Inflammation and Intestinal Homeostasis-Associated Genes in Colorectal Cancer
Sammanfattning: Colorectal cancer (CRC) is a global ‘killer’ and every year more than 1.2 million new individuals are affected and approximately 600 000 succumb to the disorder. Several mechanisms such as inactivation of tumor suppressor genes, activation of oncogenes and dysregulation of cell fate determinating pathways e.g. Wnt and Notch can initiate a cancerous cell growth and promote colorectal tumorigenesis. In addition, most tumors are exposed to an inflammatory environment, which together with the presence of mitogenic and angiogenic signals may sustain several hallmarks of cancer. Genetic alterations in inflammatory genes are associated with chronic inflammatory bowel disease, which is a strong risk factor of developing CRC. Scientists have for a long time looked for ‘the Key’ that would unlock the ‘cancer door’ but more likely cancer should be considered as not one but many diseases where almost every single patient is genetically and clinically unique. Hence recent research has turned to identify such inter-individual discrepancies and to find disease markers and strategies for guiding clinicians when tailoring individual management and optimized therapy. A deeper understanding of the regulation and genetic variation of inflammation and intestinal-homeostasis associated genes is pivotal to find potential targets for future therapies.The present thesis focuses on genetic variation and alterations in inflammatory genes as well as genes specifically involved in maintaining intestinal homeostasis. The most common anti-inflammatory drugs, NSAIDs, inhibit the prostanoid-generating COX-enzymes and are associated with decreased CRC risk when administered for a long time. Unfortunately, continuous NSAID treatment may lead to severe side-effects such as gastrointestinal bleeding, possibly through the ablation of non-PGE2 prostanoids. Therefore, a more specific inhibition of PGE2 has been suggested to be superior to classical NSAIDs. In papers I and II, the terminal PGE2 generating enzyme mPGES1 was studied in the context of intestinal cancer. Unexpectedly, ApcMin/+ mice with a targeted deletion of the mPGES1 encoding gene displayed significantly more and larger intestinal adenomas as compared to their wilde-type (wt) littermates. Probably this was due to the redirected generation of PGE2 towards non-PGE2 prostanoids seen in the murine tumors, resulting in enhanced pro-tumorigenic activity of these transmitter substances. Next, with a battery of functional and descriptive assays we investigated whether the outcome of mPGES1 expression and activity could depend on the genetic profile of the tumor e.g. the Apc mutational status. Indeed, high expression of mPGES1 was associated with the presence of wt-Apc, both in vitro and in vivo, most likely depending on mPGES1 mRNA stabilization rather than upregulation through ?–catenin/Lef/Tcf4 signaling.NF?B is a major regulator of inflammation e.g. through the production of inflammatory cytokines. Variations in genes controlling inflammation and angiogenesis could potentially be used as biomarkers to identify patients with increased risk of CRC development, and/or to identify those with high risk of a rapidly progressing disease. Further, such analyzes have been suggested to select patients, which may benefit from specific anti-inflammatory or anti-angiogenic therapies. In paper III, genetic alterations in NF?B associated genes were studied among CRC patients and healthy controls. The NF?B negative regulator TNFAIP3 was found to exert tumor suppressive functions in CRC and moreover, homozygous mutant TNFAIP3 (rs6920220), homozygous mutant NF?B -94 ATTG ins/del and heterozygous NLRP3 (Q705K) were identified as prognostic markers for identifying CRC patients with a high risk of rapid progression. Further studies, which focus on the potential to treat such patients with anti-inflammatory IL-1? targeting therapies, are warranted.In the intestinal epithelium, Notch and Wnt signaling function in synergy to maintain homeostasis and together these pathways promote stem cell renewal and drive proliferation. Thus, dysregulation and/or overactivation of one of the two pathways could potentially lead to simultaneous activation of the other. While the genetic mechanisms explaining aberrant Wnt signaling in CRC are well-known, the reasons for the Notch pathway activation are less so. Further, relatively little is known about the mechanisms linking the two pathways in CRC. In paper IV, we addressed this question with a set of experimental in vitro assays, hereby identifying Notch2 together with several additional genes classically belonging to the Notch pathway, as putative targets for canonical and non-canonical Wnt signaling. We therefore suggest that aberrant Notch signaling in colon cancer cells may be the result of dysregulated Wnt signaling.In summary, the results here presented add a couple of pieces to the immensely complex jigsaw puzzle connecting intestinal homeostasis, inflammation and CRC. These results may aid in identifying future biomarkers or potential drug targets that could take us to the next level in the war against cancer.
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