Nuclear Hormone Receptors in Intestinal Biology

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DESCRIPTION (provided by applicant): Carcinoma of the colon and rectum is the second-leading cause of cancer related deaths in the United States and thus represents a major public health concern. The disease develops due to a loss of the normal regulatory pathways that govern the transition from undifferentiated stem cell to mature colon epithelial cell. The nuclear hormone receptor (NHR) super family of transcription factors are important regulators of many biological pathways including cellular differentiation. One family of NHRs, the peroxisome proliferator-activated receptors (PPARs), is activated by fatty acids and certain fatty acid metabolites and plays important role in metabolic homeostasis. For example, the PPAR subtype is a central regulator of adipocyte differentiation and synthetic agonists of the receptor enhance insulin sensitivity and are being widely used in the treatment of type ll diabetes mellitus. We and others have found PPAR to be strongly expressed in post-mitotic, differentiated colon epithelial cells and have demonstrated that activation of PPAR in a broad spectrum of colorectal cancer cell lines induces growth inhibition associated with a delay in the 01 phase of the cell cycle. Activation of the receptor also leads to an increase in several markers of intestinal epithelial cell differentiation. Consistent with an anti-oncogenic, pro-differentiation role for PPAR in the colon, loss of function mutations in the receptor have been identified in a subset of colorectal tumors and cell lines. Collectively, these findings raise the possibility that PPAR agonists may have clinical value as a form of differentiation therapy for colorectal cancer. However, despite these advances our understanding of PPAR signaling in the colon remains superficial. The major focus of this proposal is to define the mechanistic basis for the biological phenotype induced by PPAR activation in colorectal cancer cells. Specifically, we hypothesize that binding of ligand to apo-PPAR causes conformational changes in the receptor leading to the recruitment of a specific set of co-regulatory molecules that then leads to the regulation of target genes that are important modifiers of colon epithelial cell growth and differentiation. Our experimental approach will focus on 1) identifying the structural domains unique to the PPAR subtype that allows the ligand-bound receptor to induce colorectal cancer cell differentiation; 2) identify the co-regulators critical for PPAR-mediated differentiation specifically in the colon; and 3) define the role of the PPAR target gene transforming growth factor-stimulated clone-22 (TSC-22) in intestinal epithelial cell function. Results from these experiments have the potential not only to lead to the design of PPAR activators with enhanced anti-tumor efficacy, but also help clarify the molecular basis of intestinal epithelial cell differentiation.