Impaired Mevalonate-Dependent NAD⁺ Regeneration Suppresses Serine Biosynthesis and Enhances Sensitivity to PHGDH Inhibition

Abstract The mevalonate pathway generates sterols and isoprenoids essential for membrane biosynthesis and signaling. Increased activity of the mevalonate pathway is a common feature of cancer and has emerged as a potential therapeutic vulnerability. Here, we showed that the mevalonate pathway sustains de novo serine biosynthesis and aspartate production by maintaining NAD⁺ regeneration through ubiquinone-dependent electron transport. Statin-mediated inhibition of the mevalonate pathway impaired oxidative phosphorylation, lowered the NAD⁺/NADH ratio, suppressed serine and aspartate biosynthesis, and activated the GCN2–eIF2α–ATF4 amino acid deprivation response. The resulting depletion of serine-derived glycine and one-carbon units, together with reduced aspartate availability, limited purine and pyrimidine biosynthesis. Genetic and pharmacological disruption of ubiquinone synthesis recapitulated the metabolic defects, whereas expression of the bacterial NADH oxidase LbNOX restored the NAD⁺/NADH ratio and reversed the metabolic and growth defects induced by statin treatment. Importantly, impairment of NAD⁺ regeneration reduced PHGDH-dependent de novo serine synthesis, thereby sensitizing neuroblastoma cells to PHGDH inhibition. Accordingly, simvastatin enhanced the anti-proliferative effects of the PHGDH inhibitor NCT-503 in vitro and exhibited elevated anti-tumor activity in combination with NCT-503 in neuroblastoma xenograft models. Together, these findings establish ubiquinone-dependent NAD⁺ regeneration as a key mechanism linking the mevalonate pathway to amino acid and nucleotide biosynthesis and provide a mechanistic rationale for combined targeting of the mevalonate pathway and serine biosynthesis in cancer.