William H. Okamura

1α,25-Dihydroxyvitamin D3 modulates human adipocyte metabolism via nongenomic action

We reported recently that suppression of the renal lα,25‐dihyroxyvitamin D3 (lα,25‐(OH)2‐D3) production in aP2‐agouti transgenic mice by increasing dietary calcium decreases adipocyte intracellular Ca2+ ([Ca2+]i), stimulates lipolysis, inhibits lipogenesis, and reduces adiposity. However, it was not clear whether this modulation of adipocyte metabolism by dietary calcium is a direct effect of inhibition of lα,25‐(OH)2‐D3‐induced [Ca2+]i. Accordingly, we have now evaluated the direct role of lα,25‐(OH)2‐D3. Human adipocytes exhibited a lα,25‐(OH)2‐D3 dose‐responsive (1–50 nM) increase in [Ca2+]i (P < 0.01). This action was mimicked by lα,25‐dihyroxylumisterol3 (lα,25‐(OH)2‐lumisterol3) (P < 0.001), a specific agonist for a putative membrane vitamin D receptor (mVDR), and completely prevented by lβ,25‐dihydroxyvitamin D3 (1β,25‐(OH)2‐D3), a specific antagonist for the mVDR. Similarly, lα,25‐(OH)2‐D3 (5 nM) caused 50%‐100% increases in adipocyte fatty acid synthase (FAS) expression and activity (P < 0.02), a 61% increase in glycerol‐3‐phosphate dehydrogenase (GPDH) activity (P < 0.01), and an 80% inhibition of isoproterenol‐stimulated lipolysis (P < 0.001), whereas lβ,25‐(OH)2‐D3 completely blocked all these effects. Notably, lα,25‐(OH)2‐lumisterol3 exerted more potent effects in modulating adipocyte lipid metabolism, with 2.5‐ to 3.0‐fold increases in FAS expression and activity (P < 0.001) and a threefold increase in GPDH activity (P < 0.001). Also l α,25‐(OH)2‐lumisterol3 was approximately twice as potent in inhibiting basal lipolysis (P < 0.025), whereas lβ,25‐(OH)2‐D3 completely blocked all these effects. These data suggest that l α,25‐(OH)2‐D3 modulates adipocyte Ca2+ signaling and, consequently, exerts a coordinated control over lipogenesis and lipolysis. Thus, a direct inhibition of l α,25‐(OH)2‐D3‐induced [Ca2+]i may contribute to an anti‐obesity effect of dietary calcium, and the mVDR may represent an important target for obesity.

1α,25-dihydroxyvitamin D3 inhibits uncoupling protein 2 expression in human adipocytes

We recently demonstrated that suppressing 1α,25‐(OH)2‐D3 by increasing dietary calcium decreases adipocyte intracellular Ca2+ ([Ca2+]i), stimulates lipolysis, and inhibits lipogenesis. High calcium diets also increase core temperature and white adipose tissue uncoupling protein 2 (UCP2) expression in aP2‐agouti transgenic mice. Accordingly, we have evaluated the role of 1α,25‐(OH)2‐D3 in regulating human adipocyte UCP2 expression. Treatment of human adipocytes for 48 h with 1 nM 1α,25‐(OH)2‐D3 inhibited UCP2 mRNA and protein levels by 50% (P<0.002) and completely blocked isoproterenol‐ or fatty acid‐stimulated two‐ to threefold increases in UCP2 expression. However, a specific agonist for the membrane vitamin D receptor (mVDR), 1α,25‐dihydroxylumisterol3, was unable to inhibit basal, isoproterenol‐stimulated, or fatty acid‐stimulated UCP2 expression, whereas a specific mVDR antagonist,1β,25dihydroxyvitamin D3, was unable to prevent the 1α,25‐(OH)2‐D3 inhibition of UCP2 expression. In contrast, nuclear vitamin D receptor (nVDR) knockout via antisense oligodeoxynucleotide (ODN) prevented the inhibitory effect of 1α,25‐(OH)2‐D3 on adipocyte UCP2 expression and protein levels. These data indicate that 1α,25‐(OH)2‐D3 exerts an inhibitory effect on adipocyte UCP2 expression via the nVDR. Thus, suppression of 1α,25‐(OH)2‐D3 and consequent up‐regulation of UCP2 may contribute to our previous observation of increased thermogenesis in mice fed with high calcium diets.