The Phosphoenolpyruvate Carboxykinase Gene Glucocorticoid Response Unit: Identification of the Functional Domains of Accessory Factors HNF3β (Hepatic Nuclear Factor-3β) and HNF4 and the Necessity of Proper Alignment of Their Cognate Binding Sites

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Abstract

Complete induction of hepatic phosphoenolpyruvate carboxykinase (PEPCK) gene transcription by glucocorticoids requires a complex glucocorticoid response unit (GRU). The GRU is comprised of two glucocorticoid receptor (GR)-binding sites (GR1 and GR2) and four accessory factor-binding sites [AF1, AF2, AF3, and cAMP response element (CRE)] that bind distinct transcription factors. Hepatic nuclear factor 4 (HNF4) and chicken ovalbumin upstream promoter transcription factor (COUP-TF) bind to the AF1 element and account for AF1 activity. Members of the hepatic nuclear factor 3 (HNF3) family bind to the AF2 element and provide AF2 activity. In this report, we show that the functions of AF1 and AF2 are dependent on their positions in the promoter, since they cannot substitute for each other nor can they be exchanged without a reduction in the response to glucocorticoids. We also identified the domains of HNF4 and HNF3β that are required for the AF1 and AF2 activities, respectively. The carboxy-terminal transactivation domain of HNF4 (amino acids 128-374) confers most of the AF1 activity, while the carboxy-terminal transactivation domain of HNF3β (amino acids 361-458) mediates AF2 activity. These domains of HNF4 and HNF3β appear to have distinct roles in the response to glucocorticoids, as there are unique structural requirements for each, as judged by the failure of most other classes of transactivation domains to serve as accessory factors. These results suggest that the regulation of the PEPCK gene by glucocorticoids requires specific interactions between GR, accessory factors, and coactivators, and that the transactivation domains of AF1 and AF2 are of fundamental importance in the assembly of this multiprotein complex.

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