Hormonal regulation of expression of ileal bile acid binding protein in suckling rats

P2Y2 purinergic receptor activation is essential for efficient hepatocyte proliferation in response to partial hepatectomy

Extracellular nucleotides via activation of P2 purinergic receptors influence hepatocyte proliferation and liver regeneration in response to 70% partial hepatectomy (PH). Adult hepatocytes express multiple P2Y (G protein-coupled) and P2X (ligand-gated ion channels) purinergic receptor subtypes. However, the identity of key receptor subtype(s) important for efficient hepatocyte proliferation in regenerating livers remains unknown. To evaluate the impact of P2Y2 purinergic receptor-mediated signaling on hepatocyte proliferation in regenerating livers, wild-type (WT) and P2Y2 purinergic receptor knockout (P2Y2-/-) mice were subjected to 70% PH. Liver tissues were analyzed for activation of early events critical for hepatocyte priming and subsequent cell cycle progression. Our findings suggest that early activation of p42/44 ERK MAPK (5 min), early growth response-1 (Egr-1) and activator protein-1 (AP-1) DNA-binding activity (30 min), and subsequent hepatocyte proliferation (24-72 h) in response to 70% PH were impaired in P2Y2-/- mice. Interestingly, early induction of cytokines (TNF-α, IL-6) and cytokine-mediated signaling (NF-κB, STAT-3) were intact in P2Y2-/- remnant livers, uncovering the importance of cytokine-independent and nucleotide-dependent early priming events critical for subsequent hepatocyte proliferation in regenerating livers. Hepatocytes isolated from the WT and P2Y2-/- mice were treated with ATP or ATPγS for 5-120 min and 12-24 h. Extracellular ATP alone, via activation of P2Y2 purinergic receptors, was sufficient to induce ERK phosphorylation, Egr-1 protein expression, and key cyclins and cell cycle progression of hepatocytes in vitro. Collectively, these findings highlight the functional significance of P2Y2 purinergic receptor activation for efficient hepatocyte priming and proliferation in response to PH.

Physiological level of norepinephrine increases adenine nucleotides hydrolysis in rat blood serum

Extracellular adenosine 5'-triphosphate (ATP) and its breakdown products, adenosine 5'-diphosphate (ADP) and adenosine, have significant effects on a variety of biological processes. NTPDase enzymes, responsible for adenine nucleotides hydrolysis, are considered the major regulators of purinergic signaling in the blood. Previous work by our group demonstrated that ATP and ADP hydrolysis in rat blood serum are higher during the dark (activity) phase compared to the light (rest) phase. In nocturnal animals (e.g., rats), important physiological changes occur during the dark phase, such as increased circulating levels of melatonin, corticosterone, and norepinephrine (NE). This study investigated the physiological effects, in vivo and in vitro, of melatonin, dexamethasone, and NE upon nucleotides hydrolysis in rat blood serum. For in vivo experiments, the animals received a single injection of saline (control), melatonin (0.05 mg/kg), dexamethasone (0.1 mg/kg), or NE (0.03 mg/kg). For in vitro experiments, melatonin (1.0 nM), dexamethasone (1.0 μM), or NE (1.0 nM) was added directly to the reaction medium with blood serum before starting the enzyme assay. The results demonstrated that ATP and ADP hydrolysis in both in vitro and in vivo experiments were significantly higher with NE treatment compared to control (in vitro: ATP = 36.63%, ADP = 22.43%, P < 0.05; in vivo: ATP = 44.1%, ADP = 37.28%, P < 0.001). No significant differences in adenine nucleotides hydrolysis were observed with melatonin and dexamethasone treatments. This study suggests a modulatory role of NE in the nucleotidases pathway, decreasing extracellular ATP and ADP, and suggests that NE might modulate its own release by increasing the activities of soluble nucleotidases.

Mild hypothermia does not affect liver regeneration after partial hepatectomy in mice

BACKGROUND

The use of mild hypothermia has been suggested to be therapeutically useful in treating acute liver failure. It is not known if hypothermia influences liver regeneration.

AIM

To assess the effect of hypothermia on liver regeneration in mice.

METHODS

After partial (70%) hepatectomy (PHx), C57BL6/J mice were randomly assigned to either a hypothermic group or a normothermic group. Controlled mild hypothermia was maintained for up to 3 h after surgery. In addition, assessment of liver mass restitution was examined by studying the induction of key cell cycle proteins (cyclin A, D1 and E) and hepatocyte proliferation [assessment of proliferating cell nuclear antigen (PCNA) protein expression] by Western blotting and DNA synthesis by measuring 5-bromo-2-deoxyuridine (BrdU) incorporation by immunohistochemical techniques 45 h after PHx.

RESULTS

Partial hepatectomy induced a vigorous proliferative response in the remnant livers of both groups of mice (normothermic and hypothermic groups), as evidenced by the induction of key cyclins, PCNA and incorporation of BrdU after PHx. The liver/body weight ratio and both cyclin and PCNA protein expression as well as BrdU incorporation did not differ between the regenerating livers of hypothermic and normothermic groups.

CONCLUSION

Mild hypothermia does not influence liver regeneration in mice.

Does abnormal bile acid metabolism contribute to NEC?

Bile acids (BAs) facilitate emulsification, absorption, and transport of fats and sterols in the intestine and liver and are essential for normal digestion. However, accumulation of BAs in the intestine can result in damage to the intestinal epithelium. Using the neonatal rat model of necrotizing enterocolitis (NEC), we have recently shown that BAs accumulate in both the ileal lumen and enterocytes of neonatal rats with NEC and the increased BA levels are positively correlated with disease severity. Importantly, when BAs are not allowed to accumulate, neonatal rat pups develop significantly less disease. In addition, BA transporters are altered during disease development. These data indicate that BAs play an important role in the development of experimental NEC, and suggest that the inability of neonatal rats to adequately regulate BA transporters may be a mechanism by which ileal damage occurs.

Role of protein kinase C-delta in the age-dependent secretagogue action of bile acids in mammalian colon

The role of specific PKC isoforms in the regulation of epithelial Cl(-) secretion by Ca(2+)-dependent secretagogues remains controversial. In the developing rabbit distal colon, the bile acid taurodeoxycholate (TDC) acts via intracellular calcium to stimulate Cl(-) transport in adult, but not in young, animals, whereas the PKC activator phorbol dibutyrate (PDB) stimulates Cl(-) transport at all ages. We tested the hypothesis that specific PKC isoforms account for the age-specific effects of TDC. The effects of conventional (cPKC) and novel (nPKC) PKC-specific inhibitors on TDC- and PDB-stimulated Cl(-) transport in adult and weanling colonocytes were assessed by using 6-methoxy-quinolyl acetoethyl ester. In adult colonocytes, the cPKC inhibitor Gö-6976 inhibited PDB action but not TDC action, whereas the cPKC and nPKC inhibitor Gö-6850 blocked both TDC and PDB actions. Additionally, rottlerin and the PKC-delta-specific inhibitor peptide (deltaV1-1) inhibited TDC- and PDB-stimulated Cl(-) transport in adult colonocytes. Rottlerin also decreased TDC-stimulated short-circuit current in intact colonic epithelia. Only Gö-6976, but neither rottlerin nor deltaV1-1, inhibited PDB-stimulated transport in weanling colonocytes. Colonic lysates express PKC-alpha, -lambda, and -iota protein equally at all ages, but they do not express PKC-gamma or -theta at any age. Expression of PKC-beta and PKC-epsilon protein was newborn>adult>weanling, whereas PKC-delta was expressed in adult but not in weanling or newborn colonocytes. TDC (1.6-fold) and PDB (2.0-fold) stimulated PKC-delta enzymatic activity in adult colonocytes but failed to do so in weanling colonocytes. PKC-delta mRNA expression showed age dependence. Thus PKC-delta appears critical for the action of TDC in the adult colon, and its low expression in young animals may account for their inability to secrete in response to bile acids.

Weanling, but not adult, rabbit colon absorbs bile acids: flux is linked to expression of putative bile acid transporters

Intestinal handling of bile acids is age dependent; adult, but not newborn, ileum absorbs bile acids, and adult, but not weanling or newborn, distal colon secretes Cl(-) in response to bile acids. Bile acid transport involving the apical Na(+)-dependent bile acid transporter (Asbt) and lipid-binding protein (LBP) is well characterized in the ileum, but little is known about colonic bile acid transport. We investigated colonic bile acid transport and the nature of the underlying transporters and receptors. Colon from adult, weanling, and newborn rabbits was screened by semiquantitative RT-PCR for Asbt, its truncated variant t-Asbt, LBP, multidrug resistance-associated protein 3, organic solute transporter-alpha, and farnesoid X receptor. Asbt and LBP showed maximal expression in weanling and significantly less expression in adult and newborn rabbits. The ileum, but not the colon, expressed t-Asbt. Asbt, LBP, and farnesoid X receptor mRNA expression in weanling colon parallel the profile in adult ileum, a tissue designed for high bile acid absorption. To examine their functional role, transepithelial [(3)H]taurocholate transport was measured in weanling and adult colon and ileum. Under short-circuit conditions, weanling colon and ileum and adult ileum showed net bile acid absorption: 1.23 +/- 0.62, 5.53 +/- 1.20, and 11.41 +/- 3.45 nmol x cm(-2) x h(-1), respectively. However, adult colon secreted bile acids (-1.39 +/- 0.47 nmol x cm(-2) x h(-1)). We demonstrate for the first time that weanling, but not adult, distal colon shows net bile acid absorption. Thus increased expression of Asbt and LBP in weanling colon, which is associated with parallel increases in taurocholate absorption, has relevance in enterohepatic conservation of bile acids when ileal bile acid recycling is not fully developed.

Fic1 is expressed at apical membranes of different epithelial cells in the digestive tract and is induced in the small intestine during postnatal development of mice

Mutations in ATP8B1 are associated with FIC1 disease, an autosomal recessive disorder in which intrahepatic cholestasis is the predominant manifestation. ATP8B1 encodes FIC1, which is expressed in several tissues, most prominently in the intestine, pancreas, and stomach and, to a much lesser extent, in the liver. In this study, Fic1 localization and expression during postnatal development was examined in healthy mice. Immunoblot and RT-PCR analysis indicated Fic1 is expressed abundantly in regions of the adult gastrointestinal tract of humans and mice. Immunohistochemistry revealed that Fic1 was localized to the apical membranes of enterocytes, pancreatic acinar cells, gastric pit epithelial cells, and hepatocytes and cholangiocytes. Subsequent analysis of early postnatal expression revealed that Fic1 expression in the small intestine was limited or absent at the age of 7 and 14 d and increased significantly with maturation. In contrast, pancreatic, hepatic, and gastric Fic1 expression was not diminished during the first 3 wk of postnatal development. In conclusion, these data show that Fic1 is expressed in a tissue-specific and developmentally regulated fashion at the apical membranes of epithelial cells. We speculate that the developing bile salt pool in the maturing intestine accounts for the increase in Fic1 protein expression in this tissue.

Bile salt transporters: molecular characterization, function, and regulation

Molecular medicine has led to rapid advances in the characterization of hepatobiliary transport systems that determine the uptake and excretion of bile salts and other biliary constituents in the liver and extrahepatic tissues. The bile salt pool undergoes an enterohepatic circulation that is regulated by distinct bile salt transport proteins, including the canalicular bile salt export pump BSEP (ABCB11), the ileal Na(+)-dependent bile salt transporter ISBT (SLC10A2), and the hepatic sinusoidal Na(+)- taurocholate cotransporting polypeptide NTCP (SLC10A1). Other bile salt transporters include the organic anion transporting polypeptides OATPs (SLC21A) and the multidrug resistance-associated proteins 2 and 3 MRP2,3 (ABCC2,3). Bile salt transporters are also present in cholangiocytes, the renal proximal tubule, and the placenta. Expression of these transport proteins is regulated by both transcriptional and posttranscriptional events, with the former involving nuclear hormone receptors where bile salts function as specific ligands. During bile secretory failure (cholestasis), bile salt transport proteins undergo adaptive responses that serve to protect the liver from bile salt retention and which facilitate extrahepatic routes of bile salt excretion. This review is a comprehensive summary of current knowledge of the molecular characterization, function, and regulation of bile salt transporters in normal physiology and in cholestatic liver disease and liver regeneration.

Kinetics of mRNA expression of alkaline phosphatase isoenzymes in hepatic tissues from glucocorticoid-treated dogs

OBJECTIVE

To clone segments of the canine liver alkaline phosphatase (LALP) and corticosteroid-induced alkaline phosphatase (CIALP) genes and use those clones to determine the tissue source of CIALP, the kinetics of LALP and CIALP mRNA expression for glucocorticoid-treated dogs, and the correlation between LALP and CIALP transcript concentrations and isoenzyme activities.

SAMPLE POPULATION

Tissues obtained from 7 dogs treated with prednisone (1 mg/kg, SC, q 24 h) for up to 32 days and 1 untreated (control) dog.

PROCEDURE

Gene segments of LALP and CIALP were obtained by reverse transcription-polymerase chain reaction (RT-PCR) assay. The tissue source of CIALP and IALP mRNA was determined by northern blot analysis of tissues from 1 of the glucocorticoid-treated dogs. Hepatic tissues and serum samples were obtained from the 6 remaining glucocorticoid-treated dogs on days 0, 2, 5, 10, and 32 of prednisone treatment, and relative expression of LALP and CIALP mRNA was correlated with LALP and CIALP activity.

RESULTS

A 2,246-base pair (bp) segment of canine LALP and a 1,338-bp segment of CIALP were cloned. Northern blot analysis revealed CIALP mRNA expression in hepatic tissues only after glucocorticoid treatment. Kinetics of LALP and CIALP mRNA expression in the liver of glucocorticoid-treated dogs paralleled liver and serum activities of LALP and CIALP.

CONCLUSIONS AND CLINICAL RELEVANCE

The liver is the most likely source for CIALP in dogs. Analysis of kinetics of serum and hepatic LALP and CIALP mRNA suggests that after glucocorticoid treatment, both are regulated by modification of mRNA transcript concentrations, possibly through differing mechanisms.

Bile salt transporters

Bile salts are the major organic solutes in bile and undergo extensive enterohepatic circulation. Hepatocellular bile salt uptake is mediated predominantly by the Na(+)-taurocholate cotransport proteins Ntcp (rodents) and NTCP (humans) and by the Na(+)-independent organic anion-transporting polypeptides Oatp1, Oatp2, and Oatp4 (rodents) and OATP-C (humans). After diffusion (bound by intracellular bile salt-binding proteins) to the canalicular membrane, monoanionic bile salts are secreted into bile canaliculi by the bile salt export pump Bsep (rodents) or BSEP (humans). Both belong to the ATP-binding cassette (ABC) transporter superfamily. Dianionic conjugated bile salts are secreted into bile by the multidrug-resistance-associated proteins Mrp2/MRP2. In bile ductules, a minor portion of protonated bile acids and monomeric bile salts are reabsorbed by non-ionic diffusion and the apical sodium-dependent bile salt transporter Asbt/ASBT, transported back into the periductular capillary plexus by Mrp3/MRP3 [and/or a truncated form of Asbt (tAsbt)], and subjected to cholehepatic shunting. The major portion of biliary bile salts is aggregated into mixed micelles and transported into the intestine, where they are reabsorbed by apical Oatp3, the apical sodium-dependent bile salt transporter (ASBT), cytosolic intestinal bile acid-binding protein (IBABP), and basolateral Mrp3/MRP3 and tAsbt. Transcriptional and posttranscriptional regulation of these enterohepatic bile salt transporters is closely related to the regulation of lipid and cholesterol homeostasis. Furthermore, defective expression and function of bile salt transporters have been recognized as important causes for various cholestatic liver diseases.

Region-specific ontogeny of alpha-2,6-sialyltransferase during normal and cortisone-induced maturation in mouse intestine

Regional differences in the ontogeny of mouse intestinal alpha-2,6-sialyltransferase activities (alpha-2,6-ST) and the influence of cortisone acetate (CA) on this expression were determined. High ST activity and alpha-2,6-ST mRNA levels were detected in immature small and large intestine, with activity increasing distally from the duodenum. As the mice matured, ST activity (predominantly alpha-2,6-ST) in the small intestine decreased rapidly to adult levels by the fourth postnatal week. CA precociously accelerated this region-specific ontogenic decline. A similar decline of ST mRNA levels reflected ST activity in the small, but not the large, intestine. Small intestinal sialyl alpha-2,6-linked glycoconjugates displayed similar developmental and CA induced-precocious declines when probed using Sambucus nigra agglutinin (SNA) lectin. SNA labeling demonstrated age-dependent diminished sialyl alpha2,6 glycoconjugate expression in goblet cells in the small (but not large) intestine, but no such regional specificity was apparent in microvillus membrane. This suggests differential regulation of sialyl alpha-2,6 glycoconjugates in absorptive vs. globlet cells. These age-dependent and region-specific differences in sialyl alpha-2,6 glycoconjugates may be mediated in part by altered alpha-2,6-ST gene expression regulated by trophic factors such as glucocorticoids.