Keiji Hirata

Nuclear and cytosolic calcium are regulated independently

Nuclear calcium (Ca2+) regulates a number of important cellular processes, including gene transcription, growth, and apoptosis. However, it is unclear whether Ca2+ signaling is regulated differently in the nucleus and cytosol. To investigate this possibility, we examined subcellular mechanisms of Ca2+ release in the HepG2 liver cell line. The type II isoform of the inositol 1,4,5-trisphosphate (InsP3) receptor (InsP3R) was expressed to a similar extent in the endoplasmic reticulum and nucleus, whereas the type III InsP3R was concentrated in the endoplasmic reticulum, and the type I isoform was not expressed. Ca2+ signals induced by low InsP3 concentrations started earlier or were larger in the nucleus than in the cytosol, indicating higher sensitivity of nuclear Ca2+ stores for InsP3. Nuclear InsP3R channels were active at lower InsP3 concentrations than InsP3R from cytosol. Enriched expression of type II InsP3R in the nucleus results in greater sensitivity of the nucleus to InsP3, thus providing a mechanism for independent regulation of Ca2+-dependent processes in this cellular compartment.

Loss of Inositol 1,4,5-Trisphosphate Receptors From Bile Duct Epithelia Is a Common Event in Cholestasis

BACKGROUND AND AIMSnCholestasis is one of the principal manifestations of liver disease and often results from disorders involving bile duct epithelia rather than hepatocytes. A range of disorders affects biliary epithelia, and no unifying pathophysiologic event in these cells has been identified as the cause of cholestasis. Here we examined the role of the inositol 1,4,5-trisphosphate receptor (InsP3R)/Ca(2+) release channel in Ca(2+) signaling and ductular secretion in animal models of cholestasis and in patients with cholestatic disorders.nnnMETHODSnThe expression and distribution of the InsP3R and related proteins were examined in rat cholangiocytes before and after bile duct ligation or treatment with endotoxin. Ca(2+) signaling was examined in isolated bile ducts from these animals, whereas ductular bicarbonate secretion was examined in isolated perfused livers. Confocal immunofluorescence was used to examine cholangiocyte InsP3R expression in human liver biopsy specimens.nnnRESULTSnExpression of the InsP3R was selectively lost from biliary epithelia after bile duct ligation or endotoxin treatment. As a result, Ca(2+) signaling and Ca(2+)-mediated bicarbonate secretion were lost as well, although other components of the Ca(2+) signaling pathway and adenosine 3,5-cyclic monophosphate (cAMP)-mediated bicarbonate secretion both were preserved. Examination of human liver biopsy specimens showed that InsP3Rs also were lost from bile duct epithelia in a range of human cholestatic disorders, although InsP3R expression was intact in noncholestatic liver disease.nnnCONCLUSIONSnInsP3-mediated Ca(2+) signaling in bile duct epithelia appears to be important for normal bile secretion in the liver, and loss of InsP3Rs may be a final common pathway for cholestasis.