Identification and characterization of the prolactin-binding protein in human serum and milk

The actions of prolactin (PRL) are mediated by its receptor, a member of the superfamily of single transmembrane cytokine receptors. High affinity binding proteins for the closely related growth hormone have been found in the sera of several species including humans and are generated by alternative splicing or proteolysis of the growth hormone receptor extracellular domain (ECD). In contrast, no conclusive evidence has been presented that an analogous prolactin-binding protein (PRLBP) is expressed in human serum. Using both monoclonal and polyclonal antibodies generated against hPRL and the ECD of the human prolactin receptor, co-immunoprecipitation analyses of human serum identified a 32-kDa hPRLBP capable of binding both hPRL and human growth hormone. A measurable fraction of circulating PRL (36%) was associated with the hPRLBP. Despite well documented sex differences in serum hPRL levels, there were no significant differences in the levels of hPRLBP found in the sera of normal adult males and females (15.3 +/- 1.3 ng/ml versus 13.4 +/- 0.8 ng/ml, respectively (mean +/- S.E.)). Immunoprecipitation studies also detected the PRLBP in human milk albeit at lower concentrations than found in sera. Deglycosylation did not alter its electrophoretic mobility, indicating an absence of carbohydrate moieties and suggesting that the hPRLBP spans most of the PRLR ECD, a result confirmed by limited proteolysis and mass spectrometry. The potential function of this serum chaperone was assessed in vitro by the addition of recombinant hPRLBP to the culture medium of the PRL-dependent Nb2 T-cell line. These studies revealed that the hPRLBP antagonizes PRL action, inhibiting PRL-driven growth in a dose-dependent manner.