The binding of lipoproteins to human muscle cells: binding and uptake of LDL, HDL, and alpha-tocopherol

Transfer Investigations of Lipophilic Drugs from Lipid Nanoemulsions to Lipophilic Acceptors: Contributing Effects of Cholesteryl Esters and Albumin as Acceptor Structures

When studying the release of poorly water-soluble drugs from colloidal drug delivery systems designed for intravenous administration, the release media should preferentially contain lipophilic components that represent the physiological acceptors present in vivo. In this study, the effect of different acceptor structures was investigated by comparing the transfer of fenofibrate, retinyl acetate, and orlistat from trimyristin nanoemulsion droplets into lipid-containing hydrogel particles, as well as to bovine serum albumin (BSA). A nanodispersion based on trimyristin and cholesteryl nonanoate was incorporated into the hydrogel particles (mean diameter ~40 µm) in order to mimic the composition of lipoproteins. The course of transfer observed utilizing the lipid-containing hydrogel particles as an acceptor was in relation to the lipophilicity of the drugs: the higher the logP value, the slower the transfer. There was no detectable amount of the drugs transferred to BSA in liquid solution, demonstrating clearly that albumin alone does not contribute substantially as acceptor for the lipophilic drugs under investigation in this study. In contrast, cholesteryl nonanoate contributes to a much greater extent. However, in all cases, the partition equilibrium of the drugs under investigation was in favor of the trimyristin emulsion droplets.

Ceramides contained in LDL are elevated in type 2 diabetes and promote inflammation and skeletal muscle insulin resistance

Dysregulated lipid metabolism and inflammation are linked to the development of insulin resistance in obesity, and the intracellular accumulation of the sphingolipid ceramide has been implicated in these processes. Here, we explored the role of circulating ceramide on the pathogenesis of insulin resistance. Ceramide transported in LDL is elevated in the plasma of obese patients with type 2 diabetes and correlated with insulin resistance but not with the degree of obesity. Treating cultured myotubes with LDL containing ceramide promoted ceramide accrual in cells and was accompanied by reduced insulin-stimulated glucose uptake, Akt phosphorylation, and GLUT4 translocation compared with LDL deficient in ceramide. LDL-ceramide induced a proinflammatory response in cultured macrophages via toll-like receptor-dependent and -independent mechanisms. Finally, infusing LDL-ceramide into lean mice reduced insulin-stimulated glucose uptake, and this was due to impaired insulin action specifically in skeletal muscle. These newly identified roles of LDL-ceramide suggest that strategies aimed at reducing hepatic ceramide production or reducing ceramide packaging into lipoproteins may improve skeletal muscle insulin action.

Vitamin E Regulatory Mechanisms

Dietary and supplemental vitamin E is absorbed and delivered to the liver, but of the various antioxidants with vitamin E activity, only alpha-tocopherol is preferentially recognized by the alpha-tocopherol transfer protein (alpha-TTP) and is transferred to plasma, while the other vitamin E forms (e.g., gamma-tocopherol or tocotrienols) are removed from the circulation. Hepatic alpha-TTP is required to maintain plasma and tissue alpha-tocopherol concentrations. The liver is the master regulator of the body's vitamin E levels in that it not only controls alpha-tocopherol concentrations, but also appears to be the major site of vitamin E metabolism and excretion. Vitamin Es are metabolized similarly to xenobiotics; they are initially omega-oxidized by cytochrome P450s, undergo several rounds of beta-oxidation, and then are conjugated and excreted. As a result of these various mechanisms, liver alpha-tocopherol and other vitamin E concentrations are closely regulated; thus, any potential adverse vitamin E effects are limited.

Anti-peroxidation effects of vitamin E on low density lipoprotein and milk fat globule membrane of lactating goats: in vivo versus metal ion challenge in vitro

Ruminants are animals with mild oxidation risk considering characteristics of their plasma. The purpose of the present study was to determine if surplus vitamin E supplementation further improved their peroxidation status. Four lactating goats (Capra ibex ibex) receiving a single intramuscular injection of 3000 IU d-alpha-tocopheryl acetate were monitored daily for a week. Plasma and milk levels of thiobarbituric acid reactive substances (TBARS) were measured to estimate the peroxidation status of overall body and mammary gland, respectively. Lipid hydroperoxide content of low density lipoprotein (LDL) and milk fat globule membrane (MFGM) were determined both immediately after isolation and after metal ion challenge to evaluate their spontaneous peroxidation in vivo and peroxidation susceptibility in vitro, respectively. The results showed that while plasma and milk levels of alpha-tocopherol peaked at day 2, the corresponding TBARS dropped to their lowest weekly levels. Content of preformed lipid hydroperoxides in LDL and MFGM remained unchanged (P>0.05) during the study period, while peroxidation in LDL and MFGM on extended exposure to Cu(2+) was prevented by vitamin E enrichment. Therefore, surplus vitamin E was beneficial to the peroxidation-resistant mechanisms of the overall body and those specifically within mammary gland of lactating goats. Furthermore, peroxidation resistance of LDL and MFGM to copper challenge in vitro also benefited from vitamin E enrichment despite the fact their endogenous peroxidation seemed unresponsive. The hypothesis that peroxidation of LDL and MFGM of lactating goats in situ, which proceeds with metal ion-independent mechanisms, is minor was discussed.

Tocopherol-binding proteins: their function and physiological significance

The present review is a continuation of earlier essays on the uptake mechanisms and the biological function of vitamin E. There are eight naturally occurring homologues of vitamin E, which differ in their structure and in biological activity in vivo and in vitro. Various studies have suggested that after normal gastrointestinal absorption of dietary vitamin E specific mechanisms favor the preferential accumulation of one of its homologues, alpha-tocopherol, in the human body. This process is thought to be mediated in part by the alpha-tocopherol transfer protein (alpha-TTP) in the liver cytoplasm. The mechanism and pathway by which alpha-TTP specifically incorporates alpha-tocopherol into plasma lipoproteins is not yet fully understood. Because alpha-tocopherol is widely distributed in tissues in various concentrations but alpha-TTP resides only in liver, its role as intracellular carrier of alpha-tocopherol seems unlikely. However, recent data indicate that a system of alpha-tocopherol-binding proteins is involved in these processes that favor the localization of alpha-tocopherol at the sites where it is required. The current status of the evidence for the regulation of alpha-tocopherol levels and their impact on cellular signaling is discussed.

Role of plasma lipoproteins in modifying the biological activity of hydrophobic drugs

The plasma lipoprotein distribution of potential drug candidates is not commonly studied. For some hydrophobic drug candidates, attainment of similar plasma free drug levels has not been associated with uniform production of pharmacological activity in different animal species. It is well-known that plasma lipoprotein lipid profiles vary considerably between different animal species. In addition, human disease states can significantly influence plasma lipoprotein profiles, resulting in altered therapeutic outcomes. Current research has shown that lipoprotein binding of drug compounds can significantly influence not only the pharmacological and pharmacokinetic properties of the drug, but the relative toxicity as well. Elucidation of drug distribution among plasma lipoproteins is expected to yield valuable insight into factors governing the pharmacological activity and potential toxicity of the drug. This paper will present an historical perspective and summarize the latest research in the area of lipoprotein-drug interactions.

Specific binding of leukemia inhibitory factor to murine myoblasts in culture

Leukemia inhibitory factor (LIF) is a member of the cytokine family of growth factors. It has been shown to exert a variety of actions on a diverse range of cell types, including neuronal, bone, and hemopoietic cells (Hilton, 1992, Trends Biochem. Sci., 17:72-76). In many of these cell types, studies have indicated the presence of specific receptors for LIF (Godard et al., 1982, J. Biol. Chem., 267: 3214-3222; Hilton et al., Proc. Natl. Acad. Sci. USA, 85:5971-5975; Hilton and Nicola, 1992, J. Biol. Chem., 267:10238-10247.). The mechanism by which these receptors act is believed to involve tyrosine phosphorylation and the signal transducing receptor component gp130. We have previously shown that LIF is capable of inducing both human and murine myoblasts to proliferate in culture (Austin et al., 1992, J. Neurol. Sci., 112:185-191). We now report that LIF binds specifically to receptors on the surface of myoblasts, with an equilibrium dissociation constant of 400 pM and the number of receptors per cell varies with cell density. Binding competition studies showed that LIF binding to these receptor sites was not competed for by a number of other growth factors which stimulate myoblast proliferation including basic fibroblast growth factor (bFGF), transforming growth factor-alpha (TGF alpha), insulin-like growth factor 1 (IGF-1), and interleukin-6 (IL-6). There was a time and concentration-dependent down-regulation of receptor numbers following preincubation of myoblasts with LIF. The processing of these receptors subsequent to binding, involves as a first step, internalization and degradation by the myoblast. LIF appeared to stimulate myoblast proliferation rather than cell survival.

Muscular involvement in lysosomal acid lipase deficiency in rats

We investigated the pathological and biochemical changes of skeletal muscle in rats with lysosomal acid lipase deficiency, which is an animal counterpart of human Wolman's disease. In the affected rats, the acid lipase activity for three different substrates, 4-methylumbelliferyl-oleate (18.9% of the normal control level), [14C]cholesteryl oleate (23.5%), and [14C]triolein (26.9%), was similarly decreased in the lysosomal fraction of skeletal muscle which was obtained by differential centrifugation. Histochemical studies showed that acid phosphatase activity was high in the endomysium and perimysium and in some muscle fibers. Some fibers showed vacuolar degeneration resembling "rimmed vacuoles". Ultrastructural studies demonstrated many membrane-bound lipid droplets in the muscle fibers, especially in the subsarcolemmal space, indicating that a low density lipoprotein (LDL) uptake pathway apparently existed in the muscle cells. However, such lipid accumulation was much greater in the interstitial cells and the endothelial cells. This distribution also suggests that LDL/cholesterol is supplied to muscle cells predominantly through endothelial cells.

Stimulation of myoblast proliferation in culture by leukaemia inhibitory factor and other cytokines

Significant stimulation of growth of myoblasts in culture is achieved by leukemia inhibitory factor (LIF). The optimum activity of this cytokine occurs at about 6 pM LIF. Interleukin-6 (IL-6) also stimulates cultured myoblasts but to a lesser degree than LIF and the effect is not maintained for extended culture periods. In addition, transforming growth factor-alpha (TGF-alpha) also increases the growth rate of myoblasts but only after a considerable lag phase. All 3 cytokines may be of value in the large scale production of myoblasts for use in the potential treatment of primary myopathies by injection of cultured myoblasts into diseased muscle to form genetically complete muscle fibres after fusion of the myoblasts in situ. Their potential use is enhanced in that at least under the conditions used here they do not stimulate fibroblast proliferation.

Therapeutic potential of vitamin E in the pathogenesis of spontaneous atherosclerosis

Spontaneous atherosclerosis is largely an occlusive disease of medium-size arteries whose progression in a hyperlipidemic environment reflects chronic interactions among injury stimuli to the vessel wall and "responses to injury" by vascular tissue and certain blood components. Development of vessel lesions in animal models of spontaneous atherosclerosis and (at least in principle) in man largely reflects responses of three major cell types (vascular endothelial cells, vascular smooth muscle cells, monocytes-macrophages) as well as the content and distribution of lipids among various lipoprotein subclasses and the increased atherogenicity of modified (e.g., oxidized) lipoproteins. The severe clinical complications associated with spontaneous atherosclerosis, along with its rather common incidence in man, have focused attention on the prevention and therapy of this vascular disease state. Some pharmacological studies in animal models of spontaneous atherosclerosis and some retrospective epidemiological studies in man suggest that vitamin E, the principal (if not sole) lipid-soluble chain-breaking tissue antioxidant, might have therapeutic benefit as an antiatherosclerotic agent. This suggestion gains support from a variety of compelling in vitro evidence demonstrating direct influences of vitamin E on cells and lipoproteins likely involved in the pathogenesis of spontaneous atherosclerosis. Biochemical and cellular data indicate that the potential antiatherogenic activity of vitamin E could reflect its activities as a regulator of endothelial, smooth muscle, or monocyte-macrophage function, an inhibitor of endothelial membrane lipid peroxidation, a modulator of plasma lipid levels and lipid distribution among circulating lipoproteins, and a preventor of lipoprotein oxidative modification. On the other hand, there is a comparative lack of conclusive evidence from animal models regarding: (a) the importance to atherogenesis of vascular and cellular processes modulated by vitamin E; (b) the influence of vitamin E on these processes in vivo and, consequently, on the initiation/progression of spontaneous atherosclerosis. Therefore, pharmacologic investigation of vitamin E (and synthetic, vitamin E-like antioxidants) in nutritional and hyperlipidemic animal models of spontaneous atherosclerosis is required to establish whether any atherosclerotic impact is associated with vitamin E and, if so, what the mechanistic basis of the therapeutic benefit is. Such a line of experimental inquiry should also increase our understanding of the pathogenesis of atherosclerotic vessel disease per se.

Hepatic uptake and antihepatotoxic properties of vitamin E and liposomes in the mouse

Intravenous administration of soybean phosphatidylcholine liposomes containing different amounts of tocopherol acetate leads to a dose and time dependent increase of mouse liver tocopherol content, which was not observed when the preparation was given orally. When benzo[a]pyrene pretreated mice intoxicated with 400 mg/kg AAP were pretreated 2 h before with 1 g/kg phosphatidylcholine liposomes containing 4 mg/kg vitamin E acetate, these animals were protected against liver damage. Vitamin E alone or liposomes lacking vitamin E showed no protection. In an inflammatory liver disease model, i.e. fulminant hepatitis induced by intraperitoneal administration of 700 mg/kg galactosamine and 1 microgram/kg lipopolysaccharide phosphatidylcholine liposomes protected at a dose of 1 g/kg i.v. In this case, however, the protection was not due to the presence of vitamin E. These findings demonstrate the usefulness of phosphatidylcholine for liver protection and show that the protective spectrum is improved when they contain vitamin E. The data suggest that phosphatidylcholine is an excellent carrier for delivery of vitamin E to the liver.