Prostate-specific antigen in milk of lactating women.

Prostate-specific antigen (PSA) is believed to be a highly specific marker for normal and cancerous prostatic tissue. We recently found that 30-40% of breast tumors produce PSA. Other data from our group suggest that normal breast can also produce PSA under conditions of stimulation by steroid hormones. In addition, we detected PSA in amniotic fluid. Here we report the presence of PSA in breast milk of lactating women. PSA concentrations in breast milk were quite variable, ranging from < 0.01 microgram/L in 4 of 38 milks to 350 micrograms/L; the median was 0.47 microgram/L. PSA concentration in breast milk was not correlated with mother's age or the sex of the newborn. It did tend to decrease with increasing time postdelivery, but was still detectable 2 weeks postdelivery. PSA in milk was equally measurable by a highly sensitive PSA assay based on time-resolved fluorometry and by the IMx automated PSA method. As confirmed by Western blot analysis, PSA in milk was present predominantly in its 33-kDa form; the PSA-alpha 1-antichymotrypsin complex (100 kDa) was also present but its concentration was < 25% of total PSA. We conclude that the female breast can produce PSA and that PSA is secreted into the milk during lactation; however, the biological role of PSA in milk is unknown. These and other data presented by our group suggest that PSA, a serine protease, may play a role in control of growth in mammary and other tissues through regulation of growth factors, cytokines, and growth-factor-binding proteins.