Bile acids and salt-sensitive hypertension: a role of the gut-liver axis

Abstract

Salt-sensitivity of blood pressure (SSBP) affects 50% of the hypertensive and 25% of the normotensive populations. Importantly, SSBP is associated with increased risk for mortality in both populations independent of blood pressure. Despite its deleterious effects, the pathogenesis of SSBP is not fully understood. Emerging evidence suggests a novel role of bile acids in salt-sensitive hypertension and that they may play a crucial role in regulating inflammation and fluid volume homeostasis. Mechanistic evidence implicates alterations in the gut microbiome, the epithelial sodium channel (ENaC), the farnesoid X receptor, and the G protein-coupled bile acid receptor TGR5 in bile acid-mediated effects on cardiovascular function. The mechanistic interplay between excess dietary sodium-induced alterations in the gut microbiome and immune cell activation, bile acid signaling, and whether such interplay may contribute to the etiology of SSBP is still yet to be defined. The main goal of this review is to discuss the potential role of bile acids in the pathogenesis of cardiovascular disease with a focus on salt-sensitive hypertension.

INTRODUCTION

The 2017 guidelines by the American College of Cardiology and the American Heart Association define hypertension as a systolic blood pressure of ≥130 mmHg or diastolic blood pressure of ≥80 mmHg. Thus, nearly half of the US population is classified as hypertensive (1). An increase in hypertension prevalence is a global phenomenon with the largest increase in incidence observed in middle- to low-income countries whereas high-income countries have seen a slight decrease in cases of hypertension (2). Although females have a lower risk of hypertension compared with males at younger ages, this trend reverses after menopausal age. Furthermore, common treatments for hypertension including angiotensin-converting enzyme inhibitors and angiotensin receptor blockers are less effective in reducing blood pressure in women (3). Interestingly, studies from different parts of the globe report different prevalences of hypertension among men and women. Men in South Asia were reported to have the lowest rate of hypertension, whereas men in Eastern Europe and Central Asia have the highest. Among women, the prevalence of hypertension is lower in high-income countries and highest in Sub-Saharan Africa (4). This variance in hypertension prevalence may be attributed to differences in environmental factors and race as a risk factor in systemic racism. Indeed, Van Laer et al.’s (5) research in the Netherlands found that women were affected more by racial differences in hypertension prevalence compared with men.

Salt sensitivity of blood pressure (SSBP) is a trait characterized as an arbitrary increase in blood pressure of 10 mmHg in response to a sodium (Na⁺) load that affects more than half of all hypertensive subjects as well as a quarter of normotensive individuals in the United States (6). Salt sensitivity increases the risk for cardiovascular disease and is an independent predictor of both cardiovascular and all-cause mortality (7). Evidence from clinical trials shows a nearly linear relationship between sodium intake and blood pressure, which is even more pronounced in hypertensive people (8). The female sex has a significant impact on the outcomes of salt-sensitive hypertension. Females have a more dramatic blood pressure response to changes in salt intake than males and are more likely to be genetically predisposed to salt sensitivity, indicating that they are typically more salt-sensitive. Although the mechanisms are not fully understood, recent findings attribute this discrepancy to a combination of higher levels of serum aldosterone and higher expression of the endothelial mineralocorticoid receptor in women with salt sensitivity (9). These findings suggest that other factors in addition to impairment of renal natriuretic system, which is postulated to be the major mechanism of salt sensitivity by Guyton et al. (10), may be implicated in the higher risk of SSBP observed in females (9).

The physiological factors of salt-sensitive hypertension and inflammation are multifaceted. Evidence suggests a potential role for extrarenal mechanisms such as the liver, the skin, and the gut in blood pressure regulation. Interestingly, studies suggest that various mechanisms may work in concert to regulate blood pressure through the gut-liver axis (Fig. 1) (11). Indeed, emerging evidence suggests that hypertension is associated with dysbiosis and dysregulation in gut-derived metabolites such as short-chain fatty acids and bile acids. Nevertheless, the mechanistic interplay between dysbiosis and SSBP remains to be investigated.

Figure 1.

An overview of bile acid synthesis through the bidirectional gut-liver axis. The liver synthesizes primary bile acids cholesterol, which is transported to the gut via the enterohepatic circulation. Bacteria in the intestinal lumen modify the primary bile acids to generate a pool of secondary bile acids, which are predominantly reabsorbed via the portal circulation. The remaining secondary bile acids are released into circulation and can affect the cardiovascular system including modulating immune mechanisms, fluid balance, and insulin sensitivity. Created with Biorender.com and published with permission.

Bile Acids

Primary bile acids, cholic acid, and chenodeoxycholic acid are synthesized by hepatocytes from cholesterol and stored in the gall bladder following their conjugation with taurine or glycine. Primary bile acids undergo gut-mediated modification and are converted to secondary bile acids, deoxycholic acid, and lithocholic acid, by bile salt hydrolases (Fig. 2) (12). Until recently, bile acids were best known for their role in aiding digestion and lipid absorption by emulsifying dietary fats. Mounting evidence now recognizes bile acids as signaling molecules and their connection to the gut microbiome. The gut microbiome is defined as the genome of the gut microbiota, which encompasses the organisms that live in the intestinal tract. There are over 50 secondary bile acids, many of which have unclear physiologic functions and are topics for future research. Nevertheless, bile acids have a strong relationship with gut microbiota. Humans with short bowel syndrome exhibit gut dysbiosis, which is associated with a disruption of enterohepatic bile acid composition (13). Germ-free mice do not have secondary bile acids whereas conventionally housed mice have a more diverse pool of bile acids (14). Degradation of intestinal flora by antibiotics is associated with decreased levels of both cholesterol and bile acids (15), indicating that gut microbiota is crucial for the synthesis and diversity of secondary bile acids. Although the particular bacteria responsible for the synthesis of each secondary bile acid remain unclear, the predominant phyla including Firmicutes and Bacteroidetes are reported to harbor bile salt hydrolases (16). Researchers have identified a group of gut strains from the bacterial phylum Bacteroidetes that exhibit selective bile salt hydrolase activity on conjugated bile acid substrates (12).

Figure 2.

Bile acids are synthesized from cholesterol and modified in the intestine by bacteria. The two main primary bile acids cholic acid and chenodeoxycholic are synthesized in the hepatocytes from cholesterol. Primary bile acid undergoes further modification in the intestine mediated by bacteria to generate the secondary bile acids deoxycholic acid and lithocholic acid. Created with Biorender.com and published with permission.

A healthy gut, with a homeostatic microbiota and structure, is crucial in maintaining balance in bile acids. In turn, bile acids and their receptors play a role in mediating gut health. Bile acids reprogram the intestinal epithelial by coordinating interstitial cells renewal and proliferation and attenuating inflammation (17). G protein-coupled receptor Gpbar1 (TGR5) upregulation improves intestinal mucosal structure and permeability (18). Administration of secondary bile acids following their deficiency because of gut dysbiosis mitigates intestinal inflammation (19). Bile acid administration to the ileum also improves oral glucose tolerance via farnesoid X receptor (FXR) signaling and may play a role in the management of obesity and diabetes (20).

Bile Acids and Cardiovascular Health

Levels of circulating bile acids during fasting range between 2 and 10 µM under healthy conditions but increase drastically under pathological conditions (21, 22). Bile acid receptors are expressed by various organs crucial to regulating cardiometabolic functions including the heart, gastrointestinal tract, liver, adrenal glands, and kidneys. Indeed, bile acids have been proposed to affect cardiovascular health by activating the nuclear receptors farnesoid X receptor (FXR), pregnane X receptor and vitamin D receptor, and the G protein-coupled receptor Gpbar1 (TGR5) pathways (23–25).

FXR is highly expressed in vascular smooth muscle cells and endothelial cells (26). Studies imply that bile acids may act as vasoactive substances within the circulation. Bile acids including lithocholic acid and their taurine conjugates increase large-conductance calcium-activated potassium channels (BK_Ca) channel activity in smooth muscle cells leading to vasodilation (27). These vasorelaxant effects are also observed in lipophilic bile acids through the release of endothelial-derived relaxant factors or stimulation of a surface membrane bile acid-binding site (28). Specifically, bile acids increase intracellular calcium concentrations and subsequent nitric oxide production in vascular endothelial cells potentially through a TGR5-dependent regulation of cystathionine-γ-lyase expression and activity both in endothelial and muscular cells (29). Treatment of endothelial cells with a FXR ligand upregulates endothelial nitric oxide implicating their role in vascular diseases (30). Activation of FXR in vascular smooth muscles plays a role in vascular disease progression by inducing cell death by apoptosis. Other mechanisms of bile acids on the vasculature include modulation of aortic calcification and regulating vascular tension mediated by calcium influx (31, 32).

Bile acid-induced activation of the FXR may also have cardiovascular implications. FXR is highly expressed in the kidneys. FXR expression is crucial for renal tubular cell survival (33) and negatively correlates with the progression of tubular interstitial fibrosis and chronic kidney disease. Similarly, a recent study showed a protective role of an FXR agonist against renal ischemia/reperfusion injury, inflammation, and oxidative stress (34). Contradictory findings have been reported by other studies, signifying the need for further research on the role of FXR in the kidney (35, 36).

ENaC, Bile Acids, and Salt-Sensitive Hypertension

The kidney regulates blood pressure through the modulation of Na⁺ reabsorption. Among various sodium transporters in renal tubular cells, the epithelial Na⁺ channel (ENaC) is crucial for the final fine-tuning of Na⁺ homeostasis. However, there is no difference between salt-sensitive and salt-resistant individuals in renal handling of a salt load or plasma volume, suggesting a role of extrarenal mechanisms in the pathogenesis of SSBP, a finding also supported by animal studies (37, 38). Recent data indicate that ENaC may also regulate blood pressure through Na⁺ handling in extrarenal systems such as the gut and skeletal muscle, both of which express bile acid receptors (39). The effects of the renin-angiotensin-aldosterone system, members of the epidermal growth factors family, arginine vasopressin, and oxidative stress on ENaC activity have been extensively studied (37, 40). However, little is known about the role of bile acids in regulating ENaC, both in renal and extrarenal systems and salt-sensitive hypertension. Interestingly, new evidence suggests that secondary bile acid signaling may play a role in salt-sensitive hypertension. Activation of FXR upregulates angiotensin II type 2 receptor (41). Agonists that activate the latter prevent the development of salt-sensitive hypertension in rats, suggesting that bile acid-induced activation of FXR may be a potential target in SSBP (42).

The gastrointestinal tract is a major organ controlling Na⁺ absorption and homeostasis (43), but its role in regulating SSBP is less understood. Excess dietary salt induces gut microbial dysbiosis and leads to activation of antigen-presenting cells and hypertension (44). Dendritic cells (DCs) are found throughout the intestine and can sense Na⁺ through ENaC. Na⁺ entry into the DCs stimulates the formation of isolevuglandins, highly reactive lipid peroxidation products, leading to the development of hypertension in response to high salt. A high-salt diet has been shown to downregulate the colonic expression of mineralocorticoid receptors and ENaC, also potentially, affecting blood pressure (45, 46). In addition, gut dysbiosis disrupts bile acids homeostasis, leading to inflammation, which has been suggested to be involved in the pathophysiology of hypertension (19, 47).

ENaC is a member of the ENaC/degenerin family of ion channels, which also includes the bile acid-sensitive ion channel (BASIC), previously known as the brain liver intestine Na⁺ ion channel. BASIC is located in the cholangiocytes, epithelial cells lining the bile ducts, as well as in the intestinal tract, and the cerebellum (48). Activation of ENaC and BASIC by bile acids requires degenerin (49, 50). Tauro-deoxycholate activates BASIC and stabilizes the channel’s open state as suggested by patch-clamp experiments showing increased BASIC mean open time after tauro-deoxycholate application (50). Structural and functional differences exist among mouse, rat, and human BASIC, implying that bile acids may have species-specific effects. Rat and human BASIC are inactive at rest and regulated by changes in divalent cation concentrations, whereas the mouse BASIC is constitutively active (51). Unlike ENaC activation, evidence suggests that the activation of BASIC by bile acids may be due to alterations in its membrane environment rather than a direct binding and requires the entire structure of the channel rather than individual subdomains (52). The cytosolic NH₂-terminal domain of BASIC inhibits its activity via a three-turn α-helix including an amphiphilic amino acid composition, which also regulates its sensitivity for the cholesterol moieties of the plasma membrane (53).

Bile acids can both activate and inhibit ENaC. Taurine-conjugated bile acids affect ENaC activity more strongly than their unconjugated counterparts, particularly in humans independent of the membrane permeability (54). Tauro-deoxycholate stimulates human ENaC by increasing the channel’s open probability whereas nonconjugated forms like cholic acid and deoxycholic acid have moderate stimulatory effects on ENaC. Nonconjugated bile acids such as chenodeoxycholate may even have inhibitory effects (49). In addition, ursodeoxycholic acid, a secondary bile acid FDA approved for the treatment of liver disease, inhibits ENaC activity (55). However, other studies demonstrate cholic acid, deoxycholic acid, ursodeoxycholic acid, and chenodeoxycholic acid to be novel ENaC agonists. Specifically, these bile acids were shown to increase ENaC currents through increased channel open probability, an effect dependent on the cholesterol moieties of the bile acids. Bile acids may have a higher affinity for ENAC than BASIC, suggesting that their role in salt-sensitive hypertension may predominantly be via ENaC rather than BASIC (56).

Bile acids play a role in regulating water and electrolytes homeostasis through additional mechanisms beyond ENaC and BASIC. FXR activation by endogenous bile acids increases urine concentrating ability via upregulation of aquaporin 2 expression in the medullary collecting duct cells (57). FXR also helps maintain the renal urine concentration during antidiuresis by promoting the survival of medullary collecting duct cells (33). In addition, FXR appears to be crucial for immune modulation in the gut. Maran et al. found that FXR deficiency in mice resulted in colonic inflammation and upregulation of IL-6 expression, which was accompanied by carcinogenic proliferation in the colon (58). Bile acids may modulate water homeostasis also through TGR5, which works to regulate renal aquaporin activity in the renal tubules and therefore urinary concentration (Fig. 3) (59). Studies show that stimulation of TGR5 by lithocholic acid increases and restores aquaporin 2 in mice following ischemia-reperfusion injury in the kidney (60).

Figure 3.

Bile acids affect blood pressure by regulating water and electrolyte homeostasis in the kidney. Activation or inhibition of the renal farnesoid X receptor (FXR), the G protein-coupled receptor TGR5, and the epithelial sodium channel (ENaC) may affect the water and sodium (Na⁺) and, therefore, blood pressure. In the kidney, bile acids increase the ENaC currents by increasing the channel’s open probability whereas they upregulate aquaporin (AQP2) mediated through the FXR and TGR5 receptors. Created with Biorender.com and published with permission.

Sex Differences in Bile Acids

Studies in humans suggest that men have a larger bile acids pool (61). Sex differences also exist among specific bile acids. For example, men have higher cholic acid levels compared with women whereas women have higher chenodeoxycholic acid (62). In a study among obese children and adolescents, glycine-conjugated bile acids’ concentrations were found higher in males compared with females (63, 64). The opposite is reported in mice where female mice display higher total, primary and secondary serum bile acids compared with male littermates (62). Interestingly, sulfation changes this pattern resulting in male mice having higher bile acid sulfated forms in circulation, urine, and liver (65). Sulfation of bile acids modulates their solubility, absorption, and elimination, therefore regulating their homeostasis, which may contribute to sex differences in various pathophysiological conditions (66). This sexual dimorphism in bile acids is linked to higher 12α-hydroxylase activity in females (67). Female mice maintain higher concentrations of total bile acids than male counterparts even during treatment with high fructose and western diets, which are known to increase bile acids synthesis, reabsorption, and decrease efflux (68). Interestingly, this sexual dimorphism may be attributed to the tissue-specific regulation of bile acids. For example, aging predominantly changes the hepatic bile acid profile in females but intestinal bile acid profile in males (69). Bile acids are synthesized from cholesterol, the metabolism of which is also sex divergent. In rodents, females have higher cholesterol precursors than males when challenged with dietary cholesterol, suggesting that they may handle cholesterol more efficiently (70). These findings emphasize that biological sex is an important factor to consider when investigating potential treatment strategies. Sex hormones are established key mediators of sex differences in hypertension (71, 72), but their role in salt-sensitive hypertension and in regulating bile acid homeostasis is less understood. Estrogen ablates high fat-induced steatosis and the associated decrease in fecal bile acid excretion (73). A potential signaling connection between sex hormones and bile acids pathways is conceivable especially considering their close structural backbone similarities (74).

Bile Acids in Inflammation

Dysregulation of bile acids signaling is reported in several inflammatory diseases. Patients with ulcerative colitis have lower secondary but higher primary bile acids, which correlate with changes in the gut microbiota and circulating proinflammatory cytokines including TNF-α and interleukin-1β (24). Bile acids specifically inhibit the NLRP3 inflammasome activation through the TGR5-cAMP-PKA axis, therefore, attenuating inflammation (75). NLRP3 inflammasome contributes to the development of elevated pressure and its inhibition attenuates salt-sensitive hypertension (76, 77). Bile acids also play a role in modulating renal inflammation (35, 78, 79). This suggests that bile acids may be a viable therapeutic strategy for salt-sensitive hypertension by inhibiting inflammasome activation.

Dendritic cells express bile acids receptors, which function to modulate inflammatory activation. Bile acids induce differentiation of interleukin-12 hypoproducing dendritic cells from monocytes through the TGR5 pathway (80). Activation of FXR may rescue depletion of splenic dendritic cells and increase the plasma concentration of anti-inflammatory cytokine interleukin-10 (81).

3β-Hydroxydeoxycholic acid (isoDCA), a secondary bile acid, diminishes the immunostimulatory response of DCs, increasing Foxp3 and Treg cell induction (15). Furthermore, ablating FXR enhanced the generation of T-regulatory cells and imposed a transcriptional profile like that induced by isoDCA, suggesting that isoDCA potentiates T-reg cell induction through an FXR dependent process (15). Similarly, other bile acids including lithocholic acid (LCA) derivative bile acids, 3-oxoLCA, and isoalloLCA were found to regulate T-cell function in mice. 3-OxoLCA inhibits the differentiation of TH17 cells by directly binding to the key transcription factor retinoid related orphan receptor-γt (RORγt), whereas isoalloLCA increases the differentiation of T-regulatory cells through the production of mitochondrial reactive oxygen species, leading to increased expression of FOXP3 (82). In the heart, bile acids activate TGR5 to protect the heart following myocardial ischemia-reperfusion injury by inhibiting IL-1β activation by blocking NF-κb signaling and upregulating heat-shock proteins (83–85).

Bile Acid Signaling Pathways as Potential Novel Targets in Salt-Sensitive Hypertension

The reported associations between bile acid signaling and mechanisms of salt-sensitive hypertension suggest their therapeutic potential (Fig. 4). Activation of FXR and TGR5 by endogenous ligands and pharmacological agents may be a potential therapeutic target for the treatment of salt-sensitive hypertension. In animal models, INT-767, a semisynthetic agonist for both FXR and TGR5, is effective in reversing obesity and atherosclerosis, pathologies that are strongly associated with hypertension (86). Chenodeoxycholic acid attenuates hypertension in rats by activating FXR (87). A western diet that is high in fat and salt is associated with dysregulated bile acid signaling characterized by increased production of proinflammatory bile acids such as DCA (88, 89). A combination of high fat and antibiotic treatment increases taurine-conjugated bile acids including T-β-MCA, an endogenous inhibitor of FXR (90). Although FXR activation by agonists such as GW4064 is widely regarded as beneficial (91–93), evidence suggests that this effect may be tissue dependent (94). Glycine-β-muricholic acid, an intestine-specific FXR inhibitor, has been demonstrated to attenuate obesity and metabolic syndrome in mice (95). Moreover, fexaramine, another intestine-specific FXR agonist, induces lithocholic acid-producing bacteria Acetatifactor and Bacteroides and improves glucose and insulin sensitivity by stimulating the secretion of glucagon-like peptide-1 (96). Interventions that ameliorate bile acid metabolism including weight loss and exercise are also associated with improved insulin sensitivity (97, 98). This further supports the role of the gut-liver signaling in metabolic disorders.

Figure 4.

Pharmacological targets in the bile acid signaling pathways. Factors that contribute to the development of hypertension including gut dysbiosis and inflammation impair bile acid signaling. Studies have demonstrated the role of selective bile acids and their agonists that activate the FXR and TGR5 pathways to promote vasodilation and attenuate inflammation and hypertension. Future studies are warranted to test the therapeutic potential of these agents in the treatment of salt-sensitive hypertension. Created with Biorender.com and published with permission.

The interplay between the liver, gut, and bile acid metabolism, widely studied as the gut-liver axis, is essential for the healthy function of many systems and its impairment has been associated with a wide range of pathologies including hypertension (11, 99, 100). Gut inflammation is associated with decreased expression of FXR and TGR5 and secondary bile acid deficiency (19, 101). Importantly, gut-derived trimethylamine-N-oxide (TMAO), a metabolite linked to vascular dysfunction and hypertension (102, 103), has been found to modulate bile acid metabolism and inhibit FXR, suggesting that impairment in bile acid metabolism may be implicated in the relationship between TMAO and hypertension, a topic for further research.

Emerging evidence demonstrates a potential for probiotics in preventing disease states by inhibiting hepatic excessive synthesis of toxic bile acids and promoting their excretion (104). Activation of the TGR5 pathway reduces macrophages migration and downregulates NF-κB signaling (105, 106), whereas aberration of TGR5 specifically in macrophages increases chemokine expression and insulin resistance (106). Bile acid-mediated modulation of macrophage activation is important since macrophages contribute to the development of salt-sensitive hypertension via endothelial insulin resistance and vascular inflammation (107). TGR signaling may be a novel target for the treatment of hypertension as it also promotes the generation of vasodilators nitric oxide and hydrogen sulfide while dampening endothelin signaling (108). Indeed, TGR5 agonists like BAR501 attenuate the development of portal hypertension (109). Similarly, FXR agonist PX20606 ameliorates portal hypertension by reducing inflammation and promoting vasodilatory mechanisms (110). Therefore, bile acids may have vasoactive properties through FXR and TGR5 activation in the portal system, which is naturally rich with bile acids. However, their vascular effects outside of the portal system are a topic for future investigation. Early studies by Tominaga et al. (111) showed that administration of deoxycholic acid to spontaneously hypertensive rats lowered blood pressure by improving vascular function, which suggests that bile acids may be vasoactive in the nonportal system as well. Besides, FXR and TGR5 are expressed in tissues that play a role in regulating sodium balance and water homeostasis including the gut (Fig. 5) and the kidney, which warrants further research to evaluate their therapeutic potential in salt-sensitive hypertension (112, 113).

Figure 5.

Secondary bile acids may mediate inflammation and salt-sensitive hypertension. Excess dietary salt induces gut microbial dysbiosis and leads to activation of dendritic cells (DC) to produce reactive oxygen species (ROS) leading to Isolevuglandins (IsoLGs) formation and secretion of proinflammatory cytokines interleukin 6 (IL-6) and interleukin 1β (IL-1β). Activation of DCs leads to T cell activation and hypertension. Dysbiosis impairs FXR activation by attenuating secondary bile acid synthesis, which may promote inflammation. Created with Biorender.com and published with permission.

CONCLUSIONS AND PERSPECTIVES

Salt-sensitive hypertension is undoubtedly dangerous for the health of men and women, young and old including even those who may otherwise seem healthy. There have been great efforts to describe the exact pathogenesis of salt-sensitive hypertension. Although we now understand some factors that play a role in its development including renal sodium handling and immune activation, they have not led us to a cure of salt-sensitive hypertension yet. This has been challenging because, based on the available scientific reports, these factors are numerous and from diverse systems. New research supports that bile acids are signaling molecules that may regulate blood pressure by regulating water homeostasis, inflammatory response, and vascular function mechanisms. Thus, bile acids may be novel targets for future studies to understand the cause and find therapeutics for salt-sensitive hypertension.

GRANTS

This study was supported by Vanderbilt Clinical and Translational Science Award Grant UL1TR002243 (to A.K.) from National Center for Advancing Translational Sciences; American Heart Association Grant 903428 (to J.A.I.); and National Heart, Lung, and Blood Institute Grants K01HL13049, R03HL155041, and R01HL144941 (to A.K.).

DISCLOSURES

No conflicts of interest, financial or otherwise, are declared by the authors.

AUTHOR CONTRIBUTIONS

J.A.I. and A.K. conceived and designed research; J.A.I. and L.A.E. prepared figures; J.A.I., T.D., and L.A.E. drafted manuscript; J.A.I., T.D., L.A.E., and A.K. edited and revised manuscript; J.A.I., T.D., L.A.E., and A.K. approved final version of manuscript.