Amyloid fibril formation of peptides derived from the C-terminus of CETP modulated by lipids

Peptide Helix-Y12 as Potential Effector for Peroxisome Proliferator-Activated Receptors

Peroxisome proliferator-activated receptors (PPARs) are nuclear receptors involved in the regulation of lipids and glucose metabolism, and immune response. Therefore, they have been considered pharmacological targets for treating metabolic diseases, such as dyslipidemia, atherosclerosis, and non-alcoholic fatty liver disease. However, the available synthetic ligands of PPARs have mild to significant side effects, generating the necessity to identify new molecules that are selective PPAR ligands with specific biological responses. This study aimed to evaluate some components of the atheroprotective and hepatoprotective HB-ATV-8 nanoparticles [the amphipathic peptide Helix-Y¹², thermozeaxanthin, thermozeaxanthin-13, thermozeaxanthin-15, and a set of glycolipids], as possible ligands of PPARs through blind molecular docking. According to the change in free energy upon protein-ligand binding, ∆G _b, thermozeaxanthins show a more favorable interaction with PPARs, followed by Helix-Y¹². Moreover, Helix-Y¹² interacts with most parts of the Y-shaped ligand-binding domain (LBD), surrounding helix 3 of PPARs, and reaching helix 12 of PPARα and PPARγ. As previously reported for other ligands, Tyr314 and Tyr464 of PPARα interact with Helix-Y¹² through hydrogen bonds. Several PPARα's amino acids are involved in the ligand binding by hydrophobic interactions. Furthermore, we identified additional PPARs' amino acids interacting with Helix-Y¹² through hydrogen bonds still not reported for known ligands. Our results show that, from the studied ligand set, the Helix-Y¹² peptide and Tzeaxs have the most significant probability of binding to the PPARs' LBD, suggesting novel ligands for PPARs.

Lipid Modulation in the Formation of β-Sheet Structures. Implications for De Novo Design of Human Islet Amyloid Polypeptide and the Impact on β-Cell Homeostasis

Human islet amyloid polypeptide (hIAPP) corresponds to a 37-residue hormone present in insulin granules that maintains a high propensity to form β-sheet structures during co-secretion with insulin. Previously, employing a biomimetic approach, we proposed a panel of optimized IAPP sequences with only one residue substitution that shows the capability to reduce amyloidogenesis. Taking into account that specific membrane lipids have been considered as a key factor in the induction of cytotoxicity, in this study, following the same design strategy, we characterize the effect of a series of lipids upon several polypeptide domains that show the highest aggregation propensity. The characterization of the C-native segment of hIAPP (residues F₂₃-Y₃₇), together with novel variants F₂₃R and I₂₆A allowed us to demonstrate an effect upon the formation of β-sheet structures. Our results suggest that zwitterionic phospholipids promote adsorption of the C-native segments at the lipid-interface and β-sheet formation with the exception of the F₂₃R variant. Moreover, the presence of cholesterol did not modify this behavior, and the β-sheet structural transitions were not registered when the N-terminal domain of hIAPP (K₁-S₂₀) was characterized. Considering that insulin granules are enriched in phosphatidylserine (PS), the property of lipid vesicles containing negatively charged lipids was also evaluated. We found that these types of lipids promote β-sheet conformational transitions in both the C-native segment and the new variants. Furthermore, these PS/peptides arrangements are internalized in Langerhans islet β-cells, localized in the endoplasmic reticulum, and trigger critical pathways such as unfolded protein response (UPR), affecting insulin secretion. Since this phenomenon was associated with the presence of cytotoxicity on Langerhans islet β-cells, it can be concluded that the anionic lipid environment and degree of solvation are critical conditions for the stability of segments with the propensity to form β-sheet structures, a situation that will eventually affect the structural characteristics and stability of IAPP within insulin granules, thus modifying the insulin secretion.

Modulation of Amyloidogenesis Controlled by the C-Terminal Domain of Islet Amyloid Polypeptide Shows New Functions on Hepatocyte Cholesterol Metabolism

The islet amyloid polypeptide (IAPP) or amylin maintains a key role in metabolism. This 37-residues-peptide could form pancreatic amyloids, which are a characteristic feature of diabetes mellitus type 2. However, some species do not form amyloid fibril structures. By employing a biomimetic approach, we generated an extensive panel of optimized sequences of IAPP, which could drastically reduce aggregation propensity. A structural and cellular characterization analysis was performed on the C-terminal domain with the highest aggregation propensity. This allowed the observation of an aggregative phenomenon dependent of the lipid environment. Evaluation of the new F₂₃R variant demonstrated inhibition of β-sheet structure and, therefore, amyloid formation on the native C-terminal, phenomenon that was associated with functional optimization in calcium and cholesterol management coupled with the optimization of insulin secretion by beta cells. When F₂₃R variant was evaluated in microglia cells, a model of amyloidosis, cytotoxic conditions were not registered. In addition, it was found that C-terminal sequences of IAPP could modulate cholesterol metabolism in hepatocytes through regulation of SREBP-2, apoA-1, ABCA1, and LDLR, mechanism that may represent a new function of IAPP on the metabolism of cholesterol, increasing the LDL endocytosis in hepatocytes. Optimized sequences with only one residue modification in the C-terminal core aggregation could diminish β-sheet formation and represent a novel strategy adaptable to other pharmacological targets. Our data suggest a new IAPP function associated with rearrangements on metabolism of cholesterol in hepatocytes.

Anti-Aging and Tissue Regeneration Ability of Policosanol Along with Lipid-Lowering Effect in Hyperlipidemic Zebrafish via Enhancement of High-Density Lipoprotein Functionality

We investigated the tissue regeneration and lipid-lowering effects of policosanol (PCO) by employing a hyperlipidemic zebrafish model. A reconstituted high-density lipoprotein containing policosanol (PCO-rHDL) facilitated greater cell growth and replication with less apoptosis and reactive oxygen species (ROS) production in BV-2 microglial cell lines. From in vivo study, injection of rHDL containing apolipoprotein A-I (ApoA-I) caused 76 ± 4% (p = 0.01) greater tissue regeneration activity than the phosphate-buffered saline (PBS) control, whereas PCO-rHDL caused 94 ± 7% (p = 0.002) increased regeneration. PCO in ethanol (EtOH) showed lower cholesteryl ester transfer protein (CETP) inhibitory ability than did anacetrapib, whereas PCO-rHDL showed higher inhibitory ability than anacetrapib, suggesting a synergistic effect between PCO and rHDL. Following 9 weeks of PCO consumption, the PCO group (0.003% PCO in Tetrabit) showed the highest survivability (80%), whereas normal diet (ND) and high-cholesterol diet (HCD) control groups showed 67% and 70% survival rates, respectively. Supplementation with a HCD resulted in two-fold elevation of CETP activity along with 3- and 2.5-fold increases in serum total cholesterol (TC) and triglycerides (TGs) levels, respectively. Consumption of PCO for 9 weeks resulted in 40 ± 5% (p = 0.01 vs. HCD) and 33 ± 4% (p = 0.02 vs. HCD) reduction of TC and TGs levels, respectively. Serum high-density lipoprotein cholesterol (HDL-C) level increased up to 37 ± 2 mg/dL (p = 0.004), whereas the percentage of HDL-C/TC increased up to 20 ± 2% from 5 ± 1% compared to the HCD control. The serum glucose level was reduced to 47 ± 2% (p = 0.002) compared to the HCD control. Fatty liver change and hepatic inflammation levels were remarkably increased upon HCD consumption and were two-fold higher than that under ND. However, the PCO group showed 58 ± 5% (p = 0.001) and 50 ± 3% (p = 0.006) reduction of inflammation enzyme levels and lipid content in hepatic tissue under HCD. In conclusion, PCO supplementation showed lipid-lowering and HDL-C-elevating effects with ameliorating fatty liver change. These in vivo anti-atherosclerotic and anti-diabetic effects of PCO are well associated with in vitro anti-apoptotic activities.

The C-terminal Domain Supports a Novel Function for CETPI as a New Plasma Lipopolysaccharide-Binding Protein

Described by our group a few years ago, the cholesteryl-ester transfer protein isoform (CETPI), exclusively expressed in the small intestine and present in human plasma, lacked a functional identification for a role of physiological relevance. Now, this study introduces CETPI as a new protein with the potential capability to recognise, bind and neutralise lipopolysaccharides (LPS). Peptides derived from the C-terminal domain of CETPI showed that CETPI not only might interact with several LPS serotypes but also might displace LPS bound to the surface of cells. Peptide VSAK, derived from the last 18 residues of CETPI, protected against the cytotoxic effect of LPS on macrophages. At high concentrations, when different cell types were tested in culture, it did not exhibit cytotoxicity by itself and it did prevent the expression of pro-inflammatory cytokines as well as the generation of oxidative stress conditions. In a rabbit model of septic shock, the infusion of peptide VSAK exerted a protective effect against the effects of LPS and reduced the presence of tumor necrosis factor-alpha (TNFα) in plasma. Therefore, CETPI is proposed as a new protein with the capability to advance the possibilities for better understanding and treatment of the dangerous effects of LPS in vivo.

Protein Folding and Mechanisms of Proteostasis

Highly sophisticated mechanisms that modulate protein structure and function, which involve synthesis and degradation, have evolved to maintain cellular homeostasis. Perturbations in these mechanisms can lead to protein dysfunction as well as deleterious cell processes. Therefore in recent years the etiology of a great number of diseases has been attributed to failures in mechanisms that modulate protein structure. Interconnections among metabolic and cell signaling pathways are critical for homeostasis to converge on mechanisms associated with protein folding as well as for the preservation of the native structure of proteins. For instance, imbalances in secretory protein synthesis pathways lead to a condition known as endoplasmic reticulum (ER) stress which elicits the adaptive unfolded protein response (UPR). Therefore, taking this into consideration, a key part of this paper is developed around the protein folding phenomenon, and cellular mechanisms which support this pivotal condition. We provide an overview of chaperone protein function, UPR via, spatial compartmentalization of protein folding, proteasome role, autophagy, as well as the intertwining between these processes. Several diseases are known to have a molecular etiology in the malfunction of mechanisms responsible for protein folding and in the shielding of native structure, phenomena which ultimately lead to misfolded protein accumulation. This review centers on our current knowledge about pathways that modulate protein folding, and cell responses involved in protein homeostasis.

Reality of a Vaccine in the Prevention and Treatment of Atherosclerosis

Atherosclerosis together with multiple sclerosis, psoriasis and rheumatoid arthritis can be used as examples of chronic inflammatory diseases associated with multifactorial components that evolve over the years. Nevertheless, an important difference between these diseases relies on the fact that atherosclerosis develops from early ages where inflammation dominates the very beginning of the disease. This review highlights the inflammatory nature of atherosclerosis and the role the immune system plays in the process of atherogenesis. Although treatment of atherosclerosis has been for years based on lipid-lowering therapies reducing a series of risk factors, the degree of success has been only limited because cardiovascular complications related to the evolution of atherosclerotic lesions continue to appear in the population worldwide. In this sense, alternative treatments for atherosclerosis have come into play where both innate and adaptive immunity have been proposed to modulate atherosclerosis-associated inflammatory phenomena. When tested for their atheroprotective properties, several immunogens have been studied through passive and active immunization with good results and, therefore, the strategy through vaccination to control the disease has been made possible. Many experimental pre-clinical studies demonstrating proof of concept that vaccination using DNA and protein with an effective use of adjuvants and the optimal route of administration now provide a tangible new therapeutic approach that sets the stage for several of these vaccines to be tested in large, randomized, long-term clinical studies. A vaccine ready for human use will only be accomplished through the close association between academia, regulatory government organizations and private industry, allowing the reality of a simple and successful therapy to reduce atherosclerosis and its severe clinical complications.

Key structural arrangements at the C-terminus domain of CETP suggest a potential mechanism for lipid-transfer activity

The cholesteryl-ester transfer protein (CETP) promotes cholesteryl-ester and triglyceride transfer between lipoproteins. We evaluated the secondary structure stability of a series of small peptides derived from the C-terminus of CETP in a wide range of pH's and lipid mixtures, and studied their capability to carry out disorder-to-order secondary structure transitions dependent of lipids. We report that while a mixture of phosphatidylcholine/cholesteryl-esters forms large aggregated particles, the inclusion of a series of CETP carboxy-terminal peptides in a stable α-helix conformation, allows the formation of small homogeneous micelle-like structures. This phenomenon of lipid ordering was directly connected to secondary structural transitions at the C-terminus domain when lysophosphatidic acid and lysophosphatidylcholine lipids were employed. Circular dichroism, cosedimentation experiments, electron microscopy, as well as molecular dynamics simulations confirm this phenomenon. When purified CETP is studied, the same type of phenomenon occurs by promoting the reorganization of lipid from large to smaller particles. Our findings extend the emerging view for a novel mechanism of lipid transfer carried out by CETP, assigning its C-terminus domain the property to accomplish lipid ordering through secondary structure disorder-to-order transitions.