Thioredoxin reductase regulates the induction of haem oxygenase-1 expression in aortic endothelial cells.

Certain selenoproteins such as GPX-1 (glutathione peroxidase-1) and TrxR1 (thioredoxin reductase-1) possess important antioxidant defence functions in vascular endothelial cells. Reduced selenoprotein activity during dietary selenium (Se) deficiency can result in a compensatory increase of other non-Se-dependent antioxidants, such as HO-1 (haem oxygenase-1) that may help to counteract the damaging effects of oxidant stress. However, the role of individual selenoproteins in regulating vascular-derived protective gene responses such as HO-1 is less understood. Using an oxidant stress model based on Se deficiency in BAECs (bovine aortic endothelial cells), we sought to determine whether TrxR1 activity may contribute to the differential regulation of HO-1 expression as a function of altered redox environment. Se-sufficient BAECs up-regulated HO-1 expression following stimulation with the pro-oxidant, 15-HPETE (15-hydroperoxyeicosatetraenoic acid), and levels of this antioxidant inversely correlated with EC apoptosis. While Se-deficient BAECs exhibited higher basal levels of HO-1, it was not up-regulated upon 15-HPETE treatment, which resulted in significantly higher levels of pro-apoptotic markers. Subsequent results showed that HO-1 induction depended on the activity of TrxR1, as proved with chemical inhibitor studies and direct inhibition with TrxR1 siRNA. Finally, restoring intracellular levels of the reduced substrate Trx (thioredoxin) in Sedeficient BAECs was sufficient to increase HO-1 activation following 15-HPETE stimulation. These data provide evidence for the involvement of the Trx/TrxR system, in the regulation of HO-1 expression in BAECs during pro-oxidant challenge.

Enhanced 15-HPETE production during oxidant stress induces apoptosis of endothelial cells.

Oxidant stress plays an important role in the etiology of vascular diseases by increasing rates of endothelial cell apoptosis, but few data exist on the mechanisms involved. Using a unique model of oxidative stress based on selenium deficiency (-Se), the effects of altered eicosanoid production on bovine aortic endothelial cells (BAEC) apoptosis was evaluated. Oxidant stress significantly increased the immediate oxygenation product of arachidonic acid metabolized by the 15-lipoxygenase pathway, 15-hydroxyperoxyeicosatetraenoic acid (15-HPETE). Treatment of -Se BAEC with TNFalpha/cyclohexamide (CHX) exhibited elevated levels of apoptosis, which was significantly reduced by the addition of a specific 15-lipoxygenase inhibitor PD146176. Furthermore, the addition of 15-HPETE to PD146176-treated BAEC, partially restored TNF/CHX-induced apoptosis. Increased exposure to 15-HPETE induced apoptosis, as determined by internucleosomal DNA fragmentation, chromatin condensation, caspase-3 activation, and caspase-9 activation, which suggests mitochondrial dysfunction. The expression of Bcl-2 protein also was decreased in -Se BAEC. Addition of a caspase-9 inhibitor (LEHD-fmk) completely blocked 15-HPETE-induced chromatin condensation in -Se BAEC, suggesting that 15-HPETE-induced apoptosis is caspase-9 dependent. Increased apoptosis of BAEC as a result of oxidant stress and subsequent production of 15-HPETE may play a critical role in a variety of inflammatory based diseases.

Selenium deficiency alters the formation of eicosanoids and signal transduction in rat lymphocytes.

Previous reports have shown that selenium (Se) nutrition alters the lipoxygenase pathway and mitogenic responses in bovine lymphocytes. In order to further understand how Se may alter lymphocyte function, we examined the effects of Se nutrition on arachidonic acid (AA) metabolism and phospholipase D (PLD) activation. Lymphocytes were isolated from the lymph nodes of rats fed either Se-deficient diet (-Se) or Se-supplemented diet (+Se) for 12 weeks. Our results revealed that calcium ionophore A23187-stimulated lymphocytes derived from -Se rats produced significantly less prostaglandins (PGs) than those obtained from +Se rats. Phospholipase D (PLD) activation by 12-O-tetradecanoylphorbol-13-acetate (TPA) was significantly lower in lymphocytes obtained from -Se rats when compared to cells from +Se rats. Furthermore, the addition of PGE2, PGD2 or PGF2alpha to suspended lymphocytes from -Se rats significantly enhanced PLD activity. The effects of TPA and PGE2 on PLD activation were additive. However, the addition of PGE2 abolished the significant difference in PLD activation between -Se and +Se cells observed in response to TPA alone. Based on these results, we postulate that dietary Se status plays an important role in the regulation of AA metabolism that subsequently affects PLD activation.