Small-Molecule Screen Identifies De Novo Nucleotide Synthesis as a Vulnerability of Cells Lacking SIRT3

Sirtuin 3 (SIRT3) is a NAD⁺-dependent deacetylase downregulated in aging and age-associated diseases such as cancer and neurodegeneration and in high-fat diet (HFD)-induced metabolic disorders. Here, we performed a small-molecule screen and identified an unexpected metabolic vulnerability associated with SIRT3 loss. Azaserine, a glutamine analog, was the top compound that inhibited growth and proliferation of cells lacking SIRT3. Using stable isotope tracing of glutamine, we observed its increased incorporation into de novo nucleotide synthesis in SIRT3 knockout (KO) cells. Furthermore, we found that SIRT3 KO cells upregulated the diversion of glutamine into de novo nucleotide synthesis through hyperactive mTORC1 signaling. Overexpression of SIRT3 suppressed mTORC1 and growth in vivo in a xenograft tumor model of breast cancer. Thus, we have uncovered a metabolic vulnerability of cells with SIRT3 loss by using an unbiased small-molecule screen.

l-Alanine activates hepatic AMP-activated protein kinase and modulates systemic glucose metabolism

Objective

AMP activated protein kinase (AMPK) is recognized as an important nutrient sensor contributing to regulation of cellular, tissue, and systemic metabolism. We aimed to identify specific amino acids which could modulate AMPK and determine effects on cellular and systemic metabolism.

Methods

We performed an unbiased amino acid screen to identify activators of AMPK. Detailed analysis of cellular signaling and metabolism was performed in cultured hepatoma cells, and in vivo glucose metabolism and metabolomic patterns were assessed in both chow-fed mice and mice made obese by high-fat diet feeding.

Results

Alanine acutely activates AMP kinase in both cultured hepatic cells and in liver from mice treated in vivo with Ala. Oral alanine administration improves systemic glucose tolerance in both chow and high fat diet fed mice, with reduced efficacy of Ala in mice with reduced AMPK activity. Our data indicate that Ala activation of AMPK is mediated by intracellular Ala metabolism, which reduces TCA cycle metabolites, increases AMP/ATP ratio, and activates NH₃ generation.

Conclusions

Ala may serve as a distinct amino acid energy sensor, providing a positive signal to activate the beneficial AMPK signaling pathway.

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Unbiased amino acid screen identified alanine as a unique activator of AMP kinase.

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Alanine acutely activates AMP kinase in both cultured cells and in vivo.

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Alanine and NH₃ metabolism contribute to regulation of AMP kinase activation.

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Effects of Ala are reduced in absence of AMP kinase.

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Oral alanine improves glucose tolerance in vivo in both chow and HFD-fed mice.

Intersections between mitochondrial sirtuin signaling and tumor cell metabolism

Cancer cells use glucose and glutamine to facilitate cell growth and proliferation, a process coined "metabolic reprograming" - an emerging hallmark of cancer. Inside the cell, these nutrients synergize to produce metabolic building blocks, such as nucleic acids, lipids and proteins, as well as energy (ATP), glutathione and reducing equivalents (NADPH), required for survival, growth and proliferation. Intense research aimed at understanding the underlying cause of the metabolic rewiring has revealed that established oncogenes and tumor suppressors involved in signaling alter cellular metabolism to contribute to the transition from a normal quiescent cell to a rapidly proliferating cancer cell. Likewise, bona fide metabolic sensors are emerging as regulators of tumorigenesis. This review will focus on one such family of sensors, sirtuins, which utilize NAD(+) as a cofactor to catalyze deacetylation, deacylation and ADP-ribosylation of their protein substrates. In this review, we will enumerate how cancer cell metabolism is different from a normal quiescent cell and highlight the emerging role of mitochondrial sirtuin signaling in the regulation of tumor metabolism.