Fatty acid- and cholesterol transporter protein expression along the human intestinal tract

Abstract

Background: Protein distribution profiles along the human intestinal tract of transporters involved in the absorption of cholesterol and long-chain fatty acids (LCFA) have been scarcely evaluated.

Methodology/principal findings: In post-mortem samples from 11 subjects, intestinal transporter distribution profiles were determined via Western Blot. Differences in transporter protein levels were statistically tested using ANOVA and Tukey's Post Hoc comparisons. Levels in all segments were expressed relative to those in duodenum. Except for ABCG5 and FATP4, levels (mean+/-SEM) were the highest in the ileum. For ABCA1, ileal levels (1.80+/-0.26) differed significantly from those in duodenum (P = 0.049) and proximal colon (0.92+/-0.14; P = 0.029). ABCG8 levels in ileum (1.91+/-0.30) differed from those in duodenum (P = 0.041) and distal colon (0.84+/-0.22; P = 0.010) and jejunum (1.64+/-0.26) tended to be higher than distal colon (0.84+/-0.22; P = 0.087). Ileal NPC1L1 levels (2.56+/-0.51) differed from duodenum levels (P = 0.019) and from distal colon (1.09+/-0.22; P = 0.030). There was also a trend (P = 0.098) for higher jejunal (2.23+/-0.37) than duodenal NPC1L1 levels. The levels of ABCG5 did not correlate with those of ABCG8. FAT/CD36 levels in ileum (2.03+/-0.42) differed from those in duodenum (P = 0.017), and proximal and distal colon (0.89+/-0.13 and 0.97+/-0.15 respectively; P = 0.011 and P = 0.014). FABPpm levels in ileum (1.04+/-0.13) differed from proximal (0.64+/-0.07; P = 0.026) and distal colon (0.66+/-0.09; P = 0.037).

Conclusions/significance: The distribution profiles showed a bell-shape pattern along the GI-tract with the highest levels in ileum for ABCA1, ABCG8, NPC1L1, FATCD36 and FABPm, suggesting a prominent role for ileum in transporter-mediated uptake of cholesterol and LCFAs.

Figure 1. Intestinal uptake of dietary cholesterol.

Dietary cholesterol (ch) and plant sterols/stanols (ps) travel, incorporated in mixed micelles, through the intestinal lumen. The sterols are transported across the brush-border membrane by NPC1L1. Once taken up, sterols are either incorporated in apoB48-rich chylomicrons (CM), which are secreted in the lymph compartment, or used to form apoA1-rich HDL cholesterol, a process that is mediated by ABCA1. Excess amounts of sterols are also excreted into the intestinal lumen by the reverse sterol transporters ABCG5 and G8. The level of transport proteins is under tight control of the liver X receptor (LXR) gene. This gene indirectly measures cellular sterol levels and regulates the transcription of sterol transporters NPC1L1, ABCA1, ABCG5 and G8.

Figure 2. Intestinal uptake of dietary fatty acids.

Dietary fatty acids (FA) pass through the intestinal lumen whilst esterified to triacylglycerols (TAG) or incorporated into mixed micelles. Gastric and hepatic lipases free the fatty acids, which are then receptive to uptake. Fatty acids are either transported across the apical membrane actively by FATP4, FAT/CD36 or FABPpm, or passively diffuse (blocked arrows) through the lipid bilayer. In the enterocyte FABP_c facilitates fatty acid transport through the cytosol. In the cytoplasm, the major part of fatty acids is re-esterified to triacylglycerols and excreted into chylomicrons (CM), whereas a small part is excreted as free fatty acids (FFA).

Figure 3. Distribution patterns of cholesterol transporter proteins along the longitudinal axis of the human intestine.

These transporter proteins included ABCA1 (panel A), ABCG5 (panel B), ABCG8 (panel C) and NPC1L1 (panel D). Panel E displays Western Blots of the different segments of a single subject, which is representative for the average distribution pattern of ABCA1, ABCG5 and ABCG8 and NPC1L1. * = P<0.05 compared to duodenum; # = P<0.05 compared to proximal colon; $ = P<0.05 compared to distal colon. D = duodenum, J = jejunum, I = ileum, PC = proximal colon and DC = distal colon.

Figure 4. Distribution patterns of fatty acid transporter proteins along the longitudinal axis of the human intestine.

These transporter proteins included FABPpm (panel A), FATP4 (panel B) and FAT/CD36 (panel C). Panel D displays Western Blots of the different segments of a single subject, which is representative for the average distribution pattern of FABPpm and FATP4. * = P<0.05 compared to duodenum; # = P<0.05 compared to proximal colon; $ = P<0.05 compared to distal colon. D = duodenum, J = jejunum, I = ileum, PC = proximal colon and DC = distal colon.