A corepressor and chicken ovalbumin upstream promoter transcriptional factor proteins modulate peroxisome proliferator-activated receptor-γ2 retinoid X receptor α-activated transcription from the murine lipoprotein lipase promoter

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-gamma(2) (PPAR gamma(2))/retinoid X receptor alpha (RXR alpha) 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 PPAR gamma(2)/RXR alpha 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 PPAR gamma in pull-down experiments. In contrast, cotransfection of SMRT repressed PPAR gamma(2)/RXR alpha-mediated LPL transcription in the absence or presence of COUP-TFII (ARP-1). The interaction between PPAR gamma(2) and SMRT localized to the receptor-interactive domain 2 (amino acids 1260-1495) of the SMRT protein based on cotransfection and pull-down assays. These in vitro data indicate that COUP-TF proteins and SMRT modulate PPAR gamma-mediated LPL transcription in the 293T cell line.