Walnut oil increases cholesterol efflux through inhibition of stearoyl CoA desaturase 1 in THP-1 macrophage-derived foam cells

Abstract

Background: Walnuts significantly decrease total and low-density lipoprotein cholesterol in normo- and hypercholesterolemic individuals. No study to date has evaluated the effects of walnuts on cholesterol efflux, the initial step in reverse cholesterol transport, in macrophage-derived foam cells (MDFC). The present study was conducted to investigate the mechanisms by which walnut oil affects cholesterol efflux.

Methods: The extract of English walnuts (walnut oil) was dissolved in DMSO and applied to cultured THP-1 MDFC cells (0.5 mg/mL). THP-1 MDFC also were treated with human sera (10%, v:v) taken from subjects in a walnut feeding study. Cholesterol efflux was examined by liquid scintillation counting. Changes in gene expression were quantified by real time PCR.

Results: Walnut oil treatment significantly increased cholesterol efflux through decreasing the expression of the lipogenic enzyme stearoyl CoA desaturase 1 (SCD1) in MDFC. Alpha-linolenic acid (ALA), the major n-3 polyunsaturated fatty acids found in walnuts, recaptured SCD1 reduction in MDFC, a mechanism mediated through activation of nuclear receptor farnesoid-X-receptor (FXR). Postprandial serum treatment also increased cholesterol efflux in MDFC. When categorized by baseline C-reactive protein (CRP; cut point of 2 mg/L), subjects in the lower CRP sub-group benefited more from dietary intervention, including a more increase in cholesterol efflux, a greater reduction in SCD1, and a blunted postprandial lipemia.

Conclusion: In conclusion, walnut oil contains bioactive molecules that significantly improve cholesterol efflux in MDFC. However, the beneficial effects of walnut intake may be reduced by the presence of a pro-inflammatory state.

Trial registration: ClinicalTrials.gov: NCT00938340.

Figures

Figure 1

Walnut oil affects cholesterol transport and storage in THP-1 macrophage-derived foam cells (MDFC). (A) Walnut oil increases cholesterol efflux. DMSO treated cells have 2.3% cholesterol efflux (of total 3H-Ch). Values of DMSO treated cells are normalized and expressed as 100%. (B) Expression change of genes related with cholesterol transport and storage following walnut oil treatment. Full names of all abbreviation are as described in Table S1 footnote. (C) Walnut oil affects gene expression related with lipogenesis and fatty acid oxidation. (D) Walnut oil treatment decreases SCD1 protein. Band intensities were determined by OptiQuant Image analysis software (Packard Instrument Co., Meriden, CT). (E) Effect of walnut oil on ABCA1 and ABCG1 protein levels. (F) Effect of SCD1 on cholesterol efflux. SCD1 overespression is shown by western blot. A total of 60 μg proteins were loaded on each lane of control (empty plasmid) and overexpression groups. Bars not sharing common letters (a, b or c) differ with P < 0.05. (G) Fatty acids affect SCD1 expression. Fatty acids are conjugated to BSA with a molar ratio of 4:1 modified from method described by Calder et al. [50]. PA: palmitic acid; OA: oleic acid; LA: linoleic acid; ALA: alpha linolenic acid. (H) Fatty acids affect cholesterol efflux. Symbol * in all figure panels indicates a significant difference from respective control with P < 0.05. The data presented are means ± SEM of triplicate wells of two independent experiments.

Figure 2

Walnut oil inhibits SCD1 through a FXR pathway in THP-1 MDFC. (A) Activation of walnut oil on nuclear receptor ligand binding domain of Dual Luciferase Reporter Assay. PPAR: peroxisome proliferator-activated receptor; LXR: liver-X-receptor; RXR: retinoic-X-receptor; PXR: pregnane-X-receptor; FXR: farnesoid-X-receptor. (B) SCD1 protein (top) and mRNA (bottom) changes following nuclear receptor agonists treatment. SCD1 protein change was detected by Western blot. Treatments are as follows: Lane 1: DMSO control; Lane 2: ciprofibrate 100 μM (PPAR-α agonist); Lane 3: GW501506 500 nM (PPAR-β agonist); Lane 4: rosiglitazone 10 μM (PPAR-γ agonist); Lane 5: TO901317 5 μM (LXR agonist); Lane 6: 9-cis RA 100 nM (RXR agonist); Lane 7: rifamipicin 25 μM (PXR agonist); Lane 8: GW4064 10 μM (FXR agonist). (C) FXR activation increases cholesterol efflux. (D) FXR activation decreases SCD1 expression. THP-1 MDFCs were treated with 10 μM GW4064 (FXR agonist), walnut oil (0.5 mg/mL), 100 μM ALA in the presence or absence of 50 μM GGS (FXR antagonist). Bars not sharing common letters are significantly different. (E) Walnut oil and ALA activate FXR and increase its target gene SHP expression. (F) Overexpression of SHP inhibits SCD1 expression in MDFC. THP-1 derived macrophages were transfected with pCMX-hSHP and empty plasmid (control) for 24 h. Symbol * in all figure panels indicates a significant difference from respective control (P < 0.05). The data presented are means ± SEM of triplicate wells of two independent experiments.

Figure 3

Human serum affects SCD1 expression and cholesterol efflux in THP-1 MDFC. Human serum affects SCD1 expression in THP-1 MDFCs. Samples were taken from 15 subjects consuming different walnut components: whole walnut, walnut oil, defatted walnut meat, and walnut skin. Baseline serum as well as postprandial serum of 1, 2, 4, 6 h were applied as treatment (10%, v:v) for 24 h in serum free media. Real time PCR value of individual serum treated sample is divided by a PCR value of a pooled serum treated sample to minimize the variation between PCR runs and to exclude a non-specific serum effect on gene expression. Value of each postprandial serum treated sample is adjusted by baseline value, which served as a control within the same dietary period. Data were logarithm transformed for analysis. Sera of fifteen participants at baseline and 1, 2, 4, and 6 h postprandially were applied as treatment. N = 75 of each dietary group.

Figure 4

Categorized by baseline CRP level (2 mg/L as a cutting point), subjects have different serum lipid response following walnut oil intake. (A) Changes in CRP between low- and high-CRP subgroups. CRP values were expressed as mean (mg/L) ± SEM. Subjects were categorized as low (8 subjects) or high (7 subjects) CRP sub-groups based on the mean of baseline CRP values (P < 0.001 between sub-groups). (B) Changes in TG between low- and high-CRP subgroups (P < 0.01 between sub-groups). (C) Changes inTC between low- and high-CRP subgroups. (D) Changes in LDL-C between low- and high-CRP subgroups. (E) Changes in HDL-C between low- and high-CRP subgroups. Lipids and lipoproteins were expressed as mean (mg/dL) ± SEM. Open square with dashed line represents the high CRP sub-group. Solid circle with straight line represents the low CRP sub-group.

Figure 5

Human sera of low and high CRP sub-groups differentially affect cholesterol efflux. (A) Sera of subjects consuming walnut oil affect cholesterol efflux. Following the induction of foam cells, human sera (10%, v:v) of baseline and 4 h postprandially were applied as treatment to induce cholesterol efflux for 24 h. Following efflux period, control cells (incubated in serum-free culture medium only) had a cholesterol efflux of 5.4%. The value is normalized and expressed as 100%. (B) Categorized by baseline CRP (cut point of 2 mg/L), changes of cholesterol efflux following treatment by human sera of walnut oil group. Experiment conditions were as described in Figure 3B. Symbol * indicates a significant difference between control and treatment groups (P < 0.05). Results are representative of two independent experiments with a duplicate at each time point (baseline and 4 h postprandially) of each subject.

Figure 6

Human sera of low and high CRP sub-groups differentially affect gene expression. (A) Changes in SCD1 mRNA expression following treatment of human sera from subjects consuming walnut oil. * indicates significant difference from baseline and between CRP sub-groups (P < 0.01). (B) Quantification of SCD1 protein. Value of baseline sera treated cells of low CRP sub-group was normalized as 100. Bars not sharing the same letters differ (P < 0.05). (C) Changes in membrane transporters ABCA1 mRNA following human sera treatment. (D) Changes in ABCA1 proteins categorized by baseline CRP level. In the GLM model, age, gender, BMI, and glucose are served as covariates and ID number as dummy variable.