Akt negatively regulates the cJun N-terminal kinase pathway in PC12 cells

The protein serine/threonine kinase Akt is a target of phosphatidylinositol 3-kinase that mediates many of the trophic actions of growth factors on ce lls. In PC12 cells, complete removal of serum leads to vapid stimulation of the cJun N-terminal kinase (JNK) pathway. Inclusion of insulin-like growth factor-1, a stimulator of Akt in PC12 cells, inhibits JNK activation in th is setting, whereas addition of wortmannin to PC12 cells in the presence of sevum stimulates JNK activity, suggesting that growth factor-mediated sign aling through the phosphatidylinositol 3-kinase/Akt pathway chronically inh ibits the JNK pathway in PC12 cells. To explore the possible role of Akt as a negative regulator of JNK activity in PC12 cells, a myristoylated, gain- of-function Akt polypeptide (Myr-Akt) was expressed by retrovirus-mediated gene transfer. Stimulation of JNK activity by sevum withdrawal or UV irradi ation in PC12 cell clones stably expressing Myr-Akt was inhibited similar t o 95% or 50%, respectively, relative to control transfected PC12 cells. Pho sphorylation of both JNKs and a proximal activator, MAP kinase kinase 4 (MK K4), in response to UV irradiation was inhibited in Myr-Akt-expressing PC12 cells. Furthermore, transient expression of Myr-Akt strongly inhibited cJu n transactivation mediated by MEKK1 or MKK7-JNK3, a gain-of-function MKK7-J NK fusion protein. Interestingly, inhibited JNK activation in the Myr-Akt-e xpressing PC12 cells is associated with marked induction of JNK-interacting protein-1 (JIP-1). We propose that negative regulation of the JNK pathway through Akt-dependent induction of specfic JIP proteins contributes to the antiapoptotic actions of Akt in neuronal cell types. (C) 2000 Wiley-Liss, I nc.