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The Two Isoforms of the Na⁺/Ca²⁺ Exchanger, NCX1 and NCX3, Constitute Novel Additional Targets for the Prosurvival Action of Akt/Protein Kinase B Pathway

Division of Pharmacology, Department of Neuroscience (L.F., M.S., A.S., R.S., V.V., P.M., G.D.R., L.A.) and Department of Biology and Molecular and Cell Pathology (P.V.), School of Medicine, "Federico II" University of Naples, Naples, Italy

The proteins NCX1, NCX2, and NCX3 expressed on the plasma membrane of neurons play a crucial role in ionic regulation because they are the major bidirectional system promoting the efflux and influx of Na⁺ and Ca²⁺ ions. Here, we demonstrate that NCX1 and NCX3 proteins are novel additional targets for the survival action of the phosphatidylinositol 3-kinase (PI3-K)/Akt pathway. Indeed, the doxycycline-dependent overexpression of constitutively active Akt1 in tetracycline (Tet)-Off PC-12 positive mutants and the exposure of Tet-Off PC-12 wild type to nerve growth factor induced an up-regulation of NCX1 and NCX3 proteins. NCX1 up-regulation induced by Akt1 activation occurred at the transcriptional level because NCX1 mRNA increased, and it was counteracted by cAMP response element-binding protein 1 inhibition through small interfering RNA strategy. In contrast, Akt1-induced NCX3 up-regulation recognized a post-transcriptional mechanism occurring at the proteasome level because 1) NCX3 transcript did not increase and 2) the proteasome inhibitor N-benzyloxycarbonyl (Z)-Leu-Leu-leucinal (MG-132) did not further enhance NCX3 protein levels in Akt1 active mutants as it would be expected if the ubiquitin-proteasome complex was not already blocked by Akt1 pathway. As expected, in PC-12 Tet-Off wild-type cells MG-132 enhanced NCX3 protein levels. This up-regulation produced an increased activity of NCX function. Furthermore, NCX1 and NCX3 up-regulation contributed to the survival action of Akt1 during chemical hypoxia because both the silencing of NCX1 or NCX3 and the pharmacological paninhibition of NCX isoforms reduced the prosurvival property of Akt1. Together, these results indicated that NCX1 and NCX3 represent novel additional molecular targets for the prosurvival action of PI3-K/Akt pathway.

Address correspondence to: Dr. Lucio Annunziato, Division of Pharmacology, Department of Neuroscience, School of Medicine, University of Naples "Federico II", Via Pansini 5, 80131 Naples, Italy. E-mail: lannunzi{at}unina.it