Proliferation of peripheral blood mononuclear cells increases riboflavin influx

Riboflavin-Targeted Drug Delivery

Active targeting can improve the retention of drugs and drug delivery systems in tumors, thereby enhancing their therapeutic efficacy. In this context, vitamin receptors that are overexpressed in many cancers are promising targets. In the last decade, attention and research were mainly centered on vitamin B9 (folate) targeting; however, the focus is slowly shifting towards vitamin B2 (riboflavin). Interestingly, while the riboflavin carrier protein was discovered in the 1960s, the three riboflavin transporters (RFVT 1-3) were only identified recently. It has been shown that riboflavin transporters and the riboflavin carrier protein are overexpressed in many tumor types, tumor stem cells, and the tumor neovasculature. Furthermore, a clinical study has demonstrated that tumor cells exhibit increased riboflavin metabolism as compared to normal cells. Moreover, riboflavin and its derivatives have been conjugated to ultrasmall iron oxide nanoparticles, polyethylene glycol polymers, dendrimers, and liposomes. These conjugates have shown a high affinity towards tumors in preclinical studies. This review article summarizes knowledge on RFVT expression in healthy and pathological tissues, discusses riboflavin internalization pathways, and provides an overview of RF-targeted diagnostics and therapeutics.

Riboflavin along with antibiotics balances reactive oxygen species and inflammatory cytokines and controls Staphylococcus aureus infection by boosting murine macrophage function and regulates inflammation

Background

Macrophages serve as intracellular reservoirs of S. aureus. Recent in vitro studies have confirmed high level resistance by S. aureus to macrophage mediated killing and the intracellular persistence of Staphylococci may play an important role in the pathogenesis. Since this localization protects them from both cell-mediated and humoral immune responses, therefore, a successful anti-staphylococcal therapy should include the elimination of intracellular bacteria, further protecting the host cells from staphylococci-induced cell death. So, only antibiotic therapy may not be helpful, successful therapy needs combination of drugs not only for elimination of pathogen but also for rescuing the host cell for S. aureus induced cell death.

Methods

In keeping with this idea an in vitro study has been done to examine the effect of Riboflavin along with antibiotics on phagocytosis, hydorgen peroxide, superoxide production, antioxidant enzyme levels, and cytokine levels in mouse macrophages for amelioration of the Staphylococcus aureus burden. The immune boosting effects of Riboflavin have been validated through perturbations of redox homeostasis and pro-inflammatory cytokines measurements.

Results

It was observed that the supplementation of Vitamin B-2 (Riboflavin) not only enhances macrophage function as previously reported but also decreases pro-inflammatory responses in Staphylococcus aureus infected macrophages. The observed influence of Riboflavin on enhanced antimicrobial effects such as enhanced phagocytosis of macrophages exposed to S. aureus, hydrogen peroxide or superoxide production when combined with either ciprofloxacin (CIP) or Azithromycin (AZM) and decrease in pro-inflammatory responses of IFN-γ, IL-6, IL-1β. Riboflavin treatment also decreased NO and TNF-α level possibly by inhibiting the NF-κβ pathway. The increased antioxidant enzymes like glutathione reductase, SOD and GSH level helped in maintaining a stable redox state in the cell.

Conclusion

Riboflavin plus antibiotic pretreatment not only enhances macrophage functions but also decreases proinflammatory responses in Staphylococcus aureus infected macrophages indicating better bacterial clearance and regulated inflammation which may be considered as a novel and important therapeutic intervention.

Involvement of riboflavin transporter RFVT2/Slc52a2 in hepatic homeostasis of riboflavin in mice

Riboflavin (vitamin B2) acts as an intermediary during various biochemical oxidation-reduction reactions in the liver. Hepatic riboflavin homeostasis is suggested to be maintained through its transporter(s). Riboflavin transporters, RFVT2/Slc52a2 and RFVT3/Slc52a3, have been identified in rodents. However, the role of each RFVT in the hepatic homeostasis of riboflavin has not yet been fully clarified. In this study, we assessed the contribution of each RFVT to riboflavin uptake into the liver using in vitro and in vivo studies. The uptake of riboflavin by mouse primary hepatocytes increased in a time-dependent and a concentration-dependent manner. Riboflavin transport was independent of extracellular Na(+). However, the uptake decreased slightly along with the extracellular pH increases. Real-time PCR analysis revealed that the mRNA level of Slc52a2, or coding for mouse (m)RFVT2, in the mouse liver was 10 times higher than that of Slc52a3 (coding for mRFVT3). The uptake of riboflavin at pH 7.4 by primary hepatocytes was significantly decreased by the transfection of Slc52a2-small interfering RNA (siRNA), but not Slc52a3-siRNA. Furthermore, we also confirmed the contribution of riboflavin transporters in vivo. The riboflavin concentrations in plasma, but not in the liver, were significantly decreased in mice fed on a riboflavin-deficient diet for 8 weeks. The expression of Slc52a2 mRNA was significantly upregulated by riboflavin deprivation. These results strongly suggest that mRFVT2 was involved in hepatic riboflavin homeostasis.

Riboflavin deprivation inhibits macrophage viability and activity – a study on the RAW 264.7 cell line

Riboflavin, or vitamin B2, as a precursor of the coenzymes FAD and FMN, has an indirect influence on many metabolic processes and determines the proper functioning of several systems, including the immune system. In the human population, plasma riboflavin concentration varies from 3·1 nm(in a moderate deficiency, e.g. in pregnant women) to 10·4 nm(in healthy adults) and 300 nm(in cases of riboflavin supplementation). The purpose of the present study was to investigate the effects of riboflavin concentration on the activity and viability of macrophages, i.e. on one of the immunocompetent cell populations. The study was performed on the murine monocyte/macrophage RAW 264.7 cell line cultured in medium with various riboflavin concentrations (3·1, 10·4, 300 and 531 nm). The results show that riboflavin deprivation has negative effects on both the activity and viability of macrophages and reduces their ability to generate an immune response. Signs of riboflavin deficiency developed in RAW 264.7 cells within 4 d of culture in the medium with a low riboflavin concentration (3·1 nm). In particular, the low riboflavin content reduced the proliferation rate and enhanced apoptotic cell death connected with the release of lactate dehydrogenase. The riboflavin deprivation impaired cell adhesion, completely inhibited the respiratory burst and slightly impaired phagocytosis of the zymosan particles. In conclusion, macrophages are sensitive to riboflavin deficiency; thus, a low riboflavin intake in the diet may affect the immune system and may consequently decrease proper host immune defence.

Identification and functional expression of a carrier-mediated riboflavin transport system on rabbit corneal epithelium

PURPOSE

To investigate the functional expression of a carrier-mediated transport mechanism for riboflavin (vitamin B2) across cultured rabbit primary corneal epithelial cells (rPCECs) and intact rabbit cornea. The secondary objective was to understand the physiological significance behind the presence of such a transport system for riboflavin on the apical side of the corneal epithelium.

METHODS

rPCECs and freshly excised rabbit corneas were selected as in vitro and ex vivo models, respectively. Transport and uptake characteristics of [3H]riboflavin were determined at various time points, concentrations, temperatures, and pH. Substrate specificity, energy, and ion dependence studies were carried out to characterize the translocation mechanism. Rabbit tear analysis was done with liquid chromatography/tandem mass spectrometry (LC-MS/MS) to understand the physiological relevance of this transporter.

RESULTS

The uptake process in rPCECs was found to be concentration dependent and saturable at higher concentrations. The process was also independent of pH, Na+, and Cl- but dependent on energy and temperature. Unlabeled riboflavin and its structural analogues caused significant inhibition, whereas unrelated vitamins did not interfere with the process. Transport of [3H]riboflavin across rabbit cornea was also saturable at higher concentration and energy dependent but Na+ independent. Substrate specificity studies across intact rabbit cornea produced results similar to the uptake studies in cultured rPCECs. LC-MS/MS analysis of rabbit tears showed the presence of riboflavin.

CONCLUSIONS

Results suggest the presence of a specialized, high-affinity transport mechanism for riboflavin that is expressed on the apical side of rabbit corneal epithelium and may in turn be responsible for influx of riboflavin from tears to cornea.

Biotin availability regulates expression of the sodium-dependent multivitamin transporter and the rate of biotin uptake in HepG2 cells

In human cells, biotin is essential to maintain metabolic homeostasis and as regulator of gene expression. The enzyme holocarboxylase synthetase (HCS) transforms biotin into its active form 5'-biotinyl-AMP and this compound is used to biotinylate five biotin-dependent carboxylases or to activate a soluble guanylate cyclase (sGC) and a cGMP-dependent protein kinase (PKG). The HCS-sGC-PKG pathway is responsible for maintaining the mRNA levels of enzymes involved in biotin utilization including HCS, carboxylases, and a biotin carrier known as sodium-dependent multivitamin transporter (SMVT). To understand the role of SMVT in the control of biotin utilization, we have studied the effect of biotin availability on SMVT protein and mRNA expression levels in HepG2 cells by Western blot analysis and rtPCR, respectively; and their functional impact on the rate of [3H]biotin uptake in human cells. Our results showed that human HepG2 cells grown in a biotin-deficient medium have a lower rate of biotin uptake than normal cells. The impairment in biotin uptake is associated with a reduction in the amount of both SMVT protein mass and mRNA levels. Transfection of HepG2 cells with a vector containing a luciferase reporter gene under the control of the rat SMVT promoter demonstrated that its transcriptional activity is regulated by biotin availability through activation of the HCS-sGC-PKG pathway. Our results support the proposed role of SMVT in the altruistic regulation of biotin utilization in liver cells that has been associated with sparing biotin depletion of the brain.

Riboflavin deficiency impairs oxidative folding and secretion of apolipoprotein B-100 in HepG2 cells, triggering stress response systems

Secretory proteins such as apolipoprotein B-100 (apoB) undergo oxidative folding (formation of disulfide bonds) in the endoplasmic reticulum (ER) before secretion. Oxidative folding depends on flavoproteins in eukaryotes. Here, human liver (HepG2) cells were used to model effects of riboflavin concentrations in culture media on folding and secretion of apoB. Cells were cultured in media containing 3.1, 12.6, and 300 nmol/L of riboflavin, representing moderately deficient, physiological, and pharmacological plasma concentrations in humans, respectively. When cells were cultured in riboflavin-deficient medium, secretion of apoB decreased by >80% compared with controls cultured in physiological medium. The nuclear translocation of the transcription factor ATF-6 increased by >180% in riboflavin-deficient cells compared with physiological controls; this is consistent with ER stress. Nuclear translocation of ATF-6 was associated with activation of the unfolded protein response. Expression of stress-response genes coding for ubiquitin-activating enzyme 1, growth arrest and DNA damage inducible gene, and glucose regulated protein of 78 kDa was greater in riboflavin-deficient cells compared with other treatment groups. Finally, phosphorylation of the eukaryotic initiation factor (eukaryotic initiation factor 2alpha) increased in riboflavin-deficient cells, consistent with decreased translational activity. We conclude 1) that riboflavin deficiency causes ER stress and activation of unfolded protein response in HepG2 cells, and 2) that riboflavin deficiency decreases protein secretion in HepG2 cells. Decreased secretion of apoB in riboflavin-deficient cells might interfere with lipid homeostasis in vivo.

Current perspectives on the cellular uptake and trafficking of riboflavin

The role of riboflavin in cell maintenance and growth, and the mechanism by which it is absorbed into various human tissues and cell lines has been extensively studied over the past decade. Evidence suggests two absorption mechanisms, a saturable-active component that dominates at near physiological vitamin concentrations and a passive component that is revealed at oversupplemented riboflavin conditions. Various transport modulator studies consistently suggest a highly riboflavin specific, temperature-dependent active transport mechanism that is regulated by the Ca2+/calmodulin pathway. The PKA and PKG pathways have also been implicated in absorption regulation. The long-standing model that riboflavin absorption involves a carrier-mediated transporter has recently been challenged through studies suggesting a receptor-mediated endocytic component. The presence of a soluble, human riboflavin binding protein in the transport stratagem has been shown to play an important role in fetal development. The relationship of this binding protein with the riboflavin specific membrane bound protein, though currently not well defined, may involve a protein-protein interaction that plays a primary role in this proposed receptor-mediated component.

Proliferation of peripheral blood mononuclear cells causes increased expression of the sodium-dependent multivitamin transporter gene and increased uptake of pantothenic acidopen star

Antigenic or mitogenic stimulation of peripheral blood mononuclear cells (PBMC) causes rapid cell proliferation. PBMC proliferation is associated with increased activities of pantothenic acid-dependent metabolic pathways, suggesting increased demand for pantothenic acid. We sought to determine whether PBMC respond to proliferation by increased cellular uptake of pantothenic acid and, if so, by what mechanism(s) the increased uptake is mediated. Uptake of pantothenic acid into PBMC was mediated by the sodium-dependent multivitamin transporter, SMVT, as judged by sodium dependency of uptake, substrate affinity and specificity, and RT-PCR of PBMC RNA. Proliferating PBMC accumulated two times more [3H]pantothenic acid than quiescent PBMC. Rates of [3H]pantothenic acid uptake paralleled rates of PBMC proliferation, as judged by uptake of [3H]thymidine. The increased uptake of [3H]pantothenic acid into proliferating PBMC was mediated by increased expression of SMVT (as judged by RT-PCR using total RNA from PBMC), leading to an increased number of transporters on the cell surface (as judged by maximal transport rates for pantothenic acid). We conclude that proliferating PBMC increase expression of the gene encoding SMVT to increase uptake of pantothenic acid.

In vivo biotin supplementation at a pharmacologic dose decreases proliferation rates of human peripheral blood mononuclear cells and cytokine release

Theoretically, vitamin supplements may either enhance or reduce protein synthesis and proliferation in peripheral blood mononuclear cells (PBMC). In the present study, we determined whether administration of a pharmacologic dose of biotin affects proliferation rates of PBMC and cytokine release. Healthy adults (n = 5) ingested 3.1 micromol biotin/d for 14 d; blood and urine were collected pre- and postsupplementation. PBMC were isolated by density gradient and incubated with the mitogen concanavalin A for up to 3 d. At timed intervals during mitogen stimulation, we measured the following: 1) cellular uptake of [(3)H]thymidine to determine proliferation rates; 2) concentrations of various cytokines released into the medium; and 3) the percentages of PBMC subsets as judged by CD surface markers. Biotin supplementation caused a significant decrease of PBMC proliferation. At 2 d after mitogen stimulation, [(3)H]thymidine uptake by postsupplementation PBMC was 66 +/- 21% of the uptake by presupplementation PBMC (P < 0.05). Similarly, concentrations of interleukin-1beta (2 d after mitogen) and interleukin-2 (1 d after mitogen) in media from postsupplementation PBMC were 65 +/- 28% and 44 +/- 23%, respectively, of those for presupplementation PBMC (P < 0.01). Percentages of PBMC subsets were not affected by 14 d of biotin supplementation. Overall, this study provides evidence that administration of pharmacologic doses of biotin for 14 d decreases PBMC proliferation and synthesis of interleukin-1beta and interleukin-2.