Role of uncoupling protein 2 (UCP2) expression and 1α,25-dihydroxyvitamin D3 in modulating adipocyte apoptosis

We previously found that 1alpha, 25-dihydroxyvitamin D-3 [1alpha, 25-(OH)(2)-D-3] modulates adipocyte lipid metabolism via a Ca2+-dependent mechanism and inhibits adipocyte UCP2 expression, indicating that the anti-obesity effects of dietary calcium are mediated by suppression of 1a, 25-(OH)(2)-D-3 levels. However, because UCP2 reduces mitochondrial potential, we have evaluated the roles of UCP2, mitochondrial uncoupling, and 1alpha, 25-(OH)(2)-D-3 in adipocyte apoptosis. Overexpressing UCP2 in 3T3-L1 cells induced marked reductions in mitochondrial potential (Deltapsi) and ATP production (P< 0.01), increases in the expression of caspases (P< 0.05), and a decrease in Bcl-2/Bax expression ratio (P< 0.01). Physiological doses of 1 alpha, 25-(OH)(2)-D-3 (0.1 - 10 nM) restored mitochondrial Delta psi in LI-UCP2 cells and protected against UCP2 overexpression-induced apoptosis ( P< 0.01), whereas a high dose ( 100 nM) stimulated apoptosis in 3T3-L1 and L1-UCP2 cells (P< 0.05). 1 alpha, 25-(OH)(2)-D-3 stimulated cytosolic Ca2+ dose-dependently in both 3T3-L1 and L1-UCP2 cells. However, physiological doses suppressed mitochondrial Ca2+ levels by similar to 50% whereas the high dose increased mitochondrial Ca2+ by 25% (P< 0.05); this explains stimulation of apoptosis by the high dose of 1alpha, 25-(OH)(2)-D-3. Using high-calcium diets to suppress 1alpha, 25-(OH)(2)-D-3 stimulated adipose tissue apoptosis in aP2 transgenic mice ( P< 0.01), suggesting that increasing dietary calcium stimulates adipose apoptosis and thereby further contributes to an anti-obesity effect of dietary calcium.