Phosphatidylcholine Transfer Protein Research Articles

RUNX1 Isoforms Regulate RUNX1 and Target-Genes Differentially in Platelets/Megakaryocytes: Association with Clinical Cardiovascular Events

RUNX1 is an essential transcription factor for hematopoiesis. Germline RUNX1 haplodeficiency is associated with thrombocytopenia, platelet dysfunction and predisposition to myeloid malignancies. Three major RUNX1 isoforms (A, B and C) are recognized and share a DNA binding RUNT domain; RUNX1A lacks the transactivation domain. Two distinct promoters regulate RUNX1 expression: distal P1 regulates RUNX1C; proximal P2 regulates RUNX1B and RUNX1A. Little is known regarding the differential effects of isoforms in RUNX1 autoregulation, target-gene regulation and association with clinical events. We performed studies in megakaryocytic HEL cells, HeLa cells (which do not express RUNX1) and in platelets of healthy subjects (n=74) and provide evidence for isoform-specific differential autoregulation and target-gene regulation by RUNX1. We show an association between expression of specific RUNX1-regulated genes in whole blood with cardiovascular events in patients. There are 5 RUNX1 consensus binding sites in P1 promoter and 1 in P2 promoter within ~1000 bp from ATG. Wild type RUNX1 P1 and P2 promoter regions were individually cloned into PGL4 luciferase promoter vector. ChIP studies using PMA-treated megakaryocytic HEL cells showed RUNX1 binding to chromatin regions encompassing the RUNX1 binding sites in both promoters. Mutations of 2 RUNX1 binding sites in intron region of P1 promoter reduced promoter activity; mutations of first 2 RUNX1 binding sites at the exon region of P1 promoter increased activity. Mutation of single RUNX1 binding site in P2 promoter increased promoter activity. Thus, RUNX1 binds to P1 and P2 promoters to regulate activities. To examine the RUNX1 autoregulation by individual isoforms, we co-transfected each isoform with P1 or P2 promoter vector in HeLa cells, which do not express endogenous RUNX1. In response to RUNX1B over-expression, both P1 and P2 promoters showed a dose-dependent decrease in promoter activity. RUNX1C over-expression increased P1 and P2 promoter activities. Thus, RUNX1B and RUNX1C regulate P1 and P2 promoters differentially. In HEL cells, which have endogenous RUNX1, RUNX1B overexpression decreased RUNX1C and RUNX1A mRNA/protein expression by ~50%. RUNX1C overexpression increased RUNX1B and RUNX1A mRNA/protein expression. Thus, RUNX1B decreases RUNX1C and RUNX1A expression; RUNX1C increases RUNX1B and RUNX1A. We studied the regulation by RUNX1 isoforms of target genes PCTP (phosphatidylcholine transfer protein), MYL9 (myosin light chain), PDE5A (phosphodiesterase 5A) and F13A (factor XIIIA) . In HeLa cells. RUNX1B overexpression increased PCTP, MYL9 and PDE5A (protein and mRNA); RUNX1C overexpression reduced PCTP and MYL9. In HEL cells RUNX1B increased PCTP, MYL9, PDE5A and F13A expression; RUNX1C reduced PCTP and MYL9 expression. To obtain evidence in vivo, we studied the relationships between RUNX1 isoforms and their relationships to RUNX1-target genes by RNAseq in leukocyte-poor platelets (74 healthy donors; ages 30-75 yrs). RUNX1B correlated negatively with RUNX1C (R= -0.413 and p=1.5e-06) and RUNX1A (R= -0.493 and p=0.00e+oo); RUNX1C correlated positively with RUNX1A (R=0.45, p<0.0001). RUNX1B correlated positively with PCTP, F131A, PDE5A and RAB1B, and negatively with MYL9 (Figure 1); no significant relationships were observed with RUNX1C or RUNX1A. To understand the relationships to cardiovascular events (MI or death), we explored whole blood gene expression using microarrays in 587 patients (2 cohorts, 190 and 397 patients) presenting to cardiac catherization for chest pain and followed for 3.8 years. In our previous studies RUNX1B transcripts were higher and RUNX1C lower in those with MI or death. We now found that higher expressions of F13A1(Figure 2) and RAB31, which have been shown by us to be RUNX1-targets, were associated with cardiovascular events. Conclusions: RUNX1 isoforms autoregulate RUNX1 expression differentially and regulate target genes in a differential manner, a pattern consistent with in vivo platelet gene expression. The relative expression of RUNX1B and RUNX1C isoforms and of their target genes associate with clinical cardiovascular events. RUNX1-isoform specific effects need to be considered in clinical scenarios where RUNX1 is involved and in its pharmacologic modulation.

Ligand dependent interaction between PC-TP and PPARδ mitigates diet-induced hepatic steatosis in male mice

Phosphatidylcholine transfer protein (PC-TP; synonym StarD2) is a soluble lipid-binding protein that transports phosphatidylcholine (PC) between cellular membranes. To better understand the protective metabolic effects associated with hepatic PC-TP, we generated a hepatocyte-specific PC-TP knockdown (L-Pctp−/−) in male mice, which gains less weight and accumulates less liver fat compared to wild-type mice when challenged with a high-fat diet. Hepatic deletion of PC-TP also reduced adipose tissue mass and decreases levels of triglycerides and phospholipids in skeletal muscle, liver and plasma. Gene expression analysis suggest that the observed metabolic changes are related to transcriptional activity of peroxisome proliferative activating receptor (PPAR) family members. An in-cell protein complementation screen between lipid transfer proteins and PPARs uncovered a direct interaction between PC-TP and PPARδ that was not observed for other PPARs. We confirmed the PC-TP– PPARδ interaction in Huh7 hepatocytes, where it was found to repress PPARδ-mediated transactivation. Mutations of PC-TP residues implicated in PC binding and transfer reduce the PC-TP-PPARδ interaction and relieve PC-TP-mediated PPARδ repression. Reduction of exogenously supplied methionine and choline reduces the interaction while serum starvation enhances the interaction in cultured hepatocytes. Together our data points to a ligand sensitive PC-TP– PPARδ interaction that suppresses PPAR activity.

Differential RUNX1-Isoform Specific Autoregulation of RUNX1 and Regulation of Target Genes (PCTP, MYL9) in Megakaryocytic Cells

RUNX1 is an essential transcription factor for hematopoiesis. Human germline RUNX1 haplodeficiency is associated with impaired megakaryocyte differentiation, thrombocytopenia, platelet dysfunction and predisposition to myeloid malignancies. The two major RUNX1 isoforms RUNX1C and RUNX1B differ at N-terminus by 14AA, and are regulated by two distinct promoters P1 and P2, respectively. RUNX1B and RUNX1C have distinct roles and tissue expression. Little is known regarding the differential effects of RUNX1 isoforms in RUNX1 autoregulation, and on their regulation of target genes. There are 5 RUNX1 consensus binding sites located in P1 promoter and one RUNX1 site in P2 promoter. To study RUNX1 promoters, both WT RUNX1 P1 and P2 promoter regions were individually cloned into PGL4 luciferase promoter vector. ChIP studies using PMA-treated megakaryocytic HEL cells showed RUNX1 binding to chromatin regions encompassing the RUNX1 binding sites in both promoters. Mutations of 2 RUNX1 binding sites in intron region of P1 promoter reduced promoter activity; mutations of first 2 RUNX1 binding sites in at the exon region of P1 promoter increased activity. Mutation of single RUNX1 binding site in P2 promoter increased promoter activity. Thus, RUNX1 binds to P1 and P2 promoters to regulate their activities. To examine the RUNX1 autoregulation by individual isoforms, we co-transfected each isoform with P1 or P2 promoter vector in HeLa cells, which express negligible endogenous RUNX1. In response to RUNX1B, P1 promoter showed a dose-dependent decrease in promoter activity. RUNX1C expression, increased P1 promoter activity in a dose-dependent manner. With constant RUNX1C expression, increase in RUNX1B co-expression led to a dose-dependent decrease in P1 promoter activity. In HeLa cells RUNX1B decreased P2 promoter activity in a dose-dependent manner. With constant RUNX1B expression, an increase in RUNX1C increased RUNX1 P2 promoter activity. Thus, RUNX1B and RUNX1C regulate both P1 and P2 promoters in a distinct manner. In studies in megakaryocytic HEL cells, which express endogenous RUNX1, RUNX1B overexpression decreased RUNX1C expression by 50%. RUNX1C overexpression increased by 2-fold RUNX1B protein. In studies using RUNX1C-specific antibody, and expression plasmids for hemagglutinin-tagged-RUNX1B and c-Myc-tagged RUNX1C, RUNX1B overexpression decreased RUNX1C; RUNX1C overexpression increased RUNX1B. These studies indicate that RUNX1 isoforms autoregulate in a differential manner at the protein level. We studied the effects of RUNX1 isoforms on two established downstream RUNX1 target genes PCTP (phosphatidylcholine transfer protein) and MYL9(myosin light chain) in HeLa cells. RUNX1B overexpression increased both PCTP and MYL9 protein on immunoblotting, while RUNX1C overexpression reduced both proteins. This suggests RUNX1B and RUNX1C have differential effects in regulating downstream genes. These findings are in line with our studies in whole blood from a large cohort of patients with CAD showing a positive correlation between PCTP (Circulation 2017136(10):927-939) with RUNX1B transcript levels and a negative relationship with those for RUNX1C. Conclusions: RUNX1 isoforms autoregulate RUNX1 expression differentially in an isoform-specific manner. Moreover, they regulate target genes (MYL9 and PCTP) in a differential manner. These RUNX1 isoform-specific effects may be important in understanding the consequences of germline RUNX1 haplodeficiency and in modulating RUNX1 levels for therapeutic purposes.

PMON184 Ligand-dependent interaction between phosphatidylcholine transfer protein and PPARδ: therapeutic implications for metabolic syndrome

Abstract Phosphatidylcholine transfer protein (PC-TP; synonym StarD2) is a soluble lipid binding protein that transports phosphatidylcholine (PC) between cellular membranes. PC-TP knockout mice are more sensitive to glucose and insulin, have increased beta oxidation and improved liver health compared to wild-type mice. To better understand the protective metabolic effects associated with PC-TP deletion, we investigated how hepatic PC-TP affects lipid metabolism, sensitivity to glucose and insulin, and energy homeostasis in a mouse model of diet-induced obesity. We generated a liver-specific PC-TP knockout mouse (L-Pctp-/-), which gains less weight and accumulates less liver fat compared to wild-type mice when challenged with a high fat diet. Hepatic deletion of PC-TP also reduces levels of triglycerides and phospholipids in skeletal muscle, suggesting that PC-TP in the liver influences lipid metabolism in the muscle. Gene expression analyses suggest that the observed metabolic changes are related to transcriptional activity of peroxisome proliferative activating receptor (PPAR) family members. An in-cell protein complementation screen between lipid transfer proteins and PPARs uncovered a novel, direct interaction between PC-TP and PPARδ that was not observed for other PPAR family members. We confirmed the PC-TP-PPARδ interaction in Huh7 hepatocytes, where it was found to repress PPARδ-mediated transactivation. Mutations of PC-TP residues implicated in PC binding and transfer combined with a reduction of exogenously supplied lipids through serum starvation relieves PC-TP mediated PPAR repression. Together our data points to a ligand sensitive PC-TP– PPARδ interaction that suppresses PPAR activity. Presentation: Monday, June 13, 2022 12:30 p.m. - 2:30 p.m.

PCTP contributes to human platelet activation by enhancing dense granule secretion

PCTP (phosphatidylcholine transfer protein) was discovered recently to regulate aggregation of human platelets stimulated with PAR4 activating peptide (PAR4AP). However, the role of PCTP following thrombin stimulation, the mechanisms by which PCTP contributes to platelet activation, and the role of PCTP with other receptors remained unknown. As mouse platelets do not express PCTP, we treated human platelets with various agonists in the presence of the specific PCTP inhibitor A1. We observed that PCTP inhibition significantly reduced dense granule secretion in response to thrombin, PAR1AP, PAR4AP, convulxin (GPVI agonist) and FcγRIIA crosslinking. In contrast, among these agonists, PCTP inhibition reduced aggregation only to low dose thrombin and PAR4AP. Unlike its effects on dense granule secretion, PCTP inhibition did not reduce alpha granule secretion in response to thrombin or PAR4AP. PCTP inhibition reduced both the increase in cytoplasmic Ca2+ as well as PKC activity downstream of thrombin. These data are consistent with PCTP contributing to secretion of dense granules, and to being particularly important to human PAR4 early signaling events. Future studies will address further these molecular mechanisms and consequences for hemostasis and thrombosis.

SAT-LB124 Phosphatidylcholine Transfer Protein Interacts With PPARδ to Modulate Activity

Obesity is one of the largest public health crises in the USA. Obesity can be lethal due to the development of cardiovascular complications such as, hypertension, heart attack, and stroke. Recently, peroxisome proliferator-activated receptor δ (PPARδ), the least-characterized member of the PPAR family of nuclear receptors (NRs), has shown great promise in treating obesity and associated cardiovascular complications. Recent reports have shown that PPARδ activation is tuned, in part, through an interaction with fatty acid binding protein 5 (FABP5) in a polyunsaturated Fatty acids (PUFAs) dependent manner. This enhancement is thought to occur due to ligand transfer, however, FABP5 only binds a subset of reported PPARδ ligands. To find other candidate lipid transport proteins (LTPs), we performed a protein complementation assay (PCA) between LTPs and NRs. We uncovered a novel interaction between PPARδ and phosphatidylcholine transfer protein (PC-TP), sensitive to cellular nutrient levels. Preliminary data show that this interaction opposes canonical PPARδ signaling, leading to a decrease in PPARδ transactivation in cells and isolated mice livers. This led me to hypothesize that PC-TP senses nutrient status through membrane composition. Specific PC molecular species drive PC-TP translocation to inhibit PPARδ transactivation of genes. Utilizing our novel PCA assay I have shown that the interaction is modulated in part by cellular levels of methionine and choline, as cells cultured in media depleted of methionine and choline (MCD) show a decreased in the interaction. Using the same assay, I have shown a requirement of full length PPARδ for the interaction with PC-TP. This analysis will be complimented by mutagenesis and chemical perturbations aimed to alter PC-TP and PPARδ function. Additionally, I have assayed the in vivo relevance of PC-TP interaction with PPARδ using livers harvested from PCTP-/- and littermate control mice fed a variety of diets. Preliminary RNA-seq characterization, show interesting alterations in gene expression that suggest more complex regulation between PC-TP and PPARδ in vivo. MCD, which was shown to reduce the inhibitory interaction between PC-TP and PPARδ in vitro, seems to lead to an increased effect on PPARδ transactivation when PC-TP is depleted. Analysis comparing the effect of diet within each genotype shows a loss of differential PPARδ regulation for both CHEA and KEGG analysis when comparing WT to PCTP -/- mice supporting a role for PC-TP in differential PPARδ regulation caused by MCD diet. These studies will further the understanding of how lipid homeostasis is sensed and maintained through PPARδ by interactions with two separate LTPs.

The phosphatidylcholine transfer protein StarD7 is important for myogenic differentiation in mouse myoblast C2C12 cells and human primary skeletal myoblasts

StarD7 is a phosphatidylcholine (PC)-specific lipid transfer protein essential for the maintenance of mitochondrial PC composition, morphogenesis, and respiration. Here, we studied the role of StarD7 in skeletal myoblast differentiation using mouse myoblast C2C12 cells and human primary myoblasts. Immunofluorescence and immuno-electron microscopy revealed that StarD7 was distributed in the cytosol, inner mitochondria space, and outer leaflet of the outer mitochondrial membrane in C2C12 cells. Unlike human kidney embryonic cell line HEK293 cells, the mitochondrial proteinase PARL was not involved in the processing and maturation of StarD7 in C2C12 cells. StarD7 was constantly expressed during myogenic differentiation of C2C12 cells. The siRNA-mediated knockdown of StarD7 in C2C12 cells and human primary myoblasts significantly impaired myogenic differentiation and reduced the expression of myomaker, myomerger and PGC-1α. The reduction in mitochondrial PC levels and oxygen consumption rates, decreased expression of myomaker, myomerger and PGC-1α, as well as impaired myogenic differentiation, were completely restored when the protein was reintroduced into StarD7-knockout C2C12 cells. These results suggest that StarD7 is important for skeletal myogenesis in mammals.

A potential role for lysophosphatidylcholine in the delivery of long chain polyunsaturated fatty acids to the fetal circulation

The mechanisms of placental transfer of long chain polyunsaturated fatty acids are poorly understood, however fatty acid transporters in the syncytiotrophoblast microvillous (MVM) and basal plasma membrane (BM) are believed to be involved. Using LC-MS/MS and samples from normal term pregnant women, we found that the concentration of lysophosphatidylcholine-docosahexaenoyl (DHA-LPC) was 4-fold higher in umbilical vein plasma compared to maternal venous concentrations while conversely phosphatidylcholine (PC) containing DHA or arachidonic acid were higher in maternal circulation. Using primary human trophoblast cells incubated with DHA we show that DHA was highly incorporated into PC and LPC species in the placenta. Protein expression of Phosphatidylcholine Transfer Protein (PCTP) was 14-fold higher in MVM and the expression of MFSD2a, an LPC transporter, was 5-fold higher in BM than in MVM. Interestingly, BM MFSD2a expression was positively correlated with DHA-LPC in the umbilical vein. These findings suggest that syncytiotrophoblast takes up PC from the maternal circulation, as well as preferentially incorporating DHA into PC. Placental PC are converted to LPC forms, which are transported across the BM mediated by MFSD2a, with a strong selectivity for DHA.

Thioesterase-mediated control of cellular calcium homeostasis enables hepatic ER stress.

The incorporation of excess saturated free fatty acids (SFAs) into membrane phospholipids within the ER promotes ER stress, insulin resistance, and hepatic gluconeogenesis. Thioesterase superfamily member 2 (Them2) is a mitochondria-associated long-chain fatty acyl-CoA thioesterase that is activated upon binding phosphatidylcholine transfer protein (PC-TP). Under fasting conditions, the Them2/PC-TP complex directs saturated fatty acyl-CoA toward β-oxidation. Here, we showed that during either chronic overnutrition or acute induction of ER stress, Them2 and PC-TP play critical roles in trafficking SFAs into the glycerolipid biosynthetic pathway to form saturated phospholipids, which ultimately reduce ER membrane fluidity. The Them2/PC-TP complex activated ER stress pathways by enhancing translocon-mediated efflux of ER calcium. The increased cytosolic calcium, in turn, led to the phosphorylation of calcium/calmodulin-dependent protein kinase II, which promoted both hepatic insulin resistance and gluconeogenesis. These findings delineate a mechanistic link between obesity and insulin resistance and establish the Them2/PC-TP complex as an attractive target for the management of hepatic steatosis and insulin resistance.

Transcription Factor RUNX1 Regulates Platelet PCTP (Phosphatidylcholine Transfer Protein): Implications for Cardiovascular Events: Differential Effects of RUNX1 Variants.

PCTP (phosphatidylcholine transfer protein) regulates the intermembrane transfer of phosphatidylcholine. Higher platelet PCTP expression is associated with increased platelet responses on activation of protease-activated receptor 4 thrombin receptors noted in black subjects compared with white subjects. Little is known about the regulation of platelet PCTP. Haplodeficiency of RUNX1, a major hematopoietic transcription factor, is associated with thrombocytopenia and impaired platelet responses on activation. Platelet expression profiling of a patient with a RUNX1 loss-of-function mutation revealed a 10-fold downregulation of the PCTP gene compared with healthy controls. We pursued the hypothesis that PCTP is regulated by RUNX1 and that PCTP expression is correlated with cardiovascular events. We studied RUNX1 binding to the PCTP promoter using DNA-protein binding studies and human erythroleukemia cells and promoter activity using luciferase reporter studies. We assessed the relationship between RUNX1 and PCTP in peripheral blood RNA and PCTP and death or myocardial infarction in 2 separate patient cohorts (587 total patients) with cardiovascular disease. Platelet PCTP protein in the patient was reduced by ≈50%. DNA-protein binding studies showed RUNX1 binding to consensus sites in ≈1 kB of PCTP promoter. PCTP expression was increased with RUNX1 overexpression and reduced with RUNX1 knockdown in human erythroleukemia cells, indicating that PCTP is regulated by RUNX1. Studies in 2 cohorts of patients showed that RUNX1 expression in blood correlated with PCTP gene expression; PCTP expression was higher in black compared with white subjects and was associated with future death/myocardial infarction after adjustment for age, sex, and race (odds ratio, 2.05; 95% confidence interval 1.6-2.7; P<0.0001). RUNX1 expression is known to initiate at 2 alternative promoters, a distal P1 and a proximal P2 promoter. In patient cohorts, there were differential effects of RUNX1 isoforms on PCTP expression with a negative correlation in blood between RUNX1 expressed from the P1 promoter and PCTP expression. PCTP is a direct transcriptional target of RUNX1. PCTP expression is associated with death/myocardial infarction in patients with cardiovascular disease. RUNX1 regulation of PCTP may play a role in the pathogenesis of platelet-mediated cardiovascular events.

Understanding phospholipid function: Why are there so many lipids?

In the 1970s, phospholipids were still considered mere building blocks of the membrane lipid bilayer, but the subsequent realization that phospholipids could also serve as second messengers brought new interest to the field. My own passion for the unique amphipathic properties of lipids led me to seek other, non-signaling functions for phospholipids, particularly in their interactions with membrane proteins. This seemed to be the last frontier in protein chemistry and enzymology to be conquered. I was fortunate to find my way to Eugene Kennedy's laboratory, where both membrane proteins and phospholipids were the foci of study, thus providing a jumping-off point for advancing our fundamental understanding of lipid synthesis, membrane protein biosynthesis, phospholipid and membrane protein trafficking, and the cellular roles of phospholipids. After purifying and characterizing enzymes of phospholipid biosynthesis in Escherichia coli and cloning of several of the genes encoding these enzymes in E. coli and Saccharomyces cerevisiae, I was in a position to alter phospholipid composition in a systematic manner during the cell cycle in these microorganisms. My group was able to establish, contrary to common assumption (derived from the fact that membrane proteins retain activity in detergent extracts) that phospholipid environment is a strong determining factor in the function of membrane proteins. We showed that molecular genetic alterations in membrane lipid composition result in many phenotypes, and uncovered direct lipid-protein interactions that govern dynamic structural and functional properties of membrane proteins. Here I present my personal "reflections" on how our understanding of phospholipid functions has evolved.

The Phosphatidylcholine Transfer Protein Stard7 is Required for Mitochondrial and Epithelial Cell Homeostasis

Mitochondria synthesize select phospholipids but lack the machinery for synthesis of the most abundant mitochondrial phospholipid, phosphatidylcholine (PC). Although the phospholipid transfer protein Stard7 promotes uptake of PC by mitochondria, the importance of this pathway for mitochondrial and cellular homeostasis represents a significant knowledge gap. Haploinsufficiency for Stard7 is associated with significant exacerbation of allergic airway disease in mice, including an increase in epithelial barrier permeability. To test the hypothesis that Stard7 deficiency leads to altered barrier structure/function downstream of mitochondrial dysfunction, Stard7 expression was knocked down in a bronchiolar epithelial cell line (BEAS-2B) and specifically deleted in lung epithelial cells of mice (Stard7epi∆/∆). Stard7 deficiency was associated with altered mitochondrial size and membrane organization both in vitro and in vivo. Altered mitochondrial structure was accompanied by disruption of mitochondrial homeostasis, including decreased aerobic respiration, increased oxidant stress, and mitochondrial DNA damage that, in turn, was linked to altered barrier integrity and function. Both mitochondrial and barrier defects were largely corrected by targeting Stard7 to mitochondria or treating epithelial cells with a mitochondrial-targeted antioxidant. These studies suggest that Stard7-mediated transfer of PC is crucial for mitochondrial homeostasis and that mitochondrial dysfunction contributes to altered barrier permeability in Stard7-deficient mice.

Phosphatidylcholine transfer protein/StarD2 promotes microvesicular steatosis and liver injury in murine experimental steatohepatitis.

Mice fed a methionine- and choline-deficient (MCD) diet develop steatohepatitis that recapitulates key features of nonalcoholic steatohepatitis (NASH) in humans. Phosphatidylcholine is the most abundant phospholipid in the surfactant monolayer that coats and stabilizes lipid droplets within cells, and choline is required for its major biosynthetic pathway. Phosphatidylcholine-transfer protein (PC-TP), which exchanges phosphatidylcholines among membranes, is enriched in hepatocytes. PC-TP also regulates fatty acid metabolism through interactions with thioesterase superfamily member 2. We investigated the contribution of PC-TP to steatohepatitis induced by the MCD diet. Pctp-/- and wild-type control mice were fed the MCD diet for 5 wk and were then euthanized for histopathologic and biochemical analyses, as well as determinations of mRNA and protein expression. Whereas all mice developed steatohepatitis, plasma alanine aminotransferase and aspartate aminotransferase activities were only elevated in wild-type mice, indicating that Pctp-/- mice were protected from MCD diet-induced hepatocellular injury. Reduced hepatotoxicity due to the MCD diet in the absence of PC-TP expression was further evidenced by decreased activation of c-Jun and reduced plasma concentrations of fibroblast growth factor 21. Despite similar total hepatic concentrations of phosphatidylcholines and other lipids, the relative abundance of microvesicular lipid droplets within hepatocytes was reduced in Pctp-/- mice. Considering that the formation of larger lipid droplets may serve to protect against lipotoxicity in NASH, our findings suggest a pathogenic role for PC-TP that could be targeted in the management of this condition.NEW & NOTEWORTHY Phosphatidylcholine-transfer protein (PC-TP) is a highly specific phosphatidylcholine-binding protein that we previously showed to regulate hepatocellular nutrient metabolism through its interacting partner thioesterase superfamily member 2 (Them2). This study identifies a pathogenic role for PC-TP, independent of Them2, in the methionine- and choline-deficient diet model of experimental steatohepatitis. Our current observations suggest that PC-TP promotes liver injury by mediating the intermembrane transfer of phosphatidylcholines, thus stabilizing more pathogenic microvesicular lipid droplets.

Identification of a functional genetic variant driving racially dimorphic platelet gene expression of the thrombin receptor regulator, PCTP

Platelet activation in response to stimulation of the Protease Activated Receptor 4 (PAR4) receptor differs by race. One factor that contributes to this difference is the expression level of Phosphatidylcholine Transfer Protein (PCTP), a regulator of platelet PAR4 function. We have conducted an expression Quantitative Trait Locus (eQTL) analysis that identifies single nucleotide polymorphisms (SNPs) linked to the expression level of platelet genes. This analysis revealed 26 SNPs associated with the expression level of PCTP at genome-wide significance (p < 5×10-8). Using annotation from ENCODE and other public data we prioritised one of these SNPs, rs2912553, for functional testing. The allelic frequency of rs2912553 is racially-dimorphic, in concordance with the racially differential expression of PCTP. Reporter gene assays confirmed that the single nucleotide change caused by rs2912553 altered the transcriptional potency of the surrounding genomic locus. Electromobility shift assays, luciferase assays, and overexpression studies indicated a role for the megakaryocytic transcription factor GATA1. In summary, we have integrated multi-omic data to identify and functionalise an eQTL. This, along with the previously described relationship between PCTP and PAR4 function, allows us to characterise a genotype-phenotype relationship through the mechanism of gene expression.

PCTP (Phosphatidylcholine Transfer Protein) is Regulated By RUNX1 in Platelets/Megakaryocytes and is Associated with Adverse Cardiovascular Events

Transcription factor (TF) mutations are increasingly recognized to play a major role in inherited platelet abnormalities. RUNX1, a major hematopoietic TF, acts in a combinatorial manner with other TFs to regulate numerous megakaryocyte (MK)/platelet genes. Human RUNX1 haplodeficiency is associated with thrombocytopenia, platelet function defects, and increased leukemia risk. We have described a patient with multiple abnormalities in platelet aggregation and secretion responses with a heterozygous RUNX1 nonsense mutation (Sun et al Blood 2004; 103; 948-54). Transcript expression profiling of patient platelets (Sun et al J Thromb Haemost 2007; 5:146-54)showed several genes were significantly downregulated, including myosin light chain (MYL9), platelet factor 4 (PF4), protein kinase C-θ (PRCKQ), and 12-lipoxygenase (ALOX12); these have been shown by us to be regulated by RUNX1. The profiling data also showed 10-fold downregulation of phosphatidylcholine transfer protein (PCTP) gene (fold change ratio 0.09, p=0.02) in the patient compared with normal controls. PCTP regulates the intermembrane transfer of phosphatidylcholine (PC), a major plasma membrane phospholipid. Platelet PCTP expression is associated with increased platelet aggregation and calcium mobilization upon activation of protease-activated receptor 4 (PAR4) thrombin receptors in black subjects as compared to white subjects (Edelstein et al Nat Med 2013; 19:1609-16). Pharmacologic inhibition of PCTP decreased platelet aggregation in response to PAR4 agonist and siRNA knockdown of PCTP in megakaryocytic cells blunted calcium mobilization induced by PAR4 (Edelstein et al Nat Med 2013; 19:1609-16). Little is known regarding the regulation of PCTP in MKs/platelets and its role in cardiovascular events.Based on the decreased platelet PCTP expression in our patient, we pursued the hypothesis that PCTP is regulated by RUNX1 and contributes to cardiovascular events. Corrected total cellular immunofluorescence with anti-PCTP antibody showed significantly reduced platelet PCTP expression by 58% in our patient compared to a normal control. In silico analysis revealed 5 RUNX1 consensus binding sites up to ~ 1 kb of the PCTP 5’ upstream region from ATG. To assess for interaction of RUNX1 with the PCTP promoter, chromatin immunoprecipitation (ChIP) assay with anti-RUNX1 antibody was performed using human erythroid leukemia (HEL) cells treated with phorbol 12-myristate 13-acetate (PMA) for 48 hours to induce megakaryocytic transformation. The ChIP studies showed RUNX1 binding to PCTP chromatin in the regions encompassing RUNX1 binding site 1 (-345/-340), site 3 (-632/-627), and encompassing sites 4 and 5 (-974/-969, -997/-992). Electrophoretic mobility shift assay (EMSA) using PMA-treated HEL cell nuclear extracts showed RUNX1 binding to DNA probes (28-37 bp) containing site 1 (-345/-340) and both sites 4 and 5 (-974/-969, -997/-992). PCTP mRNA and protein expression were increased with RUNX1 overexpression and reduced with RUNX1 knockdown in HEL cells, indicating that PCTP is regulated by RUNX1. To assess the clinical relevance of the findings, the relationship between RUNX1 and PCTP in peripheral blood RNA, and PCTP and death or myocardial infarction (MI) events were assessed in two separate patient cohorts (n = 587 total patients) with cardiovascular disease. RUNX1 is transcribed from two alternate promoters (P1 and P2) resulting in different isoforms. In both patient cohorts, there was strong correlation between RUNX1 and PCTP expression in a promoter specific manner. RUNX1 P1 probe sets were strongly and inversely correlated with PCTP expression (p < 0.0001), while the P2 probe sets were not. PCTP expression was associated with death or MI in both patient cohorts (odds ratio 2.1, 95% CI [1.61-2.95], P-value < 0.0001) independent of age, sex, race, platelet count, and cardiovascular risk factors.Conclusions: Our results provide evidence that PCTP is regulated by RUNX1 (potentially in a promoter specific manner), and that PCTP expression is associated with death or myocardial infarction in patients with cardiovascular disease. RUNX1 regulation of PCTP may play a role in the pathogenesis of platelet-mediated cardiovascular events. DisclosuresNo relevant conflicts of interest to declare.

Structural elements that govern Sec14-like PITP sensitivities to potent small molecule inhibitors

Sec14-like phosphatidylinositol transfer proteins (PITPs) play important biological functions in integrating multiple aspects of intracellular lipid metabolism with phosphatidylinositol-4-phosphate signaling. As such, these proteins offer new opportunities for highly selective chemical interference with specific phosphoinositide pathways in cells. The first and best characterized small molecule inhibitors of the yeast PITP, Sec14, are nitrophenyl(4-(2-methoxyphenyl)piperazin-1-yl)methanones (NPPMs), and a hallmark feature of NPPMs is their exquisite targeting specificities for Sec14 relative to other closely related Sec14-like PITPs. Our present understanding of Sec14::NPPM binding interactions is based on computational docking and rational loss-of-function approaches. While those approaches have been informative, we still lack an adequate understanding of the basis for the high selectivity of NPPMs among closely related Sec14-like PITPs. Herein, we describe a Sec14 motif, which we term the VV signature, that contributes significantly to the NPPM sensitivity/resistance of Sec14-like phosphatidylinositol (PtdIns)/phosphatidylcholine (PtdCho) transfer proteins. The data not only reveal previously unappreciated determinants that govern Sec14-like PITP sensitivities to NPPMs, but enable predictions of which Sec14-like PtdIns/PtdCho transfer proteins are likely to be NPPM resistant or sensitive based on primary sequence considerations. Finally, the data provide independent evidence in support of previous studies highlighting the importance of Sec14 residue Ser173 in the mechanism by which NPPMs engage and inhibit Sec14-like PITPs.

A selective calix[6]arene-based fluorescent chemosensor for phosphatidylcholine type lipids.

The development of chemosensors that can selectively detect phosphatidylcholines (PCs) in biological samples is of medical relevance considering the importance of these phospholipids in cell growth and survival. Their selective sensing over phosphatidylethanolamines (PEs) is however a challenging task. We report here on the chemosensing capacities of calix[6]tris-pyrenylurea 1, which is able to selectively interact with phosphatidylcholine-type lipids in organic media. Host 1 also binds them in a biphasic chloroform/water solution, opening the way to the design of selective chemosensors for these lipids in biological media. The results obtained by NMR, fluorescence spectroscopy and modelling studies show that the selectivity is the result of the high degree of complementarity between the lipids' zwitterionic phosphatidylcholine headgroup and the receptor's H-bonding donor site and hydrophobic pocket. The mode of recognition is reminiscent of natural systems, such as human phosphatidylcholine transfer proteins (PC-TPs), validating the biomimetic approach adopted in our work.

Transcription Factor RUNX1 Regulates Pctp (Phosphatidylcholine Transfer Protein) in Platelets: A Potential Role in Regulating Platelet Function

Transcription factor (TF) mutations are increasingly recognized to play a major role in inherited platelet abnormalities. RUNX1, a critical hematopoietic TF, acts in a combinatorial manner with other TFs to regulate numerous megakaryocyte (MK)/platelet genes. Human RUNX1 haplodeficiency is associated with thrombocytopenia, numerous platelet function defects, and increased leukemia risk. We have previously described a patient with a heterozygous RUNX1 nonsense mutation in the conserved runt domain necessary for DNA binding (Sun et al Blood 2004;103;948-54). The patient's platelet abnormalities included impaired aggregation and secretion in response to multiple agonists, granule deficiency, and decreased phosphorylation of myosin light chain and pleckstrin, activation of GPIIb-IIIa, 12-HETE production, and platelet protein kinase C-θ. Transcript expression profiling of patient platelets (Sun et al, J Thromb Haemost 2007;5:146-54) showed numerous genes were significantly downregulated, including myosin light chain (MYL9), platelet factor 4 (PF4), protein kinase C-θ (PRCKQ), and 12-lipoxygenase (ALOX12); these have been shown by us to be regulated by RUNX1. The profiling data also showed 10-fold downregulation of phosphatidylcholine transfer protein (PCTP) gene (fold change ratio 0.09, p=0.02) in the patient compared with normal controls. PCTP is a member of the START (Steroidogenic Acute Regulatory Protein-Related Transfer) domain superfamily and is responsible for the intermembrane transfer of phosphatidylcholine (PC), a major plasma membrane phospholipid. Several findings indicate that PCTP is important for platelet function. PC is the main fraction of platelet phospholipids and source of arachidonic acid (AA) upon activation. Release of free AA from PC is the rate-limiting step in thromboxane production. PC is also a substrate for phospholipase D, which yields phosphatidic acid that can generate the second messenger diacylglycerol. Importantly, platelet PCTP has recently been demonstrated to have a major role in the racial differences in platelet responses - increased PCTP expression has been linked to the higher platelet aggregation and calcium mobilization induced by thrombin receptor protease-activated receptor 4 (PAR4) in blacks as compared to whites (Edelstein et al Nat Med 2013;19:1609-16). Little is known regarding the regulation of PCTP in MKs/platelets.Based on the decreased platelet PCTP expression in our patient, we pursued the hypothesis that PCTP is regulated by RUNX1. Corrected total cellular immunofluorescence with anti-PCTP antibody showed significantly reduced platelet PCTP expression by 58% in our patient compared to a normal control. In silico analysis revealed 5 RUNX1 consensus binding sites up to 995 bp of the PCTP 5' upstream region from ATG. To assess for interaction of RUNX1 with the PCTP promoter, chromatin immunoprecipitation (ChIP) assay with anti-RUNX1 antibody was performed using human erythroid leukemia (HEL) cells treated with phorbol 12-myristate 13-acetate (PMA) for 24 hours to induce megakaryocytic transformation. The ChIP studies showed RUNX1 binding to PCTP chromatin in the regions encompassing RUNX1 binding sites 1 (-232/-227) and 2 (-345/-340), at site 3 (-519/-514), and encompassing sites 4 and 5 (-861/-856, -884/-879). Electrophoretic mobility shift assay (EMSA) using PMA-treated HEL cell nuclear extracts showed RUNX1 binding to DNA probes (28-37 bp) containing site 1 (-232/-227) and both sites 4 and 5 (-861/-856, -884/-879). To assess the effect of modulating RUNX1 expression on PCTP expression, PMA-treated HEL cells were transfected with RUNX1 overexpression plasmid or siRNA. PCTP mRNA and protein expression were increased with RUNX1 overexpression and reduced with RUNX1 knockdown, suggesting that PCTP is regulated by RUNX1.Conclusions: Our results provide evidence that PCTP is a direct transcriptional target of and regulated by RUNX1, and a cogent molecular mechanism for downregulation of platelet PCTP in our patient with RUNX1 haplodeficiency. Regulation of PCTP by RUNX1 may be relevant to the platelet dysfunction in RUNX1 haplodeficiency as well as to racial differences in platelet responses linked to the differential platelet expression of PCTP. DisclosuresNo relevant conflicts of interest to declare.

Transmembrane START domain proteins: in silico identification, characterization and expression analysis under stress conditions in chickpea (Cicer arietinum L.)

Steroidogenic acute regulatory related transfer (StART) proteins that are involved in transport of lipid molecules, play a myriad of functions in insects, mammals and plants. These proteins consist of a modular START domain of approximately 200 amino acids which binds and transfers the lipids. In the present study we have performed a genome-wide search for all START domain proteins in chickpea. The search identified 36 chickpea genes belonging to the START domain family. Through a phylogenetic tree reconstructed with Arabidopsis, rice, chickpea, and soybean START proteins, we were able to identify four transmembrane START (TM-START) proteins in chickpea. These four proteins are homologous to the highly conserved mammalian phosphatidylcholine transfer proteins. Multiple sequence alignment of all the transmembrane containing START proteins from Arabidopsis, rice, chickpea, and soybean revealed that the amino acid residues to which phosphatidylcholine binds in mammals, is also conserved in all these plant species, implying an important functional role and a very similar mode of action of all these proteins across dicots and monocots. This study characterizes a few of the not so well studied transmembrane START superfamily genes that may be involved in stress signaling. Expression analysis in various tissues showed that these genes are predominantly expressed in flowers and roots of chickpea. Three of the chickpea TM-START genes showed induced expression in response to drought, salt, wound and heat stress, suggesting their role in stress response.

Selective Recognition of Phosphatidylcholine Lipids by a Biomimetic Calix[6]tube Receptor.

Phosphatidylcholines (PCs) and phosphatidylethanolamines (PEs) are usually the most abundant phospholipids in membranes. Only a few examples of artificial macrocyclic receptors capable of binding these zwitterionic lipids were reported, and in most cases, their mode of action differs from that of natural receptors. NMR studies show that calix[6]arenes 4-6 behave as heteroditopic receptors that can efficiently bind 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) in nonpolar solvents. Similarly to natural systems, the recognition proceeds through the establishment of specific interactions with the zwitterionic head of the lipid. In a protic environment, calix[6]tube 4 binds DOPC much more strongly than 5 and 6, thanks to the higher acidity of its H-bonding thiourea groups and the better preorganization of its binding site. Moreover, 4 is reluctant to the corresponding PE, highlighting a unique selectivity for PCs over PEs. A high selectivity for DOPC over dodecylphosphocholine (DPC) was also observed, and computer modeling studies showed that it may likely originate from the curved shape of the tubular recognition system of 4, which is well-adapted to the native conformation of DOPC. From a biomimetic point of view, the complex 4⊃DOPC shows remarkable similarities with a natural complex formed between a PC and the human phosphatidylcholine transfer protein.