Membrane proteins from lymphoblastoid cells showing cross-affinity for alpha-fetoprotein and albumin. Isolation and characterization

The adenocarcinoma cell surface mucin receptor for alpha-fetoprotein: is the same receptor present on circulating monocytes and macrophages? A commentary

The mucin family of proteins is largely expressed on sedentary epithelial cells lining the gastrointestinal, pulmonary, and reproductive tracts and their associated organs and malignant tumors. It is less well-known that mucins are also expressed on circulatory cells of the immune and inflammatory systems, such as monocytes, macrophages, leukemic, and lymphoma cells. The epithelial mucins function in (a) protection and lubrication of mucosal linings, (b) cell adhesion and cell-to-cell contact, (c) cell migration and metastasis, and (d) signal transduction. It would be logical to presume that mucins expressed on circulating mononuclear cells could perform similar functions. Recently, it was proposed that the alpha-fetoprotein (AFP) receptor, known to be present on solid epithelial-derived malignant tumor cells, can be identified as a mucin glycoprotein. Interestingly, it was also reported that AFP binds to a receptor on circulating cells and sedentary tumor cells of lymphoreticular origin, especially monocytes associated with lymphomas and leukemias. The primary objective of the present commentary is to present literature-based evidence that some of the cell surface mucins on sedentary epithelial tumor cells and certain mucins expressed on circulating monocytes/macrophages are identical to the AFP receptor. The secondary objective is to discuss the role of AFP and its derived peptides in the growth suppression of adenocarcinomas and lymphomas using the AFP-mucin receptor concept as a key to the mechanism of tumor growth inhibition.

Alpha-fetoprotein contributes to THP-1 cell invasion and chemotaxis via protein kinase and Gi-protein-dependent pathways

Alpha-fetoprotein (AFP) for long was known as immunomodulator and tumor marker having multifaceted actions on the activity of normal and transformed cells. In present study, we have investigated the involvement of AFP in regulation of THP-1 cell line invasion and underlying mechanisms. Treatment with human recombinant AFP causes up-regulation of MMP9 expression, chemotaxis and calcium mobilization, and increases invasion through Matrigel, with no significant impact on THP-1 cell growth or viability. Using small molecule inhibitors, we have shown that the rhAFP-induced MMP9 expression depends on the activation of ERK1,2, JNK and Akt kinases, with the involvement of NFκB and likely, AP-1 transcription factors. In contrast, inhibition of p38 kinase, but not of JNK, had dramatic suppressive effect on the rhAFP-triggered chemotaxis. In addition, rhAFP-induced MMP9 expression and calcium response were completely blocked by pertussis toxin, indicating that Gi-protein-coupled receptor(s) has a mediatory role in these processes. CCR5 chemokine receptor is the only known Gi-protein binding to AFP. The action of CCR5 inhibitor Maraviroc results in partial suppression of MMP9 up-regulation and calcium response suggesting that CCR5 might be involved in these effects.

Review of the adenocarcinoma cell surface receptor for human alpha-fetoprotein; proposed identification of a widespread mucin as the tumor cell receptor

The identification of a tumor cell receptor for alpha-fetoprotein (AFP) has long been sought in the field of medicine. The uptake and endocytosis of AFP by rat tumor cells in 1983 sparked a series of confirmatory reports which were extended to include multiple tumor types in rats, mice, and humans. The following year, French investigators characterized the binding properties of the AFP receptor but they did not purify and characterize the receptor. It was not until 1991-1992 that an AFP receptor was partially purified and characterized from both human monocytes and breast cancer cells. By 1993, monoclonal antibodies had been raised against the "AFP receptor" derived from breast cancer extracts with claims that the receptor was a widespread oncoprotein biomarker for cancer. To date, that receptor has yet to be identified due to its complex multimeric structure and carbohydrate composition. The present report will review the literature of the multiple AFP receptors previously including their cellular uptake, transmembrane passage, and partial biochemical characterization. . In addition, evidence derived from computer modeling, proteolytic/fragmentation cleavage patterns, domain structure analysis, and protein binding software analysis will be presented in a proposed identification of a widespread protein/gene family of transmembrane proteins which fits many, if not most, of the criteria attributed to the AFP receptor. The proposed receptor protein family is tentatively identified as an epithelial cell surface mucin constituting one (or more) of many classes of single-pass transmembrane proteins. Present data do not support the concept that the AFP receptor is a "universal" tumor receptor and/or biomarker, but rather a widespread mucin protein that functions primarily in protecting and lubricating epithelial mucosal layers, and engaging in signal transduction; the mucin only binds AFP as a molecule serving in a subordinate or ancillary function.

Combination of CA125 and RECAF biomarkers for early detection of ovarian cancer

Ovarian cancer can be cured in up to 90% of cases if diagnosed early. CA125, the most studied ovarian cancer biomarker, exhibits poor sensitivity for detecting early disease stages and low specificity to malignancy. RECAF, the alpha-fetoprotein receptor, is a wide-spectrum oncofetal antigen with clinical potential for cancer diagnosis, screening, and monitoring. This study evaluated the performance of RECAF as a diagnostic tool and the sensitivity of a combination of RECAF and CA125 to detect early stages of ovarian cancer at a cutoff resulting in 100% specificity among healthy women. This retrospective case–control study was designed to measure the serum levels of RECAF and CA125 in normal individuals (n = 106) and cancer patients stages I/II (RECAF, n = 32; CA125, n = 35) and III/IV (RECAF, n = 49; CA125, n = 51). A competitive chemiluminescence assay was developed to measure the circulating RECAF. To eliminate any false positives, we classified as positive any patient with a RECAF or a CA125 value higher than their respective 100% specificity cutoff. We have shown that RECAF discriminated cancer and healthy donors better than CA125, particularly in the early stages (AUC_RECAF = 0.96 and AUC_CA125 = 0.805). CA125 sensitivity was lower in the early stages than in the advance stages; RECAF sensitivity was high at all stages. A combination of CA125 and RECAF detected three out of four early-stage patients, with no false positives. In conclusion, the combination of RECAF and CA125 serum values provides the specificity and the sensitivity necessary to screen for ovarian cancer and in particular, to detect early stages of the disease.

Targeted delivery of anti-cancer growth inhibitory peptides derived from human alpha-fetoprotein: review of an International Multi-Center Collaborative Study

The alpha-fetoprotein derived growth inhibitory peptide (GIP) is a 34-amino acid peptide composed of three biologically active subfragments. GIP-34 and its three constituent segments have been synthesized, purified, and studied for biological activity. The GIP-34 and GIP-8 have been characterized as anticancer therapeutic peptides. An multicenter study was initiated to elucidate the means by which these peptide drugs could be targeted to tumor cells. The study first established which cancer types were specifically targeted by the GIP peptides in both in vitro and in vivo investigations. It was next demonstrated that radiolabeled peptide ((125)I GIP-34) is specifically localized to rodent breast tumors at 24 h post-injection. The radionuclide studies also provided evidence for a proposed cell surface receptor; this was confirmed in a further study using fluorescent-labeled GIP-nanobeads which localized at the plasma membrane of MCF-7 breast cancer cells. Finally, it was readily demonstrated that GIP conjugated to either fluorescein or doxorubicin (DOX) underwent tumor cell uptake; subsequently, DOX-GIP conjugates induced cytotoxic cell destruction indicating the utility of GIP segments as cancer therapeutic agents. Following a discussion of the preceding results, a candidate cell surface receptor family was proposed which correlated with previous published reports for a putative AFP/GIP receptor.

Review of the putative cell-surface receptors for alpha-fetoprotein: identification of a candidate receptor protein family

The identification of a receptor for alpha-fetoprotein (AFP) has long been sought in the field of medicine. The uptake and endocytosis of AFP by rat tumor cells in 1984 sparked a series of confirmatory reports and the original studies were then extended to include multiple tumor types in rats, mice, and humans. The following year, French investigators partially characterized the binding properties of the AFP receptor, but they were not able to purify the receptor. It was not until 1991-1992 that an AFP receptor was partially purified and characterized from both human monocytes and breast cancer cells. By 1993, a monoclonal antibody had been raised against the AFP receptor produced from a breast cancer extract with claims that the receptor was a widespread (universal) oncofetal biomarker for cancer. However, that receptor has yet to be cloned and/or purified due to its complex multimeric binding interactions and associations. The present report will review the literature of the multiple putative AFP receptors described to date, the cellular uptake and endocytosis of AFP, and the biochemical characterization of these putative cell-surface proteins. In addition, evidence derived from computer modeling, proteolytic degradation patterns, and amino acid sequence analysis will be presented in a proposed identification of a family of multi-ligand binding receptors; this family fits many, if not most, of the criteria required for an AFP receptor. The purposed receptor protein family is tentatively identified as the Scavenger receptors which comprise several classes of single- and double-pass integral transmembrane proteins. Present data do not support the concept that the AFP receptor is a "universal" tumor receptor and/or biomarker.

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.

Uptake of radiolabelled alpha-fetoprotein by experimental mammary adenocarcinoma and adenoma: in vivo and in vitro studies

The biodistribution of iodine-labelled alpha-fetoprotein ( I-AFP) in experimental mammary tumours was studied. C3H mice with subcutaneously transplanted mammary adenocarcinoma and Sprague-Dawley rats treated with -methyl- -nitrosourea for mammary adenoma induction were used as animal models. The accumulation of labelled I-AFP in mouse mammary adenocarcinoma was significantly higher than that in rat mammary adenoma. The tumour/muscle radioactivity ratios increased with time and, 48 h after intravenous injection, were estimated as 23.4 and 6.7, respectively. For experiments, extracts from both mammary tumours were prepared. The extracts were subjected to sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE), transferred to polyvinylidene difluoride (PVDF) membranes and incubated with I-AFP. A single major AFP-binding protein with a molecular weight of about 30 kDa was detected in both extracts. The amount of AFP-binding protein was clearly higher for adenocarcinoma than for adenoma. In the presence of cross-linking reagent, I-AFP formed a complex (about 100 kDa) with adenocarcinoma proteins.

A direct role for serum albumin in the cellular uptake of long-chain fatty acids

The interaction of long-chain fatty acids with cells is important for their uptake and metabolism, as well as their involvement in signalling processes. The majority of long-chain fatty acids circulating in plasma exist as complexes with serum albumin. Thus an understanding of the involvement of serum albumin in these processes is vitally important. The effect of serum albumin on the uptake of long-chain fatty acids was studied in 3T3-L1 adipocytes. Serum albumin had a stimulatory effect on oleate uptake at all ratios of oleate: serum albumin tested. Furthermore, the rate of oleate uptake was saturable with increasing concentrations of serum albumin when the oleate: serum albumin ratio, and therefore the concentration of uncomplexed oleate, remained constant. This was not due to uptake being limited by dissociation of oleate from serum albumin, because oleate did not appear to be limiting. Furthermore, at very high ratios of oleate: serum albumin, when the concentration of uncomplexed oleate was predicted to be large relative to the amount of oleate taken up by cells, the rate of oleate uptake was still dependent on the albumin concentration. Serum albumin, covalently labelled with the photoreactive fatty acid 11-m-diazirinophenoxy[11-3H]undecanoate, bound to cells in a manner exhibiting both saturable (Kd 66.7 microM) and non-saturable processes. These results indicate that the stimulatory effect of serum albumin on the rate of oleate uptake is due to a direct interaction of serum albumin with the cells and point to an involvement of albumin binding sites in the cell surface in the cellular uptake of long-chain fatty acids.

Alpha-fetoprotein binding proteins: implications for transmembrane passage and subcellular localization

Alpha-fetoprotein (AFP) is an oncofetal protein classified in a super-family together with albumin and Vitamin-D binding (Gc) protein which present as globular proteins comprised of three domains. Several subdomain regions on AFP have been previously proposed to serve as dimerization interfaces for nuclear receptors or perhaps other co-factors/inhibitors. The cellular uptake and internalization of AFP together with its subcellular compartmentalization is now well documented in a variety of cell types. A myriad of reports have emerged which have detected, identified, and characterized various binding proteins associated with AFP in different cellular compartments. However, the literature is devoid of any attempts to summarize, categorize, and relate these proteins to the various physiological activities attributed to this fetal protein. It is conceivable that AFP could interact and/or bind cytoplasmic chaperone proteins that normally escort nuclear factors or transcription co-factors through the cytoplasm toward organelle interfaces. A dual concept proposing binding or escort proteins for AFP together with subdomain dimerization interfaces on the AFP molecule can be reconciled into a composite hypothesis to formulate a rationale for the growth regulating properties ascribed to AFP during the last decade. Thus, AFP might serve as a modulator/modifier of various cell growth regulatory pathways during embryonic and fetal development in vertebrates.

Endocytosis of three serum proteins of a multigene family and of arachidonic acid in human lectin-stimulated T lymphocytes

Alpha-fetoprotein (AFP), serum albumin (SA), and vitamin D binding protein (DBP) are members of a multigene family of proteins showing high structural homology. AFP and SA exhibit a reciprocal relation during development and carry mostly fatty acids, while DBP carries vitamin D and its metabolites in the plasma. Covalent conjugates of these proteins with horseradish peroxidase (HRP) were used to follow by cytochemistry, at the electron microscope level, the protein uptake and intracellular pathways in peripheral blood human lymphocytes stimulated to blast formation by phytohemagglutinin (PHA). Transferrin (Tf), an iron-binding plasma protein, was used as a control. Combined with the results of competition and saturability experiments reported elsewhere, the ultrastructural observations are in favour of a specific endocytosis of the four proteins through cell surface receptors. Tf and AFP enter the cells via small vesicles and endosomes and move to multivesicular bodies (MVBs) and tubular vesicular elements located in the Golgi-centrosphere region to be finally recycled back into the medium. A noncovalent conjugate of AFP-HRP with 3H arachidonic acid [3H-(20:4)] is strongly internalized at 37 degrees C in PHA-stimulated lymphocytes; the autoradiographic labelling, localized in cellular membranes and mostly in lipid droplets, was only occasionally associated with organelles where the presence of AFP-HRP was cytochemically detected. SA, which competes with AFP for a common binding site on the surface of activated T cells, is endocytosed through small vesicles, endosomes, and MVBs before being released in a degraded form from the cells, in agreement with the localization of SA-HRP in lysosome-like organelles. DBP-HRP is poorly internalized through noncoated vesicles, endosomes, and MVBs and is finally routed to lysosomes. The physiological role of AFP and SA would be to mediate the transfer of fatty acids into cells, while that of DBP would be to facilitate the intracellular delivery of vitamin D.

Carrier-protein-mediated enhancement of fatty-acid binding and internalization in human T-lymphocytes

Albumin and alpha-fetoprotein (AFP) are members of a multigene family which also includes vitamin-D-binding protein. Previous work in our laboratory has provided experimental support for the suggestion that the entry of unsaturated fatty acids into growing, normal and neoplastic cells may be regulated by AFP. In the actual study we have examined the role of human serum albumin (HSA) as a carrier protein, when compared to AFP, on the uptake (binding and internalization) of fatty acids by resting and PHA-activated human lymphocytes. Radioiodinated human HSA and tritiated oleic and arachidonic acids were used under different experimental conditions to follow the binding of the protein and fatty acids (FA) to cells. Time-course uptake at 4 degrees C of HSA and of oleic and arachidonic acids bound to HSA (FA/HSA molar ratio = 1) by either resting or activated T-lymphocytes exhibited a steady state of binding. The amount of FA bound was much greater than the corresponding amount of HSA, suggesting that T-lymphocytes bear distinct binding sites for albumin and fatty acids. A saturable process of FA binding was observed at constant unbound FA concentration in the incubation medium when the HSA-to-FA molar ratio was fixed at 1 and the concentrations of both HSA and FA were increased simultaneously. This saturable component of binding reflects an intrinsic regulatory effect of HSA, probably operating throughout the interaction of the protein with specific cell receptors. At varying unbound FA concentrations, binding curves showed two distinct components: a non-linear portion which could indicate the presence of a saturable process operating at low concentrations of unbound, free FA, followed by a second part which increased linearly with the concentration of unbound FA. The amount of FA bound at 4 degrees C and bound and internalized at 37 degrees C by both types of cell was considerably higher in the presence than in the absence of carrier proteins. On the contrary, carrier proteins were without effect on the distribution pattern of internalized oleic or arachidonic acid. Taken together, these observations suggest that: (i) the binding and entry of FA into cells is enhanced by the two carrier-proteins at low concentrations of free, unbound fatty acids in the vicinity of the cell surface, and (ii) fatty-acid uptake seems regulated by a dual-receptor mechanism involving HSA and/or AFP receptors as well as plasma-membrane FA-binding proteins.