Studies on human alpha-fetoprotein. Isolation and characterization of monomeric and polymeric forms and amino-terminal sequence analysis

An ultra-sensitive SPR immunosensor for quantitative determination of human cartilage oligomeric matrix protein biomarker

This paper reports the development of a novel surface plasmon resonance (SPR) immunosensor for ultra-sensitive quantitative determination of human articular cartilage oligomeric matrix protein (COMP), a major component of the extracellular matrix and an exploratory biomarker. Capture antibodies against human COMP (anti-COMP_16F12) were covalently immobilized on an 11-mercaptoundecanoic acid (11-MUA) self-assembled monolayer (SAM)-coated SPR sensor disk and a dual sandwich-type signal amplification strategy using biotinylated detection antibodies against COMP (anti-COMP_17C10-biot) and streptavidin-conjugated quantum dots (SAv‒QDs) were used for the development of an immunosensor. The binding of high-mass SAv‒QDs via biotin-streptavidin interaction to the surface of the immunosensor resulted in a drastic increase in the sensitivity. The developed immunosensor was able to detect concentrations of COMP in a range from 2.80 to 680.54 fM with a limit of detection (LOD) and a limit of quantification (LOQ) of 0.15 and 0.50 fM, respectively. The immunosensor exhibited good repeatability (relative standard deviation (RSD) 8.05%) and reproducibility (RSD 9.88%) as well as excellent operational stability (2.14 % decrease in SPR signal after 13 days). In addition, the analysis of secretomes of human knee articular cartilage explants from patients with osteoarthritis revealed that the immunosensor has good accuracy (analytical error less than 5 %). These results indicate that the immunosensor developed may be suitable for quantitative determination of COMP derived from articular cartilage and other synovial joint tissues in clinical studies.

Ultrasensitive Detection of Dual Cancer Biomarkers with Integrated CMOS-Compatible Nanowire Arrays

A direct, rapid, highly sensitive and specific biosensor for detection of cancer biomarkers is desirable in early diagnosis and prognosis of cancer. However, the existing methods of detecting cancer biomarkers suffer from poor sensitivity as well as the requirement of enzymatic labeling or nanoparticle conjugations. Here, we proposed a two-channel PDMS microfluidic integrated CMOS-compatible silicon nanowire (SiNW) field-effect transistor arrays with potentially single use for label-free and ultrasensitive electrical detection of cancer biomarkers. The integrated nanowire arrays showed not only ultrahigh sensitivity of cytokeratin 19 fragment (CYFRA21-1) and prostate specific antigen (PSA) with detection to at least 1 fg/mL in buffer solution but also highly selectivity of discrimination from other similar cancer biomarkers. In addition, this method was used to detect both CYFRA21-1 and PSA real samples as low as 10 fg/mL in undiluted human serums. With its excellent properties and miniaturization, the integrated SiNW-FET device opens up great opportunities for a point-of-care test (POCT) for quick screening and early diagnosis of cancer and other complex diseases.

Discovery and diagnostic value of a novel oncofetal protein: glypican 3

Glypican 3 is a membrane-bound heparan sulfate proteoglycan, which has recently been identified as a marker for liver cancer and germ cell malignancies. Individuals with loss-of-function mutations for the glypican 3 gene exhibit Simpson-Golabi-Behmel syndrome, a rare X-linked overgrowth disorder. Expression of glypican 3 mRNA and protein is normally silenced in most adult organs and may reappear during malignant transformation. In the past few years, immunohistochemical and molecular characteristics of glypican 3 in hepatocellular carcinoma have been elucidated. More recently, glypican 3 has been emerging as a new diagnostic marker for germ cell tumors and especially testicular and ovarian yolk sac tumors. However, in other tumors such as renal cell carcinomas, squamous cell carcinomas, and melanomas, studies disagree on the level of glypican 3 expression. Finally, there is the controversial notion of glypican 3 as a tumor suppressor gene. In this review article, we update current knowledge on glypican 3 expression in normal and neoplastic tissues, evaluate its utility as a tumor marker in clinical practice, and explore its role as a novel oncofetal protein with clinical implications. Our focus is on the diagnostic value of glypican 3 in germ cell tumors and other neoplasms in addition to hepatocellular carcinoma. In conclusion, glypican 3 has been proven to be a useful immunohistochemical marker in distinguishing yolk sac tumors, choriocarcinomas, and Wilms tumors from other malignancies histologically mimicking these primitive tumors. Clinically, we recommend that glypican 3 be used as part of a panel of markers in subtyping testicular germ cell tumors.

Structure and function of alpha-fetoprotein: a biophysical overview

alpha-Fetoprotein (AFP) is a large serum glycoprotein belonging to the intriguing class of onco-developmental proteins. AFP has attracted considerable attention since it was shown that the change in its serum level during pregnancy is a hallmark of the development of numerous embryonic disorders, while the increase in its content in the plasma of adults correlates with the appearance of several pathological conditions. Over the past 30 years, some 11000 papers have been published concerning AFP, an average rate of over a publication a day since 1969. The majority of publications are about the application of the protein in diagnostics, or about other uses of AFP in biomedicine; though some of them describe the biochemical and functional properties of AFP, two aspects have been extensively reviewed. However, surprisingly little is currently known about structural properties of this protein as well as about the molecular mechanism of its function. The present review pursues the aim to describe the current state of the art in studies of structural properties and conformational stability of AFP. An attempt to establish the relationship between conformational transformations in AFP and its function is also made.

Biochemical characterization of alpha-fetoprotein and other serum proteins produced by a uterine endometrial adenocarcinoma

A high serum alpha-fetoprotein (AFP) level was found in a patient with endometrial adenocarcinoma of the uterus, which appeared to be hepatoid on histological examination. The AFP of this unusual patient was purified by immunoaffinity chromatography and characterized. The electrophoretic profiles on sodium dodecyl sulfate-polyacrylamide get electrophoresis both before and after glycopeptidase F treatment were indistinguishable from those of a hepatoma AFP. This indicates that the patient's AFP was also composed of a single polypeptide chain of Mr 67,000 and an N-linked sugar chain of Mr 3,000. Amino acid sequence analyses of this AFP, and of AFP from hepatoma and umbilical cord serum indicated that the N-terminal sequences were essentially the same. The sequence, Arg-Thr-Leu-His-Arg-Asn-Glu-Tyr-Gly-Ile, was slightly different from previous reports, but matched that deduced from the cDNA sequence. AFP isoforms due to microheterogeneity of the sugar chain were analyzed by lectin affinity electrophoresis using a series of lectins. The AFP isoform profiles were distinct from those of proteins derived from cord serum, hepatoma, yolk sac tumor and gastric cancer. The reverse-transcription of RNA from the tumor tissue followed by a polymerase chain reaction using primers with AFP-specific sequences gave a product of the size and nucleotide sequence expected for AFP. mRNAs possessing the requisite sequences for albumin and transferrin syntheses were also detected in the tumor. The expression of these hepatocyte-specific proteins supported the hepatoid nature of this tumor.

Chemistry and biology of alpha-fetoprotein

Alpha-Fetoprotein (AFP) is a product of specific fetal tissues and of neoplastic cells of hepatocyte or germ cell origin in adults. This protein belongs to a gene family that is phylogenetically most closely related to serum albumin. Its primary, secondary, and tertiary structural aspects appear similar to the three-domain concept proposed for the latter protein. The primary sequence of AFP departs most widely from serum albumin in the first 135 amino acid residues, with about 42% of the remaining 590 residues of the human proteins being identical. Some evidence exists that there are limited sequence differences in the AFP of a given animal species. AFP shows considerable charge heterogeneity that appears to relate mostly to its glycoid moiety. The proteins of some species such as the rat show more pronounced heterogeneities than that of humans. The variations in extent and type of glycosylations are evidenced by differences in the binding to various lectins. These interactions are being extensively explored in attempts to differentiate the sources of the protein produced by various normal and neoplastic cells and may provide valuable diagnostic methods. AFP, like serum albumin, shows relatively strong binding affinities for a variety of ligands. The most notable difference is the strong preferential binding of polyunsaturated fatty acids by AFP. This protein may play a role in transporting these substances to developing and to malignant cells. Various agents affect the synthesis of this protein both by specific fetal tissues and by neoplastic cells. Marked differences in the responses of cells, particularly those of neoplastic types, are indicative of variations in the genetic factors responsible for control of its synthesis. The subject of the genomic repression of the synthesis of AFP seen in fetal life upon maturation of the liver and the reoccurrence of synthesis upon malignant conversion of hepatocytes and of certain germ cells are of particular interest. The regulation of the closely related AFP and albumin genes is providing a powerful and attractive model to examine molecular events in the activation and inactivation of specific genes during development and in oncogenic processes. Extensive measurements of AFP during pregnancy and in the course of neoplasias, notably hepatoma, are being made to aid in following changes in such developments. Various specific physiological roles for this protein are also being proposed. One of these is its possible action in the regulation of immune processes.(ABSTRACT TRUNCATED AT 400 WORDS)

Human alfa-fetoprotein: isolation and production of monoclonal antibodies

Alfa-fetoprotein from human cord serum was purified in a single step by hydrophobic interaction chromatography on Phenyl Sepharose CL-4B with a final recovery of alfa-fetoprotein of about 90% and a purification factor of 900. The purified preparation was homogeneous on SDS-PAGE and native PAGE running with a relative molecular weight of 72,000. Monoclonal antibodies against this purified preparation were raised by hybridoma technology using Sp2/0 myeloma cells as a fusion partner. 50% of culture wells exhibited hybrid growth and 7% of these wells contained anti-AFP secreting hybrids. Positive hybrid cells were cloned twice by the limiting dilution method and 8 clones were obtained that secreted monoclonal antibodies. Five of these cell lines (3F6H10, 3F6H4, 3F6H1, 3F6G5 and 3F6G10) were selected at random for purification and characterization purposes. All 5 cell lines secreted monoclonal antibodies of IgG1 subclass which were purified by affinity chromatography on Protein A- Sepharose CL-4B column with a final recovery of 80% and a purification factor of about 13. The purified preparations were homogeneous on SDS-PAGE, native PAGE and IEF. The monoclonal antibodies were highly specific for human alfa-fetoprotein as determined by Western blotting. The affinity constants (K) of these Mab ranged from 10(6) to 10(9) l/mol.

A sensitive blotting system for detection of alpha-fetoprotein variants with monoclonal and polyclonal antibodies

Human alpha-fetoprotein (AFP) variants from cord sera were separated by isoelectric focusing in agarose gels under native conditions, transferred to nitrocellulose paper and detected with polyclonal and monoclonal antibodies (Moabs). Rabbit anti-AFP recognized up to 9 individual electrophoretic variants in the range of pH 4.5 to pH 5.2. The reactivity of 8 Moabs ranged from weak to strong and showed variability in the pattern of AFP bands recognized. Moabs were separated into 3 groups according to the number of bands detected: group 1 detected 6 to 7 bands; group 2 recognized only one band; and group 3 recognized 4 bands. The sensitivity of the system with polyclonal antibodies was 0.15 ng of AFP in complete cord serum and varied between 300 and 0.2 ng with Moabs.

Primary structures of human alpha-fetoprotein and its mRNA

DNA complementary to human alpha-fetoprotein (AFP)mRNA was cloned in the plasmid pBR322. Analysis of three overlapping cDNA clones revealed most of the nucleotide sequence of AFP mRNA, and the remaining nucleotides at the 5' end of the mRNA were elucidated from a cloned genomic DNA fragment. The amino acid sequence was deduced from the nucleotide sequence, which revealed 19 amino acids in the signal sequence and 590 amino acids in mature AFP. There are 15 regularly spaced disulfide bridges, which generate a folding structure having three repeating domains. There is one potential N-glycosylation site, Asn-Phe-Thr, in the amino acid sequence. In comparison with mouse AFP, 66% of the amino acid sequence was conserved, with the highest identity (72%) in domain 3, followed by domain 2 (67%) and domain 1 (59%). In comparison with human albumin, a 39% conservation of primary structure was found. Again, the similarity was the highest in domain 3 and the lowest in domain 1. Human AFP and human albumin are similar in overall structure, but certain parts of the molecules differ significantly in their predicted secondary structure.

Identification of alpha-fetoprotein polymers. Artifacts of the isolation procedure

By combining polyacrylamide gel electrophoresis and radioimmunoassay, a method has been developed to detect polymeric forms of alpha-fetoprotein (AFP) during the isolation procedure. The method is capable of detecting AFP polymers in the presence of other serum proteins in serum samples that contain as little as 100 ng of AFP. Using this method, we have found that AFP polymers do not exist in cord serum; if encountered, we feel they should be considered to be artifacts of the isolation procedure AFP polymers appear in 7.5% polyacrylamide gel as multiple bands. We believe the multiple bands to be AFP polymers and not serum protein contaminants for the following reasons: (1) the electrophoretic pattern is very similar to that of serum albumin; (2) the polymers do exhibit AFP activity, although less than the monomeric form does; (3) the molecular weight of the dimer is exactly twice that of the monomer; (4) in the presence of sodium dodecyl sulfate and mercaptoethanol, the polymers move to the same position as do the monomers in polyacrylamide gel electrophoresis; (5) the polymers cannot be removed by repeated use of an immunoadsorbent affinity column designed to remove contaminant serum proteins.

The sequence of human serum albumin cDNA and its expression in E. coli

A recombinant plasmid has been constructed which contains the mature protein coding region of the human serum albumin (HSA) gene. Bacteria containing this plasmid synthesize HSA protein under control of the E. coli trp promoter-operator. The DNA sequence and predicted protein sequence of HSA were determined from the cDNA plasmid and are compared to existing data obtained from direct protein sequencing. The DNA sequence predicts a mature protein of 585 amino acids preceded by a 24 amino acid "prepro" peptide.

Purified human plasma proteins of unknown function

The development of new analytic and preparative techniques in the field of protein chemistry has essentially extended our knowledge about the variety of human plasma proteins in the last 15 years. In many cases plasma proteins have been particularly determined by immunologic techniques, partially highly purified and physicochemically well characterized, before knowing the biological function. To these proteins belong among others the alpha 1-antitrypsin, Cl-inactivator, alpha 2-macroglobulin, Gc-globulin and the cold-insoluble globulin. Today we know more than 100 proteins being isolated from human plasma and among these are nearly 20, of which the biological function is not yet known. To this group are belonging proteins, which are known since many years, like the alpha 1-acid glycoprotein and the C-reactive proteins as well as proteins, which have only been described in the last years and which partially have an interesting chemical structure, e.g. the histidinerich 3,8S-alpha 2-glycoprotein and the leucine-rich 3,1S-alpha 2-glycoprotein, of which every fifth amino acid is formed by leucine. It is to be hoped that the special chemical structure of some of these human plasma proteins as well as the quantitative immunologic determination in different patient sera will give hints to their biological function.

Alpha-fetoprotein on human peripheral blood lymphocytes does not block complement-dependent lymphocytotoxicity

Using a direct immunofluorescence assay, we showed that alpha-fetoprotein (AFP) both in purified form and in hepatocellular carcinoma (HCC) sera was capable of binding onto 10-20% of T lymphocytes and 5-10% of B lymphocytes in human peripheral blood when these lymphocytes were preincubated in AFP-positive fluids at 4 degrees C in the presence of sodium azide. But when the preincubation temperature was raised to 37 degrees C, most of the membrane-bound AFP was internalized or shed, and, consequently, less than 3% of the cells showed positive membrane fluorescence. In addition, binding of AFP onto lymphocyte surface membrane and the continuous presence of large amounts of AFP in these lymphocyte cultures did not interfere with the action of cytotoxic antibodies directed against HLA determinants on the lymphocyte surface.

Microheterogeneity of rat glycogen phosphorylase liver-type isozyme

We devised a method of polyacrylamide gel electrophoresis at pH 7.3, modified by omitting base catalyst, N,N,N',N'-tetramethylethylenediamine, in the preparation of separating gels. Using this method, both liver and liver-like types of rat glycogen phosphorylase (1,4-alpha-D-glucan:orthophosphate alpha-glucosyl-transferase, EC 2.4.1.1) were resolved into multiple forms, about 6-10, although either of them was purified to a single protein with the same molecular size on sodium dodecyl sulfate gel electrophoresis. The microheterogeneity of these two types was also confirmed by isoelectric focusing in polyacrylamide gels (pH 5-8). The major isoelectric points of the liver type phosphorylase were between 5.72 and 5.86, but those of the liver-like type were between 5.86 and 5.92, and so the former had slightly but significantly lower isoelectric points than the latter. However, the both types were not distinguished immunologically. The brain and muscle types of rat phosphorylase did not show such a distinct heterogeneity by the same electrophoresis methods.

Microheterogeneity of rat, mouse and human alpha1-fetoprotein as revealed by polyacrylamide gel electrophoresis and by crossed immuno-affino-electrophoresis with different lectins

Polyacrylamide gel electrophoresis and crossed immuno-affino-electrophoresis with several free lectins have been used to characterize and to compare the molecular heterogeneity of rat, mouse and human alpha1-fetoproteins. Each alpha1-fetoprotein contains a variable number of electrophoretic variants depending on the gel porosity. In SDS electrophoresis, two molecular size populations are present in rat alpha1-fetoprotein (Mr = 74 000 and 72 000) and in mouse alpha1-fetoprotein (Mr = 73 000 and 72 000) but only one is observed in human alpha1-fetoprotein (Mr = 70 000). The crossed immuno-affino-electrophoresis patterns square with affinity chromatography results and reveal a marked and characteristic heterogeneity for the three alpha1-fetoprotein species with Concanavalin A, Ricinus communis and Lens culinaris lectins. No lectin-alpha-fetoprotein interaction is apparent with Ulex, Lotus and wheat germ lectins. Since similar patterns are obtained whether with purified alpha1-fetoprotein or with unfractionated fresh fetal sera, it is likely that this heterogeneity is not a consequence of artefactual molecular modifications arising during the purification procedure.

Isolation of human alpha-fetoprotein in two fractionation steps and demonstration of homogeneity

Human alpha-fetoprotein (hAFP) has been isolated from cord serum in 40% yield using an isolation procedure consisting of only two major steps: affinity chromatography followed by preparative polyacrylamide gel electrophoresis (PAGE). The final product appeared homogeneous on the basis of five independent criteria for purity. Sodium dodecyl sulfate gel electrophoresis (SDS-PAGE) demonstrated a single polypeptide chain with molecular weight of 71,000. The protein exhibited an apparent isoelectric point (pI') of 4.85, molecular radius of 3.0 nm and a valence (net H+/molecule) of 21.9 derived from computation of analytical PAGE data. The two-step isolation procedure made it possible for a single operator to isolate milligram amounts of hAFP in a matter of weeks.

Heterogeneity of human alpha-fetoprotein as revealed by isoelectric focusing in urea-containing gels

The heterogeneity of human alpha-fetoprotein has been studied by analytical isoelectric focusing in polyacrylamide gel slabs in the presence of 8 M urea. Six major isoelectric variants could be identified over a pH range of 6.0--6.2. Verification of their identity was achieved by crossed immunoelectrophoresis into agarose gel containing monospecific antiserum to human alpha-fetoprotein. Complete desialylation of the protein did not abolish the heterogeneity; a complex pattern of major alpha-fetoprotein bands persisted over a more alkaline pH range. We have been able to correlate the pattern of alpha-fetoprotein heterogeneity seen following extended agarose gel electrophoresis with that obtained during isoelectric focusing in the presence of urea. The quantity of certain alpha-fetoprotein charge isomers in various alpha-fetoprotein isolates may be important in considering certain biological functions of this protein.

Human alpha-fetoprotein: immunosuppressive activity and microheterogeneity

Purified human alpha-fetoprotein (HAFP) from five patients with hepatoma, one with gastric carcinoma, one with an embryonal cell tumor, and from fetal liver has demonstrated immunosuppressive potencies in vitro which vary over three orders of magnitude. A reversible association of HAFP with the cell surface and a predominant effect on T cells are suggested. No evidence of complex formation between HAFP and mitogen has been found. The microheterogeneity of HAFP has been detailed with crossed immunoelectrophoresis and isoelectric focusing in polyacrylamide gels containing 8M urea, and the immunosuppressive potency of HAFP isolated from a given source can be correlated with the proportion of certain HAFP species contained in it.