Folate and vitamin B12 imbalance induces endoplasmic reticulum stress and cholesterol biosynthesis gene expression in human adipocytes

Rationale: Low B12 has been shown to play an important role in the prediction of metabolic risk, but its significance and mechanism in the development of adiposity and adipose tissue dysfunction is largely unknown. Objective: To investigate the role of B12 and folic acid in the development of adipocyte dysfunction. Methods and Results: Microarray analysis of human adipocytes (CHUB-S7 cell line) cultured and differentiated in customised media with varying concentrations of B12 and folic acid led to the identification of two important pathways: cholesterol synthesis and unfolded protein response (UPR). Adipocytes cultured in media with low B12 (150 pmol/L) or no B12 had increased intracellular total cholesterol, higher secreted homocysteine levels, induced UPR and reduced glucose uptake capacity compared to adipocytes cultured in normal media with higher B12. The folate concentrations had either no or little effect on the measured functions. Further analysis of these adipocytes for overall DNA methylation showed that the promoter regions of sterol regulatory element-binding transcription factor 1 (SREBF1) and low density lipoprotein receptor (LDLR) were hypomethylated in the low and no B12 conditions. The SREB proteins (SREBP1 and 2) and mRNA expressions (SREBF1 and LDLR) were also increased in the same conditions. Conclusion: The data suggest that low B12 can lead to adipocyte dysfunction by inducing excess cholesterol biosynthesis, homocysteine production and induction of UPR and overall adipocyte dysfunction. Both of these pathways and adipocyte dysfunction play a significant role in the development of cardiovascular diseases. Independent replicate samples of the human adipocyte cell line CHUB-S7 were treated with four different concentrations of B12 and folate.

研究背景与依据：已有研究证实，低维生素B12在代谢风险预测中发挥重要作用，但其在肥胖及脂肪组织功能障碍发生发展中的意义与机制仍未明确。
研究目的：探讨维生素B12与叶酸在脂肪细胞功能障碍发生过程中的作用。
方法与结果：对在含不同浓度维生素B12与叶酸的定制培养基中培养并分化的人脂肪细胞（CHUB-S7细胞系）开展微阵列分析，最终鉴定出两条关键通路：胆固醇合成通路与未折叠蛋白反应（unfolded protein response, UPR）。与在高维生素B12正常培养基中培养的脂肪细胞相比，在低维生素B12（150 pmol/L）或无维生素B12培养基中培养的脂肪细胞，其细胞内总胆固醇水平升高、分泌型同型半胱氨酸水平上升、未折叠蛋白反应被激活，且葡萄糖摄取能力降低。叶酸浓度对上述检测的细胞功能无显著影响或影响极微。对该类脂肪细胞进行全基因组DNA甲基化分析后发现，在低维生素B12及无维生素B12培养条件下，固醇调节元件结合转录因子1（sterol regulatory element-binding transcription factor 1, SREBF1）与低密度脂蛋白受体（low density lipoprotein receptor, LDLR）的启动子区域呈低甲基化状态。在此培养条件下，固醇调节元件结合蛋白（SREB蛋白，包含SREBP1与SREBP2）的蛋白表达及SREBF1、LDLR的mRNA表达量均显著升高。
研究结论：本研究数据表明，低维生素B12可通过诱导过量胆固醇生物合成、同型半胱氨酸生成及未折叠蛋白反应激活，进而引发脂肪细胞功能障碍。上述两条通路及脂肪细胞功能障碍均在心血管疾病的发生发展中发挥重要作用。后续将使用四种不同浓度的维生素B12与叶酸处理人脂肪细胞系CHUB-S7的独立重复样本。