Hypoinsulinemia alleviates the GRF1/Ras/Akt anti-apoptotic pathway and induces alterations of mitochondrial ras trafficking in neuronal cells

Recent observations have established that interruption of insulin production causes deficits in learning and memory formation. We have studied the mechanism of insulin's neuroprotective effect on primary neuronal cells and in streptozotocin (STZ)-induced diabetic rat brain. We have found that in hippocampal neuronal cells insulin increases the content of farnesylated Ras and phosphorylated form of Akt. Besides, the treatment of cells by insulin leads to the activation of mitochondrial cytochrome oxidase, which is inhibited by manumycin, a farnesyltransferase inhibitor. During experimental diabetes, the content of membrane-bound GRF1 was decreased in rat hippocampus that was correlated with the reduction in mitochondrial Ras and phosphorylated forms of Akt. This redistribution in Ras-GRF system was accompanied by the alteration in the activities of CREB, NF-kB (p65) and c-Rel transcription factors. We have proposed that hypoinsulinemia induces the inhibition of Ras signalling in the neuronal cells additionally by abnormality of Ras trafficking into mitochondria.

Inhibition of nitric oxide synthase and farnesyltransferase change the activities of several transcription factors

Several types of cellular proteins can be modified by farnesylation and nitrosylation, of which the most significant is Ras. We used manumycin, a farnesyltransferase inhibitor, and L-NAME (Nomega-nitro-L-arginine methyl ester), a nitric oxide synthase (NOS) inhibitor, for characterization of Ras-dependent downstream targets activities. Our results suggest that change of the steady-state levels of nitric oxide and inhibition of farnesylation modified the activities of several transcription factors. We have found that the inhibition of farnesylation by manumycin decreased the DNA-binding activity of nuclear factor (NF)-kappaB, did not change the DNA-binding activities of STAT, Sp1, ATF-2, and CREB, and increased the activities of c-Fos, JunD, and c-Jun. Under such conditions, phosphorylation of Akt was decreased, whereas phosphorylation of extracellular signal-regulated kinase (ERK) was increased and phosphorylation of JNK did not change. Furthermore, our results show that reduction of intracellular concentration of nitric oxides by L-NAME increases the activities of c-Fos, ATF-2 and JunD and decreases the activities of CREB, STAT, Sp1, and c-Jun. The activities of all of these transcription factors are restored to normal levels in the presence of manumycin, suggesting that simultaneous modifications of proteins by farnesylation and nitrosylation change the direction of Ras-controlled downstream pathways. Our results provide further evidence of the significance of posttranslational modifications of Ras for the specificity of transducing cascade networks and physiological outcome.