Adenomatous polyposis coli regulation of WNT signaling via retinoic acid and cycloxygenase-2

Genetic and pharmacological data support the conclusion that cyclooxygenase-2 (COX-2) is negatively regulated by the tumor suppressor adenomatous polyposis coli (APC) and that the loss of APC function and subsequent dysregulation of COX-2 expression promotes turnorigenesis. APC is a negative regulator of ?-catenin and it has been hypothesized that upregulation of COX-2 following loss of APC function is a consequence of dysregulated expression of ?-catenin. These observations are challenged by recent studies suggesting that a product of COX-2, prostaglandin E2 (PGE2), stimulates ?-catenin signaling. These important findings imply that PGE2 and COX-2 are upstream signaling molecules necessary for activation of the ?-catenin/TCF/LEF pathway. In this dissertation I provide evidence supporting the existence of a functional association between APC and COX-2 through a mechanism that involves retinoic acid. The work presented here also demonstrates that PGE2 is a key determinant of ?-catenin stabilization when the function of APC is compromised. Using zebrafish as a model system 1 demonstrate that COX-2 is upregulated in APC mutant zebrafish owing to a deficiency in retinoic acid and that this does not depend on the ability of APC to control ?-catenin. Second, I present data showing that retinoic acid regulates COX-2 by controlling expression of the transcription factor C/EBP-?. Third, I report that retinoic acid is a negative regulator of ?-catenin owing to its ability to suppress COX-2 expression and decrease PGE2 synthesis. My studies demonstrate that in wild-type zebrafish PGE2 stabilizes ?-catenin and that this stabilization is mediated by disassembly of the ?-catenin destruction complex. Conversely, inhibition of PGE2 production in APC mutant zebrafish restores the association between axin and ?-catenin. Interestingly, I found that truncated APC can form functional complexes that result in ?-catenin degradation and that the stability of these entities is regulated by the levels of PGE2. The data I present support a sequence of events in which mutations in APC impair retinoic acid biosynthesis, cause elevated levels of C/EBP-P, upregulation of COX-2, PGE2 accumulation and activation of the ?-catenini/Wnt signaling pathway. The study has important implications for our understanding of the molecular mechanisms that participate in colon cancer progression.