Endotoxin downregulates rat hepatic ntcp gene expression via decreased activity of critical transcription factors.

Abstract

Sodium-dependent uptake of bile acids across the hepatic basolateral membrane is rapidly and profoundly diminished during sepsis, thus contributing to the pathogenesis of sepsisassociated cholestasis. This effect is mediated by endotoxin or effector cytokines, which reduce expression of several hepatobiliary transporters, including the sodium-dependent bile acid transporter gene, ntcp . We test here the hypothesis that endotoxin treatment leads to impaired binding activity of ntcp promoter trans -acting factors, resulting in reduction of ntcp mRNA expression. After endotoxin administration, ntcp mRNA levels reached their nadir by 16 h, and nuclear run-on assays demonstrated a marked reduction in ntcp gene transcription. At 16 h after treatment, nuclear binding activities of two key factors that transactivate the ntcp promoter, hepatocyte nuclear factor (HNF) 1 and Footprint B binding protein (FpB BP), decreased to 44 and 47% of pretreatment levels, respectively, while levels of the other known ntcp promoter transactivator, signal transducer and activator of transcription 5, were unaffected. In contrast, the universal inflammatory response factors nuclear factor B and activating protein 1 were both upregulated significantly. Examination of nuclear extracts obtained at sequential time points revealed that the maximal decrease in nuclear activities of both HNF1 and FpB BP preceded the nadir of ntcp mRNA expression by 6-10 h. Furthermore, these two nuclear factors returned towards normal levels before the recovery of ntcp mRNA levels observed by 48 h. Since HNF1 ␣ mRNA levels were unchanged at all time points, HNF1 is likely to be regulated posttranscriptionally by endotoxin. We conclude that the downregulation of ntcp gene expression by endotoxin is mediated at the level of transcription through tandem reductions in the nuclear binding activity of two critical transcription factors. These findings provide new insight into the coordinated downreg-

Hepatobiliary transporter expression in percutaneous liver biopsies of patients with cholestatic liver diseases

“…42,43 mRNA levels of the recently cloned OATP2 (SCL21A6) were also significantly reduced, consistent with recent findings in common bile duct-ligated rats. 44 Hepatic NTCP and OATP2 mRNA levels correlated inversely with serum bile salt levels, suggesting a causal relationship between retention of bile salts and down-regulation of these basolateral bile salt-uptake systems.…”

Short-term regulation of bile acid uptake by microfilament-dependent translocation of rat ntcp to the plasma membrane

“…ntcp transcription and thus ntcp expression is decreased in response to stimulation with endotoxin, and transcriptional factors responsible for this have recently been identified. 4 However, transcriptional regulation of ntcp is not sufficient to account for all of the observed changes in ntcp activity or membrane protein content, suggesting a posttranslational component. 3 Although much is known about long-term regulation of ntcp expression, little is known about short-term regulation and trafficking of the formed protein.…”

Regulation of Plasma Membrane Localization of the Na+-taurocholate cotransporting polypeptide (Ntcp) by Hyperosmolarity and Tauroursodeoxycholate

“…At the canalicular membrane, bile salt excretion is mediated by the bile salt export pump (Bsep) 2 (7) and to a lesser extent by the multidrug resistance-associated protein 2 (Mrp2) (8). The expression of Bsep and Mrp2 in the canalicular membrane of the hepatocyte is regulated by oxidative stress (9 -11), lipopolysaccharide (LPS) (12)(13)(14), drugs (15,16), and ambient osmolarity (1,10,(17)(18)(19). The cellular hydration state is a dynamic parameter that can change physiologically within minutes depending on the ambient osmolarity, nutrient supply, influence of hormones, and oxidative stress.…”