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

Modulation of miR-29 expression by α-fetoprotein is linked to the hepatocellular carcinoma epigenome

Globally, hepatocellular carcinoma (HCC) accounts for 70%-85% of primary liver cancers and ranks as the second leading cause of male cancer death. Serum alpha-fetoprotein (AFP), normally highly expressed in the liver only during fetal development, is reactivated in 60% of HCC tumors and associated with poor patient outcome. We hypothesize that AFP+ and AFP- tumors differ biologically. Multivariable analysis in 237 HCC cases demonstrates that AFP level predicts poor survival independent of tumor stage (P<0.043). Using microarray-based global microRNA (miRNA) profiling, we found that miRNA-29 (miR-29) family members were the most significantly (P<0.001) down-regulated miRNAs in AFP+ tumors. Consistent with miR-29's role in targeting DNA methyltransferase 3A (DNMT3A), a key enzyme regulating DNA methylation, we found a significant inverse correlation (P<0.001) between miR-29 and DNMT3A gene expression, suggesting that they might be functionally antagonistic. Moreover, global DNA methylation profiling reveals that AFP+ and AFP- HCC tumors have distinct global DNA methylation patterns and that increased DNA methylation is associated with AFP+ HCC. Experimentally, we found that AFP expression in AFP- HCC cells induces cell proliferation, migration, and invasion. Overexpression of AFP, or conditioned media from AFP+ cells, inhibits miR-29a expression and induces DNMT3A expression in AFP- HCC cells. AFP also inhibited transcription of the miR-29a/b-1 locus, and this effect is mediated through c-MYC binding to the transcript of miR-29a/b-1. Furthermore, AFP expression promotes tumor growth of AFP- HCC cells in nude mice.

CONCLUSION

Tumor biology differs considerably between AFP+ HCC and AFP- HCC; AFP is a functional antagonist of miR-29, which may contribute to global epigenetic alterations and poor prognosis in HCC.

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.

Abrogation of p53 by its antisense in MCF-7 breast carcinoma cells increases cyclin D1 via activation of Akt and promotion of cell proliferation

The p53 protein has been a subject of intense research interest since its discovery as about 50% of human cancers carry p53 mutations. Mutations in the p53 gene are the most frequent genetic lesions in breast cancers suggesting a critical role of p53 in breast cancer development, growth and chemosensitivity. This report describes the derivation and characterization of MCF-7As53, an isogenic cell line derived from MCF-7 breast carcinoma cells in which p53 was abrogated by antisense p53 cDNA. Similar to MCF-7 and simultaneously selected hygromycin resistant MCF-7H cells, MCF-7As53 cells have consistent basal epithelial phenotype, morphology, and estrogen receptor expression levels at normal growth conditions. Present work documents investigation of molecular variations, growth kinetics, and cell cycle related studies in relation to absence of wild-type p53 protein and its transactivation potential as well. Even though wild-type tumor suppressor p53 is an activator of cell growth arrest and apoptosis-mediator genes such as p21, Bax, and GADD45 in MCF-7As53 cells, no alterations in expression levels of these genes were detected. The doubling time of these cells decreased due to depletion of G0/G1 cell phase because of constitutive activation of Akt and increase in cyclin D1 protein levels. This proliferative property was abrogated by wortmannin, an inhibitor of PI3-K/Akt signaling pathway. Therefore this p53 null cell line indicates that p53 is an indispensable component of cellular signaling system which is regulated by caveolin-1 expression, involving Akt activation and increase in cyclin D1, thereby promoting proliferation of breast cancer cells.

Novel human alpha-fetoprotein mRNA isoform lacking exon 1 identified in ovarian yolk sac tumor

OBJECTIVE

Alpha-fetoprotein (AFP) is a major fetal serum protein, the biologic role of which has not been not fully elucidated. Recently, existence of a novel AFP mRNA isoform (del.1 AFP mRNA isoform), which is transcribed from the intron A (the intron between exons 1 and 2), has been reported in murine yolk sac and fetal liver. In the present study, we intended to identify the human homologue of the murine AFP mRNA isoform in the yolk sac tumor.

METHODS

To investigate the existence of the mRNA isoform (which we termed the "AFP-C mRNA isoform"), reverse transcription-polymerase chain reaction (RT-PCR) was used. Moreover, the expression analysis of the AFP-C cDNA isoform using the AFP-negative human cell line was carried out.

RESULTS

RT-PCR revealed the existence of the AFP-C mRNA isoform in the yolk sac tumor and human hepatocellular carcinoma cells. The expression analysis clarified that the molecular size of the AFP-C was approximately 65 kd, and that the protein was not secreted, in contrast to the traditional AFP.

CONCLUSION

From these results, the existence of the AFP-C mRNA isoform has been demonstrated for the first time in humans. The AFP-C located in cytoplasm possibly plays physiologic/pathogenic roles distinct from those of the traditional AFP in the yolk sac tumor and hepatocellular carcinoma.

Interactions between Prostaglandin E2, Liver Receptor Homologue-1, and Aromatase in Breast Cancer

Local synthesis of estrogens within breast adipose tissue by cytochrome P450 aromatase contributes to the growth of postmenopausal breast cancers. One of the major stimulators of aromatase expression in breast is prostaglandin E2 (PGE2) derived from tumorous epithelium and/or infiltrating macrophages. Recently, the orphan nuclear receptor, liver receptor homologue-1 (LRH-1), has also been shown to regulate aromatase expression in breast adipose tissue. We therefore examined the expression of, and correlations between, aromatase and LRH-1 mRNA in a panel of breast carcinoma tissues and adjacent adipose tissue. LRH-1 mRNA expression was low in normal breast tissue but markedly elevated in both breast carcinoma tissue and adipose tissue surrounding the tumor invasion (thereby paralleling aromatase expression). Laser capture microdissection localized the site of LRH-1 expression to tumor epithelial cells but not to intratumoral stromal cells. A strong correlation between LRH-1 and aromatase mRNA levels was observed in tumor-containing adipose tissue but not in tumor tissue. Ectopic expression of LRH-1 in primary human adipose stromal cells strongly activated endogenous aromatase mRNA expression and enzyme activity. Finally, treatment of adipose stromal cells with PGE2 induced expression of both LRH-1 and aromatase. We suggest that PGE2 derived from breast tumor tissue may increase aromatase expression in the surrounding adipose stroma in part by inducing LRH-1 in these cells. The roles of LRH-1 in breast cancer proliferation merit further study.

Alpha-fetoprotein structure and function: relevance to isoforms, epitopes, and conformational variants

Mammalian alpha-fetoprotein (AFP) is classified as a member of the albuminoid gene superfamily consisting of albumin, AFP, vitamin D (Gc) protein, and alpha-albumin. Molecular variants of AFP have long been reported in the biomedical literature. Early studies identified isoelectric pH isoforms and lectin-binding variants of AFP, which differed in their physicochemical properties, but not in amino acid composition. Genetic variants of AFP, differing in mRNA kilobase length, were later extensively described in rodent models during fetal/perinatal stages, carcinogenesis, and organ regeneration. With the advent of monoclonal antibodies in the early 1980s, multiple antigenic epitopes on native AFP were detected and categorized, culminating in the identification of six to seven major epitopes. During this period, various AFP-binding proteins and receptors were reported to inhibit certain AFP immunoreactions. Concomittantly, human and rodent AFP were cloned and the amino acid sequences of the translated proteins were divulged. Once the amino acid composition of the AFP molecule was known, enzymatic fragments could be identified and synthetic peptide segments synthesized. Following discovery of the molten globule form in 1981, the existence of transitory, intermediate forms of AFP were acknowledged and their physiological significance was realized. In the present review, the various isoforms and variants of AFP are discussed in light of their potential biological relevance.

Stimulation of tumor-cell growth by alpha-fetoprotein

It has been recognized that alpha-fetoprotein (AFP), as an oncofetal antigen, is re-expressed in large amounts in adult tumor cells and serves clinically useful purposes in tumor-marker assays. However, its biological activities are still undefined. In the present study, the ability of AFP to stimulate tumor-cell growth was observed by an in vitro experimental system. Mouse ascites cancer cells derived from hepatoma-22(H-22) or Ehrlich ascites carcinoma(EAC) were extracted intraperitoneally and cultured in RPMI 1640 medium containing 10% newborn calf serum for 48 hr. Cell growth was quantitated by a colorimetric assay using a MTT microculture tetrazolium dye. The results demonstrated that AFP significantly increased H-22-cell proliferation, with stimulation per cents of 122 to 156%. A similar growth-promoting effect of AFP was observed using EAC cells, with stimulation per cents of 86 to 210%. Moreover, the growth-stimulatory activity of AFP could be abrogated with anti-AFP antibodies. In addition, 5-fluorouracil could obviously inhibit AFP-induced proliferation of H-22 or EAC cells in vitro. These results suggest that AFP is associated with tumor-cell growth and may serve as an important target of tumor therapy.

Stimulation of tumor-cell growth by alpha-fetoprotein

It has been recognized that alpha-fetoprotein (AFP), as an oncofetal antigen, is re-expressed in large amounts in adult tumor cells and serves clinically useful purposes in tumor-marker assays. However, its biological activities are still undefined. In the present study, the ability of AFP to stimulate tumor-cell growth was observed by an in vitro experimental system. Mouse ascites cancer cells derived from hepatoma-22(H-22) or Ehrlich ascites carcinoma(EAC) were extracted intraperitoneally and cultured in RPMI 1640 medium containing 10% newborn calf serum for 48 hr. Cell growth was quantitated by a colorimetric assay using a MTT microculture tetrazolium dye. The results demonstrated that AFP significantly increased H-22-cell proliferation, with stimulation per cents of 122 to 156%. A similar growth-promoting effect of AFP was observed using EAC cells, with stimulation per cents of 86 to 210%. Moreover, the growth-stimulatory activity of AFP could be abrogated with anti-AFP antibodies. In addition, 5-fluorouracil could obviously inhibit AFP-induced proliferation of H-22 or EAC cells in vitro. These results suggest that AFP is associated with tumor-cell growth and may serve as an important target of tumor therapy.

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.

Synergistic interaction of growth factors and albumin in regulating estradiol synthesis in breast cancer cells

Estradiol 17 beta-hydroxysteroid dehydrogenase acts to convert estrone to the biologically active estrogen, estradiol, in breast tumors and MCF-7 breast cancer cells in vitro. In this study we have examined the ability of albumin to influence the effect of growth factors (insulin-like growth factor-I (IGF-I), epidermal growth factor (EGF), transforming growth factor-alpha (TGF alpha)) and cytokines (interleukin (IL)-1, IL-6) on estradiol 17 beta-hydroxysteroid dehydrogenase activity in MCF-7 cells. IGF-I (80 ng/ml) or albumin (30 micrograms/ml) stimulated estradiol 17 beta-hydroxysteroid dehydrogenase activity by 144% and 102% (p less than 0.01). The combination of IGF-I and albumin, however, produced a marked (704%) synergistic stimulation of estradiol 17 beta-hydroxysteroid dehydrogenase activity. EGF or TGF alpha failed to stimulate estradiol 17 beta-hydroxysteroid dehydrogenase activity and no synergism with albumin was detected. IL-1 (10 ng/ml), but not IL-6, also stimulated estradiol 17 beta-hydroxysteroid dehydrogenase activity and acted synergistically with albumin to stimulate enzyme activity. MCF-7 cells were shown to specifically bind 125I-albumin and binding is increased by pretreatment of cells with IGF-I (80 ng/ml) for 48 h. It is concluded that the synergism that results from treating MCF-7 cells with albumin and IGF-I may result from increased albumin uptake and subsequent biological effect.

Human mammary tumor cell proliferation: primary role of platelet-derived growth factor and possible synergism with human alpha-fetoprotein

Human mammary medullary carcinoma cells (passages 16 to 21) were cultured for 2 days to allow for attachment, followed by 6 days of culture in either fetal calf serum, human cord blood, human amniotic fluid, or growth factors in the presence or absence of purified human alpha-fetoprotein (AFP). When growth factors were tested alone, only platelet-derived growth factor produced a significant increase in cell proliferation. Although up to 40% amniotic fluid had no effect on cell proliferation, human cord blood was two-fold more potent than fetal calf serum at similar concentrations. The addition of 10 ng/ml of platelet-derived growth factor increased the proliferative activity of human cord blood 1.5- to 2.5-fold. Ablation of endogenous AFP by affinity chromatography reduced the proliferative activity of cord blood by 75%. Similarly, the mitogenic activity of cord blood plus platelet-derived growth factor was reduced by 56% when AFP was removed. Purified AFP dose-dependently enhanced the proliferative activity of platelet-derived growth factor. This synergistic effect was specific for platelet-derived growth factor. We conclude that platelet-derived growth factor is a major growth factor controlling the proliferation of these tumor cells and that AFP may enhance growth factor proliferative activity and human mammary tumor growth.

Alpha-fetoprotein gene expression in human lymphoblastoid cells and in PHA-stimulated normal T-lymphocytes

Alpha-fetoprotein (AFP) is mainly synthesized by the fetal liver, the yolk sac and, to a much lower extent, by a few non-hepatic fetal tissues (i.e. kidney, pancreas, lung). This property is considered to be lost in mature quiescent cells of the adult. In the present we have studied the expression of AFP mRNA sequences in phytohemagglutinin (PHA)-stimulated normal human T-lymphocytes and in several human lymphoma cell lines. The amount of mRNA transcripts detected in quiescent T-lymphocytes by dot and Northern blot analysis was very low. It increased rapidly after PHA-activation, reached a maximum at 72 hours (six fold the level observed for quiescent T-lymphocytes) and decreased thereafter. The lymphoma cell lines Daudi, Raji, Rh6 et CEM, all expressed elevated levels of AFP mRNA. The transcripts had the size expected for human AFP, suggesting that they were functional and probably translated into protein. The possible role of AFP synthesis in lymphocyte blastogenesis and in lymphoma growth is discussed in relation with the strong binding affinity of this protein for polyunsaturated fatty acids.

Comparison of AFP and beta-hCG levels in infiltrating duct mammary carcinoma at different stages of malignancy

In order to assess the likely clinical value of AFP and beta-hCG levels as markers of the extent of spread of infiltrating duct mammary carcinoma, we compared (Mann-Whitney U test) the immunoreactive tumour tissue concentrations of the respective antigens, measured by specific RIA, after mastectomy in Chinese women at stages I (n = 9), II (n = 13) and III (n = 7) with that of normal breast tissue (n = 11). Elevated concentrations of the oncofetal antigens were found at all stages of malignancy. The discovery of an hCG-like material in the extract of normal breast tissues indicates limited value of beta-hCG assay in mammary tumours. However, our findings generally suggest that measurement of the tumour tissue AFP levels warrant consideration as a useful biochemical marker for in-vitro evaluation of tumour spread at all stages of malignancy.

Specific cytoplasmic alpha-fetoprotein binding protein in MCF-7 human breast cancer cells and primary breast cancer tissue

Direct evidence was obtained for the existence of a specific high affinity alpha-fetoprotein (AFP)-binding protein in the cytosol of both MCF-7 human breast cancer cultured cells and primary breast cancer tissue from postmenopausal women using a nitrocellulose blotting assay. Scatchard analysis of the binding data for MCF-7 cells at 37 degrees C revealed the presence of a single class of AFP binding sites with an apparent Kd of 4.5 x 10(-8) M, and 75,000 binding sites per cell. All 9 primary breast cancer cytosols obtained from postmenopausal women also contained measureable levels of this specific AFP-binding protein. The number of AFP molecules specifically bound varied considerably between patients and ranged from 29-250 fmol per mg cytosol protein. Levels of AFP-binding protein levels and estrogen receptor measured in these same breast cancer cytosols showed a positive statistical correlation (r = 0.85). Taken together, the present evidence for the existence of a specific cytoplasmic AFP-binding protein in MCF-7 cells and previously reported evidence for de novo synthesis of free immunoreactive and bound nonimmunoreactive forms of cytoplasmic AFP by MCF-7 cells is consistent with the conclusion that most of the endogenous AFP synthesized in breast cancer cells is rapidly bound to specific cytoplasmic AFP-receptors, and that binding of AFP to these receptors masks its immunoreactivity.(ABSTRACT TRUNCATED AT 250 WORDS)