Phospholipase C-mediated hydrolysis of phosphatidylcholine is a target of transforming growth factor beta 1 inhibitory signals

Tricyclodecan-9-yl-Xanthogenate (D609): Mechanism of Action and Pharmacological Applications

Tricyclodecan-9-yl xanthogenate (D609) is a synthetic tricyclic compound possessing a xanthate group. This xanthogenate compound is known for its diverse pharmacological properties. Over the last three decades, many studies have reported the biological activities of D609, including antioxidant, antiapoptotic, anticholinergic, anti-tumor, anti-inflammatory, anti-viral, anti-proliferative, and neuroprotective activities. Its mechanism of action is extensively attributed to its ability to cause the competitive inhibition of phosphatidylcholine (PC)-specific phospholipase C (PC-PLC) and sphingomyelin synthase (SMS). The inhibition of PCPLC or SMS affects secondary messengers with a lipidic nature, i.e., 1,2-diacylglycerol (DAG) and ceramide. Various in vitro/in vivo studies suggest that PCPLC and SMS inhibition regulate the cell cycle, block cellular proliferation, and induce differentiation. D609 acts as a pro-inflammatory cytokine antagonist and diminishes Aβ-stimulated toxicity. PCPLC enzymatic activity essentially requires Zn²⁺, and D609 might act as a potential chelator of Zn²⁺, thereby blocking PCPLC enzymatic activity. D609 also demonstrates promising results in reducing atherosclerotic plaque formation, post-stroke cerebral infarction, and cancer progression. The present compilation provides a comprehensive mechanistic insight into D609, including its chemistry, mechanism of action, and regulation of various pharmacological activities.

Role of UEV-1, an inactive variant of the E2 ubiquitin-conjugating enzymes, in in vitro differentiation and cell cycle behavior of HT-29-M6 intestinal mucosecretory cells

By means of differential RNA display, we have isolated a cDNA corresponding to transcripts that are down-regulated upon differentiation of the goblet cell-like HT-29-M6 human colon carcinoma cell line. These transcripts encode proteins originally identified as CROC-1 on the basis of their capacity to activate transcription of c-fos. We show that these proteins are similar in sequence, and in predicted secondary and tertiary structure, to the ubiquitin-conjugating enzymes, also known as E2. Despite the similarities, these proteins lack a critical cysteine residue essential for the catalytic activity of E2 enzymes and, in vitro, they do not conjugate or transfer ubiquitin to protein substrates. These proteins constitute a distinct subfamily within the E2 protein family and are highly conserved in phylogeny from yeasts to mammals. Therefore, we have designated them UEV (ubiquitin-conjugating E2 enzyme variant) proteins, defined as proteins similar in sequence and structure to the E2 ubiquitin-conjugating enzymes but lacking their enzymatic activity (HW/GDB-approved gene symbol, UBE2V). At least two human genes code for UEV proteins, and one of them, located on chromosome 20q13.2, is expressed as at least four isoforms, generated by alternative splicing. All human cell types analyzed expressed at least one of these isoforms. Constitutive expression of exogenous human UEV in HT-29-M6 cells inhibited their capacity to differentiate upon confluence and caused both the entry of a larger proportion of cells in the division cycle and an accumulation in G2-M. This was accompanied with a profound inhibition of the mitotic kinase, cdk1. These results suggest that UEV proteins are involved in the control of differentiation and could exert their effects by altering cell cycle distribution.

Regulation of phosphatidylethanolamine degradation by enzyme(s) of subcellular fractions from cerebral cortex

Hydrolysis of 1-acyl-2-[14C]arachidonoyl-sn-glycero-3-phosphoethanolamine was studied in cerebral cortex homogenate and subcellular fractions. The enzyme(s) confined to the synaptic plasma membrane (SPM) hydrolyze(s) [14C-arachidonoyl]phosphatidylethanolamine (PE) in the presence of EGTA to [14C-arachidonoyl]diacylglycerol (DAG) and a small amount of [14C]arachidonic acid (AA). Degradation of PE is time-, protein- and substrate-dependent with a pH optimum of 7.8. The highest activity of PE degradation was observed in the presence of 10 mM EGTA. Under this condition GTP gamma S has no effect on PE hydrolysis. In the presence of Ca2+ ions degradation of PE was significantly lower as compared to the conditions with EGTA. However, the percentage distribution of free AA in the sum of both products of PE hydrolysis (AA + DAG) increases from 16 and 20% observed in the presence of EGTA 2 mM and 10 mM to 34% and 43% in the presence of 0.5 mM CaCl2 alone and together with GTP gamma S, respectively. Cytosolic enzymes also degrade PE in the presence of 2 mM EGTA with the formation of DAG and AA. Radioactivity in the AA represents about 80% of the total radioactivity of the products of PE degradation. The hydrolysis of PE by cytosolic enzymes is almost completely inhibited by neomycin but the hydrolysis by the SPM-bound enzyme(s) is inhibited only 70%. Other studies with quinacrine indicated that only a small pool of PE is degraded by SPM-bound Ca(2+)-independent phospholipase A2 (PLA2). All of these data suggest that PE in cerebral cortex is mainly degraded by cytosolic and SPM-bound Ca(2+)-independent phospholipase C. Further studies towards a better understanding of the mechanisms of cerebral degradation and the physiological significance of Ca(2+)-independent pathways of PE hydrolysis are necessary.

The co-mitogenic combination of transforming growth factor beta 1 and bombesin protects protein kinase C-delta from late-phase down-regulation, despite synergy in diacylglycerol accumulation

Bombesin induces the down-regulation of protein kinase C-delta (PKC-delta) and PKC-epsilon in Swiss 3T3 cells. Simultaneous addition of transforming growth factor beta 1 (TGF beta 1) selectively blocks PKC-delta down-regulation at mid-S-phase, whereas PKC-epsilon levels continue to decline. Northern blot analysis shows that PKC-epsilon levels could be controlled in part at the level of transcription; PKC-delta mRNA levels remained constant at these later times. Bombesin induces a sustained elevation of some species of diacylglycerol (DAG), consistent with the observed loss of PKC-delta and PKC-epsilon. Interestingly, the combination of bombesin and TGF-beta 1 produces an even greater DAG response. Flow cytometric analysis demonstrates that bombesin induces only 15% of the cells to enter the cell cycle, in contrast to the combination of TGF beta 1 plus bombesin which induces 75-80% of the cells to progress through the cycle. The protection of PKC-delta from down-regulation under conditions of sustained DAG elevation correlates with the mitogenic response and implies that the down-regulation process itself is regulated. Consistent with this, it is demonstrated that bombesin plus TGF beta 1 protects PKC-delta from phorbol ester-induced down-regulation.

Subcellular distribution of protein kinase C (pKC) in erythrocytes and concentration of D-myo-inositol-1,4,5-trisphosphate (IP3) in platelets and monocytes of force-fed zinc-deficient rats

The purpose of the present study was to investigate whether alimentary zinc (Zn) deficiency affects the activities of the Zn metalloenzymes protein kinase C (pKC) and the phosphatidylinositol-specific phospholipase C (PLC) in force-fed Zn-deficient rats. The in vivo activity of pKC was determined by measuring the subcellular distribution of the enzyme between the cytosolic and the particulate fraction of erythrocytes, whereas the activity of PLC was measured indirectly through the concentration of its metabolite inositol-1,4, 5-trisphosphate (IP3) in platelets and monocytes. For this purpose, 24 male Sprague-Dawley rats with an average live mass of 126 g were divided into 2 groups of 12 animals each. The Zn-deficient and the control rats received a semisynthetic casein diet with a Zn content of 1.2 and 24.1 ppm, respectively. All animals were fed the same amount of the diet (10.8 g dry matter [DM]/d and rat) four times daily by gastric tube. After 12 d, the depleted rats were in a state of severe Zn deficiency, as demonstrated by a 70% lower Zn concentration and a 66% reduction in the serum activity of alkaline phosphatase. The radio-immunologically determined concentration of IP3 was reduced by a significant 55% in the platelets of the Zn-deficient rats (8.4 pmol IP3/ 5 x 10(8)) as compared with the control rats (18.8 pmol IP3/5 x 10(8)), whereas the IP3 concentration in the monocytes was not affected by the alimentary Zn supply (1.4 vs 1.2 pmol IP3/10(6)), nor was there any difference between the Zn-deficient and the control rats with regard to the radioenzymatically determined specific activity of pKC, either in the cytosolic fraction (32.7 vs 32.5 pmol P/min/mg protein) or in the particulate fraction (38.1 vs 36.5 pmol P/min/mg protein) of the erythrocytes.

NIH 3T3 cells stably transfected with the gene encoding phosphatidylcholine-hydrolyzing phospholipase C from Bacillus cereus acquire a transformed phenotype

In order to determine whether chronic elevation of intracellular diacylglycerol levels generated by hydrolysis of phosphatidylcholine (PC) by PC-hydrolyzing phospholipase C (PC-PLC) is oncogenic, we generated stable transfectants of NIH 3T3 cells expressing the gene encoding PC-PLC from Bacillus cereus. We found that constitutive expression of this gene (plc) led to transformation of NIH 3T3 cells as evidenced by anchorage-independent growth in soft agar, formation of transformed foci in tissue culture, and loss of contact inhibition. The plc transfectants displayed increased intracellular levels of diacylglycerol and phosphocholine. Expression of B. cereus PC-PLC was confirmed by immunoperoxidase and immunofluorescence staining with an affinity-purified anti-PC-PLC antibody. The NIH 3T3 clones expressing plc induced DNA synthesis, progressed through the cell cycle in the absence of added mitogens, and showed significant growth in low-concentration serum. Transfection with an antisense plc expression vector led to a loss of PC-PLC expression accompanied by a complete reversion of the transformed phenotype, suggesting that plc expression was required for maintenance of the transformed state. Taken together, our results show that chronic stimulation of PC hydrolysis by an unregulated PC-PLC enzyme is oncogenic to NIH 3T3 cells.

NIH 3T3 cells stably transfected with the gene encoding phosphatidylcholine-hydrolyzing phospholipase C from Bacillus cereus acquire a transformed phenotype

In order to determine whether chronic elevation of intracellular diacylglycerol levels generated by hydrolysis of phosphatidylcholine (PC) by PC-hydrolyzing phospholipase C (PC-PLC) is oncogenic, we generated stable transfectants of NIH 3T3 cells expressing the gene encoding PC-PLC from Bacillus cereus. We found that constitutive expression of this gene (plc) led to transformation of NIH 3T3 cells as evidenced by anchorage-independent growth in soft agar, formation of transformed foci in tissue culture, and loss of contact inhibition. The plc transfectants displayed increased intracellular levels of diacylglycerol and phosphocholine. Expression of B. cereus PC-PLC was confirmed by immunoperoxidase and immunofluorescence staining with an affinity-purified anti-PC-PLC antibody. The NIH 3T3 clones expressing plc induced DNA synthesis, progressed through the cell cycle in the absence of added mitogens, and showed significant growth in low-concentration serum. Transfection with an antisense plc expression vector led to a loss of PC-PLC expression accompanied by a complete reversion of the transformed phenotype, suggesting that plc expression was required for maintenance of the transformed state. Taken together, our results show that chronic stimulation of PC hydrolysis by an unregulated PC-PLC enzyme is oncogenic to NIH 3T3 cells.

Protein kinase C-dependent stimulation of phospholipase D in phospholipase C-treated fibroblasts

Treatment of [14C]choline- or [14C]ethanolamine-labeled NIH 3T3 fibroblasts with Bacillus cereus phosphatidyl-choline-specific phospholipase C (PLC) enhanced phospholipase D (PLD)-mediated hydrolysis of the respective 14C-labeled phospholipids. PLD activity was stimulated by 1.5 U/mL of PLC and by 100 nM of the protein kinase C (PKC) activator phorbol 12-myristate 13-acetate (PMA) to similar extents. Treatment of [14C]palmitic acid-labeled fibroblasts with PLC in the presence of ethanol also enhanced PLD-mediated formation of phosphatidylethanol; the effects of PLC and PMA were nonadditive. PLC had no effect on PLD activity in fibroblasts in which PKC was down-regulated by prolonged (24 h) treatment with 300 nM PMA. These data indicate that treatment of fibroblasts with exogenous PLC results in PKC-dependent activation of PLD.

Phosphatidylcholine hydrolysis and c-myc expression are in collaborating mitogenic pathways activated by colony-stimulating factor 1

Stimulation of diglyceride production via phospholipase C (PLC) hydrolysis of phosphatidylcholine was an early event in the mitogenic action of colony-stimulating factor 1 (CSF-1) in the murine macrophage cell line BAC1.2F5 and was followed by a second phase of diglyceride production that persisted throughout the G1 phase of the cell cycle. Addition of phosphatidylcholine-specific PLC (PC-PLC) from Bacillus cereus to the medium of quiescent cells raised the intracellular diglyceride concentration and stimulated [3H]thymidine incorporation, although PC-PLC did not support continuous proliferation. PC-PLC treatment did not induce tyrosine phosphorylation or turnover of the CSF-1 receptor. The major protein kinase C (PKC) isotype in BAC1.2F5 cells was PKC-delta. Diglyceride production from PC-PLC did not target PKC-delta, since unlike phorbol esters, PC-PLC treatment neither decreased the electrophoretic mobility of PKC-delta nor increased the amount of GTP bound to Ras, and PC-PLC was mitogenically active in BAC1.2F5 cells in which PKC-delta was downregulated by prolonged treatment with phorbol ester. PC-PLC mimicked CSF-1 action by elevating c-fos and junB mRNAs to 40% of the level induced by CSF-1; however, PC-PLC induced c-myc mRNA to only 5% of the level in CSF-1-stimulated cells. PC-PLC addition to CSF-1-dependent BAC1.2F5 clones that constitutively express c-myc increased [3H]thymidine incorporation to 86% of the level evoked by CSF-1 and supported slow growth in the absence of CSF-1. Therefore, PC-PLC is a component of a signal transduction pathway leading to transcription of c-fos and junB that collaborates with c-myc and is independent of PKC-delta and Ras activation.

Phosphatidylcholine hydrolysis and c-myc expression are in collaborating mitogenic pathways activated by colony-stimulating factor 1

Stimulation of diglyceride production via phospholipase C (PLC) hydrolysis of phosphatidylcholine was an early event in the mitogenic action of colony-stimulating factor 1 (CSF-1) in the murine macrophage cell line BAC1.2F5 and was followed by a second phase of diglyceride production that persisted throughout the G1 phase of the cell cycle. Addition of phosphatidylcholine-specific PLC (PC-PLC) from Bacillus cereus to the medium of quiescent cells raised the intracellular diglyceride concentration and stimulated [3H]thymidine incorporation, although PC-PLC did not support continuous proliferation. PC-PLC treatment did not induce tyrosine phosphorylation or turnover of the CSF-1 receptor. The major protein kinase C (PKC) isotype in BAC1.2F5 cells was PKC-delta. Diglyceride production from PC-PLC did not target PKC-delta, since unlike phorbol esters, PC-PLC treatment neither decreased the electrophoretic mobility of PKC-delta nor increased the amount of GTP bound to Ras, and PC-PLC was mitogenically active in BAC1.2F5 cells in which PKC-delta was downregulated by prolonged treatment with phorbol ester. PC-PLC mimicked CSF-1 action by elevating c-fos and junB mRNAs to 40% of the level induced by CSF-1; however, PC-PLC induced c-myc mRNA to only 5% of the level in CSF-1-stimulated cells. PC-PLC addition to CSF-1-dependent BAC1.2F5 clones that constitutively express c-myc increased [3H]thymidine incorporation to 86% of the level evoked by CSF-1 and supported slow growth in the absence of CSF-1. Therefore, PC-PLC is a component of a signal transduction pathway leading to transcription of c-fos and junB that collaborates with c-myc and is independent of PKC-delta and Ras activation.