Prostaglandin F2α stimulates the expression and secretion of transforming growth factor B1 via induction of the early growth response 1 gene (EGR1) in the bovine corpus luteum

In most mammals, prostaglandin F2 alpha (PGF2 alpha) is believed to be a trigger that induces the regression of the corpus luteum (CL), whereby progesterone synthesis is inhibited, the luteal structure involutes, and the reproductive cycle resumes. Studies have shown that the early growth response 1 (EGR1) protein can induce the expression of proapoptotic proteins, suggesting that EGR1 may play a role in luteal regression. Our hypothesis is that EGR1 mediates the actions of PGF2 alpha by inducing the expression of TGF beta 1 (TGFB1), a key tissue remodeling protein. The levels of EGR1 mRNA and protein were up-regulated in the bovine CL during PGF2 alpha-induced luteolysis in vivo and in PGF2 alpha-treated luteal cells in vitro. Using chemical and genetic approaches, the RAF/MAPK kinase (MEK) 1/ERK pathway was identified as a proximal signaling event required for the induction of EGR1 in PGF2 alpha-treated cells. Treatment with PGF2 alpha increased the expression of TGFB1 mRNA and protein as well as the binding of EGR1 protein to TGFB1 promoter in bovine luteal cells. The effect of PGF2 alpha on TGFB1 expression was mimicked by a protein kinase C (PKC)/RAF/MEK1/ERK activator or adenoviral-mediated expression of EGR1. The stimulatory effect of PGF2 alpha on TGFB1 mRNA and TGFB1 protein secretion was inhibited by blockade of MEK1/ERK signaling and by adenoviral-mediated expression of NAB2, an EGR1 binding protein that inhibits EGR1 transcriptional activity. Treatment of luteal cells with TGFB1 reduced progesterone secretion, implicating TGFB1 in luteal regression. These studies demonstrate that PGF2 alpha stimulates the expression of EGR1 and TGFB1 in the CL. We suggest that EGR1 plays a role in the expression of genes whose cognate proteins coordinate luteal regression.