INTERACTIONS AMONG JANUS KINASES AND THE PROLACTIN (PRL) RECEPTOR IN THE REGULATION OF A PRL RESPONSE ELEMENT

PRL regulates milk gene expression, at least in part, by activating JA K2 kinase and STATE (signal transducer and activator of transcription 5), initially termed mammary gland factor (MGF), These experiments wer e initiated to gain a better understanding of the mechanisms of transc riptional activation via PRL receptor (PRL-R) signaling, Binding of PR L to the recombinant pigeon PRL-R-activated transcription driven by a 2.8 kbp 5'-fragment of the rat beta-casein gene, PRL enhanced the expr ession of chimeric reporters containing the beta-casein PRL response e lement (PRE), but not the c-fos sis-inducible element, when the report ers were transfected into Chinese hamster ovary cells with the PRL-R, Wild type receptor, which contains a duplication of the entire extrace llular ligand-binding domain, was only slightly more effective than a truncation mutant with a single extracellular domain, Transfection wit h either JAK1, JAK2, or JAK3 increased basal transcription through bot h the PRE and sis-inducible element, Coexpression of JAK2 with PRL-R r esulted in amplification of the induction of the PRE by PRL, whereas J AKs 1 and 3 did not amplify the PRL effect, Overexpression of JAK2 mut ants blocked PRE activation by PRL, Mutant JAK2 also interfered with P RE activation by JAK3 but did not affect JAK1's stimulatory effect.