G proteins in rat liver proliferation during cholestasis

After damage of large bile ducts by gamma-aminobutyric acid, small ducts replenish the biliary tree by amplification of calcium-dependent signaling and de novo acquisition of large cholangiocyte phenotypes

Large cholangiocytes secrete bicarbonate in response to secretin and proliferate after bile duct ligation by activation of cyclic adenosine 3', 5'-monophosphate signaling. The Ca(2+)-dependent adenylyl cyclase 8 (AC8, expressed by large cholangiocytes) regulates secretin-induced choleresis. Ca(2+)-dependent protein kinase C (PKC) regulates small cholangiocyte function. Because gamma-aminobutyric acid (GABA) affects cell functions by activation of both Ca(2+) signaling and inhibition of AC, we sought to develop an in vivo model characterized by large cholangiocyte damage and proliferation of small ducts. Bile duct ligation rats were treated with GABA for one week, and we evaluated: GABA(A), GABA(B), and GABA(C) receptor expression; intrahepatic bile duct mass (IBDM) and the percentage of apoptotic cholangiocytes; secretin-stimulated choleresis; and extracellular signal-regulated kinase1/2 (ERK1/2) phosphorylation and activation of Ca(2+-)dependent PKC isoforms and AC8 expression. We found that both small and large cholangiocytes expressed GABA receptors. GABA: (i) induced apoptosis of large cholangiocytes and reduced large IBDM; (ii) decreased secretin-stimulated choleresis; and (iii) reduced ERK1/2 phosphorylation and AC8 expression in large cholangiocytes. Small cholangiocytes: (i) proliferated leading to increased IBDM; (ii) displayed activation of PKCbetaII; and (iii) de novo expressed secretin receptor, cystic fibrosis transmembrane regulator, Cl(-)/HCO(3)(-) anion exchanger 2 and AC8, and responded to secretin. Therefore, in pathologies of large ducts, small ducts replenish the biliary epithelium by amplification of Ca(2+)-dependent signaling and acquisition of large cholangiocyte phenotypes.

Taurocholate feeding prevents CCl4-induced damage of large cholangiocytes through PI3-kinase-dependent mechanism

Bile acids are cytoprotective in hepatocytes by activating phosphatidylinositol-3-kinase (PI3-K) and its downstream signal AKT. Our aim was to determine whether feeding taurocholate to CCl(4)-treated rats reduces cholangiocyte apoptosis and whether this cytoprotective effect is dependent on PI3-K. Cholangiocyte proliferation, secretion, and apoptosis were determined in cholangiocytes from bile duct ligation (BDL), CCl(4)-treated BDL rats, and CCl(4)-treated taurocholate-fed rats. In vitro, we tested whether CCl(4) induces apoptosis and whether loss of cholangiocyte proliferation and secretion is dependent on PI3-K. The CCl(4)-induced cholangiocyte apoptosis and loss of cholangiocyte proliferation and secretion were reduced in CCl(4)-treated rats fed taurocholate. CCl(4)-induced cholangiocyte apoptosis, loss of cholangiocytes secretion, and proliferation were prevented by preincubation with taurocholate. Taurocholate cytoprotective effects were ablated by wortmannin. Taurocholate prevented, in vitro, CCl(4)-induced decrease of phosphorylated AKT protein expression in cholangiocytes. The cytoprotective effects of taurocholate on CCl(4) effects on cholangiocyte proliferation and secretion were abolished by wortmannin. Taurocholate protects cholangiocytes from CCl(4)-induced apoptosis by a PI3-K-dependent mechanism. Bile acids are important in the prevention of drug-induced ductopenia in cholangiopathies.

Regulation of cholangiocyte proliferation

Intrahepatic bile duct epithelial cells (i.e., cholangiocytes) are the target cells of chronic cholestatic liver diseases (i.e., cholangiopathies), which makes these cells of great interest to clinical hepatologists. This review will focus on "typical" cholangiocyte proliferation, whereas "atypical" (extension of cholangiocyte proliferation into parenchyma), and premalignant "oval" cell proliferation are reviewed elsewhere. The bile duct ligated (BDL) rat model, where most of the known mechanisms of cholangiocyte proliferation have been illustrated, was the first and remains the prototype animal model for "typical" cholangiocyte proliferation. Following a short overview of cholangiocyte functions, we briefly discuss the: (i) in vivo models [i.e., BDL (Fig. 1 and 4), chronic alpha-naphthylisothiocyanate (ANIT) or bile acid feeding (Fig. 2), acute carbon tetrachloride (CCl4) feeding and partial hepatectomy; and (ii) in vitro experimental tools [e.g., purified cholangiocytes and isolated intrahepatic bile duct units (IBDU)] that are key to the understanding of the mechanisms of "typical" cholangiocyte growth. In the second part of the review, we discuss a number of potential factors or conditions [e.g., gastrointestinal hormones, nerves, estrogens, blood supply, and growth factors] as well as the intracellular mechanisms [e.g., adenosine 3',5'-monophosphate (cAMP), and protein kinase C (PKC)] that may regulate "typical" cholangiocyte hyperplasia.

Lack of effect of incretin hormones on insulin release from pancreatic islets in the bile duct-ligated rats

Hyperglycemia associated with obstructive jaundice seriously affects the prognosis of patients with hepatobiliary diseases. We investigated secretory properties of isolated islets from bile duct-ligated (BDL) rats. Pancreatic islets from BDL rats lost their secretory responses to glucagon-like peptide-1 (GLP-1), although their responses to glucose were normal. Loss of potentiation of insulin release was also observed in glucagon and glucose-dependent insulinotropic peptide (GIP), whereas modulation of the release by forskolin, dibutyryl cAMP, or epinephrine remained unaffected. cAMP production by BDL islets was not increased by these insulinotropic hormones. Serum levels of glucagon, but not GIP, were increased in BDL rats. GLP-1 levels were also elevated, although they did not reach statistical significance. Immunoblotting of trimeric G protein subunits demonstrated that G(s)alpha L and G(s)alpha S, but not G(i)alpha 1/2 and G(i)alpha 3/o alpha, were less expressed in BDL islets. Therefore, unresponsiveness of the beta-cell to cAMP-raising hormones is involved in glucose intolerance under cholestasis. It results from diminished expression of alpha-subunits of the relevant G protein, G(s), and desensitization of receptors of these hormones.

Changes in G protein expression account for impaired modulation of hepatic cAMP formation after BDL

The regulation of cAMP synthesis by hormones and bile acids is altered in isolated hamster hepatocytes 2 days after bile duct ligation (BDL) [Y. Matsuzaki, B. Bouscarel, M. Le, S. Ceryak, T. W. Gettys, J. Shoda, and H. Fromm. Am. J. Physiol. 273 (Gastrointest. Liver Physiol. 36): G164-G174, 1997]. Therefore, studies were undertaken to elucidate the mechanism(s) responsible for this impaired modulation of cAMP formation. Hepatocytes were isolated 48 h after either a sham operation or BDL. Both preparations were equally devoid of cholangiocyte contamination. Although the basal cAMP level was not affected after BDL, the ability of glucagon to maximally stimulate cAMP synthesis was decreased by approximately 40%. This decreased glucagon effect after BDL was not due to alteration of the total glucagon receptor expression. However, this effect was associated with a parallel 50% decreased expression of the small stimulatory G protein alpha-subunit (GsalphaS). The expression of either the large subunit (GsalphaL) or the common beta-subunit remained unchanged. The expression of Gialpha2 and Gialpha3 was also decreased by 25 and 46%, respectively, and was associated with the failure of ANG II to inhibit stimulated cAMP formation. Therefore, alterations of the expression of GsalphaS and Galphai are, at least in part, responsible for the attenuated hormonal regulation of cAMP synthesis. Because cAMP has been reported to stimulate both bile acid uptake and secretion, impairment of cAMP synthesis and bile acid uptake may represent an initial hepatocellular defense mechanism during cholestasis.

Cholera and pertussis toxins increase acidification of endocytic vesicles without altering ion conductances

Acidification of endocytic vesicles, driven by the vacuolar H+ pump, is affected by parallel ion transporters. Because adenosine 3',5'-cyclic monophosphate (cAMP) and heterotrimeric G proteins may alter ion transporters, I tested whether cholera and pertussis toxins affected acidification of rat liver endosomes. Fluorescein-labeled dextran-loaded "10-min" endosomes from cholera toxin-treated rats exhibited ATP-dependent rates of acidification in the presence and absence of Cl- or K+ that were approximately 60-120% (P < 0.05) faster than rates from control endosomes. This increase was greater for "older" "20-min" endosomes and less for 'early" "2-min" endosomes. Ion transport functions of 10-min and 20-min toxin-exposed endosomes were similar to those of 2-min control endosomes. Cholera toxin also increased ATP-dependent steady-state intravesicular H+ concentration by 38-218% (P < 0.05). Pertussis toxin increased endosome acidification rates by 20-54% (P < 0.05). Both toxins increased liver cAMP content, and endosomes prepared from perfused livers exposed to 0.75 mM dibutyryl cAMP exhibited similar increases in acidification rates. These studies indicate that both cholera and pertussis toxins markedly alter the function of rat liver endosomes. The mechanism is unlikely to reflect major changes in vesicle ion transporters but rather may indicate either an increase in the number of H+ pumps per endosome and/or changes in fusion, remodeling, and maturation of early endocytic vesicles in response to cAMP.