The promoter elements and transcription factors necessary for triiodothyronine (T-3) induction of hepatic HMG-CoA reductase (HMGR) were investigated by transfecting rat livers with wild type and mutant HMGR promoter-luciferase constructs using in vivo electroporation. Mutations in the sterol response element (SRE), nuclear factor-y (NF-Y) site, and the newly identified upstream transcription factor-2 (USF-2) site essentially abolished the T-3 response. Chromatin immunoprecipitation (ChIP) analysis demonstrated that T-3 treatment caused a 4-fold increase in in vivo binding of USF-2 to the HMGR promoter. Co-transfection of the wild type HMGR promoter with siRNAs to USF-2, SREBP-2, or NF-Y nearly abolished the T-3 induction, as measured by promoter activity. These data provide in vivo evidence for functional roles for USF-2, SREBP-2, and NF-Y in mediating the T-3-induction of hepatic HMGR transcription. (C) 2009 Elsevier Inc. All rights reserved.