Quantitative enzyme-linked immunosorbent assay for the estrogen-regulated Mr 24,000 protein in human breast tumors: correlation with estrogen and progesterone receptors.

A competitive enzyme-linked immunoassay has been developed to quantitate the Mr 24,000 estrogen-regulated protein (p24) in human breast tumors and tumor cell lines. The concentration of monoclonal antibody used to coat polystyrene microtiter assay plates was critical in optimizing assay sensitivity versus precision and demonstrated marked differences between plate types. Analysis of 114 human breast tumor cytosols revealed a 4-log range in p24 concentration with a median value of 1 microgram/mg cytosol protein. The abundance of p24 in breast tumors in vivo is therefore similar to that observed previously in hormone-responsive breast tumor cells in vitro. Elevated expression of p24 in these specimens was found to correlate well with presence of estrogen and progesterone receptors (P = 0.0002 by Pearson chi 2 analysis) with the correlation coming primarily through estrogen receptor. This result was confirmed by immunoblot analysis of p24 in 76 additional tumor cytosols and suggests that p24 may be a valuable marker for differentiation in human breast cancer.