Abstract
Nicotinamide nucleotide transhydrogenase, NNT, is a ubiquitous protein of the inner mitochondrial membrane with a key role in mitochondrial redox balance. NNT produces high concentrations of NADPH for detoxification of reactive oxygen species by glutathione and thioredoxin pathways. In humans, NNT dysfunction leads to an adrenal-specific disorder, glucocorticoid deficiency. Certain substrains of C57BL/6 mice contain a spontaneously occurring inactivating Nnt mutation and display glucocorticoid deficiency along with glucose intolerance and reduced insulin secretion. To understand the underlying mechanism(s) behind the glucocorticoid deficiency, we performed comprehensive RNA-seq on adrenals from wild-type (C57BL/6N), mutant (C57BL/6J) and BAC transgenic mice overexpressing Nnt (C57BL/6JBAC). The following results were obtained. Our data suggest that Nnt deletion (or overexpression) reduces adrenal steroidogenic output by decreasing the expression of crucial, mitochondrial antioxidant (Prdx3 and Txnrd2) and steroidogenic (Cyp11a1) enzymes. Pathway analysis also revealed upregulation of heat shock protein machinery and haemoglobins possibly in response to the oxidative stress initiated by NNT ablation. In conclusion, using transcriptomic profiling in adrenals from three mouse models, we showed that disturbances in adrenal redox homeostasis are mediated not only by under expression of NNT but also by its overexpression. Further, we demonstrated that both under expression or overexpression of NNT reduced corticosterone output implying a central role for it in the control of steroidogenesis. This is likely due to a reduction in the expression of a key steroidogenic enzyme, Cyp11a1, which mirrored the reduction in corticosterone output.
Highlights
Adrenal insufficiency is a rare, potentially fatal, endocrine disorder resulting from a failure of the adrenal cortex to respond to hormonal stimuli
The activities of steroidogenic cytochrome P450 (CYP) enzymes are reliant upon electron-donating redox partners; for mitochondrial enzymes, electrons are transferred from the reduced form of nicotinamide adenine dinucleotide phosphate (NADPH) by ferredoxin reductase (FDXR) and ferredoxin (FDX1), whereas the microsomal enzymes (Type 2) use P450 oxidoreductase (POR) as their redox partner
We have previously reported that 3-month-old mice carrying a spontaneous Nnt 5-exon deletion (C57BL/6J, Nnt−/−) have significantly lower levels of corticosterone (50%) than their wild-type counterparts (C57BL/6N, Nnt+/+) (Meimaridou et al 2012)
Summary
Adrenal insufficiency is a rare, potentially fatal, endocrine disorder resulting from a failure of the adrenal cortex to respond to hormonal stimuli. Under the control of hypothalamic corticotropin-releasing hormone (CRH) and arginine vasopressin (AVP), the pituitary releases adrenocorticotropic hormone (ACTH), which acts on the adrenal via the ACTH receptor (otherwise known as MC2R) to produce glucocorticoids, mainly cortisol. This NADPH is regenerated from NADP by a few pathways including the thioredoxin and glutathione pathways, which are enabled by NNT (Fig. 2)
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