Inhibition of mitogen-activated protein kinase kinase induces apoptosis ofhuman chondrocytes

We previously have reported that the mitogen-activated protein kinase (MAPK ) pathway is stimulated by adhesion of human chondrocytes to anti-beta (1)- integrin antibodies or collagen type II in vitro. These mechanisms most lik ely prevent chondrocyte dedifferentiation to fibroblast-like cells and chon drocyte death. To investigate whether this pathway plays an essential role for the differentiation, phenotype, and survival of chondrocytes, we blocke d mitogen-activated protein kinase/extracellular signal-regulated kinase (E rk) (MEK), a kinase upstream of the kinase Erk by using U0126, Exposure of chondrocytes to U0126 caused activation of caspase-3 in a dose-dependent ma nner, Western blot analysis with an antibody specific for dually phosphoryl ated Erk shows that collagen type II induced phosphorylation of Erkl/2 was specifically blocked by U0126 in a dose-dependent manner. Immunohistochemic al analysis showed that treated chondrocytes were caspase-3 positive. In tr eated chondrocytes, the cleavage of 116-kDa poly(ADP-ribose)polymerase resu lted in the 85-kDa apoptosis-related cleavage fragment and was associated w ith caspase-3 activity, Analysis by electron microscopy showed typical morp hological signs of apoptosis, such as crescent-shaped clumps of heterochrom atin, and a degraded' pericellular matrix. Thus, these results indicate tha t the MEK/Erk signal transduction pathway is involved in the maintenance of chondrocytes differentiation and survival. These data stimulate further in vestigations on the role of mitogen-activated protein kinase pathways in hu man chondrocytes.