Dominant negative mutants of mitogen-activated protein kinase pathway

One of the gene families most commonly found mutated in human tumors is the ras gene family. Ras has been shown to regulate several pathways that contribute to cellular transformation, including the Rac and Rho pathways and the Raf-MEK-MAPK pathway (MEK, MAPK/ERK kinase; MAPK, mitogen activated protein kinase; ERK, extracellular signal-regulated kinase).Raf-MEK-MAPK pathway is necessary for Ras transformation in epithelial cells. The immediate biochemical effect of expressing dominant negative MEK in the tumor cells is a reduction in endogenous Raf-1 and MEK-1 activity through the formation of nonproductive complexes between endogenous Raf-1 and dominant negative MEK. An unexpected finding is that there is no direct correlation between dominant negative MEK expression levels and the percentage reduction of MEK activity in the cells. The most striking aspect of the in vivo studies is the finding that the inhibitory effect of dominant negative MEK on the measured end points was dose dependent. It was consistently observed that clones expressing the highest levels of dominant negative MEK formed the smallest tumors and the fewest metastases and exhibited significant increases in survival. However, it could be argued that high levels of dominant negative MEK might nonspecifically interfere with pathways other than the Raf- MEK-MAPK pathway and perhaps affect in vivo tumor growth. This possibility is negated by the observation that clones expressing low levels of dominant negative MEK were able to elicit all the biological responses measured, suggesting that altered MEK activity alone can have a profound effect on biological outcome.