The orphan receptor COUP-TF binds to a third glucocorticoid accessory factor element within the phosphoenolpyruvate carboxykinase gene promoter.

The phosphoenolpyruvate carboxykinase (PEPCK) gene promoter contains a glucocorticoid response unit (GRU) that includes, as a linear array, two accessory factor binding sites (AF1 and AF2) and two glucocorticoid receptor binding sites. All of these elements are required for a complete glucocorticoid response. AF1 and AF2 also partially account for the response of the PEPCK gene to retinoic acid and insulin, respectively. A second retinoic acid response element was recently located just downstream of the GRU. In this study we show that mutation of the 3' half-site of this element results in a 60% reduction of the glucocorticoid response of PEPCK promoter-chloramphenicol acetyltransferase (CAT) fusion constructs in transient transfection assays, thus the half-site is now termed AF3. A variety of assays were used to show that chicken ovalbumin upstream promoter transcription factor (COUP-TF) binds specifically to AF3 and that upstream stimulatory factor (USF) binds to an E-box motif located 2 base pairs downstream of AF3. Mutations of AF3 that diminish binding of COUP-TF reduce the glucocorticoid response, but mutation of the USF binding site has no effect. The functional roles of AF1, AF2, and AF3 in the glucocorticoid response were explored using constructs that contained combinations of mutations in all three elements. All three elements are required for a maximal glucocorticoid response, and mutation of any two abolish the response.