Phosphoinositide-Dependent Signaling in Cancer: A Focus on Phospholipase C Isozymes

Phosphoinositides (PI) form just a minor portion of the total phospholipid content in cells but are significantly involved in cancer development and progression. In several cancer types, phosphatidylinositol 3,4,5-trisphosphate [PtdIns(3,4,5)P3] and phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P2] play significant roles in regulating survival, proliferation, invasion, and growth of cancer cells. Phosphoinositide-specific phospholipase C (PLC) catalyze the generation of the essential second messengers diacylglycerol (DAG) and inositol 1,4,5 trisphosphate (InsP3) by hydrolyzing PtdIns(4,5)P2. DAG and InsP3 regulate Protein Kinase C (PKC) activation and the release of calcium ions (Ca2+) into the cytosol, respectively. This event leads to the control of several important biological processes implicated in cancer. PLCs have been extensively studied in cancer but their regulatory roles in the oncogenic process are not fully understood. This review aims to provide up-to-date knowledge on the involvement of PLCs in cancer. We focus specifically on PLCβ, PLCγ, PLCδ, and PLCε isoforms due to the numerous evidence of their involvement in various cancer types.

Developmental and Metabolic Effects of Disruption of the Mouse CTP:Phosphoethanolamine Cytidylyltransferase Gene (Pcyt2)

ABSTRACT The CDP-ethanolamine pathway is responsible for the de novo biosynthesis of ethanolamine phospholipids, where CDP-ethanolamine is coupled with diacylglycerols to form phosphatidylethanolamine. We have disrupted the mouse gene encoding CTP:phosphoethanolamine cytidylyltransferase, Pcyt2, the main regulatory enzyme in this pathway. Intercrossings of Pcyt2 +/ − animals resulted in small litter sizes and unexpected Mendelian frequencies, with no null mice genotyped. The Pcyt2 − / − embryos die after implantation, prior to embryonic day 8.5. Examination of mRNA expression, protein content, and enzyme activity in Pcyt2 +/ − animals revealed the anticipated 50% decrease due to the gene dosage effect but rather a 20 to 35% decrease. [14C]ethanolamine radiolabeling of hepatocytes, liver, heart, and brain corroborated Pcyt2 gene expression and activity data and showed a decreased rate of phosphatidylethanolamine biosynthesis in heterozygotes. Total phospholipid content was maintained in Pcyt2 +/ − tissues; however, this was not due to compensatory increases in the decarboxylation of phosphatidylserine. These results establish the necessity of Pcyt2 for murine development and demonstrate that a single Pcyt2 allele in heterozygotes can maintain phospholipid homeostasis.

Compositional correlates of metabolic depression in the mitochondrial membranes of estivating snails

The phospholipid and protein compositions of mitochondrial membranes from hepatopancreas of active and estivating terrestrial snails ( Cepaea nemoralis) were compared. Mitochondria from estivating snails contained 82.7% less cardiolipin, and this was associated with an 83.9% reduction in cytochrome- c oxidase activity. Substantial changes also occurred in the proportional amounts of other individual phospholipid classes and their constituent fatty acids, including a 72% loss of total mitochondrial phospholipids, a 37% increase in monoenes, and 49% fewer n–3 fatty acids in membranes of estivating snails. These changes are consistent with those correlated with lowered metabolic rate and lower rates of proton leak in other animal models. Estivating snail hepatopancreas showed no change in total phospholipid content, indicating that the phospholipids lost from mitochondrial membranes may be sequestered elsewhere within the cell. We suggest that estivating snails remodel mitochondrial membranes as part of a coordinated, reversible suppression of mitochondrial membrane-associated processes, which may include a concomitant reduction in rates of proton pumping and leaking.

Effect of fetal exposure to gentamicin on phosphate transport in young rat kidney

The renal phosphate (Pi)-transporting capacity normally increases, due to increased carrier system affinity, during the third postnatal week in rats. However, the tubular Pi reabsorption of rat pups born from gentamicin-treated mothers does not increase during this period. This study determines whether exposure to gentamicin in utero selectively alters the postnatal maturation of the carrier affinity for Pi. Pi and glucose transports by proximal tubule brush-border membrane (BBM) were studied. The maximal rate of uptake (Vmax) of Na-Pi cotransport was significantly lower (536 +/- 169 pmol.mg protein-1.10 s-1; n = 6, P < 0.01) in gentamicin-exposed rats than in controls (1,021 +/- 167 pmol.mg protein-1.10 s-1, n = 6), whereas the Michaelis constant (Km) values were the same. Gentamicin exposure had no effect on plasma parathyroid hormone concentration or on BBM glucose transport activity. The total phospholipid content of BBM, their phospholipid composition, cholesterol content, and cholesterol-to-total phospholipid mole ratio were unaltered, suggesting that membrane fluidity was unchanged. The Vmax of BBM alkaline phosphatase was lower in gentamicin-exposed rats than in controls.

Metabolic basis of diethylaminoethoxyhexestrol-induced phospholipid fatty liver

Diethylaminoethoxyhexestrol caused a foam cell lipidosis in humans characterized by phospholipid storage in the liver, spleen, and other tissues, and this represents the first description of acquired lipidosis caused by a drug. It has been proposed that diethylaminoethoxyhexestrol causes phospholipid fatty liver by concentrating in lysosomes and inhibiting phospholipases but it has not previously been possible to measure the intralysosomal concentration of diethylaminoethoxyhexestrol. In this paper we report for the first time the intralysosomal concentration of this drug in rats. After a single oral dose of diethylaminoethoxyhexestrol (100 mg/kg) the intralysosomal concentration was 7.9 mM at 2.5 h, 15.6 mM at 12 h, and 20.9 mM at 24 h, respectively. The total phospholipid content of lysosomes in drug-treated rats increased 1.9-, 6.0-, and 7.6-fold over control at 2.5, 12, and 24 h, respectively. Purified lysosomal phospholipase A1 was strongly inhibited by diethylaminoethoxyhexestrol in vitro. In phospholipid fatty liver, phospholipid accumulation in lysosomes appears to be caused by the presence of diethylaminoethoxyhexestrol in lysosomes at concentrations estimated to be 7.9–20 mM, because drug levels above 1 mM completely block the activity of purified lysosomal phospholipase A1 in vitro.

Shape changes in human erythrocytes induced by replacement of the native phosphatidylcholine with species containing various fatty acids.

Phosphatidylcholine-specific transfer protein from beef liver has been used to replace native phosphatidylcholine (PC) molecules from intact human erythrocytes by a variety of PC species differing in fatty acid composition. These replacements changed neither the total phospholipid content of the membrane, nor the composition of this fraction in terms of the various phospholipid classes. The morphology of the erythrocyte was not modified when native PC was replaced by 1-palmitoyl,2-oleoyl PC, 1-palmitoyl,2-linoleoyl PC, egg PC, or PC isolated from rat liver microsomes. Replacement with the disaturated species 1,2-dimyristoyl PC, 1,2-dipalmitoyl PC, and 1,2-distearoyl PC resulted in the formation of echinocytes and, at higher levels of replacement, in spheroechinocytes. Echinocyte-like erythrocytes were also observed after replacement with 1-palmitoyl,2-arachidonoyl PC, whereas stomatocytes were formed upon replacement with PC species containing two unsaturated fatty acids, e.g., 1,2-dioleoyl PC and 1,2-dilinoleoyl PC. The observations show that the erythrocyte membrane structure and the overall discoid cell shape of the human erythrocyte are optimally stabilized by PC species that contain one saturated and one mono- or diunsaturated fatty acid, and that the cell tolerates only limited variations in the species composition of its PC.

Sarcolemmal Na+-K+-ATPase activity in diabetic rat heart

Heart sarcolemmal membranes were isolated by the hypotonic shock-LiBr treatment from rats with chronic diabetes induced by a streptozotocin (65 mg/kg, iv) injection. Sarcolemmal Mg2+-dependent ATPase activity was elevated, whereas 5'-nucleotidase and K+-p-nitrophenylphosphatase activities in diabetic heart were depressed in comparison to control preparations. Although patent Na+-K+-ATPase and patent ouabain-sensitive Na+-K+-ATPase activities were unaltered, latent Na+-K+-ATPase activities, as determined in membranes after alamethicin or deoxycholate treatments, were found to be significantly depressed in diabetic animals. A depression in the latent Na+-K+-ATPase activity in diabetic preparations was also observed in membranes prepared by the sucrose density gradient method. Insulin-treated diabetic rats were observed to have normalized latent Na+-K+-ATPase activities. Total phospholipid content did not differ, but cholesterol content of the sarcolemmal membranes was significantly increased in diabetic heart preparations. Sarcolemmal Na+-K+-ATPase activity in diabetic heart was more resistant to treatments with filipin, an agent known to bind with cholesterol residues. These results suggest that chronic experimental diabetes is associated with some defects in sarcolemmal enzymatic activities and composition.

Thyroid hormones, malnutrition, and biochemical composition of developing rat lung

We studied the effects of thyroid hormones and malnutrition on protein, DNA, and phospholipid content of the developing rat lung during the first month of life. Neonatal hypothyroidism significantly decreases the lung phospholipid content by 30-45% between 5 and 30 days of age whether the results are expressed per milligram DNA or per gram tissue. Administration of thyroxine for 3 days to hypothyroid rats increases significantly (20%) their total phospholipid content, mainly through its preferential effect on phosphatidylcholine (50% increase). Adult animal response to thyroid hormones is markedly different from that observed in young rats for most parameters examined. In malnourished rats, lung tissue phospholipids are decreased per cell but not per unit cell mass after 11 days of age. These results show that hypothyroidism has a specific effect on lung phospholipids during the neonatal period.

Effects of centrifugation, storage, and contamination of amniotic fluid on its total phospholipid content.

The total phospholipid content of the supernatant fluid decreases rapidly with increasing relative centrifugal force when amniotic fluid is centrifuged. Possible explanations for this are discussed. The effect of storage on the total phospholipid content of amniotic fluid at different temperatures and the influence of contamination with blood and meconium are also described.

Effects of centrifugation, storage, and contamination of amniotic fluid on its total phospholipid content.

The total phospholipid content of the supernatant fluid decreases rapidly with increasing relative centrifugal force when amniotic fluid is centrifuged. Possible explanations for this are discussed. The effect of storage on the total phospholipid content of amniotic fluid at different temperatures and the influence of contamination with blood and meconium are also described.