PPAR-gamma is a member of the nuclear receptor gene family that has been shown to play a key role in adipogenesis. PPAR-gamma is also expressed at very high levels in the colonic epithelium. Since colon cancer is a leading cause of cancer deaths, it is of great interest to understand the role of this receptor in the development of the colonic epithelium. The investigator's previous work has indicated that ligand activation of PPAR-gamma in human colon cancer cells induces a differentiative-like response including the cessation of cell growth and expression of genes characteristic of mature colonic epithelium. Most recently, he has shown that human colon cancer is associated with loss of function mutations in PPAR-gamma. Paradoxically, others have shown that TZD ligands, when applied to the polyp-prone min mouse that are genetically deficient in APC, develop more colon polyps. The present proposal will critically evaluate the role of PPAR-gamma in normal and abnormal colon development, and study the transcriptional mechanisms that underlie these functions. Firstly, the investigator will determine how PPAR-gamma intersects pathways known to regulate colon cell biology. Preliminary data show a strong connection between PPAR-gamma and TGF-beta signaling; hence, biochemical and genetic studies will focus on how Smad proteins interact with PPAR-gamma, and the relative contributions these interactions make to both signaling systems in the colon. The investigator will also study the transcriptional mechanisms that enable PPAR-gamma to induce a distinct program of gene expression when it is activated in colon cells. This will utilize both yeast 2-hybrid screens and biochemical purification to isolate components which interact with PPAR-gamma in a colon- or epithelium-selective manner. The function of novel proteins will be studied to discern their transcriptional and biological properties by expressing them in various mammalian cells. The program of colon-selective genes regulated by PPAR-gamma will be better characterized through the use of SAGE analysis, applied to mRNA from colon cells treated with PPAR-gamma ligands. This work, done in collaboration with Dr. Ken Kinzler, will be used to characterize the expression of known genes, and to clone novel genes. Again, the function of downstream targets of PPAR-gamma will be characterized for their ability to regulate colon cell growth and differentiation. Finally, the role of PPAR-gamma in modulating colon cancer will be critically evaluated in transgenic mice, using potential dominant-negative alleles with a colon-selective promoter, and heterozygous PPAR-gamma KO mice that are already available. The initial studies will evaluate carcinogenesis using the chemical carcinogen DMH, though other transgenic methods of inducing colon cancer will also be evaluated. In summary, the investigator hopes that these studies will to provide a unique insight into the role of PPAR-gamma in normal and malignant colon development. Since PPAR-gamma ligands are clinically available now, this work may provide a framework in which to contemplate new therapeutic approaches to colon cancer.
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| Drori, Stavit; Girnun, Geoffrey D; Tou, Liqiang et al. (2005) Hic-5 regulates an epithelial program mediated by PPARgamma. Genes Dev 19:362-75 |
| Girnun, Geoffrey D; Smith, Wendy M; Drori, Stavit et al. (2002) APC-dependent suppression of colon carcinogenesis by PPARgamma. Proc Natl Acad Sci U S A 99:13771-6 |
| Rosen, E D; Spiegelman, B M (2001) PPARgamma : a nuclear regulator of metabolism, differentiation, and cell growth. J Biol Chem 276:37731-4 |
