Prostate-specific antigen found in Type I breast cyst fluids is a secretory product of the apocrine cells lining breast gross cysts

Prostate-specific antigen (PSA), a serine protease thought to be exclusivel y produced by the prostate epithelial cells, has been recently found in hum an breast tissues and fluids. PSA in breast cancer is associated with the p resence of steroid-hormones and receptors, and its presence seems to be a f avourable prognostic indicator. In order to clarify whether the cells linin g breast cysts may represent the source of PSA found in human breast cyst f luid, we performed an ultrastructural immunolocalization of PSA in the cell s surrounding Type I breast cysts, obtained from breast cyst fluids of wome n affected by breast gross cystic disease, the most commonly occurring beni gn breast lesions associated with increased cancer risk. These apocrine cel ls show morphological features typical of actively synthesizing and secreti ng cells, and a PSA labelling distributed on free ribosomes, RER cisternae, and secretory granules, indicating that the metabolically active apocrine cells lining the Type I cysts are responsible for the production and secret ion of PSA in Type I breast cyst fluids. The synthesis and intracystic accu mulation of this serine protease in biosynthetically active apocrine Type I cysts can play an important role in the natural history of breast gross cy stic disease as well as in the mechanism of cyst evolution.