Effects of Heme on Kidney NAD+ Content In Vivo and In Vitro and Restorative Effects of NAD+ Boosting

Background: Heme proteins such as myoglobin and hemoglobin contribute to acute kidney injury (AKI) through oxidative stress, inflammation, mitochondrial dysfunction, and cellular senescence. Heme proteins contribute to these adverse effects in AKI, at least in part, through the release of their heme prosthetic group. In our prior studies, we demonstrated that heme protein-induced AKI is attended by reduced kidney content of NAD+. Nicotinamide mononucleotide (NMN), a precursor in the NAD+ biosynthetic pathway, has emerged as a potential therapeutic agent due to its role in cellular metabolism and stress resistance. Methods: We investigated the effects of the administration of myoglobin or heme on kidney content of NAD+ in the otherwise intact murine kidney. Additionally, we examined the effect of heme (hemin – Ferriprotoporphyrin IX chloride), with or without concomitant exposure to NMN, on human kidney-2 (HK-2) cells to assess NAD+ levels, inflammatory markers, and senescence-associated gene expression. Results: NAD+ levels were significantly reduced in mouse kidneys following administration of hemin or myoglobin to mice with intact kidneys. Exposing HK-2 cells to heme led to a reduction in cellular NAD+ content, the latter restored when these cells were concomitantly exposed to NMN as well as heme. Heme exposure also upregulated selected pro-inflammatory cytokines (TNF-α and IL-8) in HK-2 cells, and such induction was significantly attenuated by NMN. Finally, the expression of the senescence marker p16Ink4a was elevated upon hemin treatment and normalized with NMN co-treatment. Conclusion: NMN supplementation mitigates heme-induced NAD+ depletion by restoring NAD+ levels and suppresses expression of markers of inflammation and cellular senescence. These findings support the therapeutic potential of NMN in preventing or treating heme protein-associated kidney injury, especially AKI. Finally, we speculate that as heme content is increased in the kidney (largely due to the release of heme from cytochrome p450 heme proteins) in models of AKI induced by ischemia, sepsis, and other nephrotoxins not involving myoglobin or hemoglobin, these findings may be broadly relevant to AKI. Funding: NIDDK Support