Caspase-2, a novel lipid sensor under the control of sterol regulatory element binding protein 2.

Caspases play important roles in apoptotic cell death and in some other functions, such as cytokine maturation, inflammation, or differentiation. We show here that the 5'-flanking region of the human CASP-2 gene contains three functional response elements for sterol regulatory element binding proteins (SREBPs), proteins that mediate the transcriptional activation of genes involved in cholesterol, triacylglycerol, and fatty acid synthesis. Exposure of several human cell lines to statins, lipid-lowering drugs that drive SREBP proteolytic activation, induced the CASP-2 gene to an extent similar to that for known targets of SREBP proteins. Adenoviral vector-mediated transfer of active SREBP-2 also induced expression of the CASP-2 gene and the caspase-2 protein and increased the cholesterol and triacylglycerol cellular content. These rises in lipids were strongly impaired following small interfering RNA-mediated silencing of the CASP-2 gene. Taken together, our results identify the human CASP-2 gene as a member of the SREBP-responsive gene battery that senses lipid levels in cells and raise the possibility that caspase-2 participates in the control of cholesterol and triacylglycerol levels.

Comparison of the cytotoxic, pro-oxidant and pro-inflammatory characteristics of different oxysterols.

Oxidized low-density lipoproteins play important roles in the development of atherosclerosis and contain several lipid-derived, bioactive molecules which are believed to contribute to atherogenesis. Of these, some cholesterol oxidation products, referred to as oxysterols, are suspected to favor the formation of atherosclerotic plaques involving cytotoxic, pro-oxidant and pro-inflammatory processes. Ten commonly occurring oxysterols (7alpha-, 7beta-hydroxycholesterol, 7-ketocholesterol, 19-hydroxycholesterol, cholesterol-5alpha,6alpha-epoxide, cholesterol-5beta,6beta-epoxide, 22R-, 22S-, 25-, and 27-hydroxycholesterol) were studied for both their cytotoxicity and their ability to induce superoxide anion production (O2*-) and IL-8 secretion in U937 human promonocytic leukemia cells. Cytotoxic effects (phosphatidylserine externalization, loss of mitochondrial potential, increased permeability to propidium iodide, and occurrence of cells with swollen, fragmented and/or condensed nuclei) were only identified with 7beta-hydroxycholesterol, 7-ketocholesterol and cholesterol-5beta,6beta-epoxide, which also induce lysosomal destabilization associated or not associated with the formation of monodansylcadaverine-positive cytoplasmic structures. No relationship between oxysterol-induced cytotoxicity and HMG-CoA reductase activity was found. In addition, the highest O2*- overproduction quantified with hydroethidine was identified with 7beta-hydroxycholesterol, 7-ketocholesterol and cholesterol-5beta,6beta-epoxide, with cholesterol-5alpha, 6alpha-epoxide and 25-hydroxycholesterol. The highest capacity to simultaneously stimulate IL-8 secretion (quantified by ELISA and by using a multiplexed, particle-based flow cytometric assay) and enhance IL-8 mRNA levels (determined by RT-PCR) was observed with 7beta-hydroxycholesterol and 25-hydroxycholesterol. None of the effects observed for the oxysterols were detected for cholesterol. Therefore, oxysterols may have cytotoxic, oxidative, and/or inflammatory effects, or none whatsoever.

Involvement of a calcium-dependent dephosphorylation of BAD associated with the localization of Trpc-1 within lipid rafts in 7-ketocholesterol-induced THP-1 cell apoptosis.

7-Ketocholesterol is a component of oxidized LDL, which plays a central role in atherosclerosis. It is a potent inducer of cell death towards a wide number of cells involved in atherosclerosis. In this study, it is reported that 7-ketocholesterol treatment induces an increase of cytosolic-free Ca(2+) in THP-1 monocytic cells. This increase is correlated with the induction of cytotoxicity as suggested from experiments using the Ca(2+) channel blockers verapamil and nifedipine. This 7-ketocholesterol-induced apoptosis appears to be associated with the dephosphorylation of serine 75 and serine 99 of the proapoptotic protein Bcl-2 antagonist of cell death (BAD). We demonstrated that this dephosphorylation results mainly from the activation of calcium-dependent phosphatase calcineurin by the oxysterol-induced increase in Ca(2+). Moreover, this Ca(2+) increase appears related to the incorporation of 7-ketocholesterol into lipid raft domains of the plasma membrane, followed by the translocation of transient receptor potential calcium channel 1, a component of the store operated Ca(2+) entry channel, to rafts.