Cyclical expression of egr-1/NGFI-A in the rat anterior pituitary: a molecular signal for ovulation?

Molecular genetic studies of egr-1/NGFI-A have recently established a key role for this immediate early gene in anterior pituitary development, and particularly in transcriptional regulation of the LH beta-subunit gene. These studies, using null mutant mice, addressed the developmental aspects of gonadotrophin gene regulation by egr-1 and, because of the limitations of this approach, did not address the role of egr-1 in adult reproductive physiology. Using the rat, we have now investigated the expression of egr-1 during the female reproductive cycle, and have found evidence of cyclical changes in anterior pituitary egr-1 expression, at mRNA, protein, and DNA binding activity levels. Specifically, we have shown that egr-1 mRNA is significantly elevated during proestrous, and conversely suppressed on the subsequent day of oestrous. We have also demonstrated significantly raised levels of an approximately 70 kDa Egr-1 immunoreactive protein band during the night of proestrous, together with markedly raised levels of a Egr-1 consensus sequence DNA binding complex. These studies are indicative of both direct egr-1 gene regulation by oestrogen, and also regulation by GnRH. Finally, we have sequenced previously uncharacterized regions of the egr-1 gene and found evidence of a potential oestrogen response element. Our findings are consistent with the hypothesis that Egr-1 forms at least part of the molecular signal for ovulation in the pituitary.