Colonic Cytokinetics and Cell Signaling: Dietary Effects

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DESCRIPTION (provided by applicant): Among dietary factors, there are cogent data indicating a protective effect of n-3 polyunsaturated fatty acids (PUFA) e.g., eicosapentaenoic acid (EPA; 20:5n-3) and docosahexaenoic acid (DHA, 22:6n-3), on colon cancer. In contrast, dietary lipids rich in n-6 PUFA, e.g., linoleic acid (18:2n-6) and arachidonic acid (20:4n-6), enhance the development of colon tumors. This is significant because the typical Western diet contains 10 to 20 times more n-6 than n-3 PUFA. Unfortunately, to date, a unifying mechanistic hypothesis addressing why n-3 PUFA selectively suppress colon cancer compared to n-6 PUFA (the major dietary form of PUFA in the U.S. diet) is lacking. We have recently shown that (i) the antitumorigenic effects of n-3 PUFA are in part the result of the coordinated upregulation of targeted apoptosis dudng the initiation phase of tumorigenesis and spontaneous apoptosis during tumor promotion; (ii) the effect is enhanced by butyrate; (iii) EPA and DHA induce compositional changes in mitochonddal membrane phospholipids which facilitate apoptosis; and (iv) n-3 PUFA suppress oncogenic Ras activation, a powerful antiapoptotic signal in the colon. Since the inhibition of apoptosis is now thought to be an integral component in the genesis of colorectal tumors, the overall goal of this proposal is to understand how n-3 PUFA promote apoptosis in colonocytes. Since n-3 PUFA are uniquely capable of altering cell membrane properties due to both the number and position of double bonds, we have hypothesized that n-3 PUFA alter colonocyte mitochondrial and plasma membrane composition and function, thereby creating a permissive environment for apoptosis. We propose to utilize a combination of experimental models (azoxymethane-injected rat, oxidatively stressed SOD2+/- mouse; normal and malignant transformed mouse and human colonocyte cell lines) in order to elucidate n-3 PUFA apoptogenic signaling in the colon. To test our hypothesis the following specific aims are proposed: Aim #1 will elucidate the mechanisms by which n-3 PUFA modulate intrinsic (mitochondria-mediated) cell death signaling; and Aim #2 will determine the mechanisms by which n-3 PUFA modulate extrinsic (non-mitochondrial) cell death signaling. At present, the molecular basis of the n-3 PUFA effects on colonocyte apoptosis is a complete black box. The proposed experiments represent the first attempt to provide an explanation of EPA and DHA action at the molecular level.