Quantitative profiling of the endonuclear glycerophospholipidome of murine embryonic fibroblasts

Lipid transfer proteins and a PI 4-kinase initiate nuclear phosphoinositide signaling

Phosphoinositide (PIP n ) messengers are present in non-membranous regions of nuclei, where they are assembled into a phosphatidylinositol (PI) 3-kinase (PI3K)/Akt pathway that is distinct from the cytosolic membrane-localized pathway. In the nuclear pathway, PI kinases/phosphatases bind the p53 tumor suppressor protein (wild-type and mutant) to generate p53-PIP n complexes that regulate Akt activation. However, this pathway is dependent on poorly characterized nuclear PIP n pools. Here we report that PI transfer proteins (PITPs), which transport PI between membranes to enable membrane-localized PIP n synthesis, accumulate in the nucleoplasm in response to stress and supply nuclear PIP n pools. PITPα/β and the PI 4-kinase PI4KIIα bind p53 and are required to generate p53-PI4P, which is further phosphorylated to synthesize p53-PIP n complexes that regulate nuclear Akt activation and stress-resistance. Remarkably, PITPα/β and PI4KIIα initiate PIP n -linkage to multiple proteins that are detectable by immunoblotting and [ 3 H] myo -inositol metabolic labeling and are resistant to denaturation, suggesting a posttranslational modification.

In brief

Phosphatidylinositol transfer proteins initiate the nuclear PIP n -linked protein network in membrane-free regions.

Environmental pollutants and phosphoinositide signaling in autoimmunity

Environmental pollution stands as one of the most critical challenges affecting human health, with an estimated mortality rate linked to pollution-induced non-communicable diseases projected to range from 20% to 25%. These pollutants not only disrupt immune responses but can also trigger immunotoxicity. Phosphoinositide signaling, a pivotal regulator of immune responses, plays a central role in the development of autoimmune diseases and exhibits high sensitivity to environmental stressors. Among these stressors, environmental pollutants have become increasingly prevalent in our society, contributing to the initiation and exacerbation of autoimmune conditions. In this review, we summarize the intricate interplay between phosphoinositide signaling and autoimmune diseases within the context of environmental pollutants and contaminants. We provide an up-to-date overview of stress-induced phosphoinositide signaling, discuss 14 selected examples categorized into three groups of environmental pollutants and their connections to immune diseases, and shed light on the associated phosphoinositide signaling pathways. Through these discussions, this review advances our understanding of how phosphoinositide signaling influences the coordinated immune response to environmental stressors at a biological level. Furthermore, it offers valuable insights into potential research directions and therapeutic targets aimed at mitigating the impact of environmental pollutants on the pathogenesis of autoimmune diseases. SYNOPSIS: Phosphoinositide signaling at the intersection of environmental pollutants and autoimmunity provides novel insights for managing autoimmune diseases aggravated by pollutants.

Nuclear Phosphoinositides as Key Determinants of Nuclear Functions

Polyphosphoinositides (PPIns) are signalling messengers representing less than five per cent of the total phospholipid concentration within the cell. Despite their low concentration, these lipids are critical regulators of various cellular processes, including cell cycle, differentiation, gene transcription, apoptosis and motility. PPIns are generated by the phosphorylation of the inositol head group of phosphatidylinositol (PtdIns). Different pools of PPIns are found at distinct subcellular compartments, which are regulated by an array of kinases, phosphatases and phospholipases. Six of the seven PPIns species have been found in the nucleus, including the nuclear envelope, the nucleoplasm and the nucleolus. The identification and characterisation of PPIns interactor and effector proteins in the nucleus have led to increasing interest in the role of PPIns in nuclear signalling. However, the regulation and functions of PPIns in the nucleus are complex and are still being elucidated. This review summarises our current understanding of the localisation, biogenesis and physiological functions of the different PPIns species in the nucleus.

Fishing for lipid lactones using selective reaction and characteristic fragmentation pattern

Extensive research has been invested in developing sensitive methods to identify lipid mediators (LMs) from multiple biological matrices. Previous studies point to the existence of a potential family of lactone-containing metabolites generated from eicosanoid families, isoprostanes, and prostanoid-like compounds that may function as LMs. However, targeted lipidomic studies do not routinely include lactone-containing lipids due to their low ionizability and instability under some common sample preparation conditions. Thus, the discovery of lactone-containing LM is limited. Herein we describe a method for selective identification of lipid lactones from within biological matrices. This method is based on a selective reaction of lactones with 1-(3-aminopropyl)imidazole, followed by cation exchange solid phase extraction and the identification of characteristic fragmentation patterns unique to reaction products of lactones in LC/MS/MS. NMR and LC/MS results indicated that saturated and unsaturated aliphatic ɣ and δ lactone model compounds mixed with human serum were successfully detected. MS/MS analyses of the reaction products revealed a unique pattern for the lactones, resulting from common neutral losses and fragmentation. When applied to esters and free fatty acids, some reaction products were observed. However, these reaction products' MS/MS fragmentation did not match the specific fragmentation of the lactones' reaction products. Confirming that lactones can be detected in a highly selective manner from within complex biological matrices when using the presented method. Thus, the presented method can selectively analyze lactones and may further complement existing lipidomic approaches to discover new LMs.

Lipid transfer proteins and instructive regulation of lipid kinase activities: Implications for inositol lipid signaling and disease

Cellular membranes are critical platforms for intracellular signaling that involve complex interfaces between lipids and proteins, and a web of interactions between a multitude of lipid metabolic pathways. Membrane lipids impart structural and functional information in this regulatory circuit that encompass biophysical parameters such as membrane thickness and fluidity, as well as chaperoning the interactions of protein binding partners. Phosphatidylinositol and its phosphorylated derivatives, the phosphoinositides, play key roles in intracellular membrane signaling, and these involvements are translated into an impressively diverse set of biological outcomes. The phosphatidylinositol transfer proteins (PITPs) are key regulators of phosphoinositide signaling. Found in a diverse array of organisms from plants, yeast and apicomplexan parasites to mammals, PITPs were initially proposed to be simple transporters of lipids between intracellular membranes. It now appears increasingly unlikely that the soluble versions of these proteins perform such functions within the cell. Rather, these serve to facilitate the activity of intrinsically biologically insufficient inositol lipid kinases and, in so doing, promote diversification of the biological outcomes of phosphoinositide signaling. The central engine for execution of such functions is the lipid exchange cycle that is a fundamental property of PITPs. How PITPs execute lipid exchange remains very poorly understood. Molecular dynamics simulation approaches are now providing the first atomistic insights into how PITPs, and potentially other lipid-exchange/transfer proteins, operate.

Nuclear Phosphoinositides-Versatile Regulators of Genome Functions

The many functions of phosphoinositides in cytosolic signaling were extensively studied; however, their activities in the cell nucleus are much less clear. In this review, we summarize data about their nuclear localization and metabolism, and review the available literature on their involvements in chromatin remodeling, gene transcription, and RNA processing. We discuss the molecular mechanisms via which nuclear phosphoinositides, in particular phosphatidylinositol (4,5)-bisphosphate (PI(4,5)P2), modulate nuclear processes. We focus on PI(4,5)P2’s role in the modulation of RNA polymerase I activity, and functions of the nuclear lipid islets—recently described nucleoplasmic PI(4,5)P2-rich compartment involved in RNA polymerase II transcription. In conclusion, the high impact of the phosphoinositide–protein complexes on nuclear organization and genome functions is only now emerging and deserves further thorough studies.

The interface between phosphatidylinositol transfer protein function and phosphoinositide signaling in higher eukaryotes

Phosphoinositides are key regulators of a large number of diverse cellular processes that include membrane trafficking, plasma membrane receptor signaling, cell proliferation, and transcription. How a small number of chemically distinct phosphoinositide signals are functionally amplified to exert specific control over such a diverse set of biological outcomes remains incompletely understood. To this end, a novel mechanism is now taking shape, and it involves phosphatidylinositol (PtdIns) transfer proteins (PITPs). The concept that PITPs exert instructive regulation of PtdIns 4-OH kinase activities and thereby channel phosphoinositide production to specific biological outcomes, identifies PITPs as central factors in the diversification of phosphoinositide signaling. There are two evolutionarily distinct families of PITPs: the Sec14-like and the StAR-related lipid transfer domain (START)-like families. Of these two families, the START-like PITPs are the least understood. Herein, we review recent insights into the biochemical, cellular, and physiological function of both PITP families with greater emphasis on the START-like PITPs, and we discuss the underlying mechanisms through which these proteins regulate phosphoinositide signaling and how these actions translate to human health and disease.

The phosphatidic acid paradox: Too many actions for one molecule class? Lessons from plants

Phosphatidic acid (PA) is a simple phospholipid observed in most organisms. PA acts as a key metabolic intermediate and a second messenger that regulates many cell activities. In plants, PA is involved in numerous cell responses induced by hormones, stress inputs and developmental processes. Interestingly, PA production can be triggered by opposite stressors, such as cold and heat, or by hormones that are considered to be antagonistic, such as abscisic acid and salicylic acid. This property questions the specificity of the responses controlled by PA. Are there generic responses to PA, meaning that cell regulation triggered by PA would be always the same, even in opposite physiological situations? Alternatively, do the responses to PA differ according to the physiological context within the cells? If so, the mechanisms that regulate the divergence of PA-controlled reactions are poorly defined. This review summarizes the latest opinions on how PA signalling is directed in plant cells and examines the intrinsic properties of PA that enable its regulatory diversity. We propose a concept whereby PA regulatory messages are perceived as complex "signatures" that take into account their production site, the availability of target proteins and the relevant cellular environments.

Fat nucleosome: Role of lipids on chromatin

Structural changes in chromatin regulate gene expression and define phenotypic outcomes. Molecules that bind to the nucleosome, the complex of DNA and histone proteins, are key modulators of chromatin structure. Most recently, the formation of condensed chromatin regions based on phase-separation in the cell, a basic physical mechanism, was proposed. Increased understanding of the mechanisms of interaction between chromatin and lipids suggest that small lipid molecules, such as cholesterol and short-chain fatty acids, can regulate important nuclear functions. New biophysical data has suggested that cholesterol interacts with nucleosome through multiple binding sites and affects chromatin structure in vitro. Regardless of the mechanism of how lipids bind to chromatin, there is currently little awareness that lipids may be stored in chromatin and influence its state. Focusing on lipids that bind to nuclear receptors, clinically relevant transcription factors, we discuss the potential interactions of the nucleosome with steroid hormones, bile acids and fatty acids, which suggest that other lipid chemotypes may also impact chromatin structure through binding to common sites on the nucleosome. Herein, we review the main impacts of lipids on the nuclear environment, emphasizing its role on chromatin architecture. We postulate that lipids that bind to nucleosomes and affect chromatin states are likely to be worth investigating as tools to modify disease phenotypes at a molecular level.

Nuclear phosphoinositide regulation of chromatin

Phospholipid signaling has clear connections to a wide array of cellular processes, particularly in gene expression and in controlling the chromatin biology of cells. However, most of the work elucidating how phospholipid signaling pathways contribute to cellular physiology have studied cytoplasmic membranes, while relatively little attention has been paid to the role of phospholipid signaling in the nucleus. Recent work from several labs has shown that nuclear phospholipid signaling can have important roles that are specific to this cellular compartment. This review focuses on the nuclear phospholipid functions and the activities of phospholipid signaling enzymes that regulate metazoan chromatin and gene expression. In particular, we highlight the roles that nuclear phosphoinositides play in several nuclear-driven physiological processes, such as differentiation, proliferation, and gene expression. Taken together, the recent discovery of several specifically nuclear phospholipid functions could have dramatic impact on our understanding of the fundamental mechanisms that enable tight control of cellular physiology.

Nuclear Inositide Signaling Via Phospholipase C

The existence of an independent nuclear inositide pathway distinct from the cytoplasmic one has been demonstrated in different physiological systems and in diseases. In this prospect we analyze the role of PI-PLCβ1 nuclear isoform in relation to the cell cycle regulation, the cell differentiation, and different physiopathological pathways focusing on the importance of the nuclear localization from both molecular and clinical point of view. PI-PLCβ1 is essential for G1/S transition through DAG and Cyclin D3 and plays also a central role in G2/M progression through Cyclin B1 and PKCα. In the differentiation process of C2C12 cells PI-PLCβ1 increases in both myogenic differentiation and osteogenic differentiation. PI-PLCβ1 and Cyclin D3 reduction has been observed in Myotonic Dystrophy (DM) suggesting a pivotal role of these enzymes in DM physiopathology. PI-PLCβ1 is also involved in adipogenesis through a double phase mechanism. Moreover, PI-PLCβ1 plays a key role in the normal hematopoietic differentiation where it seems to decrease in erythroid differentiation and increase in myeloid differentiation. In Myelodysplastic Syndromes (MDS) PI-PLCβ1 has a genetic and epigenetic relevance and it is related to MDS patients' risk of Acute Myeloid Leukemia (AML) evolution. In MDS patients PI-PLCβ1 seems to be also a therapeutic predictive outcome marker. In the central nervous system, PI-PLCβ1 seems to be involved in different pathways in both brain cortex development and synaptic plasticity related to different diseases. Another PI-PLC isozyme that could be related to nuclear activities is PI-PLCζ that is involved in infertility processes. J. Cell. Biochem. 118: 1969-1978, 2017. © 2017 Wiley Periodicals, Inc.

5,6-δ-DHTL, a stable metabolite of arachidonic acid, is a potential EDHF that mediates microvascular dilation

OBJECTIVE

Prominent among the endothelium-derived hyperpolarizing factors (EDHFs) are the Cytochrome P450 (CYP) epoxygenase-derived arachidonic acid metabolites-the epoxyeicosatrienoic acids (EETs), that are known as vasodilators in the microcirculation. Among the EET isomers, 5,6-EET undergoes rapid lactonization in aqueous solution to the more stable 5,6-δ DHTL (5,6-dihydroxytrienoic lactone) isomer. It is unclear whether this metabolic transformation maintains its vasodilator potential and what is the mechanism of action. Thus, the aim of this study was to investigate the capacity of the lactone isomer, 5,6- δ DHTL, to induce dilation of arterioles and explore the endothelial Ca²⁺ response mechanism.

APPROACH AND RESULTS

In isolated human microvessels 5,6- δ DHTL induced a dose dependent vasodilation, that was inhibited by mechanical denudation of the endothelial layer. This 5,6- δ DHTL -dependent dilation was partially reduced in the presence of L-NAME (NOS inhibitor) or the NO-scavenger, cPTIO (by 19.7%, which was not statistically significantly). In human endothelial cells, 5,6- δ DHTL induced an increase in intracellular Ca²⁺([Ca²⁺]i) in a dose dependent manner. This increase in [Ca²⁺]i was similar to that induced by the 5,6-EET isomer, and significantly higher than observed by administering the hydrolytic dihydroxy isomer, 5,6-DHET. Further experiments aimed to investigate the mechanism of action revealed, that the 5,6-δ DHTL-mediated ([Ca²⁺]i elevation was reduced by IP3 and ryanodine antagonists, but not by antagonists to the TRPV4 membrane channel. Similar to their effect on the dilation response in the arteries, NO inhibitors reduced the 5,6-δ DHTL-mediated ([Ca²⁺]i elevation by 20%. Subsequent 5,6-δ DHTL -dependent K⁺ ion efflux from endothelial cells, was abolished by the inhibition of small and intermediate conductance K_Ca.

CONCLUSIONS

The present study shows that 5,6-δ DHTL is a potential EDHF, that dilates microvessels through a mechanism that involves endothelial dependent Ca²⁺ entry, requiring endothelial hyperpolarization. These results suggest the existence of additional lactone-containing metabolites that can be derived from the PUFA metabolism and which may function as novel EDHFs.