globular protein domains with marginal stability, high conformational flexibility, and proteolytic sensitivity. The physical nature of the behavior of this domain is similar to that of intrinsically unstructured proteins, providing a further example of functionally relevant ligand-induced reorganization of a polypeptide chain.

Collagen and calcium-binding EGF domains 1 is frequently inactivated in ovarian cancer by aberrant promoter hypermethylation and modulates cell migration and survival

Background:

Collagen and calcium-binding EGF domains 1 (CCBE1) is an uncharacterised gene that has down-regulated expression in breast cancer. As CCBE1 maps to 18q21.32, a region frequently exhibiting loss of heterozygosity in ovarian cancer, the aim of this study was to determine the expression and function of CCBE1 in ovarian cancer.

Methods:

Expression and methylation patterns of CCBE1 were determined in ovarian cancer cell lines and primary tumours. CCBE1 contains collagen repeats and an aspartic acid/asparagine hydroxylation/EGF-like domain, suggesting a function in extracellular matrix remodelling and migration, which was determined using small-interfering RNA (siRNA)-mediated knockdown and over-expression of CCBE1 in cell lines.

Results:

CCBE1 is expressed in normal ovary, but is reduced in ovarian cancer cell lines and primary carcinomas. Pharmacological demethylation/deacetylation in ovarian cancer cell lines re-induced CCBE1 expression, indicating that epigenetic mechanisms contribute to its silencing in cancer. CCBE1 promoter hypermethylation was detected in 6/11 (55%) ovarian cancer cell lines and 38/81 (41%) ovarian carcinomas. siRNA-mediated knockdown of CCBE1 in ovarian cancer cell lines enhanced their migration; conversely, re-expression of CCBE1 reduced migration and survival. Hence, loss of CCBE1 expression may promote ovarian carcinogenesis by enhancing migration and cell survival.

Conclusions:

These data suggest that CCBE1 is a new candidate tumour suppressor in ovarian cancer.

Minor sarcoplasmic reticulum membrane components that modulate excitation-contraction coupling in striated muscles

In striated muscle, activation of contraction is initiated by membrane depolarisation caused by an action potential, which triggers the release of Ca(2+) stored in the sarcoplasmic reticulum by a process called excitation-contraction coupling. Excitation-contraction coupling occurs via a highly sophisticated supramolecular signalling complex at the junction between the sarcoplasmic reticulum and the transverse tubules. It is generally accepted that the core components of the excitation-contraction coupling machinery are the dihydropyridine receptors, ryanodine receptors and calsequestrin, which serve as voltage sensor, Ca(2+) release channel, and Ca(2+) storage protein, respectively. Nevertheless, a number of additional proteins have been shown to be essential both for the structural formation of the machinery involved in excitation-contraction coupling and for its fine tuning. In this review we discuss the functional role of minor sarcoplasmic reticulum protein components. The definition of their roles in excitation-contraction coupling is important in order to understand how mutations in genes involved in Ca(2+) signalling cause neuromuscular disorders.

Aspartyl-asparagyl beta hydroxylase over-expression in human hepatoma is linked to activation of insulin-like growth factor and notch signaling mechanisms

Aspartyl-(asparagyl)-beta-hydroxylase (AAH) is overexpressed in various malignant neoplasms, including hepatocellular carcinomas (HCCs). The upstream regulation of AAH and its functional role in Notch-mediated signaling and motility in HCC cells was accessed. The mRNA transcript levels of AAH, insulin receptor substrate (IRS), insulin and insulin-like growth factor (IGF) receptors and polypeptides, Notch, Jagged, and HES were measured in 15 paired samples of HCC and adjacent HCC-free human liver biopsy specimens using real-time quantitative RT-PCR and Western blot analysis. Overexpression of AAH was detected in 87% of the HCC relative to the paired HCC-free liver tissue. IRS-1, IRS-2, and IRS-4 were each overexpressed in 80% of the HCC samples, and IGF-I and IGF-2 receptors were overexpressed in 40% and 100% of the HCCs, respectively. All HCC samples had relatively increased levels of Notch-1 and HES-1 gene expression. Overexpression of AAH led to increased levels of Notch, and co-immunoprecipitation experiments demonstrated a direct interaction between AAH and Notch as well as its ligand Jagged. In conclusion, contributions to the malignant phenotype of HCC is due to activation of IGF-I and IGF-II signaling that results in over-expression of both AAH and Notch. The functional role of AAH in relation to cell motility has been linked to increased activation of the Notch signaling pathway.

Aspartyl-(asparaginyl)-beta-hydroxylase regulates hepatocellular carcinoma invasiveness

BACKGROUND/AIMS

We measured aspartyl (asparaginyl)-beta-hydroxylase (AAH) gene expression in human hepatocelluar carcinoma and surrounding uninvolved liver at both the mRNA and protein level and examined the regulation and function of this enzyme.

METHODS

Since growth of HCC is mediated by signaling through the insulin-receptor substrate, type 1 (IRS-1), we examined-if AAH is a downstream gene regulated by insulin and IGF-1 in HCC cells. In addition, IRS-1 regulation of AAH was examined in a transgenic (Tg) mouse model in which the human (h) IRS-1 gene was over-expressed in the liver, and an in vitro model in which a C-terminus truncated dominant-negative hIRS-1 cDNA (hIRS-DeltaC) was over-expressed in FOCUS HCC cells. The direct effects of AAH on motility and invasiveness were examined in AAH-transfected HepG2 cells.

RESULTS

Insulin and IGF-1 stimulation increased AAH mRNA and protein expression and motility in FOCUS and Hep-G2 cells. These effects were mediated by signaling through the Erk MAPK and PI3 kinase-Akt pathways. Over-expression of hIRS-1 resulted in high levels of AAH in Tg mouse livers, while over-expression of hIRS-DeltaC reduced AAH expression, motility, and invasiveness in FOCUS cells. Finally, over-expression of AAH significantly increased motility and invasiveness in HepG2 cells, whereas siRNA inhibition of AAH expression significantly reduced directional motility in FOCUS cells.

CONCLUSIONS

The results suggest that enhanced AAH gene activity is a common feature of human HCC and growth factor signaling through IRS-1 regulates AAH expression and increases motility and invasion of HCC cells. Therefore, AAH may represent an important target for regulating tumor growth in vivo.

Antisense oligodeoxynucleotides directed against aspartyl (asparaginyl) beta-hydroxylase suppress migration of cholangiocarcinoma cells

BACKGROUND

Aspartyl (asparaginyl) beta-hydroxylase (AAH) is an alpha-ketoglutarate-dependent dioxygenase that hydroxylates aspartate and asparagine residues in EGF-like domains of proteins. The consensus sequence for AAH beta-hydroxylation occurs in signaling molecules such as Notch and Notch homologs, which have roles in cell migration.

AIM

This study evaluated the potential role of AAH in cell migration using cholangiocarcinoma cell lines as models due to their tendency to widely infiltrate the liver.

METHODS

Five human cholangiocarcinoma cell lines established from human tumors were examined for AAH expression and motility. The effect of antisense oligodeoxynucleotide inhibition of AAH on cholangiocarcinoma cell migration was investigated.

RESULTS

Western blot analysis detected the approximately 86 kDa AAH protein in all five cholangiocarcinoma cell lines, and higher levels of AAH in cell lines derived from moderately or poorly differentiated compared with well-differentiated tumors. Immunocytochemical staining and fluorescence activated cell sorting analysis revealed both surface and intracellular AAH immunoreactivity. Using the phagokinetic non-directional migration assay and a novel ATPLite luminescence-based directional migration assay, we correlated AAH expression with motility. Correspondingly, antisense and not sense or mutated antisense AAH oligodeoxynucleotides significantly inhibited AAH expression and motility in cholangiocarcinoma cells.

CONCLUSIONS

AAH over-expression may contribute to the infiltrative growth pattern of cholangiocarcinoma cells by promoting motility.

Antisense oligonucleotides selectively regulate aspartyl beta-hydroxylase and its truncated protein isoform in vitro but distribute poorly into A549 tumors in vivo

Alternative splicing of the human beta-aspartyl (asparaginyl) hydroxylase (BAH) gene results in the expression of humbug, a truncated form of BAH that lacks the catalytic domain of the enzyme. Overexpression of BAH and humbug has been associated with a variety of human cancers, and although humbug lacks enzymatic activity, it is expressed at levels comparable with that of BAH in various cancer cell lines. Phosphorothioate antisense oligonucleotides (ONs) were designed to dissect out the function of these hydroxylase protein isoforms. In A549 cells, these ONs differentially down-regulated BAH and humbug at the mRNA and protein level. Phosphorothioate ON uptake and antisense studies were conducted in parallel in nude mice bearing A549 tumor xenografts. Microscopic examination of the tumor after administration of a fluorescein-labeled ON showed strong labeling of the outer layers of the tumor connective tissue but cells within the interior of the tumor were sparsely labeled. A modest but significant effect on tumor growth was observed in animals treated with an antisense ON directed against both BAH and humbug transcripts. However, Northern analysis of tumor RNA did not indicate a down-regulation of the targeted mRNA species. These results demonstrate the successful development of antisense ONs that selectively differentiate between the closely related beta-hydroxylase protein isoforms. However, determination of the biological function of these proteins in vivo was limited by the poor uptake properties of phosphorothioate ONs in A549 tumors.

Absence of post-translational aspartyl beta-hydroxylation of epidermal growth factor domains in mice leads to developmental defects and an increased incidence of intestinal neoplasia

The BAH genomic locus encodes three distinct proteins: junctin, humbug, and BAH. All three proteins share common exons, but differ significantly based upon the use of alternative terminal exons. The biological roles of BAH and humbug and their functional relationship to junctin remain unclear. To evaluate the role of BAH in vivo, the catalytic domain of BAH was specifically targeted such that the coding regions of junctin and humbug remained undisturbed. BAH null mice lack measurable BAH protein in several tissues, lack aspartyl beta-hydroxylase activity in liver preparations, and exhibit no hydroxylation of the epidermal growth factor (EGF) domain of clotting Factor X. In addition to reduced fertility in females, BAH null mice display several developmental defects including syndactyly, facial dysmorphology, and a mild defect in hard palate formation. The developmental defects present in BAH null mice are similar to defects observed in knock-outs and hypomorphs of the Notch ligand Serrate-2. In this work, beta-hydroxylation of Asp residues in EGF domains is demonstrated for a soluble form of a Notch ligand, human Jagged-1. These results along with recent reports that another post-translational modification of EGF domains in Notch gene family members (glycosylation by Fringe) alters Notch pathway signaling, lends credence to the suggestion that aspartyl beta-hydroxylation may represent another post-translational modification of EGF domains that can modulate Notch pathway signaling. Previous work has demonstrated increased levels of BAH in certain tumor tissues and a role for BAH in tumorigenesis has been proposed. The role of hydroxylase in tumor formation was tested directly by crossing BAH KO mice with an intestinal tumor model, APCmin mice. Surprisingly, BAH null/APCmin mice show a statistically significant increase in both intestinal polyp size and number when compared with BAH wild-type/APCmin controls. These results suggest that, in contrast to expectations, loss of BAH catalytic activity may promote tumor formation.

The role of the individual domains in the structure and function of the catalytic region of a modular serine protease, C1r

The first enzymatic event in the classical pathway of complement activation is autoactivation of the C1r subcomponent of the C1 complex. Activated C1r then cleaves and activates zymogen C1s. C1r is a multidomain serine protease consisting of N-terminal alpha region interacting with other subcomponents and C-terminal gammaB region mediating proteolytic activity. The gammaB region consists of two complement control protein modules (CCP1, CCP2) and a serine protease domain (SP). To clarify the role of the individual domains in the structural and functional properties of the gammaB region we produced the CCP1-CCP2-SP (gammaB), the CCP2-SP, and the SP fragments in recombinant form in Escherichia coli. We successfully renatured the inclusion body proteins. After renaturation all three fragments were obtained in activated form and showed esterolytic activity on synthetic substrates similar to each other. To study the self-activation process in detail zymogen mutant forms of the three fragments were constructed and expressed. Our major statement is that the ability of autoactivation and C1s cleavage is an inherent property of the SP domain. We observed that the CCP2 module significantly increases proteolytic activity of the SP domain on natural substrate, C1s. Therefore, we propose that CCP2 module provides accessory binding sites. Differential scanning calorimetric measurements demonstrated that CCP2 domain greatly stabilizes the structure of SP domain. Deletion of CCP1 domain from the CCP1-CCP2-SP fragment results in the loss of the dimeric structure. Our experiments also provided evidence that dimerization of C1r is not a prerequisite for autoactivation.

Aspartyl beta -hydroxylase (Asph) and an evolutionarily conserved isoform of Asph missing the catalytic domain share exons with junctin

The mouse aspartyl beta-hydroxylase gene (Asph, BAH) has been cloned and characterized. The mouse BAH gene spans 200 kilobase pairs of genomic DNA and contains 24 exons. Of three major BAH-related transcripts, the two largest (6,629 and 4,419 base pairs) encode full-length protein and differ only in the use of alternative polyadenylation signals. The smallest BAH-related transcript (2,789 base pairs) uses an alternative 3' terminal exon, resulting in a protein lacking a catalytic domain. Evolutionary conservation of this noncatalytic isoform of BAH (humbug) is demonstrated in mouse, man, and Drosophila. Monoclonal antibody reagents were generated, epitope-mapped, and used to definitively correlate RNA bands on Northern blots with protein species on Western blots. The gene for mouse junctin, a calsequestrin-binding protein, was cloned and characterized and shown to be encoded from the same locus. When expressed in heart tissue, BAH/humbug preferably use the first exon and often the fourth exon of junctin while preserving the reading frame. Thus, three individual genes share common exons and open reading frames and use separate promoters to achieve differential expression, splicing, and function in a variety of tissues. This unusual form of exon sharing suggests that the functions of junctin, BAH, and humbug may be linked.

One Active C1r Subunit Is Sufficient for the Activity of the Complement C1 Complex: Stabilization of C1r in the Zymogen Form by Point Mutations

The binding of C1 (the first component of complement) to immune complexes leads to the autoactivation of C1r through the cleavage of the Arg463-Ile464 bond in the catalytic domain. Spontaneous activation of C1r (and C1) also occurs in the fluid phase, preventing the characterization of the zymogen form of C1r. To overcome this difficulty, the zymogen form of human C1r was stabilized by mutating the Arg in the Arg463-Ile464 bond to Gln. This mutant was designated as mutant QI. Recombinant C1r (wild type (wt) or mutant) was expressed in insect cells using serum-free medium in functionally pure form; therefore, the cell culture supernatant was suitable to reconstruct C1 for the hemolytic assay. Mutant QI was a stable, nonactivable zymogen and showed no hemolytic activity in reconstituted C1. However, this stable zymogen C1r mutant could form an active mixed dimer with the wt C1r, indicating that one active C1r subunit in the C1 complex is sufficient for the full activity of the entire complex. Our experiments also showed that the exchange of C1r monomers between the C1r dimers is completed in less than 16 h even at pH 7 and 4°C. Two other mutants were also constructed by changing Arg463 to Lys, or Ile464 to Phe, and were designated as mutants KI and RF, respectively. Although these substitutions did increase the stability of the proenzyme in the cell culture supernatant, the mutant proteins retained their ability to autoactivate, and both had a wt-like hemolytic activity.

The five cysteine residues located in the active site region of bovine aspartyl (asparaginyl) beta-hydroxylase are not essential for catalysis

In previous chemical modification studies on bovine aspartyl (asparaginyl) beta-hydroxylase, cysteines were implicated as critical catalytic residues. Using site-directed mutagenesis, the five cysteine residues located in a highly conserved region of the enzyme identified as the active site were individually mutated to alanine. Substitutions at cysteine 637, 644, 656, 681, and 696 resulted in active mutant enzymes indicating that these residues are not required for catalysis.

Structure and function of the serine-protease subcomponents of C1: protein engineering studies

Our protein engineering studies on human C1r and C1s revealed important characteristics of the individual domains of these multidomain serine-proteases, and supplied evidence about the cooperation of the domains to create binding sites, and to control the activation process. We expressed the recombinant subcomponents in the baculovirus-insect cell system and checked the biological activity. Deletions and point mutants of C1r were constructed and C1r-C1s chimeras were also produced. Our deletion mutants demonstrated that the N-terminal CUB domain and the EGF-like domain of C1r together are responsible for the calcium dependent C1r-C1s interaction. It seems very likely that these two modules form the calcium-binding site of the C1r alpha-fragment and participate in the tetramer formation. The deletion mutants also demonstrated that the N-terminal region of the C1r molecule contains essential elements involved in the control of activation of the serine-protease module. The substrate specificity of the serine-protease is also determined by the five N-terminal noncatalytic domain of C1r/C1s chimera, which contains the catalytic domain of C1s preceded by the N-terminal region of C1r, could replace the C1r in the hemolytically active C1 complex. The C1s/C1r chimera, in which the alpha-fragment of the C1r was replaced for that of the C1s exibits both C1r- and C1s-like characteristics. We stabilized the zymogen form of human C1r by mutating the Arg(463)-Ile(464) bond. Using our stable zymogen C1r we showed that one active C1r in the C1 complex is sufficient for the full activity of the entire complex. Further experiment with this mutant could provide us with important information about the structure of the C1 complex.

Immunological approach to hepatocellular carcinoma

A library of monoclonal antibodies (MoAbs) has been produced against a human hepatocellular carcinoma (HCC) cell line designated FOCUS in order to study the antigenic properties of transformed hepatocytes. Several monoclonal antibodies (MoAbs) were initially selected for study since they bound to antigens which were overexpressed in HCC tissues compared with the adjacent uninvolved normal liver counterpart; in addition, these MoAbs revealed low level antigen expression on other normal human tissues. Subsequently, HCC cell lines were metabolically labelled and the antigens further characterized by immunoprecipitation and Western blot analysis. If the MoAb recognized a primary linear epitope on a protein, cloning was performed using a lambda GT11 cDNA expression library prepared from the FOCUS HCC cell line. These studies characterized the HCC associated antigen(s) at the molecular level. This review illustrates the value of such an experimental approach to search for and identify HCC associated antigens and emphasizes the biological properties of novel proteins may be defined and characterized by these techniques. More important, our investigations have described unique proteins that may not only be important in the pathogenesis of HCC but also demonstrates how such antigen-antibody systems may be used to develop strategies for immunotargetting and gene therapy of HCC.

Overexpression of human aspartyl(asparaginyl)beta-hydroxylase in hepatocellular carcinoma and cholangiocarcinoma

To characterize genes that become upregulated with malignant transformation of human hepatocytes, a library of monoclonal antibodies was produced against the FOCUS hepatocellular carcinoma cell line. Antibody FB-50 reacted with an antigen that was highly expressed in 4 of 10 primary hepatocellular carcinomas, in all 20 cholangiocarcinomas we studied, and in a variety of transformed cell lines. This antigen was also highly expressed in neoplastic epithelial cells of breast and colon carcinomas in contrast to its low level of expression in normal hepatocytes and in non-neoplastic epithelial cells. Among the normal adult tissues studied, high levels were observed only in proliferating trophoblastic cells of the placenta and in adrenal glands. A 636-bp partial cDNA, isolated from a gamma GT11 expression library generated with HepG2 human hepatoblastoma cells, and a complete cDNA, generated by reverse transcriptase-PCR, identified the antigen as the human form of aspartyl(asparaginyl)beta-hydroxylase. This enzyme catalyzes posttranslational hydroxylation of beta carbons of specific aspartyl and asparaginyl residues in EGF-like domains of certain proteins. Analyses of extracts prepared from several human tumor cell lines compared to their normal tissue counterparts indicate that the increase in hydroxylase, approximately 10-fold, is controlled at the level of transcription and the protein is expressed in an enzymatically active form. In similar analyses, comparing hepatocellular carcinomas to adjacent uninvolved liver from five patients, enzymatic activity was much higher in the tumor tissue from the four patients whose immunoblots revealed increased hydroxylase protein in the malignant tissue. EGF repeats in the extracellular domain of Notch or its homologs contain the consensus sequence for hydroxylation. Deletion mutants lacking this domain are gain-of-function mutants, suggesting that the domain modulates signal transduction by the cytoplasmic domain. While the function imparted by beta hydroxylation is unknown, our studies raise the possibility that beta hydroxylation is regulated in proteins like the mammalian Notch homologs, whose cytoplasmic domains have been shown to be oncogenic.

Functional effects of domain deletions in a multidomain serine protease, C1r

The C1r subcomponent of the first component of complement is a complex, multidomain glycoprotein containing five regulatory or binding modules in addition to the serine protease domain. To reveal the functional role of the N-terminal regulatory domains, two deletion mutants of C1r were constructed. One mutant comprises the N-terminal half of domain I joined to the second half of the highly homologous domain III, resulting in one chimeric domain in the N-terminal region, instead of domains I-III. In the second mutant most of the N-terminal portion of domain I was deleted. Both deletion mutants were expressed in the baculovirus-insect cell expression system with yields typical of wild type C1r. Both mutants maintained the ability of the wild type C1r to dimerize. The folding and secretion of the recombinant proteins was not affected by these deletions, and C1-inhibitor binding was not impaired. The stability of the zymogen was significantly decreased however, indicating that the N-terminal region of the C1r molecule contains essential elements involved in the control of activation of the serine protease module. Tetramer formation with C1s in the presence of Ca2+ was abolished by both deletions. We suggest that the first domain of C1r is essential for tetramer formation, since the deletion of domain I from C1r impairs this interaction.

A fully active catalytic domain of bovine aspartyl (asparaginyl) beta-hydroxylase expressed in Escherichia coli: characterization and evidence for the identification of an active-site region in vertebrate alpha-ketoglutarate-dependent dioxygenases

The alpha-ketoglutarate-dependent dioxygenase aspartyl (asparaginyl) beta-hydroxylase (EC 1.14.11.16) specifically hydroxylates one aspartic or asparagine residue in certain epidermal growth factor-like domains of a number of proteins. The expression in Escherichia coli, purification, characterization of a fully active catalytic domain, and evidence for the identification of an active-site region of this enzyme are described. Sequence alignment analyses among the vertebrate alpha-ketoglutarate-dependent dioxygenases and chemical modification studies were undertaken aimed at locating specific regions of 52-kDa recombinant aspartyl (asparaginyl) beta-hydroxylase involved in substrate binding and/or catalysis. Based upon these studies, an alignment of the C-terminal regions of prolyl and lysyl hydroxylase and of aspartyl (asparaginyl) beta-hydroxylase is proposed. When histidine-675, an invariant residue located in a region of homology within this alignment, was mutated to an alanine residue in aspartyl (asparaginyl) beta-hydroxylase (H675A), no enzymatic activity was detected. Chemical modification studies show that the wild-type protein is protected from iodo[14C]acetamide labeling by Fe2+/alpha-ketoglutarate whereas the H675A mutant protein is not, suggesting that this mutant does not bind Fe2+/alpha-ketoglutarate.