While Western diets are implicated in increased colon cancer risk, molecular underpinnings of these dietary effects remain largely unknown. The azoxymethane (AOM) and Apc+/min mouse models mimic many features of human colon cancer, including tumor promotion by Western diet. We showed that Western diet up-regulated ligands for epidermal growth factor receptors (EGFR). Furthermore, EGFR was required for tumor promotion. Several EGFR ligands are released from membrane-bound pro-ligands by the lipid-raft-associated metalloproteinase ADAM17. Our recent studies indicate that ADAM17 is down-regulated by microRNA-145 (miR-145), whereas K-ras, an EGFR effector is suppressed by miR-143. These co-transcribed miRNAs are down-regulated in human colon cancer. We recently showed that EGFR signals downregulate miR-143 and miR-145 in AOM and Apc+/min tumors. Furthermore, these miRNA reductions are necessary for EGFR mitogenic effects. Based on our data we hypothesize that ADAM17 up-regulation and miR-143 and miR-145 down-regulation play essential roles in Western diet-induced tumor promotion. We propose several aims to address this hypothesis:
Aim 1 : Elucidate the requirement for ADAM17 in diet-promoted colonic tumorigenesis. We hypothesize that ADAM17 inhibition or deletion will suppress diet-related tumor promotion. We will use 1a) the AOM model in conditional ADAM17-deleted mice; 1b) the Apc+/min model with a novel ADAM17 pharmacological inhibitor INCB3619 to dissect the role of ADAM17 in diet-promoted tumorigenesis; 1c) in vitro studies of lipid rafts to dissect fatty acid effects on ADAM17 in colon cancer cells.
Aim 2 : To determine contributions of miR-143 and miR-145 in diet-promoted colonic tumorigenesis. We hypothesize that loss of these miRNAs is necessary for diet-induced tumor promotion. We will employ Apc+/min mouse interbred with 2a) transgenic mice expressing villin-promoter regulated pre-miR-143 and pre-miR-145; or with 2b) miR-143 null mice or with 2c) miR-145 null mice to uncover the role of these miRNAs in diet- promoted tumorigenesis.
In aim 2 d), we will examine other miRNAs implicated in ADAM17 regulation and/or diet-related tumorigenesis, including miR-1, -31, -148, and -152.
Aim 3 : Determine the regulation of miR-143 and miR-145 by Western diet and tumorigenesis. We hypothesize that Western diet and malignant transformation suppress transcription, while neoplastic transformation also deranges processing. We will 3a) assess effects of ADAM17, diet and neoplastic stage on pri-, pre- and mature levels of miR-143, -145 in in vivo models; 3b) dissect EGFR and fatty acid effects on miR-143/-145 promoter activity using mutant deletions to identify cis regulatory elements; 3c) Determine proteins differentially co-associating with biotinylated miR-143 or miR-145 in murine processing-competent YAMC and processing-incompetent CT26 colon cancer cells to discover deregulated processing factors. Our proposal will clarify the role of ADAM17 and test a novel hypothesis that EGFR and these miRNAs form a self-amplifying loop that drives diet-promoted tumorigenesis.
Colon cancer is a leading cause of cancer-related deaths in the US. Western diets increase the risk of colon cancer in part by increasing growth factor signals. Elucidating how diet controls growth factors will help us understand how tumors develop and suggest new approaches to block their growth.
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