Cytosolic NADK is conditionally essential for folate-dependent nucleotide synthesis

Area of Science:

• Biochemistry
• Cell Biology
• Metabolic Pathways

Background:

• Nicotinamide adenine dinucleotide kinase (NADK) phosphorylates NAD+ to NADP+, producing NADPH, a key cofactor for reductive metabolism.
• Cancer cells utilize both cytosolic (NADK) and mitochondrial (NADK2) isoforms, which are generally dispensable for growth in standard media.
• CRISPR screens revealed a conditional growth defect upon NADK deletion in a human plasma-like medium.

Purpose of the Study:

• To investigate the conditional essentiality of NADK in cancer cells.
• To elucidate the specific metabolic pathway and nutrient dependency linked to NADK function.
• To understand the role of NADK in folate metabolism and NADPH production.

Main Methods:

• CRISPR screening across multiple cancer cell lines.
• Cell growth assays in conventional and human plasma-like media.
• Metabolic analyses focusing on folate metabolism and cofactor availability.

Main Results:

• NADK deletion specifically impaired cancer cell growth under low folic acid conditions.
• NADK is required for cytosolic NADPH production, essential for dihydrofolate reductase (DHFR) activity.
• Sufficient DHFR activity, driven by NADPH, is critical for folate-dependent nucleotide synthesis under nutrient stress.
• Folate availability dictates whether DHFR relies on NADPH or alternative donors like NADH.

Conclusions:

• Conditional essentiality of NADK is linked to folic acid availability.
• NADK plays a critical role in maintaining folate metabolism via NADPH production, particularly under nutrient-limited conditions.
• Targeting NADK could be a strategy for cancers with specific folate dependencies.