Role In Phospholipid Metabolism Research Articles

Efektivitas Citicoline sebagai Agen Neuroprotektif dalam Tata Laksana Traumatic Brain Injury

Background: Traumatic brain injury (TBI) is a condition resulting from physical trauma to the head which can cause structural damage to the brain and interfere with cognitive, motor and emotional functions. Citicoline is a natural compound that plays an important role in phospholipid metabolism and neurotransmitter synthesis in the brain. This evidence-based case report (EBCR) study aims to provide scientific evidence of the potential benefits of citicoline in improving clinical outcomes in TBI patients. Method: Literature search using the PICO format. The formulation of clinical questions was arranged by case. Search for articles on March 6–7th 2023 using the Publish or Perish (PoP) software with the target of searching articles for the last 10 years (2013-2023) on the Scopus, Pubmed, Google Scholar databases, and manual searches on the Cochrane database, using the keyword 'traumatic brain' injury' AND 'therapy' AND 'citicoline' AND 'neuroprotective' AND 'treatment' AND 'outcome' along with synonyms and related terms. Results: Four studies were obtained, with the level of evidence 1a which resulted from a meta-analysis or systematic review of various randomized controlled trials, which were then critically reviewed. Two studies reported no significance of clinical assessment and the Glasgow Coma Scale (GCS); two other studies showed significant results at the level of independence. Conclusion: This study shows that citicoline is beneficial in the independence of TBI patients but does not produce a significant difference in the Glasgow Coma Scale assessment.

Ablation of pigment epithelium-derived factor receptor (PEDF-R/Pnpla2) causes photoreceptor degeneration

Photoreceptor cells express the patatin-like phospholipase domain-containing 2 (PNPLA2) gene that codes for pigment epithelium-derived factor receptor (PEDF-R) (also known as ATGL). PEDF-R exhibits phospholipase activity that mediates the neurotrophic action of its ligand PEDF. Because phospholipids are the most abundant lipid class in the retina, we investigated the role of PEDF-R in photoreceptors by generating CRISPR Pnpla2 knock-out mouse lines in a retinal degeneration-free background. Pnpla2−/− mice had undetectable retinal Pnpla2 gene expression and PEDF-R protein levels as assayed by RT-PCR and immunofluorescence, respectively. The photoreceptors of mice deficient in PEDF-R had deformities as examined by histology and transmission electron microscopy. Pnpla2 knockdown diminished the PLA2 enzymatic activity of PEDF-R in the retina. Lipidomic analyses revealed the accumulation of lysophosphatidyl choline-DHA and lysophosphatidyl ethanolamine-DHA in PEDF-R-deficient retinas, suggesting a possible causal link to photoreceptor dysfunction. Loss of PEDF-R decreased levels of rhodopsin, opsin, PKCα, and synaptophysin relative to controls. Pnpla2−/− photoreceptors had surface-exposed phosphatidylserine, and their nuclei were TUNEL positive and condensed, revealing an apoptotic onset. Paralleling its structural defects, PEDF-R deficiency compromised photoreceptor function in vivo as indicated by the attenuation of photoreceptor a- and b-waves in Pnpla2−/− and Pnpla2+/− mice relative to controls as determined by electroretinography. In conclusion, ablation of PEDF-R in mice caused alteration in phospholipid composition associated with malformation and malperformance of photoreceptors. These findings identify PEDF-R as an important component for photoreceptor structure and function, highlighting its role in phospholipid metabolism for retinal survival and its consequences.

Disruption in phosphate transport affects membrane lipid and lipid droplet homeostasis in Saccharomyces cerevisiae.

Phosphate plays a crucial role in phospholipid metabolism and it is transported by the phosphate (Pi) transporters. Phospholipids are building blocks of the cell membrane, and essential for cell growth; however, the role of phosphate transporters in lipid metabolism remains elusive. The present study shows that the deletion of Pi transporters exhibited an increase in both phospholipid and neutral lipid levels when compared to wild type. The mRNA expressions of genes involved in phospholipid synthesis (CKI1, EKI1, CHO2, and OPI3) were increased due to de-repression of the transcription factors (INO2 and INO4). Neutral lipid levels (triacylglycerol and sterol ester) and their synthesizing genes (LRO1, ARE2, ACC1, and FAS1) were also increased, resulting in lipid droplet accumulation in Pi transporter mutants. Interestingly, phospholipase (PLC1) and histone acetyltransferase genes (ESA1, EAF1, YNG1, YNG2, and GCN5) were also found to be significantly increased, leading to dysregulation of lipid levels in Pi transporter mutants. In summary, our results suggest that the Pi transporters are involved in lipid droplet and membrane lipid homeostasis.

Mutational Analysis of OsPLDα1 Reveals Its Involvement in Phytic Acid Biosynthesis in Rice Grains.

Phospholipids and phytic acid are important phosphorus (P)-containing compounds in rice grains. Phytic acid is considered as a major antinutrient, because the negatively charged phytic acid chelates cations, including essential micronutrients, and decreases their bioavailability to human beings and monogastric animals. To gain an insight into the interplay of these two kinds of phosphorus-containing metabolites, we used the CRISPR/Cas9 system to generate mutants of a phospholipase D gene (OsPLDα1) and analyzed the mutational effect on metabolites, including phytic acid in rice grains. Metabolic profiling of two ospldα1 mutants revealed depletion in the phosphatidic acid production and lower accumulation of cytidine diphosphate diacylglycerol and phosphatidylinositol. The mutants also showed significantly reduced phytic acid content as compared to their wild-type parent, and the expression of the key genes involved in the phytic acid biosynthesis was altered in the mutants. These results demonstrate that OsPLDα1 not only plays an important role in phospholipid metabolism but also is involved in phytic acid biosynthesis, most probably through the lipid-dependent pathway, and thus revealed a potential new route to regulate phytic acid biosynthesis in rice.

Choline transport for phospholipid synthesis: An emerging role of choline transporter-like protein 1.

This review will provide a summary of recent advances in choline transport research and highlight important novel areas of focus in the field.

LPCAT3 deficiency in hematopoietic cells alters cholesterol and phospholipid homeostasis and promotes atherosclerosis

Background and aimsLPCAT3 plays a major role in phospholipid metabolism in the liver and intestine. However, the impact of LPCAT3 on hematopoietic cell and macrophage functions has yet to be described. Our aim was to understand the functions of LPCAT3 in macrophages and to investigate whether LPCAT3 deficiency in hematopoietic cells may affect atherosclerosis development. MethodsMice with constitutive Lpcat3 deficiency (Lpcat3−/−) were generated. We used fetal hematopoietic liver cells to generate WT and Lpcat3−/− macrophages in vitro and to perform hematopoietic cell transplantation in recipient Ldlr−/− mice. ResultsLpcat3-deficient macrophages displayed major reductions in the arachidonate content of phosphatidylcholines, phosphatidylethanolamines and, unexpectedly, plasmalogens. These changes were associated with altered cholesterol homeostasis, including an increase in the ratio of free to esterified cholesterol and a reduction in cholesterol efflux in Lpcat3−/− macrophages. This correlated with the inhibition of some LXR-regulated pathways, related to altered cellular availability of the arachidonic acid. Indeed, LPCAT3 deficiency was associated with decreased Abca1, Abcg1 and ApoE mRNA levels in fetal liver cells derived macrophages. In vivo, these changes translated into a significant increase in atherosclerotic lesions in Ldlr−/− mice with hematopoietic LPCAT3 deficiency. ConclusionsThis study identifies LPCAT3 as a key factor in the control of phospholipid homeostasis and arachidonate availability in myeloid cells and underlines a new role for LPCAT3 in plasmalogen metabolism. Moreover, our work strengthens the link between phospholipid and sterol metabolism in hematopoietic cells, with significant consequences on nuclear receptor-regulated pathways and atherosclerosis development.

Ageing sensitized by iPLA2β deficiency induces liver fibrosis and intestinal atrophy involving suppression of homeostatic genes and alteration of intestinal lipids and bile acids

Ageing is a major risk factor for various forms of liver and gastrointestinal (GI) disease and genetic background may contribute to the pathogenesis of these diseases. Group VIA phospholipase A2 or iPLA2β is a homeostatic PLA2 by playing a role in phospholipid metabolism and remodeling. Global iPLA2β−/− mice exhibit aged-dependent phenotypes with body weight loss and abnormalities in the bone and brain. We have previously reported the abnormalities in these mutant mice showing susceptibility for chemical-induced liver injury and colitis. We hypothesize that iPLA2β deficiency may sensitize with ageing for an induction of GI injury. Male wild-type and iPLA2β−/− mice at 4 and 20–22months of age were studied. Aged, but not young, iPLA2β−/−mice showed increased hepatic fibrosis and biliary ductular expansion as well as severe intestinal atrophy associated with increased apoptosis, pro-inflammation, disrupted tight junction, and reduced number of mucin-containing globlet cells. This damage was associated with decreased expression of intestinal endoplasmic stress XBP1 and its regulator HNF1α, FATP4, ACSL5, bile-acid transport genes as well as nuclear receptors LXRα and FXR. By LC/MS-MS profiling, iPLA2β deficiency in aged mice caused an increase of intestinal arachidonate-containing phospholipids concomitant with a decrease in ceramides. By the suppression of intestinal FXR/FGF-15 signaling, hepatic bile-acid synthesis gene expression was increased leading to an elevation of secondary and hydrophobic bile acids in liver, bile, and intestine. In conclusions, ageing sensitized by iPLA2β deficiency caused a decline of key intestinal homeostatic genes resulting in the development of GI disease in a gut-to-liver manner.

Chronic intermittent hypoxia affects the cytosolic phospholipase A2α/cyclooxygenase 2 pathway via β2-adrenoceptor-mediated ERK/p38 stimulation.

Cardiac resistance against acute ischemia/reperfusion (I/R) injury can be enhanced by adaptation to chronic intermittent hypoxia (CIH), but the changes at the molecular level associated with this adaptation are still not fully explored. Phospholipase A2 (PLA2) plays an important role in phospholipid metabolism and may contribute to membrane destruction under conditions of energy deprivation during I/R. The aim of this study was to determine the effect of CIH (7000m, 8h/day, 5weeks) on the expression of cytosolic PLA2α (cPLA2α) and its phosphorylated form (p-cPLA2α), as well as other related signaling proteins in the left ventricular myocardium of adult male Wistar rats. Adaptation to CIH increased the total content of cPLA2α by 14% in myocardial homogenate, and enhanced the association of p-cPLA2α with the nuclear membrane by 85%. The total number of β-adrenoceptors (β-ARs) did not change but the β2/β1 ratio markedly increased due to the elevation of β2-ARs and drop in β1-ARs. In parallel, the amount of adenylyl cyclase decreased by 49% and Giα proteins increased by about 50%. Besides that, cyclooxygenase 2 (COX-2) and prostaglandin E2 (PGE2) increased by 36 and 84%, respectively. In parallel, we detected increased phosphorylation of protein kinase Cα, ERK1/2 and p38 (by 12, 48 and 19%, respectively). These data suggest that adaptive changes induced in the myocardium by CIH may include activation of cPLA2α and COX-2 via β2-AR/Gi-mediated stimulation of the ERK/p38 pathway.

IPLA2β deficiency attenuates obesity and hepatic steatosis in ob/ob mice through hepatic fatty-acyl phospholipid remodeling

PLA2G6 or GVIA calcium-independent PLA2 (iPLA2β) is identified as one of the NAFLD modifier genes in humans, and thought to be a target for NAFLD therapy. iPLA2β is known to play a house-keeping role in phospholipid metabolism and remodeling. However, its role in NAFLD pathogenesis has not been supported by results obtained from high-fat feeding of iPLA2β–null (PKO) mice. Unlike livers of human NAFLD and genetically obese rodents, fatty liver induced by high-fat diet is not associated with depletion of hepatic phospholipids. We therefore tested whether iPLA2β could regulate obesity and hepatic steatosis in leptin-deficient mice by cross-breeding PKO with ob/ob mice to generate ob/ob–PKO mice. Here we observed an improvement in ob/ob–PKO mice with significant reduction in serum enzymes, lipids, glucose, insulin as well as improved glucose tolerance, and reduction in islet hyperplasia. The improvement in hepatic steatosis measured by liver triglycerides, fatty acids and cholesterol esters was associated with decreased expression of PPARγ and de novo lipogenesis genes, and the reversal of β-oxidation gene expression. Notably, ob/ob livers contained depleted levels of lysophospholipids and phospholipids, and iPLA2β deficiency in ob/ob–PKO livers lowers the former, but replenished the latter particularly phosphatidylethanolamine (PE) and phosphatidylcholine (PC) that contained arachidonic (AA) and docosahexaenoic (DHA) acids. Compared with WT livers, PKO livers also contained increased PE and PC containing AA and DHA. Thus, iPLA2β deficiency protected against obesity and ob/ob fatty liver which was associated with hepatic fatty-acyl phospholipid remodeling. Our results support the deleterious role of iPLA2β in severe obesity associated NAFLD.

Modulatory Effect of Phytoestrogens and Curcumin on Induction of Annexin 1 in Human Peripheral Blood Mononuclear Cells and their Inhibitory Effect on Secretory Phospholipase A<sub>2</sub>

Purpose: To investigate the modulatory effects of phytoestrogens (coumestrol, daidzein and genistein) and curcumin on the induction and secretion of annexin-1 (ANXA-1) in human peripheral blood mononuclear cells (PBMCs) under inflammatory and non-inflammatory conditions, as well as their effect on the activity of phospholipase A2-V (sPLA2-V). Methods: The modulatory effects of phytoestrogens and curcumin on the induction ofANXA1 were investigated via sandwich ELISA method, while their effects on the activity of sPLA2-V were determined by photometric assays. Besides, the cell viability of these compounds was determined by standard trypan blue exclusion method using PBMCs. Results: The results indicate a significant increase (p < 0.05) in the total content ofANXA1, particularly by coumestrol (p < 0.01), in both inflammatory and non-inflammatory cells. Besides, the compounds also exhibited a dose-dependent inhibition of sPLA2-V activity; however, among these compounds, curcumin and genistein were the strongest inhibitors with an IC50 value of 11.1 ± 0.3 µM and13.6 ± 0.6 µM respectively. Conclusion: The investigated compounds have a potential to induce synthesis and secretion ofANXA1 as well as inhibitory activity of sPLA2-V, suggesting their inhibitory role in phospholipid metabolism and inflammation.

Interactions of Platinum and Ruthenium Coordination Complexes with Pancreatic Phospholipase A2 and Phospholipids Investigated by MALDI TOF Mass Spectrometry

Phospholipase A2 is involved in propagation of inflammatory processes and carcinogenesis through its role in phospholipid metabolism, and release of arachidonic acid and lysophospholipids. Recent findings on correlation between elevated PLA2 activity and metastatic cancer render this enzyme an attractive target for cancer therapy. On the other hand, due to a broad range of oxidation states under physiological conditions and a high affinity for protein binding, platinum and ruthenium coordination complexes are promising candidates for PLA2 inhibitors. In this article, we discuss the interactions of Pt and Ru coordination complexes with PLA2 and phospholipids, as well as the application of MALDI-TOF mass spectrometry for screening PLA2 inhibitors. Owing to the ability of this technique to simultaneously detect and monitor changes in substrate and product concentrations, the inhibitor mechanisms of both Pt and Ru complexes with various ligands were determined.

Conserved domains and evolution of secreted phospholipases A2

Secreted phospholipases A(2) (sPLA(2) s) are lipolytic enzymes present in organisms ranging from prokaryotes to eukaryotes but their origin and emergence are poorly understood. We identified and compared the conserved domains of 333 sPLA(2) s and proposed a model for their evolution. The conserved domains were grouped into seven categories according to the in silico annotated conserved domain collections of 'cd00618: PLA(2) _like' and 'pfam00068: Phospholip_A2_1'. PLA(2) s containing the conserved domain cd04706 (plant-specific PLA(2) ) are present in bacteria and plants. Metazoan PLA(2) s of the group (G) I/II/V/X PLA(2) collection exclusively contain the conserved domain cd00125. GIII PLA(2) s of both vertebrates and invertebrates contain the conserved domain cd04704 (bee venom-like PLA(2) ), and mammalian GIII PLA(2) s also contain the conserved domain cd04705 (similar to human GIII PLA(2) ). The sPLA(2) s of bacteria, fungi and marine invertebrates contain the conserved domain pfam09056 (prokaryotic PLA(2) ) that is the only conserved domain identified in fungal sPLA(2) s. Pfam06951 (GXII PLA(2) ) is present in bacteria and is widely distributed in eukaryotes. All conserved domains were present across mammalian sPLA(2) s, with the exception of cd04706 and pfam09056. Notably, no sPLA(2) s were found in Archaea. Phylogenetic analysis of sPLA(2) conserved domains reveals that two main clades, the cd- and the pfam-collection, exist, and that they have evolved via gene-duplication and gene-deletion events. These observations are consistent with the hypothesis that sPLA(2) s in eukaryotes shared common origins with two types of bacterial sPLA(2) s, and their persistence during evolution may be related to their role in phospholipid metabolism, which is fundamental for survival.

Role of Phospholipid Metabolism and G Protein in the Action Induced by Clostridium Perfringens Alpha-Toxin

Alpha-toxin (370 residues) of Clostridium perfringens is the key virulence determinant in gas gangrene and has also been implicated in the pathogenesis of sudden death syndrome in young animals. Alpha-toxin possesses phospholipase C (PLC), sphingomyelinase (SMase) and biological activities causing hemolysis and lethality. The structure of the toxin reveals two domains: the N-terminal domain containing the catalytic active site and the C-terminal domain involving the binding to membranes. Recent research data showed that alpha-toxin-induced biological activities are responsible for the activation of phospholipid metabolism via a pertussis toxin (PT)-sensitive GTP-binding protein, Gi. In this review, we summary the role of phospholipid metabolism and G protein in the biological activities induced by alpha-toxin. Discussed are activations of the arachidonic acid cascade (Section 1), the phospholipid metabolism (Section 2), the sphingomeylin metabolism (Section 3) and TrkA signaling (Section 4) induced by alpha-toxin.

The Saccharomyces cerevisiae PHM8 Gene Encodes a Soluble Magnesium-dependent Lysophosphatidic Acid Phosphatase

Phosphate is the essential macronutrient required for the growth of all organisms. In Saccharomyces cerevisiae, phosphatases are up-regulated, and the level of lysophosphatidic acid (LPA) is drastically decreased under phosphate-starved conditions. The reduction in the LPA level is attributed to PHM8, a gene of unknown function. phm8Delta yeast showed a decreased LPA-hydrolyzing activity under phosphate-limiting conditions. Overexpression of PHM8 in yeast resulted in an increase in the LPA phosphatase activity in vivo. In vitro assays of the purified recombinant Phm8p revealed magnesium-dependent LPA phosphatase activity, with maximal activity at pH 6.5. The purified Phm8p did not hydrolyze any lipid phosphates other than LPA. In silico analysis suggest that Phm8p is a soluble protein with no transmembrane domain. Site-directed mutational studies revealed that aspartate residues in a DXDXT motif are important for the catalysis. These findings indicated that LPA plays a direct role in phosphate starvation. This is the first report of the identification and characterization of magnesium-dependent soluble LPA phosphatase.

Dexamethasone enhances phospholipase D activity in M-1 cells.

Phospholipase D (PLD) is an enzyme involved in signal transduction and widely distributed in mammalian cells. The signal transduction pathways and role for phospholipid metabolism during hormonal response in cortical collecting duct remain partly undefined. It has been reported that dexamethasone increases transepithelial transport in M-1 cells that are derived from the mouse cortical collecting duct. We investigated the expression and activity of PLD in M-1 cells. Basal PLD activity of M-1 cells cultured in the presence of dexamethasone (5 microM) was higher than in the absence of dexamethasone. Dexamethasone and ATP activated PLD in M-1 cells but phorbol ester did not stimulate PLD activity. Vasopressin, bradykinin, dibutyryl cyclic AMP, and ionomycin were ineffective in activating PLD of the cells. The PLD2 isotype was detected by immunoprecipitation but PLD1 was not detected in M-1 cells. Addition of GTPgammaS and ADP-ribosylation factor or phosphatidylinositiol 4,5-bisphosphate to digitonin-permeabilized cells did not augment PLD activity. In intact cells PLD activity was increased by sodium oleate but there was no significant change between dexamethasone treated- and untreated cells by oleate. These results suggest that at least two types of PLD are present in M-1 cells and PLD plays a role in the corticosteroid-mediated response of cortical collecting duct cells.

Dermatophyte lipids-Composition and regulation of phospholipids.

This review presents the extensive work carried out on lipid components of dermatophytes, their biosynthesis, turnover and regulation. It emerges from the work done so far that the pathways of lipid biosynthesis/ degradation and the lipid composition in dermatophytes are similar to those in yeasts and other fungi. Second messengers (Ca(2+), cAMP) were demonstrated to have a regulatory role in phospholipid metabolism and they mainly act by stimulating Ca(2+)/CaM or cAMP dependent protein kinase(s). Both these kinases were purified and characterized inMicrosporum gypseum. Further work is being carried out to elucidate the molecular mechanism of regulation of phospholipid metabolism by these second messengers.

Differential expression of type II, IV and cytosolic PLA2 messenger RNA in human intrauterine tissues at term.

The involvement of phospholipase A2 (PLA2) enzymes in the formation of biologically-active phospholipid metabolites by human gestational tissues has principally been characterized by the use of enzyme activity assays. While such assays have established the presence of functional PLA2 activity, there is a paucity of information concerning the tissue distribution and relative contribution to net activity made by specific PLA2 isozymes. In particular, both secretory and cytosolic isozymes may be involved in gestational tissue phospholipid metabolism. Thus, the aim of this study was to test the hypothesis that phospholipase A2 mRNA transcripts encoding Type II, Type IV and cytosolic PLA2 are tissue-specifically expressed in human amnion, choriodecidua and placenta obtained at term. The relative expression of polyA+ mRNA encoding these PLA2 isozymes was determined by Northern blot analysis and laser densitometry. The data obtained confirm the tissue-specific expression of PLA2 mRNA in human intrauterine tissues. Cytosolic PLA2 mRNA was most abundantly expressed in amnion when compared to either choriodecidua (which was 5-fold less than amnion; P < 0.001) or placenta (72-fold less than amnion; P < 0.001). In contrast, the secretory PLA2 mRNA transcripts (i.e. Type II and Type IV) were most abundantly expressed in placenta. Type II PLA2 mRNA expression in choriodecidua and amnion was 30-fold less than that observed in placenta (both P < 0.001). Type IV PLA2 mRNA expression was 37-fold (P < 0.001) and 73-fold (P < 0.001) less in choriodecidua and amnion respectively. These data support the conclusion that cytosolic PLA2 is the principal PLA2 isozyme mediating phospholipid metabolism and the liberation of fatty acid substrate (i.e. arachidonic acid) in term amnion, while secretory PLA2 isozymes, and in particular, Type II PLA2 play a major role in phospholipid metabolism in term placenta.

Bradykinin highlights the role of phospholipid metabolism in the control of nerve excitability

Bradykinin highlights the role of phospholipid metabolism in the control of nerve excitability

Elevated levels of diacylglycerol and decreased phorbol ester sensitivity in ras-transformed fibroblasts.

Diacylglycerol (DG) plays a central role in phospholipid metabolism and is an endogenous activator of protein kinase C. We have suggested that constitutive activation of this kinase is one mechanism by which oncogenes transform cells. The ras-encoded proteins are similar to regulatory G-proteins and are candidates for the unknown G-protein that modulates phosphatidylinositol (PI) turnover. Differences in polyphosphoinositide metabolism have been reported for ras-transformed cells. But because these experiments were performed on confluent cultures of established cell lines, the differences are difficult to attribute to ras transformation. Here we show that exponentially growing NIH 3T3 fibroblasts recently transformed by Ha-ras or Ki-ras possess elevated DG concentrations without significant alterations in the levels of other polyphosphoinositide metabolites. The basal phosphorylation of protein kinase C substrate of relative molecular mass (Mr) 80,000 (80K) is significantly increased in all the ras-transformed cell lines. Surprisingly, however, further phosphorylation of this protein on addition of phorbol ester was greatly reduced. Ha-ras cells also show less binding of phorbol ester than control cells, suggesting that elevation of DG causes partial down-regulation in addition to activation of protein kinase C.

An update on the role of phospholipid metabolism in the action of steroidogenic agents

Most steroidogenic agents which bind to cell surface receptors activate adenylate cyclase and/or phospholipase C. Activation of either signaling system may also be associated with rapid increases in de novo phosopholipid synthesis, hut it is at present uncertain whether this is a secondary or parallel event. Activation of phospholipase C leads to hydrolysis of phosphatidylinositol-4',5'-PO 4 (PIP 2) and generation of two second messengers, inositol-triphosphate and diacylglycerol (DAG), which mobilize Ca 2+ and activate protein kinase C, respectively. Increases in de novo phospholipid synthesis lead to rapid increases in phosphatidic acid, DAG and C-kinase activity. The PIP 2-phospholipase C system appears to initiate the steroidogenic response to certain agents, such as angiotensin-II, and this may be amplified by concomitant increases in phospholipid synthesis. With other agonists, the role of phospholipase C activation and de novo phospholipid (and DAG) synthesis is less certain. In some tissues, activation of protein kinase C by exogeneously added DAG analogues provokes an increase in steroidogenesis. However, this is not observed in other tissues, and it is uncertain whether this rules out involvement of the C-kinase system for steroidogenesis in these tissues, or whether endogenously produced DAG is a more effective activator of the relevant C-kinase system then exogenously added DAG analogues. The role of other potential intracellular signaling substances that may be derived from phospholipase C activation and de novo phospholipid synthesis is also at present uncertain, as are the interrelationships between these two phospholipid responses, cyclic nucleotides, and other steroidogenic factors.