Differential regulation of DNA synthesis by nitric oxide and hydroxyurea in vascular smooth muscle cells

Detailed mechanism of the autoxidation of N-hydroxyurea catalyzed by a superoxide dismutase mimic Mn(III) porphyrin: formation of the nitrosylated Mn(II) porphyrin as an intermediate

The in vitro autoxidation of N-hydroxyurea (HU) is catalyzed by Mn(III)TTEG-2-PyP(5+), a synthetic water soluble Mn(III) porphyrin which is also a potent mimic of the enzyme superoxide dismutase. The detailed mechanism of the reaction is deduced from kinetic studies under basic conditions mostly based on data measured at pH = 11.7 but also including some pH-dependent observations in the pH range 9-13. The major intermediates were identified by UV-vis spectroscopy and electrospray ionization mass spectrometry. The reaction starts with a fast axial coordination of HU to the metal center of Mn(III)TTEG-2-PyP(5+), which is followed by a ligand-to-metal electron transfer to get Mn(II)TTEG-2-PyP(4+) and the free radical derived from HU (HU˙). Nitric oxide (NO) and nitroxyl (HNO) are minor intermediates. The major pathway for the formation of the most significant intermediate, the {MnNO} complex of Mn(II)TTEG-2-PyP(4+), is the reaction of Mn(II)TTEG-2-PyP(4+) with NO. We have confirmed that the autoxidation of the intermediates opens alternative reaction channels, and the process finally yields NO(2)(-) and the initial Mn(III)TTEG-2-PyP(5+). The photochemical release of NO from the {MnNO} intermediate was also studied. Kinetic simulations were performed to validate the deduced rate constants. The investigated reaction has medical implications: the accelerated production of NO and HNO from HU may be utilized for therapeutic purposes.

Activity of hydroxyurea against Leishmania mexicana

Leishmania mexicana is a protozoan parasite that causes a disease in humans with frequent relapses after treatment. It is also highly resistant to the currently available drugs. For this reason, there is an urgent need for more effective antileishmanial drugs. Hydroxyurea, an anticancer drug, is toxic to replicating eukaryotic cells and has been proven to be effective in arresting the Leishmania major cell cycle. In this study, hydroxyurea was tested in an in vitro model of intracellular Leishmania infection in macrophages. The parasite density in infected macrophages was measured by microscopy after incubation for various times and treatment with hydroxyurea at different concentrations. Viable parasites that could be transformed into promastigotes by shifting the temperature to 26 degrees C were counted every other day after the replacement of hydroxyurea with fresh medium. Meglumine antimoniate, the standard drug treatment for Leishmania mexicana, was used as a reference drug under the same experimental conditions. Hydroxyurea completely eliminated Leishmania parasites when it was used at a dosage of 10 or 100 microg/ml. Differences in the length of treatment needed to achieve elimination were as follows: the 10-microg/ml doses required 9 days, while 3 days was sufficient when 100 microg/ml was used. Hydroxyurea had a 50% effective dose of 0.015 microg/ml in vitro, which was observed on day 6 after exposure. Hydroxyurea is highly effective in killing intracellular amastigotes in vitro.

Hydroxyurea induces fetal hemoglobin by the nitric oxide-dependent activation of soluble guanylyl cyclase

Hydroxyurea treatment of patients with sickle-cell disease increases fetal hemoglobin (HbF), which reduces hemoglobin S polymerization and clinical complications. Despite its use in the treatment of myeloproliferative diseases for over 30 years, its mechanism of action remains uncertain. Recent studies have demonstrated that hydroxyurea generates the nitric oxide (NO) radical in vivo, and we therefore hypothesized that NO-donor properties might determine the hemoglobin phenotype. We treated both K562 erythroleukemic cells and human erythroid progenitor cells with S-nitrosocysteine (CysNO), an NO donor, and found similar dose- and time-dependent induction of gamma-globin mRNA and HbF protein as we observed with hydroxyurea. Both hydroxyurea and CysNO increased cGMP levels, and the guanylyl cyclase inhibitors ODQ, NS 2028, and LY 83,538 abolished both the hydroxyurea- and CysNO-induced gamma-globin expression. These data provide strong evidence for an NO-derived mechanism for HbF induction by hydroxyurea and suggest possibilities for therapies based on NO-releasing or -potentiating agents.

Inhibition of gastric cancer cell proliferation by resveratrol: role of nitric oxide

Resveratrol is a dietary phytochemical that has been shown to inhibit proliferation of a number of cell lines, and it behaves as a chemopreventive agent in assays that measure the three stages of carcinogenesis. We tested for its chemopreventive potential against gastric cancer by determining its interaction with signaling mechanisms that contribute to the proliferation of transformed cells. Low levels of exogenous reactive oxygen (H(2)O(2)) stimulated [(3)H]thymidine uptake in human gastric adenocarcinoma SNU-1 cells, whereas resveratrol suppressed both synthesis of DNA and generation of endogenous O(2)(-) but stimulated nitric oxide (NO) synthase (NOS) activity. To address the role of NO in the antioxidant action of resveratrol, we measured the effect of sodium nitroprusside (SNP), an NO donor, on O(2)(-) generation and on [(3)H]thymidine incorporation. SNP inhibited DNA synthesis and suppressed ionomycin-stimulated O(2)(-) generation in a concentration-dependent manner. Our results revealed that the antioxidant action of resveratrol toward gastric adenocarcinoma SNU-1 cells may reside in its ability to stimulate NOS to produce low levels of NO, which, in turn, exert antioxidant action. Resveratrol-induced inhibition of SNU-1 proliferation may be partly dependent on NO formation, and we hypothesize that resveratrol exerts its antiproliferative action by interfering with the action of endogenously produced reactive oxygen. These data are supportive of the action of NO against reactive oxygen and suggest that a resveratrol-rich diet may be chemopreventive against gastric cancer.

A redox-based mechanism for nitric oxide-induced inhibition of DNA synthesis in human vascular smooth muscle cells

1. The current study explored potential redox mechanisms of nitric oxide (NO)-induced inhibition of DNA synthesis in cultured human and rat aortic smooth muscle cells. 2. Exposure to S-nitrosothiols, DETA-NONOate and NO itself inhibited ongoing DNA synthesis and S phase progression in a concentration-dependent manner, as measured by thymidine incorporation and flow cytometry. Inhibition by NO donors occurred by release of NO, as detected by chemiluminescence and judged by the effects of NO scavengers, haemoglobin and cPTIO. 3. Co-incubation with redox compounds, N-acetyl-L-cysteine, glutathione and L-ascorbic acid prevented NO inhibition of DNA synthesis. These observations suggest that redox agents may alternatively attenuate NO bioactivity extracellularly, interfere with intracellular actions of NO on the DNA synthesis machinery or restore DNA synthesis after established inhibition by NO. 4. Recovery of DNA synthesis after inhibition by NO was similar with and without redox agents suggesting that augmented restoration of DNA synthesis is an unlikely mechanism to explain redox regulation. 5. Study of extracellula interactions revealed that all redox agents potentiated S-nitrosothiol decomposition and NO release. 6. Examination of intracellular NO bioactivity showed that as opposed to attenuation of NO inhibition of DNA synthesis by redox agents, there was no inhibition (potentiation in the presence of ascorbic acid) of soluble guanylate cyclase (sGC) activation judged by cyclic GMP accumulation in rat cells. 7. These data provide evidence that NO-induced inhibition of ongoing DNA synthesis is sensitive to redox environment. Redox processes might protect the DNA synthesis machinery from inhibition by NO, in the setting of augmented liberation of biologically active NO from NO donors.