The role of vitamin B12 deficiency on hepatic metabolism of lipids

Background: Obesity is currently a worldwide epidemic that increases the risk of developing metabolic disorders like diabetes, hypertension and cardiovascular diseases (CVDs) which causes great public health concern. A series of studies involving animal models and epidemiological investigations have demonstrated a relationship between the importance of vitamin B12 (B12) and various components of metabolic syndrome. High prevalence of B12 deficiency has been shown in Europeans (27%) and South Indians (32%) with type 2 diabetes mellitus (T2DM) as well as several T2DM patients on prolonged metformin treatment. Similarly, studies in human adipose tissues showed evidence of dysregulation in lipids by low B12, accounting for higher adiposity and adipocyte dysfunction. However, the liver is the principal storage organ for B12, and hepatic contribution to the lipogenesis of the entire body is also significantly higher compared to adipose tissue. Several clinical studies have shown that metformin, the first drug of choice for T2DM treatment and proposed to reduce lipid levels in patients, had no effect on intrahepatic triglyceride levels. Metformin treatment has also shown an association with low B12 levels. Some studies in the liver have also reported no expression of adiponectin in the liver, unlike its receptors (adipoR1 and adipoR2). Adiponectin is an adipose tissue-derived hormone that regulates glucose and lipid metabolism and is known to upregulate glycolysis and fatty acid oxidation (FAO) as well as reduces gluconeogenesis in liver. Therefore, we hypothesized that low B12 may dysregulate hepatic metabolism of lipids and reduce the lipid lowering effect of metformin in the liver that might be improved by adiponectin treatment. Methods: Hep G2 cell line was cultured using custom-made B12 deficient Eagle’s Minimal Essential Medium (EMEM) and seeded in different concentrations of B12 media including 500nM (control) and 25pM (low) B12. Oil Red O (ORO) staining, RT-qPCR, total intracellular triglyceride (TG), radioactive flux assay, fatty acid profiling using gas chromatography and extracellular seahorse XF24 flux assay were employed to examine the effect of B12 on lipid metabolism. Results: The intrahepatic uptake of B12 was increased in lower circulating B12 concentrations by increased expression of B12 receptors (CD320) and transporters (TCN2), whereas decreased expressions of CD320 and TCN2 accounted for reduced B12 uptake in higher circulating B12 levels. Low B12 increased de novo lipogenesis and levels of fatty acid (FA) groups associated with higher CVD risk. There was further decrease in FAO and mitochondrial functional efficiency, accounting for high hepatic lipid accumulation in low B12. Similarly, the lipid lowering effect of metformin was decreased by low B12, but improved via adiponectin, in the metformin-treated hepatocyte cell line. Conclusion: Our data, therefore, provides novel evidence that B12 deficiency dysregulates lipid metabolism leading to hepatic lipid accumulation.