Characterization of posttranslational modifications of human A33 antigen, a novel palmitoylated surface glycoprotein of human gastrointestinal epithelium

Role of non-canonical post-translational modifications in gastrointestinal tumors

Post-translational modifications (PTMs) of proteins contribute to the occurrence and development of tumors. Previous studies have suggested that canonical PTMs such as ubiquitination, glycosylation, and phosphorylation are closely implicated in different aspects of gastrointestinal tumors. Recently, emerging evidence showed that non-canonical PTMs play an essential role in the carcinogenesis, metastasis and treatment of gastrointestinal tumors. Therefore, we summarized recent advances in sumoylation, neddylation, isoprenylation, succinylation and other non-canonical PTMs in gastrointestinal tumors, which comprehensively describe the mechanisms and functions of non-classical PTMs in gastrointestinal tumors. It is anticipated that targeting specific PTMs could benefit the treatment as well as improve the prognosis of gastrointestinal tumors.

Inclusion of a Furin Cleavage Site Enhances Antitumor Efficacy against Colorectal Cancer Cells of Ribotoxin α-Sarcin- or RNase T1-Based Immunotoxins

Immunotoxins are chimeric molecules that combine the specificity of an antibody to recognize and bind tumor antigens with the potency of the enzymatic activity of a toxin, thus, promoting the death of target cells. Among them, RNases-based immunotoxins have arisen as promising antitumor therapeutic agents. In this work, we describe the production and purification of two new immunoconjugates, based on RNase T1 and the fungal ribotoxin α-sarcin, with optimized properties for tumor treatment due to the inclusion of a furin cleavage site. Circular dichroism spectroscopy, ribonucleolytic activity studies, flow cytometry, fluorescence microscopy, and cell viability assays were carried out for structural and in vitro functional characterization. Our results confirm the enhanced antitumor efficiency showed by these furin-immunotoxin variants as a result of an improved release of their toxic domain to the cytosol, favoring the accessibility of both ribonucleases to their substrates. Overall, these results represent a step forward in the design of immunotoxins with optimized properties for potential therapeutic application in vivo.

Heterogeneous expression of A33 in colorectal cancer: possible explanation for A33 antibody treatment failure

The A33 protein, expressed in colorectal tumors, is a target for improving treatment of patients with colorectal cancer. Over the last decade, studies have tested anti-A33 antibody as a therapeutic agent for these patients. Preclinical results were promising, but clinical trials did not confirm positive results. Here, immunohistochemistry in colorectal cancer tissue showed that samples from well-differentiated tumors presented a strong A33 membrane staining, whereas poorly differentiated tumors and mucinous adenocarcinomas showed weak cytoplasmic and nuclear staining. Moderately differentiated tumors presented variable staining. We suggest that in future clinical trials, patients should be selected on the basis of membrane expression of A33.

α-sarcin and RNase T1 based immunoconjugates: the role of intracellular trafficking in cytotoxic efficiency

Toxins have been thoroughly studied for their use as therapeutic agents in search of an improvement in toxic efficiency together with a minimization of their undesired side effects. Different studies have shown how toxins can follow different intracellular pathways which are connected with their cytotoxic action inside the cells. The work herein presented describes the different pathways followed by the ribotoxin α-sarcin and the fungal RNase T1, as toxic domains of immunoconjugates with identical binding domain, the single chain variable fragment of a monoclonal antibody raised against the glycoprotein A33. According to the results obtained both immunoconjugates enter the cells via early endosomes and, while α-sarcin can translocate directly into the cytosol to exert its deathly action, RNase T1 follows a pathway that involves lysosomes and the Golgi apparatus. These facts contribute to explaining the different cytotoxicity observed against their targeted cells, and reveal how the innate properties of the toxic domain, apart from its catalytic features, can be a key factor to be considered for immunotoxin optimization.

Quantitative intratumoural microdistribution and kinetics of (131)I-huA33 antibody in patients with colorectal carcinoma

BACKGROUND

The ability of recombinant antibodies to adequately penetrate into tumours is a key factor in achieving therapeutic effect; however, the behaviour of antibodies at a cellular level in tumours is poorly understood. The purpose of this study was to investigate those factors that influence the macroscopic and microscopic intratumoural distribution of an IgG1-humanized antibody, huA33, in colorectal tumours.

METHODS

Twelve patients were infused with radiolabelled huA33 at 7 days prior to elective surgery for colorectal carcinoma. Macroscopic huA33 uptake was determined by both gamma well counter and autoradiography measurements of the resected tumour specimens. Microscopic uptake was then quantitated at a cellular level and compared to vascular penetrance. The impact of variation in tumour antigen (GPA33) expression, tumour size, specimen type (primary vs metastatic), presence of macroscopic necrosis, and tumour vasculature on huA33 uptake were examined.

RESULTS

The I-huA33 uptake in whole tumour sections was (mean ± SD) 5.13 ± 2.71 × 10(-3)% injected dose per gram (ID/g). GPA33 was expressed in all viable tumour cells, and huA33 uptake was excellent regardless of tumour size and specimen type. In tumours with macroscopically evident central necrosis (n = 5), huA33 uptake in tumour necrotic centres was lower than in viable peripheries (0.606 ± 0.493 vs 2.98 ± 2.17 × 10(-3)%ID, p = 0.06). However, when corrected for low cell viability in necrotic centres, uptake of huA33 at the cellular level was highly comparable to that in the more viable tumour periphery (7.10 ± 5.10 × 10(-9) vs 3.82 ± 3.67 × 10(-9)%ID/cell, p = 0.4). In the five patients who exhibited macroscopic necrosis in their tumours, huA33 showed excellent tissue penetration, with a maximum penetration distance of 26 μm in peripheral tumour regions and 118 μm in central regions. No correlation was observed between (131)I-huA33 uptake in tumour on a cellular basis and tumour vascularity.

CONCLUSIONS

In patients with colorectal carcinoma, monoclonal antibody huA33 effectively targets viable tumour cells in all cellular milieus examined, including effective penetration into necrotic tumour centres, a novel and therapeutically important finding.

Antibody-based therapy in colorectal cancer

Treatment in patients with nonresectable and resectable colorectal cancer at the advanced stage is challenging, therefore intensive strategies such as chemotherapy, signaling inhibitors and monoclonal antibodies (mAbs) to control the disease are required. mAbs are particularly promising tools owing to their target specificities and strong antitumor activities through multiple mechanisms, as shown by rituximab in B-cell non-Hodgkin's lymphoma and trastuzumab in breast cancer. Three mAbs (cetuximab, bevacizumab and panitumumab) have been approved for the treatment of colorectal cancer in the USA and many other mAbs are being tested in clinical trials. The potential of antibody therapy is associated with several mechanisms including interference of vital signaling pathways targeted by the antibody and immune cytotoxicity selectively directed against tumor cells by tumor-bound antibody through the Fc portion of the antibody, such as antibody-dependent cellular cytotoxicity and complement-dependent cytotoxicity. Moreover, recent experimental findings have shown that immune complexes formed by therapeutic mAbs with tumor-released antigens could augment the induction of tumor-specific cytotoxic CD8(+) T cells through activation of APCs. In addition, antibodies targeting immune checkpoints on hematopoietic cells have recently opened a new avenue for the treatment of cancer. In this review, we focus on mAb treatment in colorectal cancer and its immunological aspects.

Gene expression profiling of ampullary carcinomas classifies ampullary carcinomas into biliary-like and intestinal-like subtypes that are prognostic of outcome

BACKGROUND

Adenocarcinomas of the ampulla of Vater are classified as biliary cancers, though the exact epithelium of origin for these cancers is not known. We sought to molecularly classify ampullary adenocarcinomas in comparison to known adenocarcinomas of the pancreas, bile duct, and duodenum by gene expression analysis.

METHODS

We analyzed 32 fresh-frozen resected, untreated periampullary adenocarcinomas (8 pancreatic, 2 extrahepatic biliary, 8 duodenal, and 14 ampullary) using the Affymetrix U133 Plus 2.0 genome array. Unsupervised and supervised hierarchical clustering identified two subtypes of ampullary carcinomas that were molecularly and histologically characterized.

RESULTS

Hierarchical clustering of periampullary carcinomas segregated ampullary carcinomas into two subgroups, which were distinctly different from pancreatic carcinomas. Non-pancreatic periampullary adenocarcinomas were segregated into two subgroups with differing prognoses: 5 year RFS (77% vs. 0%, p = 0.007) and 5 year OS (100% vs. 35%, p = 0.005). Unsupervised clustering analysis of the 14 ampullary samples also identified two subgroups: a good prognosis intestinal-like subgroup and a poor prognosis biliary-like subgroup with 5 year OS of 70% vs. 28%, P = 0.09. Expression of CK7+/CK20- but not CDX-2 correlated with these two subgroups. Activation of the AKT and MAPK pathways were both increased in the poor prognostic biliary-like subgroup. In an independent 80 patient ampullary validation dataset only histological subtype (intestinal vs. pancreaticobiliary) was significantly associated with OS in both univariate (p = 0.006) and multivariate analysis (P = 0.04).

CONCLUSIONS

Gene expression analysis discriminated pancreatic adenocarcinomas from other periampullary carcinomas and identified two prognostically relevant subgroups of ampullary adenocarcinomas. Histological subtype was an independent prognostic factor in ampullary adenocarcinomas.

Exosomes: extracellular organelles important in intercellular communication

In addition to intracellular organelles, eukaryotic cells also contain extracellular organelles that are released, or shed, into the microenvironment. These membranous extracellular organelles include exosomes, shedding microvesicles (SMVs) and apoptotic blebs (ABs), many of which exhibit pleiotropic biological functions. Because extracellular organelle terminology is often confounding, with many preparations reported in the literature being mixtures of extracellular vesicles, there is a growing need to clarify nomenclature and to improve purification strategies in order to discriminate the biochemical and functional activities of these moieties. Exosomes are formed by the inward budding of multivesicular bodies (MVBs) and are released from the cell into the microenvironment following the fusion of MVBs with the plasma membrane (PM). In this review we focus on various strategies for purifying exosomes and discuss their biophysical and biochemical properties. An update on proteomic analysis of exosomes from various cell types and body fluids is provided and host-cell specific proteomic signatures are also discussed. Because the ectodomain of ~42% of exosomal integral membrane proteins are also found in the secretome, these vesicles provide a potential source of serum-based membrane protein biomarkers that are reflective of the host cell. ExoCarta, an exosomal protein and RNA database (http://exocarta.ludwig.edu.au), is described.

KLF4-dependent, PPARgamma-induced expression of GPA33 in colon cancer cell lines

The glycoprotein A33 (GPA33) is a colon cancer antigen. Phase I trials with 131I and 125I monoclonal antibody A33 in colon carcinoma patients showed excellent localization to colorectal cancer and some evidence of tumor response. Using DNA microarrays, we have identified the GPA33 gene as a target of PPARgamma in HT29-Cl.16E colon cancer cells. Treatment of HT29-Cl.16E, Caco2, SW1116 and LS174T colon cancer cells with the PPARgamma agonist GW7845 induced a 2- to 6-fold increase in GPA33 mRNA as determined by real-time PCR. This induction was also found in HT29-Cl.16E cells treated with rosiglitazone and ciglitazone and was prevented by cotreatment with the PPARgamma antagonist GW9662, indicating that this regulation was PPARgamma dependent. No canonical PPAR responsive element was found in the GPA33 promoter. We therefore analyzed the expression of transcription factors involved in GPA33 expression. CDXl, CDX2 and KLF5 expression was not modified by PPARgamma activation. By contrast, a significant increase in KLF4 was seen, both at mRNA and protein levels. Furthermore, chromatin immunoprecipitation studies demonstrated that an increased amount of KLF4 protein was bound to the GPA33 promoter in cells treated with rosiglitazone. Finally, downregulation of KLF4 expression by siRNA reduced rosiglitazone-induced GPA33 expression. This indicates that PPARgamma activation induces KLF4 expression, which in turn increases GPA33 expression. We also demonstrate that PPARgamma activation leads to increased (p21WAF1/Cip1 and keratin 19) or decreased (cyclin D1) expression of known KLF4 targets, suggesting that KLF4 is a nodal player in a network of PPARgamma-regulated genes.

Proteomics analysis of A33 immunoaffinity-purified exosomes released from the human colon tumor cell line LIM1215 reveals a tissue-specific protein signature

Exosomes are 40-100-nm-diameter nanovesicles of endocytic origin that are released from diverse cell types. To better understand the biological role of exosomes and to avoid confounding data arising from proteinaceous contaminants, it is important to work with highly purified material. Here, we describe an immunoaffinity capture method using the colon epithelial cell-specific A33 antibody to purify colorectal cancer cell (LIM1215)-derived exosomes. LC-MS/MS revealed 394 unique exosomal proteins of which 112 proteins (28%) contained signal peptides and a significant enrichment of proteins containing coiled coil, RAS, and MIRO domains. A comparative protein profiling analysis of LIM1215-, murine mast cell-, and human urine-derived exosomes revealed a subset of proteins common to all exosomes such as endosomal sorting complex required for transport (ESCRT) proteins, tetraspanins, signaling, trafficking, and cytoskeletal proteins. A conspicuous finding of this comparative analysis was the presence of host cell-specific (LIM1215 exosome) proteins such as A33, cadherin-17, carcinoembryonic antigen, epithelial cell surface antigen (EpCAM), proliferating cell nuclear antigen, epidermal growth factor receptor, mucin 13, misshapen-like kinase 1, keratin 18, mitogen-activated protein kinase 4, claudins (1, 3, and 7), centrosomal protein 55 kDa, and ephrin-B1 and -B2. Furthermore, we report the presence of the enzyme phospholipid scramblase implicated in transbilayer lipid distribution membrane remodeling. The LIM1215-specific exosomal proteins identified in this study may provide insights into colon cancer biology and potential diagnostic biomarkers.

Exosomes: proteomic insights and diagnostic potential

Exosomes are 40-100-nm diameter membrane vesicles of endocytic origin that are released by most cell types upon fusion of multivesicular bodies with the plasma membrane, presumably as a vehicle for cell-free intercellular communication. While early studies focused on their secretion from diverse cell types in vitro, exosomes have now been identified in body fluids such as urine, amniotic fluid, malignant ascites, bronchoalveolar lavage fluid, synovial fluid, breast milk, saliva and blood. Exosomes have pleiotropic biological functions, including immune response, antigen presentation, intracellular communication and the transfer of RNA and proteins. While they have also been implicated in the transport and propagation of infectious cargo, such as prions, and retroviruses, including HIV, suggesting a role in pathological situations, recent studies suggest that the presence of such infectious cargo may be artefacts of exosome-purification strategies. Improvements in mass spectrometry-based proteomic tools, both hardware and software, coupled with improved purification schemes for exosomes, has allowed more in-depth proteome analyses, contributing immensely to our understanding of the molecular composition of exosomes. Proteomic cataloguing of exosomes from diverse cell types has revealed a common set of membrane and cytosolic proteins, suggesting the evolutionary importance of these membrane particles. Additionally, exosomes express an array of proteins that reflect the originating host cell. Recent findings that exosomes contain inactive forms of both mRNA and microRNA that can be transferred to another cell and be functional in that new environment, have initiated many microRNA profiling studies of exosomes circulating in blood. These studies highlight the potential of exosomal microRNA profiles for use as diagnostic biomarkers of disease through a noninvasive blood test. The exacerbated release of exosomes in tumor cells, as evidenced by their increased levels in blood during the late stage of a disease and their overexpression of certain tumor cell biomarkers, suggests an important role of exosomes in diagnosis and biomarker studies. The aim of this article is to provide a brief overview of exosomes, including methods used to isolate and characterize exosomes. New advances in proteomic methods, and both mass spectrometry hardware and informatics tools will be covered briefly.

Surface expression of gpA33 is dependent on culture density and cell-cycle phase and is regulated by intracellular traffic rather than gene transcription

The cell-surface marker, gpA33, a new member of the immunoglobulin superfamily, is expressed by gastrointestinal cells and by 95% of colon cancers. It has become a promising target of immunologic therapy strategies, but its biologic function and potential role in tumorigenesis are unknown. In this study, we have investigated the expression of gpA33 on the mRNA and cell-surface protein levels by quantitative reverse transcriptase polymerase chain reaction and flow cytometry, respectively, in response to cell density in the culture and to cell-cycle arrest in the G1, S, or G2/M phases. As internalization of the surface protein had previously been reported, we also investigated the binding and intracellular migration of an anti-gpA33 fluobody with green fluorescent protein (A33scFv::GFP) by laser confocal microscopy. Contrary to intuition, we found that gpA33 surface expression and mRNA levels do only partly correlate under the conditions tested. Dependent on cell density in culture, gpA33 surface expression peaked at the point of confluence. Dependent on cell-cycle phase, it peaked in the G2/M phase but was lowest in the S phase, whereas mRNA levels were highest in S, but almost absent in G1. Laser confocal microscopy clearly demonstrated the intracellular uptake of A33scFv::GFP and showed the migration of microvesicles over time. These findings are, in part, concordant with the putative role of gpA33 as an adhesion molecule. However, intracellular traffic and recycling to the cell surface appear to play a major role in its function and to have an influence on its surface density superseding translational regulation.

A phase I radioimmunolocalization trial of humanized monoclonal antibody huA33 in patients with gastric carcinoma

In order to determine the in vivo characteristics of huA33, an open label dose escalation biopsy-based phase I clinical trial and radioimmunolocalization study were conducted with a complement determinant region-grafted humanized monoclonal antibody against the A33 antigen in patients with gastric carcinoma. Thirteen patients were entered onto one of four dose levels (1.0, 2.0, 5.0 or 10.0 mg/m(2)). Patients with locally advanced (UICC-TNM [International Union Against Cancer-tumor, node, metastasis] stage over 2 but resectable at clinical diagnosis) gastric carcinoma received a single infusion of (131)I-huA33 1 week prior to surgery. Adverse events were monitored, and imaging studies with gamma camera plus ex vivo scintigraphy of the resected specimen, biodistribution study by dosimetry analysis of the biopsied and resected tissues, and immunohistochemical analysis were carried out and evaluated. No dose-limiting toxicity was observed during the trial. Therefore, the maximum tolerated dose was not reached. Although cancer tissues with + intensity and <25% extent by immunostaining in biopsied frozen sections did not show positive imaging or postoperative dosimetry findings, cancers with ++ or +++ intensity or wide (>25%) extent by frozen and paraffin sections in the biopsied specimen showed positive ex vivo tumor images and positive antigen expression in resected gastric cancer specimens, and the biodistribution analysis showed tumor uptake of (131)I-huA33. In conclusion, humanized monoclonal antibody huA33 demonstrated selective localization to gastric cancer that expressed A33 antigen strongly. These excellent targeting characteristics of huA33 indicate potential for targeted therapy of advanced gastric cancer that is refractory to cytotoxic therapy, and could also be exploitable for curatively resected early gastric cancer in an adjuvant setting.

Antibody-guided radiation therapy of cancer

Radioimmunotherapy (RIT) using radiolabeled monoclonal antibodies (MAbs) directed against tumor-associated antigens has evolved from an appealing concept to one of the standard treatment options for patients with non-Hodgkin's lymphoma (NHL). Inefficient localization of radiolabeled MAbs to nonhematological cancers due to various tumor-related factors, however, has refrained RIT from outgrowing the experimental stage in solid tumors. Still, small volume or minimal residual disease has been recognized as a potentially suitable target for radiolabeled antibodies. Several strategies are being explored aimed at improving the targeting of radiolabeled MAbs to solid tumors thus improving their therapeutic efficacy. In this review, a historical overview of the application of RIT is given and various aspects of the application of radiolabeled MAbs as anti-cancer agents are discussed. Finally, the clinical results of RIT of NHL, colorectal cancer, ovarian cancer, breast cancer, and renal cell cancer are reviewed.

A phase I trial of humanized monoclonal antibody A33 in patients with colorectal carcinoma: biodistribution, pharmacokinetics, and quantitative tumor uptake

PURPOSE

To determine the in vivo characteristics of huA33, a CDR-grafted humanized antibody against the A33 antigen, we have conducted an open-label, dose escalation, biopsy-based phase I trial of huA33 in patients with colorectal carcinoma.

EXPERIMENTAL DESIGN

Patients with colorectal carcinoma were infused with [131I]huA33 (400 MBq: 10 mCi) and [125I]huA33 (40 MBq: 1 mCi) 1 week before surgery. There were four huA33 dose levels (0.25, 1.0, 5.0, and 10 mg/m2). Adverse events, pharmacokinetics, biodistribution, tumor biopsies, and immune responses to huA33 were evaluated.

RESULTS

There were 12 patients entered into the trial (6 males and 6 females; age range, 39-66 years). No dose-limiting toxicity was observed. The biodistribution of huA33 showed excellent uptake of [131I]huA33 in metastatic colorectal carcinoma. Pharmacokinetic analysis showed no significant difference in terminal half-life (T1/2beta) between dose levels (mean +/- SD, 86.92 +/- 22.12 hours). Modeling of colon uptake of huA33 showed a T1/2 of elimination of 32.4 +/- 8.1 hours. Quantitative tumor uptake ranged from 2.1 x 10(-3) to 11.1 x 10(-3) %ID/g, and tumor/normal tissue and tumor/serum ratios reached as high as 16.3:1 and 4.5:1, respectively. Biosensor analysis detected low-level human anti-human antibody responses in four patients following huA33 infusion.

CONCLUSIONS

huA33 shows selective and rapid localization to colorectal carcinoma in vivo and penetrates to the center of large necrotic tumors, and colon elimination half-life of huA33 is equivalent to basal colonocyte turnover. The excellent targeting characteristics of this humanized antibody indicate potential for the targeted therapy of metastatic colorectal cancer in future trials.

Phase I Trial of 131I-huA33 in Patients with Advanced Colorectal Carcinoma

PURPOSE

Humanized monoclonal antibody A33 (huA33) targets the A33 antigen which is expressed on 95% of colorectal cancers. A previous study has shown excellent tumor-targeting of iodine-131 labeled huA33 (131I-huA33). Therefore, we did a phase I dose escalation trial of 131I-huA33 radioimmunotherapy.

EXPERIMENTAL DESIGNS

Fifteen patients with pretreated metastatic colorectal carcinoma each received two i.v. doses of 131I-huA33. The first was an outpatient trace-labeled "scout" dose for biodistribution assessment, followed by a second "therapy" dose. Three patients were treated at 20, 30, and 40 mCi/m2 dose levels, and six patients at 50 mCi/m2 to define the maximum tolerated dose.

RESULTS

Hematologic toxicity was 131I dose-dependent, with one episode of grade 4 neutropenia and two episodes of grade 3 thrombocytopenia observed at 50 mCi/m2. The maximum tolerated dose was determined to be 40 mCi/m2. There were no acute infusion-related adverse events, and gastrointestinal toxicity was not observed despite uptake of 131I-huA33 in bowel. Seven patients developed pruritus or rash, which was not related to 131I dose. There was excellent tumor-targeting of 131I-huA33 shown in all patients. The serum T1/2beta of 131I-huA33 was (mean +/- SD) 135.2 +/- 46.9 hours. The mean absorbed tumor dose was 6.49 +/- 2.47 Gy/GBq. Four patients developed human anti-human antibodies. At restaging, 4 patients had stable disease, whereas 11 patients had progressive disease.

CONCLUSION

Radioimmunotherapy using 131I-huA33 shows promise in targeting colorectal tumors, and is deliverable at a maximum tolerated dose of 40 mCi/m2. Further studies of 131I-huA33 in combination with chemotherapy are planned.

Radioimmunotherapy and colorectal cancer

BACKGROUND

Despite the success of radioimmunotherapy (RIT) using radiolabelled monoclonal antibodies (Mabs) directed against tumour-associated antigens in the treatment of non-Hodgkin's lymphoma, therapeutic success in solid tumours has been modest. In the past decade, a dozen Mabs have been investigated clinically for their potential usefulness in RIT of colorectal cancer.

METHODS

The application of radiolabelled Mabs for the treatment of solid cancers is discussed, and clinical trials investigating RIT for colorectal cancer listed in the Medline and Embase databases are reviewed.

RESULTS

Uptake of radiolabelled Mabs in tumour and, consequently, the therapeutic efficacy of RIT is inversely correlated with tumour size. The bone marrow is the most important dose-limiting organ. Twenty-three phase I/II studies were found that investigated the feasibility and efficacy of RIT using five radionuclides and 15 Mabs against carcinoembryonic antigen, tumour-associated glycoprotein 72, epithelial cellular adhesion molecule, A33 or colon-specific antigen p, mainly in patients with advanced colorectal cancer. A few responses were recorded but no particular antibody construct seemed superior.

CONCLUSION

RIT might be an effective adjuvant treatment modality in colorectal cancer. Future studies should focus on its application in patients with small-volume or minimal residual disease.

Serological identification of breast cancer-related antigens from aSaccharomyces cerevisiae surface display library

A yeast cell surface display technology was used for the isolation and characterization of tumor antigens recognised by autologous or allogeneic breast cancer serum. More than 100 clones recognized by patient serum were isolated using high-through-put fluorescence activated cell sorting. Combined serological and sequence analysis confirmed that a number of proteins known to be overexpressed in breast cancer tissue could be detected. A recently identified small breast epithelial mucin almost exclusively expressed in mammary gland tissue was isolated as a mutated protein variant. Subsequent serological analysis using the yeast expression system for the wild-type and mutant form showed a strong recognition by patient sera, whereas no significant recognition was observed for the respective prokaryotically expressed proteins. The small breast epithelial mucin is present to a large extent in a membrane bound format and might be used for tumor targeting strategies.

Comparative biodistribution of the radiohalogenated (Br, I and At) antibody A33. Implications for in vivo dosimetry

The alpha-emitter astatine-211 (T(1/2) = 7.2 h) has great potential for use in targeted radionuclide therapy. Its potent alpha-radiation makes (211)At unsuitable for dose planning. Its x-rays can be used for gamma-camera monitoring of the radioactivity distribution during therapy but not for accurate estimation of absorbed dose in critical organs. This study was intended to establish whether the absorbed dose delivered by astatinated antibody could be accurately determined by analogue labeling with radiohalogens, better suited for quantitative measurements in vivo. PET facilitates quantitative pharmacokinetics; possible halogen labels are, e.g., (76)Br (T(1/2) = 16.2 h) and (124)I (T(1/2) = 4.18 d). Antibody A33 was labeled with (76)Br, (125)I and (211)At using N-succinimidyl-p-halobenzoates. The conjugates were co-injected into Sprague-Dawley rats. Radioactivity concentrations in different organs and tissues were measured at three time points. Pharmacokinetic data were used to calculate absorbed doses. (125)I and (76)Br reflected the biokinetics of astatine reasonably well. The absorbed doses in bladder, kidney, pancreas, liver, bone and brain were determined with 10% accuracy. The absorbed doses in stomach, spleen and thyroid were underestimated by a factor 2-3. Positron-emitting analogues can be used to predict the astatine-derived dose in critical organs. Correction factors should be used for stomach, spleen and thyroid.

Proteomic analysis of the human colon carcinoma cell line (LIM 1215): development of a membrane protein database

The proteomic definition of plasma membrane proteins is an important initial step in searching for novel tumor marker proteins expressed during the different stages of cancer progression. However, due to the charge heterogeneity and poor solubility of membrane-associated proteins this subsection of the cell's proteome is often refractory to two-dimensional electrophoresis (2-DE), the current paradigm technology for studying protein expression profiles. Here, we describe a non-2-DE method for identifying membrane proteins. Proteins from an enriched membrane preparation of the human colorectal carcinoma cell line LIM1215 were initially fractionated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE, 4-20%). The unstained gel was cut into 16 x 3 mm slices, and peptide mixtures resulting from in-gel tryptic digestion of each slice were individually subjected to capillary-column reversed phase-high performance liquid chromatography (RP-HPLC) coupled with electrospray ionization-ion trap-mass spectrometry (ESI-IT-MS). Interrogation of genomic databases with the resulting collision-induced dissociation (CID) generated peptide ion fragment data was used to identify the proteins in each gel slice. Over 284 proteins (including 92 membrane proteins) were identified, including many integral membrane proteins not previously identified by 2-DE, many proteins seen at the genomic level only, as well as several proteins identified by expressed sequence tags (ESTs) only. Additionally, a number of peptides, identified by de novo MS sequence analysis, have not been described in the databases. Further, a "targeted" ion approach was used to unambiguously identify known low-abundance plasma membrane proteins, using the membrane-associated A33 antigen, a gastrointestinal-specific epithelial cell protein, as an example. Following localization of the A33 antigen in the gel by immunoblotting, ions corresponding to the theoretical A33 antigen tryptic peptide masses were selected using an "inclusion" mass list for automated sequence analysis. Six peptides corresponding to the A33 antigen, present at levels well below those accessible using the standard automated "nontargeted" approach, were identified. The membrane protein database may be accessed via the World Wide Web (WWW) at http://www.ludwig. edu.au/jpsl/jpslhome.html.

Characterization of mouse A33 antigen, a definitive marker for basolateral surfaces of intestinal epithelial cells

The murine A33 antigen is emerging as a definitive marker of intestinal epithelial cells. Cloning and sequence determination of cDNAs encoding mA33 antigen predict a novel type 1 transmembrane protein of 298 amino acids, comprising an extracellular domain with two immunoglobulin-like domains, a single-span transmembrane domain, and a highly acidic cytoplasmic domain. On the basis of conservation of amino acid sequence and genomic structure, the mA33 antigen is a member of a growing subfamily within the immunoglobulin superfamily, which includes transmembrane proteins CTX/ChT1, CTM/CTH, and CAR. During embryonic development, mA33 antigen expression is first observed in the inner cell mass of blastocysts before implantation. Intestinal expression of mA33 antigen is initiated in the hindgut at E14.5 and increases steadily throughout late embryonic and postnatal life into adulthood. The protein is specifically expressed on the basolateral surfaces of intestinal epithelial cells of all lineages, independent of their position along the rostrocaudal and crypt-villus axes. Thus the mA33 antigen appears to be a novel marker for both proliferating and differentiating intestinal epithelial cells.

The rabbit antibody repertoire as a novel source for the generation of therapeutic human antibodies

The rabbit antibody repertoire, which in the form of polyclonal antibodies has been used in diagnostic applications for decades, would be an attractive source for the generation of therapeutic human antibodies. The humanization of rabbit antibodies, however, has not been reported. Here we use phage display technology to select and humanize antibodies from rabbits that were immunized with human A33 antigen which is a target antigen for the immunotherapy of colon cancer. We first selected rabbit antibodies that bind to a cell surface epitope of human A33 antigen with an affinity in the 1 nm range. For rabbit antibody humanization, we then used a selection strategy that combines grafting of the complementarity determining regions with framework fine tuning. The resulting humanized antibodies were found to retain both high specificity and affinity for human A33 antigen.

Proteomic analysis of the human colon carcinoma cell line (LIM 1215): development of a membrane protein database

The proteomic definition of plasma membrane proteins is an important initial step in searching for novel tumor marker proteins expressed during the different stages of cancer progression. However, due to the charge heterogeneity and poor solubility of membrane-associated proteins this subsection of the cell's proteome is often refractory to two-dimensional electrophoresis (2-DE), the current paradigm technology for studying protein expression profiles. Here, we describe a non-2-DE method for identifying membrane proteins. Proteins from an enriched membrane preparation of the human colorectal carcinoma cell line LIM1215 were initially fractionated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE, 4-20%). The unstained gel was cut into 16 x 3 mm slices, and peptide mixtures resulting from in-gel tryptic digestion of each slice were individually subjected to capillary-column reversed phase-high performance liquid chromatography (RP-HPLC) coupled with electrospray ionization-ion trap-mass spectrometry (ESI-IT-MS). Interrogation of genomic databases with the resulting collision-induced dissociation (CID) generated peptide ion fragment data was used to identify the proteins in each gel slice. Over 284 proteins (including 92 membrane proteins) were identified, including many integral membrane proteins not previously identified by 2-DE, many proteins seen at the genomic level only, as well as several proteins identified by expressed sequence tags (ESTs) only. Additionally, a number of peptides, identified by de novo MS sequence analysis, have not been described in the databases. Further, a "targeted" ion approach was used to unambiguously identify known low-abundance plasma membrane proteins, using the membrane-associated A33 antigen, a gastrointestinal-specific epithelial cell protein, as an example. Following localization of the A33 antigen in the gel by immunoblotting, ions corresponding to the theoretical A33 antigen tryptic peptide masses were selected using an "inclusion" mass list for automated sequence analysis. Six peptides corresponding to the A33 antigen, present at levels well below those accessible using the standard automated "nontargeted" approach, were identified. The membrane protein database may be accessed via the World Wide Web (WWW) at http://www.ludwig. edu.au/jpsl/jpslhome.html.

Micropreparative ligand fishing with a cuvette-based optical mirror resonance biosensor

We have previously demonstrated the role of an optical biosensor (BIAcore 2000) as a specific detector to monitor chromatographic fractions during the purification and characterisation of ligands for orphan biomolecules. We have now extended this application to perform micropreparative ligand fishing directly on the sensor surface using an automated cuvette-based optical biosensor (Iasys Auto+) equipped with a high-capacity carboxymethyldextran surface (surface area 16 mm2). Using a F(ab)2' fragment of the A33 monoclonal antibody as bait, we have recovered microgram quantities of essentially homogeneous A33 ligand from the sensor surface in a form suitable for subsequent sensitive and specific down stream analysis (micropreparative HPLC, sodium dodecyl sulphate-polyacrylamide gel electrophoresis and Western blotting). The design of the cuvette-based system facilitates recovery of desorbed material from the constrained workspace in small volumes at high concentration. The use of on-surface detection allows the surface viability to be continuously monitored and permits direct quantitation of both bound and recovered material.

High yield direct 76Br-bromination of monoclonal antibodies using chloramine-T

Monoclonal antibody (MAb) A33 was labeled with the positron emitter 76Br (T(1/2) = 16.2 h). Direct labeling was done using the conventional chloramine-T method. After optimization of the labeling conditions, a maximum yield (mean +/- max error) of 77 +/- 2% was obtained at pH 6.8. In vitro binding of 76Br-A33 to SW1222 colonic cancer cells showed that the immunoreactivity was retained. Also, the MAbs 38S1 and 3S193 and the peptide hEGF were 76Br-labeled, resulting in labeling yields (mean +/- max error) of 75 +/- 3%, 63 +/- 4%, and 73 +/- 0.1%, respectively. We conclude that antibodies and peptides can be labeled conveniently with 76Br for the purpose of whole-body tumour imaging by positron emission tomography.

Instrumental biosensors: new perspectives for the analysis of biomolecular interactions

The use of instrumental biosensors in basic research to measure biomolecular interactions in real time is increasing exponentially. Applications include protein-protein, protein-peptide, DNA-protein, DNA-DNA, and lipid-protein interactions. Such techniques have been applied to, for example, antibody-antigen, receptor-ligand, signal transduction, and nuclear receptor studies. This review outlines the principles of two of the most commonly used instruments and highlights specific operating parameters that will assist in optimising experimental design, data generation, and analysis.

CTX, a Xenopus thymocyte receptor, defines a molecular family conserved throughout vertebrates

CTX, a cortical thymocyte marker in Xenopus, is an immunoglobulin superfamily (Igsf) member comprising one variable and one constant C2-type Igsf domain, a transmembrane segment and a cytoplasmic tail. Although resembling that of the TCR and immunoglobulins, the variable domain is not encoded by somatic rearrangement of the gene but by splicing of two half-domain exons. The C2 domain, also encoded by two exons, has an extra pair of cysteines. The transmembrane segment is free of charged residues, and the cytoplasmic tail (70 amino acids) contains one tyrosine and many glutamic acid residues. ChT1, a chicken homologue of CTX, has the same structural and genetic features, and both molecules are expressed on the thymocyte surface. We cloned new mouse (CTM) and human (CTH) cDNA and genes which are highly homologous to CTX/ChT1 but not lymphocyte specific. Similarity with recently described human cell surface molecules, A33 antigen and CAR (coxsackie and adenovirus 5 receptor), and a number of expressed sequence tags leads us to propose that CTX defines a novel subset of the Igsf, conserved throughout vertebrates and extending beyond the immune system. Strong homologies within vertebrate sequences suggest that the V and C2 CTX domains are scions of a very ancient lineage.