Nitric oxide inhibits the proliferation of murine microglial MG5 cells by a mechanism involving p21 but independent of p53 and cyclic guanosine monophosphate

Nitric oxide attenuates microglia proliferation by sequentially facilitating calcium influx through TRPV2 channels, activating NFATC2, and increasing p21 transcription

Microglia proliferation is critical for proper development and function of the central nervous system (CNS), while dysregulation of proliferation contributes to pathology. We recently reported that male inducible nitric oxide synthase knockout (iNOS^-/-) mice displayed significantly more proliferating microglia in their postnatal cortex than age-matched wildtype (WT) male mice. Moreover, nitric oxide (NO) signaling in mouse microglia greatly upregulates calcium entry through transient receptor potential vanilloid type 2 (TRPV2) channels. Considering that TRPV2 activity restricts astrocytic proliferation within glioma tissues, we investigated the roles of iNOS/NO signaling and TRPV2 expression in the regulation of microglial proliferation in vitro using assays of calcium imaging, immunocytochemistry, western blot, and polymerase chain reaction. Results showed that non-dividing microglia exhibited substantially higher expression of TRPV2 on the plasma membrane and significantly larger calcium influx through TRPV2 channels in comparison to dividing microglia. Additionally, non-dividing WT microglia exhibited significantly more NO production than dividing WT microglia. Furthermore, the NO-donor NOC18 increased the nuclear translocation of nuclear factor of activated T-cells cytoplasmic 2 (NFATC2) and the mRNA of the cyclin-dependent kinase inhibitor p21 and decreased the percentage of dividing WT and iNOS^-/- microglia in culture. Importantly, the presence of the TRPV2 inhibitor tranilast abolished these effects of NOC18. Together, results from this study indicated that iNOS/NO signaling inhibits microglial proliferation through TRPV2-mediated calcium influx, nuclear translocation of the transcription factor NFATC2, and p21 expression. [Figure: see text].

Alternatively activated microglia and macrophages in the central nervous system

Macrophages are important players in the fight against viral, bacterial, fungal and parasitic infections. From a resting state they may undertake two activation pathways, the classical known as M1, or the alternative known as M2. M1 markers are mostly mediators of pro-inflammatory responses whereas M2 markers emerge for resolution and cleanup. Microglia exerts in the central nervous system (CNS) a function similar to that of macrophages in the periphery. Microglia activation and proliferation occurs in almost any single pathology affecting the CNS. Often microglia activation has been considered detrimental and drugs able to stop microglia activation were considered for the treatment of a variety of diseases. Cumulative evidence shows that microglia may undergo the alternative activation pathway, express M2-type markers and contribute to neuroprotection. This review focuses on details about the role of M2 microglia and in the approaches available for its identification. Approaches to drive the M2 phenotype and data on its potential in CNS diseases are also reviewed.

STAT3 regulation by S-nitrosylation: implication for inflammatory disease

AIMS

S-nitrosylation and S-glutathionylation, redox-based modifications of protein thiols, are recently emerging as important signaling mechanisms. In this study, we assessed S-nitrosylation-based regulation of Janus-activated kinase 2/signal transducer and activator of transcription 3 (JAK2/STAT3) pathway that plays critical roles in immune/inflammatory responses and tumorigenesis.

RESULTS

Our studies show that STAT3 in stimulated microglia underwent two distinct redox-dependent modifications, S-nitrosylation and S-glutathionylation. STAT3 S-nitrosylation was associated with inducible nitric oxide synthase (iNOS)-produced nitric oxide (NO) and S-nitrosoglutathione (GSNO), whereas S-glutathionylation of STAT3 was associated with cellular oxidative stress. NO produced by iNOS or treatment of microglia with exogenous GSNO inhibited STAT3 activation via inhibiting STAT3 phosphorylation (Tyr(705)). Consequently, the interleukin-6 (IL-6)-induced microglial proliferation and associated gene expressions were also reduced. In cell-free kinase assay using purified JAK2 and STAT3, STAT3 phosphorylation was inhibited by its selective preincubation with GSNO, but not by preincubation of JAK2 with GSNO, indicating that GSNO-mediated mechanisms inhibit STAT3 phosphorylation through S-nitrosylation of STAT3 rather than JAK2. In this study, we identified that Cys(259) was the target Cys residue of GSNO-mediated S-nitrosylation of STAT3. The replacement of Cys(259) residue with Ala abolished the inhibitory role of GSNO in IL-6-induced STAT3 phosphorylation and transactivation, suggesting the role of Cys(259) S-nitrosylation in STAT3 phosphorylation.

INNOVATION

Microglial proliferation is regulated by NO via S-nitrosylation of STAT3 (Cys(259)) and inhibition of STAT3 (Tyr(705)) phosphorylation.

CONCLUSION

Our results indicate the regulation of STAT3 by NO-based post-translational modification (S-nitrosylation). These findings have important implications for the development of new therapeutics targeting STAT3 for treating diseases associated with inflammatory/immune responses and abnormal cell proliferation, including cancer.

Nitric oxide: a regulator of eukaryotic initiation factor 2 kinases

Generation of nitric oxide (NO(•)) can upstream induce and downstream mediate the kinases that phosphorylate the α subunit of eukaryotic initiation factor 2 (eIF2α), which plays a critical role in regulating gene expression. There are four known eIF2α kinases (EIF2AKs), and NO(•) affects each one uniquely. Whereas NO(•) directly activates EIF2AK1 (HRI), it indirectly activates EIF2AK3 (PERK). EIF2AK4 (GCN2) is activated by depletion of l-arginine, which is used by nitric oxide synthase (NOS) during the production of NO(•). Finally EIF2AK2 (PKR), which can mediate inducible NOS expression and therefore NO(•) production, can also be activated by NO(•). The production of NO(•) and activation of EIF2AKs coordinately regulate physiological and pathological events such as innate immune response and cell apoptosis.

Redox control of G(1)/S cell cycle regulators during nitric oxide-mediated cell cycle arrest

Redox regulation of cell cycle progression during nitric oxide (NO) mediated cytostasis is not well-understood. In this study, we investigated the role of the intracellular antioxidant glutathione (GSH) in regulating specific signaling events that are associated with NO-mediated cell cycle arrest. Manipulation of intracellular GSH content through pharmacological inhibition of glutamate-cysteine ligase (GCL) indicated that GSH depletion potentiated nitrosative stress, DNA damage, phosphorylation of the tumor suppressor p53 (Ser-18) and upregulation of p21(cip1/waf1) upon NO stimulation. However, we found that neither overexpression of a dominant negative p53 nor pharmacological inhibition of p53 with cyclic pifithrin-alpha (cPFT-alpha) was sufficient to reverse NO-mediated cell cycle arrest or hypophosphorylation of retinoblastoma protein (Rb). We found that the decrease in cyclin D1 levels induced by NO was GSH-sensitive implying that the redox regulation of NO-mediated cytostasis was a multifaceted process and that both p53/p21(cip1/waf1) and p53 independent cyclin D1 pathways were involved. Together, our results demonstrate that GSH serves as an important component of cellular protective mechanisms against NO-derived nitrosative stress to regulate DNA damage checkpoint.

Nitric oxide and cell proliferation

Nitric oxide (NO*) has been proposed to be a physiological modulator of cell proliferation, able to promote in most cases cell cycle arrest. In this review I explore the molecular basis of this mechanism of action. The modulatory action of NO* on the intracellular concentration of cGMP and the machinery directly involved in the control of cell cycle progression, including the expression and activity of diverse cyclins and cyclin-dependent kinases, their physiological inhibitors, and the master transcriptional regulator retinoblastoma protein, will be discussed. The role of NO* in proliferation mediated by tyrosine kinase receptors such as the epidermal growth factor receptor and downstream signalling pathways will also be considered. Finally, the involvement of NO* in proliferative processes relevant for normal development will be outlined.

Nitric oxide-modulated marker gene expression of signal transduction pathways in lung endothelial cells

Nitric oxide (NO) is a signal molecule involved in regulation of physiological and pathophysiological functions of the vascular endothelium such as apoptosis. We examined whether NO-modulates marker gene expression of signal transduction pathways in cultured pulmonary artery endothelial cell (PAEC). Cells were exposed to a NO donor, 1 mM NOC-18, for 0.5, 5, and 24 h, thereafter, expression levels of 96 marker genes associated with 18 signal transduction pathways were assessed using a signal transduction pathway-finder microarray analysis system. NO modulation of apoptotic pathways and nuclear factor (NF) microarray were further analyzed. Gene array analyses revealed that 17 genes in 13 signal pathways were up- or down-regulated in cells exposed to NO, four of which were significantly altered by NO and are associated with apoptotic pathways. Apoptotic pathways resulted in identification of 11 genes in this group. Nuclear factor microarray studies demonstrated that NO-modulated expression of these signal transduction genes was associated with regulation of NF-binding activities. Gel shift analysis verified the effects of NO on DNA-binding activity of NF. These results demonstrated that NO signaling modulates at least 13 signal transduction pathways including apoptosis-related families in PAEC.

HIV associated neurodegeneration requires p53 in neurons and microglia

HIV infection of the central nervous system leads to HIV-associated dementia (HAD) in a substantial subset of infected individuals. The pathogenesis of neuronal dysfunction in HAD is not well understood, but previous studies have demonstrated evidence for activation of apoptotic pathways. The tumor suppressor transcription factor p53 is an apical mediator of neuronal apoptosis following a variety of injurious stimuli. To determine whether p53 participates in HAD, we exposed cerebrocortical cultures from wild-type and p53 deficient mice to the neurotoxic HIV envelope protein gp120. Using neuron/microglia co-culture of mixed p53 genotype, we observed that both neurons and microglia require p53 for gp120 induced neuronal apoptosis. Additionally, accumulation of p53 protein in neurons was recently reported in post-mortem cortical tissue from a small group of HAD patients. Using a much larger cohort of HAD cases, we extend this finding and report that p53 protein also increases in non-neuronal cells, including microglia. Taken together these findings demonstrate a novel role for p53 in the microglial response to gp120. Additionally, these findings, in conjunction with a recent report that monocytes expressing HIV-Tat also secrete neurotoxins that promote p53 activation, suggest that distinct HIV proteins may converge on the p53 pathway to promote neurotoxicity.