Caffeine is the most popular neurostimulant consumed worldwide. It is included not only in drinks like coffee, tea, and cola but also in a variety of foods and medicine. Therefore, the excess of caffeine use could become a common occurrence. We recently reported that caffeine intake may influence bone composition and potentially induces osteoporosis in rats. In the present study, we conducted an in vitro system to examine the effects of caffeine on the viability of bone cells using the osteogenic sarcoma cell line (UMR106). This cell line exhibits a number of osteoblastic characteristics, including its ability to respond to osteogenesis hormone. The results indicated that caffeine exposures caused a significant loss in the viability of UMR106. At high concentrations (0.5-3.0 mM), the cell numbers were decreased with time and in a dose-dependent manner (within 24 hours, 75% loss in 0.5 mM, 80% loss in 1.0 mM, and 93% loss in 3.0 mM; and within 72 hours, 84% loss in 0.5 mM, 93% loss in 1.0 mM, and 99% loss in 3.0 mM). Interestingly, the lower concentrations (0.05-0.1 mM), which could be reached in the plasma of individuals with a daily intake of four cups of coffee, caused significant loss of cell numbers (within 24 hours, 58% loss in 0.05 mM and 59% loss in 0.1 mM). Caffeine induced internucleosomal DNA fragmentation, which is consistent with an apoptotic response in this cell line. Internucleosomal DNA fragmentation was accompanied with a clear activation of caspase 3 and cleavage of its substrate PARP-1 as assessed by Western blotting analysis, in both low- and high-concentration groups. These results indicate that caffeine treatment induces apoptosis through the caspase 3 pathway. Using cell fractionation studies, we show that caffeine at low and high concentrations induced release of cytochrome c and is a critical component necessary for caspase 3 activation, from mitochondrial compartments to cytosol of treated cells. These results clearly suggest that caffeine induces a cytochrome c-caspase 3-mediated apoptosis in UMR106 cells. Using an ex vivo system, we examined the potential effect of caffeine on bone by subjecting fetal bones to 1 mM caffeine for 7 days. Similar to data attained using UMR106 cells, caffeine caused a marked induction of caspase 3 as assessed by Western immunoblot analysis of homogenized tissues. Tissue exposed to media-alone caspase 3 induction was not detected. Altogether, these results may suggest that excessive caffeine exposure may induce traits of bone loss and potentially osteoporosis. We intend in our future studies to examine the potential effect of caffeine exposure during embryonic bone development.
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