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

Abstract

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)

Monoclonal Antibodies Directed against a Widespread Oncofetal Antigen: The Alpha-Fetoprotein Receptor

“…However, since both 167H.1 and 167H.4 secrete antibodies of the IgM class as revealed by the Zymed kit, it was pos sible that they could sterically hinder the binding of AFP to its receptor even if they were directed against a neighboring epitope. To explore this possibility, the reciprocal inhi bition experiment was carried out to assess the ability of AFP, HSA or OA to inhibit the reaction of the 167H.1 and 167H.4 MAbs against PE or the breast cancer preparations which reportedly contain an AFP binding protein [31]. Figures la and lb that HSA does not prevent AFP binding to the soluble form of the AFP receptor.…”

“…AFP might be released through a process that is facilitated by the high cytoplas mic KCI concentration. The resulting free re ceptor could account, at least in part, for the receptor found in membrane-free materials such as cord serum, the pleural effusion from a metastatic mammary cancer and the cyto sols used in this work and others [29,31 ].…”

Nonsecreted cytoplasmic alpha-fetoprotein: a newly discovered role in intracellular signaling and regulation. An update and commentary

“…In subsequent studies, AFP synthesis lacking subsequent secretion was shown to occur in other cell and tissue types such as immature rodent uterine cells in vivo, MCF-7 human breast cancer cell cultures, serum of postmenopausal women, and in human breast cancer cytosols [10][11][12][13]. In addition, direct evidence was reported for the presence of specific high affinity (kD 4.5×10 −8 M) AFP-binding proteins (molecular mass, 42 kD) in the cytosols of cultured MCF-7 breast cancer cells and in primary breast cancer serum and tissues from postmenopausal women [14,15]. Taken together, the above data are consistent with the conclusion that endogenous synthesized AFP retained in the cytoplasm might be rapidly bound to one or more unknown cytoplasmic AFP-binding proteins as addressed below.…”

“…It is evident from early studies that CyAFP, produced in certain malignant and nonmalignant cells, is synthesized in the cytoplasm but not secreted (see [10][11][12][13][14]). Since CyAFP was shown by 14 C-leucine autoradiography to be synthesized within the cell of a nonsecreting hepatoma cell line, it is logical to presume that cell-bound AFP does not possess the amino-terminus signal leader sequence or the sequence itself was masked.…”

Novel Human Alpha-Fetoprotein mRNA Isoform Lacking Exon 1 Identified in Ovarian Yolk Sac Tumor

“…26 Although it is unclear that the cAFP receptor exists in the yolk sac tumor and hepatocellular carcinoma cells, it is possible that the AFP-C synthesized in the tumor and cells might be bound by the cAFP receptor to affect the proliferative capacity of the tumor and cells. Further study will reveal the physiologic/ pathogenic role of the intracellular AFP-C protein in the yolk sac tumor and hepatocellular carcinoma, as well as the regulatory mechanism of expression of the protein.…”