Division of Metabolic and Vascular Health, Warwick Medical School, University of Warwick, Coventry CV2 2DX, UK.
Department of Biosciences, School of Science and Technology, Nottingham Trent University, Nottingham NG11 8NS, UK.
Int J Mol Sci. 2021 Mar 17;22(6):3089. doi: 10.3390/ijms22063089.
The liver mass constitutes hepatocytes expressing receptors for vitamin B12 (B12)-bound transporters in circulation. However, intrahepatic and circulating B12 interrelationship levels remain unclear. We assessed the intracellular B12 levels at various circulating B12 concentrations in human HepG2 cell-line and liver tissue levels of B12 in the C57BL/6 mouse model. In HepG2 cells treated with a range of B12 concentrations, the intracellular and circulatory B12 levels, transcript and protein levels of B12 receptor (CD320) and transporter (TCN2) were determined using immunoassays, qRT-PCR and Western blot, respectively. Similar assessments were done in plasma and liver tissue of C57BL/6 mice, previously fed a diet of either a high or low B12 (30.82 µg B12/kg and 7.49 µg B12/kg, respectively) for 8-10 weeks. The physiological B12 status (0.15-1 nM) resulted in increased levels of intracellular B12 in HepG2 cells compared to supraphysiological levels of B12 (>1 nM). Gene and protein expression of CD320 and TCN2 were also higher at physiological levels of B12. Progressively increasing extracellular B12 to supraphysiological levels led to relative decreased levels of intracellular B12, lower expression of gene and protein levels of CD320 and TCN2. Similar results were observed in liver tissue from mice fed on a low B12 diet verses high B12 diet. These findings suggest that unlike supraphysiological B12, physiological levels of B12 in the extracellular media or circulation accelerates active transport of B12, and expression of CD320 and TCN2, resulting in higher relative uptake of B12 in hepatocytes.
肝组织由表达维生素 B12（B12）结合转运体受体的肝细胞组成，循环中的 B12。然而，肝内和循环 B12 之间的相互关系水平仍不清楚。我们评估了不同循环 B12 浓度下人 HepG2 细胞系和 C57BL/6 小鼠模型肝组织中 B12 的细胞内 B12 水平。在用不同浓度 B12 处理的 HepG2 细胞中，用免疫测定法、qRT-PCR 和 Western blot 分别测定细胞内和循环中的 B12 水平、B12 受体（CD320）和转运体（TCN2）的转录物和蛋白水平。在之前用高或低 B12（分别为 30.82 µg B12/kg 和 7.49 µg B12/kg）喂养 8-10 周的 C57BL/6 小鼠的血浆和肝组织中进行了类似的评估。生理 B12 状态（0.15-1 nM）导致 HepG2 细胞中的细胞内 B12 水平升高，与超生理 B12 水平相比（>1 nM）。CD320 和 TCN2 的基因和蛋白表达也在生理 B12 水平下更高。细胞外 B12 逐渐增加到超生理水平导致细胞内 B12 水平相对降低，CD320 和 TCN2 的基因和蛋白水平表达降低。在低 B12 饮食与高 B12 饮食喂养的小鼠肝组织中观察到类似的结果。这些发现表明，与超生理 B12 不同，细胞外介质或循环中的生理 B12 水平加速了 B12 的主动转运，以及 CD320 和 TCN2 的表达，导致肝细胞中 B12 的相对摄取增加。
