1Alpha,25-dihydroxyvitamin D3 modulation of adipocyte reactive oxygen species production

We have previously shown 1alpha,25-dihydroxyvitamin D3 [1alpha,25-(OH)2D3] to inhibit mitochondrial uncoupling protein 2 (UCP2) expression in adipocytes and that in vivo suppression of calcitriol levels with calcium-rich diets increases UCP2 expression. Because UCP2 plays a significant role in the clearance of reactive oxygen species (ROS), we studied the effect of calcitriol on ROS production and ROS-induced adipocyte proliferation.

ROS production in human and murine adipocytes was stimulated by high glucose (30 mM) or H2O2 (100 nM).

Both approaches resulted in increased ROS production by 27% to 100% (p < 0.05) and increased cell proliferation by 15% to 39% (p < 0.03). These effects were augmented by the addition of mitochondrial uncoupling inhibitor guanosine 5'-diphosphate (GDP; 100 microM) or 1alpha,25-(OH)2D3 (10 nM) and attenuated by UCP2 overexpression, suggesting that inhibition of mitochondrial uncoupling suppresses clearance of ROS and increases adipocyte proliferation. The addition of alpha +/- tocopherol (1 microM) inhibited cell proliferation in adipocytes treated with either H2O2 or high glucose, indicating that ROS plays a major role in the regulation of cell proliferation in adipocytes. Moreover, stimulation of ROS with high glucose and H2O2 resulted in a 2- to 5-fold increase in adipocyte intracellular calcium ([Ca2+]i; p < 0.001), and calcium channel antagonism (nifedipine, 10 microM) suppressed ROS induced calcium influx and cell proliferation, indicating that [Ca2+]i may also regulate ROS production and exert a mitogenic effect in adipocytes.

These data support a role of 1alpha,25-(OH)2D3, UCP2, and [Ca2+]i in the regulation of adipocyte ROS production.