Preferential localization of rat liver D-myo-inositol 1,4,5-trisphosphate/1,3,4,5-tetrakisphosphate 5-phosphatase in bile-canalicular plasma membrane and 'late' endosomal vesicles

Subcellular organization of calcium signalling in hepatocytes and the intact liver

Hepatocytes respond to inositol 1,4,5-trisphosphate (InsP3)-linked agonists with frequency-modulated oscillations in the intracellular free calcium concentration ([Ca2+]i), that occur as waves propagating from a specific origin within each cell. The subcellular distribution and functional organization of InsP3-sensitive Ca2+ pools has been investigated, in both intact and permeabilized cells, by fluorescence imaging of dyes which can be used to monitor luminal Ca2+ content and InsP3-activated ion permeability in a spatially resolved manner. The Ca2+ stores behave as a luminally continuous system distributed throughout the cytoplasm. The structure of the stores, an important determinant of their function, is controlled by the cytoskeleton and can be modulated in a guanine nucleotide-dependent manner. The nuclear matrix is devoid of Ca2+ stores, but Ca2+ waves in the intact cell propagate through this compartment. The organization of [Ca2+]i signals has also been investigated in the perfused liver. Frequency-modulated [Ca2+]i oscillations are still observed at the single cell level, with similar properties to those in the isolated hepatocyte. The [Ca2+]i oscillations propagate between cells in the intact liver, leading to the synchronization of [Ca2+]i signals across part or all of each hepatic lobule.

The versatility of inositol phosphates as cellular signals

Cells from across the phylogenetic spectrum contain a variety of inositol phosphates. Many different functions have been ascribed to this group of compounds. However, it is remarkable how frequently several of these different inositol phosphates have been linked to various aspects of signal transduction. Therefore, this review assesses the evidence that inositol phosphates have evolved into a versatile family of second messengers.

Comparison of the subcellular distribution of G-proteins in hepatocytes in situ and in primary cultures

The subcellular localization of the heterotrimeric G-proteins in hepatocytes in situ was compared to that in hepatocytes in primary culture. The ability of various ligands to activate adenylyl cyclase (AC) in membrane preparations was also investigated. In hepatocytes in situ the G proteins were mainly localized at the plasma membrane while in hepatocytes in culture they were predominantly cytoplasmic. The localization of the G-proteins in hepatocytes in situ correlates with their role in signal transduction. In homogenates prepared from the cultured cells, ligands which stimulate AC via Gs alpha were without effect, which was consistent with the localization of Gs alpha in the cytoplasmic and nuclear compartments. The "relocalization" of the G proteins to the cytoplasm when cells are cultured suggests that transmembrane signalling may be regulated by cell differentiation and cell-cell and cell-extracellular matrix interactions.

Post-translational modification of human brain type I inositol-1,4,5-trisphosphate 5-phosphatase by farnesylation

In brain, type I inositol-1,4,5-trisphosphate 5-phosphatase (InsP3 5-phosphatase) is the major isoenzyme hydrolyzing the calcium-mobilizing second messenger InsP3. Activity of this enzyme could be measured in both soluble and particulate fractions of tissue homogenates. The protein sequence showed a putative C-terminal isoprenylation site (CVVQ). In this study, two mutants have been generated. The first mutant (C409S) has a serine replacing a cysteine at position 409 of the wild-type enzyme. The second mutant (K407D1) is a deletion mutant that lacks the last five C-terminal amino acids. These constructs were individually expressed by transfection in COS-7 cells. Western blot analysis of wild-type transfected cells indicated that both soluble and particulate fractions had a 43-kDa immunoreactive band, with a higher proportion of the original homogenate associated with the particulate part. On the contrary, when the two mutated constructs were transfected in COS-7 cells, the phosphatase was predominantly soluble. Confocal immunofluorescence studies showed the wild-type enzyme to be present on the cell surface of transfected COS-7 cells and in subcellular compartments around the nucleus. This was not observed for the two mutants, where uniform immunofluorescence labeling was observed throughout the cytosol. Recombinant type I InsP3 5-phosphatase expressed in Escherichia coli was a substrate of purified farnesyltransferase. Altogether, the data therefore suggest a direct participation of Cys-409 in a C-terminally anchored InsP3 5-phosphatase by farnesylation.

Tissue distribution and intracellular localisation of the 75-kDa inositol polyphosphate 5-phosphatase

The 75-kDa inositol polyphosphate 5-phosphatase (75-kDa 5-phosphatase) hydrolyses several important mediators of intracellular calcium homeostasis, including inositol 1,4,5-trisphosphate [Ins(1,4,5)P3], inositol 1,3,4,5-tetrakisphosphate [Ins(1,3,4,5)P4] and phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P2]. Northern analysis of various human tissues revealed the 75-kDa 5-phosphatase has a ubiquitous expression, where differential splicing may occur in specific tissues. Prominent expression of a 4.4-kb transcript was noted in human lung, thymus, testes and placenta, and a 4.6-kb transcript was observed in heart, brain, kidney, ovary and colon. Determination of the intracellular location of the enzyme by indirect immunofluorescence, demonstrated that the 75-kDa 5-phosphatase was associated with mitochondrial and cytosolic cellular compartments. Immunoprecipitation of the total cell homogenate of human lung carcinoma cells (A549) with anti-(recombinant 75-kDa 5-phosphatase) antibodies revealed that the 75-kDa 5-phosphatase is the major PtdIns(4,5)P2 5-phosphatase in this cell line. Analysis of PtdIns(4,5)P2 5-phosphatase activity in subcellular fractions of A549 cells revealed peak 75-kDa 5-phosphatase enzyme activity in the cytosolic and mitochondrial enriched fractions. Immunoblot analysis further confirmed the mitochondrial location of the enzyme. This study demonstrates the tissue distribution and intracellular location of the 75-kDa 5-phosphatase and reveals a novel location for an enzyme involved in phosphatidylinositol turnover.

Functional identification of three major phosphoproteins in endocytic fractions from rat liver. A comparative in vivo and in vitro study

Liver plasma membranes originating from the sinusoidal, lateral and canalicular domains and 'early' and 'late' endosomes were prepared from rats injected with [32P]orthophosphate. The phosphorylated polypeptides in these subcellular fractions, resolved by gel electrophoresis, were analysed and compared with those obtained by in vitro phosphorylation of the fractions by endogenous protein kinases. The polypeptides phosphorylated in vitro were different in plasma membranes, endosomes and lysosomes. Three of the major phosphoproteins in the endocytic membranes were shown to be the polymeric immunoglobulin receptor, the beta subunit of the insulin receptor and the 550-kDa low-density-lipoprotein-receptor-related protein (LRP). An additional 35-kDa polypeptide of unknown function was a major phosphorylated component and thus emerges as a candidate marker protein of hepatic endosomes. Phosphoserine was shown to be the major amino acid phosphorylated in vitro in the phosphoproteins of endocytic membranes. The subcellular distribution in liver tissue of protein kinase activity was also investigated and activity shown to be recovered mainly in blood-sinusoidal and lateral plasma membranes; bile canalicular plasma membranes and endosomes contained low protein kinase activities. The results show that receptor phosphorylation is an 'early' event in endocytosis and the trafficking of ligands that is sustained especially in early endosomes in liver, and emphasizes the biochemical and thus functional distinctiveness of the plasma membrane and the endosomal and lysosomal compartments with regard to their population of phosphorylated proteins.

Bovine testis and human erythrocytes contain different subtypes of membrane-associated Ins(1,4,5)P3/Ins(1,3,4,5)P4 5-phosphomonoesterases

1. We have purified membrane-associated Ins(1,4,5)P3/Ins(1,3,4,5)P4 5-phosphatases from bovine testis and human erythrocytes by chromatography on several media, including a novel 2,3-bisphosphoglycerate affinity column. 2. The enzymes have apparent molecular masses of 42 kDa (testis) and 70 kDa (erythrocyte), as determined by SDS/PAGE, and affinities for Ins(1,4,5)P3 of 14 microM and 22 microM respectively. 3. The two enzymes hydrolyse both Ins(1,4,5)P3 and Ins(1,3,4,5)P4 and are therefore type I Ins(1,4,5)P3 5-phosphatases [nomenclature of Hansen, Johanson, Williamson and Williamson (1987) J. Biol. Chem. 262, 17319-17326]. 4. On chromatofocusing, the partially purified testicular enzyme migrates as two peaks of activity, with pI values of about 5.8 and 5.5. The erythrocyte enzyme exhibits only the latter peak. 5. The testis 5-phosphatase is labile at 37 degrees C, but its activity can be maintained in the presence of 50 mM phorbol dibutyrate (PdBu). After PdBu treatment, a third form of the enzyme, with pI about 6.2, appears on chromatofocusing, but without change in its Km or Vmax. 6. Consideration of the properties of these enzymes and of the 5-phosphatases from other tissues suggests that type I Ins(1,4,5)P3 5-phosphatases are of two well-defined subtypes. We propose that these be termed type Ia [typified by the testis enzyme: approximately 40 kDa, higher affinity for Ins(1,4,5)P3] and Type Ib [typified by the erythrocyte enzyme: approximately 70 kDa, lower affinity for Ins(1,4,5)P3].

Relationships between the degree of cross-linking of surface immunoglobulin and the associated inositol 1,4,5-trisphosphate and Ca2+ signals in human B cells

Cross-linking of surface immunoglobulin (Ig) receptors on human B cells leads to the activation of a tyrosine kinase. The activated tyrosine kinase subsequently phosphorylates a number of substrates, including phospholipase C-gamma. This enzyme breaks down phosphoinositol bisphosphate to form two intracellular messengers, diacylglycerol and inositol 1,4,5-trisphosphate, leading to the activation of protein kinase C and the release of intracellular Ca2+ respectively. We have used h.p.l.c. and flow cytometry to measure accurately the inositol phosphate turnover and Ca2+ release in anti-Ig-stimulated human B cells. In particular, we have examined the effect of dose of the cross-linking antibody on the two responses. The identity of putative messenger inositol phosphates has been verified by structural analysis, and the amounts of both inositol phosphates and Ca2+ present have been quantified. In the Ramos Burkitt lymphoma, which is very sensitive to stimulus through its Ig receptors, both inositol phosphate production and Ca2+ release were found to be related to the dose of anti-Ig antibody applied. This suggests that phospholipase C-mediated signal transduction in human B cells converts the degree of cross-linking of the immunoglobulin receptor quantitatively into intracellular signals.

Purification of bovine brain inositol-1,4,5-trisphosphate 5-phosphatase

In bovine brain, two soluble inositol-1,4,5-trisphosphate (InsP3) 5-phosphatases, which catalyse the dephosphorylation of InsP3 to inositol 1,4-bisphosphate, have been separated by DEAE-Sephacel. Type I, i.e. the first eluted enzyme, is the main soluble form and is reminiscent of the membrane-bound enzyme by multiple criteria. Type I was purified to apparent homogeneity by a method involving chromatography on DEAE-Sephacel, Blue-Sepharose, Sephacryl S-200, phosphocellulose, and C18 HPLC. A single protein band of 42-43 kDa was identified by SDS/PAGE, corresponding to the peak of maximal activity. InsP3 5-phosphatase was purified to apparent homogeneity to a final yield of 45-50 micrograms protein. The minimal estimate value of the Vmax for InsP3 5-phosphatase was in the range 20-35 mumol.min-1.mg protein-1.

Differential expression of asialoglycoprotein receptor subunits in the endocytic compartment during liver regeneration

Asialoglycoprotein receptors, responsible for the removal of circulating asialoglycoproteins by the liver, are located in at least two different membrane locations in hepatocytes. Receptors on the cell surface account only for a minor proportion (20-36%), for the majority of receptors in the liver are located intracellularly, mainly in the endocytic membrane networks. An understanding of the basis of receptor distribution and the underlying trafficking of receptors between the hepatocyte's polarised cell surface and the endocytic compartment would be aided if biochemical differences between the receptors in these pools were established. We now show, using three antibodies that recognise the receptor subunits in rat liver (RHL-1, RHL-2 and RHL-3), that the asialoglycoprotein receptors located in the plasma membrane domains and the endocytic compartment differ in oligomeric composition, sialic acid content, and solubility in Triton X-114 using two-phase systems. It is well established that the expression of the asialoglycoprotein receptor is down-regulated in livers regenerating after a partial hepatectomy. We demonstrate that the levels of the receptor subtype that is located mainly in the endocytic compartment (RHL-1, 42 kDa) was elevated in regenerating liver by agents that regulate cAMP production, whereas the levels of the other receptor subtypes remained unchanged. The asialoglycoprotein receptor subtypes that are present in different subcellular locations are thus regulated independently.

Multiple isomers of inositol pentakisphosphate in Epstein-Barr-virus- transformed (T5-1) B-lymphocytes. Identification of inositol 1,3,4,5,6-pentakisphosphate, D-inositol 1,2,4,5,6-pentakisphosphate and L-inositol 1,2,4,5,6-pentakisphosphate

Substantial amounts of three [3H]InsP5 isomers were detected in [3H]inositol-labelled human lymphoblastoid (T5-1) cells. Their structures were determined by h.p.l.c. [Phillippy & Bland (1988) Anal. Biochem. 175, 162-166], and by utilizing a stereospecific D-inositol 1,2,4,5,6-pentakisphosphate 3-kinase from Dictyostelium discoideum [Stephens & Irvine (1990) Nature (London) 346, 580-583]. The structures were: inositol 1,3,4,5,6-pentakisphosphate, D-inositol 1,2,4,5,6-pentakisphosphate and L-inositol 1,2,4,5,6-pentakisphosphate. The relative proportions of these isomers (approx. 73:14:14 respectively) were unaffected by cross-linking anti-IgD receptors. The T5-1 cells also contained InsP6 and three Ins P4s, which were identified as the 1,3,4,5, 1,3,4,6 and 3,4,5,6 isomers. In incubations with permeabilized T5-1 cells, both 1,3,4,6 and 3,4,5,6 isomers of InsP4 were phosphorylated solely to Ins(1,3,4,5,6)P5. Permeabilized cells also dephosphorylated InsP6, even in the presence of a large excess of glucose 6-phosphate to saturate non-specific phosphatases. In the latter experiments the following isomers of InsP5 accumulated: D- and/or L-Ins(1,2,3,4,5)P5, plus D- and/or L-Ins(1,2,4,5,6)P5. This demonstration that multiple isomers of InsP5 may be formed in vivo and in vitro by a transformed lymphocyte cell line adds a new level of complexity to the study of inositol polyphosphate metabolism and function.

Activation of Ca2+ entry into acinar cells by a non-phosphorylatable inositol trisphosphate

In many cell types, receptor activation of phosphoinositidase C results in an initial release of intracellular Ca2+ stores followed by sustained Ca2+ entry across the plasma membrane. Inositol 1,4,5-trisphosphate is the mediator of the initial Ca2+ release, although its role in the mechanism underlying Ca2+ entry remains controversial. We have now used two techniques to introduce inositol phosphates into mouse lacrimal acinar cells and measure their effects on Ca2+ entry: microinjection into cells loaded with Fura-2, a fluorescent dye which allows the measurement of intracellular free calcium concentration by microspectrofluorimetry, and perfusion of patch clamp pipettes in the whole-cell configuration while monitoring the activity of Ca(2+)-activated K+ channels as an indicator of intracellular Ca2+. We report here that inositol 1,4,5-trisphosphate serves as a signal that is both necessary and sufficient for receptor activation of Ca2+ entry across the plasma membrane in these cells.

Subcellular distribution of the calcium-storing inositol 1,4,5-trisphosphate-sensitive organelle in rat liver. Possible linkage to the plasma membrane through the actin microfilaments

The role of Ins(1,4,5)P3 in the mobilization of Ca2+ from intracellular stores of non-muscle cells has been extensively demonstrated; however, the nature of the organelle releasing the Ca2+ is still poorly understood. The distributions of the Ins(1,4,5)P3-binding sites and of the Ins(1,4,5)P3-sensitive Ca2+ pool were investigated in subcellular fractions obtained from rat liver and compared with those of other markers. The Ins(1,4,5)P3-binding vesicles appeared to be completely distinct from the endoplasmic-reticulum-derived microsomes and were enriched in the same fractions which were enriched in alkaline phosphodiesterase I activity. This co-purification of the plasma-membrane marker with the Ins(1,4,5)P3-binding sites was dramatically altered after freezing or after treatment of the homogenate with the microfilament-disruptive drug cytochalasin B, suggesting that the Ins(1,4,5)P3-sensitive organelle may be linked to the plasma membrane through the actin microfilaments. No correlation was observed between the Ins(1,4,5)P3-binding capacity and the portion of the Ca2+ pool that was released by Ins(1,4,5)P3. This may result from the disruption of the native organelle during homogenization, leading to the formation of vesicles containing the Ins(1,4,5)P3 receptor, but lacking the Ca2+ pump. These results are consistent with the idea of a specialized Ins(1,4,5)P3-regulated organelle distinct from the endoplasmic reticulum, and we propose a model of the structural organization of this organelle, in which the anchorage to the cytoskeleton as well as the spatial separation of the Ca2+ pump from the Ins(1,4,5)P3 receptor have important functional significance.

Regulation of the metabolism of 1,2-diacylglycerols and inositol phosphates that respond to receptor activation

This review assimilates information on the regulation of the metabolism of those inositol phosphates and diacylglycerols that respond to receptor activation. Particular emphasis is placed on the regulation of specific enzymes, the occurrence of isoenzymes, and metabolic compartmentalization; the overall aim is to demonstrate the significance of these activities in relation to the physiological impact of the various cell signalling processes.

A two-dimensional electrophoretic analysis of the proteins and glycoproteins of liver plasma membrane domains and endosomes. Implications for endocytosis and transcytosis

1. Polypeptides of liver plasma membrane fractions enriched in three surface domains of hepatocytes, blood-sinusoidal, lateral and bile canalicular, were analysed by isoelectric focusing (IEF) and non-equilibrium pH gel electrophoresis (NEPHGE) across a wide pH range, followed by SDS/PAGE. The overall Coomassie Blue-stained polypeptide patterns in the fractions were different. lateral plasma membrane fractions contained a characteristically higher number of polypeptides focusing at the basic pH range, whereas few basic polypeptides were present in sinusoidal plasma membrane fractions. The glycoproteins in these plasma membrane fractions stained by a lectin overlay technique with radio-iodinated concanavalin A, wheat-germ agglutinin and a slug lectin, were also different. 2. The polypeptides and glycoproteins of 'early' and 'late' endosome fractions were also compared by two-dimensional electrophoresis. Their composition was shown by Coomassie Blue staining, lectin overlay staining and in membranes metabolically labelled with [35S]methionine to be generally similar. The glycoproteins of sinusoidal plasma membranes and early and late endosomes were generally similar, but major differences in polypeptides of molecular mass 20-50 kDa, pI 7.5-8.5, in plasma membranes and endosomes were demonstrated, with a specific population of basic (pI 8-9) low-molecular-mass polypeptides being present at highest levels in 'late' endosomal fractions (shown by Coomassie Blue staining). 3. Analysis of the distribution of three specific membrane glycoproteins identified by using immunoblotting techniques showed that the asialoglycoprotein and the divalent-cation-sensitive mannose 6-phosphate receptors were present in sinusoidal plasma membrane and in early and late endocytic fractions: they were not detected in canalicular plasma membrane fractions. In contrast, 5'-nucleotidase was detected in all fractions examined. The role of the endocytic compartment in regulating trafficking pathways between the plasma membrane domains of the hepatocyte is discussed.

Highly purified bile-canalicular vesicles and lateral plasma membranes isolated from rat liver on Nycodenz gradients. Biochemical and immunolocalization studies

1. A liver canalicular plasma-membrane fraction enriched 115-155-fold in five marker enzymes relative to the tissue homogenate was obtained by sonication of liver plasma membranes followed by fractionation in iso-osmotic Nycodenz gradients. 2. Two lateral-plasma membrane fractions were also collected by this procedure; the lighter-density fraction was still associated with canalicular membranes, as assessed by enzymic and polypeptide analysis. 3. The polypeptide composition of the domain-defined plasma-membrane fractions was evaluated. It was demonstrated by immunoblotting that the 41 kDa alpha-subunit of the inhibitory G-protein, associated in high relative amounts with canalicular plasma-membrane fractions, was partially lost in the last stage of purification; however, this subunit was retained by lateral plasma membranes. 4. Antibodies to the proteins of bile-canalicular vesicles were shown to localize to the hepatocyte surface in thin liver sections examined by immunofluorescent and immuno-gold electron microscopy. Two subsets of antigens were identified, one present on both sinusoidal and canalicular plasma-membrane domains and another, by using antisera pre-absorbed with sinusoidal plasma membranes, that was confined to the bile-canalicular domain.

Priority targeting of glycosyl-phosphatidylinositol-anchored proteins to the bile-canalicular (apical) plasma membrane of hepatocytes. Involvement of 'late' endosomes

1. Liver plasma membranes originating from the sinusoidal, lateral and canalicular surface domains of hepatocytes were covalently labelled with sulpho-N-hydroxysuccinamide-biotin. After solubilization in Triton X-114, treatment with a phosphatidylinositol-specific phospholipase C (PI-PLC), two-phase partitioning and 125I-streptavidin labelling of the proteins resolved by PAGE, six major polypeptides (molecular masses 110, 85, 70, 55, 38 and 35 kDa) were shown to be anchored in bile canalicular membrane vesicles by a glycosyl-phosphatidylinositol (G-PI) 'tail'. 2. Permeabilized 'early' and 'late' endocytic vesicles isolated from liver were also examined. Two polypeptides (110 and 35 kDa) were shown to be anchored by a G-PI tail in 'late' endocytic vesicles. 3. Analysis of marker enzymes in bile-canalicular vesicles treated with PI-PLC showed that 5'-nucleotidase and alkaline phosphatase, but not leucine aminopeptidase and ecto-Ca2(+)-ATPase activities were released from the membrane. A low release and recovery of alkaline phosphodiesterase activity was noted. The cleavage from the membrane of 5'-nucleotidase as a 70 kDa polypeptide was confirmed by Western blotting using an antibody to this enzyme. 4. Antibodies raised to proteins released from bile-canalicular vesicles by PI-PLC treatment, and purified by partitioning in aqueous and Triton X-114 phases, localized to the bile canaliculi in thin liver sections. Antibodies to proteins not hydrolysed by this treatment stained by immunofluorescence the sinusoidal and canalicular surface regions of hepatocytes. 5. Antibodies generated to proteins cleaved by PI-PLC treatment of canalicular vesicles were shown to identify, by Western blotting, a major 110 kDa polypeptide in these vesicles. Two polypeptides (55 and 38 kDa) were detected in MDCK and HepG-2 cultured cells. 6. Since two of the six G-PI-anchored proteins targeted to the bile-canalicular plasma membrane were also detected in 'late' endocytic vesicles, the results suggest that a junction where exocytic and endocytic traffic routes meet occurs in a 'late' endocytic compartment.

Distribution of polypeptides binding guanosine 5'-[gamma-[35S]thio]triphosphate and anti-(ras protein) antibodies in liver subcellular fractions. Evidence for endosome-specific components

The subcellular distribution in rat liver of polypeptides binding guanosine 5'-[gamma-[35S]thio]triphosphate [( 35S]GTP[S]) and seven antibodies against ras oncoproteins was evaluated. Multiple low-Mr (21,000-28,000) GTP-binding proteins were detected, but their relative distribution among the membrane fractions varied. A more specific compartmentation of polypeptides which bind antibodies generated against ras proteins was evident, with an Mr-28,000 polypeptide and a probable Mr-56,000 dimer, identified by six of the antibodies tested, being confined mainly to endosomes. An Mr-23,000 polypeptide was detected by some of the antibodies in all of the membrane fractions, but especially in the plasma membranes.

Polarized subcellular distribution of the 1-, 4- and 5-phosphatase activities that metabolize inositol 1,4,5-trisphosphate in intestinal epithelial cells

In intestinal epithelial cells, Ins(1,4,5)P3 is metabolized both by an intracellular 5-phosphatase and by less specific extracellular phosphatases [Rubiera, Velasco, Michell, Lazo & Shears (1988) Biochem. J. 255, 131-137]. A total of 91% of intracellular Ins(1,4,5)P3 5-phosphatase was particulate, and was preferentially associated with plasma membranes rather than with other subcellular organelles. A soluble Ins(1,4,5)P3 3-kinase activity was also characterized, further supporting the idea that inositol phosphates are important in enterocyte function. We have studied the distribution of Ins(1,4,5)P3 phosphatase activities in basolateral and brush-border domains of the plasma membrane. Compared with homogenates, the extracellular phosphatases were 13-17-fold enriched in brush-border membranes, but only 2-fold enriched in basolateral membranes. The 1- and 4-phosphates of Ins(1,4,5)P3 were hydrolysed at equal rates by the extracellular phosphatases; these enzymes are proposed to have digestive functions. The intracellular particulate 5-phosphatase was 2-fold enriched in brush-border membranes and 13-fold enriched in basolateral membranes, at the same pole of the cell where Ins(1,4,5)P3 is believed to be generated. This is opposite to the polarized distribution of particulate 5-phosphatase in hepatocytes [Shears, Evans, Kirk & Michell (1988) Biochem. J. 256, 363-369]; these differences in subcellular distribution may be important in determining cell-specific metabolism of Ins(1,4,5)P3.

Rat liver contains a potent endogenous inhibitor of inositol 1,3,4,5-tetrakisphosphate 3-phosphatase

When Ins(1,3,4,5)P4 was incubated with a rat liver 100,000 g supernatant, about 93% of the substrate was metabolized by a 5-phosphatase, and only 7% by a 3-phosphatase. Ion-exchange chromatography of the supernatant specifically increased its 3-phosphatase activity 72 +/- 3-fold. This activated enzyme was inhibited by a heat-stable factor present in both the soluble and particulate portions of the cell.

Kinetic consequences of the inhibition by ATP of the metabolism of inositol (1,4,5) trisphosphate and inositol (1,3,4,5) tetrakisphosphate in liver. Different effects upon the 3- and 5-phosphatases

A kinetic analysis was undertaken of the inhibition by 5 mM MgATP of Ins(1,4,5)P3 5-phosphatase in 100,000 g particulate fractions prepared from liver homogenates. The Km for Ins(1,4,5)P3 was increased by 44% (from 16 to 23 microM). The competitive nature of the inhibition was confirmed with a Dixon plot. The effect of MgATP on 5-phosphatase was also studied at physiological concentrations of Ins(1,4,5)P3 and Ins(1,3,4,5)P4 (i.e. 1.5 microM); the rate of substrate hydrolysis was inhibited by over 30%. Ins(1,3,4,5)P4 was also hydrolysed by a 3-phosphatase, but this enzyme was unaffected by 5 mM MgATP. Thus, ATP, by differentially affecting Ins(1,3,4,5)P4 3- and 5-phosphatase, may increase the flux through the futile cycle that interconverts Ins(1,4,5)P3 and Ins(1,3,4,5)P4.

Cellular mechanisms of intrahepatic cholestasis

Most forms of intrahepatic cholestasis are caused by a failure of hepatocytes to secrete osmotically active bile constituents into the minute channels of bile canaliculi. This overall vectorial bile secretory process is dependent upon a variety of polarised active transport functions at the basolateral (sinusoidal and lateral) and canalicular plasma membrane domains, as well as upon the coordinated vectorial movement of intracellular vesicles. Although considerable progress has been made in recent years in the identification, characterisation and exact localisation of a number of polarised hepatocellular transport systems, the primary mechanisms and targets leading to defective bile secretion and cholestasis are still not completely understood. For example, not all reported experimental data are compatible with the concept that estrogen-induced cholestasis represents a predominant sinusoidal disease process. In addition, the pathophysiological significance of disturbed transcytotic pathways and/or disrupted intracellular calcium homeostasis are not yet clear. For many forms of cholestasis, it remains uncertain as to whether leaky tight junctions represent a primary cause rather than a secondary phenomenon of the cholestatic state. However, the ongoing progress in the understanding of the normal mechanisms involved in the establishment, maintenance and regulation of ion homeostasis and polar transport functions in hepatocytes will, undoubtedly, improve our knowledge of the pathogenesis of intrahepatic cholestasis and, it is hoped, lead to better therapeutic strategies in the near future.

G-proteins of rat liver membranes. Subcellular compartmentation and disposition in the plasma membrane

The distribution of the alpha- and beta-subunits of G-proteins and their disposition in rat liver plasma and intracellular membranes was investigated. Western blotting, using antibodies that recognised the alpha-subunit of the inhibitory and the beta-subunits of most G-proteins, identified 41 and 36 kDa polypeptides respectively in all plasma membrane functional domains, in endosomes as well as in Golgi membranes. Lysosomes were devoid of these subunits. The highest levels of G-protein subunits were found in bile canalicular plasma membranes prepared by density gradient centrifugation followed by free-flow electrophoresis. Separation of membrane proteins into extrinsic and intrinsic components was carried out by extraction of the membranes at pH 11.0 and by partitioning the membranes in Triton X-114/aqueous phases. The results demonstrated that the alpha- and beta-subunits were tightly associated with the hepatic membranes but they could be solubilised by extraction with detergent, e.g. SDS. Prolonged incubation in the presence of GTP analogues also released up to approximately 50% of the alpha-subunit of inhibitory G-proteins from membranes. The beta-subunit was still associated with membranes after alkaline extraction. The results emphasise the strong association of G-protein subunits with liver membranes, and show that these proteins are distributed widely in the plasma membrane and along the endocytic pathways of hepatocytes.

Distribution of G-proteins in rat liver plasma-membrane domains and endocytic pathways

1. The distribution of the alpha- and beta-subunits of nucleotide-binding G-proteins among rat liver sinusoidal, lateral and canalicular plasma membranes, endosomes, Golgi membranes and lysosomes was investigated. 2. Pertussis-toxin-catalysed ADP-ribosylation identified a 41 kDa inhibitory alpha-subunit in all liver plasma-membrane functional domains as well as in endosomes. An antibody to a synthetic peptide corresponding to a C-terminal sequence of the inhibitory alpha-subunit also identified the 41 kDa polypeptide in all plasma-membrane domains, in 'early' and 'late' endosomes and in Golgi membranes; this polypeptide was not detected in lysosomes. The antibody-binding studies showed that bile-canalicular plasma membranes had the highest content of the inhibitory alpha-subunit. 3. Immunofluorescent microscopy confirmed the presence of the inhibitory alpha-subunit in all regions of the hepatocyte's cell surface. 4. An antibody recognizing the beta-subunit showed that a 36 kDa polypeptide was present in all plasma membranes and in 'early' and 'late' endosomes; it was not detected in lysosomes. The relative distribution among the fractions of this polypeptide was similar to the distribution of the inhibitory alpha-subunit. 5. The presence of high levels of the G-protein inhibitory alpha-subunit in bile-canalicular plasma membranes was confirmed by demonstration of its co-fractionation with marker enzymes in Nycodenz gradients and by free-flow electrophoresis. The significance of this location is discussed.