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Peroxisome Proliferator-activated Receptor (PPAR) is a nuclear hormone receptor that binds specific response elements (PPRE) in the promoter of target genes. Three PPAR isoforms (α,β/δ,γ), each coded by a different gene, have been reported. We have previously shown that PPAR γ1 is highly over expressed in human breast cancer cells, and the high expression of PPAR γ1 in human breast cancer cells is driven by a tumor specific promoter. These data indicate that the pA1 promoter is the major promoter used in human breast cancer cell lines for PPAR γ1 transcription. We cloned a 3kb promoter region that is upstream of exon A1 of PPAR γ1 and identified a highly GC-rich (90%) 263bp minimum promoter fragment of pA1 by using standard deletion analysis. To obviate the difficulty of working with such a GC-rich promoter and to speed the identification of the transcription factor(s) mediating tumor specific expression of PPARγ1,we have merged microarry analysis with the Transcription Element Search Software, TESS analysis. These analysis identified 29 potential transcription factors that could bind to one of 209 different binding site within the 263bp fragment. These data were used in conjunction with microarry data comparing the expression of thousand of gene between MCF-7 and Normal Human Mammary Epitheial Cell (HMEC). We identified the only one of the transcription factor, MAZ (Myc-associated zinc finger protein, Genebank Accession: BC041629), that was highly over expressed in MCF-7 compared to HMEC. Gel shift assay demonstrated that MAZ is able to bind to the putative MAZ response element in the tumor specific promoter but not in the PPAR γ1 promoter used in HMEC. Using both MAZ over expression and silencing RNA approaches, these data indicates that MAZ plays a critical role in the tumor specific transcriptional activation of PPAR γ1 by binding to the tumor specific promoter region and possibly promoting PPAR γ1 promoter switching in tumor cells. Our gene chip data also showed GADD45 (the Growth Arrest and DNA Damage-inducible gene 45) mRNA level is regulated by PPAR γ1. When MAZ is transiently expressed in HMEC, GADD45 mRNA level also changes dramatically. As GADD45 is already known as an important regulator of cell cycle progression, apoptosis and DNA repair, and this points to one possible mechanism of PPARγ1 action in breast cancer. Together these data begin to define the mechanism of the tumor-specific expression of PPARγ1 provides unique insights into novel targets for therapeutic intervention.

[Proc Amer Assoc Cancer Res, Volume 46, 2005]