Impaired bacterial killing in early obstructive jaundice

The bile acid membrane receptor TGR5: a novel pharmacological target in metabolic, inflammatory and neoplastic disorders

TGR5 is the G-protein-coupled bile acid-activated receptor, found in many human and animal tissues. Considering different endocrine and paracrine functions of bile acids, the current review focuses on the role of TGR5 as a novel pharmacological target in the metabolic syndrome and related disorders, such as diabetes, obesity, atherosclerosis, liver diseases and cancer. TGR5 ligands improve insulin sensitivity and glucose homeostasis through the secretion of incretins. The bile acid/TGR5/cAMP signaling pathway increases energy expenditure in brown adipose tissue and skeletal muscle. Activation of TGR5 in macrophages inhibits production of proinflammatory cytokines and attenuates the development of atherosclerosis. This receptor has been detected in many cell types of the liver where it has anti-inflammatory effects, thus reducing liver steatosis and damage. TGR5 also modulates hepatic microcirculation and fluid secretion in the biliary tree. In cell culture models TGR5 has been linked to signaling pathways involved in metabolism, cell survival, proliferation and apoptosis, which suggest a possible role of TGR5 in cancer development. Despite the fact that TGR5 ligands may represent novel drugs for prevention and treatment of different aspects of the metabolic syndrome, clinical studies are awaited with the perspective that they will complete TGR5 biology and identify efficient and safe TGR5 agonists.

Perspective: TGR5 (Gpbar-1) in liver physiology and disease

Bile acids are signaling molecules with diverse endocrine functions. Bile acid effects are mediated through the nuclear receptor farnesoid X receptor (FXR), the G-protein coupled receptor TGR5 (Gpbar-1) and various other bile acid sensing molecules. TGR5 is almost ubiquitously expressed and has been detected in different non-parenchymal cells of human and rodent liver. Here, TGR5 has anti-inflammatory, anti-apoptotic and choleretic functions. Mice with targeted deletion of TGR5 are protected from the development of cholesterol gallstones. Administration of specific TGR5 agonists lowers serum and liver triglyceride levels thereby reducing liver steatosis. Furthermore, activation of TGR5 promotes intestinal glucagon-like peptide-1 (GLP-1) release, thereby modulating glucose homeostasis and energy expenditure in brown adipose tissue and skeletal muscle. Additionally, TGR5 exerts anti-inflammatory actions resulting in decreased liver injury in animal models of sepsis. These beneficial effects make TGR5 an attractive therapeutic target for metabolic diseases, such as diabetes, obesity, atherosclerosis and steatohepatitis.

Interleukin-1 receptor type I gene-deficient bile duct-ligated mice are partially protected against endotoxin

Cholestatic liver injury is associated with an increased susceptibility toward endotoxin-induced toxicity. To determine the role of interleukin 1 (IL-1) herein, extrahepatic cholestasis was induced by bile duct ligation (bdl) in IL-1 receptor type I gene-deficient (IL-1R(-/-)) mice, which are unresponsive to IL-1alpha and IL-1beta, and normal IL-1R(+/+) mice. Bdl elicited increases in hepatic IL-1alpha and IL-1beta messenger RNA (mRNA) and protein. Hepatocellular injury at 2 weeks after bdl was similar in IL-1R(-/-) and IL-1R(+/+) mice as shown by clinical chemistry and histopathology. Administration of endotoxin to cholestatic mice at 2 weeks after bdl was associated with enhanced cytokine release, more severe liver damage, and occurrence of death when compared with sham-operated mice. Endotoxin effects in sham-operated IL-1R(-/-) and IL-1R(+/+) mice were largely similar, but cholestatic IL-1R(-/-) mice were better protected against toxic effects of endotoxin, as reflected by lowered cytokine release, less profound liver injury, and reduced mortality. These data indicate that IL-1alpha and IL-1beta are produced in the liver after bdl, but that these cytokines do not play a significant role in cholestatic liver damage; however, endogenous IL-1 activity is an important denominator of enhanced endotoxin sensitivity that is observed during cholestasis induced by bdl.

Reversible Kupffer cell suppression in biliary obstruction is caused by hydrophobic bile acids

BACKGROUND/AIM

Biliary obstruction is associated with suppressed Kupffer cell clearance of bacteria and intracellular bactericidal activity of the phagocytes. We studied the superoxide generation of Kupffer cell in biliary obstruction and after incubation with bile acids to elucidate the mechanism of impaired intracellular killing of these phagocytes.

METHODS

Kupffer cells were extracted from rats with common bile duct ligation or sham-operation. The extracted cells were tested for superoxide production immediately after extraction, 2 h and 24 h post-extraction. Superoxide generation from Kupffer cells after incubation with five bile acids (cholic acid, taurocholic acid, deoxycholic acid, chenodeoxycholic acid and ursodeoxycholic acid) at two different concentrations (0.1 mM and 1.0 mM) were also studied. Cell viability was monitored by trypan blue exclusion.

RESULTS

Kupffer cells extracted from common duct-ligated animals had significantly lower superoxide production (-1.37+/-0.24 nmol O2(-)/10(6) cells) compared to that from sham-operated rats (2.54+/-0.58 nmol O2(-)/10(6) cells) and non-operated rats (2.15+/-0.76 nmol O2(-)/10(6) cells) (p<0.05). After 2 h of resting in culture medium, these cells recovered significantly in superoxide production to 2.72+/-0.63 nmol O2(-)/10(6) cells (p<0.01). A dose-related reduction in superoxide production was demonstrated when Kupffer cells were incubated with bile acids. Hydrophobic bile acids (deoxycholic acid and chenodeoxycholic acid) caused more significant suppression than with hydrophilic bile acids (cholic acid, taurocholic acid, ursodeoxycholic acid). The drop in superoxide production after bile acid treatment was not due to cell death. Washing in Hank's balanced salt solution resulted in partial recovery of Kupffer cell superoxide production.

CONCLUSION

High blood levels of hydrophobic bile acids are likely to be the cause of impaired intracellular bactericidal activity of Kupffer cells in biliary obstruction.

Role of endotoxin and nitric oxide in the pathogenesis of renal failure in obstructive jaundice

BACKGROUND

There is an increased incidence of postoperative renal failure in patients with obstructive jaundice. The purpose of this study was to investigate the role of endotoxaemia and nitric oxide in this association.

METHODS

In bile duct-ligated, sham-operated and control rats, plasma total bilirubin levels, creatinine clearance and plasma endotoxin were determined. Endothelium-dependent vasodilatation to acetylcholine, and endothelium-independent vasodilatation to nitroglycerine and forskolin were evaluated in isolated perfused rat kidney.

RESULTS

Twenty-one of 27 bile-duct ligated rats had endotoxaemia. Plasma bilirubin levels were higher and creatinine clearance was significantly reduced in the bile duct-ligated endotoxin-positive group compared with values in the other groups. Furthermore, in the isolated perfused rat kidney from rats with endotoxaemia, basal perfusion pressure and renal vascular relaxation to acetylcholine and nitroglycerine which is mediated by guanosine cyclic 3',5'-cyclic monophosphate (cGMP) were significantly reduced, but relaxation to forskolin mediated by adenosine cyclic 3',5'-cyclic monophosphate did not change.

CONCLUSION

Endotoxaemia in obstructive jaundice may induce overproduction of nitric oxide that may lead to impairment of cGMP-associated vasodilatation and disrupt autoregulation of the renal vascular bed. This may contribute to renal failure in obstructive jaundice.

Effects of ursodeoxycholic acid on hemodynamic and renal function abnormalities induced by obstructive jaundice in rats

The mechanism of renal function abnormalities in experimental biliary cirrhosis can be partially explained by the absence of gastrointestinal bile flow, which predisposes to translocation of intestinal endotoxin, a potent renal vasoconstrictor. Since bile acids prevent the absorption of intestinal endotoxins, we aimed to evaluate the effects of ursodeoxycholic acid (UDCA) administration on renal function and hemodynamic abnormalities induced by 1 week of obstructive jaundice in rats.

METHODS

Fifty-two rats were used; 30 had ligation of the common bile duct, 22 were sham operated. Bile duct ligated rats were randomly and blindly assigned to receive UDCA (25 mg/kg/day, n = 14) or placebo (n = 16) during 1 week. Sham rats received no treatment. Portal pressure (PP) as well as creatinine clearance (CrCl), urinary sodium (US), and plasma renin activity (PRA) were evaluated. Results are mean +/- SEM, with a significant value of p < 0.05.

RESULTS

Portal pressure (10.4 +/- 1.1 vs. 12.1 +/- 0.8 mm Hg) was significantly lower in UDCA than in placebo-treated rats. ALT serum levels were also significantly lower in bile duct ligated rats receiving UDCA (77.3 +/- 28 IU/L) than in placebo-treated rats (162 +/- 65 IU/L). US (1.1 +/- 0.5 vs. 2.1 +/- 0.3 mEq/24 h) was significantly lower and PRA (6.0 +/- 2.6 vs. 1.9 +/- 1.0 ng Ang 1/mL/h) higher in bile duct ligated than in sham-operated rats. No differences were found between UDCA or placebo-treated bile duct ligated rats. CrCl was similar between sham (0.39 +/- 0.12 mL/min/100 g BW) and UDCA (0.32 +/- 0.16) but significantly lower in placebo-treated (0.28 +/- 0.07) than sham-operated rats (p < 0.05).

CONCLUSION

UDCA administration had very mild effects on renal function abnormalities induced by experimental obstructive jaundice in rats. However, portal hypertension and biochemical abnormalities were partially improved.