Mechanism of the activation of arachidonic acid release by oxysterols in NRK 49F cells: role of calcium

Quantitative detection of free 24S-hydroxycholesterol, and 27-hydroxycholesterol from human serum

Background

Cholesterol metabolism is important for the maintenance of myelin and neuronal membranes in the central nervous system. Blood concentrations of the brain specific cholesterol metabolite 24S-hydroxysterol to the peripheral metabolite 27-hydroxycholesterol may be useful surrogate markers for neurodegenerative diseases including Alzheimer’s disease, Huntington’s disease, HIV-Associated Neurocognitive Disorders, and Multiple Sclerosis. However, current methods to isolate hydroxycholesterols are labor intensive, prone to produce variable extraction efficiencies and do not discriminate between free and esterfied forms of hydroxycholesterols. Since free hydroxycholesterols are the biologically active form of these sterols, separating free from esterfied forms may provide a sensitive measure to identify disease-associated differences in brain sterol metabolism.

Results

We found that average human serum concentrations were 12.3 ± 4.79 ng/ml for free 24(s)-hydroxycholesterol and 17.7 ± 8.5 ng/ml for 27-hydroxycholesterol.

Conclusion

Serum measurements of these biologically active oxysterols may be useful surrogate measures for brain health in a variety of neurodegenerative conditions.

Oxysterols regulate differentiation of mesenchymal stem cells: pro-bone and anti-fat

Pluripotent mesenchymal stem cells can undergo lineage-specific differentiation in adult organisms. However, understanding of the factors and mechanisms that drive this differentiation is limited. We show the novel ability of specific oxysterols to regulate lineage-specific differentiation of mesenchymal stem cells into osteogenic cells while inhibiting their adipogenic differentiation. Such effects may have important implications for intervention with osteoporosis.

INTRODUCTION

Oxysterols are products of cholesterol oxidation and are formed in vivo by a variety of cells including osteoblasts. Novel pro-osteogenic and anti-adipogenic effects of specific oxysterols on pluripotent mesenchymal cells are demonstrated in this report. Aging and osteoporosis are associated with a decrease in the number and activity of osteoblastic cells and a parallel increase in the number of adipocytic cells.

MATERIALS AND METHODS

The M2-10B4 pluripotent marrow stromal cell line, as well as several other mesenchymal cell lines and primary marrow stromal cells, was used to assess the effects of oxysterols. All results were analyzed for statistical significance using ANOVA.

RESULTS AND CONCLUSION

Pro-osteogenic and anti-adipogenic effects of specific oxysterols were assessed by the increase in early and late markers of osteogenic differentiation, including alkaline phosphatase activity, osteocalcin mRNA expression and mineralization, and the decrease in markers of adipogenic differentiation including lipoprotein lipase and adipocyte P2 mRNA expression and adipocyte formation. Complete osteogenic differentiation of M2 cells into cells expressing early and late markers of differentiation was achieved only when using combinations of specific oxysterols, whereas inhibition of adipogenesis could be achieved with individual oxysterols. Oxysterol effects were in part mediated by extracellular signal-regulated kinase and enzymes in the arachidonic acid metabolic pathway, i.e., cyclo-oxygenase and phospholipase A(2). Furthermore, we show that these specific oxysterols act in synergy with bone morphogenetic protein 2 in inducing osteogenic differentiation. These findings suggest that oxysterols may play an important role in the differentiation of mesenchymal stem cells and may have significant, previously unrecognized, importance in stem cell biology and potential therapeutic interventions.

Oxysterol and 9-cis-retinoic acid stimulate the group IIA secretory phospholipase A2 gene in rat smooth-muscle cells

The inflammation that occurs during rheumatoid arthritis or atherosclerosis is characterized by the release of large amounts of sPLA(2) (group IIA secretory phospholipase A(2)). We have shown previously that the sPLA(2) promoter in SMC (smooth-muscle cells) is activated by interleukin-1beta and cAMP-signalling pathways, through the interplay of multiple transcription factors [Antonio, Brouillet, Janvier, Monne, Bereziat, Andreani, and Raymondjean (2002) Biochem. J. 368, 415-424]. In the present study, we have investigated the regulation of sPLA(2) gene expression in rat aortic SMCs by oxysterols. We found that oxysterol ligands that bind to the LXR (liver X receptor), including 25-HC (25-hydroxycholesterol) and 22( R )-HC, cause the accumulation of sPLA(2) mRNA and an increased enzyme activity. Transient transfection experiments demonstrated that the sPLA(2) promoter is synergistically activated by 22( R )-HC in combination with 9- cis -retinoic acid, a ligand for the LXR heterodimeric partner RXR (retinoid X receptor). Promoter activity was also increased in a sterol-responsive fashion when cells were co-transfected with LXRalpha/RXRalpha or LXRbeta/RXRalpha. Mutagenesis studies and gel mobility-shift assays revealed that LXR/RXR heterodimers regulate sPLA(2) transcription directly, by interacting with a degenerated LXRE (LXR response element) at position [-421/-406] of the sPLA(2) promoter. Chromatin immunoprecipitation revealed the in vivo occupancy of LXR on the sPLA(2) promoter. In addition, the orphan nuclear receptor LRH-1 (liver receptor homologue-1) potentiated the sterol-dependent regulation of the sPLA(2) promoter by binding to an identified promoter element (TCAAGGCTG). Finally, we have demonstrated that oxysterols act independent of interleukin-1beta and cAMP pathways to activate the sPLA(2) promoter. In the present study, we have identified a new pathway activating sPLA(2) gene expression in SMCs.

Oxysterol-induced activation of macrophage NADPH-oxidase enhances cell-mediated oxidation of LDL in the atherosclerotic apolipoprotein E deficient mouse: inhibitory role for vitamin E

In the present study we provide evidence, both direct and circumstantial, that macrophage oxysterols induce translocation of p47phox from the cytosol to the cell's plasma membrane, forming an active NADPH-oxidase complex which produces superoxide anion and facilitates cell-mediated oxidation of LDL. The study was performed on macrophages from atherosclerotic apolipoprotein E deficient (E(0)) mice, which are under oxidative stress. The oxysterol content in peritoneal macrophages (MPM) from E(0) mice was significantly higher (by 50-80%) than that observed in MPM from control (C57BL6) mice. E(0) MPM release 2-fold more superoxide anions and oxidize LDL by 2.5-fold more than control MPM. Furthermore, macrophage protein kinase C (PKC) activity and arachidonic acid (AA) release (which are both involved in NADPH-oxidase activation) were elevated by 60 and 70%, respectively, in E(0) MPM compared with control MPM. Dietary supplementation of vitamin E (40 mg/kg per day for 2 months) to E(0) mice resulted in a reduction in MPM total oxysterols content (-27%) and this effect was associated with a reduction in PKC activity (-36%), AA release (-39%), cytosolic p47phox translocation to the plasma membrane (-30%), superoxide anion release (-25%) and MPM-mediated LDL oxidation (-28%), compared with unsupplemented E(0) mice. Enrichment of MPM from control mice with the major oxysterols found in E(0) MPM (7-ketocholesterol, beta-epoxycholesterol and 7beta-hydroxycholesterol) resulted in a dose-dependent increase (60-80%) in PKC activity, AA release, p47phox translocation, superoxide anion release and cell-mediated oxidation of LDL. These data clearly demonstrate for the first time that under oxidative stress, cellular lipids are oxidized, and that macrophage enrichment with oxysterols (as exists in E(0) mice) activates the NADPH-oxidase system and enhances cell-mediated oxidation of LDL, a key event during early atherogenesis.

Oxysterols: modulators of cholesterol metabolism and other processes

Oxygenated derivatives of cholesterol (oxysterols) present a remarkably diverse profile of biological activities, including effects on sphingolipid metabolism, platelet aggregation, apoptosis, and protein prenylation. The most notable oxysterol activities center around the regulation of cholesterol homeostasis, which appears to be controlled in part by a complex series of interactions of oxysterol ligands with various receptors, such as the oxysterol binding protein, the cellular nucleic acid binding protein, the sterol regulatory element binding protein, the LXR nuclear orphan receptors, and the low-density lipoprotein receptor. Identification of the endogenous oxysterol ligands and elucidation of their enzymatic origins are topics of active investigation. Except for 24, 25-epoxysterols, most oxysterols arise from cholesterol by autoxidation or by specific microsomal or mitochondrial oxidations, usually involving cytochrome P-450 species. Oxysterols are variously metabolized to esters, bile acids, steroid hormones, cholesterol, or other sterols through pathways that may differ according to the type of cell and mode of experimentation (in vitro, in vivo, cell culture). Reliable measurements of oxysterol levels and activities are hampered by low physiological concentrations (approximately 0.01-0.1 microM plasma) relative to cholesterol (approximately 5,000 microM) and by the susceptibility of cholesterol to autoxidation, which produces artifactual oxysterols that may also have potent activities. Reports describing the occurrence and levels of oxysterols in plasma, low-density lipoproteins, various tissues, and food products include many unrealistic data resulting from inattention to autoxidation and to limitations of the analytical methodology. Because of the widespread lack of appreciation for the technical difficulties involved in oxysterol research, a rigorous evaluation of the chromatographic and spectroscopic methods used in the isolation, characterization, and quantitation of oxysterols has been included. This review comprises a detailed and critical assessment of current knowledge regarding the formation, occurrence, metabolism, regulatory properties, and other activities of oxysterols in mammalian systems.

Potentiation by cholesterol and vitamin D3 oxygenated derivatives of arachidonic acid release and prostaglandin E2 synthesis induced by the epidermal growth factor in NRK 49F cells: the role of protein kinase C

We have previously demonstrated that oxysterols and calcitriol potentiate arachidonic acid (AA) release and prostaglandin (PG) synthesis when NRK cells (fibroblastic clone 49F) are activated by foetal calf serum. As serum is essential for a full oxysterol effect, we hypothesized that these compounds could act on one or more of the events triggered by serum growth factor binding to their specific receptors and leading to PLA2 activation; we showed that the oxysterol effect on AA release is synergistic with, but not fully dependent on, protein kinase C (PKC) activity and Ca2+ ion fluxes, suggesting that oxysterols could effect early events in the cell signalling pathway. In the present paper, we investigated the effect of some oxysterols and calcitriol on epidermal growth factor (EGF)-induced AA release and PGE2 synthesis in NRK cells. The clear potentiation of EGF effect by most of the oxygenated sterols--chiefly when polyoxidized--cannot be explained by a modification of EGF high affinity binding site number which was only moderately increased after a 4 h incubation of cells with these compounds, and moreover was not related to the ability of a given oxysterol to increase PLA2 activity; whatever the compound, the dissociation constant (Kd) of either a high or low affinity binding site was unchanged (respectively, 3.5 x 10(-11) M and 4.4 x 10(-10) M). Genistein, a known inhibitor of EGF receptor tyrosine kinase, changed neither the EGF effect on AA release nor its potentiation by oxysterol, whereas it inhibited PGE2 synthesis in both situations. PKC activation by phorbol ester TPA increased the effect of EGF alone as well as the oxysterol potentiating effect, whereas PKC down-regulation strongly decreased both of these effects, showing that both are dependent on PKC activity. Nevertheless staurosporine, a PKC inhibitor, did not reproduce the effects of PKC down-regulation on EGF activation: stimulatory when AA release was induced by EGF alone, inhibitory when AA release is induced by TPA alone, this compound did not modify the oxysterol potentiating effect. In conclusion, the potentiating effect of oxysterols on AA release seems to be exerted downstream to the growth factor receptor (as demonstrated here with EGF) and probably at the PKC level, but not exclusively.

Oxysterol activation of arachidonic acid release and prostaglandin E2 biosynthesis in NRK 49F cells is partially dependent on protein kinase C activity [corrected]

We previously demonstrated that the oxysterol potentiation of arachidonic acid release and prostaglandin biosynthesis induced by foetal calf serum activation of normal rat kidney (NRK) cells (fibroblastic clone 49F) was not related to a direct effect of oxysterols on cell free Ca2+ level. Since both Ca2+ variations and protein kinase C are involved in arachidonic acid release in some models, we looked for a possible modulation by protein kinase C in the oxysterol effect on arachidonic acid release. We show that when the phorbol ester 12-O-tetradecanoyl-phorbol-13-acetate (TPA), a protein kinase C activator, was added to the culture medium, the oxysterol effect on arachidonic acid release and prostaglandin synthesis clearly increased. Moreover, the effect of TPA was dose-dependent and TPA EC50 (4 x 10(-9) M) was unchanged in the presence of the oxysterol. Preincubation of cells with TPA for 24 h prevented the arachidonic acid release induced by TPA alone, whereas the oxysterol effect was decreased but not abolished. In the absence of serum, TPA and ionomycin added together induced the same noticeable (arachidonic acid) release and PGE2 synthesis as serum alone. Nevertheless, the potentiating effect of cholest-5-ene-3 beta, 25-diol was much higher when serum itself was used to activate NRK cells than it was in the present serum-mimicking experimental conditions. Thus, the presence of growth factors is probably required to obtain a full oxysterol effect. We conclude that the oxysterol effect was synergistic with, but not fully dependent on, protein kinase C and Ca2+ ion fluxes, therefore oxysterols could affect earlier events triggered by serum growth factor binding to their cell membrane receptors.