Mitochondrial disorders are devastating
genetic diseases for which efficacious
therapies are still an unmet need. Recent studies report that increased availability of intracellular
NAD obtained by inhibition of the
NAD-consuming
enzyme poly(ADP-ribose) polymerase (PARP)-1 or supplementation with the
NAD-precursor
nicotinamide riboside (NR) ameliorates energetic derangement and symptoms in mouse models of
mitochondrial disorders. Whether these pharmacological approaches also improve bioenergetics of human cells harboring
mitochondrial defects is unknown. It is also unclear whether the same signaling cascade is prompted by PARP-1 inhibitors and NR supplementation to improve mitochondrial homeostasis. Here, we show that human fibroblasts mutant for the
NADH dehydrogenase (ubiquinone) Fe-S
protein 1 (NDUFS1) subunit of
respiratory complex I have similar
ATP,
NAD, and mitochondrial content compared with control cells, but show reduced mitochondrial membrane potential. Interestingly, mutant cells also show increased transcript levels of
mitochondrial DNA but not nuclear
DNA respiratory complex subunits, suggesting activation of a compensatory response. At variance with prior work in mice, however, NR supplementation, but not PARP-1 inhibition, increased intracellular
NAD content in NDUFS1 mutant human fibroblasts. Conversely, PARP-1 inhibitors, but not NR supplementation, increased transcription of
mitochondrial transcription factor A and
mitochondrial DNA-encoded respiratory complexes constitutively induced in mutant cells. Still, both NR and PARP-1 inhibitors restored mitochondrial membrane potential and increased organelle content as well as oxidative activity of NDUFS1-deficient fibroblasts. Overall, data provide the first evidence that in human cells harboring a mitochondrial respiratory defect exposure to NR or PARP-1, inhibitors activate different signaling pathways that are not invariantly prompted by
NAD increases, but equally able to improve energetic derangement.