Depletion and repletion of biotinyl enzymes in liver of biotin-deficient rats: evidence of a biotin storage system

Pretargeted radioimmunotherapy using genetically engineered antibody-streptavidin fusion proteins for treatment of non-hodgkin lymphoma

PURPOSE

Pretargeted radioimmunotherapy (PRIT) using streptavidin (SAv)-biotin technology can deliver higher therapeutic doses of radioactivity to tumors than conventional RIT. However, "endogenous" biotin can interfere with the effectiveness of this approach by blocking binding of radiolabeled biotin to SAv. We engineered a series of SAv FPs that downmodulate the affinity of SAv for biotin, while retaining high avidity for divalent DOTA-bis-biotin to circumvent this problem.

EXPERIMENTAL DESIGN

The single-chain variable region gene of the murine 1F5 anti-CD20 antibody was fused to the wild-type (WT) SAv gene and to mutant SAv genes, Y43A-SAv and S45A-SAv. FPs were expressed, purified, and compared in studies using athymic mice bearing Ramos lymphoma xenografts.

RESULTS

Biodistribution studies showed delivery of more radioactivity to tumors of mice pretargeted with mutant SAv FPs followed by (111)In-DOTA-bis-biotin [6.2 ± 1.7% of the injected dose per gram (%ID/gm) of tumor 24 hours after Y43A-SAv FP and 5.6 ± 2.2%ID/g with S45A-SAv FP] than in mice on normal diets pretargeted with WT-SAv FP (2.5 ± 1.6%ID/g; P = 0.01). These superior biodistributions translated into superior antitumor efficacy in mice treated with mutant FPs and (90)Y-DOTA-bis-biotin [tumor volumes after 11 days: 237 ± 66 mm(3) with Y43A-SAv, 543 ± 320 mm(3) with S45A-SAv, 1129 ± 322 mm(3) with WT-SAv, and 1435 ± 212 mm(3) with control FP (P < 0.0001)].

CONCLUSIONS

Genetically engineered mutant-SAv FPs and bis-biotin reagents provide an attractive alternative to current SAv-biotin PRIT methods in settings where endogenous biotin levels are high.

Biotin regulates the expression of holocarboxylase synthetase in the miR-539 pathway in HEK-293 cells

Holocarboxylase synthetase (HCS) catalyzes the covalent binding of biotin to carboxylases and histones. In mammals, the expression of HCS depends on biotin, but the mechanism of regulation is unknown. Here we tested the hypothesis that microRNA (miR) plays a role in the regulation of the HCS gene. Human embryonic kidney cells were used as the primary model, but cell lines from other tissues and primary human cells were also tested. In silico searches revealed an evolutionary conserved binding site for miR-539 in the 3 prime -untranslated region (3 prime -UTR) of HCS mRNA. Transgenic cells and reporter gene constructs were used to demonstrate that miR-539 decreases the expression of HCS at the level of transcription rather than translation; these findings were corroborated in nontransgenic cells. When miR-539 was overexpressed in transgenic cells, the abundance of both HCS and biotinylated histones decreased. The abundance of miR-539 was tissue dependent: fibroblasts gt kidney cells gt intestinal cells gt lymphoid cells. Dose-response studies revealed that the abundance of miR-539 was significantly higher at physiological concentrations of biotin than both biotin-deficient and biotin-supplemented media in all cell lines tested. In kidney cells, the expression of HCS was lower in cells in physiological medium than in deficient and supplemented medium. In contrast, in fibroblasts, lymphoid cells, and intestinal cells, there was no apparent link between miR-539 abundance and HCS expression, suggesting that factors other than miR-539 also contribute to the regulation of HCS expression in some tissues. Collectively, the results of this study suggest that miR-539 is among the factors sensing biotin and regulating HCS.

Biotin requirements are lower in human Jurkat lymphoid cells but homeostatic mechanisms are similar to those of HepG2 liver cells

The following proteins are candidates for maintaining biotin homeostasis in humans: the biotin transporters sodium-dependent multivitamin transporter (SMVT) and monocarboxylate transporter 1, the biotinyl-protein ligase holocarboxylase synthetase (HCS), and the lysine-epsilon-biotin hydrolase biotinidase. Liver cells are supplied through the portal vein with high levels of water-soluble vitamins compared with those of peripheral tissues. We hypothesized that the mechanisms of biotin homeostasis are qualitatively and quantitatively different in cells derived from human liver (HepG2 cells) and lymphoid tissues (Jurkat cells). Cells were cultured in biotin-defined media, representing deficient (D), normal (N), and supplemented (S) individuals. Biotinylation of carboxylases depended on biotin availability in both cell types, but HepG2 cells required 3 times more biotin than Jurkat cells to maintain normal levels of holocarboxylases. The expression of biotin transporters was less in both types in medium S compared with cells in media D and N; in contrast, the expression of HCS was higher in cells in medium S compared with the other cells. The abundance of 3-methylcrotonyl-CoA carboxylase mRNA was lower in cells in medium D than cells in media N and S. The enrichment of biotinylated histones was higher at the SMVT promoter 1 in HepG2 and Jurkat cells in medium S compared with the corresponding cells in media D and N, presumably repressing the SMVT gene. The mechanisms of biotin homeostasis are qualitatively similar but quantitatively different in HepG2 and Jurkat cells; HCS, histone biotinylation, and biotin transporters play a role in homeostasis in both.

Biotin

Biotin is a water-soluble vitamin and serves as a coenzyme for five carboxylases in humans. Biotin is also covalently attached to distinct lysine residues in histones, affecting chromatin structure and mediating gene regulation. This review describes mammalian biotin metabolism, biotin analysis, markers of biotin status, and biological functions of biotin. Proteins such as holocarboxylase synthetase, biotinidase, and the biotin transporters SMVT and MCT1 play crucial roles in biotin homeostasis, and these roles are reviewed here. Possible effects of inadequate biotin intake, drug interactions, and inborn errors of metabolism are discussed, including putative effects on birth defects.

Biotin and biotinidase deficiency

Biotin is a water-soluble vitamin that serves as an essential coenzyme for five carboxylases in mammals. Biotin-dependent carboxylases catalyze the fixation of bicarbonate in organic acids and play crucial roles in the metabolism of fatty acids, amino acids and glucose. Carboxylase activities decrease substantially in response to biotin deficiency. Biotin is also covalently attached to histones; biotinylated histones are enriched in repeat regions in the human genome and appear to play a role in transcriptional repression of genes and genome stability. Biotin deficiency may be caused by insufficient dietary uptake of biotin, drug-vitamin interactions and, perhaps, by increased biotin catabolism during pregnancy and in smokers. Biotin deficiency can also be precipitated by decreased activities of the following proteins that play critical roles in biotin homeostasis: the vitamin transporters sodium-dependent multivitamin transporter and monocarboxylate transporter 1, which mediate biotin transport in the intestine, liver and peripheral tissues, and renal reabsorption; holocarboxylase synthetase, which mediates the binding of biotin to carboxylases and histones; and biotinidase, which plays a central role in the intestinal absorption of biotin, the transport of biotin in plasma and the regulation of histone biotinylation. Symptoms of biotin deficiency include seizures, hypotonia, ataxia, dermatitis, hair loss, mental retardation, ketolactic acidosis, organic aciduria and also fetal malformations. This review focuses on the deficiencies of both biotin and biotinidase, and the medical management of such cases.