Harmony F. Turk

Dietary n-3 polyunsaturated fatty acids (PUFA) decrease obesity-associated Th17 cell-mediated inflammation during colitis.

Clinical and experimental evidence suggests that obesity-associated inflammation increases disease activity during colitis, attributed in part to the effects of Th17 cells. Using a model of concurrent obesity and colitis, we monitored changes in critical immune cell subsets and inflammatory biomarker expression in three key tissues: visceral adipose tissue, colon (local inflammatory site) and spleen (systemic inflammatory site), and we hypothesized that n-3 PUFA would reduce the percentage of inflammatory immune cell subsets and suppress inflammatory gene expression, thereby improving the disease phenotype. Obesity was induced in C57BL/6 mice by feeding a high fat (HF) diet (59.2% kcal) alone or an isocaloric HF diet supplemented with fish oil (HF-FO) for 12 weeks. Colitis was induced via a 2.5% trinitrobenzene sulfonic acid (TNBS) enema. The HF-FO diet improved the obese phenotype by reducing i) serum hormone concentrations (leptin and resistin), ii) adipose tissue mRNA expression of inflammatory cytokines (MCP-1, IFNγ, IL-6, IL17F and IL-21) and iii) total (F4/80+ CD11b+) and inflammatory adipose tissue M1 (F4/80+ CD11c+) macrophage content compared to HF (P<0.05). In addition, the HF-FO diet reduced both colitis-associated disease severity and colonic mRNA expression of the Th17 cell master transcription factor (RORγτ) and critical cytokines (IL-6, IL-17A, IL-17F, IL-21, IL-23 and IFNγ) versus HF (P<0.05). Compared to HF, the percentage of both splenic Th17 and Th1 cells were reduced by the HF-FO group (P<0.05). Under ex vivo polarizing conditions, the percentage of HF-FO derived CD4+ T cells that reached Th17 cell effector status was suppressed (P = 0.05). Collectively, these results indicate that n-3 PUFA suppress Th1/Th17 cells and inflammatory macrophage subsets and reconfigure the inflammatory gene expression profile in diverse tissue sites in obese mice following the induction of colitis.

Alteration of EGFR Spatiotemporal Dynamics Suppresses Signal Transduction

The epidermal growth factor receptor (EGFR), which regulates cell growth and survival, is integral to colon tumorigenesis. Lipid rafts play a role in regulating EGFR signaling, and docosahexaenoic acid (DHA) is known to perturb membrane domain organization through changes in lipid rafts. Therefore, we investigated the mechanistic link between EGFR function and DHA. Membrane incorporation of DHA into immortalized colonocytes altered the lateral organization of EGFR. DHA additionally increased EGFR phosphorylation but paradoxically suppressed downstream signaling. Assessment of the EGFR-Ras-ERK1/2 signaling cascade identified Ras GTP binding as the locus of the DHA-induced disruption of signal transduction. DHA also antagonized EGFR signaling capacity by increasing receptor internalization and degradation. DHA suppressed cell proliferation in an EGFR-dependent manner, but cell proliferation could be partially rescued by expression of constitutively active Ras. Feeding chronically-inflamed, carcinogen-injected C57BL/6 mice a fish oil containing diet enriched in DHA recapitulated the effects on the EGFR signaling axis observed in cell culture and additionally suppressed tumor formation. We conclude that DHA-induced alteration in both the lateral and subcellular localization of EGFR culminates in the suppression of EGFR downstream signal transduction, which has implications for the molecular basis of colon cancer prevention by DHA.

Linoleic acid and butyrate synergize to increase Bcl-2 levels in colonocytes

The biological properties of polyunsaturated fatty acid (PUFA) classes have been the source of much contention. For example, n‐3 PUFA are chemoprotective, whereas n‐6 PUFA may promote tumor development. Since dietary components can have combinatorial effects, we further examined the apoptotic properties of n‐3 or n‐6 fatty acids when combined with different fiber sources. Mice were fed diets supplemented with either fish oil (FO; enriched in n‐3 PUFA) or corn oil (CO; enriched in n‐6 PUFA) and nonfermentable (cellulose) or fermentable (pectin) fiber sources. In complementary experiments, immortalized young adult mouse colonic (YAMC) cells were treated with docosahexaenoic acid (DHA; 22:6n‐3) or linoleic acid (LA; 18:2n‐6) with or without butyrate. Mice fed a FO and pectin diet had significantly (p < 0.05) increased levels of apoptosis in colonocytes compared to all other diets. Similarly, apoptosis was highly induced in DHA and butyrate cotreated YAMC cells. In contrast, in both YAMC and mouse models, LA/CO with butyrate/pectin treatment reduced apoptosis and enhanced expression of bcl‐2. The LA and butyrate induced antiapoptotic phenotype was reversed by knocking down bcl‐2 using targeted siRNA. In comparison, overexpression of bcl‐2 blocked the proapoptotic effect of DHA and butyrate. These data provide new mechanistic insights into the regulation of apoptosis by dietary PUFA and fiber.

Inhibitory effects of omega-3 fatty acids on injury-induced epidermal growth factor receptor transactivation contribute to delayed wound healing

Epidermal growth factor receptor (EGFR)-mediated signaling is required for optimal intestinal wound healing. Since n-3 polyunsaturated fatty acids (PUFA), specifically docosahexaenoic acid (DHA), alter EGFR signaling and suppress downstream activation of key signaling pathways, we hypothesized that DHA would be detrimental to the process of intestinal wound healing. Using a mouse immortalized colonocyte model, DHA uniquely reduced EGFR ligand-induced receptor activation, whereas DHA and its metabolic precursor eicosapentaenoic acid (EPA) reduced wound-induced EGFR transactivation compared with control (no fatty acid or linoleic acid). Under wounding conditions, the suppression of EGFR activation was associated with a reduction in downstream activation of cytoskeletal remodeling proteins (PLCγ1, Rac1, and Cdc42). Subsequently, DHA and EPA reduced cell migration in response to wounding. Mice were fed a corn oil-, DHA-, or EPA-enriched diet prior to intestinal wounding (2.5% dextran sodium sulfate for 5 days followed by termination after 0, 3, or 6 days of recovery). Mortality was increased in EPA-fed mice and colonic histological injury scores were increased in EPA- and DHA-fed mice compared with corn oil-fed (control) mice. Although kinetics of colonic EGFR activation and downstream signaling (PLCγ1, Rac1, and Cdc42) were delayed by both n-3 PUFA, colonic repair was increased in EPA- relative to DHA-fed mice. These results indicate that, during the early response to intestinal wounding, DHA and EPA uniquely delay the activation of key wound-healing processes in the colon. This effect is mediated, at least in part, via suppression of EGFR-mediated signaling and downstream cytoskeletal remodeling.

n3 PUFAs Reduce Mouse CD4+ T-Cell Ex Vivo Polarization into Th17 Cells

Little is known about the impact of n3 (ω3) PUFAs on polarization of CD4(+) T cells into effector subsets other than Th1 and Th2. We assessed the effects of dietary fat [corn oil (CO) vs. fish oil (FO)] and fermentable fiber [cellulose (C) vs. pectin (P)] (2 × 2 design) in male C57BL/6 mice fed CO-C, CO-P, FO-C, or FO-P diets for 3 wk on the ex vivo polarization of purified splenic CD4(+) T cells (using magnetic microbeads) into regulatory T cells [Tregs; forkhead box P3 (Foxp3(+)) cells] or Th17 cells [interleukin (IL)-17A(+) and retinoic acid receptor-related orphan receptor (ROR) γτ(+) cells] by flow cytometry. Treg polarization was unaffected by diet; however, FO independently reduced the percentage of both CD4(+) IL-17A(+) (P < 0.05) and CD4(+) RORγτ(+) cells (P < 0.05). Moreover, expression of another critical Th17-cell-related transcription factor, signal transducer and activator of transcription 3, was reduced by FO. Dietary FO reduced the surface expression of both IL-6R and IL-23R on polarized Th17 cells (P ≤ 0.05), thus interfering with the promotive effects of these critical cytokines on Th17 polarization. Additionally, C57BL/6 mice fed diets enriched in eicosapentaenoic acid (EPA), docosahexaenoic acid (DHA), or DHA + EPA similarly reduced Th17-cell polarization in comparison to CO by reducing expression of the Th17-cell signature cytokine (IL-17A; P = 0.0015) and transcription factor (RORγτ P = 0.02), whereas Treg polarization was unaffected. Collectively, these data show that n3 PUFAs exert a direct effect on the development of Th17 cells in healthy mice, implicating a novel n3 PUFA-dependent, anti-inflammatory mechanism of action via the suppression of the initial development of this inflammatory T-cell subset.

n-3 Polyunsaturated Fatty Acids and Mechanisms to Mitigate Inflammatory Paracrine Signaling in Obesity-Associated Breast Cancer

Globally, the prevalence of obesity is increasing which subsequently increases the risk of the development of obesity-related chronic diseases. Low-grade chronic inflammation and dysregulated adipose tissue inflammatory mediator/adipokine secretion are well-established in obesity, and these factors increase the risk of developing inflammation-associated cancer. Breast cancer is of particular interest given that increased inflammation within the subcutaneous mammary adipose tissue depot can alter the local tissue inflammatory microenvironment such that it resembles that of obese visceral adipose tissue. Therefore, in obese women with breast cancer, increased inflammatory mediators both locally and systemically can perpetuate inflammation-associated pro-carcinogenic signaling pathways, thereby increasing disease severity. Herein, we discuss some of these inflammation-associated pro-carcinogenic mechanisms of the combined obese breast cancer phenotype and offer evidence that dietary long chain n-3 polyunsaturated fatty acids (PUFA) may have utility in mitigating the severity of obesity-associated inflammation and breast cancer.

Immunomodulatory action of dietary fish oil and targeted deletion of intestinal epithelial cell PPARδ in inflammation-induced colon carcinogenesis

The ligand-activated transcription factor peroxisome proliferator-activated receptor (PPAR)-δ is highly expressed in colonic epithelial cells; however, the role of PPARδ ligands, such as fatty acids, in mucosal inflammation and malignant transformation has not been clarified. Recent evidence suggests that the anti-inflammatory/chemoprotective properties of fish oil (FO)-derived n-3 polyunsaturated fatty acids (PUFAs) may be partly mediated by PPARδ. Therefore, we assessed the role of PPARδ in modulating the effects of dietary n-3 PUFAs by targeted deletion of intestinal epithelial cell PPARδ (PPARδ(ΔIEpC)). Subsequently, we documented changes in colon tumorigenesis and the inflammatory microenvironment, i.e., local [mesenteric lymph node (MLN)] and systemic (spleen) T cell activation. Animals were fed chemopromotive [corn oil (CO)] or chemoprotective (FO) diets during the induction of chronic inflammation/carcinogenesis. Tumor incidence was similar in control and PPARδ(ΔIEpC) mice. FO reduced mucosal injury, tumor incidence, colonic STAT3 activation, and inflammatory cytokine gene expression, independent of PPARδ genotype. CD8(+) T cell recruitment into MLNs was suppressed in PPARδ(ΔIEpC) mice. Similarly, FO reduced CD8(+) T cell numbers in the MLN. Dietary FO independently modulated MLN CD4(+) T cell activation status by decreasing CD44 expression. CD11a expression by MLN CD4(+) T cells was downregulated in PPARδ(ΔIEpC) mice. Lastly, splenic CD62L expression was downregulated in PPARδ(ΔIEpC) CD4(+) and CD8(+) T cells. These data demonstrate that expression of intestinal epithelial cell PPARδ does not influence azoxymethane/dextran sodium sulfate-induced colon tumor incidence. Moreover, we provide new evidence that dietary n-3 PUFAs attenuate intestinal inflammation in an intestinal epithelial cell PPARδ-independent manner.

Antagonizing Arachidonic Acid-Derived Eicosanoids Reduces Inflammatory Th17 and Th1 Cell-Mediated Inflammation and Colitis Severity

During colitis, activation of two inflammatory T cell subsets, Th17 and Th1 cells, promotes ongoing intestinal inflammatory responses. n-6 polyunsaturated fatty acid- (PUFA-) derived eicosanoids, such as prostaglandin E2 (PGE2), promote Th17 cell-mediated inflammation, while n-3 PUFA antagonize both Th17 and Th1 cells and suppress PGE2 levels. We utilized two genetic mouse models, which differentially antagonize PGE2 levels, to examine the effect on Th17 cells and disease outcomes in trinitrobenzene sulfonic acid- (TNBS-) induced colitis. Fat-1 mice contain the ω3 desaturase gene from C. elegans and synthesize n-3 PUFA de novo, thereby reducing the biosynthesis of n-6 PUFA-derived eicosanoids. In contrast, Fads1 Null mice contain a disrupted Δ5 desaturase gene and produce lower levels of n-6 PUFA-derived eicosanoids. Compared to Wt littermates, Fat-1 and Fads1 Null mice exhibited a similar colitic phenotype characterized by reduced colonic mucosal inflammatory eicosanoid levels and mRNA expression of Th17 cell markers (IL-17A, RORγτ, and IL-23), decreased percentages of Th17 cells and, improved colon injury scores (P ≤ 0.05). Thus, during colitis, similar outcomes were obtained in two genetically distinct models, both of which antagonize PGE2 levels via different mechanisms. Our data highlight the critical impact of n-6 PUFA-derived eicosanoids in the promotion of Th17 cell-mediated colonic inflammation.

Analysis of Epidermal Growth Factor Receptor Dimerization by BS 3 Cross-Linking

Dimerization of receptor tyrosine kinases is a well-characterized process. It is imperative for the activation of many receptors, including the epidermal growth factor receptor (EGFR). EGFR has been shown to be regulated by a number of factors, including lipid raft localization. For example, alteration of the lipid raft localization of EGFR has been demonstrated to modify receptor dimerization. This protocol describes an assay to quantify EGFR dimers using BS(3) cross-linking. BS(3) cross-linking is well suited for this purpose because of its length, water solubility, and membrane impermeability. Although this protocol is written specifically for EGFR, the assay can be extrapolated in order to characterize dimerization of other receptor tyrosine kinases.

Omega-3 Fatty Acids in Cancer Prevention and Control: A Membrane Perspective

Long-chain n-3 polyunsaturated fatty acids (PUFA) have been shown to provide health benefits in a number of diseases including several forms of cancer. In this chapter we will discuss in detail some of the prominent mechanisms through which n-3 PUFA and its metabolites are believed to function in the prevention of colon tumorigenesis. At the plasma membrane, n-3 PUFA antagonize the production of inflammatory and procarcinogenic n-6 PUFA (i.e., arachidonic acid)-derived metabolites. Additionally, the highly unsaturated nature of n-3 PUFA impacts cell membrane properties and dynamics thereby altering numerous cellular functions, including intracellular signaling, cell growth, survival, and proliferation. Due to the sterically incompatible relationship between docosahexaenoic acid, sphingolipids, and cholesterol, the major constituents of lipid rafts, n-3 PUFA modulate these crucial membrane microdomains and perturb efficient signal transduction thereby eliciting the same biological effects exploited by some anti-cancer therapies. Moreover, we discuss how alterations in lipid rafts and downstream signaling impact both epithelial cells and the activation of immune cells. This is noteworthy, because chronic inflammation plays a critical role in tumorigenesis. Therefore, we also present an overview regarding the anti-inflammatory and immunomodulatory mechanisms through which n-3 PUFA perturb the tumor microenvironment and downregulate the activation of critical transcription factors and target genes with an established role in cancer development. Finally, we discuss recent evidence suggesting that n-3 PUFA in combination with other dietary bioactive nutrients, such as soluble fiber and curcumin, could be beneficial in cancer prevention. Collectively, we demonstrate that dietary n-3 PUFA have utility in the prevention of cancer development through mechanisms centered at both the molecular, cellular (plasma membrane), and tissue level.