Stimulation of phospholipid metabolism in embryonic muscle cells treated with phospholipase C

From masochistic enzymology to mechanistic physiology and disease

The pioneering work of Eugene Kennedy in the 1950s established the choline pathway for phosphatidylcholine (PC) biosynthesis. However, the regulation of PC biosynthesis was poorly understood at that time. When I started my lab at the University of British Columbia in the 1970s, this was the focus of my research. This article provides my reflections on these studies that began with enzymology and the use of cultured mammalian cells, and progressed to utilize the techniques of molecular biology and gene-targeted mice. The research in my lab and others demonstrated that the regulated and rate-limiting step in the choline pathway for PC biosynthesis was catalyzed by CTP:phosphocholine cytidylyltransferase. This enzyme is regulated by its movement from a soluble form (largely in the nucleus) to a membrane-associated form where the enzyme becomes activated. Gene targeting in mice subsequently demonstrated that this gene is essential for development of mouse embryos. The other mammalian pathway for PC biosynthesis is catalyzed by phosphatidylethanolamine N-methyltransferase (PEMT) that converts phosphatidylethanolamine to PC. Understanding of the regulation and function of the integral membrane protein PEMT was improved when the enzyme was purified (a masochistic endeavor) in 1987, leading to the cloning of the Pemt cDNA. Generation of knock-out mice that lacked PEMT showed that they were protected from atherosclerosis, diet-induced obesity, and insulin resistance. The protection from atherosclerosis appears to be due to decreased secretion of lipoproteins from the liver. We continue to investigate the mechanism(s) by which Pemt^-/- mice are protected from weight gain and insulin resistance.

Myoblast fusion and inositol phospholipid breakdown: causal relationship or coincidence?

The fusion of embryonic chick myoblasts has been examined in culture. Cells were prepared from 12-day-old chick embryonic breast muscle and cultured for 50 h at a Ca2+ concentration in the medium of 10(-7) M. During this period they attain fusion competence. Addition of 1.4 mM-Ca2+ to these cells elicits rapid fusion. Changes in the metabolism of myoblast phospholipids in response to the raised Ca2+ concentration have been examined. Only the inositol phospholipids are affected. Phosphatidylinositol, phosphatidylinositol 4-phosphate and phosphatidylinositol 4,5-bisphosphate are rapidly broken down and 1,2-diacylglycerol and phosphatidic acid are synthesized. Myoblast fusion has also been found to be stimulated by a factor present in chick embryo extract, probably of neuronal origin. A receptor-mediated mechanism for myoblast fusion is proposed. This envisages the polyphosphoinositides acting as a fusion block, either themselves or by their binding to membrane proteins. The inositol phospholipid breakdown could result in a more fluid membrane and the breakdown products 1,2-diacylglycerol and phosphatidic acid, two known fusogens, could stimulate fusion.

Transcriptional regulation of phosphatidylcholine biosynthesis

Phosphatidylcholine biosynthesis in animal cells is primarily regulated by the rapid translocation of CTP:phosphocholine cytidylyltransferase alpha between a soluble form that is inactive and a membrane-associated form that is activated. Until less than 10 years ago there was no information on the transcriptional regulation of phosphatidylcholine biosynthesis. Research has identified the transcription factors Sp1, Rb, TEF4, Ets-1 and E2F as enhancing the expression of the cytidylyltransferase and Net as a factor that represses cytidylyltransferase expression. Key transcription factors involved in cholesterol or fatty acid metabolism (SREBPs, LXRs, PPARs) do not have a major role in transcriptional regulation of the cytidylyltransferase. Rather than being linked to cholesterol or energy metabolism, regulation of the cytidylyltransferase is linked to the cell cycle, cell growth and differentiation. Transcriptional regulation of phospholipid biosynthesis is more elegantly understood in yeast and involves responses to inositol, choline and zinc in the culture medium.

Fundamental research is the basis for understanding and treatment of many human diseases

There are numerous examples of how fundamental research has been required to understand and treat human disease. This article focuses on three human diseases of lipid metabolism in which advancements in understanding and treatment would not have been possible without basic research. Fabry disease is an inherited metabolic disorder caused by the lack of a specific enzyme in glycosphingolipid catabolism. Cardiovascular disease is a complex and multifactorial disease but as many as half of the cases can be attributed to abnormal levels of plasma cholesterol. The incidence of liver disease is increasing due to the current epidemic of obesity. It is only recently that curiosity-driven research has yielded valuable insight into the mechanism by which liver disease evolves.

Regulatory enzymes of phosphatidylcholine biosynthesis: a personal perspective

Phosphatidylcholine is a prominent constituent of eukaryotic and some prokaryotic membranes. This Perspective focuses on the two enzymes that regulate its biosynthesis, choline kinase and CTP:phosphocholine cytidylyltransferase. These enzymes are discussed with respect to their molecular properties, isoforms, enzymatic activities, and structures, and the possible molecular mechanisms by which they participate in regulation of phosphatidylcholine levels in the cell.

Effects of Clostridium perfringens phospholipase C in mammalian cells

Clostridium perfringens phospholipase C (Cp-PLC), the major virulence factor in the pathogenesis of gas gangrene, is a Zn(2+) metalloenzyme with lecithinase and sphingomyelinase activities. Its structure shows an N-terminal domain containing the active site, and a C-terminal Ca(2+) binding domain required for membrane interaction. Although the knowledge of the structure of Cp-PLC and its interaction with aggregated phospholipids has advanced significantly, an understanding of the effects of Cp-PLC in mammalian cells is still incomplete. Cp-PLC binds to artificial bilayers containing cholesterol and sphingomyelin or phosphatidylcholine (PC) and degrades them, but glycoconjugates present in biological membranes influence its binding or positioning toward its substrates. Studies with Cp-PLC variants harboring single amino-acid substitutions have revealed that the active site, the Ca(2+) binding region, and the membrane interacting surface are required for cytotoxic and haemolytic activity. Cp-PLC causes plasma membrane disruption at high concentrations, whereas at low concentrations it perturbs phospholipid metabolism, induces DAG generation, PKC activation, Ca(2+) mobilization, and activates arachidonic acid metabolism. The cellular susceptibility to Cp-PLC depends on the composition of the plasma membrane and the capacity to up-regulate PC synthesis. The composition of the plasma membrane determines whether Cp-PLC can bind and acquire its active conformation, and thus the extent of phospholipid degradation. The capacity of PC synthesis and the availability of precursors determine whether the cell can replace the degraded phospholipids. Whether the perturbations of signal transduction processes caused by Cp-PLC play a role in cytotoxicity is not clear. However, these perturbations in endothelial cells, platelets and neutrophils lead to the uncontrolled production of intercellular mediators and adhesion molecules, which inhibits bacterial clearance and induces thrombotic events, thus favouring bacterial growth and spread in the host tissues.

Phosphocholine and phosphoethanolamine during chick embryo myogenesis: a (31)P-NMR study

Elevated contents of phosphoethanolamine (Etn-P) and/or phosphocholine (Cho-P), a common feature of most tumours with respect to normal counterparts, may also occur in non-cancerous proliferating tissues. The significance of these alterations in relation to cell proliferation, differentiation and maturation is scarcely understood. In this work, the Cho-P and Etn-P pools were measured by (31)P-NMR in extracts of chick embryo pectoral muscle at different days of development. The average concentration of these metabolites exhibited the highest values (respectively, 1.5 and 3.0 micromol/mg DNA) on days 9-11 and decreased at later stages of myogenesis. While, however, Cho-P maintained substantial levels (above 1.0 micromol/mg DNA) also during myotube formation (days 11-18) and stepwise decreased (to about 0.5 micromol/mg DNA) upon fibres' maturation, Etn-P gradually decreased between day 11 and hatching time (down to about 0.2 micromol/mg DNA). These results demonstrate that significant changes may occur in the steady-state pools of these metabolites during normal in vivo cellular development and differentiation, and are consistent with: (a) high rates of phospholipid biosynthesis reported in the literature for proliferating myoblasts; (b) sustained phosphatidylcholine synthesis maintained also during myoblast fusion; and (c) decreased requirement of phospholipid synthesis in the last phase of in ovo myofibre maturation.

The progression of spermatogenesis in the developing rat testis followed by 31P MR spectroscopy

To evaluate the use of human testicular 31P MR spectroscopy as a diagnostic tool to differentiate between several stages of male infertility, we have studied the testicular levels of several phosphorus containing compounds in the rat in relation to the condition of spermatogenesis and the cell types present in the seminiferous tubules of the testis. During testicular maturation several characteristic changes occur in the 31P MR spectrum of the testis of male Wistar rats. The phosphomonoester/adenosine triphosphate (PM/ATP) ratio shows a decline from 1.61 to 1.02 between the age of 3 and 12 weeks, whereas the phosphodiester (PD)/ATP ratio increases from 0 to 0.72. The testicular pH increases in the same time from 7.06 to 7.32. Testicular MR data obtained after 12 weeks of age onward do not show significant change anymore. The high PM/ATP ratio is associated by a relative high amount of proliferating spermatogonia and spermatocytes during meiosis in the testis, whereas the PD peak seems to be correlated with the release and maintenance of spermatozoa. The MR spectra show a specific fingerprint in all developmental stages of the rat testis as a result of the different cell types in the testis.

Coupled and uncoupled induction of fos and jun transcription by different second messengers in cells of hematopoietic origin

The nuclear oncoproteins fos and jun are associated as a heterodimer which binds to TPA (PMA or TPA: phorbol 12-myristate 13-acetate)- responsive promoter elements (TRE), the recognition site for the transcription factor AP-1. The fos/jun heterodimer has a higher affinity to the TRE and stimulates transcription of responsive genes more than the jun homodimer. The association of these two oncoproteins may play a central role in signal transduction and regulation of cell proliferation and differentiation. We further defined the regulation of fos and jun by studying their inducibility by second messengers in cells of hematopoietic origin. In THP-1 monocytic leukemia cells fos and jun mRNA levels are regulated in a coupled manner by second messengers activated after membrane phospholipid turnover. Addition of phospholipase C to cells, as well as stimulation of protein kinase C and release of intracellular Ca2+, caused a rapid induction of fos and jun mRNA levels, but the induction of jun mRNA showed a more persistant and less transient pattern than fos. In contrast to the phosphoinositol system, stimulation of the adenylate cyclase pathway in THP-1 cells induced only fos transcription whereas jun mRNA levels remained unchanged. A similar uncoupling of fos and jun inducibility was found after phorbol ester addition to the human erythroleukemia cell line HEL and the human promyelocytic cell line HL-60. The uncoupling of fos and jun levels might predispose cells to the formation of combinatorial transcription complexes of a different composition and activity than the fos/jun heterodimer. Indeed, nuclear extracts from THP-1 cells before or after activation of the phosphinositol or adenylate cyclase second messenger pathways revealed a correlation in fos and jun expression and specific binding of the heterocomplex to a TRE sequence.

Concurrent changes in Dunaliella salina ultrastructure and membrane phospholipid metabolism after hyperosmotic shock

Hyperosmotic shock, induced by raising the NaCl concentration of Dunaliella salina medium from 1.71 to 3.42 M, elicited a rapid decrease of nearly one-third in whole cell volume and in the volume of intracellular organelles. The decrease in cell volume was accompanied by plasmalemma infolding without overall loss of surface area. This contrasts with the dramatic increase in plasmalemma surface area after hypoosmotic shock (Maeda, M., and G. A. Thompson. 1986. J. Cell Biol. 102:289-297). Although plasmalemma surface area remained constant after hyperosmotic shock, the nucleus, chloroplast, and mitochondria lost membrane surface area, apparently through membrane fusion with the endoplasmic reticulum. Thus the endoplasmic reticulum serves as a reservoir for excess membrane during hyperosmotic stress, reversing its role as membrane donor to the same organelles during hypoosmotically induced cell expansion. Hyperosmotic shock also induced rapid changes in phospholipid metabolism. The mass of phosphatidic acid dropped to 56% of control and that of phosphatidylinositol 4,5-bisphosphate rose to 130% of control within 4 min. Further analysis demonstrated that within 10 min after hyperosmotic shock, there was 2.5-fold increase in phosphatidylcholine turnover, a twofold increase in lysophosphatidylcholine mass, a four-fold increase in lysophosphatidate mass, and an elevation in free fatty acids to 124% of control, all observations suggesting activation of phospholipase A. The observed biophysical and biochemical phenomena are likely to be causally interrelated in providing mechanisms for successful accommodation to such severe osmotic extremes.

The fusion of myoblasts

Images

Phorbol esters, phospholipase C, and growth factors rapidly stimulate the phosphorylation of a Mr 80,000 protein in intact quiescent 3T3 cells

Addition of biologically active phorbol esters to intact quiescent 3T3 mouse cells stimulates an extremely rapid (detectable within seconds) phosphorylation of a Mr 80,000 cellular protein (termed "80k"). Phorbol 12,13-dibutyrate enhances 80k phosphorylation in a dose-dependent manner; half-maximal effect is obtained at 32 nM. The possibility that this phosphorylation is related to the activation of Ca2+-activated phospholipid-dependent protein kinase is suggested by the fact that phospholipid breakdown induced by exogenous treatment of the cells with phospholipase C from Clostridium perfringens or with platelet-derived growth factor, which is a potent activator of endogenous phospholipase C activity, also causes a rapid enhancement of 80k phosphorylation. Moreover, prolonged pretreatment of the cells with phorbol 12,13-dibutyrate, which leads to a marked decrease in the number of specific phorbol ester binding sites, prevents the phosphorylation of 80k stimulated by phorbol esters, phospholipase C, and platelet-derived growth factor. These findings provide evidence obtained with intact cells that implicate the stimulation of Ca2+-activated phospholipid-dependent protein kinase in the action of phorbol esters and other growth factors.

Combination chemotherapy with Clostridium perfringens phospholipase C and cytosine antimetabolites: complementary inhibition directed at membrane lipids

Tumor cell membranes were susceptible to the action of Clostridium perfringens phospholipase C, and this was reflected by inhibition of cellular replication in culture. The differential susceptibility of two neoplastic cell lines to this enzyme was studied in detail. The growth of sarcoma 180 cells cultured in Fischer's medium was markedly inhibited by phospholipase C; whereas, in contrast, cultured L1210 leukemia cells were relatively resistant to the cytotoxic effects of this enzyme. The differential sensitivity of these two neoplastic cell lines to phospholipase C was corroborated by dye-exclusion tests. Thus, leukemia L1210 cells exposed to a concentration of 0.2 mg of phospholipase C per ml of Fischer's medium for 30 min at 37 degrees C were able to exclude Trypan Blue; whereas, only about 21% of sarcoma 180 cells treated under identical conditions were able to exclude the dye. That the cytotoxicity of phospholipase C to sarcoma 180 was the result of hydrolysis of phospholipids of the plasma membrane was supported by measurements of the rate of hydrolysis of radioactivity from the phospholipid of neoplastic cells prelabeled with [3H]choline. Eighty-two percent of incorporated radioactive choline was released from sarcoma 180 cells treated with phospholipase C in Fischer's medium, whereas, only 20% of the label from [3H]choline was solubilized from L1210 leukemia cells treated with the enzyme under similar conditions. Scanning electron microscopy revealed significant damage to sarcoma 180 cells exposed to phospholipase C in Fischer's medium, which was characterized by alterations in size and shape of cells, disappearance of microvilli, and appearance of fistulas in cell membranes; relatively resistant L1210 leukemic cells did not appear to be markedly damaged by comparable enzyme treatment. Exposure of leukemia L1210 cells to phospholipase C in Puck's saline A increased the sensitivity of these cells to enzymatic action. Under these conditions, a comparable amount of phospholipid was hydrolyzed from surface membranes of sarcoma 180 and leukemia L1210 cells, and the degree of membrane damage appeared to be similar, as measured by the capacity of the tumor cell lines to exclude Trypan Blue and by scanning electron microscopy. The extensive damage to membranes by hydrolysis of phospholipids was not accompanied by a change in the degree of specific binding of [3H]concanavalin A(ConA).(ABSTRACT TRUNCATED AT 400 WORDS)

Changes in galactosyltransferase activity in chick pectoral muscle during embryonic development

The two major vertebrate galactosyltransferases have been investigated in developing chick muscle in ovo and in vitro, and in cultured chick fibroblasts. The two enzymes were UDP-galactose-N-acetylglucosamine galactosyltransferase (galactosyltransferase I) and UDP-galactose-N-acetylgalactosamine galactosyltransferase (galactosyltransferase II). Both activities fell during muscle development in ovo. Galactosyltransferase I activity was constant from day 7 to day 16, after which it declined 5-fold, whereas galactosyltransferase II activity fell markedly from day 9 to 13 and 16 to 20, displaying an overall 8-fold decrease. In primary muscle cultures, galactosyltransferase I activity fell slightly during 7 days in culture, whereas galactosyltransferase II increased 2-fold during the same period. No significant change in activity of either galactosyltransferase was observed during intercellular recognition and fusion. Analysis of muscle cultures treated with cytosine arabinoside and of fibroblast cultures revealed that the majority of galactosyltransferase I activity in primary muscle cultures is associated with fibroblasts, whereas the majority of galactosyltransferase II activity is muscle-associated. The addition of 5-bromodeoxyuridine to primary muscle cultures resulted in a 3-fold rise in activities of both transferases.