CD36 is important for chylomicron formation and secretion and may mediate cholesterol uptake in the proximal intestine

Abstract

Background & aims: Studies are aimed to determine the role of CD36 in intestinal lipid absorption.

Methods: Knock-out (KO) and wild-type (WT) lymph fistula mice were used to study fatty acids (FA) and cholesterol uptake, and chylomicron formation and secretion. Uptake of FA and cholesterol was studied by using sucrose polybehenate and fecal dual isotope methods, respectively.

Results: The CD36 KO exhibited significant accumulation of dietary cholesterol in the intestinal lumen at the end of 6-hour lipid infusion and significant reduction of dietary cholesterol transport into the lymph. Fecal dual isotope studies, however, did not show any significant difference in cholesterol uptake, suggesting that given sufficient time, the KO intestine could compensate for the reduced cholesterol uptake observed in the acute lymph fistula studies. Recovery of dietary FA in the intestinal lumen was comparable between WT and KO, consistent with the sucrose polybehenate study. However, the KO mice accumulated more, albeit not significantly, dietary triacylglycerols in the intestine, followed by a significant reduction in lymphatic transport. The ratio of intestinal dietary triacylglycerols to FA was not higher in WT than KO, arguing against impaired lipid esterification. It is rather a deficiency in the formation and secretion of chylomicrons, as supported by the significantly less apolipoprotein B-48 and the smaller, albeit not significantly, lipoprotein particles secreted into the lymph of the KO.

Conclusions: CD36 may play an important role in chylomicron formation and secretion and may also facilitate cholesterol uptake in the proximal intestine.

Figure 1

Analysis of FA (A) and cholesterol (B) uptake by the small intestines of CD36 null and WT mice. (A) A minimum of 9 fecal samples from each group were analyzed by the SPB method to determine the ratio of fecal FA vs fecal nonabsorbable fat marker. (B) Mice were gavaged with 20% intralipid containing 0.35 mg cholesterol, [¹⁴C] cholesterol, and [³H] sitosterol. Fecal samples were collected 24 hours postgavage and analyzed. No statistical significance was found. Values are means ± SE.

Figure 2

(A) [³H]-TG, (B) TG mass, and (C) [¹⁴C]-cholesterol transport into the lymph during continuous intraduodenal lipid infusion. Mice were equipped with lymph and duodenal cannulas, and were intraduodenally infused with a lipid emulsion containing labeled TG (the label was on all FA molecules of the TG) and cholesterol for a period of 6 hours. Lymph was collected hourly and analyzed. *P < .05 as determined by Tukey posttest analysis of 2-way repeated measures ANOVA. Values are means ± SE.

Figure 3

Fate of dietary TG (A) and cholesterol (B) at the end of 6-hour continuous infusion. Mice were equipped with lymph and duodenal cannulas, and were intraduodenally infused with a lipid emulsion containing labeled TG (the label was on all FA molecules of the TG) and cholesterol for a period of 6 hours. The recovery of radioactivity in the small intestine, lymph, stomach, intestinal lumen, and colon were determined at the end of the 6 hours by scintillation counter. *P < .05. Values are means ± SE.

Figure 4

Distribution of dietary TG (A) and cholesterol (B) along the segments of the small intestine. Mice were equipped with lymph and duodenal cannulas, and were intraduodenally infused with a lipid emulsion containing labeled TG (the label was on all FA molecules of the TG) and cholesterol for a period of 6 hours. At the end of the study, small intestines (n = 5) were harvested and divided into 4 equal length segments, from proximal to distal: M1, M2, M3, and M4. The amounts of radioactivity in these segments were determined by scintillation counter. No statistical significance was found between the WT and KO. Values are means ± SE.

Figure 5

Thin-layer chromatography analysis of [³H]-labeled lipid fraction of the small intestine. Mice were equipped with lymph and duodenal cannulas, and were intraduodenally infused with a lipid emulsion containing labeled TG (the label was on all FA molecules of the TG) and cholesterol for a period of 6 hours. At the end of the study, the small intestines were divided into 4 equal segments. Only the analysis of the first 2 (most proximal) segments was shown here: (A) M1 and (B) M2. Extracted lipid fraction was separated by thin-layer chromatography into CE, TG, FA, DG, and MG+PL. CE, cholesteryl esters; DG, diacylglycerols; MG+PL, monoacylglycerols and phospholipids. *P < .05. Values are means ± SE.

Figure 6

Thin-layer chromatography analysis of [¹⁴C]-labeled lipid fraction of the small intestine. Mice were equipped with lymph and duodenal cannulas, and were intraduodenally infused with a lipid emulsion containing labeled TG (the label was on all FA molecules of the TG) and cholesterol for a period of 6 hours. At the end of the study, the small intestines were divided into 4 equal segments. Only the analysis of the first 2 (most proximal) segments was shown here: (A) M1 and (B) M2. Extracted lipid fraction was separated by thin-layer chromatography into cholesterol and CE. No statistical significance was found between the WT and KO. Values are means ± SE.

Figure 7

Lipoprotein particle size of the lymph of fasted mice. Lipoproteins analyzed were from lymph collected 1 hour prior to lipid infusion (during glucose/saline infusion). Electron micrographs of the lipoprotein particles from CD36 WT (A) and KO (B) mice, and their size distribution (C) and relative VLDL/chylomicron ratio (D) are shown. Particles with diameters of <800 Å are considered VLDL, and ≥800 Å are considered chylomicrons. Standard bars represent 5000 Å (500 nm). No statistical significance was found in the VLDL/chylomicron ratio between the WT and KO. Values are means ± SE.

Figure 8

Lipoprotein particle size of lymph of lipid fed mice. Lipoproteins analyzed were from lymph collected during the 5th–6th hour lipid infusion. Electron micrographs of the lipoprotein particles from CD36 WT (A) and KO (B) mice, and their size distribution (C) and relative VLDL/chylomicron ratio (D) are shown. Particles with diameters of <800 Å are VLDL, and ≥800 Å are chylomicrons. Standard bars are 5000 Å (500 nm). No statistical significance was found in the VLDL/chylomicron ratio between the WT and KO. Values are means ± SE.

Figure 9

Lipid composition of chylomicrons from lipid fed mice. Chylomicrons were isolated from lymph collected during 6 hours of lipid infusion. Each chylomicron lipid, TG (white), total cholesterol (gray), and PL (black) is expressed as a percentage of the total content. No statistical significance was found.

Figure 10

Apolipoprotein secretion into lymph during fasting and lipid feeding. Fasting samples were from lymph collected 1 hour prior to lipid infusion (during glucose/saline infusion); lipid-feeding samples were from lymph collected during the 3rd–4th hour of lipid infusion. Amounts of lymphatic apolipoprotein A-I (A), A-IV (B), and B (C) were determined by ELISA. See Methods for details. *P = .009 for the genotype factor (2-way ANOVA). Values are means ± SE.