Cholestasis

In contrast with urine formation, bile flow is not dependent on hydrostatic forces, but driven by osmotic pressure of solutes secreted across the apical membrane of hepatocytes and bile duct epithelial cells. This secretory process is mediated by a set of primary active transporters that use ATP hydrolysis to pump solutes against the concentration gradient. The most important solutes in bile are bile salts, lipids, electrolytes, and organic anions. The direct consequence of the osmotic mechanism of bile formation is that impaired function of these pumps leads to impaired bile flow-that is, cholestasis. The function of these pumps is highlighted by a number of inherited cholestatic diseases, which are caused by mutations in these genes. Identification of the molecular defect in these diseases was not only important for diagnostic reasons but also emphasised that impaired transporter function has pathological consequences. Indeed, it is now becoming clear that impaired or downregulated transporter function is also involved in the pathogenesis of acquired cholestatic syndromes.

Mammalian ABC transporters in health and disease

The ATP-binding cassette (ABC) transporters are a family of large proteins in membranes and are able to transport a variety of compounds through membranes against steep concentration gradients at the cost of ATP hydrolysis. The available outline of the human genome contains 48 ABC genes; 16 of these have a known function and 14 are associated with a defined human disease. Major physiological functions of ABC transporters include the transport of lipids, bile salts, toxic compounds, and peptides for antigen presentation or other purposes. We review the functions of mammalian ABC transporters, emphasizing biochemical mechanisms and genetic defects. Our overview illustrates the importance of ABC transporters in human physiology, toxicology, pharmacology, and disease. We focus on three topics: (a) ABC transporters transporting drugs (xenotoxins) and drug conjugates. (b) Mammalian secretory epithelia using ABC transporters to excrete a large number of substances, sometimes against a steep concentration gradient. Several inborn errors in liver metabolism are due to mutations in one of the genes for these pumps; these are discussed. (c) A rapidly increasing number of ABC transporters are found to play a role in lipid transport. Defects in each of these transporters are involved in human inborn or acquired diseases.

Pretreatment levels of urinary deoxypyridinoline as a potential marker in patients with prostate cancer with or without bone metastasis

OBJECTIVE

To assess the predictive role of the bone markers alkaline phosphatase (ALP) and urinary deoxypyridinoline (DPD), as indicators of bone turnover, at baseline in patients with prostate cancer.

PATIENTS, SUBJECTS AND METHODS

Urinary DPD, serum ALP and prostate-specific antigen (PSA) were evaluated in 23 patients with benign prostatic hyperplasia (BPH), 115 with prostatic carcinoma, of whom 21 had bone metastasis, and in 16 age-matched control subjects.

RESULTS

Patients with newly diagnosed prostate cancer and bone metastasis had a higher urinary excretion of DPD, and a higher serum PSA and ALP than had patients with BPH and those with prostate cancer but no metastasis. Receiver operating curve analysis for PSA, ALP and DPD showed a significant discriminating ability for positive and negative bone scans (P = 0.0684). However, from logistic regression of the combinations, only serum ALP was a significant independent predictor of bone metastasis in patients with prostate cancer.

CONCLUSION

Serum ALP or urinary DPD are the best predictors of bone metastasis in patients with prostate cancer; further studies with more patients are required.

Endotoxin-induced mortality in bile duct-ligated rats after administration of reconstituted high-density lipoprotein

Cholestatic patients have substantial morbidity because of increased susceptibility to endotoxin (lipopolysaccharide [LPS]). Although reconstituted high-density lipoprotein (rHDL) can bind and neutralize LPS, cholestasis is associated with a near complete absence of HDL. Effects of rHDL infusion on the outcome of LPS-induced inflammatory responses in cholestatic rats were determined. Bile duct-ligated (BDL) and sham rats were treated with rHDL or saline and challenged with LPS. Distribution of cholesterol over the lipoprotein subclasses changed by ligation: levels in low-density lipoprotein (LDL) and very low-density lipoprotein (VLDL) were increased 2-fold and 5-fold, respectively, and were decreased in HDL 2-fold. rHDL treatment did not affect cholesterol distribution. LPS was mainly found in the HDL fraction, and ligation affected only levels of HDL-bound LPS (50% decrease; P<.05). Although rHDL infusion effectively normalized the lipoprotein-bound LPS distribution, it resulted in increased sensitivity (mortality: 88% in the ligation + rHDL group versus 44% in the ligation + saline group, 25% in the sham + saline group, and 0% in the sham + rHDL group, P <.05). In accordance with these results, plasma tumor necrosis factor (TNF) was significantly highest in the BDL + rHDL group at several hours after LPS challenge as well as the accumulation of LPS in the liver (P<.05). rHDL infusion leads to increased LPS-induced mortality in cholestatic rats. These results sharply contrast with the protective effects of rHDL suppletion in experimental endotoxemia in animals and human volunteers without biliary obstruction and suggest that there may be danger in administration of rHDL to cholestatic patients.

Intestinal excretion of unconjugated bilirubin in man and rats with inherited unconjugated hyperbilirubinemia

Patients with Crigler-Najjar syndrome and Gunn rats cannot form bilirubin glucuronides owing to a lack of bilirubin UDP-glucuronosyltransferase activity. Because increased serum and tissue bilirubin levels remain constant, an alternative excretory route has to substitute for this deficiency. Gunn rats excrete in bile only 2-13% of the bilirubins eliminated in Wistar rats. In contrast, the biliary excretion rate of urobilinogen in Gunn and Wistar rats is comparable. The sum of bilirubins and urobilinogen excreted in the bile of Gunn rats amounts to 10-30% of pigments excreted in Wistar rats. Despite this low biliary excretion, the intestinal content and fecal excretion of bile pigments in Gunn and Wistar rats were similar. These data support an extrabiliary entrance of unconjugated bilirubin into the intestine. Additional proof for this was found in that the intestinal lumen of Gunn rats still contains a high amount of bilirubins and urobilinogen after 3 d of external biliary drainage. A similar procedure in Wistar rats resulted in the complete disappearance of bile pigments from the intestine. The direct transmural transport of bilirubin from blood to all parts of the intestinal lumen was demonstrated by injecting 14C-bilirubin i.v. into Gunn rats with isolated parts of small and large intestine. In Crigler-Najjar and Gilbert's syndrome patients, the biliary excretion of bile pigments has previously been shown to be strongly reduced. Their stools, however, contained approximately the same amount of bile pigments as in normal subjects. Although only traces of unconjugated bilirubin were detected in the stool of normal persons (4 +/- 3% of total bile pigments), higher amounts were found in patients with Crigler-Najjar disease (20 +/- 12&). These results suggest a direct intestinal permeation of unconjugated bilirubin in severe unconjugated hyperbilirubinemia both in man and rats.

Assignment of the human UDP glucuronosyltransferase gene (UGT1A1) to chromosome region 2q37

UDP glucuronosyltransferases (UGTs) comprise a multigene family of drug-metabolizing enzymes. The sub-family of UGTs that conjugate bilirubin and phenolic compounds with glucuronic acid has been termed UGT1A1. In man, UGT1A1 isoforms are encoded by a single gene, UGT1A1. Protein isoforms encoded by UGT1A1 originate by alternative splicing. In the present study, we used the cDNA of UGT1A1*4, a bilirubin-conjugating isoform, to localize the UGT1A1 locus in the human genome. The UGT1A1 gene was assigned by in situ hybridization to chromosome region 2q37.

Biosynthesis of acid alpha-glucosidase in late-onset forms of glycogenosis type II (Pompe's disease)

Cultured human skin fibroblasts from control persons and from patients with the generalized and late-onset forms of Pompe's disease were labelled with radioactive leucine and the incorporation of radioactivity into acid alpha-glucosidase and cathepsin D was analysed by immunoprecipitation, gel electrophoresis and fluorography. When the labelling was carried out for 6-12 h in the presence of NH4Cl, the labelling of secreted alpha-glucosidase relative to that of secreted cathepsin D in fibroblasts from patients with the late-onset form of Pompe's disease was less than 15% of that in fibroblasts from control persons. However, when the fibroblasts were labelled for less than 1 h, the relative rate of incorporation of radioactivity into acid alpha-glucosidase was rather similar in the two types of fibroblasts. In fibroblasts from patients with the generalized form of Pompe's disease no incorporation of radioactivity into acid alpha-glucosidase could be detected.

Accumulation of weak bases in relation to intralysosomal pH in cultured human skin fibroblasts

The volume of the lysosomal compartment in cultured human skin fibroblasts was estimated from the distribution between the cells and the medium of tracer amounts of labelled methylamine and chloroquine, which accumulate in the lysosomes, 2,2-dimethyloxazolidine-2,4-dione, which accumulates in the soluble cytoplasmic compartment relative to the lysosomes, and sucrose, which is excluded by the cells. In a foetal fibroblast line, the fractional volume of the lysosomal compartment was 0.044 +/- 0.007 (n = 8). In fibroblasts from a patient with the I-cell disease, the fractional volume was 0.15. The fractional volume of the lysosomal compartment was used to calculate the intralysosomal pH from the accumulation of the weak bases in the cells. The mean value obtained was 5.29 +/- 0.04 (n = 8). In fibroblasts incubated with various concentrations of chloroquine, the fractional volume of the lysosomal compartment and the accumulation of chloroquine in the cells were used to calculate the concentration of chloroquine in the lysosomes. The intralysosomal concentration increased from 3 to 114 mM as the extracellular concentration increased from 1 to 100 microM. Concomitantly, the intralysosomal pH increased from 5.3 in the absence of chloroquine to 5.9 in the presence of 100 microM chloroquine. A similar increase in intralysosomal pH could be calculated in fibroblasts incubated with different concentrations of ammonia.