Sirtuin 3 downregulation links metabolic reprogramming, mitochondrial oxidative stress and cell death in macrophages infected with Mycobacterium tuberculosis
Smulan L1, Martinez N1, Kiristy MC1, Kativhu C1, Cavallo K1, Sassetti CM1, Singhal A2, Remold HG3, Kornfeld H1

1University of Massachusetts Medical School, MA; 2Agency for Science, Technology and Research (A*STAR), Singapore; 3Brigham and Women’s Hospital, Harvard Medical School, MA

Mycobacterium tuberculosis (M. tuberculosis) rewires macrophage metabolism whereby glucose metabolism is shifted from oxidative phosphorylation (OXPHOS) towards glycolysis, possibly as a means to fuel a rapid inflammatory response. Although metabolic reprogramming, initiated by the bacillus, has been identified in human and mouse macrophages and in murine lungs, the mechanisms driving M. tuberculosis-mediated metabolic reprogramming remain unknown. Sirtuin (SIRT) 3, an NAD+-dependent protein deacetylase, is an important regulator of mitochondrial metabolism and cellular redox homeostasis. We hypothesized that modulation of SIRT3 in macrophages contributes to M. tuberculosis-mediated mitochondrial metabolic rewiring and stress. Here, we investigate the effect of M. tuberculosis infection on SIRT3 and downstream targets in murine macrophages.

Following infection of macrophages, we found reduced SIRT3 mRNA and protein levels which was mediated through TLR signaling. This downregulation was associated with reduced expression of the SIRT3 target, isocitrate dehydrogenase 2 (IDH2), a key enzyme in the tricarboxylic acid (TCA) cycle. The IDH2 substrate, isocitrate, was also found to be increased indicating a block within the TCA cycle following infection. Expression of electron transport chain complex I subunits were also downregulated, resulting in reduced complex I activity. Coinciding with decreased OXPHOS, infected macrophages had decreased glutathione levels, and accumulation of mitochondrial reactive oxygen species (mtROS) leading to macrophage death. We found these alterations to be SIRT3-dependent as Sirt3-/- macrophages showed decreased expression of IDH2 and complex I subunits, along with decreased glutathione and increased mtROS compared to wild type macrophages. Lungs of M. tuberculosis-infected Sirt3-/- mice had greater bacterial burden and immune pathology compared to wild type mice showing relevance to tuberculosis (TB) disease in vivo.

All together, our findings suggest that M. tuberculosis downregulates SIRT3 through TLR signaling resulting in an altered mitochondrial metabolic state and oxidative stress which ultimately impacts TB pathogenesis.

This work was supported by NIH grants HL081149 and HL127384.

Lorissa J. Smulan PhD

UMASS Medical School

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