PLACENTAL LACTOGEN-I (PL-I) TARGET TISSUES IDENTIFIED WITH AN ALKALINE PHOSPHATASE-PL-I FUSION PROTEIN

The rat placenta expresses a family of genes related to prolactin (PRL ). Target tissues and physiological roles for many members of the PRL family have yet to be determined. In this investigation we evaluated t he use of an alkaline phosphatase (AP) tag for monitoring the behavior of a prototypical member of the PRL family, placental lactogen-l (PL- I). A probe was generated consisting of a fusion protein of human plac ental AP and rat PL-I (AP-PL-I). The AP-PL-I construct was stably expr essed in 293 human fetal kidney cells, as was the unmodified AP Vector that served as a control. AP activity was monitored with a colorimetr ic assay in conditioned medium from transfected cells. Immunoreactivit y and PRL-like biological activities of the AP-PL-I fusion protein wer e demonstrated by immunoblotting and the Nb2 lymphoma cell proliferati on assay, respectively. AP-PL-I specifically bound to tissue sections known to express the PRL receptor, including the ovary, liver, and cho roid plexus. Binding of AP-PL-I to tissues was specific and could be c ompeted with ovine PRL. The results indicate that AP is an effective t ag for monitoring the behavior of PL-I and suggest that this labeling system may also be useful for monitoring the actions of other members of the PRL family.