HEPATIC NUCLEAR FACTOR-3 IS AN ACCESSORY FACTOR REQUIRED FOR THE STIMULATION OF PHOSPHOENOLPYRUVATE CARBOXYKINASE GENE-TRANSCRIPTION BY GLUCOCORTICOIDS

Transcription of the hepatic phosphoenolpyruvate carboxykinase gene is stimulated by glucocorticolds and inhibited by insulin. The glucocort icoid response is mediated by a complex glucocorticold response unit t hat consists of two glucocorticoid receptor (GR)-binding sites (GR1 an d GR2) and two accessory factor-binding sites (AF1 and AF2), The compl ete unit is required for the full glucocorticoid response. The dominan t insulin effect is mediated in part through an insulin response seque nce that is coincident with the AF2 element. Members of the hepatic nu clear factor 3 (HNF3) and CCAAT enhancer binding protein (C/EBP) famil ies bind to the AF2 element; however, there is no correlation between binding of these factors and the ability of the AF2 element to mediate an insulin response, We show here that binding of HNF3 does correlate with the stimulation of the glucocorticoid response by the AF2 elemen t and that C/EBP is apparently not involved in this effect. This requi rement for HNF3 is quite specific since the substitution of elements k nown to enhance the action of the GR in other promoters fails to recap itulate AF2 accessory factor activity. By contrast, an HNF3-binding si te from the transthyretin gene is able to substitute for the wild type AF2 sequence and elicit a maximal glucocorticoid response. Based on c urrent and previous observations, the glucocorticoid response unit con sists of four DNA elements that bind four different proteins. These ar e: AF1 (hepatic nuclear factor 4/chicken ovalbumin upstream promoter t ranscription factor), AF2 (HNF3), GR1 (GR), and GR2 (GR).