Mitochondrial lactate metabolism is involved in antioxidative defense in human astrocytoma cells

Although lactate has traditionally been known to be an end product of anaerobic metabolism, recent studies have revealed its disparate biological functions. Oxidative energy production and cell signaling are two important roles assigned to this monocarboxylic acid. Here we demonstrate that mitochondrial lactate metabolism to pyruvate mediated by lactate dehydrogenase (LDH) in a human astrocytic cell line is involved in antioxidative defense. The pooling of this α-ketoacid helps to detoxify reactive oxygen species, with the concomitant formation of acetate. In-gel activity assays following blue native PAGE electrophoresis were utilized to demonstrate the increase in mitochondrial LDH activity coupled to the decrease in pyruvate dehydrogenase activity in the cells challenged by oxidative stress. The enhanced production of pyruvate with the concomitant formation of acetate in astrocytoma cells was monitored by high-performance liquid chromatography. The ability of pyruvate to fend off oxidative stress was visualized by fluorescence microscopy with the aid of the dye 2',7'-dichlorodihydrofluorescein diacetate. Immunoblotting helped confirm the presence of elevated levels of LDH in cells exposed to oxidative stress, and recovery experiments were performed with pyruvate to diminish the oxidative burden on the astrocytoma. The acetate, generated as a consequence of the antioxidative attribute of pyruvate, was subsequently channeled toward the production of lipids, a process facilitated by the upregulation in activity of acetyl-CoA synthetase and acetyl-CoA carboxylase, as demonstrated by in-gel activity assays. The mitochondrial lactate metabolism mediated by LDH appears to play an important role in antioxidative defence in this astrocytic system.