Kong Juan, Li Yan Chun
Dept. of Medicine, Univ. of Chicago, 5841 S. Maryland Ave., Chicago, IL 60637, USA.
Am J Physiol Endocrinol Metab. 2006 May;290(5):E916-24. doi: 10.1152/ajpendo.00410.2005. Epub 2005 Dec 20.
We have investigated the molecular mechanism whereby 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] inhibits adipogenesis in vitro. 1,25(OH)2D3 blocks 3T3-L1 cell differentiation into adipocytes in a dose-dependent manner; however, the inhibition is ineffective 24-48 h after the differentiation is initiated, suggesting that 1,25(OH)2D3 inhibits only the early events of the adipogenic program. Treatment of 3T3-L1 cells with 1,25(OH)2D3 does not block the mitotic clonal expansion or C/EBPbeta induction; rather, 1,25(OH)2D3 blocks the expression of C/EBPalpha, peroxisome proliferator-activated receptor-gamma (PPARgamma), sterol regulatory element-binding protein-1, and other downstream adipocyte markers. The inhibition by 1,25(OH)2D3 is reversible, since removal of 1,25(OH)2D3 from the medium restores the adipogenic process with only a temporal delay. Interestingly, although the vitamin D receptor (VDR) protein is barely detectable in 3T3-L1 preadipocytes, its levels are dramatically increased during the early phase of adipogenesis, peaking at 4-8 h and subsiding afterward throughout the rest of the differentiation program; 1,25(OH)2D3 treatment appears to stabilize the VDR protein levels. Consistently, adenovirus-mediated overexpression of human (h) VDR in 3T3-L1 cells completely blocks the adipogenic program, confirming that VDR is inhibitory. Inhibition of adipocyte differentiation by 1,25(OH)2D3 is ameliorated by troglitazone, a specific PPARgamma antagonist; conversely, hVDR partially suppresses the transacting activity of PPARgamma but not of C/EBPbeta or C/EBPalpha. Moreover, 1,25(OH)2D3 markedly suppresses C/EBPalpha and PPARgamma mRNA levels in mouse epididymal fat tissue culture. Taken together, these data indicate that the blockade of 3T3-L1 cell differentiation by 1,25(OH)2D3 occurs at the postclonal expansion stages and involves direct suppression of C/EBPalpha and PPARgamma upregulation, antagonization of PPARgamma activity, and stabilization of the inhibitory VDR protein.
我们研究了1,25 - 二羟基维生素D3 [1,25(OH)2D3]在体外抑制脂肪生成的分子机制。1,25(OH)2D3以剂量依赖的方式阻断3T3 - L1细胞向脂肪细胞的分化；然而，在分化开始后24 - 48小时这种抑制作用无效，这表明1,25(OH)2D3仅抑制脂肪生成程序的早期事件。用1,25(OH)2D3处理3T3 - L1细胞并不阻断有丝分裂克隆扩增或C/EBPβ的诱导；相反，1,25(OH)2D3阻断C/EBPα、过氧化物酶体增殖物激活受体 - γ（PPARγ）、固醇调节元件结合蛋白 - 1以及其他下游脂肪细胞标志物的表达。1,25(OH)2D3的抑制作用是可逆的，因为从培养基中去除1,25(OH)2D3仅使脂肪生成过程有暂时延迟但能恢复。有趣的是，尽管在3T3 - L1前脂肪细胞中几乎检测不到维生素D受体（VDR）蛋白，但其水平在脂肪生成的早期阶段显著增加，在4 - 8小时达到峰值，随后在整个其余分化程序中下降；1,25(OH)2D3处理似乎能稳定VDR蛋白水平。一致地，腺病毒介导的人（h）VDR在3T3 - L1细胞中的过表达完全阻断脂肪生成程序，证实VDR具有抑制作用。曲格列酮（一种特异性PPARγ拮抗剂）可改善1,25(OH)₂D₃对脂肪细胞分化的抑制作用；相反，hVDR部分抑制PPARγ的反式作用活性，但不抑制C/EBPβ或C/EBPα的活性。此外，1,25(OH)2D3显著抑制小鼠附睾脂肪组织培养物中C/EBPα和PPARγ的mRNA水平。综上所述，这些数据表明1,25(OH)2D3对3T3 - L1细胞分化的阻断发生在克隆扩增后阶段，涉及直接抑制C/EBPα和PPARγ的上调、拮抗PPARγ活性以及稳定抑制性VDR蛋白。
