We further demonstrate that the two compounds bind to ⌬N3-S218E/S222D MEK in a mutually exclusive fashion, suggesting that they may share a common or overlapping binding site(s). Quantitative evaluation of the steady state kinetics of MEK inhibition by these compounds reveals that U0126 has approximately 100-fold higher affinity for ⌬N3-S218E/S222D MEK than does PD098059. We further tested the effects of these compounds on the activity of wild type MEK isolated after activation from stimulated cells. Surprisingly, we observe a significant diminution in affinity of both compounds for wild type MEK as compared with the ⌬N3-S218E/S222D mutant enzyme. These results suggest that the affinity of both compounds is mediated by subtle conformational differences between the two activated MEK forms. The MEK affinity of U0126, its selectivity for MEK over other kinases, and its cellular efficacy suggest that this compound will serve as a powerful tool for in vitro and cellular investigations of mitogen-activated protein kinase-mediated signal transduction.

Two cyclooxygenase isozymes catalyze conversion of arachidonic acid to prostaglandin H2: constitutive COX-1 and inducible COX-2. To assess the role of COX-2 in colorectal tumorigenisis, we determined the effects of COX-2 gene (Ptgs2) knockouts and a novel COX-2 inhibitor on Apc delta716 knockout mice, a model of human familial adenomatous polyposis. A Ptgs2 null mutation reduced the number and size of the intestinal polyps dramatically. Furthermore, treating Apc delta716 mice with a novel COX-2 inhibitor reduced the polyp number more significantly than with sulindac, which inhibits both isoenzymes. These results provide direct genetic evidence that COX-2 plays a key role in tumorigenesis and indicate that COX-2-selective inhibitors can be a novel class of therapeutic agents for colorectal polyposis and cancer.

Mitogen-activated protein kinase (MAPK) is an integral component of cellular signaling during mitogenesis and differentiation of mitotic cells. Recently MAPK activation in post-mitotic cells has been implicated in hippocampal long-term potentiation (LTP), a potential cellular mechanism of learning and memory. Here we investigate the involvement of MAPK in learning and memory in behaving animals. MAPK activation increased in the rat hippocampus after an associative learning task, contextual fear conditioning. Two other protein kinases known to be activated during hippocampal LTP, protein kinase C and alpha-calcium/calmodulin protein kinase II, also were activated in the hippocampus after learning. Inhibition of the specific upstream activator of MAPK, MAPK kinase (MEK), blocked fear conditioning. Thus, classical conditioning in mammals activates MAPK, which is necessary for consolidation of the resultant learning.