Background: Vitamin 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 affect lipogenesis leading to risk of cardiovascular diseases. In clinical studies, B12 deficiency is associated with higher BMI and dyslipidaemia (high triglycerides and low HDL). 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 Eagles 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, RT-qPCR, total intracellular triglyceride (TG) assay and de novo TG biosynthesis using radioactive flux assay were employed to examine the effect of B12 on lipogenesis
Results: Hepatocytes in low B12 (25pM) had more lipid droplets compared with control (500 nM). Total intracellular TG levels were higher in low B12 hepatocytes. Gene expressions of nuclear transcription factors sterol regulatory element binding protein (SREBF1) and low density lipoprotein receptor (LDLR) were higher in low B12 conditions than 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 very-long-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)] were upregulated in low B12 conditions. Cellular uptake of radio-labelled fatty acid (14C-oleate) for de novo TG biosynthesis assessed by scintillation was about 80% higher in low B12 hepatocytes.
Conclusion: Our data provide novel evidence that B12 deficiency dysregulates lipid metabolism in hepatocytes.