['Mice that carry the Klotho mutation (KL(-) (/) (-) ) manifest diverse age-related disorders similar to those observed in humans. Thus, the Klotho protein might function as an anti-aging hormone in mammals. Recently, we reported that Klotho recombinant protein attenuated apoptosis and cellular senescence in endothelial cells, but the mechanism remained unclear. Here, we designed an in vitro study to test whether inhibitors of extracellular signal-regulated kinase and mitogen-activated kinase kinase could affect Klotho regulation of apoptosis and cellular senescence.', 'Cellular senescence was investigated in human umbilical vein endothelial cells treated with or without Klotho recombinant protein, and with or without inhibitors of mitogen-activated kinases. Senescence was quantified by staining with senescence-associated β-galactosidase and by evaluating western blots probed for phosphorylation of mitogen-activated kinases. Apoptosis was assayed on western probed for p53, p21, and caspase-3 and -9.', 'The Klotho recombinant protein induced transient phosphorylation of mitogen-activated kinases within a few minutes. Application of inhibitors of mitogen-activated kinases attenuated the ability of Klotho to interfere with apoptosis and senescence in endothelial cells.'] Does klotho protein diminish endothelial apoptosis and senescence via a mitogen-activated kinase pathway?