Sirt4 regulates ATP homeostasis and mediates a retrograde signaling via AMPK
The yeast silent information regulator 2 (Sir2)-like group of enzymes (sirtuins) are nicotinamide adenine dinucleotide (NAD+)-dependent protein deacetylases/mono-ADP-ribosyltransferases evolutionally conserved in the 250 amino acid catalytic domains. Sirtuins can extend the lifespan of several lower model organisms including yeast, worms and flies. The seven mammalian sirtuins, SIRT1 to SIRT7, have emerged as key metabolic sensors that directly link environmental signals to mammalian metabolic homeostasis and stress response. Among three mitochondrial sirtuins, SIRT4 is the least characterized in terms of mechanism, substrates and regulating pathways in metabolism. SIRT4 functions as an adenosine diphosphate (ADP)-ribosyltransferase on the mitochondrial enzyme glutamate dehydrogenase (GDH) and negatively regulates insulin secretion in islet beta cells. However, ADP-ribosylation of sirtuins may be an inefficient side reaction of deacetylation, with little physiological relevance. Recently, it has been reported that SIRT4 deacetylates malonyl-CoA decarboxylase (MCD) and regulates its activity and malonyl-CoA levels to repress fatty acid oxidation (FAO) while promoting lipid anabolism in regulation of lipid homeostasis. SIRT4 also has a role in glutamine metabolism and this pathway is involved in regulating tumor biology and DNA damage response.
Efficient coupling of cellular energy production to metabolic demand is crucial to maintain organismal homeostasis. Here, we report that SIRT4 regulates mitochondrial adenosine triphosphase (ATP) homeostasis. We found that SIRT4 affects mitochondrial uncoupling via the adenine nucleotide translocator 2 (ANT2). Loss of SIRT4 expression leads to decreased cellular ATP levels in vitro and in vivo while SIRT4 overexpression is associated with increased ATP levels. Further, we provide evidence that lack of SIRT4 activates a retrograde signaling response from the mitochondria to the nucleus that includes 5' adenosine monophosphate-activated protein kinase (AMPK), peroxisome proliferator-activated receptor-gamma coactivator (PGC)-1α, key regulators of β-oxidation, such as Acetyl-CoA carboxylase, and components of the mitochondrial respiratory machinery. This study highlights the ability of SIRT4 to regulate ATP levels via ANT2 and a feedback loop involving AMPK.