Transcriptional regulation of the cholesteryl ester transfer protein gene by the orphan nuclear hormone receptor apolipoprotein AI regulatory protein-1.
We have defined a 105-base pair tissue-restricted promoter for the cholesteryl ester transfer protein (CETP) gene that contains a nuclear hormone receptor response element essential for transcriptional activity. DNaseI protection and electrophoretic mobility shift assays showed specific binding of nuclear extracts from HepG2 (hepatic) and Caco-2 (intestinal) cells (expressing cell types) to 3 sites (designated A (-26 to -57), B (-59 to -87), and C (-93 to -118)) within the 105-base pair minimal promoter element between -138 and -33. Mutagenesis studies indicated that the function of the promoter was dependent upon synergistic interactions between transcription factors bound to these sites. Mutation of site C reduced transcription by 50 and 80%, respectively, in HepG2 and Caco-2 cells, and electrophoretic mobility shift assays showed that nuclear hormone receptors, including ARP-1 and its homologue Ear-3/COUP-TF, were occupants of site C in both of these cell types. Overexpression of ARP-1 or Ear-3/COUP-TF with CETP promoter/chloramphenicol acetyltransferase gene reporter plasmids repressed transcriptional activity of the CETP promoter containing sequences up to -300, but activated transcription in the context of larger constructs containing sequences up to -636. Thus ARP-1 may assume a dichotomous role as both a transcriptional repressor and a transcriptional activator dependent on the promoter context. In addition, the architecture of the CETP gene promoter suggests that its expression is under the control of multiple transcriptional signaling pathways mediated by inducible transcription factors as well as nuclear hormone receptors.
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