Human glandular kallikrein in breast milk, amniotic fluid, and breast cystfluid

Background: Human glandular kallikrein (hK2) belongs to the serine protease family of enzymes and has high sequence homology with prostate-specific an tigen (PSA). The physiological role of hK2 has not as yet been determined, but there is evidence that it can regulate the proteolytic activity of PSA through processing and activating pro-PSA, an inactive precursor. Thus, it is conceivable that these two secreted proteins may coexist in biological f luids. Currently, hK2 is considered an androgen-regulated and prostate-spec ific protein. Recently, it has been demonstrated that hK2 is expressed in t he breast cancer cell line T-47D after stimulation by steroid hormones, and we reported that hK2 can be detected in a subset of breast tumor extracts. These data suggest that hK2 may be expressed in tissues other than the pro state, such as those in which PSA has already been detected. Because hK2 is a secreted protein, it may be present in various biological fluids. Methods: We analyzed milk samples from lactating women, amniotic fluid from pregnant women, and breast cyst fluid from patients with gross breast cyst ic disease, using a highly sensitive and specific immunoassay for hK2. Results: hK2 was present in all three biological fluids. We suggest that th e female breast may produce hK2 and provide evidence that hK2 may have valu e as an additional marker for the discrimination between type I and type II breast cysts. Conclusions: The female breast produces hK2 in addition to PSA. More studie s are necessary to establish the role of this kallikrein in nondiseascd bre ast, gross breast cystic disease, and breast cancer. (C) 1999 American Asso ciation for Clinical Chemistry.