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Biomedical Research
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Volume 29
Biomed Res 2018, Volume 29 | DOI: 10.4066/biomedicalresearch-C7-020
LOW VITAMIN B12 INDUCES
DE NOVO
LIPOGENESIS IN HUMAN
HEPATOCYTES
J Boachie
1
, A Adaikalakoteswari
1
, J Samavat
1
, V Zammit
1
, Z Irshad
1
and
P Saravanan
1
1Division of Metabolic and Vascular Health, Clinical Sciences Research Laboratories, Warwick Medical School, University of Warwick, Univer-
sity Hospital-Walsgrave Campus, United Kingdom
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 fa-
vours 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 hypometh-
ylation 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. There-
fore, 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 ex-
pressions 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 very-
long-chain fatty acid (ELOVL6)], cholesterol biosynthesis [3-hydroxy-3-methylglutaryl-CoA reductase (HMGCR), 3-hydroxy-3-meth-
ylglutaryl-CoA synthase 1 (HMCS1), Isopentenyl-Diphosphate delta Isomerase 1 (IDL1)] and TG biosynthesis [stearoyl CoA desat-
urase (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-la-
belled 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.