Abstract
Obesity is a major disease risk factor due to obesity-associated hyperinflammation. We found that obesity induced Nod-like receptor pyrin domain–containing 3 (NLRP3) inflammasome hyperactivation and excessive interleukin (IL)–1β production in macrophages by disrupting SAM and HD domain–containing protein 1 (SAMHD1), a deoxynucleoside triphosphate (dNTP) hydrolase crucial for nucleotide balance. This caused aberrant accumulation of dNTPs, which can be transported into mitochondria, and initiated mitochondrial DNA (mtDNA) neosynthesis, which increased the presence of oxidized mtDNA and triggered NLRP3 hyperactivation. Deletion of SAMHD1 promoted NLRP3 hyperactivation in cells isolated from zebrafish, mice, and humans. SAMHD1-deficient mice showed elevated circulating IL-1β, insulin resistance, and metabolic dysfunction–associated steatohepatitis. Blocking dNTP mitochondrial transport prevented NLRP3 hyperactivation in macrophages from obese patients and SAMHD1-deficient mice. Our study revealed that obesity by inhibiting SAMHD1 rewired macrophage nucleotide metabolism, thereby triggering NLRP3 inflammasome hyperactivation to drive disease progression.
Structured Abstract
INTRODUCTION
Obesity has emerged as a major public health crisis, particularly in developed nations, owing to its strong association with chronic diseases such as type 2 diabetes, metabolic dysfunction–associated steatotic liver disease, cardiovascular disorders, neurodegenerative diseases, and cancer. A hallmark of obesity is persistent, low-grade inflammation, which exacerbates disease progression. However, the precise molecular mechanisms linking obesity to immune dysregulation remain elusive.
RATIONALE
The Nod-like receptor pyrin domain–containing 3 (NLRP3) inflammasome, which predominantly functions in macrophages, plays a central role in regulating sterile inflammation in the context of obesity-related diseases. In response to sterile tissue injury or stress, NLRP3 activation by damage-associated molecular patterns triggers the production of proinflammatory cytokines, such as interleukin (IL)–1β, which disrupts insulin signaling, fuels adipose and hepatic inflammation, and accelerates metabolic disease progression. Although it is known that oxidized mitochondrial DNA (ox-mtDNA) can activate the NLRP3 inflammasome, whether and how obesity affects macrophage mtDNA and NLRP3 inflammasome sensitivity remain poorly understood.
RESULTS
In comparison to cells isolated from lean individuals and mice, macrophages isolated from obese individuals and mice fed on a high fat diet had NLRP3 inflammasome hyperactivation and excessive IL-1β production. This hyperactive phenotype correlated with an increase in mtDNA abundance in macrophages from obese individuals and mice. The obesity-induced mtDNA elevation occurred independently of the cytidine monophosphate kinase 2 (CMPK2)–dependent mitochondrial deoxynucleoside triphosphate (dNTP) salvage pathway. Instead, obesity impaired the function of SAM and HD domain–containing protein 1 (SAMHD1), a highly conserved dNTP hydrolase, leading to aberrant cytosolic dNTP accumulation. These excess dNTPs could be transported into mitochondria through nucleotide transporters, fueling uncontrolled mtDNA synthesis, excessive ox-mtDNA production, and subsequent NLRP3 hyperactivation. Consistently, genetic ablation of SAMHD1 caused NLRP3 hyperactivation in zebrafish and mice and predisposed mice to obesity-driven metabolic disorders. Blocking dNTP transport into mitochondria abolished NLRP3 hyperactivation in macrophages from both obese individuals and SAMHD1-deficient mice.
CONCLUSION
We conclude that obesity rewires macrophage nucleotide metabolism to enable NLRP3 inflammasome hyperactivation and uncontrolled inflammation, thereby precipitating disease progression. Moreover, we also identified SAMHD1 as a macrophage-intrinsic inhibitor restraining NLRP3 inflammasome activation from fish to humans and delineated the underlying mechanism of action. Because blocking dNTP mitochondrial transport alleviated NLRP3 inflammasome hyperactivation in macrophages isolated from obese individuals and those from SAMHD1-deficient mice, our results suggested that targeting mitochondrial dNTP transport may offer a therapeutic strategy to mitigate obesity-induced inflammation and its associated diseases.
Obesity increased SAMHD1 phosphorylation and thereby compromised its deoxynucleoside triphosphatase function, ultimately leading to aberrant accumulation of cytosolic dNTPs in macrophages. These dNTPs are then transported into mitochondria through nucleotide transporters to provide excessive building blocks for new mtDNA synthesis, resulting in the bypass of CMPK2-mediated nucleotide salvage pathway, ultimately leading to uncontrolled mtDNA neosynthesis, overproduction of ox-mtDNA, and subsequent NLRP3 inflammasome hyperactivation.
