We are investigating the phenomenon of ectopic protein synthesis of placental proteins in neoplasms because of the link between the mechanisms associated with the derepression of the genes synthesizing embryonic proteins and those involved in neoplastic transformation. Our work has concentrated on two placental membrane proteins with emphasis on alkaline phosphatase and the transferrin receptor. We have investigated the regulation of alkaline phosphatase activity by 1,25 dihydroxyvitamin D?3? and showed specific activation of the phenotypic liver/kidney/bone enzyme in osteogenic sarcoma cells and in MDA-MB-157 breast carcinoma cells. Another breast carcinoma cell line, BT20, ectopically expresses the placental alkaline phosphatase which can be down-regulated by 1,25 dihydroxyvitamin D?3? in conjunction with its effects to promote differentiation. Bovine tranferrin was demonstrated by affinity chromatography to bind specifically to the human transferrin receptor on K562 cells grown in serum-free medium. The K?a? was estimated at 5 x 10?-5?M?-1? based on the competitive binding of unlabeled bovine transferrin with ?125?I-labeled human transferrin. Immunocytochemical localization by indirect immunofluorescence revealed that bovine transferrin is internalized in the same manner as the human ligand. Recycling of the transferrin-transferrin receptor complex was demonstrated to be partially Golgi-dependent (monensin-sensitive pathway) by the addition of 10?-5? M monensin to K562 cells. This resulted in a 50% decrease of cell-surface binding sites that was not affected by new protein synthesis. Evidence for a second recycling pathway (monensin-resistant) was found when kinetic studies were performed with monensin followed by ?125?I-human transferrin. K562 cells continued to internalize transferrin at 50% of control level and the recycling time was approximately doubled. Levels of human transferrin receptor (TR) were measured in 94 cases of primary infiltrating ductal carcinma. The mean level for these cases was approximately 5 times the level for 7 samples of benign non-neoplastic breast disease. There was a negative correlation between TR levels and estrogen (ER) levels with tumors from premenopausal patients having low levels of unsaturated ER but high TR levels indicating the possibility of faster-than-average growth. (M)
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