Complex physiological stimuli differentially regulate the tissue- specific transcription of the lipoprotein lipase (LPL) gene. A conserved DNA recognition element (-171 to -149 bp) within the promoter functions as a transcriptional enhancer when bound by the peroxiSome proliferator-activated receptor-γ2 (PPARγ2)/retinoid X receptor α (RXRα) heterodimer, but serves as a transcriptional silencer in the presence of unidentified double and single stranded DNA-binding proteins. To address this apparent paradox, the current study examined the effect of two classes of candidate comodulatory proteins, COUP-TF (chicken ovalbumin upstream promoter transcriptional factor) and the corepressor SMRT (silencing mediator of retinoic acid receptor and thyroid receptor). The expression of COUP-TF was detected by Western and Northern blots in a preadipocyte 3T3-L1 cell model during periods corresponding to increased LPL transcription. Cotransfection of COUP-TF expression constructs in the renal epithelial 293T cell line significantly increased transcription from the LPL promoter in synergy with PPARγ2/RXRα, heterodimers. The COUP-TFII (ARP-1) protein specifically bound the LPL PPAR recognition element in electromobility shift assays and interacted directly with the ligand-binding domain of PPARγ in pull-down experiments. In contrast, cotransfection of SMRT repressed PPARγ2/RXRα- mediated LPL transcription in the absence or presence of COUP-TFII (ARP-1). The interaction between PPARγ2 and SMRT localized to the receptor- interactive domain 2 (amino acids 12601495) of the SMRT protein based on cotransfection and pull-down assays. These in vitro data indicate that COUP- TF proteins and SMRT modulate PPARγ-mediated LPL transcription in the 293T cell line.
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