All-trans retinoic acid regulates hepatic bile acid homeostasis

Abstract

Retinoic acid (RA) and bile acids share common roles in regulating lipid homeostasis and insulin sensitivity. In addition, the receptor for RA (retinoid x receptor) is a permissive partner of the receptor for bile acids, farnesoid x receptor (FXR/NR1H4). Thus, RA can activate the FXR-mediated pathway as well. The current study was designed to understand the effect of all-trans RA on bile acid homeostasis. Mice were fed an all-trans RA-supplemented diet and the expression of 46 genes that participate in regulating bile acid homeostasis was studied. The data showed that all-trans RA has a profound effect in regulating genes involved in synthesis and transport of bile acids. All-trans RA treatment reduced the gene expression levels of Cyp7a1, Cyp8b1, and Akr1d1, which are involved in bile acid synthesis. All-trans RA also decreased the hepatic mRNA levels of Lrh-1 (Nr5a2) and Hnf4α (Nr2a1), which positively regulate the gene expression of Cyp7a1 and Cyp8b1. Moreover, all-trans RA induced the gene expression levels of negative regulators of bile acid synthesis including hepatic Fgfr4, Fxr, and Shp (Nr0b2) as well as ileal Fgf15. All-trans RA also decreased the expression of Abcb11 and Slc51b, which have a role in bile acid transport. Consistently, all-trans RA reduced hepatic bile acid levels and the ratio of CA/CDCA, as demonstrated by liquid chromatography-mass spectrometry. The data suggest that all-trans RA-induced SHP may contribute to the inhibition of CYP7A1 and CYP8B1, which in turn reduces bile acid synthesis and affects lipid absorption in the gastrointestinal tract.

Introduction

The role of retinoids in lipid metabolism is an emerging field in retinoid research due to their ability to regulate lipid homeostatic processes including adipogenesis, lipolysis, and fatty acid oxidation. Natural and synthetic retinoids are promising treatments for metabolic syndrome, obesity, and diabetes mellitus [1], [2], [3], [4], [5]. It has been shown that all-trans retinoic acid (all-trans RA), the active metabolite of vitamin A, can reduce body fat and improve insulin sensitivity in both lean and obese rodents [6]. Similarly, there is an inverse relationship between vitamin A intake and adiposity in healthy people [7]. Moreover, using genome-wide approaches, we showed that all-trans RA reduces serum lipid in vivo [8].

While there are beneficial effects of retinoids, published data also demonstrated their negative impact in human patients. For example, body weight gain and hyperlipidemia are common side effects when all-trans RA is used to treat acute promyelocytic leukemia [9], [10]. Likewise, hypertriglyceridemia is common in patients who receive retinoid treatment for dermatological disorders [11]. The controversial findings may be explained, in part, by the models used. The conflicting effects of RA treatments may also be due to the fact that retinoid x receptor (RXR), the RA receptor, is a permissive or active partner for the receptors of fatty acids (peroxisome proliferator activated receptors), oxysterols (liver x receptors), and bile acids (farnesoid x receptor) [12]. These receptors regulate lipid homeostasis and often have opposing effects. For example, PPARα (NR1C1), which is highly expressed in the liver, is responsible for lipid oxidation, while PPARγ (NR1C3) is involved in lipogenesis [13]. Thus, RA-mediated activation of one pathway can result in deactivation of another in a tissue-specific manner. Taken together, the role of retinoids in lipid homeostasis is complicated and warrants further investigation.

Bile acids are responsible for lipid absorption in the gastrointestinal tract. They are synthesized in the liver by cholesterol catabolism via the classical (CYP7A1 and CYP8B1) and the acidic pathways (CYP27A1 and CYP7B1) [14]. Upon ingestion of fat and protein, intestinal cholecystokinin stimulates the release of bile containing digestive enzymes and primary bile acids from the gallbladder [15]. In the liver, bile acids activate FXR, which induces the expression of SHP to inhibit the activity of LRH-1 and HNF4α, which are the positive regulators of Cyp7a1 [16], [17]. In the small intestine, bile acid-activated FXR can induce the expression of fibroblast growth factor 15/19 (FGF15/19, rodents/humans), which binds hepatic FGFR4 and activates ERK1/2 (MAPK1/3) and JNK1/2 (MAPK8/9) to inhibit hepatic bile acid synthesis [18], [19], [20], [21], [22]. Bile acids are actively reabsorbed from the ileum by IBAT (SLC10A2) and circulate back to the liver through the hepatic portal vein [23]. This highly efficient process ensures that the majority of bile acids are recycled [24]. SLC51A/B (OSTα/β) a heteromeric complex, functions as a major basolateral transporter of bile acids [25], and is essential for intestinal bile acid transportation as well [26]. FXR not only regulates bile acid homeostasis, it also has a role in controlling insulin resistance and glucose homeostasis [27]. Since both RA and bile acids regulate lipid homeostasis and insulin sensitivity, the current study examines the pharmacological effect of all-trans RA on bile acid homeostasis.

Using non-biased approaches, we have conducted microarray study, bile acid quantification, and proteomic study to analyze the action of all-trans RA in bile acid homeostasis. Our data show that all-trans RA has a significant effect in regulating bile acid homeostasis in vivo. All-trans RA inhibits bile acid synthesis by down regulating the expression of key bile acid synthesis enzymes, CYP7A1 and CYP8B1. All-trans RA also modulates the expression of genes that have a role in the transportation of bile acids. All-trans RA treatment reduced the CA/CDCA ratio and may have an impact on lipid absorption.