Interrelationship between SIRT1 function and biotin homeostasis : implications in metabolic ageing

Abstract

SIRT1 (sirtuin 1) is a mammalian homolog of the longevity regulator Sir2p in yeast which catalyzes the removal of acetyl groups from protein substrates. Mammalian SIRT1 acts as an energy and stress sensor contributing to the beneficial effects of calorie restriction by regulating the acetylation status of different intracellular protein targets. Selective over-expressing SIRT1 in adipose tissue of mice prevents ageing induced insulin insensitivity and enhances energy homeostasis by inhibiting ageing related biotin accumulation and reduces the level of biotinylated protein including acetyl-CoA carboxylase (ACC) which is a major reservoir of biotin in adipose tissues. On the other hand, chronic biotin supplementation can facilitate the accumulation of ACC and abolish adipose SIRT1-mediated beneficial effects on insulin sensitivity and lipid metabolism. However, the role of biotin in regulating adipose SIRT1-mediated beneficial effects is still elusive and needs further investigations.

The present study shows that overexpression of SIRT1 in adipose tissue downregulates not only the total protein expression of ACC, but also the acetylation and biotinylation of this enzyme. After chronic biotin treatment, both acetylation and biotinylation of ACC are increased, accompanied by elevated total protein expression of this enzyme. Further study using both synthetic peptide and BCCP mutant suggests that the presence of biotin and the biotinylation status of ACC could both influence to the capacity of SIRT1 to regulate the stability of this enzyme.

Then the reciprocal causal relationship between ACC, biotin and SIRT1 in both 3T3-L1 adipocytes and mice adipose tissues is established. The reduced ACC can significantly attenuate the accumulation of biotin and enhance the SIRT1 activity in both 3T3-L1 cells and mice adipose tissues suggesting ACC acts as a central regulator of biotin homeostasis in cells and adipose tissues.

The in vivo study demonstrates that overexpression of adipose SIRT1 significantly reduces the acetylation but not biotinylation level of histone. The biotin supplementation increases the both biotinylation and acetylation level of histone in adipose tissues. The synthetic peptide study further confirms that direct biotinylation of histone inhibits SIRT1 mediated deacetylation. Biotin also regulates the expression and acetylation of two non-biotinylated SIRT1 substrates P53 and LKB1 differentially.

Finally, overexpression dominant negative deacetylase mutant SIRT1 in adipose tissues of mice accelerates ageing induced deterioration of insulin sensitivity and lipid metabolic dysfunction which is restored by fed with biotin deficient diet.

Taken in conjunction, the above findings reveal that biotin antagonizes the beneficial effects of SIRT1 by modulating its deacetylation of diversified substrates which provides a potential therapeutic target for the treatment of ageing related metabolic disorders.