Linoleic acid (LA) consumption is high in humans. LA increases azoxymethane (AOM)-induced colorectal tumorigenesis in rodents, but the impact of LA on colorectal cancer (CRC) in humans is unknown. 15- Lipoxygenase-1 (15-LOX-1) is the rate-limiting enzyme for the oxidative metabolism of LA to generate 13-S- HODE, a natural activating ligand of PPAR-gamma. PPAR-gamma suppresses aberrant Wnt/B-catenin signaling, a critical event initiating and driving CRC tumorigenesis. 15-LOX-1 is downregulated in human colorectal polyps and CRCs, but the influence of 15-LOX-1 on dietary LA modulation of CRC risk is unknown. In mice with intestinally targeted human 15-LOX-1 (15-LOX-1-Gut), we found that transgenic 15-LOX-1 expression suppressed CRC tumorigenesis, and 15-LOX-1 expression was downregulated in all experimentally-induced tumors. Furthermore, the increase in AOM-induced CRC with high LA dietary concentrations was repressed in 15-LOX-1-Gut mice; and 15-LOX-1 inhibited B-catenin activation. We therefore hypothesize that 15-LOX-1 loss in colonic epithelial cells is critical for excess LA to promote CRC tumorigenesis via augmenting Wnt/B-catenin signaling. We will test this hypothesis via 3 specific aims:
Aims 1 and 2: Determine the effects of 15-LOX-1 expression (aim 1) and loss of expression (aim 2) in colonic epithelial cells on dietary LA promotion of CRC tumorigenesis and aberrant Wnt/B-catenin signaling.
For aim 1, we will breed 15-LOX-1-Gut and Apc580mu mice to generate Apc580mu-15-LOX-1-Gut mice and examine the effects of 15-LOX-1 expression in mice fed high- or low-LA-content diets on CRC (tumor incidence and multiplicity), crypt proliferative zone length (Ki-67 IHC), activated B-catenin protein levels, and Wnt/B-catenin target gene (c-Myc, Cyclin D1, and Axin2) mRNA levels.
For aim 2, we will replace 12-S-LOX in intestinal epithelial cells of 12/15-LOX knockout mice to generate mice with functional 15-LOX-1 loss and examine the effects in mice fed high- or low-LA-content diets on AOM-induced CRC, crypt proliferative zone length, and 13- HODE levels. We will also evaluate the effect of 15-LOX-1 gain of function (aim 1) and downregulation (aim 2) in human colonic cancerous and normal organoids (isolated from patients' colonic crypts) and cultured with various concentrations of LA on 13-HODE generation, cell proliferation, cell differentiation, and Wnt/B-catenin signaling.
Aim 3 : Determine the tempora and spatial effects of 15-LOX-1 expression in colonic epithelial cells on CRC tumorigenesis in relation to dietary LA intake. We will generate mice with conditional 15-LOX-1 expression, treat them with AOM, and feed them high- or low-LA-content diets. 15-LOX-1 expression will be induced during initiation or progression of CRC tumorigenesis, and we will examine the effects on the outcomes measured in aim 1. We expect these studies to provide important new information to direct development of novel interventions for CRC prevention and to identify subjects with low 15-LOX-1 expression as having increased risk of CRC tumorigenesis with high LA intake.
Animal studies suggest that excess linoleic acid increases colon cancer risk, but human studies have not provided conclusive evidence of this relationship. 15-lipoxygenase-1 (15-LOX-1), the enzyme that metabolizes linoleic acid, is commonly lost in human colon cancers, but how 15-LOX-1 loss modifies the influence of linoleic acid on colon cancer risk is unknown. This project will examine whether 15-LOX-1 loss increases colon cancer risk in the presence of dietary linoleic acid and could show that subjects with low colonic 15-LOX-1 level should avoid excess dietary linoleic acid.
| Tian, Rui; Zuo, Xiangsheng; Jaoude, Jonathan et al. (2017) ALOX15 as a suppressor of inflammation and cancer: Lost in the link. Prostaglandins Other Lipid Mediat 132:77-83 |
