Sodium-dependent multivitamin transporter gene is regulated at the chromatin level by histone biotinylation in human Jurkat lymphoblastoma cells

Biotin rescues mitochondrial dysfunction and neurotoxicity in a tauopathy model

Mitochondrial and metabolic dysfunction are often implicated in neurological disease, but effective mechanism-based therapies remain elusive. We performed a genome-scale forward genetic screen in a Drosophila model of tauopathy, a class of neurodegenerative disorders characterized by the accumulation of the protein tau, and identified manipulation of the B-vitamin biotin as a potential therapeutic approach in tauopathy. We show that tau transgenic flies have an innate biotin deficiency due to tau-mediated relaxation of chromatin and consequent aberrant expression of multiple biotin-related genes, disrupting both carboxylase and mitochondrial function. Biotin depletion alone causes mitochondrial pathology and neurodegeneration in both flies and human neurons, implicating mitochondrial dysfunction as a mechanism in biotin deficiency. Finally, carboxylase biotin levels are reduced in mammalian tauopathies, including brains of human Alzheimer's disease patients. These results provide insight into pathogenic mechanisms of human biotin deficiency, the resulting effects on neuronal health, and a potential therapeutic pathway in the treatment of tau-mediated neurotoxicity.

TASK-1 Regulates Apoptosis and Proliferation in a Subset of Non-Small Cell Lung Cancers

Lung cancer is the leading cause of cancer deaths worldwide; survival times are poor despite therapy. The role of the two-pore domain K⁺ (K2P) channel TASK-1 (KCNK3) in lung cancer is at present unknown. We found that TASK-1 is expressed in non-small cell lung cancer (NSCLC) cell lines at variable levels. In a highly TASK-1 expressing NSCLC cell line, A549, a characteristic pH- and hypoxia-sensitive non-inactivating K⁺ current was measured, indicating the presence of functional TASK-1 channels. Inhibition of TASK-1 led to significant depolarization in these cells. Knockdown of TASK-1 by siRNA significantly enhanced apoptosis and reduced proliferation in A549 cells, but not in weakly TASK-1 expressing NCI-H358 cells. Na⁺-coupled nutrient transport across the cell membrane is functionally coupled to the efflux of K⁺ via K⁺ channels, thus TASK-1 may potentially influence Na⁺-coupled nutrient transport. In contrast to TASK-1, which was not differentially expressed in lung cancer vs. normal lung tissue, we found the Na⁺-coupled nutrient transporters, SLC5A3, SLC5A6, and SLC38A1, transporters for myo-inositol, biotin and glutamine, respectively, to be significantly overexpressed in lung adenocarcinomas. In summary, we show for the first time that the TASK-1 channel regulates apoptosis and proliferation in a subset of NSCLC.

Recent development of biotin conjugation in biological imaging, sensing, and target delivery

Despite encouraging results from preliminary studies of anticancer therapies, the lack of tumor specificity remains an important issue in the modern pharmaceutical industry. New findings indicate that biotin or biotin-conjugates could be favorably assimilated by tumor cells that over-express biotin-selective transporters. Furthermore, biotin can form stable complexes with avidin and its bacterial counterpart streptavidin. The strong bridging between avidin and biotin moieties on other molecules is a proven adaptable tool with broad biological applications. Under these circumstances, a biotin moiety is certainly an attractive choice for live-cell imaging, biosensing, and target delivery.

Biotin sensing at the molecular level

Biotin influences transcription in organisms from bacteria to humans. The enzyme, biotin protein ligase, which catalyzes post-transcriptional biotin addition to biotin-dependent carboxylases, plays a central roll in transmitting the demand for biotin to gene expression. The molecular mechanism of this communication in bacteria is well understood and involves competing protein:protein interactions. Biochemical measurements indicate that this competition is kinetically controlled. In humans, the biochemistry of biotin sensing at the transcriptional level is not well characterized. However, the biotin holoenzyme ligase (holocarboxylase synthetase) is proposed to both catalyze biotin addition to carboxylases and to histones in its metabolic and transcriptional roles, respectively. Control of human holocarboxylase synthetase function is, however, considerably more complex than the simple competitive protein protein interactions observed in bacterial systems.

Cell and molecular aspects of human intestinal biotin absorption

Humans cannot synthesize biotin and thus must obtain this vitamin from exogenous sources. The intestine is exposed to 2 sources of biotin: a dietary source and a bacterial source, which is normal microflora of the large intestine. Dietary protein-bound biotin is converted to free biotin prior to absorption. Absorption of free biotin in the small and large intestine involves a saturable and Na(+)-dependent carrier-mediated process that is shared with pantothenic acid and lipoate. For this reason, the involved transport system is referred to as the sodium-dependent multivitamin transporter (SMVT); in humans, it is designated as hSMVT. The hSMVT system has been cloned, demonstrated to be exclusively expressed at the apical membrane of enterocytes, and shown, by means of gene-specific short interfering RNA, to be the main biotin uptake system that operates in human intestinal epithelial cells. The 5'-regulatory region of the hSMVT gene has also been cloned and characterized both in vitro and in vivo. Further, the human intestinal biotin uptake process was adaptively up-regulated in biotin deficiency via a transcriptionally mediated mechanism(s) that involves Kruppel-like factor 4 sites. Studies on cell biology of hSMVT have shown a region in the cytoplasmic C-terminal domain of the polypeptide to be essential for its targeting to the apical membrane domain of epithelial cells. Intracellular trafficking of the hSMVT protein appears to involve distinct trafficking vesicles that require an intact microtubules network and the motor protein dynein for their mobility.

Marginal biotin deficiency is common in normal human pregnancy and is highly teratogenic in mice

In studies of marginal biotin deficiency induced experimentally in adults, increased urinary excretion of 3-hydroxyisovaleric acid (3HIA), which likely reflects decreased activity of the biotin-dependent enzyme beta-methylcrotonyl-CoA carboxylase, and decreased activity of the biotin-dependent enzyme propionyl-CoA carboxylase (PCC) in peripheral blood lymphocytes have been validated as indices of biotin status. About half of pregnant women excrete increased amounts of urinary 3HIA. However, interpretation of urinary 3HIA excretion rates is problematic, because renal function is altered by pregnancy per se. In a recent pilot study, activity of PCC in peripheral blood lymphocytes was decreased in 18 of 22 pregnant women. In 4 of 4 pregnant women with decreased PCC activity, biotin supplementation caused increased PCC activity by a mean of 95%. Taken together, such studies provide evidence that a substantial proportion of pregnant women are marginally biotin deficient. In mice, degrees of biotin deficiency that are metabolically similar to those seen in pregnant women are very teratogenic. Moreover, in mice, a marginal degree of biotin deficiency in the dam causes a much more severe degree of deficiency in the fetus. These observations further raise concerns that biotin deficiency does occur and does cause human birth defects.