Manganese superoxide dismutase (MnSOD) in the mitochondria plays an important role in cellular defense against oxidative damage. Homozygous MnSOD knockout (Sod2-/-) mice are neonatal lethal, indicating the essential role of MnSOD in early development. To investigate the potential cellular abnormalities underlying the aborted development of Sod2-/- mice, we examined the growth of isolated mouse embryonic fibroblasts (MEFs) from Sod2-/- mice. We found that the proliferation of Sod2-/- MEFs was significantly decreased compared with wild-type MEFs despite the absence of morphological differences. The Sod2-/- MEFs produced less cellular ATP, had lower O2 consumption, generated more superoxide, and expressed less Prdx3 protein. Furthermore, the loss of MnSOD dramatically altered several markers involved in cell proliferation and growth, including decreased growth stimulatory function of mTOR signaling and enhanced growth inhibitory function of GSK-3β signaling. Interestingly, the G-protein-coupled receptor-mediated intracellular Ca2+ signal transduction was also severely suppressed in Sod2-/- MEFs. Finally, the ratio of microtubule-associated protein light chain 3 (LC3)-II/LC3-I, an index of autophagic activity, was increased in Sod2-/- MEFs, consistent with a reduction in mTOR signal transduction. These data demonstrate that MnSOD deficiency results in alterations in several key signaling pathways, which may contribute to the lethal phenotype of Sod2-/- mice. © 2010.