Association between bcl-x(L) and 5-lipoxygenase activating protein (FLAP) levels in IL-3-dependent FL5.12 cells

P2X7 receptor activation in rat brain cultured astrocytes increases the biosynthetic release of cysteinyl leukotrienes

Astrocytes have been recognized as important elements in controlling inflammatory as well as immune processes in the central nervous system (CNS). Recently, glial cells have been shown to produce cysteinyl leukotrienes (CysLTs) which are known lipid mediators of inflammation and whose extracellular concentrations rise under different pathological conditions in the brain. In the same conditions also extracellular concentrations of ATP dramatically increase reaching levels able to activate P2X7 ionotropic receptors for which an emerging role in neuroinflammation and neurodegeneration has been claimed. RTPCR analysis showed that primary cultures of rat brain astrocytes express P2X7 receptors. Application of the selective P2X7 agonist benzoyl benzoly ATP (BzATP) markedly increased [Ca2+]i which was mediated by a calcium influx from the extracellular milieu. The P2X7 antagonist, oATP, suppressed the BzATP-induced calcium increase. Consistent with the evidence that increased calcium levels activate the leukotriene biosynthetic pathway, challenge of astrocytes with either the calcium ionophore A23187 or BzATP significantly increased CysLT production and the cell pre-treatment with EGTA abolished these effects. Again the P2X7 antagonist prevented the BzATP-mediated CysLT efflux, whereas the astrocyte pretreatment with MK-571, a CysLT1 receptor antagonist, was ineffective. The astrocyte pre-treatment with a cocktail of inhibitors of ATP binding cassette (ABC) proteins reduced the BzATP-mediated CysLT production confirming that ABC transporters are involved in the release of CysLTs. The astrocyte P2X7- evoked rise of CysLT efflux was abolished in the presence of MK-886, an inhibitor of 5-lipoxygenase activating protein (FLAP) whose expression, along with that of 5-lipoxygenase (5-LO) was reported by Northern Blot analysis. The stimulation of P2X7 induced an up-regulation of FLAPmRNA that was reduced by the antagonist oATP. These data suggest that in rat brain cultured astrocytes P2X7ATP receptors may participate in the control of CysLT release thus further supporting a role for extracellular ATP as an integral component of the inflammatory brain response.

Bax-induced cell death in yeast depends on mitochondrial lipid oxidation

The oxidant function of pro-apoptotic protein Bax was investigated through heterologous expression in yeast. Direct measurements of fatty acid content show that Bax-expression induces oxidation of mitochondrial lipids. This effect is prevented by the coexpression of Bcl-xL. The oxidation actually could be followed on isolated mitochondria as respiration-induced peroxidation of polyunsaturated cis-parinaric acid and on whole cells as the increase in the amount of thiobarbituric acid-reactive products. Treatments that increase the unsaturation ratio of lipids, making them more sensitive to oxidation, increase kinetics of Bax-induced death. Conversely, inhibitors of lipid oxidation and treatments that decrease the unsaturation ratio of fatty acids decrease kinetics of Bax-induced death. Taken together, these results show that Bax-induced mitochondrial lipid oxidation is relevant to Bax-induced cell death. Conversely, lipid oxidation is poorly related to the massive Bax-induced superoxide and hydrogen peroxide accumulation, which occurs at the same time, as chemical or enzymatic scavenging of ROS does not prevent lipid oxidation nor has any effects on kinetics of Bax-induced cell death. Whatever the origin of mitochondrial lipid oxidation, these data show that it represents a major step in the cascade of events leading to Bax-induced cell death. These results are discussed in the light of the role of lipid oxidation both in mammalian apoptosis and in other forms of cell death in other organisms.

Acrolein causes transcriptional induction of phase II genes by activation of Nrf2 in human lung type II epithelial (A549) cells

Acrolein, an alpha,beta-unsaturated aldehyde, is by far the strongest electrophile present in cigarette smoke which is involved in several lung pathophysiological conditions. Acrolein depletes glutathione and creates thiol imbalance. Acrolein due to thiol imbalance as well as covalent modification of cysteine is known to inhibit the activity of redox sensitive transcription factors such as NF-kappaB and AP-1. Exposure of human type II lung epithelial (A549) cells to non-lethal dose of acrolein (150 fmol/cell for 1 h) depletes 80% of intracellular glutathione and increases the transcription of gamma-glutamylcysteine synthetase (gamma-GCS) at 6-12 h post-treatment, which helps in replenishing the glutathione to normal level. Acrolein treatment activates transcription of phase II genes in general, as indicated by an increase in mRNA for NAD (P) H:quinone oxidoreductase (NQO1). Western blot analysis revealed the increased level of the transcription factor, Nrf2 in the nuclear extract from acrolein treated cells. Electrophoretic mobility shift assay shows increased binding of nuclear proteins to human antioxidant response element (ARE) consensus sequence after treatment with acrolein. The involvement of Nrf2 in ARE mediated transcriptional activation in response to acrolein exposure has been confirmed by human NQO1-ARE reporter assay. The ability of acrolein to transcriptionaly activate genes responsible for phase II enzymes may form the basis of resistance against cell death and can have implications in cigarette smoke related lung carcinogenesis.

Fatty acid oxidation and signaling in apoptosis

It is well established that fatty acid metabolites of cyclooxygenase, lipoxygenase (LOX), and cytochrome P450 are implicated in essential aspects of cellular signaling including the induction of programmed cell death. Here we review the roles of enzymatic and non-enzymatic products of polyunsaturated fatty acids in controlling cell growth and apoptosis. Also, the spontaneous oxidation of polyunsaturated fatty acids yields reactive aldehydes and other products of lipid peroxidation that are potentially toxic to cells and may also signal apoptosis. Significant conflicting data in terms of the role of LOX enzymes are highlighted, prompting a re-evaluation of the relationship between LOX and prostate cancer cell survival. We include new data showing that LNCaP, PC3, and Du145 cells express much lower levels of 5-LOX mRNA and protein compared with normal prostate epithelial cells (NHP2) and primary prostate carcinoma cells (TP1). Although the 5-LOX activating protein inhibitor MK886 killed these cells, another 5-LOX inhibitor AA861 hardly showed any effect. These observations suggest that 5-LOX is unlikely to be a prostate cancer cell survival factor, implying that the mechanisms by which LOX inhibitors induce apoptosis are more complex than expected. This review also suggests several mechanisms involving peroxisome proliferator activated receptor activation, BCL proteins, thiol regulation, and mitochondrial and kinase signaling by which cell death may be produced in response to changes in non-esterified and non-protein bound fatty acid levels. Overall, this review provides a context within which the effects of fatty acids and fatty acid oxidation products on signal transduction pathways, particularly those involved in apoptosis, can be considered in terms of their overall importance relative to the much better studied protein or peptide signaling factors.