ANGPTL4 promotes bile acid absorption during taurocholic acid supplementation via a mechanism dependent on the gut microbiota

Highlights

• ANGPTL4 is not responsible for the triglyceride-lowering effect of bile acids.

• ANGPTL4 influences bile acid metabolism during taurocholic acid supplementation.

• Angptl4^−/− mice have a different gut microbiota.

• An intricate relationship exists between ANGPTL4, the gut microbiota, and bile acids.

Abstract

Angiopoietin-like 4 (ANGPTL4) raises plasma triglyceride levels by inhibiting lipoprotein lipase. A set of compounds that are able to reduce plasma triglyceride levels are bile acids (BA). Because BA have been shown to decrease ANGPTL4 secretion by intestinal cells, we hypothesized that BA lower plasma triglycerides (partly) via ANGPTL4. To test that hypothesis, wild-type and Angptl4^−/− mice were fed chow supplemented with taurocholic acid (TCA) for seven days. TCA supplementation effectively lowered plasma triglycerides in wild-type and Angptl4^−/− mice, indicating that ANGPTL4 is not required for plasma triglyceride-lowering by BA. Intriguingly, however, plasma and hepatic BA concentrations were significantly lower in TCA-supplemented Angptl4^−/− mice than in TCA-supplemented wild-type mice. These changes in the Angptl4^−/− mice were accompanied by lower BA levels in ileal scrapings and decreased expression of FXR-target genes in the ileum, including the BA transporter Slc10a2. By contrast, faecal excretion of specifically primary BA was higher in the Angptl4^−/− mice, suggesting that loss of ANGPTL4 impairs intestinal BA absorption. Since the gut microbiota converts primary BA into secondary BA, elevated excretion of primary BA in Angptl4^−/− mice may reflect differences in gut microbial composition and/or functionality. Indeed, colonic microbial composition was markedly different between Angptl4^−/− and wild-type mice. Suppression of the gut bacteria using antibiotics abolished differences in plasma, hepatic, and faecal BA levels between TCA-supplemented Angptl4^−/− and wild-type mice. In conclusion, 1) ANGPTL4 is not involved in the triglyceride-lowering effect of BA; 2) ANGPTL4 promotes BA absorption during TCA supplementation via a mechanism dependent on the gut microbiota.

Introduction

Dietary triglycerides are digested in the small intestine primarily via the action of the enzyme pancreatic lipase. The activity of pancreatic lipase and subsequent breakdown of triglycerides is supported by bile acids (BA). BA are amphipathic molecules that are able to emulsify triglycerides and stimulate pancreatic lipase activity. The resulting fatty acids are taken up by enterocytes and re-esterified into triglycerides, which in turn are packed in chylomicrons and secreted into the lymph [1], [2], [3]. Circulating triglycerides—as intestine-derived chylomicrons or liver-derived very low-density lipoproteins—are cleared from the bloodstream by hydrolysis catalyzed by lipoprotein lipase (LPL), an enzyme attached to the capillary wall in adipose tissue and muscle [4], [5]. The activity of LPL is regulated by a number of proteins, including the ubiquitously expressed angiopoietin-like 4 (ANGPTL4). Numerous studies have shown that ANGPTL4 potently inhibits the activity of LPL, likely by unfolding and thereby destabilizing LPL, leading to its degradation [6], [7], [8], [9]. Accordingly, mice lacking Angptl4 have reduced plasma triglyceride levels, whereas mice overexpressing Angptl4 have elevated plasma triglyceride levels as compared with wild-type mice [6], [10], [11], [12]. Overall, it is becoming increasingly apparent that ANGPTL4 is the main factor governing the physiological changes in LPL activity in white adipose tissue during fasting, in brown adipose tissue during cold exposure, and in muscle during exercise [6], [13], [14].

In addition to supporting the function of pancreatic lipase in lipid digestion, BA also have major regulatory roles in the control of lipid, glucose and energy metabolism through activation of the bile acid receptors FXR and TGR5 [15]. Importantly, the various BA species that are present in rodents and humans show differential potencies towards the different BA signaling pathways [16]. In addition, BA may influence biological processes via interactions with the gut microbiota. An intricate interrelationship exists between the gut microbiota, BA, and host metabolism. While the gut microbiota influences BA composition by deconjugating BA and transforming primary BA into secondary BA, alterations in BA composition may in turn affect the gut microbiota [17], [18], [19], [20]. By modulating BA composition, the gut microbiota may significantly impact BA signaling and, thereby, host metabolism [15], [21].

An important manifestation of the regulatory role of BA in lipid metabolism is their ability to influence plasma triglyceride levels. The hypertriglyceridemic effect of BA-binding resins and the hypotriglyceridemic effects of oral BA therapies are well-documented in humans [22], [23], [24], [25], [26]. Direct and dose-dependent suppression of the production of very low-density lipoproteins by BA has been demonstrated in cultured human and rat hepatocytes [27], [28]. Evidence has also been presented indicating that the hypotriglyceridemic effect of BA is mediated by hepatic FXR activation, leading to up- and down-regulation of Apoc2 and Apoc3 expression, respectively, collectively resulting in enhanced triglyceride clearance [29], [30]. In addition, hepatic FXR activation has been suggested to lead to reduced hepatic lipogenesis via SREBP1c [25]. Nevertheless, complete insight into the mechanisms underlying the triglyceride-lowering actions of BA is still lacking. Previously, we observed that BA potently lower ANGPTL4 secretion from the human duodenal cell line Hutu-80. Based on this observation and given the prominent role of ANGPTL4 in the regulation of plasma triglyceride levels, we hypothesized that the triglyceride-lowering effect of BA may, at least in part, be mediated via down-regulation of ANGPTL4 in intestine and/or liver [31].

To test this hypothesis, wild-type and Angptl4^−/− mice were supplemented with the primary BA taurocholic acid (TCA) for seven days by adding it to the feed. TCA effectively lowered circulating triglycerides in both wild-type and Angptl4^−/− mice, indicating that ANGPTL4 is not required for the plasma triglyceride-lowering action of BA. Interestingly, however, our studies revealed a novel and unexpected interaction between ANGPTL4 and BA metabolism that is dependent on the gut microbiota.