Role of 12-lipoxygenase and protein kinase C in modulating the activation state of the integrin alpha IIb beta 3 on human tumor cells

Activation of 12-lipoxygenase in proinflammatory cytokine-mediated beta cell toxicity

AIMS/HYPOTHESIS

Beta cell inflammation and cytokine-induced toxicity are central to autoimmune diabetes development. Lipid mediators generated upon lipoxygenase (LO) activation can participate in inflammatory pathways. 12LO-deficient mice are resistant to streptozotocin-induced diabetes. This study sought to characterise the cellular processes involving 12LO-activation lipid inflammatory mediator production in cytokine-treated pancreatic beta cells.

METHODS

Islets and beta cell lines were treated with a combination of IL-1beta, IFN-gamma and TNF-alpha, or the 12LO product 12(S)-hydroxyeicosatetraenoic acid (HETE). Insulin secretion was measured using an enzyme immunoassay, and cell viability was evaluated using an in situ terminal deoxynucleotidyl transferase-mediated dUTP nick-end labelling assay. 12LO activity was evaluated and 12LO protein levels were determined using immunoblotting with a selective leucocyte type 12LO antibody. Cellular localisation of 12LO was evaluated using immunocytochemistry.

RESULTS

Basal expression of leucocyte type 12LO protein was found in human and mouse islets and in several rodent beta cell lines. In mouse beta-TC3 cells, and in human islets, cytokines induced release of 12-HETE within 30 min. Cytokine addition also induced a rapid translocation of 12LO protein from the cytosol to the nucleus of beta-TC3 cells as shown by subcellular fractionation and immunostaining. Cytokine-induced cell death and inhibition of insulin secretion were partially reversed by baicalein, a 12LO inhibitor. 12(S)-HETE inhibited beta-TC3 cell insulin release in a time- and concentration-dependent manner. Incubating beta-TC3 cells with 100 nmol/l of 12(S)-HETE resulted in a 57% reduction in basal insulin release (6 h), and a 17% increase in cell death (18 h) as compared with untreated cells. 12(S)-HETE activated the stress-activated protein kinase c-Jun N-terminal kinase and p38 within 15 min, as judged by increased kinase protein phosphorylation.

CONCLUSIONS/INTERPRETATION

The data suggest that inflammatory cytokines rapidly activate 12LO and show for the first time that cytokines induce 12LO translocation. The effects of 12-HETE on insulin secretion, cytotoxicity and kinase activation were similar to the effects seen with cytokines. The results provide mechanistic information of cytokine-induced toxic effects on pancreatic beta cells and support the hypothesis that blocking 12LO activation could provide a new therapeutic way to protect pancreatic beta cells from autoimmune injury.

Modulation of cell-substrate adhesion by arachidonic acid: lipoxygenase regulates cell spreading and ERK1/2-inducible cyclooxygenase regulates cell migration in NIH-3T3 fibroblasts

Adhesion of cells to an extracellular matrix is characterized by several discrete morphological and functional stages beginning with cell-substrate attachment, followed by cell spreading, migration, and immobilization. We find that although arachidonic acid release is rate-limiting in the overall process of adhesion, its oxidation by lipoxygenase and cyclooxygenases regulates, respectively, the cell spreading and cell migration stages. During the adhesion of NIH-3T3 cells to fibronectin, two functionally and kinetically distinct phases of arachidonic acid release take place. An initial transient arachidonate release occurs during cell attachment to fibronectin, and is sufficient to signal the cell spreading stage after its oxidation by 5-lipoxygenase to leukotrienes. A later sustained arachidonate release occurs during and after spreading, and signals the subsequent migration stage through its oxidation to prostaglandins by newly synthesized cyclooxygenase-2. In signaling migration, constitutively expressed cyclooxygenase-1 appears to contribute approximately 25% of prostaglandins synthesized compared with the inducible cyclooxygenase-2. Both the second sustained arachidonate release, and cyclooxygenase-2 protein induction and synthesis, appear to be regulated by the mitogen-activated protein kinase extracellular signal-regulated kinase (ERK)1/2. The initial cell attachment-induced transient arachidonic acid release that signals spreading through lipoxygenase oxidation is not sensitive to ERK1/2 inhibition by PD98059, whereas PD98059 produces both a reduction in the larger second arachidonate release and a blockade of induced cyclooxygenase-2 protein expression with concomitant reduction of prostaglandin synthesis. The second arachidonate release, and cyclooxygenase-2 expression and activity, both appear to be required for cell migration but not for the preceding stages of attachment and spreading. These data suggest a bifurcation in the arachidonic acid adhesion-signaling pathway, wherein lipoxygenase oxidation generates leukotriene metabolites regulating the spreading stage of cell adhesion, whereas ERK 1/2-induced cyclooxygenase synthesis results in oxidation of a later release, generating prostaglandin metabolites regulating the later migration stage.

12-Lipoxygenase products increase monocyte:endothelial interactions

In summary, we suggest that hyperglycemia causes upregulation of 12-lipoxygenase activity. The increased production of 12-LO products, 12(S) and 15(S)-HETE, activates monocyte integrins which result in enhanced adhesion of monocytes to endothelium. The binding of monocytes to endothelium is a key early event in development of atherosclerosis. Upregulation of this process by vascular cells exposed to chronic elevations in glucose may be one explanation for the accelerated atherosclerosis observed in patients with Type 2 diabetes.

Lipoxygenase products increase monocyte adhesion to human aortic endothelial cells

The development of atherosclerosis is accelerated in individuals with type 2 diabetes. Adhesion of monocytes to the vascular endothelium is a key initial step in atherogenesis. We have previously shown that monocyte adhesion to human aortic endothelial cells (HAECs) cultured long-term in high-glucose medium (25 mmol/L, 2 passages) is increased compared with cells grown in normal glucose (5 mmol/L). One potential mechanism for increased monocyte adhesion to HAECs under hyperglycemic conditions is via the 12-lipoxygenase (12-LO) pathway. In this study, we demonstrated in HAECs that the major LO metabolite of arachidonic acid was the 12-LO product, 12(S)-hydroxyeicosatetraenoic acid [12(S)-HETE], which was increased severalfold in HAECs cultured under high-glucose conditions. Furthermore, treatment of HAECs with 12(S)-HETE induced monocyte, but not neutrophil, adhesion an average of 3-fold (range of 1.5- to 5-fold) compared with untreated cells (75+/-5 versus 26+/-1 monocytes per field, respectively, P<0.001). Expression of the adhesion molecules vascular cell adhesion molecule-1, E-selectin, and intercellular adhesion molecule-1 was not significantly increased. However, both glucose and 12(S)-HETE induced a 60% increase in HAEC surface expression of connecting segment-1 (ie, CS-1) fibronectin, a ligand for very late-acting antigen-4 (VLA-4). The antibodies used to block monocyte integrin VLA-4 and leukocyte function-related antigen-1, a monocytic counterreceptor for intercellular adhesion molecule-1, inhibited the ability of both 12-LO products and high glucose to induce monocyte adhesion. These results definitively demonstrate for the first time in HAECs that the 12-LO pathway can induce monocyte-endothelial cell interaction and that the effects of glucose may be mediated, at least in part, through this pathway. Thus, these results suggest that the 12-LO pathway may play a role in the increased susceptibility of diabetics to atherosclerosis.