Breast tumor cytosol has been analyzed for the presence of a progesterone-binding protein (PBCP), commonly present in benign breast cysts in huge concentrations. In 377 primary carcinomas investigated PBCP was present in measurable quantities in 60.7% using “rocket” immuno-electrophoresis. Concentrations of PBCP ranged from 0 to 12.4% of total cytosol protein with an average of 4.0 µg/mg cytosol protein. The distribution of PBCP values seems to suggest two tumor populations, one of which is lacking in PBCP and the other showing a lognormal distribution. In malignant tumors PBCP levels were negatively correlated (P = 0.024) to estrogen receptor but not to (P = 0.38) progestin receptor levels. There was a highly significant (P < 0.001) positive correlation to cytosol albumin concentration which suggests an extracellular localization of PBCP possibly caused by restricted lymphatic drainage of tumor tissue. In benign breast tumors, mainly fibroadenomas, both PBCP incidence (81%) and average concentration (13.5 µg/mg protein) was higher than in malignant tumors. A positive correlation to sex-hormone receptor levels were observed indicating that PBCP production could be under hormonal control in this type of tumor development. In 71 metastatic tumors examined PBCP incidence was far less than in primary tumors (P < 0.001) and the levels seen were also considerably lower. PBCP holds promise as a marker of tumors in an early stage of development and/or with a low metastatic potential.

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This work was supported by the Norwegian Society for Fighting Cancer (Norsk Forening til Kreftens Bekjempelse).

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