Isolation and characterization of a distinct immunoregulatory isoform of alpha-fetoprotein produced by the normal fetus

Integrated analysis of the rhesus monkey liver transcriptome during development and human primary HCC AFP-related gene expression

Embryonic development and tumorigenesis have a certain degree of similarity. Alpha-fetoprotein (AFP), a protein related to embryonic development, is a well-known biomarker for the diagnosis and prognosis of hepatocellular carcinoma (HCC). In this study, we analyzed the differences in gene expression profiles and molecular mechanisms in human HCC tissues from patients in AFP^high (serum AFP level ≥ 25 ng/mL) and AFP^low (serum AFP level < 25 ng/mL) groups. The results indicated that AFP^high HCC has more malignant biological characteristics. Single-sample gene set enrichment analysis (ssGSEA) showed significantly higher levels of genes expressed in dendritic cells, neutrophils, and natural killer cells in the AFP^low group than in the AFP^high group. Then, we defined a rhesus monkey fetal liver developmental landscape and compared it to the HCC gene expression profile. The gene signatures of AFP^high HCC tissues were similar to those of early embryonic liver tissues. In this study, we comprehensively analyzed the rhesus monkey liver transcriptome during development and human primary HCC AFP-related gene expression profiles and clarified the function of AFP in the occurrence and development of HCC from the perspective of developmental biology, which might provide a new perspective on the pathogenesis of HCC.

Alpha-fetoprotein: a renaissance

Alpha-fetoprotein (AFP) is a major mammalian embryo-specific and tumor-associated protein that is also present in small quantities in adults at normal conditions. Discovery of the phenomenon of AFP biosynthesis in carcinogenesis by G. Abelev and Yu. Tatarinov 50 years ago, in 1963, provoked intensive studies of this protein. AFPs of some mammalian species were isolated, purified and physico-chemically and immunochemically characterized. Despite the significant success in study of AFP, its three-dimensional structure, mechanisms of receptor binding along with a structure of the receptor itself and, what is the most important, its biological role in embryo- and carcinogenesis remain still obscure. Due to difficulties linked with methodological limitations, research of AFP was to some extent extinguished by the 1990 s. However, over the last decade a growing number of investigations of AFP and its usage as a tumor-specific biomarker have been observed. This was caused by the use of new technologies, primarily, computer-based and genetic engineering approaches in studying of this very important oncodevelopmental protein. Our review summarizes efforts of different scientific groups throughout the world in studying AFP for 50 years with emphasis on detailed description of recent achievements in this field.

Amniotic fluid α-fetoprotein microheterogeneity in the prenatal diagnosis of congenital disorders of glycosylation type Ia

BACKGROUND

Congenital disorders of glycosylation are a group of clinically and biochemically diverse defects. The current screening method (based on analysis of transferrin), which is used postnatally for the most frequent types, is however not suitable for prenatal diagnosis. The aim of the study was to investigate whether alterations in the microheterogeneity of α-fetoprotein would provide more reliable results.

METHODS

During the 14th-19th weeks of gestation, 140 amniotic fluid samples were obtained by amniocentesis and tested for fetal developmental abnormalities. α-Fetoprotein was analyzed using isoelectric focusing on Immobiline DryPlate pH 4-7, rehydrated in urea (8 mol/L), and molecular forms of the glycoprotein were detected by immunofixation and silver staining.

RESULTS

A difference in the relative proportion of individual α-fetoprotein bands (particularly increase of band II density) was found in a case where a congenital disorder of glycosylation was diagnosed postnatally, and in two other samples from pregnancies which resulted in termination, without further examination.

CONCLUSIONS

Our potential for further testing is limited; thus far, no other congenital disorders of glycosylation-positive samples have been available. Verification of our results in another laboratory with the exclusion of several potentially pertinent variables is advisable.

Structural and functional mapping of alpha-fetoprotein

Alpha-fetoprotein (AFP) is a major mammalian oncofetal protein, which is also present in small quantities in adults. It is a member of the albuminoid gene superfamily, which consists of AFP, serum albumin, vitamin D binding protein, and alpha-albumin (afamin). Although physicochemical and immunological properties of AFP have been well-studied, its biological role in embryo- and carcinogenesis and in adult organisms as well as mechanisms underlying its functioning remain unclear. During the recent decades, the biological role of AFP has been evaluated by identification of its functionally important sites. Comparison of primary structure of AFP and some physiologically active proteins revealed similarity of some polypeptide regions. This has been used for prediction of AFP functions (i.e., its multifunctionality). Localization of functionally important sites followed by determination of their amino acid composition and type of biological activity has provided valuable information for structural-functional mapping of AFP. Some peptide fragments of AFP have been synthesized and tested for biological activity. This review summarizes data on structural-functional interrelationships. We also describe functionally important AFP sites found by various groups during the last decade of structural-functional mapping of AFP with experimentally confirmed and putative biologically active sites.

Similarity between natural and recombinant human alpha-fetoprotein as inhibitors of estrogen-dependent breast cancer growth

Alpha-fetoprotein (AFP) isolated from rodent amniotic fluid or human cord sera, upon incubation with a molar excess of estradiol, is converted to a form which inhibits estrogen-stimulated tissue growth. The purpose of this study was to determine whether recombinant human AFP produced in an E. coli expression system retained this function. The recombinant protein was similar to the natural protein isolated from pooled human cord sera in all functional aspects evaluated. It was detected by monoclonal and polyclonal antibodies to the natural protein. Following exposure to estradiol, it was converted to an inhibitor of estrogen-stimulated growth of immature mouse uterus yielding a dose/response curve similar to that of the natural protein. It inhibited the growth of estrogen-dependent (MCF-7) but not estrogen-independent (MDA-MB-231) breast cancer xenografts with the same schedule dependency and resultant histological changes as the natural protein. Availability of large quantities of homogeneous, biologically active recombinant human AFP will facilitate further studies of structure/function, mechanism, and therapeutic potential of this agent as a regular of breast cancer growth.

Purification and characterization of human and mouse recombinant alpha-fetoproteins expressed in Escherichia coli

Alpha-fetoprotein (AFP) is a tumor-associated embryonic molecule whose precise biological function(s) remains unclear. A more complete analysis of the physiological activities of this oncofetal protein has, until now, been severely limited by the lack of an appropriate source from which to obtain pure AFP in any sizeable quantity. In the present investigation, we obviate this problem by cloning and efficiently overexpressing mature mouse and human AFP cDNA's in Escherichia coli. For recombinant mouse AFP (rMoAFP), large segments of the coding region were excised from the preexisting plasmids pAFP1 and pAFP2, which together encompass 90% of the AFP sequence. The mouse cDNA was made complete by the addition of N- and C-terminal encoding oligonucleotides. Mouse AFP cDNA was expressed directly as a full-length molecule in vector pTrp4 or as fusion proteins in plasmids pMALc and pRX1 under the transcriptional control of trp or tac promoters. Accumulation of rMoAFP was significantly increased in protease-deficient E. coli strains over nonprotease-deficient strains, > or = 10% of total cell protein. Of the gene fusion proteins examined, none offered significant advantage over the direct expression product in terms of recombinant protein stability, overall levels of synthesis, or facilitated purification. Recombinant AFP polypeptides expressed by pTrp4 were as expected, deposited in bacterial inclusion bodies. Subsequent to resolubilization/refolding, rMoAFP was first enriched by passage over Q-Sepharose resin followed by final purification using immobilized copper-chelate affinity chromatography. Protein sequencing of the N-terminus revealed that purified rMoAFP had a deletion of the first nine amino acids coded for by the full-length mouse AFP cDNA. Similar N-terminal deletions are observed with AFP isolates originating from natural sources. A complete human AFP cDNA was generated from a fetal liver cDNA library and was cloned into vector pTrp4. Recombinant human AFP (rHuAFP) was expressed under the identical conditions employed for rMoAFP but purification had to be modified to include preparative Mono Q anion exchange chromatography. N-terminal sequencing, amino acid compositional analysis, and electrospray mass spectrometry revealed that purified rHuAFP was intact and unaltered and that the initiator methionine was completely removed. The biological activity of recombinant AFP, as judged by its inhibitory effects on in vitro lymphocyte proliferation, was equivalent to that of the native protein. The availability of large quantities of mouse and human recombinant AFP molecules should now permit detailed structure-function analyses of this important oncofetal protein to proceed in a manner unimpeded by previous limitations in both quantity and quality of the native proteins.

Downregulation of phorbol 12-myristate 13-acetate-induced tumor necrosis factor-alpha and interleukin-1 beta production and gene expression in human monocytic cells by human alpha-fetoprotein

We previously identified a specific receptor of alpha-fetoprotein (AFP) on human monocytes. Although AFP alters many immune cell functions, the effect of AFP on monocyte cytokine production is unknown. Because tumor necrosis factor--alpha (TNF-alpha) and interleukin-1 beta (IL-1 beta) are important cytokines in immunoregulation, we investigated whether AFP could modulate TNF-alpha and IL-1 beta production in U937, a human monocytic cell line. Our results showed that U937 cells secreted TNF-alpha and IL-1 beta in response to either phorbyl 12-myristate 13-acetate (PMA) or IFN-gamma + LPS. In contrast, AFP significantly suppressed PMA-induced TNF-alpha and IL-1 beta production by U937 cells in a time and dose dependent fashion. Pretreatment of U937 cells with AFP resulted in maximal inhibition of PMA-stimulated TNF-alpha and IL-1 beta production by 58% and 67% respectively. AFP also inhibited interferon-gamma plus lipopolysaccharide (IFN-gamma + LPS)-induced TNF-alpha and IL-1 beta production. Furthermore, Northern blot analysis showed that AFP suppressed PMA-mediated TNF-alpha and IL-1 beta messenger RNA (mRNA) expression. PMA-induced prostaglandin E2 (PGE2) production by U937 cells was enhanced by AFP. Pretreatment with indomethacin, a cyclooxygenase inhibitor, reversed AFP-inhibited TNF-alpha production by 78%. Thus, we conclude that AFP downregulates TNF-alpha and IL-1 beta production via a PGE2-dependent mechanism.

Evidence that immunosuppression is an intrinsic property of the alpha-fetoprotein molecule

Among the proteins that comprise the albumin family, alpha-fetoprotein (AFP) is the only member which exhibits immunoregulatory properties. However, some investigations have argued that AFP-mediated immunosuppression is not an inherent property of the molecule itself, but is instead, hypothesized to be either a function of a low molecular weight inhibitor bound to AFP or to a post-translational modification of the protein. AFP cannot be isolated from natural sources in quantities sufficient for the detailed biochemical and functional analyses required to resolve these issues. We have therefore produced recombinant forms of the protein (rAFP) by cloning the cDNA's for mouse and human AFP in both eukaryotic and prokaryotic expression systems. As described in this report, we were able to abundantly express rAFP's in bacterial, baculovirus and yeast expression systems. Recombinant proteins derived from each expression system were recognized by polyclonal and monoclonal anti-AFP antibodies as determined by immunoblot analysis. Pure recombinant protein samples, as characterized by polyacrylamide gel analyses, N-terminal sequencing and FPLC and HPLC chromatography, were evaluated for their immunoregulatory properties in murine and human in vitro immunological assays. The results of these studies establish that rAFP is functionally equivalent to natural fetal derived AFP molecules. Importantly, the data reported here demonstrate that AFP-mediated immunoregulation is an activity intrinsic to the molecule itself and cannot be attributed to either putative non-covalently bound moieties or to post-translational modifications such as glycosylation and sialylation. These studies provide a basis for initiating detailed investigations into the potential clinical usefulness of AFP as an immunotherapeutic agent.

Biochemical characterization of bovine alpha-fetoprotein and comparison with human alpha-fetoprotein

This study compares the molecular, charge and lectin microheterogeneity of bovine alpha-fetoprotein (bAFP) with human (h) AFP. Molecular weights of bAFP (81 kDa) and hAFP (69 kDa) were detected by Western immunoblotting. Marked crossreactivity was found between bAFP and hAFP by Western immunoblotting but no crossreactivity was noticed by radioimmunoassays. At least seven charge isoforms of bAFP and three isoforms of hAFP were consistently detected by chromatofocusing in a mixture of fetal bovine serum (FBS) and human cord blood (hCB), while only three isoforms of bAFP and hAFP were detected in a mixture of bovine (bAF) and human amniotic fluid (hAF). Using concanavalin A (Con A) chromatography, 50% of bAFP was Con A reactive and 50% non-reactive, while more than 98% of hAFP was Con A reactive in a mixture of FBS and hCB. However, in AFs, 72% of bAFP was Con A reactive, while 89% of hAFP was Con A reactive. These data indicate that marked differences exist in both the charge and lectin microheterogeneity of bovine and human AFP.

The immunomodulatory activity of human amniotic fluid can be correlated with transforming growth factor-beta 1 (TGF-beta 1) and beta 2 activity

The role of alphafetoprotein (AFP) in the immunomodulatory activity of amniotic fluids (AF) from normally progressing human pregnancy (weeks 14-16) was investigated. A panel of 42 AF (25% v/v) reduced significantly phytohaemagglutinin (PHA)-induced peripheral blood mononuclear cell (PBMC) proliferation in serum-free cultures with a mean per cent inhibition of 68.4 +/- 5.5%. In contrast, AFP preparations, with one exception (U.AFP), failed to display inhibitory activity. Pretreatment of AF with anti-TGF-beta 1 and beta 2 antibodies used alone resulted in the mean per cent loss of inhibition of 33.1 +/- 3.9% and 52.3 +/- 7.5%, respectively. A summative loss of AF-mediated inhibition was detected when anti-TGF-beta 1 and beta 2 antibodies were used in combination, but immunomodulation was rarely abolished 100% by this treatment. Anti-TGF-beta 2 antibody treatment, unlike anti-TGF-beta 1 antibody treatment, reversed the inhibitory activity of U.AFP. The amount of TGF-beta 1 and beta 2 contained in human AF was studied by growth inhibition of Mv1 Lu cells. The mean levels of TGF-beta 1 and beta 2 in AF were 11 +/- 0.9 U/ml and 2.3 +/- 0.4 U/ml, respectively, which corresponds with a mean per cent inhibition of 49 +/- 4.7%. U.AFP also significantly inhibited Mv1 Lu cell growth. To investigate the mechanism of AF-mediated inhibition, the effect of AF and AFP on IL-2 production by concanavalin A (Con A)-stimulated PBMC blasts was determined by the CTLL-2 cell bioassay. IL-2 production was reduced 55.5% in AF-treated blasts and 61% in U.AFP-treated blasts compared with controls. Our findings indicate that the immunomodulatory activity of human AF can be correlated with TGF-beta 1 and beta 2 and not with AFP, the inhibitory activity of U.AFP preparation reflecting copurifying TGF-beta 2 activity.

Purification of alpha-fetoprotein from human cord serum with demonstration of its antiestrogenic activity

Alpha-fetoprotein (AFP) was purified from pooled human cord serum to determine whether it would be similar to purified mouse AFP in its ability to be transformed into an antiestrogen by incubation with estradiol (E2). Greater purity was attained with a three-step purification procedure of chromatofocusing, Blue-Sepharose chromatography and immunoaffinity chromatography than with a two-step procedure of polyacrylamide gel electrophoresis followed by Blue-Sepharose chromatography. Nevertheless, both procedures rendered AFP in a form that was transformable by E2 to an antiestrogen, although the product of the three-step procedure afforded more consistent biological activity. Removal of albumin from AFP was crucial for transformation of AFP to an antiestrogen. Thus, human AFP is similar to mouse AFP in being transformed to an antiestrogen upon incubation with E2, even though there is only 66% structural homology between the two proteins, and human AFP lacks the high-affinity binding site for E2 present in the mouse AFP molecule.

A transforming growth factor beta 2 (TGF-beta 2)-like immunosuppressive factor in amniotic fluid and localization of TGF-beta 2 mRNA in the pregnant uterus

This report describes a murine amniotic fluid (MAF) immunosuppressive factor that has properties similar to transforming growth factor beta (TGF-beta). The MAF factor exhibits TGF-beta-like activity in stimulating soft agar colony formation by AKR-2B cells and inhibiting thymidine uptake by Mv1Lu cells. We demonstrate that both the immunosuppressive and TGF-beta-like activities of the MAF factor are completely neutralized by anti-TGF-beta 2-specific antibodies and not by anti-TGF-beta 1-specific antisera. The immunosuppressive factor in MAF is novel in that it appears to be identical or very closely related to TGF-beta 2 and is active in its native state. This active and anti-TGF-beta 2-neutralizable factor chromatographs at approximately 70 kD on Sephadex at neutral pH and appears to be able to complex with alpha-fetoprotein in native amniotic fluid. Chromatography of native MAF under acidic conditions demonstrates a lower molecular mass protein that chromatographs on BioGel in the same position as the mature 25-kD TGF-beta. This protein has the biological properties of TGF-beta and is immunosuppressive. Both of these activities are neutralizable with anti-TGF-beta 2 but not with anti-TGF-beta 1 or other antisera. By Northern analysis, we find high levels of TGF-beta 2 mRNA (with little or no TGF-beta 1) in the pregnant uterus that peak around day 15 of gestation and then fall rapidly by day 19 as birth approaches. The TGF-beta 2-like factor could possibly play a role in maternal immunity, in the retention of the fetal allograft, as well as in regulating fetal and neonatal immunological competence.

Analytical- and preparative-scale separation of molecular variants of alpha-fetoprotein by anion-exchange chromatography on Monobead resins

A rapid and reliable purification procedure is described that is useful for both analytical detection and quantitative recovery of milligram amounts of individual molecular variants of mouse alpha-fetoprotein (AFP). The appropriate separation conditions were developed with an analytical-size Mono Q anion-exchange column linked to an automated Fast Protein Liquid Chromatography system. Effective separations of fetal-derived AFP variants was accomplished within 20 min under mild conditions with an L-histidine buffer. Employing the optimal separation conditions established on the Mono Q HR 5/5 column we upscaled the procedure by using a preparative Mono Q HR 16/10 column in order to obtain milligram quantities of each molecular variant of AFP. Seven distinct isomeric forms of AFP could be recovered on the preparative anion exchanger in a highly reproducible manner. Each of the seven protein peaks eluted from the Mono Q column were confirmed to be distinct isoforms of AFP by isoelectric focusing and Western blotting developed with monospecific anti-AFP antisera. This method in its scaled up version offers the benefit of providing milligram quantities of immunochemically pure AFP isomers for structure and function studies.