Participation of JAK, STAT and unknown proteins in human placental lactogen-induced signaling: a unique signaling pathway different from prolactin and growth hormone.

The signal transduction mechanism involved in human placental lactogen (hPL) was studied. We have identified that hPL rapidly stimulated the tyrosine phosphorylation of at least 7 proteins including Janus Kinases (JAK1 and JAK2) and a signal transducer and activator of transcription protein (Stat3). This is the first evidence that the JAK-STAT pathway is involved in the hPL signaling. Moreover, two unknown proteins which were different from STAT proteins (Stat1, 3 and 5) in sizes were predominantly tyrosine-phosphorylated. Because human growth hormone (hGH) activates Stat1, 3, 5 and human prolactin (hPRL) activates Stat5, these results show that hPL uses a unique signal transduction pathway which is different from hGH and hPRL.