Control of growth of estrogen-sensitive cells: role for alpha-fetoprotein

Proof-of-Concept Study of an Alpha-Fetoprotein-Derived Peptide for the Management of Canine Mammary Cancer

Novel, well-tolerated drugs are needed for the management of canine mammary cancer. Many of these cancers are promoted in their growth by estrogen. Alpha-fetoprotein (AFP) is a ubiquitous mammalian protein that has anti-estrogenic properties. AFPep (the anti-estrogenic site of AFP) has been developed into a readily synthesizable drug. AFPep has been shown to have anti-mammary cancer activity in several models of this disease, both in cell culture and in rodents. The purpose of the study reported herein was to determine the tolerability of AFPep in normal and tumor-bearing dogs. AFPep was given to dogs via both parenteral and oral routes in a single application and in repeated daily doses. Full clinical chemistry and hematology values were determined before and after drug administration. Blood levels of the drug were achieved in dogs that had been previously found to be oncostatic in rodents. No changes in clinical chemistry, hematology, and clinical behaviors were found in dogs following drug administration. The data support the further development of AFPep for clinical use against canine mammary cancer.

A proposed unified mechanism for the reduction of human breast cancer risk by the hormones of pregnancy

Parity in women is associated with reduced lifetime risk of breast cancer, and hormones of pregnancy [estrogen (E), progesterone (P), human chorionic gonadotropin (hCG)] are implicated. Parity also reduces mammary cancer risk in carcinogen-exposed rats, and administering pregnancy hormones to these animals is similarly effective. Because pregnancy hormones are also able to stimulate cancer growth, we proposed to resolve this dichotomy by determining whether administered pregnancy hormones elicit the cancer-inhibiting agent alpha-fetoprotein (AFP) from the liver, which would implicate AFP as a proximal effector of hormonal anticancer activity. Accordingly, we treated groups of nitrosomethylurea-exposed rats with saline, E(3), E(2) + P, E(3) + P, hCG, or allowed them to experience pregnancy, and then monitored mammary cancer incidence and serum levels of AFP over time. Each hormone treatment reduced mammary cancer incidence and elevated serum AFP levels. To challenge human tissues, human HepG2 liver cells in culture were treated with the same hormonal agents. Each hormone regimen increased the levels of AFP in the culture medium. Medium containing AFP elicited by hCG inhibited the E(2)-stimulated proliferation of cultured human MCF7 breast cancer cells, whereas hCG alone did not inhibit their growth. Furthermore, antibodies to AFP neutralized the growth-inhibiting effect of AFP-containing HepG2 medium. We conclude that in the treatment of carcinogen-exposed rats with the hormones of pregnancy, and by inference in women who have experienced pregnancy, that AFP is a proximal agent that inhibits mammary gland cancer.

Estradiol stimulates progenitor cell division in the ventricular and subventricular zones of the embryonic neocortex

Two distinct populations of cerebral cortical progenitor cells that generate neurons during embryogenesis have been identified: radial glial cells and intermediate progenitor cells. Despite advances in our understanding of progenitor cell populations, we know relatively little about factors that regulate their proliferative behaviour. 17-beta-Estradiol (E2) is present in the adult and developing mammalian brain, and plays an important role in central nervous system processes such as neuronal differentiation, survival and plasticity. E2 also stimulates neurogenesis in the adult dentate gyrus. We examined the role of E2 during embryonic cortical neurogenesis through immunohistochemistry, in situ hybridization, functional enzyme assay, organotypic culture and in utero administration of estradiol-blocking agents in mice. We show that aromatase, the E2 synthesizing enzyme, is present in the embryonic neocortex, that estrogen receptor-alpha is present in progenitor cells during cortical neurogenesis, that in vitro E2 administration rapidly promotes proliferation, and that in utero blockade of estrogen receptors decreases proliferation of embryonic cortical progenitor cells. Furthermore, the E2 inhibitor alpha-fetoprotein is expressed at high levels by radial glial cells but at lower levels by intermediate progenitor cells, suggesting that E2 differentially influences the proliferation of these cortical progenitor cell types. These findings demonstrate a new functional role for E2 as a proliferative agent during critical stages of cerebral cortex development.

Expression and localization of alpha-fetoprotein mRNA and protein in human early villous trophoblasts

Alpha-fetoprotein (AFP) is a major plasma protein produced during human fetal life. It is a good marker for several possible disorders affecting gestation. We previously reported that afp gene expression, which takes place mainly in yolk sac and fetal liver, also occurs in normal human placenta, specifically in early pregnancy. The aim of the present study was to determine the precise location of AFP synthesis sites within the placental villi. In situ hybridization and immunohistochemical experiments were performed on sections obtained from placentas of first-trimester and full-term pregnancies. We found that the pattern of afp gene expression was restricted to specific villous trophoblastic areas in early placentas. Both afp transcripts and AFP protein were mainly located in discontinuous regions, at junctions between two villi and at budding sites. In contrast, no AFP expression was detected in the cytotrophoblastic extravillous proliferative zone or in other placental cell types. According to the earlier studies, no AFP synthesis was detected in placental villous tissue from full-term pregnancies, using in situ hybridization and immunohistochemistry.

Mammalian phytoestrogens: enterodiol and enterolactone

The mammalian phytoestrogens enterodiol (END) and enterolactone (ENL) are produced in the colon by the action of bacteria on the plant precursors matairesinol (MAT), secoisolariciresinol (SECO), their glycosides, and other precursors in the diet. Both END and ENL have been shown to possess weakly estrogenic and antiestrogenic activities, and it has been suggested that the high production of these antiestrogenic mammalian lignans in the gut may serve to protect against breast cancer in women and prostate cancer in men. Various in vitro experiments suggested END and ENL significantly inhibited the growth of human colon tumor cells, and the E2-induced proliferation of MCF-7 breast cancer cells was inhibited by ENL. The protective effects of mammalian lignans may be due to their ability to compete with E2 for the type II estrogen receptor, to induce sex hormone binding globulin (SHBG), to inhibit placental aromatase, and to act as antioxidants. This review mainly deals with the chemistry, quantitative analysis, biological properties and health effects of END and ENL.

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.

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.

Biosynthesis of alpha fetoprotein by MCF-7 human breast cancer cells

Direct evidence was obtained for de novo synthesis of AFP by MCF-7 human breast cancer cells per se. Synthesis was demonstrated by L-14C-leucine and L-35S-methionine incorporation into immunochemically isolated AFP, and confirmed by radioimmunodiffusion and radioimmunoelectrophoresis. This information indicates that AFP synthesis is associated with normal and neoplastic cells of several different histotypes, and suggests that AFP detected and measured previously in primary human breast cancer tissue cytosol (Sarcione et al., 1983) also resulted from in situ biosynthesis by breast cancer cells per se rather than uptake of exogenous AFP originating from extracellular sources. Evidence that AFP obtained after treatment of 14C-leucine radiolabelled MCF-7 breast cancer cell protein with 0.4 M KCl contained 2.6 times more radioactivity than did AFP obtained before such salt treatment is interpreted as indicating that two different molecular species of de novo synthesized AFP existed in breast cancer cells: (1) larger amount of non-immunoreactive AFP which became immunoreactive and measurable after KCl treatment, and (2) smaller amounts of free immunoreactive AFP. 14C-radiolabelled AFP obtained before and after treatment of cell protein with 0.4 M KCl codiffused, comigrated with alpha 1 electrophoretic mobility and gave an identical radioimmunologic reaction both with each other and with added carrier human cord serum AFP. Furthermore, preliminary studies indicated that radiolabelled non-immunoreactive AFP could be separated from lower-molecular-weight free AFP by chromatography on Sephadex G-200. Taken together, these findings suggest that synthesized free AFP was bound as a non-immunoreactive high-molecular-weight macromolecular complex rather than being covalently linked. Our current working hypothesis is that most of the de novo synthesized endogenous AFP in MCF-7 human breast cancer cells was rapidly and reversibly bound by hydrophobic bonding to a specific cytoplasmic AFP-receptor.

The role of nonesterified fatty acids and of alpha 1-fetoprotein in estrogen-dependent endocrine systems

We discuss the various experimental findings to date that indicate that AFP may be considered as a positive or negative modulator of estrogenic action. Moreover, we show that this protein, even when nonestrogenophilic, is able to bind other hydrophobic ligands, in particular nonesterified fatty acids. These fatty acids inhibit the binding of estrogens to murine AFP as well as to the cytosolic estrogen receptors. Thus, the AFPs of all species--whether or not estrogenophilic--might play an endocrinologic role through the intermediary of the unsaturated fatty acids to which they associate with high affinity.

Estradiol-activated alpha-fetoprotein suppresses the uterotropic response to estrogens

The binding of estrogen to alpha-fetoprotein (AFP) in the plasma cannot account for the impaired estrogen response seen in immature rodents because estradiol (E2) doses that far exceed the total body burden of AFP will stimulate only modest uterine growth. We investigated this phenomenon in immature female mice by determining their uterine weights 23 hr after intraperitoneal injection of estrogens or AFP or both. Administration of either 0.5 micrograms of E2 or 10 ng of moxestrol (MOX) approximately doubled the uterine weight. Giving 1 microgram of AFP 1 hr before injection of either estrogen did not alter that response. Combining the E2 and AFP just prior to injection resulted in decreased uterine growth (34% inhibition). Preincubating the estrogens with purified AFP (0.1-50 micrograms) did not affect the growth response to moxestrol but markedly decreased the response to E2. This was not due to sequestering of hormone because maximal reduction of the E2 response (ca. 65% inhibition) required only 1.0 microgram of AFP (AFP/E2 molar ratio, 1:130), and higher AFP doses inhibited less. About 40% of the growth elicited by injection of either 0.5 micrograms of E2 or 10 ng of MOX was inhibited when these doses were preceded by injection of the preincubated AFP/E2 mixture but not when preceded by either of the components. In each experiment, the mitotic index of luminal epithelium was affected to the same degree as uterine weight. AFP and E2 incubated for 1 hr thus produce a potent inhibitor of estrogen-stimulated mitotic activity and growth. This inhibitor might act upon estrogen-responsive cells at specific sites at which competition by an inactive component of AFP can block the process.

Cell multiplication in metazoans: evidence for negative control of initiation in rat fibroblasts

Experiments were performed to clarify the circumstances under which a rat fibroblast-like cell line proliferated in culture or when injected into syngeneic hosts. Rat FR3T3 and SV40FR3T3 cells in culture multiply exponentially at comparable rates regardless of the horse serum concentration in the culture medium. The concentration of horse serum in the medium determines the length of the exponential phase of cell multiplication. Heat inactivation of the serum for periods as long as 24 hr does not impair the ability of the serum to sustain maximal cell multiplication rates. At low serum concentrations the exponential phase may be prolonged by refeeding the cultures daily with fresh medium. In medium supplemented with 50% fresh rat serum, FRT3T3 and SV403T3 cells are prevented from multiplying by yet undefined component(s) in this serum. This correlates well with the lack of tumor formation by these cells when inoculated into syngeneic hosts. The evidence obtained in culture strongly suggests that the multiplication ability of established cells in culture is a dominant constitutive character of these cells that is expressed whenever they are provided with optimal supply of nutrients and when no specific inhibitory conditions are present. Based on the data presented, we postulate that cell multiplication is a repressible function. These data also are compatible with a negative control mechanism for cell multiplication.