In vitro expression of long and short ovine prolactin receptors:: activation of Jak2/STAT5 pathway is not sufficient to account for prolactin signal transduction to the ovine β-lactoglobulin gene promoter

The recent finding that sheep had long (l-oPRLR) and short (s-oPRLR) prolactin receptors provided new tools to further explore prolactin signaling to target genes. Here we used CHO cells transfected with l-oPRLR or s-oPRLR cDNAs to compare the activation of known key steps of prolactin signaling by the two receptors. We found that prolactin stimulated 1-oPRLR tyrosine phosphorylation, although it lacked the last tyrosine residue found in other long prolactin receptors. In addition, l-oPRLR and s-oPRLR both responded to prolactin stimulation by (1) Janus kinase 2 (Jak2) tyrosine phosphorylation, (2) DNA-binding activation of signal transducer and activator of transcription 5 (STAT5), (3) stimulation of transcription from a promoter made of six repeats of STAT5-responsive sequence. However, although it contains STAT5-binding consensus sequences, the ovine P-lactoglobulin promoter (-4000 to +40) was transactivated by 1-oPRLR, but not by s-oPRLR. Taken together, our results indicate that activation of Jak2/STAT5 pathway alone is not sufficient to account for prolactin-induced transcription of this milk protein gene, and that sequences of its promoter, other than STAT5-specific sequences, account for the opposite transcriptional activation capabilities of 1-oPRLR and s-oPRLR.