Output of lysosomal contents and cholesterol into bile can be stimulated by taurodehydrocholate

Intracellular supply of phospholipids for biliary secretion: evidence for a nonvesicular transport component

Phospholipids (PL) for biliary secretion could be supplied from the endoplasmic reticulum (ER) to the plasma membrane by cytosolic transfer proteins or transport vesicles. Therefore, we studied whether biliary secretions of PL and apolipoprotein A-I (apo A-I), as markers for the ER-to-Golgi vesicular transport pathway, are tightly coupled in isolated perfused rat livers with enhanced secretion (+60%) of PL after withdrawal of the cholesterol synthesis inhibitor pravastatin (0.1% of chow, fed for 7 days). Blocking agents dissociated the secretion of apo A-I and PL. Brefeldin A as well as cycloheximide inhibited biliary secretion of apo A-I (-52%; -68%), however, not of PL. Both bilirubin ditaurate and taurodehydrocholic acid reduced biliary secretion of PL (-27%; -79%), but not of apo A-I. The data support the concept that PL destined for biliary secretion bypass the vesicular transport pathway of apo A-I through the Golgi compartment, most likely via cytosolic transfer proteins.

Evidence for secretory coupling of phosphatidylcholine molecular species to cholesterol in rat bile

BACKGROUND/AIMS

Hepatocytes secrete cholesterol into bile within lipid vesicles of selected phosphatidylcholines, mainly palmitoyl-linoleoyl-phosphatidylcholines, palmitoleoyl-oleoyl-phosphatidylcholines and palmitoleoyl-arachidonyl-phosphatidylcholines, which could in part determine the secreted amount of cholesterol.

AIMS

To study whether increased secretion of cholesterol, as caused by manipulation of cholesterol synthesis rate, changes the composition of phosphatidylcholines secreted in bile.

METHODS

Livers from control rats (Control), rats fed pravastatin for 7 days (Pravastatin) and livers isolated 5-7 or 8-11 hours after pravastatin had been withdrawn (Rebound5-7h; Rebound8-11h) were isolated perfused during infusion of taurocholic acid (400 nmol/min/100 g rat), to study biliary secretion of bile salts, cholesterol and phosphatidylcholine molecular species.

RESULTS

Bile salt secretion rate was similar in all four groups, secretion of cholesterol and phosphatidylcholines was similar in Control and Pravastatin. With duration of pravastatin withdrawal the secretion rates of phosphatidylcholine and cholesterol progressively increased by +38% and +122% in Rebound5-7h and by +70% and +300% in Rebound8-11h (vs Control), respectively. In parallel, the secretion rates of palmitoleoyl-oleoyl- and palmitoleoyl-arachidonyl-phosphatidylcholines rose up to sixfold and twofold, respectively, while the secretion rate of palmitoyl-linoleoylphospatidylcholines remained constant. The secretion rate of cholesterol was correlated (p < 0.01) with the secretion rates of palmitoleoyl-oleoyl-phosphatidylcholines (r = 0.83) and palmitoleoyl-arachidonyl-phosphatidylcholines (r = 0.81). Bilirubin ditaurate or taurodehydrocholate reduced (p < 0.05) biliary secretion of phosphatidylcholines (-33%; -72%) without changes in cholesterol/phosphatidylcholine secretory ratio or phosphatidylcholine species.

CONCLUSIONS

The secretion of the major molecular species of phosphatidylcholine in bile could be coregulated with the amount of cholesterol destined for biliary secretion.

Taurolithocholate can inhibit the biliary discharge of lysosomes in the rat

The natural bile salt taurolithocholate (TLC) impairs the biliary excretion of lipids and proteins, which are known to reach the canaliculus via vesicles. In this study we examined whether these observations could be extended to the exocytic discharge of lysosomal contents into bile. The single intravenous injection of a cholestatic dose of TLC, 3 micromol/100 g body wt., markedly inhibited the biliary excretion of the lysosomal enzymes acid phosphatase and beta-glucuronidase, despite the excretion of bile salts being normalized after a transient diminution. Under such a condition, TLC did not affect the normal transport to and the processing in lysosomes of the exogenously administered [14C]sucrose-labeled horseradish peroxidase. However, the biliary excretion of the radioactive lysosomal metabolites of the protein was significantly reduced. The results indicate that TLC can inhibit the biliary discharge of lysosomes in the rat without altering the functional integrity of these organelles. Possible explanations for these findings are discussed.

Regulation of protein secretion into bile: studies in mice with a disrupted mdr2 p-glycoprotein gene

BACKGROUND & AIMS

Protein is secreted into bile via several independent pathways. The aim of this study was to investigate whether these pathways are influenced by secretion of biliary lipid.

METHODS

Protein secretion and biliary lipid output were studied in wild-type mice (+/+), heterozygotes (+/-), and homozygotes (-/-) for mdr2 gene disruption. Biliary lipid and protein output were varied by infusion with taurocholate (TC) and tauroursodeoxycholate (TUDC).

RESULTS

Exocytosis and transcytosis were unaltered in (-/-) mice. Infusion with TC strongly induced secretion of alkaline phosphatase in (-/-) mice but had little effect in (+/-) and (+/+) mice. Infusion with TUDC had little effect on alkaline phosphatase output. In contrast, both TUDC and TC strongly stimulated secretion of aminopeptidase N and lysosomal enzymes in (+/+) mice but had no effect in (-/-) animals. Aminopeptidase N secretion correlated with phospholipid output, but only at high flux. At low flux, aminopeptidase N was secreted independently from both phospholipid and bile salts.

CONCLUSIONS

The canalicular membrane enzymes alkaline phosphatase and aminopeptidase N are secreted via separate pathways. Part of alkaline phosphatase output is controlled by bile salt hydrophobicity, whereas at high lipid flux, aminopeptidase N secretion seems to be coupled to phospholipid output. Lysosomal enzymes follow the latter pathway.

Fractionation of livers following diosgenin treatment to elevate biliary cholesterol

The plant saponin, diosgenin, is known to induce a marked increase in biliary cholesterol/phospholipid ratio. We reasoned that putative biliary lipid supply vesicles might be similarly enriched with cholesterol. Seven-day diosgenin feeding to rats resulted in significantly increased biliary cholesterol and cholesterol/phospholipid ratio, but had no effect on total cholesterol or phospholipid content of the liver. Subcellular fractionation of livers showed no selective increase in any fraction (nuclear, mitochondrial, lysosomal, microsomal) of the homogenate. Further subfractionation of microsomal or nuclear (plasma membrane) fractions also showed no difference between control and diosgenin groups. Thus, no intracellular vesicle fraction has been identified with the provision of the enhanced biliary cholesterol and the results are discussed in terms of the possible involvement of cytosolic lipid-binding proteins as putative lipid carriers to the canalicular membrane as an alternative to the presence of the lipid in lipid supply vesicles.

Bile acid-dependent vesicular transport of lysosomal enzymes into bile in the rat

BACKGROUND

Bile acids may stimulate the movement of hepatocyte vesicles and enhance their fusion with the biliary canaliculus. The present study examined the effects of various bile acids on the exocytosis of the contents of hepatocyte lysosomes into the biliary canaliculus.

METHODS

The effects of various bile acids on hepatocyte lysosome movement and on exocytosis of the contents of hepatocyte lysosomes into the biliary canaliculus were determined from the distribution of fluorescein isothiocyanate-dextran--labeled lysosomes in hepatocyte couplets and by quantitating biliary lysosomal enzyme output in rats.

RESULTS

Hydrophobic as well as hydrophilic and nonmicellar bile acids were found to stimulate to a similar degree the output of lysosomal enzymes into bile, indicating that bile acid-induced change of canalicular or lysosomal membrane fluidity is not responsible for enhanced exocytosis. The taurocholate-dependent increase in lysosomal enzyme excretion was completely blocked by either microtubule or microfilament inhibition, suggesting that these subcellular structures are involved in bile acid-dependent vesicular transport. Fluorescent microscopy studies showed that taurocholate causes a microtubule-dependent translocation of lysosomes towards the canaliculus in hepatocyte couplets, which occurred at the same time as increased output of lysosomal enzymes into bile.

CONCLUSIONS

The results suggest that bile acids modulate vesicle traffic towards the canaliculus by a mechanism unrelated to bile acid interaction with the vesicle membrane.

Biliary excretion of polyethylene glycol molecular weight 900. Evidence for a bile salt-stimulated vesicular transport mechanism

Polyethylene glycol molecular weight 900 (PEG-900) has been used as a marker of vectorial water transport into bile canaliculus. However, the mechanisms by which this compound is excreted have not been clarified. To gain more information on this process, we studied the biliary excretion of [3H]PEG-900 in rats during choleresis induced by canalicular choleretics. In addition, the effects of the microtubule inhibitors colchicine and vinblastine, and of the acidotropic agent chloroquine, on PEG-900 excretion were studied to determine whether a vesicular pathway is involved. Continuous i.v. infusion of either dehydrocholate (DHC, a non-micelle forming bile salt choleretic) or 4-methylumbelliferone (4-MU, a non-bile salt canalicular choleretic) at stepwise-increasing rates [0.7, 1.0 and 1.2 mumol.min-1.(100 g body wt)-1] induced a gradual increment in bile flow, whereas a transient increment of [3H]PEG-900 excretion was observed only during DHC-induced choleresis. Furthermore, studies in which two consecutive i.v. injections of DHC (10 mumol/100 g body wt) were administered showed that [3H]PEG-900 excretion induced by a second administration of DHC was 54% lower than that induced by the first one, despite a similar excretion in bile flow. Finally, colchicine (0.5 mumol/100 g body wt), vinblastine (0.5 mumol/100 g body wt) and chloroquine (50 mg/kg body wt) pretreatments inhibited the DHC-induced increment in biliary [3H]PEG-900 output, while DHC-induced choleresis was almost unaffected. Conversely, excretion of [14C]sucrose, when coadministered with [3H]PEG-900, was not impaired by the treatments. These results suggest that, unlike sucrose, PEG-900 excretion is not associated with canalicular water movements. Instead, it may be related to a vesicular transport process followed by a bile acid-stimulated discharge of secretory vesicles into bile through the lysosomal compartment.

Inhibition of biliary cholesterol and phospholipid secretion by cefmetazole. The role of vesicular transport and of canalicular events

A number of organic anions selectively inhibit the biliary secretion of cholesterol and phospholipids without affecting bile acid secretion. We studied the effect of cefmetazole, a third-generation cephalosporin, on biliary lipid secretion in the rat. Injection of cefmetazole at a dose of 200 mumol/kg body wt. induced a choleretic effect and a significant decrease in the biliary output of cholesterol and phospholipid, without changes in bile acid secretion. The decrease was more marked for cholesterol than for phospholipid secretion, with a significant decrease in their molar ratio in bile. The effects were apparently unrelated to an inhibition of intracellular vesicular transport because, after injection of horseradish peroxidase, both the time course and total amount secreted of the protein did not significantly differ between control animals and those receiving cefmetazole. The secretory rate of the lysosomal marker acid phosphatase was not affected by cefmetazole administration. Biliary outputs of the plasma-membrane enzymes alkaline phosphatase and gamma-glutamyltransferase were significantly decreased by the antibiotic. These results point to an effect of cefmetazole at the level of the canalicular membrane.

Effect of chloroquine on biliary lipid and lysosomal enzyme output in the isolated perfused rat liver at low bile salt output rates

Chloroquine, when introduced into isolated perfused rat livers, caused a substantial output of cholesterol into bile, occurring after 30 min and peaking at 60 min, whereas the biliary output of acid phosphatase and beta-glucuronidase increased only after 90 min. The origins of this bile-salt-independent cholesterol are discussed.