Insulin-like growth factor-1-dependent maintenance of neuronal metabolism through the phosphatidylinositol 3-kinase-Akt pathway is inhibited by C ₂-ceramide in CAD cells

Ceramide is a lipid second-messenger generated in response to stimuli associated with neurodegeneration that induces apoptosis, a mechanism underlying neuronal death in Parkinson's disease. We tested the hypothesis that insulin-like growth factor-1 (IGF-1) could mediate a metabolic response in CAD cells, a dopaminergic cell line of mesencephalic origin that differentiate into a neuronal-like phenotype upon serum removal, extend processes resembling neurites, synthesize abundant dopamine and noradrenaline and express the catecholaminergic biosynthetic enzymes tyrosine hydroxylase and dopamine β-hydroxylase, and that this process was phosphatidylinositol 3-kinase (PI 3-K)-Akt-dependent and could be inhibited by C₂-ceramide. The metabolic response was evaluated as real-time changes in extracellular acidification rate (ECAR) using microphysiometry. The IGF-1-induced ECAR response was associated with increased glycolysis, determined by increased NAD(P)H reduction, elevated hexokinase activity and Akt phosphorylation. C ₂-ceramide inhibited all these changes in a dose-dependent manner, and was specific, as it was not induced by the inactive C₂-ceramide analogue C₂-dihydroceramide. Inhibition of the upstream kinase, PI 3-K, also inhibited Akt phosphorylation and the metabolic response to IGF-1, similar to C₂-ceramide. Decreased mitochondrial membrane potential occurred after loss of Akt phosphorylation. These results show that IGF-1 can rapidly modulate neuronal metabolism through PI 3-K-Akt and that early metabolic inhibition induced by C₂-ceramide involves blockade of the PI 3-K-Akt pathway, and may compromise the first step of glycolysis. This may represent a new early event in the C₂-ceramide-induced cell death pathway that could coordinate subsequent changes in mitochondria and commitment of neurons to apoptosis.