Prolactin-independent modulation of the beta-casein response element by Erk2 MAP kinase

The MAP kinases have been suggested to play a role in intracellular signall ing by PRL. A reporter gene construct, PRE3-CAT, which manifests PRL respon siveness through a Stat5-binding site (PRE), was induced by PRL in CHO cell s expressing the PRL-R. A fusion protein (Gal4-Stat5(695)), containing the C-terminal domain of Stat5a (amino acids 695-794) linked to the DNA-binding domain of Gal4 (Gal4 DBD), strongly activated transcription of a luciferas e reporter gene. Therefore, the Stat5 C-terminus, which contains a potentia l MAP kinase phosphorylation site, exhibits a modular transactivating funct ion. A kinase-defective mutant of Erk2 (iMAPK) caused a dose dependent supp ression of PRL-stimulated PRE3-CAT, and also inhibited the induction of PRE 3-CAT by Jak2 over-expression. Correspondingly, over-expression of the MAP kinase activator v-Src increased the PRL stimulated level of PRE3-CAT. Gal4 -Stat5(695) activity was not modulated by PRL or Jak2, consistent with the absence of the relevant tyrosine phosphorylation site at residue 694. Gal4- Stat5(695) was not inhibited by iMAPK, indicating that the C-terminal trans activation region of Stat5a is not sensitive to direct modulation of a MAP kinase pathway. These results suggest that alteration of Erk2 activity by g rowth factors may modulate PRL-induced gene expression by a mechanism upstr eam of Stat5. CELL SIGNAL 11;3:205-210 1999. (C) 1999 Elsevier Science Inc.