Dietary Effects on Monocyte Phenotypes in Subjects With Hypertriglyceridemia and Metabolic Syndrome

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SUMMARY
In patients with hypertriglyceridemia, a short-term low-saturated fat vs high-saturated fat diet induced lower plasma lipids and improved monocyte phenotypes. These findings highlight the role of diet fat content and composition for monocyte phenotypes and possibly cardiovascular disease risk in these patients. H ypertriglyceridemia with metabolic syndrome is causally associated with the development of atherosclerotic cardiovascular disease (ASCVD). [1][2][3][4][5] The mechanisms linking hypertriglyceridemia and metabolic syndrome with ASCVD risk are not fully understood but may include inflammation, which has been implicated in ASCVD and correlates with hypertriglyceridemia and metabolic syndrome. 2,[5][6][7][8][9][10][11][12][13] Diet intervention is an important strategy for ASCVD prevention, and changes in diet composition may affect ASCVD risk. 14 However, the method by which immune cells respond to different dietary compositions and may subsequently affect ASCVD risk is still ill defined.
Monocytes, one of the most important immune cell types in the circulation, respond to changes in plasma lipid profiles and play pivotal roles in atherosclerosis. 15 In humans, hypertriglyceridemia with metabolic syndrome or postprandial hypertriglyceridemia induced by a single high-saturated fat meal increases lipid accumulation within monocytes, causing formation of foamy monocytes, which contain intracellular lipid droplets and exhibit inflammatory phenotypes with enhanced adhesion to endothelium. [16][17][18][19][20] In mouse models of atherosclerosis, a high-saturated fat, high-cholesterol diet induces early formation of circulating foamy monocytes, which infiltrate into arterial walls and contribute to the development of atherosclerosis. 21,22 In contrast, replacing saturated fat with unsaturated fat in the diet results in decreased formation of foamy monocytes, which correlates with reduced monocyte adhesion and decreased atherosclerotic plaque formation. 23 These observations in mouse models of atherosclerosis suggest that dietary composition may affect atherosclerosis development by regulating foamy monocyte formation and phenotypes.
More than one-third of Americans have metabolic syndrome, which is present in almost one-half the population at age 60 years and has been increasing in younger adults. 24 Diets high in fat and protein but low in carbohydrates have gained increasing popularity and may help reduce body weight in patients with metabolic syndrome. 14,25 How monocytes respond to changes in di- The authors attest they are in compliance with human studies committees and animal welfare regulations of the authors'  Figure 1). Each diet period included 2 study phases: a 4-day diet phase (days 1-4) and a postprandial phase (day 5). Blood was drawn on day 1 after an overnight fast (before diet intervention) and on day 5 fasting (0 hour) and 4 and 6 hours after a test meal, which contained w900 kcal and was similar in composition to the respective study diet. An aliquot of blood samples was sent to a local Quest Diagnostics laboratory for complete blood count analysis, and the remainder of the blood samples were processed immediately to analyze monocyte phenotypes and isolate plasma samples; these samples were stored at -80 C in small aliquots and used later for biochemical assays.   PHENOTYPES. Monocyte phenotypes were analyzed by using flow cytometry as described previously. 16,18,28 Briefly, for cell surface marker analysis, 100 mL of EDTA blood was incubated with a mixture of antibodies (Supplemental Table 1) in 2 different protocols (Protocols 1 and 2) (Supplemental Table 2) at 4 C for 30 minutes and then  intensity levels of monocytes/subsets after coincubation with DiI-labeled oxLDL. 23 ON-CHIP ADHESION ASSAY. Monocyte adhesion assay was performed by using whole blood as previously reported. 23,29,30 Briefly, coverslips that were    To examine the effects of LSFD vs HSFD, each participant received HSFD and LSFD ( Table 2) in a randomized order separated by a 4-to 6-week washout period ( Figure 1). Each diet period included 2 study phases: a 4-day diet phase (days 1-4) and a postprandial phase (day 5). Daily calorie intake was not significantly different between LSFD and HSFD in the 4-day diet phase, but the calorie intake of the test meal on day 5 was slightly but significantly lower for LSFD than for HSFD. As designed, fat intake was significantly lower, and protein and carbohydrate intake was higher with LSFD than with HSFD. Of the fat intake, saturated fatty acid intake was significantly lower with LSFD than with HSFD, whereas monounsaturated fatty acid and polyunsaturated fatty acid intake showed no or minor differences.
With HSFD, w52% of calories were from fat and w24% of calories were from saturated fat in the 4-day diet phase, and w57% of calories were from fat and w34% of calories were from saturated fat in the test meal; with LSFD, w26% of calories were from fat and w5% of calories were from saturated fat in the 4-day diet phase, and w23% of calories were from fat and w3.8% of calories were from saturated fat in the test meal.
Cholesterol intake was lower with LSFD than with HSFD.  Table 3).
Total fatty acid levels did not change in the 4-day diet phase but were increased in the postprandial phase, with no differences between the 2 diets ( Figure 2F,  thickness. [36][37][38] We observed that despite the changes in total monocyte counts, the percentage of each monocyte subset in total monocytes did not change, with no diet or time effects. Lipid accumulation in monocytes, leading to foamy monocyte formation, occurs in humans and mice with hyperlipidemia and may contribute to atherosclerosis development. [16][17][18][19][20][21][22]39 We therefore examined lipid     Analyses of chemokine receptors showed that at baseline, CCR2, the receptor for CCL2, was higher, but CX3CR1, the receptor for CX3CL1, was lower on cM than on nM, with iM expressing intermediate CCR2 and CX3CR1 levels (Supplemental Figure 3), consistent with previous reports. 43,44 The changes in cM CCR2 levels tended to have a diet Â time interaction in the 4-day diet phase, with lower cM CCR2 levels  It has been recognized that triglyceride-rich lipoprotein-derived RLP can penetrate arterial walls and promote foam cell formation and is therefore atherogenic. 2,6 It is also important to note that in addition to macrophage uptake of lipids and foam cell formation in atherosclerotic plaques, monocytes can take up lipoproteins and become lipid-laden foamy monocytes in the circulation in hypertriglyceridemia and/or hypercholesterolemia. [16][17][18][19][20][21][22]39 In humans, postprandial hyperlipidemia after a single high-saturated fat meal can induce foamy monocyte formation in the circulation. [17][18][19][20]