Docosahexaenoic acid alters epidermal growth factor receptor related signaling by disrupting its lipid raft association

Docosahexaenoic acid (DHA), a 22:6 n-3 polyunsaturated fatty acid (PUFA), is the longest and most highly unsaturated PUFA found in most membranes, and has been shown to inhibit cancer cell growth in part by modifying cell signaling. In the current study, supplementation with DHA altered the membrane fatty acid composition both in vitro and in vivo of the WiDr colon carcinoma. Additionally there were observed changes in tumor and serum cholesterol. Alterations to epidermal growth factor receptor (EGFR) signaling upon DHA supplementation were examined in A549 lung adenocarcinoma, WiDr colon carcinoma and MDA-MB-231 breast carcinoma cell lines. Interestingly, EGFR phosphorylation, most notably at the tyrosine 1068 residue, is dramatically upregulated, and EGFR association with the Sos1 guanine nucleotide exchange factor is concomitantly increased upon DHA supplementation. However, GTP-bound Ras and phosphorylated Erk 1/2 are paradoxically downregulated in the same treatments. Previous reports have noted changes in membrane microdomains upon DHA supplementation, and our findings confirmed that EGFR, but not Ras, is excluded from lipid raft fractions in DHA treated cells resulting in a decreased association of Ras with Sos1 and the subsequent downregulation of Erk signaling. Xenografts of the A549 cell line implanted in athymic mice fed a control high fat diet or a diet high in DHA confirmed our in vitro data. These results demonstrate for the first time a functional consequence of decreased EGFR protein in lipid raft microdomains as a result of DHA treatment in three different cancer models. In addition, we report the ability of DHA to enhance the efficacy of EGFR inhibitors on anchorage-independent cell growth (soft agar), providing evidence for the potential development of enhanced combination therapies.