Metabolic and Transcriptomic Reprogramming during Transition to Quiescence from a Proliferative State

Metabolic rewiring during the proliferation-to-quiescence transition is poorly understood. Here, we conducted 13C-metabolic flux analysis (13C-MFA) in proliferating (P) and quiescent (Q) mouse embryonic fibroblasts (MEFs) to investigate this process. Q cells exhibit reduced glycolysis but increased TCA cycle flux and respiration. Reduced glycolysis in Q cells correlates with reduced glycolytic enzyme expression mediated by yes-associated protein (YAP) inhibition. The increased TCA cycle activity and respiration in Q cells is mediated by induced mitochondrial pyruvate carrier (MPC) expression, and by reduced NAD+ demand. The malate-to-pyruvate flux, generating NADPH, is markedly reduced by modulating malic enzyme 1 (ME1) dimerization in Q cells. Conversely, the malate dehydrogenase 1 (MDH1)-mediated oxaloacetate-to-malate flux is reversed and elevated in Q cells, driven by high mitochondrial-derived malate, reduced cytosolic oxaloacetate, elevated MDH1 expression, and a high cytosolic NAD+/NADH ratio. Transcriptomics revealed Q cells inducing genes, especially extracellular matrix (ECM)-related ones including collagen biosynthesis. Additionally, 13C-MFA noted increased proline synthesis, key component for collagen. We suggest Q cells require active TCA cycle flux and respiration to produce essential ATP and amino acids for de-novo ECM protein synthesis.