Tumor necrosis factor α (TNF-α) is a pleiotropic
cytokine mediating inflammatory as well as cell death activities, and is thought to induce chondrocytic chondrolysis in inflammatory and degenerative
joint diseases.
Selective estrogen receptor modulators (
SERMs), such as
raloxifene, which are commonly used in clinical settings act as
estrogen agonists or antagonists. It is assumed that
estrogens have a potential role in cartilage protection; however, the precise molecular mechanism for the protective effects of
estrogens is unclear. This study was designed to examine whether
raloxifene inhibits TNF-α-induced apoptosis in human chondrocytes and to clarify the mechanisms involved. We also investigated the signaling pathways responsible for the anti-apoptotic effect of
raloxifene. Apoptosis in chondrocytes was determined by DNA fragmentation assay and
caspase-3 activation.
Raloxifene significantly inhibited TNF-α-induced
caspase-3 activation and cell DNA fragmentation levels in chondrocytes. The inhibitory effect of
raloxifene was abolished by the
estrogen receptor antagonist ICI 182,780.
Extracellular signal-regulated kinase 1/2 (ERK1/2) regulates apoptosis, acting as an apoptotic or anti-apoptotic signal. TNF-α-induced apoptosis was significantly enhanced by the ERK1/2 pathway inhibitor
PD98059.
Raloxifene stimulated a further increase in ERK1/2 phosphorylation in TNF-α-treated chondrocytes. Furthermore, the anti-apoptotic effects of
raloxifene were inhibited by
PD98059. In addition, the anti-apoptotic effects of
raloxifene were completely abolished in ERK1/2
siRNA-treated chondrocytes. These results suggest that
raloxifene prevents caspase-3-dependent apoptosis induced by TNF-α in human chondrocytes by activating
estrogen receptors and the ERK1/2 signaling pathway.