ISSN: 0970-938X (Print) | 0976-1683 (Electronic)
An International Journal of Medical Sciences
J Boachie, A Adaikalakoteswari, J Samavat, V Zammit, Z Irshad and P Saravanan
Division of Metabolic and Vascular Health, Clinical Sciences Research Laboratories, Warwick Medical School, University of Warwick, University Hospital-Walsgrave Campus, United Kingdom
Posters & Accepted Abstracts : Biomed Res
DOI: 10.4066/biomedicalresearch-C7-020
Background: There is increasing evidence that lipid metabolism in humans may be regulated by environmental factors including nutrients such as vitamin B12 (B12). B12 deficiency results in disturbance of 1-carbon metabolites [methylmalonyl coenzyme A (MMA), homocysteine and S-adenosyl homocysteine (SAH), S-adenosyl methionine (SAM) and methionine] that collectively favours lipogenesis leading to risk of cardiovascular diseases. In clinical studies, B12 deficiency is associated with higher BMI and dyslipidaemia (high triglycerides and low HDL). In vitro experiments in human adipocytes showed that low B12 results in hypomethylation of SREBF1, a master regulator of cholesterol biosynthesis. If similar effects happen in hepatocytes, this may explain the observation of dyslipidaemia in humans. In addition, the role of B12 in hepatic metabolism of lipids in humans is unexplored. Therefore, we investigated whether B12 deficiency affect hepatic de novo lipogenesis. Methods: Human HepG2 cell line was cultured using custom made B12 deficient Eagle’s Minimal Essential Medium (EMEM) and seeded in four different concentrations of B12 media such as 500nM (control), 1000pM, 100pM and 25pM (low) B12. Oil Red O (ORO) staining, gene expression assay using RT-qPCR, total intracellular triglyceride (TG) assay with commercial kit and de novo TG biosynthesis using radioactive flux assay were employed to examine the effect of B12 on lipogenesis. Results: HepG2 cells in low B12 (25pM) had more lipid droplets that were intensely stained with ORO compared with less stained few oil droplets in control B12 (500nM) condition. Total intracellular TG levels were higher in low B12 hepatocytes. The gene expressions of nuclear transcription factors sterol regulatory element binding protein (SREBF1) and low density lipoprotein receptor (LDLR) were higher in low B12 conditions compared with control. Similarly, the gene expressions of the enzymes involved in de novo fatty acid synthesis [ATP citrate lyase (ACLY), Acetyl CoA carboxylase (ACC), fatty acid synthase (FASN) and elongation-of verylong- chain fatty acid (ELOVL6)], cholesterol biosynthesis [3-hydroxy-3-methylglutaryl-CoA reductase (HMGCR), 3-hydroxy-3-methylglutaryl- CoA synthase 1 (HMCS1), Isopentenyl-Diphosphate delta Isomerase 1 (IDL1)] and TG biosynthesis [stearoyl CoA desaturase (SCD), glycerol-3-phosphate acyltransferase (GPAT), acylglycerol-3-phosphate acyltransferase (AGPAT), phosphatidic acid phosphatase-1 (Lipin1) and diacylglycerol acyl transferase 2 (DGAT2)] in low B12 conditions. Lastly, cellular uptake of radio-labelled fatty acid (14C-oleate) for de novo TG biosynthesis assessed by scintillation was about 80% higher in HepG2 cells cultured in low B12 condition. Conclusion: Our data provide novel evidence that B12 deficiency dysregulates lipid metabolism in hepatocytes. Further studies are required to quantify the effect of this on circulating levels of lipid fractions as well as its epigenetic role on hepatocyte function.
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