Riboflavin transporter SLC52A1, a target of p53, suppresses cellular senescence by activating mitochondrial complex II

Cellular senescence is a state of permanent proliferative arrest induced by a variety of stresses, such as DNA damage. The transcriptional activity of p53 has been known to be essential for senescence induction. It remains unknown, however, whether among the downstream genes of p53, there is a gene that has anti-senescence function. Our recent studies have indicated that the expression of SLC52A1 (also known as GPR172B/RFVT1), a riboflavin transporter, is upregulated specifically in senescent cells depending on p53, but the relationship between senescence and SLC52A1 or riboflavin has not been described. Here, we examined the role of SLC52A1 in senescence. We found that knockdown of SLC52A1 promoted senescence phenotypes induced by DNA damage in tumor and normal cells. The senescence suppressive-action of SLC52A1 was dependent on its riboflavin transport activity. Furthermore, elevation of intracellular riboflavin led to activation of mitochondrial membrane potential (MMP) mediated by the mitochondrial electron transport chain complex II. Finally, the SLC52A1-dependent activation of MMP inhibited the AMPK-p53 pathway, a central mediator of mitochondria dysfunction-related senescence. These results suggest that SLC52A1 contributes to suppress senescence through the uptake of riboflavin and acts downstream of p53 as a negative feedback mechanism to limit aberrant senescence induction.

P035 Inflammatory myopathy and metabolic disorders causing myopathies

Background/Aims Myopathies due to inborn errors of metabolism can be difficult to differentiate from inflammatory myopathies. Careful history, examination and laboratory tests are required to establish the diagnosis. We present a case of Riboflavin Transport Deficiency (Brown-Vialetto-Van Laere syndrome) masquerading as an inflammatory myopathy. Methods A 37-year-old lady presented with severe proximal muscle weakness. She had background of sensory neuropathy and chronic anaemia. Notably, her sister had a history of similar symptoms. Creatine Kinase (CK) was 360 IU/L and lactate dehydrogenase (LDH) was 1700 IU/L. Inflammatory markers were normal. The Ro52 antibody was weakly positive. Electromyography showed evidence of a sensory neuropathy with myopathic features. There was symmetrical fatty infiltration and atrophy of the thigh muscles on magnetic resonance imaging (MRI). Positron emission tomography (PET-CT) scan showed widespread intense uptake in skeletal muscle groups. She was given 3 pulses of IV methyprednisolone followed by oral prednisolone which did not provide clinical benefit. Intravenous immune globulin was given when she developed bulbar weakness, with difficulty swallowing and breathing. She required non-invasive ventilation and nasogastric feed. There were necrotic and regenerative muscle fibres on the muscle biopsy, in keeping with rhabdomyolysis. Electron microscopy showed abundant lipid accumulation, suggestive of a metabolic disorder. Urinary organic acids were raised, triggering an acylcarnitines blood spot test, which were increased. This was compatible with riboflavin transport 'Brown-Vialetto-Van-Laere syndrome'. Riboflavin 500mg TDS was started resulting in significant clinical improvement. Prednisolone was weaned, genetic testing sent, and she was transferred for neurorehabilitation. Results Riboflavin Transport Deficiency (Brown-Vialetto-Van Laere syndrome) is an autosomal recessive neurodegenerative genetic disorder. It affects females and males equally. Symptoms can appear in infants as well as adults. These include hearing and visual loss, bulbar palsy leading to dysphagia and speech problems. Paralysis of diaphragm may cause breathing difficulty. Initially it affects the proximal muscles and then generalized muscle weakness. Molecular genetic testing is required to confirm diagnosis. Patients may have abnormal plasma levels of flavin or acylcarnitine. Acylcarnitines are biological intermediates, used in the diagnosis of fatty acid oxidation disorders. Treatment includes riboflavin supplementation and supportive measures. Response to treatment is variable. Conclusion This lady was initially managed as inflammatory myopathy but did not respond to high dose methylprednisolone. There were atypical features including normal inflammatory markers, MRI thighs showing predominantly fatty infiltration and muscle atrophy and the muscle biopsy with abundant lipid accumulation suggestive of a metabolic disorder. We are awaiting full results of genetic testing. This case is a reminder of the importance of tissue diagnosis and reassessing the initial diagnosis if the clinical picture changes or patients do not respond as expected to treatment. Disclosure M. Malik: None. A. Mason: None. B. Davidson: None. J. Furby: None.

Reconstitution in Proteoliposomes of the Recombinant Human Riboflavin Transporter 2 (SLC52A2) Overexpressed in E. coli

Background: the SLC52A2 gene encodes for the riboflavin transporter 2 (RFVT2). This transporter is ubiquitously expressed. It mediates the transport of Riboflavin across cell membranes. Riboflavin plays a crucial role in cells since its biologically active forms, FMN and FAD, are essential for the metabolism of carbohydrates, amino acids, and lipids. Mutation of the Riboflavin transporters is a risk factor for anemia, cancer, cardiovascular disease, neurodegeneration. Inborn mutations of SLC52A2 are associated with Brown-Vialetto-van Laere syndrome, a rare neurological disorder characterized by infancy onset. In spite of the important metabolic and physio/pathological role of this transporter few data are available on its function and regulation. Methods: the human recombinant RFVT2 has been overexpressed in E. coli, purified and reconstituted into proteoliposomes in order to characterize its activity following the [3H]Riboflavin transport. Results: the recombinant hRFVT2 showed a Km of 0.26 ± 0.07 µM and was inhibited by lumiflavin, FMN and Mg2+. The Riboflavin uptake was also regulated by Ca2+. The native protein extracted from fibroblast and reconstituted in proteoliposomes also showed inhibition by FMN and lumiflavin. Conclusions: proteoliposomes represent a suitable model to assay the RFVT2 function. It will be useful for screening the mutation of RFVT2.

Riboflavin - properties, occurrence and its use in medicine

Riboflavin is built on an isoalloxazin ring, which contains three sixcarbon rings: benzoic, pyrazine and pyrimidine. Riboflavin is synthesized by some bacteria, but among humans and animals, the only source of flavin coenzymes (FAD, FMN) is exogenous riboflavin. Riboflavin transport in enterocytes takes place via three translocators encoded by the SLC52 gene. Deficiency of dietary riboflavin has wide ranging implications for the efficacy of other vitamins, the mechanism of cellular respiration, lactic acid metabolism, hemoglobin, nucleotides and amino acid synthesis. In studies it was found that, pharmacologic daily doses (100 mg) have the potential to react with light, which can have adverse cellular effects. Extrene caution should be exercised when using riboflavin as phototherapy in premature newborns. At the cellular level, riboflavin deficiency leads to increased oxidative stress and causes disorders in the glutathione recycling process. Risk factors for developing riboflavin deficinecy include pregnancy, malnutrition (including anorexia and other eating disorders, vegitarianism, veganism and alcoholism. Furthermore, elderly people and atheletes are also at risk of developing this deficiency. Widespread use of riboflavin in medicine, cancer therapy, treatment of neurodegenerative diseases, corneal ectasia and viral infections has resulted in the recent increased interest in this flavina.

Uptake and Metabolism of Antibiotics Roseoflavin and 8-Demethyl-8-Aminoriboflavin in Riboflavin-Auxotrophic Listeria monocytogenes

ABSTRACTThe riboflavin analogs roseoflavin (RoF) and 8-demethyl-8-aminoriboflavin (AF) are produced by the bacteriaStreptomyces davawensisandStreptomyces cinnabarinus. Riboflavin analogs have the potential to be used as broad-spectrum antibiotics, and we therefore studied the metabolism of riboflavin (vitamin B2), RoF, and AF in the human pathogenListeria monocytogenes, a bacterium which is a riboflavin auxotroph. We show that theL. monocytogenesprotein Lmo1945 is responsible for the uptake of riboflavin, RoF, and AF. Following import, these flavins are phosphorylated/adenylylated by the bifunctional flavokinase/flavin adenine dinucleotide (FAD) synthetase Lmo1329 and adenylylated by the unique FAD synthetase Lmo0728, the first monofunctional FAD synthetase to be described in bacteria. Lmo1329 generates the cofactors flavin mononucleotide (FMN) and FAD, whereas Lmo0728 produces FAD only. The combined activities of Lmo1329 and Lmo0728 are responsible for the intracellular formation of the toxic cofactor analogs roseoflavin mononucleotide (RoFMN), roseoflavin adenine dinucleotide (RoFAD), 8-demethyl-8-aminoriboflavin mononucleotide (AFMN), and 8-demethyl-8-aminoriboflavin adenine dinucleotide (AFAD).In vivoreporter gene assays andin vitrotranscription/translation experiments show that theL. monocytogenesFMN riboswitch Rli96, which controls expression of the riboflavin transport genelmo1945, is negatively affected by riboflavin/FMN and RoF/RoFMN but not by AF/AFMN. Treatment ofL. monocytogeneswith RoF or AF leads to drastically reduced FMN/FAD levels. We suggest that the reduced flavin cofactor levels in combination with concomitant synthesis of inactive cofactor analogs (RoFMN, RoFAD, AFMN, and AFAD) explain why RoF and AF contribute to antibiotic activity inL. monocytogenes.IMPORTANCEThe riboflavin analogs roseoflavin (RoF) and 8-demethyl-8-aminoriboflavin (AF) are small molecules which are produced byStreptomyces davawensisandStreptomyces cinnabarinus. RoF and AF were reported to have antibacterial activity, and we studied how these compounds are metabolized by the human bacterial pathogenListeria monocytogenes. We found that theL. monocytogenesprotein Lmo1945 mediates uptake of AF and RoF and that the combined activities of the enzymes Lmo1329 and Lmo0728 are responsible for the conversion of AF and RoF to toxic cofactor analogs. Comparative studies with RoF and AF (a weaker antibiotic) suggest that the reduction in FMN/FAD levels and the formation of inactive FMN/FAD analogs explain to a large extent the antibiotic activity of AF and RoF.