Conformational changes in rodent and human alpha-fetoprotein: influence of fatty acids

Comprehensive approach to cancer immunotherapy - Simultaneous targeting of myeloid-derived suppressor cells and cancer cells with AFP conjugates

The immune system plays a pivotal role in combating diseases, including cancer, with monocytes emerging as key regulators of immune response dynamics. This article describes a novel strategy for cancer treatment centered on depleting myeloid-derived suppressor cells (MDSCs), to enhance the overall immune response while simultaneously targeting cancer cells directly. Alpha-fetoprotein (AFP) is an oncofetal protein that plays an important role in delivering nutrients to immature monocytes, embryonic, and cancer cells in a targeted manner. AFP can be repurposed, making it a vehicle for delivering toxins, rather than nutrients to kill cancer cells and deplete MDSCs in the tumor microenvironment (TME). Depleting monocytes not only stimulates the immune system but also improves the lymphocyte-to-monocyte ratio (LMR), often low in cancer patients. AFP combined with cytotoxic drugs, offers dual benefit-immune stimulation and targeted chemotherapy. Studies in xenograft models demonstrated high efficacy and safety of AFP-toxin conjugates, surpassing conventional targeted chemotherapy. Such conjugates have also been reported to provide superior efficacy and safety in cancer patients compared to chemotherapy. This approach, using AFP conjugated with toxins, either covalently or non-covalently, presents a safe and highly effective option for cancer immuno/chemotherapy.

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.

Maternal and fetal serum transformed alpha-fetoprotein levels in normal pregnancy

AIM

To evaluate transformed alpha-fetoprotein (t-AFP) (a new molecular conformation of alpha-fetoprotein) levels in maternal serum and fetal serum in normal pregnancy.

METHODS

Prospective longitudinal study. Fifty pregnant women were studied in two groups: 25 were evaluated in each trimester of pregnancy and near term (12, 20, 32 and 36 weeks) and the other 25 were evaluated at the time of planned cesarean section at term. In the first group, maternal serum t-AFP was measured and in the second group, maternal and fetal serum t-AFP were analyzed.

RESULTS

Maternal serum t-AFP levels (medians) were 14.73 ng/mL in the first trimester, 28.29 ng/mL in the second trimester, 30.45 ng/mL in the early third trimester and 8.06 ng/mL in late pregnancy. t-AFP levels were significantly higher in maternal than in fetal serum (P < 0.001). There were no significant correlations between AFP and t-AFP levels in maternal versus fetal serum.

CONCLUSIONS

t-AFP increases during pregnancy until the early third trimester and then falls before delivery. t-AFP levels are higher in maternal than in fetal serum which suggests that native AFP is transformed to t-AFP either in the mother or in the placenta.

The alpha-fetoprotein-derived growth inhibitory peptide 8-mer fragment: review of a novel anticancer agent

This review describes the antigrowth and anticancer activities of the alpha-fetoprotein (AFP)-derived growth inhibitory peptide (GIP) 8-mer fragment. The 8-amino acid peptide (GIP-8) comprises the carboxy-terminal portion of a 34-amino acid peptide (GIP-34) previously identified as an occult epitopic segment of the full-length human AFP molecule. The GIP-8 segment has been chemically synthesized, purified, characterized, and bioassayed. The purified 8-mer segment was characterized as a random coil (disordered) structure extending from a C-terminal beta-hairpin that forms a horseshoe-shaped partially cyclic octapeptide; this structure can be formulated into a fully cyclic form by the addition of asparagine or glutamine residues. The pharmacophore of the octo- and nanopeptide forms is largely composed of a PXXP motif known to interact with Src-3 (SH3) domains of serine/theronine kinases. The GIP-8 has been shown to be growth-suppressive largely in estradiol (E2)-dependent neonatal and tumor-cell proliferation models and to inhibit tumor-cell adhesion to extracellular matrices. The 8-mer GIP displays antigrowth properties in immature mouse uterine cells and anticancer cell proliferation traits in estrogen receptor positive (ER(+)), but not (ER()) negative breast tumor cells. Even though its mechanism of action has not been fully elucidated, GIP-8 has been shown by computer modeling to dock with the extracellular loops of G-coupled seven transmembrane helical-like receptors, which could possibly interfere with signal transduction through MAP kinase pathways. It was apparent that the GIP-8 derived from the 34-mer GIP fragment of HAFP represented an E2-sensitive growth inhibitory motif, which allows the participation in cellular events, such as receptor binding, contact inhibition, extracellular matrix adhesion, angiogenesis, and T-cell activation. Thus, it was proposed that the 8-mer fragment derived from GIP could potentially serve as a lead compound for targeted cancer therapeutic agents of the biologic-response modifier type.

Biological roles of alpha-fetoprotein during pregnancy and perinatal development

The use of alpha-fetoprotein (AFP) as a serum marker in cancer actually predates its employment in the detection of congenital defects; however, the latter use of AFP as a fetal defect marker has propelled its clinical utilization. Although the serum-marker capacity of AFP has long been exploited, less is known of the biological activities of this oncofetal protein during fetal and perinatal development. In the present review, the biological activities of AFP are discussed in light of this glycoprotein's presence in various biological fluid compartments: embryonic and fetal tissues, serum, urine, and reproductive fluids. After a review of the histochemical detection of AFP in various cells and tissues during development, AFP concentrations within various biological fluids were discussed in the context of gestational age and anatomic location. Discussion follows concerning the relationships and roles of AFP in developmental events such as erthyropoiesis, histogenesis/organogenesis, and ligand binding and in developmental disorders such as hypothyroidism, folate deficiencies, and acquired immunodeficiency disorder (AIDS). Based on its association with so many types of birth defects, malformations, and congenital anomalies, AFP can be viewed as a molecular "troubleshooter" until signal transduction pathways are established during pregnancy and prenatal development. The review concludes with a discussion of the place of AFP in the rapidly expanding field of proteomics.

Alpha-fetoprotein gene expression in early and full-term human trophoblast

Alpha-fetoprotein (AFP) is a major serum glycoprotein synthesized during fetal life mainly by the yolk sac and the fetal liver. At term, it reaches high concentrations in the maternal intervillous blood, which is in direct contact with the placental trophoblastic microvillous membrane, and this suggests the placental origin of the AFP at the fetal-maternal interface. We used several experimental approaches to investigate the expression of AFP gene and fetal protein production in early gestation and term placentas. RT-PCR and immunological studies clearly identified AFP messenger RNA and AFP protein in the placental villi from first trimester of pregnancy. The AFP gene was also expressed in highly purified cytotrophoblasts from early placentas, and enzymo-immunoassay showed that AFP protein was synthesized and secreted by early cytotrophoblasts. AFP was also detected in the cytoplasm of these cells by immuno-cytochemistry. However, none of these methods detected any expression of the AFP gene in full-term placental villi or in cultured trophoblasts. These findings demonstrate that both AFP mRNA and protein are present in trophoblastic cells early in pregnancy. The absence of AFP gene expression in term placental villi also suggests, that the AFP at the fetal-maternal interface is attributable to a notable transplacental passage of AFP from fetal blood in late pregnancy.

Fatty acids, alpha-fetoprotein, and cystic fibrosis

Docosahexaenoic acid, found lacking in animal models of cystic fibrosis, has been proposed as a dietary supplement therapy for this genetic disorder. Alpha-fetoprotein, which binds and transports docosahexaenoic acid, may be a useful marker to improve the management and follow-up in newborn screening programs for cystic fibrosis, because only 20% of such infants are diagnosed at birth.

Studies on analogs of a peptide derived from alpha-fetoprotein having antigrowth properties

A 34-amino acid portion of the third domain of alpha-fetoprotein possesses antigrowth and anticancer activities. Three analogs of this sequence were chemically synthesized, in which the two cysteines of the original sequence were replaced by alanines, glycines or serines. The original cysteine and alanine peptides formed trimers at 0.20 g/L in pH 7.4 phosphate buffer, and the glycine and serine peptides formed dimers. Trimer preparations were more potent in inhibiting estrogen-induced growth in the mouse uterine assays than the two dimeric oligomers. Of salient importance is that the alanine peptide retained its trimeric form in solution much longer than the cysteine peptide. Antigrowth assays were performed starting with stock solutions at a peptide concentration of 0.20 g/L, because at very high peptide concentration (8.0 g/L) the peptides aggregated extensively. All the peptides, although differing in biological activity, had almost identical secondary structures. Unlike alpha-fetoprotein, the three peptides have low amounts of alpha-helix. Trifluoroethanol has the ability to convert peptides into a helical conformation when they have a propensity for that structure. At trifluoroethanol concentrations of 20% and higher, the alanine and glycine peptides were changed into highly helical structures.

Studies on a growth-inhibitory peptide derived from alpha-fetoprotein and some analogs

A 34-amino acid synthetic peptide was derived from the third domain of human alpha-fetoprotein, and the peptide was shown to inhibit estrogen-stimulated growth. Under certain conditions, however, the peptide lost growth-inhibitory activity. A biophysical study of the peptide was undertaken with a goal of obtaining completely reliable preparations. The peptide was studied using gel-filtration column chromatography as a function of peptide concentration and age of solution, and was found to exhibit complex aggregation behaviors. During the early period (0-3 h) after dissolving lyophilized peptide into pH 7.4 buffer, solutions were composed mostly of trimers. At higher peptide concentrations (> or = 3.0 g/L), the trimers aggregated extensively to a large aggregate (minimum size approximately 102 peptides). At 5.0-8.0 g/L, these large aggregates increased in size (up to approximately 146 peptides) until trimers were largely exhausted from solution. During the later times (>3 h) after sample preparation, the trimeric oligomer of the peptide dissociated slowly to form dimers for samples at 0.10-3.0 g/L. After their build-up, a very small number of dimers associated to form hexamers. Disulfide bonds stabilized the dimers as indicated by the conversion of dimers to trimers upon the addition of a reducing agent, and the failure of dimers to form in the presence of reducing agent. Reducing agent did not affect trimer or large aggregate formation. Trimers were found to be active in an assay monitoring inhibition of estrogen-stimulated growth, whereas dimers and large aggregates were inactive. The two cysteines in the peptide were modified to either S-methylcysteine or S-(2-aminoethyl)cysteine, and both derivatives showed significant growth-inhibition activity. A serine analog in which both cysteines were replaced had very different aggregation behavior than the cysteine peptide and lacked its growth inhibitory ability. Peptide aggregation is critically important in establishing the ability of the peptide to inhibit growth and have anticancer activity, but the state of its two cysteines is of little influence.

Docosahexaenoic acid-containing phosphatidylcholine affects the binding of monoclonal antibodies to purified Kb reconstituted into liposomes

Class I major histocompatibility complex (MHC I) molecules are transmembrane proteins that bind and present peptides to T-cell antigen receptors. The role of membrane lipids in controlling MHC I structure and function is not understood, although membrane lipid composition influences cell surface expression of MHC I. We reconstituted liposomes with purified MHC I (Kb) and probed the effect of lipid composition on MHC I structure (monoclonal anti-MHC I antibody binding). Four phospholipids were compared; each had a phosphocholine head group, stearic acid in the sn-1 position, and either oleic, alpha-linolenic, arachidonic, or docosahexaenoic acid (DHA) in the sn-2 position. The greatest binding of monoclonal antibody AF6-88.5, which detects a conformationally sensitive epitope in the extracellular region of the MHC I alpha-chain, was achieved with DHA-containing proteoliposomes. Other epitopes (CTKb, 5041.16.1) showed some sensitivity to lipid composition. The addition of beta2-microglobulin, which associates non-covalently with the alpha-chain and prevents alpha-chain aggregation, did not equalize antibody binding to proteoliposomes of different lipid composition, suggesting that free alpha-chain aggregation was not responsible for disparate antibody binding. Thus, DHA-containing membrane lipids may facilitate conformational change in the extracellular domains of the alpha-chain, thereby modulating MHC I function through effects on that protein's structure.

Regulation of alpha-foetoprotein gene expression by fatty acids and fibrates

Alpha-foetoprotein (AFP), the major plasma protein in the foetus, is mainly synthesized by yolk sac and foetal liver. It binds polyunsaturated fatty acids and probably controls their metabolism and action. We investigated the effects of fatty acids and fibrates on expression of the AFP gene using two complementary approaches. Treatment with 5-8-11-14 eicosatetraynoic acid (ETYA), an analogue of arachidonic acid, specifically led to lower AFP mRNA levels in cultured rat yolk sac explants whereas treatment with palmitic or oleic acid did not. Clofibric acid and fenofibrate also gave lower AFP mRNA levels. Transient transfection experiments with HepG2 hepatoma cells showed that ETYA and clofibric acid decreased the transcriptional activity of the 7 kb regulatory region of the rat AFP gene. The 330 bp AFP promoter was identified as a target for these down regulating effects.

Does high polyunsaturated free fatty acid level at the feto-maternal interface alter steroid hormone message during pregnancy?

Polyunsaturated fatty acids (PUFA) are important in pregnancy, fetal development and parturition. We measured free fatty acids (FFA), albumin and alpha-fetoprotein (AFP) in the maternal and fetal circulations of women undergoing elective Caesarean section at term. We also studied the impact of PUFAs on estrogen (ER) and progesterone receptors (PR) binding properties in vitro in the myometria of pregnant women and ex vivo in human myometrial cells in culture. FFA in intervillous blood (I) (feto-maternal interface) and maternal peripheral blood (M) were similar, while those in the umbilical vein (V) and arteries (A) were 2-4 fold lower (P<0.001). PUFA levels were low in M and 3 fold higher in I, A and V (P< 0.001); consequently C20:4 and C22:6 were most abundant in intervillous space. Albumin was uniformly distributed throughout the maternal-fetal unit, but there was a transplacental gradient in AFP. The AFP in the intervillous space had a special conformation (less immuno-reactive, more anionic), suggesting loading with PUFA. Physiological concentrations of C20:4 stimulated estradiol binding, but inhibited progestin binding. C20:4 inhibited progesterone binding by decreasing the number of binding sites, with no change in apparent affinity, in vitro in myometrial tissue and ex vivo in myometrial cells. Thus PUFA may modulate the steroid hormone message, so that the high C20:4 concentration at the maternal-fetal interface at term may help amplify the estrogen signal and inhibit the progesterone signal.

Interactions between phytoestrogens and human sex steroid binding protein

The interactions of human Sex steroid binding protein (SBP) and the lignans [Nordihydrogaiaretic acid (NDGA) enterolactone (Ent), enterodiol (End)] and isoflavonoid phytoestrogens [Equol (Eq), diazein Dad), genistein (Gen)] were studied. The phytoestrogens had different dose-dependent inhibitory effects on steroid binding by SBP. Their relative efficiencies were: Ent> or = NDGA = Eq > Gen for displacing E2 and Eq > Ent > NDGA > Gen for displacing T. End and Dad were much less active. Scatchard analysis suggested that NDGA had similar non- competitive effects on T and E2 binding by reducing the number of binding sites without changing the association constants. But Eq seemed to inhibit E2 binding non-competitively and T binding competitively. NDGA binding to SBP reduced the immunorecognition of SBP by monospecific anti-SBP antibodies, suggesting that NDGA changed SBP immunoreactivity. Unlike NDGA, Eq binding to SBP caused no immunological changes in SBP, indicating qualitative differences in the effects of the lignan and isoflavonoid. Our results indicate that phytoestrogens may modulate the SBP activity and so influence the role of this protein in the delivery of hormonal information to sex steroid-dependent cells.

Modulation of glucocorticoid binding to rat liver cytosol receptor by lipid-soluble extracts from the serum of AIDS patients

The total liposoluble extract of sera from AIDS patients, IVC1 and IVD stages, containing cortisol and free fatty acids (FFA) inhibited [3H]dexamethasone binding to a lesser extent than did the same quantity of total liposoluble extract of sera from healthy men. FFA isolated from extracts of AIDS sera by Sephadex LH20 chromatography had less effect on [3H]dexamethasone binding to rat liver glucocorticoid receptor than those extracted from sera of healthy men. These results suggest the presence in sera of AIDS patients of a liposoluble substance which could be limiting the inhibitory effect of FFA on [3H]dexamethasone binding to glucocorticoid receptor by inducing a conformational change in glucocorticoid receptor that could alter the biological action of glucocorticoids. The pathological consequence could be the apparent contradiction of high cortisolemia and clinical symptoms of adrenal insufficiency that have been observed in AIDS patients.

Alterations in the concentrations and binding properties of sex steroid binding protein and corticosteroid-binding globulin in HIV+patients

The abnormal concentrations of steroid hormones and free fatty acids in the plasma of HIV-infected subjects are associated with qualitative and quantitative alterations in two of the major steroid hormones carrier proteins, sex steroid-binding protein (SBP) and corticosteroid-binding globulin (CBG). The properties of SBP and CBG in the sera of two age-matched groups of 67 men healthy blood donors (controls) and 64 HIV+subjects: 11 CDC group II and III (ASY), 6 CDC group IVA and 47 groups IV C1+D (AIDS) were analyzed. The HIV+patients had SBP concentrations 39-51% above those of controls. The sera of AIDS patients had higher SBP association constants (Ka) for testosterone than did those of the II, III and IVA groups and controls. In contrast, the CBG concentrations in all the HIV+subjects were similar to those of the controls. However, the binding properties of HIV+CBG were abnormal: the Ka's for cortisol and 17 alpha hydroxyprogesterone binding were 50% below normal, while the number of binding sites was significantly higher. Such changes in these carrier proteins could result from conformational transitions; they may cause abnormal transfer of hormonal information and/or steroid hormones metabolism, thus modifying the impact of steroids on the immune response in HIV+subjects.

In vivo transient rise in plasma free fatty acids alters the functional properties of alpha-fetoprotein

Previous in vitro studies have shown that unsaturated fatty acids (UFA) induce conformational changes in rodent and human alpha-fetoprotein (AFP). To determine whether such changes in the binding and immunological properties of rat AFP also occur in vivo, plasma free fatty acid (FFA) concentrations were increased in young male rats (15, 21 and 28 days old) by acute i.v. injection of heparin (200 IU/kg). Plasma estrogens (estrone and estradiol) did not change after injection of heparin. There was a large increase in plasma FFA 10-20 min post-heparin injection, with a return to normal 60 min later. This transient rise in FFA plasma was associated with a 50% drop (P less than 0.001) in the binding of estradiol to rat AFP of 15-, 21- and 28-day-old rats by reducing the number of binding sites (P less than 0.001), leaving the affinity constant (Ka) unchanged. FFA extracts from post-heparin plasma induced similar changes in estradiol binding to purified rat AFP. The rise in plasma FFA induced a loss of AFP immunoreactivity, in 21- (P less than 0.001) and 28-day-old rats (P less than 0.001), but not in 15-day-old rats. This age-dependent response correlated with the FFA/AFP molar ratio (38 in 15-day-old rats, 388 in 21-day-old rats, and 5600 in 28-day-old rats). These results indicate that an in vivo rise in FFA induces rapid and reversible conformational changes in AFP which may modulate the endocrine and immune function of this oncofetal protein.

Effect of alpha-fetoprotein and albumin on the uptake of polyunsaturated fatty acids by rat hepatoma cells and fetal rat hepatocytes

The uptake of polyunsaturated fatty acids by rat hepatoma cells and fetal hepatocytes has been studied using albumin and alpha-fetoprotein (AFP) as carrier proteins. Both types of cells took up linoleic (18:2(n-6)) and linolenic (18:3(n-3)) fatty acids at the same rate when they were added complexed either to albumin or AFP at a 1:1 molar ratio. At 37 degrees C a greater incorporation of arachidonic acid (20:4(n-6)) and mainly docosahexaenoic acid (22:6(n-3)) was observed when these acids were bound to albumin (6.5 nmol 20:4(n-6); 6.4 nmol 22:6(n-3) /million cells) as compared to AFP (5.5 nmol 20:4(n-6); 4.3 nmol 22:6(n-3)/million cells). The 20:4(n-6) and 22:6(n-3) uptake seems to be inversely related to the apparent association constants (k'a) between these fatty acids and both proteins (1.3 and 1.5 x 10(-7) M-1 20:4(n-6) and 22:6(n-3) for albumin; 6.4 and 54.0 x 10(-7) M-1 20:4(n-6) and 22:6(n-3) for AFP). Experiments carried out at 4 degrees C indicated that binding of 20:4(n-6) (0.83 nmol/million cells in presence of albumin; 2.16 nmol/million cells in presence of AFP) and 22:6(n-3) (0.83 nmol/million cells in presence of albumin; 1.32 nmol/million cells in presence of AFP) by cell membranes was also inversely related to the k'a of these proteins. At 4 degrees C, the k'a of AFP and albumin for 20:4(n-6) and 22:6(n-3) changed considerably (12.7 and 9.6 x 10(-7) M-1 20:4(n-6) and 22:6(n-3) for albumin; 3.9 and 14.6 x 10(-7) M-1 20:4(n-6) and 22:6(n-3) for AFP) with respect to the k'a calculated at 37 degrees C. Hence, k'a values were higher for albumin and lower for AFP than the corresponding values at 37 degrees C. It was concluded that uptake by cells and interaction of fatty acids with cell membranes depend mainly on the k'a of fatty acids and carrier proteins at equilibrium.

Localization of the estrogen-binding site of alpha-fetoprotein in the chimeric human-rat proteins

Rat alpha-fetoprotein (AFP) has been demonstrated to bind estrogen, whereas human AFP lacks the activity. We constructed four chimeric molecules from cDNAs encoding these AFPs with the use of two restriction sites common to them and expressed them as well as rat and human AFP cDNA in yeast. The recombinant molecules were purified, characterized, and found to have the predicted structures. Analyses of estrogen binding indicated that a rat AFP sequence composed of residues 423-506 that contains 31 rat-specific amino acids is essential for the activity.