Potential role for phosphatidylinositol transfer protein (PITP) family in lipid transfer during phospholipase C signalling

Expanding functions of the phosphatidylinositol/phosphatidate lipid transporter, PITPNC1 in physiology and in pathology

PITPNC1 was the last of the PITPs to be identified and has been characterized as a binding protein for phosphatidylinositol and phosphatidate. In mammals, PITPNC1 is expressed as two splice variants whilst in zebrafish is expressed from two separate genes. The two splice variants have different expression profiles with the long splice variant having a prominent role in the brain. Several physiological functions have been identified including neuronal and metabolic functions. PITPNC1 also plays a significant role in cancer and has been identified as a risk factor in type 2 diabetes. Here, we review our current understanding of PITPNC1 in cell physiology and pathology.

Mammalian START-like phosphatidylinositol transfer proteins - Physiological perspectives and roles in cancer biology

PtdIns and its phosphorylated derivatives, the phosphoinositides, are the biochemical components of a major pathway of intracellular signaling in all eukaryotic cells. These lipids are few in terms of cohort of unique positional isomers, and are quantitatively minor species of the bulk cellular lipidome. Nevertheless, phosphoinositides regulate an impressively diverse set of biological processes. It is from that perspective that perturbations in phosphoinositide-dependent signaling pathways are increasingly being recognized as causal foundations of many human diseases - including cancer. Although phosphatidylinositol transfer proteins (PITPs) are not enzymes, these proteins are physiologically significant regulators of phosphoinositide signaling. As such, PITPs are conserved throughout the eukaryotic kingdom. Their biological importance notwithstanding, PITPs remain understudied. Herein, we review current information regarding PITP biology primarily focusing on how derangements in PITP function disrupt key signaling/developmental pathways and are associated with a growing list of pathologies in mammals.

Non-Vesicular Lipid Transport Machinery in Leishmania donovani: Functional Implications in Host-Parasite Interaction

Eukaryotic cells have distinct membrane-enclosed organelles, each with a unique biochemical signature and specialized function. The unique identity of each organelle is greatly governed by the asymmetric distribution and regulated intracellular movement of two important biomolecules, lipids, and proteins. Non-vesicular lipid transport mediated by lipid-transfer proteins (LTPs) plays essential roles in intra-cellular lipid trafficking and cellular lipid homeostasis, while vesicular transport regulates protein trafficking. A comparative analysis of non-vesicular lipid transport machinery in protists could enhance our understanding of parasitism and basis of eukaryotic evolution. Leishmania donovani, the trypanosomatid parasite, greatly depends on receptor-ligand mediated signalling pathways for cellular differentiation, nutrient uptake, secretion of virulence factors, and pathogenesis. Lipids, despite being important signalling molecules, have intracellular transport mechanisms that are largely unexplored in L. donovani. We have identified a repertoire of sixteen (16) potential lipid transfer protein (LTP) homologs based on a domain-based search on TriTrypDB coupled with bioinformatics analyses, which signifies the presence of well-organized lipid transport machinery in this parasite. We emphasized here their evolutionary uniqueness and conservation and discussed their potential implications for parasite biology with regards to future therapeutic targets against visceral leishmaniasis.

Identification and Expression Analysis of Phosphatidylinositol Transfer Proteins Genes in Rice

The family of phosphatidylinositol transfer proteins (PITPs) is able to bind specific lipids to carry out various biological functions throughout different stages of plant life. But the function of PITPs in rice plant is unclear. In this study, 30 PITPs were identified from rice genome, which showed differences in physicochemical properties, gene structure, conservation domains, and subcellular localization. The promoter region of the OsPITPs genes included at least one type of hormone response element, such as methyl jasmonate (Me JA) and salicylic acid (SA). Furthermore, the expression level of OsML-1, OsSEC14-3, OsSEC14-4, OsSEC14-15, and OsSEC14-19 genes were significantly affected by infection of rice blast fungus Magnaporthe oryzae. Based on these findings, it is possible that OsPITPs may be involved in rice innate immunity in response to M. oryzae infection through the Me JA and SA pathway.

Structural organization of RDGB (retinal degeneration B), a multi-domain lipid transfer protein: a molecular modelling and simulation based approach

Lipid transfer proteins (LTPs) that shuttle lipids at membrane contact sites (MCS) play an important role in maintaining cellular homeostasis. One such important LTP is the Retinal Degeneration B (RDGB) protein. RDGB is localized at the MCS formed between the endoplasmic reticulum (ER) and the apical plasma membrane (PM) in Drosophila photoreceptors where it transfers phosphatidylinositol (PI) during G-protein coupled phospholipase C signalling. Previously, the C-terminal domains of RDGB have been shown to be essential for its function and accurate localization. In this study, using in-silico integrative modelling we predict the structure of entire RDGB protein in complex with the ER membrane protein VAP. The structure of RDGB has then been used to decipher the structural features of the protein important for its orientation at the contact site. Using this structure, we identify two lysine residues in the C-terminal helix of the LNS2 domain important for interaction with the PM. Using molecular docking, we also identify an unstructured region USR1, immediately c-terminal to the PITP domain that is important for the interaction of RDGB with VAP. Overall the 10.06 nm length of the predicted RDGB-VAP complex spans the distance between the PM and ER and is consistent with the cytoplasmic gap between the ER and PM measured by transmission electron microscopy in photoreceptors. Overall our model explains the topology of the RDGB-VAP complex at this ER-PM contact site and paves the way for analysis of lipid transfer function in this setting.Communicated by Ramaswamy H. Sarma.

pitpβ_w Encoding Phosphatidylinositol Transfer Protein Is Involved in Female Differentiation of Chinese Tongue Sole, Cynoglossus semilaevis

Phosphatidylinositol transfer protein (pitp) plays an important role in phospholipid transfer in animals. A pitp variant (pitpβ_w) in Chinese tongue sole was identified by transcriptomic analysis for its female-biased expression. The coding sequence of pitpβ_w was 816 bp, encoding a 371-amino-acid protein. pitpβ_w showed female-biased expression and was relatively high in brain, muscle, and ovary tissues. In different developmental stages of the ovary, pitpβ_w could be detected from 40 days until 3 years post hatching, and the highest expression was observed at 90 days. In situ hybridization revealed that pitpβ_w was predominantly localized in early-stage oocytes (I-III stages). After siRNA-mediated knockdown of pitpβ_w in an ovarian cell line, the expression of sox9a was reduced, while that of figla_tv1 and sox9b was significantly increased. Our findings suggest that pitpβ_w might be involved in female differentiation and early oogenesis.

Lipin modulates lipid metabolism during reproduction in the cabbage beetle

Lipids are a critical source of stored energy in insects, and their metabolism is essential for growth, development, and reproduction. Adequate provisioning of lipids and yolk proteins in the oocytes is essential to ensure reproductive output. Therefore, it is particularly important to understand the molecular mechanisms linking lipid metabolism and reproduction. Lipin proteins are emerging as pivotal modulators of lipid metabolism. They exert a dual function as phosphatidate phosphatase enzymes involved in lipid synthesis and as transcriptional coactivators of genes related to lipid metabolism. However, the functional relationship between lipid metabolism and reproduction remains unclear. In this study, the role of lipin protein in the reproduction of female cabbage beetle Colaphellus bowringi was examined. It was found that Lipin was broadly expressed in the tissues of adult females, with relatively high transcript levels in the head, midgut, fat body, malpighian tubules, and epidermis. RNA interference experiments were conducted using double-stranded RNA against Lipin in C. bowringi females. Lipin silencing blocked ovarian development and strongly suppressed transcription of vitellogenin and vitellogenin receptor genes. In addition, the reduction in Lipin expression led to a rapid increase in lipid storage in the fat body and also promoted the expression of genes related to lipid synthesis and stress tolerance. Overall, these results suggest that a Lipin-mediated lipolytic system is essential for maintaining lipid homeostasis during reproduction in C. bowringi. The findings of this study provide a foundation for future studies on the relationship between lipid metabolism and reproduction in invertebrates.

An Efficient Screen for Cell-Intrinsic Factors Identifies the Chaperonin CCT and Multiple Conserved Mechanisms as Mediating Dendrite Morphogenesis

Dendritic morphology is inextricably linked to neuronal function. Systematic large-scale screens combined with genetic mapping have uncovered several mechanisms underlying dendrite morphogenesis. However, a comprehensive overview of participating molecular mechanisms is still lacking. Here, we conducted an efficient clonal screen using a collection of mapped P-element insertions that were previously shown to cause lethality and eye defects in Drosophila melanogaster. Of 280 mutants, 52 exhibited dendritic defects. Further database analyses, complementation tests, and RNA interference validations verified 40 P-element insertion genes as being responsible for the dendritic defects. Twenty-eight mutants presented severe arbor reduction, and the remainder displayed other abnormalities. The intrinsic regulators encoded by the identified genes participate in multiple conserved mechanisms and pathways, including the protein folding machinery and the chaperonin-containing TCP-1 (CCT) complex that facilitates tubulin folding. Mutant neurons in which expression of CCT4 or CCT5 was depleted exhibited severely retarded dendrite growth. We show that CCT localizes in dendrites and is required for dendritic microtubule organization and tubulin stability, suggesting that CCT-mediated tubulin folding occurs locally within dendrites. Our study also reveals novel mechanisms underlying dendrite morphogenesis. For example, we show that Drosophila Nogo signaling is required for dendrite development and that Mummy and Wech also regulate dendrite morphogenesis, potentially via Dpp- and integrin-independent pathways. Our methodology represents an efficient strategy for identifying intrinsic dendrite regulators, and provides insights into the plethora of molecular mechanisms underlying dendrite morphogenesis.

Lipid Exchangers: Cellular Functions and Mechanistic Links With Phosphoinositide Metabolism

Lipids are amphiphilic molecules that self-assemble to form biological membranes. Thousands of lipid species coexist in the cell and, once combined, define organelle identity. Due to recent progress in lipidomic analysis, we now know how lipid composition is finely tuned in different subcellular regions. Along with lipid synthesis, remodeling and flip-flop, lipid transfer is one of the active processes that regulates this intracellular lipid distribution. It is mediated by Lipid Transfer Proteins (LTPs) that precisely move certain lipid species across the cytosol and between the organelles. A particular subset of LTPs from three families (Sec14, PITP, OSBP/ORP/Osh) act as lipid exchangers. A striking feature of these exchangers is that they use phosphatidylinositol or phosphoinositides (PIPs) as a lipid ligand and thereby have specific links with PIP metabolism and are thus able to both control the lipid composition of cellular membranes and their signaling capacity. As a result, they play pivotal roles in cellular processes such as vesicular trafficking and signal transduction at the plasma membrane. Recent data have shown that some PIPs are used as energy by lipid exchangers to generate lipid gradients between organelles. Here we describe the importance of lipid counter-exchange in the cell, its structural basis, and presumed links with pathologies.

PITPNC1 fuels radioresistance of rectal cancer by inhibiting reactive oxygen species production

BACKGROUND

Neoadjuvant radiotherapy is a commonly used method for the current standard-of-care for most patients with rectal cancer, when the effects of radioresistance are limited. The phosphatidylinositol transfer protein, cytoplasmic 1 (PITPNC1), a lipid-metabolism-related gene, has previously been proved to manifest pro-cancer effects in multiple types of cancer. However, whether PITPNC1 plays a role for developing radioresistance in rectal cancer patients is still unknown. Therefore, this study aims to investigate the role of PITPNC1 in rectal cancer radioresistance.

METHODS

Patient-derived tissue were used to detect the difference in the expression level of PITPNC1 between radioresistant and radiosensitive patients. Bioinformatic analyses of high-throughput gene expression data were applied to uncover the correlations between PITPNC1 level and oxidative stress. Two rectal cancer cell lines, SW620, and HCT116, were selected in vitro to investigate the effect of PITPNC1 on radioresistance, reactive oxygen species (ROS) generation, apoptosis, and proliferation in rectal cancer.

RESULTS

PITPNC1 is highly expressed in radioresistant patient-derived rectal cancer tissues compared to radiosensitive tissue; therefore, PITPNC1 inhibits the generation of ROS and improves the extent of radioresistance of rectal cancer cell lines and then inhibits apoptosis. Knocking down PITPNC1 facilitates the production of ROS while application of the ROS scavenger, N-acetyl-L-cysteine (NAC), could reverse this effect.

CONCLUSIONS

PITPNC1 fuels radioresistance of rectal cancer via the inhibition of ROS generation.

Lipid Transfer Proteins and Membrane Contact Sites in Human Cancer

Lipid-transfer proteins (LTPs) were initially discovered as cytosolic factors that facilitate lipid transport between membrane bilayers in vitro. Since then, many LTPs have been isolated from bacteria, plants, yeast, and mammals, and extensively studied in cell-free systems and intact cells. A major advance in the LTP field was associated with the discovery of intracellular membrane contact sites (MCSs), small cytosolic gaps between the endoplasmic reticulum (ER) and other cellular membranes, which accelerate lipid transfer by LTPs. As LTPs modulate the distribution of lipids within cellular membranes, and many lipid species function as second messengers in key signaling pathways that control cell survival, proliferation, and migration, LTPs have been implicated in cancer-associated signal transduction cascades. Increasing evidence suggests that LTPs play an important role in cancer progression and metastasis. This review describes how different LTPs as well as MCSs can contribute to cell transformation and malignant phenotype, and discusses how “aberrant” MCSs are associated with tumorigenesis in human.

In vitro lipid transfer assays of phosphatidylinositol transfer proteins provide insight into the in vivo mechanism of ligand transfer

Fluorescence resonance energy transfer (FRET) assays and membrane binding determinations were performed using three phosphatidylinositol transfer proteins, including the yeast Sec14 and two mammalian proteins PITPα and PITPβ. These proteins were able to specifically bind the fluorescent phosphatidylcholine analogue NBD-PC ((2-(12-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)amino)dodecanoyl-1-hexadecanoyl-sn-glycero-3-phosphocholine)) and to transfer it to small unilamellar vesicles (SUVs). Rate constants for transfer to vesicles comprising 100% PC were slower for all proteins than when increasing percentages of phosphatidylinositol were incorporated into the same SUVs. The rates of ligand transfer by Sec14 were insensitive to the inclusion of equimolar amounts of another anionic phospholipid phosphatidylserine (PS), but the rates of ligand transfer by both mammalian PITPs were strikingly enhanced by the inclusion of phosphatidic acid (PA) in the receptor SUV. Binding of Sec14 to immobilized bilayers was substantial, while that of PITPα and PITPβ was 3-7 times weaker than Sec14 depending on phospholipid composition. When small proportions of the phosphoinositide PI(4)P were included in receptor SUVs (either with PI or not), Sec14 showed substantially increased rates of NBD-PC pick-up, whereas the PITPs were unaffected. The data are supportive of a role for PITPβ as functional PI transfer protein in vivo, but that Sec14 likely has a more elaborate function.

Non-vesicular Lipid Transport Machinery in Entamoeba histolytica

Eukaryotic cells are organized into separate membrane-bound compartments that have specialized biochemical signature and function. Maintenance and regulation of distinct identity of each compartment is governed by the uneven distribution and intra-cellular movement of two essential biomolecules, lipids, and proteins. Non-vesicular lipid transport mediated by lipid transfer proteins plays a pivotal role in intra-cellular lipid trafficking and homeostasis whereas vesicular transport plays a central role in protein trafficking. Comparative study of lipid transport machinery in protist helps to better understand the pathogenesis and parasitism, and provides insight into eukaryotic evolution. Amebiasis, which is caused by Entamoeba histolytica, is one of the major enteric infections in humans, resulting in 40–100 thousand deaths annually. This protist has undergone remarkable alterations in the content and function of its sub-cellular compartments as well represented by its unique diversification of mitochondrion-related organelle, mitosome. We conducted domain-based search on AmoebaDB coupled with bioinformatics analyses and identified 22 potential lipid transfer protein homologs in E. histolytica, which are grouped into several sub-classes. Such in silico analyses have demonstrated the existence of well-organized lipid transport machinery in this parasite. We summarized and discussed the conservation and unique features of the whole repertoire of lipid transport proteins in E. histolytica.

Phospholipid subcellular localization and dynamics

Membrane biology seeks to understand how lipids and proteins within bilayers assemble into large structures such as organelles and the plasma membranes. Historically, lipids were thought to merely provide structural support for bilayer formation and membrane protein function. Research has now revealed that phospholipid metabolism regulates nearly all cellular processes. Sophisticated techniques helped identify >10,000 lipid species suggesting that lipids support many biological processes. Here, we highlight the synthesis of the most abundant glycerophospholipid classes and their distribution in organelles. We review vesicular and nonvesicular transport pathways shuttling lipids between organelles and discuss lipid regulators of membrane trafficking and second messengers in eukaryotic cells.

Topological organisation of the phosphatidylinositol 4,5-bisphosphate-phospholipase C resynthesis cycle: PITPs bridge the ER-PM gap

Phospholipase C (PLC) is a receptor-regulated enzyme that hydrolyses phosphatidylinositol 4,5-bisphosphate (PI(4,5)P₂) at the plasma membrane (PM) triggering three biochemical consequences, the generation of soluble inositol 1,4,5-trisphosphate (IP₃), membrane-associated diacylglycerol (DG) and the consumption of PM PI(4,5)P₂ Each of these three signals triggers multiple molecular processes impacting key cellular properties. The activation of PLC also triggers a sequence of biochemical reactions, collectively referred to as the PI(4,5)P₂ cycle that culminates in the resynthesis of this lipid. The biochemical intermediates of this cycle and the enzymes that mediate these reactions are topologically distributed across two membrane compartments, the PM and the endoplasmic reticulum (ER). At the PM, the DG formed during PLC activation is rapidly converted into phosphatidic acid (PA) that needs to be transported to the ER where the machinery for its conversion into PI is localised. Conversely, PI from the ER needs to be rapidly transferred to the PM where it can be phosphorylated by lipid kinases to regenerate PI(4,5)P₂ Thus, two lipid transport steps between membrane compartments through the cytosol are required for the replenishment of PI(4,5)P₂ at the PM. Here, we review the topological constraints in the PI(4,5)P₂ cycle and current understanding how these constraints are overcome during PLC signalling. In particular, we discuss the role of lipid transfer proteins in this process. Recent findings on the biochemical properties of a membrane-associated lipid transfer protein of the PITP family, PITPNM proteins (alternative name RdgBα/Nir proteins) that localise to membrane contact sites are discussed. Studies in both Drosophila and mammalian cells converge to provide a resolution to the conundrum of reciprocal transfer of PA and PI during PLC signalling.

Homeostatic regulation of the PI(4,5)P2-Ca(2+) signaling system at ER-PM junctions

The phosphatidylinositol 4,5-bisphosphate (PI(4,5)P2)-Ca(2+) signaling system is important for cell activation in response to various extracellular stimuli. This signaling system is initiated by receptor-induced hydrolysis of PI(4,5)P2 in the plasma membrane (PM) to generate the soluble second messenger inositol 1,4,5-trisphosphate (IP3). IP3 subsequently triggers the release of Ca(2+) from the endoplasmic reticulum (ER) store to the cytosol to activate Ca(2+)-mediated responses, such as secretion and proliferation. The consumed PM PI(4,5)P2 and ER Ca(2+) must be quickly restored to sustain signaling responses, and to maintain the homeostasis of PI(4,5)P2 and Ca(2+). Since phosphatidylinositol (PI), the precursor lipid for PM PI(4,5)P2, is synthesized in the ER membrane, and a Ca(2+) influx across the PM is required to refill the ER Ca(2+) store, efficient communications between the ER and the PM are critical for the homeostatic regulation of the PI(4,5)P2-Ca(2+) signaling system. This review describes the major findings that established the framework of the PI(4,5)P2-Ca(2+) signaling system, and recent discoveries on feedback control mechanisms at ER-PM junctions that sustain the PI(4,5)P2-Ca(2+) signaling system. Particular emphasis is placed on the characterization of ER-PM junctions where efficient communications between the ER and the PM occur, and the activation mechanisms of proteins that dynamically localize to ER-PM junctions to provide the feedback control during PI(4,5)P2-Ca(2+) signaling, including the ER Ca(2+) sensor STIM1, the extended synaptotagmin E-Syt1, and the PI transfer protein Nir2. This article is part of a Special Issue entitled: The cellular lipid landscape edited by Tim P. Levine and Anant K. Menon.

Roles of EGFR and KRAS and their downstream signaling pathways in pancreatic cancer and pancreatic cancer stem cells

Pancreatic cancer is currently the fourth most common cancer, is increasing in incidence and soon will be the second leading cause of cancer death in the USA. This is a deadly malignancy with an incidence that approximates the mortality with 44,000 new cases and 36,000 deaths each year. Surgery, although only modestly successful, is the only curative option. However, due the locally aggressive nature and early metastasis, surgery can be performed on less than 20% of patients. Cytotoxic chemotherapy is palliative, has significant toxicity and improves survival very little. Thus new treatment paradigms are needed desperately. Due to the extremely high frequency of KRAS gene mutations (>90%) detected in pancreatic cancer patients, the roles of the epidermal growth factor receptor (EGFR), Ras/Raf/MEK/ERK and PI3K/PTEN/Akt/mTORC1/GSK-3 pathways have been investigated in pancreatic cancer for many years. Constitutively active Ras can activate both of these pathways and there is cross talk between Ras and EGFR which is believed to be important in driving metastasis. Mutant KRAS may also drive the expression of GSK-3 through Raf/MEK/ERK-mediated effects on GSK-3 transcription. GSK-3 can then regulate the expression of NF-kappaB which is important in modulating pancreatic cancer chemoresistance. While the receptors and many downstream signaling molecules have been identified and characterized, there is still much to learn about these pathways and how their deregulation can lead to cancer. Multiple inhibitors to EGFR, PI3K, mTOR, GSK-3, Raf, MEK and hedgehog (HH) have been developed and are being evaluated in various cancers. Current research often focuses on the role of these pathways in cancer stem cells (CSC), with the goal to identify sites where therapeutic resistance may develop. Relatively novel fields of investigation such as microRNAs and drugs used for other diseases e.g., diabetes, (metformin) and malaria (chloroquine) have provided new information about therapeutic resistance and CSCs. This review will focus on recent advances in the field and how they affect pancreatic cancer research and treatment.

Phosphoinositide-specific phospholipase C in health and disease

Phospholipases are widely occurring and can be found in several different organisms, including bacteria, yeast, plants, animals, and viruses. Phospholipase C (PLC) is a class of phospholipases that cleaves phospholipids on the diacylglycerol (DAG) side of the phosphodiester bond producing DAGs and phosphomonoesters. Among PLCs, phosphoinositide-specific PLC (PI-PLC) constitutes an important step in the inositide signaling pathways. The structures of PI-PLC isozymes show conserved domains as well as regulatory specific domains. This is important, as most PI-PLCs share a common mechanism, but each of them has a peculiar role and can have a specific cell distribution that is linked to a specific function. More importantly, the regulation of PLC isozymes is fundamental in health and disease, as there are several PLC-dependent molecular mechanisms that are associated with the activation or inhibition of important physiopathological processes. Moreover, PI-PLC alternative splicing variants can play important roles in complex signaling networks, not only in cancer but also in other diseases. That is why PI-PLC isozymes are now considered as important molecules that are essential for better understanding the molecular mechanisms underlying both physiology and pathogenesis, and are also potential molecular targets useful for the development of innovative therapeutic strategies.