Cyclic AMP stimulates sorting of the canalicular organic anion transporter (Mrp2/cMoat) to the apical domain in hepatocyte couplets

Molecular Mechanisms in Bile Formation

Active canalicular secretion of bile salts and non-bile salt organic anions represents the major driving force of hepatic bile formation. The most important carriers involved have now been cloned on both the basolateral and canalicular sides of hepatocytes. Elucidation of their structure, transport properties, and regulation is an important step forward in the ultimate understanding of the molecular physiology of bile formation.

MRP2, a human conjugate export pump, is present and transports fluo 3 into apical vacuoles of Hep G2 cells

The multidrug resistance protein 2 (MRP2, symbol ABCC2) transports anionic conjugates and certain amphiphilic anions across the apical membrane of polarized cells. Human hepatoma Hep G2 cells retain hepatic polarity and form apical vacuoles into which cholephilic substances are secreted. Immunofluorescence microscopy showed that human MRP2 was expressed in the apical vacuole membrane of polarized Hep G2 cells, whereas the isoform MRP3 was localized to the lateral membrane. Expression of both MRP2 and MRP3 was confirmed by immunoblotting and reverse transcription PCR. Fluo 3 secretion into the apical vacuoles was inhibited by cyclosporin A but not by selective inhibitors of multidrug resistance 1 P-glycoprotein. In addition, carboxyfluorescein, rhodamine 123, and the fluorescent bile salt derivatives ursodeoxycholyl-(Nepsilon-nitrobenzoxadiazolyl)-lysine and cholylglycylamido-fluorescein were secreted into the apical vacuoles; the latter two probably via the bile salt export pump. We conclude that MRP2 mediates fluo 3 secretion into the apical vacuoles of polarized Hep G2 cells. Thus the function of human MRP2 and the action of inhibitors can be analyzed by the secretion of fluorescent anions such as fluo 3.

Characterization of the 5'-flanking region of the human multidrug resistance protein 2 (MRP2) gene and its regulation in comparison withthe multidrug resistance protein 3 (MRP3) gene

The multidrug resistance proteins MRP2 (symbol ABCC2) and MRP3 (symbol ABCC3) are conjugate export pumps expressed in hepatocytes. MRP2 is localized exclusively to the apical membrane and MRP3 to the basolateral membrane. MRP2 mRNA is expressed at a high level under normal conditions, whereas MRP3 mRNA expression is low and increases only when secretion across the apical membrane by MRP2 is impaired. We studied some of the regulatory properties of the two human genes using transient transfection assays with promoter-luciferase constructs in HepG2 cells and cloned fragments of 1229 nucleotides and 1287 nucleotides of the MRP2 and MRP3 5'-flanking regions, respectively. The sequence between nucleotides -517 and -197 was decisive for basal MRP2 expression. Basal promoter activity of MRP3 was only 4% of that measured for MRP2. At submicromolar concentrations, the histone deacetylase inhibitor trichostatin A reduced the MRP2 reporter gene activity and expression of the protein. Disruption of microtubules with nocodazole decreased gene and protein expression of MRP2 and increased MRP3 reporter gene activity. The genotoxic 2-acetylaminofluorene decreased the activity of the human MRP2 reporter gene construct, but increased MRP3 gene activity and enhanced the amounts of mRNA and protein of MRP2 and MRP3. Thus, regulation of the expression of these ATP-dependent conjugate export pumps is not co-ordinate, but in part inverse. The inverse regulation of the two MRP isoforms is consistent with their distinct localization, their different mRNA expression under normal and pathophysiological conditions, and their different directions of substrate transport in polarized cells.

Conjugate export pumps of the multidrug resistance protein (MRP) family: localization, substrate specificity, and MRP2-mediated drug resistance

The membrane proteins mediating the ATP-dependent transport of lipophilic substances conjugated to glutathione, glucuronate, or sulfate have been identified as members of the multidrug resistance protein (MRP) family. Several isoforms of these conjugate export pumps with different kinetic properties and domain-specific localization in polarized human cells have been cloned and characterized. Orthologs of the human MRP isoforms have been detected in many different organisms. Studies in mutant rats lacking the apical isoform MRP2 (symbol ABCC2) indicate that anionic conjugates of endogenous and exogenous substances cannot exit from cells at a sufficient rate unless an export pump of the MRP family is present in the plasma membrane. Several mutations in the human MRP2 gene have been identified which lead to the absence of the MRP2 protein from the hepatocyte canalicular membrane and to the conjugated hyperbilirubinemia of Dubin-Johnson syndrome. Overexpression of recombinant MRP2 confers resistance to multiple chemotherapeutic agents. Because of its function in the terminal excretion of cytotoxic and carcinogenic substances, MRP2 as well as other members of the MRP family, play an important role in detoxification and chemoprevention.

Export pumps for glutathione S-conjugates

The release of glutathione S-conjugates from cells is an ATP-dependent process mediated by integral membrane glycoproteins belonging to the recently discovered multidrug-resistance protein (MRP) family. Many lipophilic compounds conjugated with glutathione, glucuronate, or sulfate are substrates for export pumps of the MRP family. In humans six MRP isoforms encoded by different genes have been cloned. Orthologs of MRP have been identified in many species including yeast, plants, and nematodes. Human MRP1 and MRP2 are currently best characterized with respect to substrate specificity by measurements of ATP-dependent transport into inside-out membrane vesicles. High-affinity substrates include the glutathione S-conjugate leukotriene C4, S-(2,4dinitrophenyl)glutathione, bilirubin glucuronosides, and 17beta-glucuronosyl estradiol. In addition, glutathione disulfide is transported by MRP1 and MRP2. Reduced glutathione may be released from cells in a process directly or indirectly mediated by members of the MRP family. Proteins of the MRP family are indispensable for transport of glutathione S-conjugates and glutathione disulfide into the extracellular space and play, therefore, a decisive role in detoxification and defense against oxidative stress.

Canalicular export pumps traffic with polymeric immunoglobulin A receptor on the same microtubule-associated vesicle in rat liver

Basolateral to apical vesicular transcytosis in the hepatocyte is an essential pathway for the delivery of compounds from the sinusoidal blood to the bile and to traffic newly synthesized resident apical membrane proteins to their site of function at the canalicular membrane front. To characterize this pathway better, microtubules in a hepatocyte homogenate were polymerized by addition of taxol, and associated membrane-bound vesicles were isolated. This fraction was enriched in polymeric immunoglobulin A receptor and contained apical membrane proteins. Immunoelectron microscopy demonstrated that polymeric immunoglobulin A receptor was localized predominantly on vesicles ranging from 100 to 160 nm and that the multidrug resistance protein 2 and the bile salt export pump co-localized on these vesicles. The minus-ended microtubule motor, dynein, was highly enriched in the fraction, and its intermediate chain could be released effectively by incubation with 1 mM ATP or GTP. However, the association of the transcytotic vesicles with the microtubules was not sensitive to hydrolyzable or non-hydrolyzable nucleotides. This study characterizes a fraction of microtubule-associated vesicles from rat hepatocytes and demonstrates that several resident apical membrane transport proteins and the polymeric immunoglobulin A receptor traffic on the same vesicle.

Glutathione S-conjugate transport in hepatocytes entering the cell cycle is preserved by a switch in expression from the apical MRP2 to the basolateral MRP1 transporting protein

The multidrug resistance protein MRP1 and its isoform MRP2 are involved in ATP-dependent glutathione S-conjugate transport and have similar substrate specificities. MRP2 mediates hepatic organic anion transport into bile. The physiological function of MRP1 in hepatocytes is unknown. Previous results show that MRP1 expression is low in quiescent hepatocytes but increased after SV40 large T antigen immortalization, suggesting a relationship with cell proliferation. Therefore, we determined mrp1 and mrp2 expression in rat hepatocytes in relation to the cell cycle. By varying cell density we obtained cultures that are mainly in G1 (high density) or have progressed into the S-phase or beyond (low density). In both cultures mrp1 mRNA and protein levels are increased, concomitantly with the disappearance of mrp2. This switch from mrp2 to mrp1 occurs in the G1 phase of the cell cycle and is associated with a decreased cell polarity. Mrp1 is located on lateral membranes or on intracellular vesicles, depending on whether cell-cell contact is established. In both locations mrp1 contributes to cellular glutathione S-conjugate efflux and protects against oxidative stress-inducing quinones. We conclude that a switch in expression from the apically located mrp2 to the basolaterally located mrp1 preserves glutathione S-conjugate transport in hepatocytes entering the cell cycle and protects against certain cytotoxic agents.

Regulation of the multidrug resistance protein 2 in the rat liver by lipopolysaccharide and dexamethasone

BACKGROUND & AIMS

Endotoxin lipopolysaccharide (LPS) induces cholestasis and down-regulates the multidrug resistance protein 2 (MRP2). This study intends to characterize the short-term effects of LPS on MRP2.

METHODS

The effects of LPS and dexamethasone on excretion of bromosulphalein (BSP), MRP2 messenger RNA (mRNA) levels, and subcellular MRP2 localization were studied by means of liver perfusion, Northern blots, and confocal microscopy.

RESULTS

LPS treatment for 3-12 hours decreased biliary BSP excretion (10 micromol/L) by 40%. Hyposmolarity stimulated BSP excretion to control levels 3 hours after LPS injection, but was ineffective after 12 hours or in saline-treated controls. LPS led to a strong decrease of MRP2 mRNA after 12 hours, but not during the first 6 hours. LPS induced the appearance of MRP2 in intracellular vesicles in the immediate vicinity of the canaliculi within 3 hours, and these vesicles were remote from the canaliculi after 6 and 12 hours. The MRP2-containing vesicles did not stain for dipeptidylpeptidase IV (DPPIV). Dexamethasone counteracted the LPS effects on MRP2 mRNA levels, subcellular distribution, and BSP excretion.

CONCLUSIONS

LPS induces cholestasis due to an early retrieval of MRP2 from the canalicular membrane, whereas down-regulation of MRP2 mRNA is a later event. LPS-induced MRP2 retrieval from the canalicular membrane is not associated with the retrieval of DPPIV, suggestive for selectivity of the process.

Hepatic sequestration and modulation of the canalicular transport of the organic cation, daunorubicin, in the Rat

In contrast to organic anions, substrates for the canalicular mdr1a and b are usually organic cations and are often sequestered in high concentrations in intracellular acidic compartments. Because many of these compounds are therapeutic agents, we investigated if their sequestration could be regulated. We used isolated perfused rat liver (IPRL), isolated rat hepatocyte couplets (IRHC), and WIF-B cells to study the cellular localization and biliary excretion of the fluorescent cation, daunorubicin (DNR). Despite rapid (within 15 minutes) and efficient (>90%) cellular uptake in the IPRL, only approximately 10% of the dose administered (0.2-20 micromol) was excreted in bile after 85 minutes. Confocal microscopy revealed fluorescence predominantly in vesicles in the pericanalicular region in IPRL, IRHC, and WIF-B cells. Treatment of these cells with chloroquine and bafilomycin A, agents that disrupt the pH gradient across the vesicular membrane, resulted in a loss of vesicular fluorescence, reversible in the case of bafilomycin A. Taurocholate (TC) and dibutyryl cAMP (DBcAMP), stimulators of transcytotic vesicular transport, increased the biliary recovery of DNR significantly above controls, by 70% and 35%, respectively. The microtubule destabilizer, nocodazole, decreased biliary excretion of DNR. No effect on secretion was noted in TR- mutant rats deficient in mrp2. Coadministration of verapamil, an inhibitor of mdr1, also decreased DNR excretion. While TC and DBcAMP did not affect the fluorescent intensity or pattern of distribution in IRHC, nocodazole resulted in redistribution of DNR to peripheral punctuate structures. These findings suggest that the organic cation, DNR, is largely sequestered in cells such as hepatocytes, yet its excretion can still be modulated.