The 20-kilodalton (kDa) human growth hormone (hGH) differs from the 22-kDahGH in the effect on the human prolactin receptor

Previously we have demonstrated that 20-kDa human GH (20K-hGH) is a full ag onist for hGH receptor (hGHR) even though its complex formation with hGHR a nd hGH-binding protein differs from that of 22-kDa human GH (22K-hGH). In t his study, we focused on the effect of 20K-hGH on human PRL receptor (hPRLR ). To elucidate the effects of 20K-hGH on hPRLR and compare them with those of 22K-hGH, we prepared two cells stably expressing full-length hPRLR, Ba/ F3-hPRLR and CHO-hPRLR. In the proliferation of Ba/F3-hPRLR cells, which ca n grow in a dose-response to lactogenic hormones, both 20K- and 22K-hGH exh ibited bell-shaped curves in the absence of exogenous zinc ion (Zn2+); howe ver, the curve of 20K-hGH was shifted to a 10-fold higher concentration tha n that of 22K-hGH in view of EC50 value (the EC50 of 20K- and 22K-hGH were 15 nM and 1.5 nM, respectively). Addition of Zn2+ up to 25 mu M increased t he activities of both 20K- and 22K-hGH; however, the enhancement by Zn2+ wa s greater in 20K-hGH than in 22K-hGH, thereby the activities of both hGH is oforms reached the same level at 25 mu M Zn2+. Nevertheless, in the presenc e of 0.25-1 mu M free Zn2+, which is equal in human serum, the activity of 20K-hGH was still lower than that of 22K-hGH. The modest effect of 20K-hGH on activating hPRLR in the absence of Zn2+ was confirmed in the rat serine protease inhibitor 2.1 (Spi2.1) gene promoter activation and JAK2/Stat5 tyr osine phosphorylation in CHO-hPRLR. In addition, in human breast cancer cel l T-47D, 20K-hGH was proved to stimulate Stat5 tyrosine phosphorylation to much lower degree than 22K-hGH via not hGHR but hPRLR. Taken together, our data suggest that 20K-hGH may be a weaker agonist for hPRLR than 22K-hGH in the human body; therefore 20K-hGH may alleviate the hPRLR-mediated side-ef fects such as breast cancer when administered to human body.