In human epidermoid carcinoma A cells lipoxygenase was

In human epidermoid carcinoma A431 cells, 12-lipoxygenase was significantly increased in glutathione-depleted cells. The relative abundance of phospholipid hydroperoxide versus glutathione peroxidase seems to play an important role in controlling the lipoxygenase reaction [57]. It has been proposed that hydroperoxide tone in human platelets and other mammalian Bufexamac regulates the disposition of 12-HPETE either reduction to yield 12-HETE or isomerization to generate hepoxilins [58].
Regulation of mRNA expression by cytokines and growth factors The platelet-type 12S-lipoxygenase mRNA was detected as a single species with a length of 3.1kb by RNA blot analysis of human erythroleukemia cells [18]. For the leukocyte-type 12S-lipoxygenases, 2.5 and 3.4kb mRNAs were present in murine peritoneal macrophages and porcine leukocytes, respectively [9], [11]. A 2.4-kb mRNA of epidermis-type 12S-lipoxygenase was detected in mouse epidermis [20]. The major transcription initiation sites were determined by primer-extension analyses to be at 306 and 19 bases upstream from the translation initiation codons of human platelet and porcine leukocyte 12S-lipoxygenase genes, respectively [23], [59]. The 5′-flanking sequences of 12S-lipoxygenase genes have been reported for the human [23], [60] and murine [10] platelet isozymes and the porcine [59] and murine [10] leukocyte isozymes. The 5′-upstream sequences are highly related in the genes of each enzyme type a 200-base stretch with 75% identity in the platelet isozyme genes and ∼110 bases with 63% identity in the leukocyte isozyme genes [10]. Analysis of potential cis-acting regulatory elements revealed that the platelet isozyme genes contained three GC boxes for potential Sp1-binding sites and one TATA-like box, but no CCAAT box, within a 200-base stretch [10]. In the leukocyte isozyme genes there are GC boxes and TATA-like boxes within ∼110 bases of the initiation codon [10]. Analysis of further upstream regions of these genes indicates that the platelet isozyme genes have CACCC boxes, AP-2-binding sequences, NF-κB sites and a glucocorticoid-responsive element, while the leukocyte isozyme genes carry AP-1 (c-jun) and AP-2-binding sequences with different combinations depending on the species [6], [10]. Although there is still scant evidence about the roles of these sequences in the regulation of 12S-lipoxygenase gene expression, we have shown that NF-κB heterodimers (p50/p65 and p50/c-Rel) negatively modulated the expression of the human platelet 12S-lipoxygenase gene [61]. Earlier studies have shown that Th2-derived interleukins (IL) such as IL-4 [62] and IL-13 [63] enhance expression of the 15-lipoxygenase. Moreover, interferon-γ, a product of Th1 lymphocytes, blocks IL-13-mediated induction of the 15-lipoxygenase [63]. It was later demonstrated that IL-4 treatment of porcine vascular smooth muscle cells led to a dose-dependent increase in 12S-lipoxygenase mRNA levels [64]. The Th-2-derived cytokines also induced upregulation of the murine macrophage 12S-lipoxygenase in vitro and in vivo, and this upregulation required expression of the transcription factor STAT6 [65], [66]. However, controversial results have been presented about the involvement of STAT6 in the IL-4/13-induced upregulation using peritoneal macrophages from STAT6 deficient mice [67]. Interestingly enough, IL-4 also induces the expression of peroxisome proliferator-activated receptor-γ (PPAR-γ) that is a ligand-dependent nuclear receptor [68]. At the same time, a physiological role of 12/15-lipoxygenase is suggested in the generation of endogenous ligands for PPAR-γ. The coordinate induction of PPAR-γ and 12/15-lipoxygenase by IL-4 suggests a paradigm for the regulation of nuclear receptors by cytokines in macrophage development and function. IL-1 is reported to increase 12-HETE production in pancreatic islets by the mechanism involving NO production, and the effect is due to the increased availability of the substrate arachidonic acid rather than enhanced expression of 12-lipoxygenase [69]. On the other hand, Bleich et al. have demonstrated that IL-1β induced the expression of 12-lipoxygenase protein and mRNA in rat pancreatic islets and in RIN m5F cells [70].