Ras signaling and therapies.

More than 25 years have passed since activating mutations in Ras genes were identified in DNA from human tumors. In this time, it has been established beyond doubt that these mutations play a direct role in causing cancer, and do so in collaboration with a number of other oncogenes and tumor suppressors. Oncogenic mutant Ras proteins are resistant to downregulation by GAP-mediated hydrolysis of bound GTP, and therefore signal persistently. Efforts to develop therapies that block Ras oncoprotein function directly have failed. The high affinity of Ras proteins for GTP has discouraged attempts to identify GTP-analogs. Ras processing enzymes have been targeted, but unfortunately, K-Ras, the Ras protein that plays the major role in human cancer, has proven refractory to these approaches. Further progress has been made with drugs that block downstream signaling: the approved drug Sorafenib inhibits Raf kinase, and its clinical benefits in liver cancer are greatest in patients in which the mitogen activated protein kinase (MAPK) signaling pathway is hyperactive. Other Raf kinase inhibitors, as well as drugs that block mitogen-activated protein kinase / extracellular signal-regulated kinase kinase (MEK) and various steps in the PI 3' kinase pathway, are under development. Here we will discuss the complexities of Ras signaling and their effects on targeting the Ras pathway in the future.