Respiration-Deficient Cells Require Pyruvate Carboxylase to Suppress Asparagine Auxotrophy
Mutations in mitochondrial DNA (mtDNA) compromise ETC activity and impair oxidative phosphorylation. Since eukaryotic cells contain multiple copies of mtDNA, the resulting phenotype depends on the proportion of mutant mitochondrial genomes (the heteroplasmy level). Using isogenic cell lines carrying similar mtDNA deletions, a linear decline in cellular respiration was observed as mitochondrial DNA heteroplasmy increased. Despite this, cellular redox imbalance did not change until heteroplasmy exceeded 50%. As heteroplasmy increased past 70%, cells also exhibited an integrated stress response (ISR) and impaired translation was observed. These defects were reversed by either addition of asparagine or overexpression of pyruvate carboxylase (PC). The dependence on exogenous asparagine in other respiration-deficient cells was found to correlate inversely with the PC expression level. For example, patient-derived thyroid tumor cells, harboring high heteroplasmy for a Complex I mtDNA mutation and low levels of PC, exhibited asparagine auxotrophy, and L-asparaginase treatment suppressed tumor growth. Together, these findings demonstrate a role for mitochondrial pyruvate carboxylase in cellular asparagine synthesis under conditions of compromised respiratory activity.