Cholesteryl ester and triglyceride hydrolysis by an acid lipase from rabbit aorta.

The properties of the triglyceride- and cholesteryl ester-hydrolyzing activity by an acid lipase from rabbit aortic tissue were compared under different experimental conditions. Radiolabeled cholesteryl oleate or triolein was incorporated into phospholipid vesicles by sonication and the resulting preparations were used for in vitro studies. No distinction was observed between triglyceride lipase and cholesterol esterase activity in the aortic cytosol fraction following either thermal inactivation, inhibition by a mercurial, fractionation by ammonium sulfate or acid precipitation, or DEAE-cellulose chromatography. Addition of rabbit lipoproteins to the assay system resulted in inhibition of both cholesterol esterase and triglyceride lipase activity. Parallel changes in the hydrolysis of both substrates also were observed when exogenously added lipids were added to the incubation system in various physical states. Specificities of the enzyme system towards different cholesteryl esters were examined. No differences in the rate of hydrolysis were observed between cholesteryl oleate, palmitate and linoleate. The data suggest that a single acid lipase, presumably of lysosomal origin, has broad specificity towards triglycerides and cholesteryl esters, and may play a role in the hydrolysis of these lipids during intralysosomal degradation of lipoproteins.

Ral and Rab3a are major GTP-binding proteins of axonal rapid transport and synaptic vesicles and do not redistribute following depolarization stimulated synaptosomal exocytosis.

We have employed high-resolution SDS polyacrylamide gels to demonstrate that there are two major low-molecular-weight GTP-binding proteins associated with axonal membranes including synaptic vesicles, rapid transported membranes and clathrin-coated vesicles. We demonstrate that one of the major proteins is Ral and that the other is Rab3A. Following the depolarization of synaptosomes resulting in increased neurotransmitter release, we see no significant dissociation of either Ral or Rab3a from synaptic vesicle derived membranes in contrast to results reported previously.

Identification and purification of a novel G protein from neutrophils.

A novel G protein which appears to couple chemotactic peptide receptors to a polyphosphoinositide phospholipase C has been purified from rabbit neutrophils. Neutrophil membranes were solubilized with sodium cholate and fractionated by successive anion exchange, gel filtration and hydrophobic chromatography. Guanosine-5'-(3-O-thio)triphosphate binding activity was purified 170-fold from the soluble extract. The alpha-subunit of the purified G protein was identified by pertussis toxin-catalyzed ADP-ribosylation, and found to have an Mr of 40,000. The beta-subunit (Mr 36,000) comigrated on SDS-polyacrylamide gel electrophoresis with the beta-subunits of bovine brain Gi and Go. The neutrophil pertussis toxin substrate is highly unstable in cholate solution unless 30% ethylene glycol is added. Structural and functional analysis of this novel G protein will advance our understanding of the molecular mechanisms of coupling of receptors to phospholipase C.

5-Aza-2'-deoxycytidine leads to down-regulation of aberrant p16INK4A RNA transcripts and restores the functional retinoblastoma protein pathway in hepatocellular carcinoma cell lines.

The inactivation of the cyclin-dependent kinase 4 and 6 (CDK4/6) inhibitor p16INK4A may be caused by gene deletion, mutation or promoter hypermethylation. We have previously reported that p16INK4A in hepatocellular carcinoma (HCC) tissues and cell lines is inactivated predominantly by promoter hypermethylation rather than genomic aberrations. In the present experiments, we have studied the effects of the demethylating agent, 5-aza-2'-deoxycytidine (5-AZA/decitabine), on the expression of aberrant p16INK4A RNA transcripts and the CDK-retinoblastoma gene pathway in HCC cell lines with p16INK4A promoter hypermethylation. The expression of aberrant p16INK4A RNA transcripts was down-regulated and p16INK4A protein was strongly re-expressed in the HCC cell lines, SNU 354, 398, 423 and 475 after 5-AZA/decitabine treatment for 5 days. The re-expressed p16INK4A was functional, because it bound to and inhibited CDK4 kinase activity, and increased the concentrations of the hypophosphorylated form of retinoblastoma protein (pRB) in cells with a wild type RB gene. Moreover, treatment with the demethylating agent led not only to G1 cell cycle arrest, but also to the increased expression of the senescence-associated marker beta-galactosidase. This up-regulation of p16INK4A mRNA and protein correlated with demethylation of the p16INK4A promoter, and with the down-regulation or disappearance of aberrant p16INK4A transcripts. These results suggest that the aberrant p16INK4A RNA transcript can be transcribed from the methylated p16INK4A gene, and endogenous reactivation of functional p16INK4A mRNA by a demethylating agent can restore the pRB pathway in HCC, and foster the terminal differentiation of the malignant cells. Therefore, demethylating agents, such as 5-AZA/decitabine, may have potential in the treatment of HCC.

Differences in the subcellular localization of calreticulin and organellar Ca(2+)-ATPase in neurons.

It has become clear that calcium is an important mediator in the transduction of signals due to ligand binding to cell surface receptors. Cytosolic calcium is typically maintained at low levels in both muscle and non-muscle cells and intracellular sequestering of calcium appears to be important in this process. The identification of intracellular calcium pools has been the subject of much recent study, and it has been proposed that such pools would contain three components: a calcium-activated pump or Ca(2+)-ATPase, a calcium channel such as the inositol trisphosphate receptor or ryanodine receptor, and a high-capacity calcium-binding protein such as calsequestrin or calreticulin. We report here on the localization of two components, the organellar Ca(2+)-ATPase (SERCA) and calreticulin, in neuronal tissues. Using immunofluorescence and subcellular fractionation, we have found that for the most part, these two proteins do not co-localize in neuron cell bodies, dendrites, or axons; but may co-localize at the axon terminal.

KDEL proteins are found on the surface of NG108-15 cells.

Although KDEL proteins are primarily localized to the endoplasmic reticulum (ER), we have employed surface biotinylation method to demonstrate that the KDEL proteins calreticulin (Crt), protein disulfide isomerase (PDI) and the 78-kDa glucose regulated protein (GRP78) are found on the surface of the NG108-15 cell line. In contrast, the 94-kDa glucose regulated protein (GRP94), another KDEL protein, is not found on the cell surface. Calnexin (Cnx), a type-1 integral transmembrane ER protein which is partially homologous to Crt but lacks the KDEL sequence, is not detected on the cell surface either. While only small amounts of the total GRP78, PDI and Crt molecules exist on the cell surface at steady state, a significant fraction of the newly synthesized molecules are transported to the cell surface and transport of these proteins is inhibited by treatment with brefeldin A. The surface GRP78 contains the KDEL sequence. On the cell surface, GRP78, PDI and Crt associate with other proteins and form complexes of different sizes. Surface Crt is found to be essential for the neurite formation when NG108-15 cells are induced to differentiate using dibutyryl cAMP.

Molecular cloning of the fMet-Leu-Phe receptor from neutrophils.

The bacterial chemotactic peptide fMet-Leu-Phe (fMLP) activates neutrophils upon binding to surface receptors. In a previous communication we reported the functional reconstitution of the fMLP receptor in Xenopus laevis oocytes (Coats, W. D., and Navarro, J. (1990) J. Biol. Chem. 265, 5964-5966). In this work we report the isolation of the cDNA encoding the fMLP receptor from neutrophils. A rabbit neutrophil cDNA library was screened with an oligonucleotide probe deduced from the nucleotide sequence of G-protein-coupled receptors, and a cDNA encoding the fMLP receptor was isolated. This cDNA was characterized according to the following criteria: 1) Analysis of the deduced amino acid sequence revealed that the clone belongs to a G-protein-coupled receptor. 2) Tissue distribution analysis of the mRNA indicated that the message is only found in neutrophils. 3) In vitro translation of the message revealed a protein corresponding in size to the deglycosylated fMLP receptor. 4) X. laevis oocytes injected with the fMLP receptor message exhibited fMLP-dependent calcium mobilization and specific binding to the fMLP analog 125I-labeled fNle-Leu-Phe-Nle-Tyr-Lys (where Nle is norleucine and fNle is formylnorleucine). The molecular cloning of the fMLP receptor should provide the framework to analyze the relationship between structure, function, and regulation of this receptor.

Effect of atherosclerosis on lysosomal cholesterol esterase activity in rabbit aorta.

Radiolabeled cholesteryl oleate, when incorporated into phospholipid vesicles, was hydrolyzed at acid pH by an enzyme present in rabbit aortic homogenates. In contrast, cholesteryl oleate presented as an acetone dispersion was not effectively hydrolyzed at acid pH under identical conditions. Using the vesicle preparation as substrate, a sensitive assay system for the acid hydrolase was developed in which hydrolysis was proportional to protein concentration and incubation time, and was independent of substrate concentration. The physical state of the vesicles was apparently not altered by the assay conditions, and no hydrolysis of the vesicle-associated phospholipid was detected. Acid cholesterol esterase activity in atherosclerotic aortic tissue was 2.5-fold greater than that of control tissue, and even greater increases were observed in the activities of other lysosomal enzymes (N-acetyl-beta-d-glucosaminidase and beta-glucuronidase). Glucose-6-phosphatase activity was also increased in aortas from cholesterol-fed animals while 5' nucleotidase activity remained unchanged. Labeled triolein also was incorporated into phospholipid vesicles and was hydrolyzed by an acid lipase in aortic tissue. Similarities between triolein and cholesteryl oleate hydrolysis existed with respect to pH optimum and the effect of cholesterol feeding on activity, suggesting that a single enzyme may hydrolyze both lipids.

Solubilization and reconstitution of the formylmethionylleucylphenylalanine receptor coupled to guanine nucleotide regulatory protein.

We describe the solubilization, resolution, and reconstitution of the formylmethionylleucylphenylalanine (fMet-Leu-Phe) receptor and guanine nucleotide regulatory proteins (G-proteins). The receptor was solubilized with 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonate. Guanine nucleotides decreased the number of high-affinity binding sites and accelerated the rate of dissociation of the receptor-ligand complex, suggesting that the solubilized receptor remained coupled to endogenous G-proteins. The solubilized receptor was resolved from endogenous G-proteins by fractionation on a wheat germ agglutinin (WGA)-Sepharose 4B column. High-affinity [3H]fMet-Leu-Phe binding to the WGA-purified receptor was diminished and exhibited reduced guanine nucleotide sensitivity. High-affinity [3H]fMet-Leu-Phe binding and guanine nucleotide sensitivity were reconstituted upon the addition of purified brain G-proteins. Similar results were obtained when the receptor was reconstituted with brain G-proteins into phospholipid vesicles by gel filtration chromatography. In addition, we also demonstrated fMet-Leu-Phe-dependent GTP hydrolysis in the reconstituted vesicles. The results of this work indicate that coupling of the fMet-Leu-Phe receptor to G-proteins converts the receptor to a high-affinity binding state and that agonist produces activation of G-proteins. The resolution and functional reconstitution of this receptor should provide an important step toward the elucidation of the molecular mechanism of the fMet-Leu-Phe transduction system in neutrophils.