1. ¹Stroke Branch, Department of Health and Human Services, National Institute of Neurological Disorders and Stroke, National Institutes of Health, Bethesda, Maryland, USA
2. ²Laboratory of Neurophysiology, National Institute of Neurological Disorders and Stroke, National Institutes of Health, Bethesda, Maryland, USA

Correspondence: Dr J Hillion, Stroke Branch National Institute of Neurological Disorders and Stroke, Department of Health and Human Services, National Institutes of Health, 49 Convent Drive MSC 4476, Building 49/Room 2A59, Bethesda, Maryland 20892-4476, USA. E-mail: hillionj@ninds.nih.gov. Current address: The Johns Hopkins University School of Medicine, 720 Rutland Avenue, Ross Research Building, Room 1015, Baltimore, Maryland 21205-2109, USA. E-mail: jhilliol@jhmi.edu

Received 18 October 2005; Revised 21 December 2005; Accepted 10 January 2006; Published online 1 March 2006.

Abstract

The serine–threonine protein kinase Akt has been identified as an important mediator of cell survival able to counteract apoptotic stimuli. However, hibernation, a model of natural tolerance to cerebral ischemia, is associated with downregulation of Akt. We previously established a model of ischemic tolerance in a PC12 cell line and using this model we now addressed the question whether ischemic tolerance also downregulates Akt in PC12 cells. Kinetic studies showed decreased Akt phosphorylation in tolerized cells. Similarly, phosphorylated levels of three major targets of Akt and well-known proapoptotic factors, the glycogen synthase kinase 3 (GSK-3), a Forkhead family member, FoxO4, and the protein murine double minute 2 (MDM2), all inactivated upon phosphorylation by Akt, were decreased in preconditioned cells. In addition, pharmacological blockade of the phosphoinositide 3-kinase (PI3K)/Akt pathway reduced cell death induced by oxygen and glucose deprivation (OGD) and increased the protective effect of preconditioning (PC). Furthermore, decreasing availability of P-Akt by transfecting PC12 cells with constructs of inactive Akt also resulted in protection against OGD and potentiation of the protective effect of PC. Depending on the environment, GSK-3, FOXO-4, and MDM2 can trigger apoptotic responses or cell cycle arrest, and thus, in a situation of reduced energy, driving the cells into a state of quiescence might be neuroprotective. This work suggests that in the context of tolerance downregulation of Akt is beneficial.