The T47D human mammary adenocarcinoma vell line in vitro releases viral particles as well as soluble proteins. Both fractions were shown to contain antigens that immunologically crossreact with the major glycoprotein (gp52) of mouse mammary tumor virus. The crossreacting antigens are located on polypeptides with apparent molecular weights of about 68,000 and 60,000. The larger one is present in viral particles whereas both are found in the soluble fraction. Both proteins are glycosylated. The human tissue culture proteins differ from gp52 not only in molecular weight but also in charge heterogeneity and in polypeptide profiles obtained after partial proteolysis. The results suggest that there is a restricted similarity between MMTV gp52 and the immunologically related T47D proteins.Evidence for an association of viral proteins and viral nucleic acid sequences with human mammary carcinomas has been accumulated increasingly over the past few years. Key similarities between the human malignancy and mouse mammary tumors have focused attention on the possible involvement of mouse mammary tumor virus (MMTV)-like retroviruses in the human disease (1, 2). An antigen immunologically related to the protein moiety of gp52 (a 52,000 Mr glycoprotein that constitutes the major envelope protein of MMTV) was detected in sections of human breast carcinoma tissue and in particulate fractions of human milk (3-5). The antigen also has been detected in established cell lines derived from pleural effusions of patients with breast cancer (6, 7). In addition, these cell lines, MCF-7 and T47D, have been shown to possess DNA sequences with partial homology to the genome of MMTV (8).The T47D cell line, which was established and subcloned in our laboratory (9), releases retrovirus-like particles (10). Therefore, it was of interest to determine whether these particles manifested proteins antigenically related to MMTV gp52. Since MMTV gp52 is shed from tumor cells in vivo in the mouse and enters the circulation, the tissue culture system also might serve as a model in the search for crossreacting antigens in vitro. An MMTV gp52 crossreacting antigen was recently found in both retrovirus-like particles and soluble proteins released by the T47D cell line (10).The present study was designed to identify and characterize these gp52-related proteins and also to determine their degree of similarity to MMTV gp52.MATERIALS AND METHODS Cells and Viruses. The T47D cell line (sub-line Cl-11) was used in this study. The cells were grown in RPMI 1640 medium containing 10% fetal calf serum, insulin (0.2 IU/ml), 2 mM glutamine, and antibiotics (complete medium). When cells were grown in the presence of steroid hormones, serum was omitted. For virus collection, the cells were seeded in RPMI 1640 with 10% fetal calf serum and incubated hr; the medium then was replaced with RPMI 1640 with 5% dialyzed serum and 1 nM 17(-estradiol. After 24 hr, the medium was replaced with RPMI 1640 containing the same amount of 17f3estradiol without serum. The next day...
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