The lipid composition of high-density lipoprotein affects its re-absorption in the kidney by proximal tubule epithelial cells

Engineered rHDL Nanoparticles as a Suitable Platform for Theranostic Applications

Reconstituted high-density lipoproteins (rHDLs) can transport and specifically release drugs and imaging agents, mediated by the Scavenger Receptor Type B1 (SR-B1) present in a wide variety of tumor cells, providing convenient platforms for developing theranostic systems. Usually, phospholipids or Apo-A1 lipoproteins on the particle surfaces are the motifs used to conjugate molecules for the multifunctional purposes of the rHDL nanoparticles. Cholesterol has been less addressed as a region to bind molecules or functional groups to the rHDL surface. To maximize the efficacy and improve the radiolabeling of rHDL theranostic systems, we synthesized compounds with bifunctional agents covalently linked to cholesterol. This strategy means that the radionuclide was bound to the surface, while the therapeutic agent was encapsulated in the lipophilic core. In this research, HYNIC-S-(CH2)3-S-Cholesterol and DOTA-benzene-p-SC-NH-(CH2)2-NH-Cholesterol derivatives were synthesized to prepare nanoparticles (NPs) of HYNIC-rHDL and DOTA-rHDL, which can subsequently be linked to radionuclides for SPECT/PET imaging or targeted radiotherapy. HYNIC is used to complexing 99mTc and DOTA for labeling molecules with 111, 113mIn, 67, 68Ga, 177Lu, 161Tb, 225Ac, and 64Cu, among others. In vitro studies showed that the NPs of HYNIC-rHDL and DOTA-rHDL maintain specific recognition by SR-B1 and the ability to internalize and release, in the cytosol of cancer cells, the molecules carried in their core. The biodistribution in mice showed a similar behavior between rHDL (without surface modification) and HYNIC-rHDL, while DOTA-rHDL exhibited a different biodistribution pattern due to the significant reduction in the lipophilicity of the modified cholesterol molecule. Both systems demonstrated characteristics for the development of suitable theranostic platforms for personalized cancer treatment.

A systematic review of the biodistribution of biomimetic high-density lipoproteins in mice

For the past two decades, biomimetic high-density lipoproteins (b-HDL) have been used for various drug delivery applications. The b-HDL mimic the endogenous HDL, and therefore possess many attractive features for drug delivery, including high biocompatibility, biodegradability, and ability to transport and deliver their cargo (e.g. drugs and/or imaging agents) to specific cells and tissues that are recognized by HDL. The b-HDL designs reported in the literature often differ in size, shape, composition, and type of incorporated cargo. However, there exists only limited insight into how the b-HDL design dictates their biodistribution. To fill this gap, we conducted a comprehensive systematic literature search of biodistribution studies using various designs of apolipoprotein A-I (apoA-I)-based b-HDL (i.e. b-HDL with apoA-I, apoA-I mutants, or apoA-I mimicking peptides). We carefully screened 679 papers (search hits) for b-HDL biodistribution studies in mice, and ended up with 24 relevant biodistribution profiles that we compared according to b-HDL design. We show similarities between b-HDL biodistribution studies irrespectively of the b-HDL design, whereas the biodistribution of the b-HDL components (lipids and scaffold) differ significantly. The b-HDL lipids primarily accumulate in liver, while the b-HDL scaffold primarily accumulates in the kidney. Furthermore, both b-HDL lipids and scaffold accumulate well in the tumor tissue in tumor-bearing mice. Finally, we present essential considerations and strategies for b-HDL labeling, and discuss how the b-HDL biodistribution can be tuned through particle design and administration route. Our meta-analysis and discussions provide a detailed overview of the fate of b-HDL in mice that is highly relevant when applying b-HDL for drug delivery or in vivo imaging applications.

In Vivo PET Imaging of HDL in Multiple Atherosclerosis Models

OBJECTIVES

The goal of this study was to develop and validate a noninvasive imaging tool to visualize the in vivo behavior of high-density lipoprotein (HDL) by using positron emission tomography (PET), with an emphasis on its plaque-targeting abilities.

BACKGROUND

HDL is a natural nanoparticle that interacts with atherosclerotic plaque macrophages to facilitate reverse cholesterol transport. HDL-cholesterol concentration in blood is inversely associated with risk of coronary heart disease and remains one of the strongest independent predictors of incident cardiovascular events.

METHODS

Discoidal HDL nanoparticles were prepared by reconstitution of its components apolipoprotein A-I (apo A-I) and the phospholipid 1,2-dimyristoyl-sn-glycero-3-phosphocholine. For radiolabeling with zirconium-89 ((89)Zr), the chelator deferoxamine B was introduced by conjugation to apo A-I or as a phospholipid-chelator (1,2-distearoyl-sn-glycero-3-phosphoethanolamine-deferoxamine B). Biodistribution and plaque targeting of radiolabeled HDL were studied in established murine, rabbit, and porcine atherosclerosis models by using PET combined with computed tomography (PET/CT) imaging or PET combined with magnetic resonance imaging. Ex vivo validation was conducted by radioactivity counting, autoradiography, and near-infrared fluorescence imaging. Flow cytometric assessment of cellular specificity in different tissues was performed in the murine model.

RESULTS

We observed distinct pharmacokinetic profiles for the two (89)Zr-HDL nanoparticles. Both apo A-I- and phospholipid-labeled HDL mainly accumulated in the kidneys, liver, and spleen, with some marked quantitative differences in radioactivity uptake values. Radioactivity concentrations in rabbit atherosclerotic aortas were 3- to 4-fold higher than in control animals at 5 days' post-injection for both (89)Zr-HDL nanoparticles. In the porcine model, increased accumulation of radioactivity was observed in lesions by using in vivo PET imaging. Irrespective of the radiolabel's location, HDL nanoparticles were able to preferentially target plaque macrophages and monocytes.

CONCLUSIONS

(89)Zr labeling of HDL allows study of its in vivo behavior by using noninvasive PET imaging, including visualization of its accumulation in advanced atherosclerotic lesions. The different labeling strategies provide insight on the pharmacokinetics and biodistribution of HDL's main components (i.e., phospholipids, apo A-I).

Therapeutic applications of reconstituted HDL: When structure meets function

Reconstituted forms of HDL (rHDL) are under development for infusion as a therapeutic approach to attenuate atherosclerotic vascular disease and to reduce cardiovascular risk following acute coronary syndrome and ischemic stroke. Currently available rHDL formulations developed for clinical use contain apolipoprotein A-I (apoA-I) and one of the major lipid components of HDL, either phosphatidylcholine or sphingomyelin. Recent data have established that quantitatively minor molecular constituents of HDL particles can strongly influence their anti-atherogenic functionality. Novel rHDL formulations displaying enhanced biological activities, including cellular cholesterol efflux, may therefore offer promising prospects for the development of HDL-based, anti-atherosclerotic therapies. Indeed, recent structural and functional data identify phosphatidylserine as a bioactive component of HDL; the content of phosphatidylserine in HDL particles displays positive correlations with all metrics of their functionality. This review summarizes current knowledge of structure-function relationships in rHDL formulations, with a focus on phosphatidylserine and other negatively-charged phospholipids. Mechanisms potentially underlying the atheroprotective role of these lipids are discussed and their potential for the development of HDL-based therapies highlighted.

The effects of extracellular nucleotides on [Ca2+]i signalling in a human-derived renal proximal tubular cell line (HKC-8)

HKC-8 cells are a human-derived renal proximal tubular cell line and provide a useful model system for the study of human renal cell function. In this study, we aimed to determine [Ca(2+)](i) signalling mediated by P2 receptor in HKC-8. Fura-2 and a ratio imaging method were employed to measure [Ca(2+)](i) in HKC-8 cells. Our results showed that activation of P2Y receptors by ATP induced a rise in [Ca(2+)](i) that was dependent on an intracellular source of Ca(2+), while prolonged activation of P2Y receptors induced a rise in [Ca(2+)](i) that was dependent on intra- and extracellular sources of Ca(2+). Pharmacological and molecular data in this study suggests that TRPC4 channels mediate Ca(2+) entry in coupling to activation of P2Y in HKC-8 cells. U73221, an inhibitor of PI-PLC, did not inhibit the initial ATP-induced response; whereas D609, an inhibitor of PC-PLC, caused a significant decrease in the initial ATP-induced response, suggesting that P2Y receptors are coupled to PC-PLC. Although P2X were present in HKC-8, The P2X agonist, alpha,beta me-ATP, failed to cause a rise in [Ca(2+)](i). However, PPADS at a concentration of 100 microM inhibits the ATP-induced rise in [Ca(2+)](i). Our results indicate the presence of functional P2Y receptors in HKC-8 cells. ATP-induced [Ca(2+)](i) elevation via P2Y is tightly associated with PC-PLC and TRP channel.

Steroidogenic acute regulatory-related lipid transfer domain protein 5 localization and regulation in renal tubules

STARD5 is a cytosolic sterol transport protein that is predominantly expressed in liver and kidney. This study provides the first report on STARD5 protein expression and distribution in mouse kidney. Immunohistochemical analysis of C57BL/6J mouse kidney sections revealed that STARD5 is expressed in tubular cells within the renal cortex and medullar regions with no detectable staining within the glomeruli. Within the epithelial cells of proximal renal tubules, STARD5 is present in the cytoplasm with high staining intensity along the apical brush-border membrane. Transmission electron microscopy of a renal proximal tubule revealed STARD5 is abundant at the basal domain of the microvilli and localizes mainly in the rough endoplasmic reticulum (ER) with undetectable staining in the Golgi apparatus and mitochondria. Confocal microscopy of STARD5 distribution in HK-2 human proximal tubule cells showed a diffuse punctuate pattern that is distinct from the early endosome marker EEA1 but similar to the ER membrane marker GRP78. Treatment of HK-2 cells with inducers of ER stress increased STARD5 mRNA expression and resulted in redistribution of STARD5 protein to the perinuclear and cell periphery regions. Since recent reports show elevated ER stress response gene expression and increased lipid levels in kidneys from diabetic rodent models, we tested STARD5 and cholesterol levels in kidneys from the OVE26 type I diabetic mouse model. Stard5 mRNA and protein levels are increased 2.8- and 1.5-fold, respectively, in OVE26 diabetic kidneys relative to FVB control kidneys. Renal free cholesterol levels are 44% elevated in the OVE26 mice. Together, our data support STARD5 functioning in kidney, specifically within proximal tubule cells, and suggest a role in ER-associated cholesterol transport.