Contribution of the phosphorylable complex I in the growth phase-dependent respiration of C6 glioma cells in vitro.

The energy metabolism of rat C6 glioma cells was investigated as a function of the growth phases. Three-dimensional cultures of C6 cells exhibited diminished respiration and respiratory capacity during the early growth phase, before reaching confluence. This decrease in respiration was neither due to changes in the respiratory complex content nor in the mitochondrial mass per se. Nevertheless, a quantitative correlation was found between cellular respiration and the rotenone-sensitive NADH ubiquinone oxidoreductase (i.e. complex I) activity. Immunoblot analysis showed that phosphorylation of the 18 kDa-subunit of this complex was associated with the growth-phase dependent modulation of complex I and respiratory activity in C6 cells. In addition, by using forskolin or dibutyryl cAMP, short-term activation of protein kinases A of C6 cells correlated with increased phosphorylation of the 18-kDa subunit of complex I, activated NADH ubiquinone oxidoreductase activity and stimulated cellular respiration. These findings suggest that complex I of C6 glioma cells is a key regulating step that modulates the oxidative phosphorylation capacity during growth phase transitions.