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Dehydroepiandrosterone sulfate (DHEAS) is the major circulating steroid in man. Pharmacologically, it exerts marked neuropsychiatric effects. Since no target receptor has been identified, we investigated whether the organic anion transporting polypeptide (OATP), a multispecific steroid carrier, transports DHEAS. Expression of the human liver OATP in Xenopus laevis oocytes resulted in high-affinity, partially Na+-dependent uptake of [3H]DHEAS (Km: 6.6 micromol/l). DHEAS transport was inhibited by bromosulfophthalein, bile acids, sulfated estrogens and dexamethasone. Northern blot analysis showed widespread expression of OATP in human brain. These data identify OATP as the first known target protein of DHEAS in human liver and brain.

The organic anion transporting polypeptide (OATP) of the basolateral hepatocyte membrane mediates multispecific uptake of anionic and other amphipathic substrates from sinusoidal blood plasma. To investigate the mechanisms controlling OATP expression, the 5'-flanking region of the human OATP gene was isolated from a P1-derived artificial chromosome genomic clone. Sequence analysis of the OATP promoter showed a number of consensus binding sites for both ubiquitous and liver-enriched transcription factors. Transfection of HepG2 cells with a series of 5'-deleted promoter-luciferase constructs identified the minimal promoter region within 91 base pairs relative to the transcription initiation site. A putative silencer element was localized in the -662/-440 region. The minimal promoter was also active in Chang liver, Madin-Darby canine kidney, and Chinese hamster ovary cells, indicating that basal promoter function is independent of liver-specific regulatory mechanisms. In transfected HepG2 cells, taurocholate (100 micromol/L) stimulated and triiodothyronine (1 micromol/L) inhibited OATP promoter activity, whereas hydrocortisone, dexamethasone, beta-estradiol, estrone-3-sulfate, and testosterone had no significant effect. Reverse-transcription polymerase chain reaction analysis showed an increase in OATP messenger RNA in the livers of four patients with chronic cholestatic liver disease compared with three noncholestatic controls. The up-regulation of OATP expression by taurocholate could serve to enhance the sinusoidal efflux of toxic intracellular compounds during chronic cholestasis.

Background/aims: The organic anion transporting polypeptide (OATP) of human liver mediates the basolateral hepatocellular uptake of numerous cholephilic anions and steroidal compounds. The aim of this study was to clone the human OATP gene and to map its chromosomal localization by fluorescence in situ hybridization., Methods: A polymerase chain reaction-amplified fragment of the human OATP gene was used to isolate a genomic OATP clone from a P1-derived artificial chromosome human genomic library. Human metaphase chromosomes were hybridized with digoxigenin-labeled DNA from the genomic OATP clone and incubated in fluoresceinated antidigoxigenin antibodies for in situ detection of specific hybridization signals., Results: Sequence analysis revealed that the isolated P1-derived artificial chromosome clone contained a large portion of the human OATP gene. Fluorescence in situ hybridization of human chromosomes with the genomic OATP clone resulted in the specific labeling of the OATP gene on the short arm of chromosome 12 at band 12p12., Conclusions: Mapping of a genomic OATP clone to chromosome 12p12 represents a first step towards the molecular characterization of the human OATP gene. While no liver disease has so far been associated with cytogenetic abnormalities of the short arm of chromosome 12, the genomic OATP sequence provides the basis for studies on gene structure and on the tissue-specific regulation of OATP gene expression.

A phospholipid translocating protein from rat liver has been expressed in Xenopus laevis oocytes. Injection of oocytes with total rat liver messenger RNA (mRNA) resulted in the function expression of saturable uptake of the water soluble phophatidylcholine derivative L-alpha-dibutyroylglycero-3-phophatidylcholine (diC4PC), Kinetic studies revealed an apparent Km value of approximately 10 mmol/L, which is similar to the value previously obtained in isolated rat liver canalicular plasma membrane vesicles for an adenosine triphosphate (ATP)-independent phosphatidylcholine translocator. Size fractionation of total rat liver mRNA yielded an active mRNA species between 1.8 and 2.6 kb, that stimulated the expressed phophatidylcholine uptake activity approximately fivefold as compared with differently sized mRNA subfractions. This active mRNA size class is too small to code for the mdr2 P-glycoprotein, which has been suggested to function as an ATP-dependent canalicular phosphatidylcholine translocator. Hence, the data indicate that there are at least two separate polypeptides involved in phospholipid translocation from hepatocytes into bile.

Bile acids are taken up into human liver by Na+-dependent and Na+-independent transport mechanisms. In hepatocarcinogenesis, numerous liver-specific functions are lost and the uptake of organic anions is markedly reduced. We have investigated the molecular and functional derangements of bile acid transport in the human hepatoblastoma cell line HepG2. Uptake of [3H]-taurocholic acid was saturable and entirely Na+ independent, with the kinetic characteristics of the human liver organic anion transporting polypeptide (OATP). OATP, but not the Na+-dependent bile acid transporter (Na+-taurocholate-cotransporting polypeptide [NTCP]), was detectable by reverse-transcription polymerase chain reaction (RT-PCR) analysis of HepG2 RNA. The level of OATP expression in HepG2 cells was determined by Northern blot analysis and was found to be 40% in comparison with normal liver. Transfection of an OATP-derived phosphorothioate (PTO)-antisense oligonucleotide into HepG2 cells resulted in 77% inhibition of temperature-dependent bile acid uptake. Injection of HepG2 messenger RNA (mRNA) into Xenopus laevis oocytes significantly stimulated Na+-independent taurocholate uptake, indicating the expression of a bile acid transport protein. We conclude that bile acid uptake into human hepatoblastoma HepG2 cells is mediated by the multi-specific organic anion transporting polypeptide OATP. Therapeutic strategies employing bile acid-derived cytostatic agents for the treatment of hepatocellular carcinomas may therefore depend upon the expression of the Na+-independent bile acid transporter OATP in hepatic malignancies.

Background & Aims: Based on a recently cloned rat liver organic anion transporter, we attempted to clone the corresponding human liver organic anion transporting polypeptide., Methods: A human liver complementary DNA library was screened with a specific rat liver complementary DNA probe. The human liver transporter was cloned by homology with the rat protein and functionally characterized in Xenopus laevis oocytes., Results: The cloned human liver organic anion transporting polypeptide consists of 670 amino acids and shows a 67% amino acid identity with the corresponding rat liver protein. Injection of in vitro transcribed complementary RNA into frog oocytes resulted in the expression of sodium-independent uptake of [35S]bromosulfophthalein (Michaelis constant [Km], approximately 20 mumol/L), [3H]cholate (Km, approximately 93 mumol/L), [3H]taurocholate (Km, approximately 60 mumol/L), [14C]glycocholate, [3H]taurochenodeoxycholate, and [3H]tauroursodeoxycholate (Km, approximately 19 mumol/L). Northern blot analysis showed cross-reactivity with messenger RNA species from human liver, brain, lung, kidney, and testes. Polymerase chain reaction analysis of genomic DNA from a panel of human-rodent somatic cell hybrids mapped the cloned human organic anion transporter to chromosome 12., Conclusions: These studies show that the cloned human liver organic anion transporter is closely related to, but probably not identical to, the previously cloned rat liver transporter. Furthermore, its additional localization in a variety of extrahepatic tissues suggests that it plays a fundamental role in overall transepithelial organic anion transport of the human body.

Comparative studies between different patient groups have suggested that cholecystectomy enhances bacterial dehydroxylation of the primary bile acid cholic acid (CA) to the secondary bile acid deoxycholic acid (DCA). DCA may exert a cocarcinogenic effect on the colonic mucosa. In a short-term follow-up study on nine female patients we found no alterations of the CA or DCA pools after cholecystectomy. However, in the long term, cholecystectomy could promote changes of the intestinal bacterial flora and thereby lead to enhanced conversion of CA to DCA, causing an expansion of the DCA pool size and a reduction of the CA pool size. To test this hypothesis, pool sizes, fractional turnover rates (FTR), and synthesis or input rates of CA, chenodeoxycholic acid (CDCA) and DCA were determined in 12 female patients before and again 5 to 8 years after cholecystectomy. In the long term, pool size and synthesis rate of CA had not changed and DCA pool size had expanded by only 7.5% (not significant [NS]). DCA input increased by 32% (NS) but was balanced by an increase in FTR of 36%. Pool size (-17%) and synthesis rate (-5%) of CDCA were not significantly diminished. Overall, the sizes of the total bile acid pool (-6%, NS; 50 +/- 8 vs. 53 +/- 13 mumol/kg) and the pool fractions of CA (44.7 +/- 10.3% vs. 42.8 +/- 7.6%) and DCA (25.5 +/- 14.1% vs. 23.6 +/- 9.3%) remained similar. In conclusion, cholecystectomy causes no changes in bile acid pool composition and thus has no adverse effects on bile acid metabolism in the long term.

To characterize the transport functions of a recently cloned basolateral organic anion transporting polypeptide of rat hepatocytes we performed further kinetic transport and substrate cis-inhibition studies in organic anion-transporting polypeptide-cRNA injected Xenopus laevis oocytes. The studies demonstrate saturable Na(+)-independent sulfobromophthalein (Michaelis-Menten constant, 1.5 mumol/L) and taurocholate (Michaelis-Menten constant, 50 mumol/L) uptake by organic anion-transporting polypeptide. Sulfobromophthalein uptake was inhibited by the following organic anions: 0.01 mmol/L bilirubin (43%), 0.1 mmol/L indocyanine green (81%), 0.1 mmol/L 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS; 52%) and 1 mmol/L probenecid (74%). Competitive inhibition was shown for indocyanine green (inhibition constant about 1.3 mumol/L). Sulfobromophthalein and taurocholate uptakes were also inhibited by cholate, chenodeoxycholate, deoxycholate and ursodeoxycholate, as well as their glycine and taurine conjugates. Organic anion-transporting polypeptide also mediated uptake of glycocholate, tauroursodeoxycholate and taurochenodeoxycholate. No cis-inhibition of sulfobromophthalein uptake was seen in the presence of ATP, para-aminohippuric acid, bumetanide, digitoxin, reduced glutathione, leukotriene C4, nicotinic acid, ouabain, oxalate, rifampicin, succinate or sulfate. Furthermore, radioactively labeled para-aminohippuric acid, alpha-ketoglutarate and reduced glutathione were not taken up by organic anion-transporting polypeptide in cRNA-injected frog oocytes. These data confirm that organic anion-transporting polypeptide represents a novel hepatocellular organic anion uptake system that can mediate Na(+)-independent transport of monovalent (e.g., bile acids) and divalent (e.g., sulfobromophthalein and indocyanine green) cholephilic organic anions.(ABSTRACT TRUNCATED AT 250 WORDS)

The effects of two different plant sterols on intestinal cholesterol absorption were compared in normal volunteers by an intestinal perfusion study during a control period followed by high dose infusion of sitosterol or sitostanol (3.6 mumol/min), to which subjects were allocated in a randomized manner. Cholesterol absorption during the control period was similar in the two groups, averaging 0.88 +/- 0.48 mumol/min (32 +/- 11%) for group I (sitosterol) and 0.68 +/- 0.33 mumol/min (29 +/- 9%) for group II (sitostanol). The infusion of a high dose of sitosterol resulted in a significant reduction of cholesterol absorption to 0.47 mumol/min (16%). Following the same dose of sitostanol, cholesterol absorption diminished significantly to 0.15 +/- 0.11 mumol/min (5.1 +/- 2.9%). Overall cholesterol absorption declined during sitosterol infusion by almost 50%, whereas sitostanol infusion caused a reduction of cholesterol absorption by almost 85%. These findings of a more effective inhibition of cholesterol absorption by sitostanol might confirm the observation recorded by others that an increase in hydrophobicity of a plant sterol results in a higher affinity but lower capacity to mixed micells. This may cause an effective displacement of cholesterol from micellar binding and therefore diminished cholesterol absorption.