POTENTIATION OF APOPTOSIS BY LOW-DOSE STRESS STIMULI IN CELLS EXPRESSING ACTIVATED MEK KINASE-1

MEK kinases (MEKKs) are serine-threonine kinases that regulate sequent ial protein phosphorylation pathways involving mitogen-activated prote in kinases (MAPKs), including members of the Jun kinase (JNK) family. MEKK1 is a 196 kDa protein that when cleaved by caspase-3-like proteas es generates an active COOH-terminal kinase domain. Expression of the MEKK1 kinase domain is sufficient to induce apoptosis. Mutation of MEK K1 to prevent its proteolytic cleavage protects cells from MEKK1-media ted cell death even though the JNK pathway is still activated, indicat ing that JNK activation is not sufficient to induce cell death. The in ducible acute expression at modest levels of the activated MEKK1 kinas e domain can be used to potentiate the apoptotic response to low dose ultraviolet irradiation and cisplatin. Similarly, in L929 fibrosarcoma cells inducible acute expression of the kinase domain of MEKK1 marked ly increased the cell death response to tumor necrosis factor alpha(TN F alpha). The findings demonstrate that acute expression of an active farm of MEKK1 can potentiate the cell death response to external stres s stimuli. Manipulation of MEKK1 proteolysis and its regulation of sig nal pathways involved in apoptosis has significant potential for antic ancer therapies when used in combination with therapeutic agents at do ses that alone have little or modest effects on cell viability.