Lipid Transfer Proteins: Introduction to the Thematic Review Series

Sepsis and high-density lipoproteins: Pathophysiology and potential new therapeutic targets

In 2020, sepsis has been defined a worldwide health major issue (World Health Organization). Lung, urinary tract and abdominal cavity are the preferred sites of sepsis-linked infection. Research has highlighted that the advancement of sepsis is not only related to the presence of inflammation or microbial or host pattern recognition. Clinicians and researchers now recognized that a severe immunosuppression is also a common feature found in patients with sepsis, increasing the susceptibility to secondary infections. Lipopolysaccharides (LPS) are expressed on the cell surface of Gram-negative, whereas Gram-positive bacteria express peptidoglycan (PGN) and lipoteichoic acid (LTA). The main mechanism by which LPS trigger host innate immune responses is binding to TLR4-MD2 (toll-like receptor4-myeloid differentiation factor 2), whereas, PGN and LTA are exogenous ligands of TLR2. Nucleotide-binding oligomerization domain (NOD)-like receptors are the most well-characterized cytosolic pattern recognition receptors, which bind microbial molecules, endogenous by-products and environmental triggers. It has been demonstrated that high-density lipoproteins (HDL), besides their major role in promoting cholesterol efflux, possess diverse pleiotropic properties, ranging from a modulation of the immune system to anti-inflammatory, anti-apoptotic, and anti-oxidant functions. In addition, HDL are able at i) binding LPS, preventing the activating of TLR4, and ii) inducing the expression of ATF3 (Activating transcription factor 3), a negative regulator of the TLR signalling pathways, contributing at justifying their capacity to hamper infection-based illnesses. Therefore, reconstituted HDL (rHDL), constituted by apolipoprotein A-I/apolipoprotein A-IMilano complexed with phospholipids, may be considered as a new therapeutic tool for the management of sepsis.

Chronic Exposure to Environmental Concentrations of Tetrabromobisphenol A Disrupts Insulin and Lipid Homeostasis in Diet-Induced Obese Mice

Tetrabromobisphenol A (TBBPA), a widely used brominated flame retardant in consumer products, has raised significant health concerns. However, the long-term metabolic effects of chronic exposure to environmentally relevant TBBPA concentrations, particularly in the context of modern high-calorie diets, remain poorly understood. Here, we show that C57BL/6J mice fed a high-fat diet and exposed to 20 or 50 nmol/kg/day TBBPA for 120 days exhibited increased body weight, aggravated fat accumulation, impaired glucose tolerance, insulin resistance, and dyslipidemia. Mechanistic investigations revealed that TBBPA exposure led to decreased norepinephrine levels, consequently reducing energy expenditure. It disrupts hepatic insulin signaling and upregulates G6Pase, thereby increasing the level of liver glucose production. Furthermore, TBBPA enhances hepatic cholesterol synthesis by elevating protein levels of HMGCR, which is the rate-limiting enzyme in cholesterol biosynthesis. This effect is mediated through increased expression of USP20, a specific deubiquitinating enzyme for HMGCR. Additionally, TBBPA modestly enhances fatty acid biosynthesis without significantly affecting lipolysis or fatty acid oxidation. Our research reveals novel molecular pathways through which environmental TBBPA exposure disrupts metabolic balance, potentially exacerbating obesity-related health issues. These findings highlight the synergistic effects between environmental pollutants and modern calorie-dense diets on metabolic health, emphasizing the importance of considering multiple factors in obesity-related disorders.

Structure-based mechanism and inhibition of cholesteryl ester transfer protein

PURPOSE OF REVIEW

Cholesteryl ester transfer proteins (CETP) regulate plasma cholesterol levels by transferring cholesteryl esters (CEs) among lipoproteins. Lipoprotein cholesterol levels correlate with the risk factors for atherosclerotic cardiovascular disease (ASCVD). This article reviews recent research on CETP structure, lipid transfer mechanism, and its inhibition.

RECENT FINDINGS

Genetic deficiency in CETP is associated with a low plasma level of low-density lipoprotein cholesterol (LDL-C) and a profoundly elevated plasma level of high-density lipoprotein cholesterol (HDL-C), which correlates with a lower risk of atherosclerotic cardiovascular disease (ASCVD). However, a very high concentration of HDL-C also correlates with increased ASCVD mortality. Considering that the elevated CETP activity is a major determinant of the atherogenic dyslipidemia, i.e., pro-atherogenic reductions in HDL and LDL particle size, inhibition of CETP emerged as a promising pharmacological target during the past two decades. CETP inhibitors, including torcetrapib, dalcetrapib, evacetrapib, anacetrapib and obicetrapib, were designed and evaluated in phase III clinical trials for the treatment of ASCVD or dyslipidemia. Although these inhibitors increase in plasma HDL-C levels and/or reduce LDL-C levels, the poor efficacy against ASCVD ended interest in CETP as an anti-ASCVD target. Nevertheless, interest in CETP and the molecular mechanism by which it inhibits CE transfer among lipoproteins persisted. Insights into the structural-based CETP-lipoprotein interactions can unravel CETP inhibition machinery, which can hopefully guide the design of more effective CETP inhibitors that combat ASCVD. Individual-molecule 3D structures of CETP bound to lipoproteins provide a model for understanding the mechanism by which CETP mediates lipid transfer and which in turn, guide the rational design of new anti-ASCVD therapeutics.

Dynamic lipid turnover in photoreceptors and retinal pigment epithelium throughout life

The retinal pigment epithelium-photoreceptor interphase is renewed each day in a stunning display of cellular interdependence. While photoreceptors use photosensitive pigments to convert light into electrical signals, the RPE supports photoreceptors in their function by phagocytizing shed photoreceptor tips, regulating the blood retina barrier, and modulating inflammatory responses, as well as regenerating the 11-cis-retinal chromophore via the classical visual cycle. These processes involve multiple protein complexes, tightly regulated ligand-receptors interactions, and a plethora of lipids and protein-lipids interactions. The role of lipids in maintaining a healthy interplay between the RPE and photoreceptors has not been fully delineated. In recent years, novel technologies have resulted in major advancements in understanding several facets of this interplay, including the involvement of lipids in phagocytosis and phagolysosome function, nutrient recycling, and the metabolic dependence between the two cell types. In this review, we aim to integrate the complex role of lipids in photoreceptor and RPE function, emphasizing the dynamic exchange between the cells as well as discuss how these processes are affected in aging and retinal diseases.

Purification and Characterization of a Nonspecific Lipid Transfer Protein 1 (nsLTP1) from Ajwain (Trachyspermum ammi) Seeds

Ajwain (Trachyspermum ammi) belongs to the family Umbelliferae, is commonly used in traditional, and folk medicine due to its carminative, stimulant, antiseptic, diuretic, antihypertensive, and hepatoprotective activities. Non-specific lipid transfer proteins (nsLTPs) reported from various plants are known to be involved in transferring lipids between membranes and in plants defense response. Here, we describe the complete primary structure of a monomeric non-specific lipid transfer protein 1 (nsLTP1), with molecular weight of 9.66 kDa, from ajwain seeds. The nsLTP1 has been purified by combination of chromatographic techniques, and further characterized by mass spectrometry, and Edman degradation. The ajwain nsLTP1 is comprised of 91 amino acids, with eight conserved cysteine residues. The amino acid sequence based predicted three dimensional (3D) structure is composed of four α-helices stabilized by four disulfide bonds, and a long C-terminal tail. The predicted model was verified by using different computational tools; i.e. ERRAT, verify 3D web server, and PROCHECK. The docking of ajwain nsLTP1 with ligands; myristic acid (MYR), and oleic acid (OLE) was performed, and molecular dynamics (MD) simulation was used to validate the docking results. The findings suggested that amino acids; Leu11, Leu12, Ala55, Ala56, Val15, Tyr59, and Leu62 are pivotal for the binding of lipid molecules with ajwain nsLTP1.