A novel peroxisome proliferator-activated receptor γ isoform with dominant negative activity generated by alternative splicing

We examined the peroxisome proliferator-activated receptor γ (PPARG) locus in an attempt to identify expressed sequence tags and/or conserved non-coding sequences in the intron sequences containing open reading frames and potentially able to encode new proteins. We identified a new PPABG transcript, defined γORF4, which harbors a readthrough in intron 4. The expected translated protein lacks the ligand-binding domain encoded by exons 5 and 6. We identified the transcript in human tumor cell lines and tissues, synthesized the cDNA, and cloned it in expression vectors. Using transient transfections, we found that yORF4 cDNA is translated into a predominantly nuclear protein that does not transactivate a reporter gene. Moreover, the isoform is dominant negative versus PPARγ. Interestingly, γORF4 was expressed in vivo in a series of sporadic colorectal cancers. In some cases, it was expressed, albeit at lower levels, also in the mucosa adjacent to the tumors, suggesting that it may be related to tumorigenesis. A tumorigenic effect of yORF4 is in line with our finding that yORF4 has not only lost the capacity to restrain cell growth but has acquired the potential to stimulate it. In conclusion, this study demonstrates that γORF4 is expressed in vivo, that it has lost some PPARγ properties, and that it affects PPARγ functioning. The ability to counteract PPARγ suggests that γORF4 plays a role in the pathogenesis of colorectal cancers.