Toll-like receptor stimulants in processed meats promote lipid accumulation in macrophages and atherosclerosis in Apoe-/- mice

Dietary intake of processed meat is a risk factor for cardiovascular disease. However, the effects of processed meats on lipid metabolism in macrophages, a key regulator of cardiovascular risk, have remained largely unexplored. Extracts of processed meats, but not their fresh non-processed equivalents, were found to promote a significant increase in macrophage lipid accumulation in vitro. Calibrated receptor-dependent reporter assays revealed that pro-inflammatory stimulants of Toll-like receptor (TLR)-2 and TLR4 were low or undetectable in fresh meats, but rose dramatically following chopping and storage at 4 °C. Lipid accumulation in response to processed meats correlated well with TLR-stimulant content, was significantly reduced in TLR4-deficient macrophages, and was absent in response to meats stored frozen to prevent bacterial growth. TLR-stimulation significantly increased the incorporation of 14C-acetate into cellular lipids, and induced lipid accumulation in the absence of exogenous lipoproteins, suggesting a key role for de novo lipid synthesis in this process. Aortic atherosclerosis was also significantly accelerated in Apoe-/- mice receiving a diet supplemented with TLR-stimulants at concentrations relevant to those measured in processed meats, compared to normal chow. The findings reveal novel mechanisms which may be of relevance to the observed connections between processed meat consumption, inflammatory markers and cardiovascular risk.

Dietary Toll-Like Receptor Stimulants Promote Hepatic Inflammation and Impair Reverse Cholesterol Transport in Mice via Macrophage-Dependent Interleukin-1 Production

Background: The mechanisms connecting dietary intake of processed foods with systemic inflammatory markers and cardiovascular risk remain poorly defined. We sought to compare the abundance of pro-inflammatory stimulants of innate immune receptors in processed foods with those produced by the murine ileal and caecal microbiota, and to explore the impact of their ingestion on systemic inflammation and lipid metabolism in vivo.

Methods and results: Calibrated receptor-dependent reporter assays revealed that many processed foods, particularly those based on minced meats, contain pro-inflammatory stimulants of Toll-like receptor (TLR)-2 and TLR4 at concentrations which greatly exceed those produced by the endogenous murine ileal microbiota. Chronic dietary supplementation with these stimulants, at concentrations relevant to those measured in the Western diet, promoted hepatic inflammation and reduced several markers of reverse cholesterol transport (RCT) in mice. Hepatocytes were found to be insensitive to TLR2- and TLR4-stimulants directly, but their secretion of functional cholesterol acceptors was impaired by interleukin (IL)-1β released by TLR-responsive hepatic macrophages. Hepatic macrophage priming by high-fat diet enhanced the impairment of RCT by ingested endotoxin, and this was reversed by macrophage depletion via clodronate liposome treatment, or genetic deficiency in the IL-1 receptor.

Conclusion: These findings reveal an unexpected mechanism connecting processed food consumption with cardiovascular risk factors, and introduce the food microbiota as a potential target for therapeutic regulation of lipid metabolism.

The Soluble Form of Toll-Like Receptor 2 Is Elevated in Serum of Multiple Sclerosis Patients: A Novel Potential Disease Biomarker

Multiple sclerosis (MS) is an immune-mediated inflammatory demyelinating disease of the central nervous system. It was previously shown that toll-like receptor (TLR)-2 signaling plays a key role in the murine experimental autoimmune encephalomyelitis (EAE) model of MS, and that TLR2-stimulation of regulatory T cells (Tregs) promotes their conversion to T helper 17 (Th17) cells. Here, we sought potential sources of TLR2 stimulation and evidence of TLR2 activity in MS patient clinical samples. Soluble TLR2 (sTLR2) was found to be significantly elevated in sera of MS patients (n = 21), in both relapse and remission, compared to healthy controls (HC) (n = 24). This was not associated with the acute phase reaction (APR) as measured by serum C-reactive protein (CRP) level, which was similarly increased in MS patients compared to controls. An independent validation cohort from a different ethnic background showed a similar upward trend in mean sTLR2 values in relapsing-remitting MS (RRMS) patients, and significant differences in sTLR2 values between patients and HC were preserved when the data from the two cohorts were pooled together (n = 41 RRMS and 44 HC, P = 0.0006). TLR2-stimulants, measured using a human embryonic kidney (HEK)-293 cells transfectant reporter assay, were significantly higher in urine of MS patients than HC. A screen of several common urinary tract infections (UTI)-related organisms showed strong induction of TLR2-signaling in the same assay. Taken together, these results indicate that two different markers of TLR2-activity—urinary TLR2-stimulants and serum sTLR2 levels—are significantly elevated in MS patients compared to HC.

Reduced dietary intake of pro-inflammatory Toll-like receptor stimulants favourably modifies markers of cardiometabolic risk in healthy men

BACKGROUND AND AIMS

Because pro-inflammatory stimulants of Toll-like receptor-2 and TLR4 (pathogen-associated molecular patterns, PAMPs), are abundant in some processed foods, we explored the effects of diets enriched or depleted in these molecules on markers of cardiometabolic risk in man.

METHODS AND RESULTS

Adherence to a low PAMP diet for 7 days reduced LDL-cholesterol (-0.69 mM, P = 0.024) and abdominal circumference (-1.6 cm, P = 0.001) in 11 habitual consumers of high PAMP foodstuffs, and these markers, together with leukocyte counts (+14%, P = 0.017) increased significantly after 4 days consuming predominantly high PAMP foods. Change in LDL-cholesterol and leukocyte counts correlated well with change in frequency of intake of high PAMP foodstuffs per individual (r = 0.540, P = 0.0095 and r = 0.6551, P = 0.0009, respectively). In an independent group of 13 healthy men, leukocyte counts and expression of the activation marker CD11b on granulocytes and monocytes were significantly reduced after a fresh onion meal (P < 0.05), but these effects were reversed by a high PAMP equivalent meal.

CONCLUSIONS

A low PAMP diet is associated with reduced levels of several cardiometabolic risk factors, while a high PAMP diet reverses these effects. These findings suggest a novel potential mechanistic explanation for the observed association between processed food consumption and risk of cardiometabolic diseases. The study is registered at clinicaltrials.org (reference NCT02430064).