Activation of different types of RTKs leads to the activation of PI3K. This causes the conversion of PIP2 to PIP3 at the plasma membrane. Inactive AKT translocate from the cytoplasm to the plasma membrane where PIP3 binds AKT, leading to activation of AKT by phosphorylation by PDK1 and mTOR. The arrows and the bars represent activation and inhibition of the following proteins, respectively.
Phosphorylation of MEK1 by cdk5/p35 down regulates the MAP kinase pathway
Map kinases transduce responses to extracellular signals by a variety of routes, and communicate with other pathways through extensive crosstalk networks. A closely studied Map kinase cascade originates with tyrosine kinase activation, and activation of Ras. Ras activates Raf, Raf activates the Map kinase kinases Mek1 and Mek2 and these kinases activate downstream Map kinases like Erk1 and Erk2. Erk1 and Erk2 in turn activate transgenes like p35 through the Map kinase activated transcription factor EGR-1. Mek1 plays a central role in many different Map kinase pathways. Factors that activate Mek1 include growth factors like NGF, cytokines, chemokines, and phorbol ester, resulting in cellular proliferation and survival. Mek1 activation may also play a role in differentiation in neuronal tissues. In cultured neuronal PC-12 cells, NGF induces neurite outgrowth via Mek1 and the map kinase pathway. Constitutive activation of Mek1 can transform cells and may play a role in cancer.The crucial role of Mek1 in a variety of pathways including cellular transformation suggests that the cell must tightly regulate its activity. Cdk5 is a kinase that regulates the activity of Mek1. Although Cdk5 is a member of the cyclin-dependent kinase gene family, the activity of Cdk5 does not appear to be regulated by cyclins, but is activated by association with p35. Cdk5 does not act as a checkpoint kinase to regulate cell cycle progression, but acts as a regulatory kinase involved in other post-mitotic processes such as neuronal activity such as neuronal migration during development and neurite outgrowth. Mice lacking Cdk5 exhibit defects in neuronal development. One target of Cdk5 is Mek1. Phosphorylation of Mek1 by Cdk5 represses Mek1 activity and blocks downstream cellular responses. The activation of p35 by Map kinase pathways followed by deactivation of Map kinase signaling by the Cdk5/p35 complex completes the loop of a feedback circuit to terminate Map kinase signaling.
