Protein kinases modulate the sensitivity of hippocampal neurons to nitric oxide toxicity and anoxia

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Abstract
Multiple processes lead to neuronal death after ischemia, but the generation of nitric oxide (NO) is a key component in this cascade of events. The mechanisms that regulate the extent of neuronal degeneration during anoxia and NO toxicity are multifactorial. Neuronal death may be modulated by the activity of signal transduction systems that influence the toxicity of NO or its metabolic products such as cGMP. The enzyme responsible for the production of NO, nitric oxide synthase (NOS), is phosphorylated by protein kinase C (PKC), the cAMP‐dependent protein kinase (PKA), and the calcium/calmodulin‐dependent protein kinase II (CaM‐II). We examined in primary cultured hippocampal neurons whether the protein kinases PKC, PKA, CaM‐II, and cGMP‐dependent protein kinase modified the toxic effects of anoxia and NO. Down‐regulation of PKC activity with PMA (1μM) increased hippocampal neuronal survival during anoxia and NO exposure from approximately 22% to 88%. Inhibitors of PKC activity 9H‐7, H‐8, sphingosine, and staurosporine also were neuroprotective. Down‐regulation of PKC activity increased survival during anoxia even in the presence of the NOS inhibitor, Nω‐methyl‐L‐arginine. Thus, although down‐regulation of PKC activity may increase neuronal survival by decreasing NOS activity, it also is likely that PKC contributes to ischemic neuronal death by mechanisms that are independent of NOS. Inhibition of the cGMP‐dependent protein kinase activity, but not the activity of the CaM‐II also was neuroprotective during NO administration. In contrast to the protective effects of inhibition of PKC and the cGMP‐dependent protein kinase, activation rather than inhibition of PKA increased hippocampal neuronal survival during NO exposure. These results indicate that neuronal survival during anoxia and NO exposure is linked to the modulation of PKC, PKA, and cGMP‐dependent protein kinase activity but is not dependent on the CaM‐II pathway. Understanding the involvement of PKC, PKA, and the cGMP‐dependent protein kinase in modulating the effect of neuronal death during ischemia and NO toxicity may help in directing future therapeutic modalities for cerebrovascular disease.

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