Mitochondrial sirtuins in the rat adrenal gland: location within the glands of males and females, hormonal and developmental regulation of gene expressions.

Introduction: Sirtuins are NAD dependent class III histone deacetylases. In adrenal cortex mitochondria are able to transform - via nicotinamide nucleotide transhydrogenase (NNT) - NAD into NADPH, which is required for steroidogenesis. These findings suggest that sirtuins expressed in mitochondria, Sirt3, Sirt4 and Sirt5, may be associated with adrenal steroidogenesis. Therefore, the purpose of this study was to characterize the expression of mitochondrial sirtuins (Sirt3-5) in individual compartments of rat adrenal cortex, their developmental regulation and to demonstrate whether their expression is dependent on adrenocorticotrophic hormone (ACTH) and Nampt (nicotinamide phosphoribosyltransferase also known as visfatin/PBEF), the rate-limiting enzyme in the regulation of mammalian NAD synthesis.

Material And Methods: Studies were performed on rat adrenal glands or on primary culture of rat adrenocortical cells. Expression of mitochondrial sirtuins (Sirt3-5) was evaluated by Affymetrix microarray system or QPCR. The bulk of data were extracted from our earlier experiments which have been reanalyzed in regard to Sirt3-5 mRNAs expression levels and - if necessary - validated by QPCR.

Results: Sirt3-5 were expressed throughout the rat adrenal, with the highest expression level of Sirt5. The level of expression of all sirtuins is higher in the zona glomerulosa (ZG) and zona fasciculata/reticularis (ZF/R) than in the adrenal medulla. Sirt3 and Sirt5 expression levels were similar in adult male and female rats, while Sirt4 expression level was higher in females. As revealed by analysis of the available open database, no significant changes in Sirt3-5 expression levels in whole adrenal glands were observed up to week 104 of life of both male and female rats. Moreover, 60 min after intraperitoneal ACTH injection the expression level of Sirt3 in the en-tire gland was elevated while Sirt5 expression level lowered. On the other hand, chronic ACTH infusion (48 h) did not change expression of studied sirtuins. In cultured cells, ACTH greatly increased the expression levels of the Sirt4 and Sirt5. In cultured cells, Fk866 - a highly specific competitive inhibitor of Nampt - reduced expression level of Sirt5 only. In enucleation-induced regenerating rat adrenal, the expression levels of all studied sirtuins were significantly reduced in relation to the control group. Finally, in primary rat adrenal culture the FCS depletion elevates the Sirt3 and Sirt4 expression levels and downregulates Sirt5 expression.

Conclusions: Sirt3-5 are expressed throughout the rat adrenal, with the highest expression levels in adrenal cortex. Performed experiments (ACTH stimulation, FCS depletion, regeneration) suggest that in the adrenal cortex, the mitochondrial Sirt5 is the primary mitochondrial sirtuin involved in regulating the biological activity of adrenocortical cells. Our results also suggest that normal levels of intracellular Nampt (iNampt) enzymatic activity are required to maintain normal (control) levels of Sirt5 mRNA in cultured cells.