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Abstract

Rotaviruses (group A rotaviruses) are the most important cause of severe gastroenteritis in infants and children worldwide. Currently, an antiviral drug is not available and information on therapeutic targets for antiviral development is limited for rotavirus infection. Previously, it was shown that lipid homeostasis is important in rotavirus replication. Since farnesoid X receptor (FXR) and its natural ligands bile acids (such as chenodeoxycholic acid [CDCA]) play major roles in cholesterol and lipid homeostasis, we examined the effects of bile acids and synthetic FXR agonists on rotavirus replication in association with cellular lipid levels. In a mouse model of rotavirus infection, effects of oral administration of CDCA on fecal rotavirus shedding were investigated.

The results demonstrate the following. First, the intracellular contents of triglycerides were significantly increased by rotavirus infection. Second, CDCA, deoxycholic acid (DCA), and other synthetic FXR agonists, such as GW4064, significantly reduced rotavirus replication in cell culture in a dose-dependent manner. The reduction of viral replication correlated positively with activation of the FXR pathway and reduction of cellular triglyceride contents (r2 0.95). Third, oral administration of CDCA significantly reduced fecal virus shedding in mice (P < 0.05). We conclude that bile acids and FXR agonists play important roles in the suppression of rotavirus replication. The inhibition mechanism is proposed to be the downregu-lation of lipid synthesis induced by rotavirus infection.

Rotaviruses are nonenveloped, icosahedral viruses with an 11-segment double-stranded RNA genome. Rotavirus particles contain six structural proteins, which comprise a core (VP1 to -3), an inner capsid (VP6), and an outer capsid (VP4 and -7). Rotaviruses are divided into 7 morphologically indistinguishable but antigenically distinct serogroups (A to G) based on VP6 (16). Group A rotaviruses are the leading cause of severe gastroenteritis in infants and children worldwide, associated with approximately 111 million episodes of gastroenteritis that have required 25 million clinic visits and 2 million hospitalizations and have resulted in over 500,000 deaths in children younger than 5 years of age (16). Even though effective live-attenuated vaccines are available for human rotavirus infection (16), rotavirus still remains the most important cause of gastroenteritis in infants and children worldwide. Since there are no specific antiviral drugs available for rotavirus infection, treatment options for rotavirus-mediated gastroenteritis are limited to restoration and maintenance of hydration until the infection resolves (17). Therefore, development of a rotavirus-specific drug is important to reduce severity of disease and duration of rotavirus-related hospitalization. However, information on the therapeutic targets for rotavirus infection is limited.

Previously, it was shown that disruption of lipid rafts and/or lipid droplets decreased infectious rotaviruses by inhibition of rotavirus morphogenesis (7, 10). Lipid rafts are specialized membrane domains enriched in glycosphingolipids, cholesterol, and protein. Lipid droplets are the major lipid storage structure enriched in triglycerides and cholesterol ester and play a crucial role in regulating cellular lipid levels. The interaction of viral proteins with these subcellular lipid bodies is also important for infectious virus particle formation in some viruses, including human hepatitis C virus (30, 32) and dengue virus (38). These findings suggest that lipid homeostasis is important for the replication of certain viruses.

In the small intestines, where rotavirus replication occurs, de novo synthesis of lipids and absorption of dietary lipids affect cellular lipid contents in the epithelial cells (19). In the intestinal lumen, bile acids emulsify fats to form micelles to aid their absorption. Bile acids are synthesized from cholesterol in the liver, stored at the gallbladder, and released into the duodenum. The primary bile acids, cholic acid (CA) and chenodeoxycholic acid (CDCA), are synthesized in the liver from cholesterol by enzymes, including cholesterol 7-hydroxylase, and subsequently conjugated with taurine or glycine to enhance affinity to both acids and bases. Primary bile acids are transformed by intestinal bacteria into secondary bile acids, deoxycholic acid (DCA), lithocholic acid (LCA), and ursodeoxycholic acid (UDCA) (11). While the secreted bile acids travel through the intestinal tracts, they are reabsorbed in the ileum and returned to the liver via the portal vein (25). This enterohepatic circulation is essential in maintaining an effective concentration of bile acids and cholesterol homeostasis.

One of the bile acid receptors is farnesoid X receptor (FXR) (12, 27, 34). The activation of FXR by bile acids induces the expression of various proteins, including small heterodimer partner (SHP), which represses the expression of cholesterol 7-hydroxylase, the rate-limiting enzyme in bile acid synthesis (27, 34). The FXR/SHP pathway is well developed in hepatic, intestinal, and renal cells and also participates in the regulation of fatty acid (including cholesterol) metabolism and glucose homeostasis (40, 41, 44). Previously, our group reported that bile acids play an important role in the replication of hepatitis C viruses and porcine enteric caliciviruses (5, 6). However, the role of bile acids and other FXR agonists in rotavirus replication has been unknown.

Here, we report that the intracellular contents of triglycerides increased significantly by rotavirus infection. Furthermore, we demonstrate that CDCA and DCA at physiological concentrations and a synthetic FXR agonist, GW4064, at a low micromolar concentration significantly reduced rotavirus replication in cells in a dose-dependent manner. The reduction of virus replication correlated positively with activation of the FXR/SHP pathway and reduction of cellular lipid contents (triglycerides). Oral administration of CDCA significantly reduced the peak quantities of virus shedding in the mouse model of rotavirus infection compared to those for the non-treatment group (P 0.We conclude that bile acids and FXR agonists play important roles in the suppression of rotavirus replication, and the inhibition mechanism is proposed to be the downregulation of lipid synthesis induced by rotavirus infection.

Source:https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3209393/