Effects of the 5-lipoxygenase activating protein inhibitor MK886 on voltage-gated and Ca2+-activated K+ currents in rat arterial myocytes

A comprehensive strategy to discover inhibitors of the translesion synthesis DNA polymerase κ

Human DNA polymerase kappa (pol κ) is a translesion synthesis (TLS) polymerase that catalyzes TLS past various minor groove lesions including N(2)-dG linked acrolein- and polycyclic aromatic hydrocarbon-derived adducts, as well as N(2)-dG DNA-DNA interstrand cross-links introduced by the chemotherapeutic agent mitomycin C. It also processes ultraviolet light-induced DNA lesions. Since pol κ TLS activity can reduce the cellular toxicity of chemotherapeutic agents and since gliomas overexpress pol κ, small molecule library screens targeting pol κ were conducted to initiate the first step in the development of new adjunct cancer therapeutics. A high-throughput, fluorescence-based DNA strand displacement assay was utilized to screen ∼16,000 bioactive compounds, and the 60 top hits were validated by primer extension assays using non-damaged DNAs. Candesartan cilexetil, manoalide, and MK-886 were selected as proof-of-principle compounds and further characterized for their specificity toward pol κ by primer extension assays using DNAs containing a site-specific acrolein-derived, ring-opened reduced form of γ-HOPdG. Furthermore, candesartan cilexetil could enhance ultraviolet light-induced cytotoxicity in xeroderma pigmentosum variant cells, suggesting its inhibitory effect against intracellular pol κ. In summary, this investigation represents the first high-throughput screening designed to identify inhibitors of pol κ, with the characterization of biochemical and biologically relevant endpoints as a consequence of pol κ inhibition. These approaches lay the foundation for the future discovery of compounds that can be applied to combination chemotherapy.

AA/12-Lipoxygenase Signaling Contributes to Inhibitory Learning in Hermissenda Type B Photoreceptors

Conditioned inhibition (CI) is a major category of associative learning that occurs when an organism learns that one stimulus predicts the absence of another. In addition to being important in its own right, CI is interesting because its occurrence implies that the organism has formed an association between stimuli that are non-coincident. In contrast to other categories of associative learning that are dependent upon temporal contiguity (pairings) of stimuli, the neurobiology of CI is virtually unexplored. We have previously described a simple form of CI learning in Hermissenda, whereby animals' phototactic behavior is increased by repeated exposures to explicitly unpaired (EU) presentations of light and rotation. EU conditioning also produces characteristic reductions in the excitability and light response, and increases several somatic K(+) currents in Type B photoreceptors. Type B photoreceptors are a major site of plasticity for classical conditioning in Hermissenda. Because arachidonic acid (AA) and/or its metabolites open diverse K(+) channels in many cell types, we examined the potential contribution of AA to CI. Our results indicate that AA contributes to one of the major effects of EU-conditioning on Type B photoreceptors: decreases in light-evoked spike activity. We find that AA increases the transient (I(A)) somatic K(+) current in Type B photoreceptors, further mimicking CI training. In addition, our results indicate that metabolism of AA by a 12-lipoxygenase enzyme is critical for these effects of AA, and further that 12-lipoxygenase metabolites are apparently generated during CI training.

A mathematical model of Ca2+ dynamics in rat mesenteric smooth muscle cell: agonist and NO stimulation

A mathematical model of calcium dynamics in vascular smooth muscle cell (SMC) was developed based on data mostly from rat mesenteric arterioles. The model focuses on (a) the plasma membrane electrophysiology; (b) Ca2+ uptake and release from the sarcoplasmic reticulum (SR); (c) cytosolic balance of Ca2+, Na+, K+, and Cl ions; and (d) IP3 and cGMP formation in response to norepinephrine(NE) and nitric oxide (NO) stimulation. Stimulation with NE induced membrane depolarization and an intracellular Ca2+ ([Ca2+]i) transient followed by a plateau. The plateau concentrations were mostly determined by the activation of voltage-operated Ca2+ channels. NE causes a greater increase in [Ca2+]i than stimulation with KCl to equivalent depolarization. Model simulations suggest that the effect of[Na+]i accumulation on the Na+/Ca2+ exchanger (NCX) can potentially account for this difference.Elevation of [Ca2+]i within a concentration window (150-300 nM) by NE or KCl initiated [Ca2+]i oscillations with a concentration-dependent period. The oscillations were generated by the nonlinear dynamics of Ca2+ release and refilling in the SR. NO repolarized the NE-stimulated SMC and restored low [Ca2+]i mainly through its effect on Ca2+-activated K+ channels. Under certain conditions, Na+-K+-ATPase inhibition can result in the elevation of [Na+]i and the reversal of NCX, increasing resting cytosolic and SR Ca2+ content, as well as reactivity to NE. Blockade of the NCX's reverse mode could eliminate these effects. We conclude that the integration of the selected cellular components yields a mathematical model that reproduces, satisfactorily, some of the established features of SMC physiology. Simulations suggest a potential role of intracellular Na+ in modulating Ca2+ dynamics and provide insights into the mechanisms of SMC constriction, relaxation, and the phenomenon of vasomotion. The model will provide the basis for the development of multi-cellular mathematical models that will investigate microcirculatory function in health and disease.

Inhibition of leukotriene synthesis with MK-886 prevents a rise in blood pressure and reduces noradrenaline-evoked contraction in L-NAME-treated rats

(1) Long-term treatment of rats with Nomega-nitro-l-arginine methyl ester (l-NAME) induces hypertension associated with inflammatory and vascular changes. Leukotrienes are proinflammatory vasoactive products that are suspected to be involved in the pathogenesis of hypertension. We investigated, in rats chronically treated with l-NAME, the involvement of leukotrienes in the in vivo regulation of blood pressure and the in vitro contraction elicited by noradrenaline in isolated aorta. (2) Rats were randomly assigned to four groups and orally treated for 3 weeks with l-NAME (1 mg ml-1), l-NAME (1 mg ml-1) plus the leukotriene biosynthesis inhibitor MK-886 (0.1 mg ml-1), MK-886 (0.1 mg ml-1) alone or vehicle (Methocel, 0.1%). All the drugs were added to the drinking fluid. (3) The mean arterial blood pressure (MABP) increased significantly in l-NAME-treated rats (173.3+/-9.4 mmHg (n=25)) vs Methocel-treated rats (110.7+/-4.8 mmHg (n=11), P<0.001). Chronic treatment with MK-886 prevented this rise in MABP. (4) Aortic rings with or without endothelium were suspended in organ baths for recording isometric changes in response to noradrenaline. Pretreatment with either MK-886 (10 microm), the CysLT1 receptor antagonist MK571 (1 microm) or the dual CysLT1/CysLT2 receptor antagonist BAY-u9773 (0.1 microm) reduced (P<0.05) noradrenaline-induced contractions in intact aortic rings from l-NAME-treated rats only. (5) Noradrenaline (0.3 microm) induced a two-fold increase in cysteinyl leukotriene (CysLT) release (measured by enzyme immunoassay) in intact aortic rings from l-NAME-treated rats only. (6) These data suggested (1) a role for the 5-lipoxygenase pathway in the regulation of blood pressure in l-NAME-treated rats and (2) the involvement of endothelial CysLTs in noradrenaline-induced contraction in aorta from l-NAME-treated rats.

Five-lipoxygenase inhibitors can mediate apoptosis in human breast cancer cell lines through complex eicosanoid interactions

Many arachidonic acid metabolites function in growth signaling for epithelial cells, and we previously reported the expression of the major arachidonic acid enzymes in human breast cancer cell lines. To evaluate the role of the 5-lipoxygenase (5-LO) pathway on breast cancer growth regulation, we exposed cells to insulinlike growth factor-1 or transferrin, which increased the levels of the 5-LO metabolite, 5(S)-hydrooxyeicosa-6E,8C,11Z,14Z-tetraenoic acid (5-HETE), by radioimmunoassay and high-performance liquid chromatography. Addition of 5-HETE to breast cancer cells resulted in growth stimulation, whereas selective biochemical inhibitors of 5-LO reduced the levels of 5-HETE and related metabolites. Application of 5-LO or 5-LO activating protein-directed inhibitors, but not a cyclooxygenase inhibitor, reduced growth, increased apoptosis, down-regulated bcl-2, up-regulated bax, and increased G1 arrest. Exposure of breast cancer cells to a 5-LO inhibitor up-regulated peroxisome proliferator-activated receptor (PPAR)a and PPARg expression, and these same cells were growth inhibited when exposed to relevant PPAR agonists. These results suggest that disruption of the 5-LO signaling pathway mediates growth arrest and apoptosis in breast cancer cells. Additional experiments suggest that this involves the interplay of several factors, including the loss of growth stimulation by 5-LO products, the induction of PPARg, and the potential activation of PPARg by interactions with shunted endoperoxides.

Modulation of arachidonic acid release and membrane fluidity by albumin in vascular smooth muscle and endothelial cells

Albumin is the major plasma protein circulating in blood. Albumin potently decreases capillary permeability, although the mechanisms are not understood completely. Albumin also effectively binds arachidonic acid (AA), which increases capillary permeability. To investigate the interactions of BSA and AA with the cell membrane, the effect of these substances on [3H]AA release and membrane fluidity was studied in vascular myocytes and endothelial cells. BSA (0.2 and 1 mg . mL-1) stimulated a significant release of [3H]AA from both intact rat aorta and cultured smooth muscle cells. This effect was not mimicked by gamma-globulin or myoglobin (both 1 mg . mL-1) in intact tissue. BSA, but not gamma-globulin and myoglobin, decreased the membrane fluidity (assessed as changes in the steady-state fluorescence anisotropy of 1,6-diphenyl-1,3, 5-hexatriene) in a concentration-dependent manner with a half-maximum concentration between 0.007 and 0.4 mg . mL-1 in both freshly isolated and cultured rat aortic myocytes and human umbilical vein endothelial cells. AA (1 to 200 micromol/L) caused the opposite effect, increasing membrane fluidity and antagonizing the effect of BSA. BSA modified at its arginine residues, which are thought to be important in AA binding, did not stimulate [3H]AA release and was significantly less potent than native BSA in altering the membrane fluidity. The effect of BSA can be explained by a high-affinity binding of AA to the protein and extraction of AA from the cell membrane. The interaction between BSA and AA could play a role in the regulation of vascular permeability.