Signaling pathways downstream to receptor tyrosine kinases: targets for cancer treatment

Mammalian cells have the ability to respond to a myriad of diverse extracellular stimuli that modulate cell function. This often involves ligands binding to cell surface receptors and subsequent activation of intracellular signaling pathways. These pathways can lead to changes in gene expression patterns that in turn regulate cell growth, differentiation, migration, and function. One important type of cell surface receptor is the receptor tyrosine kinase (RTK). In response to in response to ligand binding, RTKs dimerize, then trans-phosphorylate each other, leading to activation of downstream pathways. While the signaling proteins in these pathways are important for normal cell growth control, when improperly regulated they can lead to uncontrolled growth and sometimes cancer. For this reason, they are often considered to be good candidates for drug targets for chemotherapeutic drugs. RTKs can activate multiple different signaling pathways. Some of the signaling proteins in these pathways can have crosstalk with other RTK activated pathways, and some of them can be activated by multiple mechanisms in addition to activation by RTKs. While there is a wide array of different signaling proteins and pathways activated by RTKs, in this review we will discuss components of several key pathways including the MAPK pathway, the Her2/Neu pathway, mTOR, and Pak kinases. We provide an overview of the roles for these pathways in cell signaling and discuss how different components of these pathways are being considered as targets for cancer treatment.