| Description |
This work explores the dysregulation of retinoid metabolism in early oncogenesis models, and the role of retinoids in early development of the intestine. APC is a tumor suppressor gene mutated or silenced in -85% of all colorectal cancers. Through the canonical pathway, APC negatively regulates protein levels of the transcriptional activator, beta catenin. Under Wnt signaling or mutated APC, beta catenin accumulates, and translocates to the nucleus where it binds to TCF/LEF transcription factors and activates Wnt target genes involved in proliferation. In this dissertation I will provide evidence that in addition to the canonical pathway and excessive proliferation, mutations in APC also lead to defects in retinoid metabolism which contributes to a lack of appropriate differentiation within the intestine, contributing to oncogenic progression. Work in our lab has focused on a role for APC in the regulation of retinoic acid (RA) metabolism; production, catabolism, and transport. Using zebrafish as a model system, I will show that certain retinol dehydrogenases (rdhs) are regulated by APC and mutations in APC lead to their loss. The defects seen in apc[mcr] mutants phenocopy antisense morpholino gene knockdowns of ape and certain rdhs, in addition to having characteristic phenotypes of RA deficiency. These phenotypes include loss of jaw and fin structures and terminal differentiation of the pancreas, intestine, liver, and eye. These phenotypes can be rescued by the addition of exogenous RA or the injection of ape or rdh mRNAs. In addition to this, I will show that through the canonical pathway, catabolism of RA is up regulated in three vertebrate model systems; apc[mcr] zebrafish, Apc[Min] mouse model, and human familial adenomatous polyposis (FAP) syndrome. Inhibition of the enzyme responsible for degradation of RA, Cyp26al, leads to increased levels of rescue of RA deficient phenotypes. Thirdly, I will report on the effects of knocking down retinol binding protein 2a (rbp2a) and retinoid transport in zebrafish and how it affects intestinal and ocular development. In Chapter 5 I will also present data on the consequences of crossing Apc'^' mice with Cyp26al knockout mice, which are embryonic lethal due to RA teratogenicity. These data, together, will support a model wherein APC regulates RA metabolism at several levels and contributes to an alternate model suggesting that loss of differentiation, in addition to increases in proliferation, facilitate the progression of colon cancer. In addition, the elucidation of these alternative pathways provides potential targets for future therapeutic and diagnostic needs. |
