Estrogen affects skeletal growth and promotes growth plate fusion in humans. High doses of estrogen have been used to limit growth in girls with predicted extreme tall stature; a treatment which has been associated with severe side effects. Selective estrogen receptor modulators (SERMs) could potentially be used as an alternative treatment. We chose to study the effects of Tamoxifen (Tam), a first generation SERM that has been used in the treatment of pubertal gynecomastia or McCune-Albright syndrome. Cultured fetal rat metatarsal bones were used to study the effects of Tam on longitudinal bone growth. In sectioned bones, chondrocyte apoptosis and proliferation were analyzed by TUNEL assay and BrdU incorporation, respectively. We also used a human chondrocytic cell line, HSC-2/8, to study the effects of Tam on apoptosis (FACS analysis and Cell Death detection ELISA) and caspase activation (caspase substrate cleavage and Western immunoblotting). Tam caused a dose-dependent growth retardation of cultured metatarsal bones. No catch-up growth was observed after Tam was removed from the culture medium. Detailed analysis of sectioned growth plate cartilage revealed increased apoptosis of chondrocytes within the resting and hypertrophic zones. HCS-2/8 cells also underwent apoptosis upon Tam treatment. Tam-induced apoptosis was caspase-dependent and completely abrogated by either caspase-8 or -9 inhibitors. A substrate assay revealed that caspase-8 is first activated followed by caspase-9 and -3. Finally, FasL secretion was stimulated by Tam and blocking of either FasL or Fas decreased Tam-induced apoptosis in chondrocytes. We here describe a novel mechanism of tamoxifen-induced apoptosis in chondrocytes, involving the activation of caspases and the FasL/Fas pathway, which diminishes the potential for bone growth.
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