Intracellular and Tissue Levels of Vitamin B12 in Hepatocytes Are Modulated by CD320 Receptor and TCN2 Transporter

Abstract

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.

Cyanocobalamin promotes muscle development through the TGF-β signaling pathway

“…Studies have found that in the physiological range, CD320 gene and protein expression gradually increased with the increase in extracellular CNCbl concentration in HepG2 cells, CNCbl accelerated the active transport of CNCbl and upregulated the expression of CD320 and TCN2, resulting in the increased uptake of CNCbl. 31 The expression of CD320 protein was upregulated with continuous CNCbl treatment during the C2C12 cell differentiation (Fig. 3A and B), which confirmed the conclusion that CNCbl concentration can affect the expression of CD320 and thus affect the uptake of CNCbl.…”

“…30 Since CD320 binding with TCN2 is an important receptor for the uptake of CNCbl by cells, studying its expression levels may help to understand the relative uptake of CNCbl by cells. 31 Intracellular CNCbl concentration is maintained by regulating the expression of its receptor CD320, 23 implying that the concentration of CNCbl affects CD320 expression. Studies have found that in the physiological range, CD320 gene and protein expression gradually increased with the increase in extracellular CNCbl concentration in HepG2 cells, CNCbl accelerated the active transport of CNCbl and upregulated the expression of CD320 and TCN2, resulting in the increased uptake of CNCbl.…”

An Integrated Analysis of Lactation-Related miRNA and mRNA Expression Profiles in Donkey Mammary Glands

“…In donkey milk, the B-complex vitamin content is higher than that of human milk, and the vitamin C content is higher than that in dairy cow milk, showing a great similarity with human milk [ 62 ]. TCN2 is essential in the transport of vitamin B12 from the blood to various tissues and organs [ 63 ]. CD320 is expressed in mammary epithelial cells and shows a high affinity for the transcobalamin—vitamin B12 complex [ 64 ].…”

Metformin‐induced vitamin B12 deficiency can cause or worsen distal symmetrical, autonomic and cardiac neuropathy in the patient with diabetes

“…This is because with physiological levels there is increased activation of the transport receptor for vitamin B12. Therefore, to maximize vitamin B12 replacement therapy regular monitoring of serum vitamin B12 levels should be performed 53 …”

“…Therefore, to maximize vitamin B12 replacement therapy regular monitoring of serum vitamin B12 levels should be performed. 53…”