K-ras p21 expression and activity in lung and lung tumors

Although K-ras is mutated in many human and mouse lung adenocarcinomas, the function of K-ras p21 in lung is not known. We sought evidence for the pre valent hypothesis that K-ras p21 activates raf, which in turn passes the si gnal through the extracellular signal regulated Kinases (Erks) to stimulate cell division, and that this pathway is upregulated when K-ras is mutated. Results from both mouse lung tumors and immortalized cultured E10 and C10 lung type II cells failed to substantiate this hypothesis. Lung tumors did not have more total K-ras p21 or K-ras p21 GTP than normal lung tissue, nor were high levels of these proteins found in tumors with mutant K-ras. Acti vated K-ras p21-GTP levels did not correlate with proliferating cell nuclea r antigen. Special features of tumors with mutant K-ras included small size of carcinomas compared with carcinomas lacking this mutation, and correlat ion of proliferating cell nuclear antigen with raf-1. In nontransformed typ e II cells in culture, both total and activated K-ras p21 increased markedl y at confluence but not after serum stimulation, whereas both Erk1/2 and th e protein kinase AKt were rapidly activated by the serum treatment. Reverse transcriptase-polymerase chain reaction (RT-PCR) assays of K-ras mRNA indi cated an increase in confluent and especially in postconfluent cells. Toget her the findings indicate that normal K-ras p21 activity is associated with growth arrest of lung type II cells, and that the exact contribution of mu tated K-ras p21 to tumor development remains to be discovered.