Cholesterol modulates PAF-stimulated Ca2(+)-mobilization in monocytic U937 cells

Effects of membrane cholesterol manipulation on excitation-contraction coupling in skeletal muscle of the toad

1. Single mechanically skinned fibres and intact bundles of fibres from the twitch region of the iliofibularis muscle of cane toads were used to investigate the effects of membrane cholesterol manipulation on excitation-contraction (E-C) coupling. The cholesterol content of membranes was manipulated with methyl-beta-cyclodextrin (MbetaCD). 2. In mechanically skinned fibres, depletion of membrane cholesterol with MbetaCD caused a dose- and time-dependent decrease in transverse tubular (t)-system depolarization-induced force responses (TSDIFRs). TSDIFRs were completely abolished within 2 min in the presence of 10 mM MbetaCD but were not affected after 2 min in the presence of a 10 mM MbetaCD-1 mM cholesterol complex. There was a very steep dependence between the change in TSDIFRs and the MbetaCD : cholesterol ratio at 10 mM MbetaCD, indicating that the inhibitory effect of MbetaCD was due to membrane cholesterol depletion and not to a pharmacological effect of the agent. Tetanic responses in bundles of intact fibres were abolished after 3-4 h in the presence of 10 mM MbetaCD. 3. The duration of TSDIFRs increased markedly soon (< 2 min) after application of 10 mM MbetaCD and 10 mM MbetaCD-cholesterol complexes, but the Ca(2+) activation properties of the contractile apparatus were minimally affected by 10 mM MbetaCD. The Ca(2+) handling abilities of the sarcoplasmic reticulum appeared to be modified after 10 min exposure to 10 mM MbetaCD. 4. Confocal laser scanning microscopy revealed that the integrity of the t-system was not compromised by either intra- or extracellular application of 10 mM MbetaCD and that a large [Ca(2+)] gradient was maintained across the t-system. 5. Membrane cholesterol depletion caused rapid depolarization of the polarized t-system as shown independently by spontaneous TSDIFRs induced by MbetaCD and by changes in the fluorescence intensity of an anionic potentiometric dye (DiBAC(4)(3)) in the presence of MbetaCD. This rapid depolarization of the t-system by cholesterol depletion was not prevented by blocking the Na(+) channels with TTX (10 microM) or the L-type Ca(2+) channels with Co(2+) (5 mM). 6. The results demonstrate that cholesterol is important for maintaining the functional integrity of the t-system and sarcoplasmic reticulum, probably by having specific effects on different membrane proteins that may be directly or indirectly involved in E-C coupling.

In situ localization of cholesterol in skeletal muscle by use of a monoclonal antibody

A common perception is that cholesterol, the major structural lipid found in mammalian membranes, is localized nearly exclusively to the plasma membrane of living cells and that it is found in much smaller quantities in internal membranes. This perception is based almost exclusively on cell fractionation studies, in which density gradient centrifugation is used for purification of discrete subcellular membrane fractions. Here we describe a monoclonal antibody, MAb 2C5-6, previously reported to detect purified cholesterol in synthetic membranes (Swartz GM Jr, Gentry MK, Amende LM, Blanchette-Mackie EJ, and Alving CR. Proc Natl Acad Sci USA 85: 1902-1906, 1988), that is capable of detecting cholesterol in situ in the membranes of skeletal muscle sections. Localization of cholesterol, the dihydropyridine receptor of the T tubule, and the Ca(2+)-ATPase of the sarcoplasmic reticulum (SERCA2) by means of double and triple immunostaining protocols clearly demonstrates that cholesterol is primarily localized to the sarcoplasmic reticulum membranes of skeletal muscle rather than the sarcolemmal or T tubule membranes. The availability of this reagent and its ability to spatially localize cholesterol in situ may provide a greater understanding of the relationship between membrane cholesterol content and transmembrane signaling in skeletal muscle.

Cholesterylphosphoserine as inhibitor of cell adhesion and actin polymerization in human T cells

To further investigate the immunosuppressive activity of cholesterylphosphoserine (CPHS), we examined a variety of human T cell responses including proliferation, adhesion and cytoskeletal organization. The CPHS-induced inhibition of T cell response is greater in the integrin-dependent mixed lymphocyte reaction than in the integrin-independent proliferation elicited by anti-TCR-CD3 or anti-CD28 antibodies in the presence of tetradecanoylphorbol acetate. Consistently, CPHS inhibits the homotypic T cell adhesion involving the integrin alphaLbeta2 (LFA-1) and the cell adhesion to fibronectin and rVCAM-1 involving the integrins of the beta1 family. Since CPHS does not change integrin expression but inhibits post-receptor events such as cell spreading and pseudopodal projections, it seems likely that the site of CPHS influence is distal to the adhesion receptors. In agreement, the steroid prevents the reorganization of actin cytoskeleton occurring when T cells are allowed to spread on immobilized anti-CD3 in the absence of integrin activation. We suggest that CPHS acts on the metabolic pathway in which signals from integrin and growth factor receptors converge to induce the reorganization of the actin cytoskeleton. Selectivity in the action of CPHS is indicated by its ineffectiveness in the integrin-mediated adhesion of the monocytic cell line U-937 to fibronectin.

Platelet-activating factor-induced inositol 1,4,5-trisphosphate generation in undifferentiated and differentiated U937 cells: role of tyrosine kinase

We compared the effect of platelet-activating factor (PAF) on inositol 1,4,5-trisphosphate (IP3) content in undifferentiated and differentiated U937 cells. In both cell types, PAF induced a rapid transient and concentration-dependent elevation of IP3 content. The production of IP3 in response to PAF was greater in differentiated than in undifferentiated cells. The increases in IP3 produced by PAF in both types of cell were inhibited by the PAF receptor antagonist, WEB 2086, as well as by the tyrosine kinase inhibitor, genistein. PAF also caused increased tyrosine phosphorylation of a 32 kDa protein substrate in both undifferentiated and differentiated cells. The magnitude of the phosphorylation was, however, greater in the differentiated cells. Genistein reduced the PAF-induced tyrosine phosphorylation of the substrate in both cell preparations. The specific binding of [3H]PAF was also markedly enhanced in differentiated cells. This effect was attenuated by genistein. The results indicate that PAF induces the production of IP3 in U937 cells via tyrosine kinase-mediated mechanisms and this process is augmented in differentiated cells.

Cholesterol and non-cardiovascular disease: basic science

Cholesterol metabolism is of fundamental biological importance. This review examines the role of cholesterol metabolism in relation to non-cardiovascular disease (non-CVD). Particular attention is paid to the question of whether or not low levels of cholesterol may have harmful effects on cell function or lead to pathological processes. Many in vitro phenomena have been demonstrated at levels of cholesterol which are very low in comparison to physiological conditions. Nevertheless, low cholesterol is more favourable than high cholesterol for most aspects of cell function. There is no evidence that any catastrophic cellular response or pathological process occurs due to exposure of organisms to low cholesterol. On the other hand, the inflammatory process is a powerful and consistent cause of decreased cholesterol levels. This, together with other confounding factors, appears to explain a major component of the association between low cholesterol levels and non-CVD.

Eicosapentaenoic acid abolishes the proatherogenic effects of cholesterol: effects on migration of bovine smooth muscle and endothelial cells in vitro

It is well known that vascular endothelial cell (EC) migration plays a major role in regeneration of the injured endothelium and also that smooth muscle cell (SMC) migration is the important step for atheromatous plaque formation. In the present study, we investigated the effects of cholesterol and eicosapentaenoic acid (EPA) on bovine carotid artery EC and SMC migration using the modified Boyden chamber technique. The migration activity of the cholesterol-enriched ECs loaded with cholesterol-rich liposomes was significantly suppressed, whereas that of the cholesterol-enriched SMCs was enhanced. Next, we examined the effects of EPA pretreatment on the migration of both cell types. When ECs and SMCs were treated with EPA (5 micrograms/ml) for 2 days, the EPA content increased from 0.55 +/- 0.04% to 11.72 +/- 0.19% and 1.22 +/- 0.09% to 9.69 +/- 0.07% in cellular phospholipids, respectively. Although pretreatment of the ECs with EPA caused a significant increase in serum-induced cell migration, pretreatment of SMCs had no effect. If both cell types were concomitantly pretreated with EPA and cholesterol-rich liposomes, EPA abolished the effects of cholesterol on the migration of both cell types, but did not affect the content of cholesterol in both cells. These data indicate the possibility that EPA counteracts the atherogenic effect of cholesterol on EC and SMC migration.