PTEN/Akt Signaling Controls Mitochondrial Respiratory Capacity through 4E-BP1. PLoS One 2012

Akt, a serine/threonine kinase has been shown to stimulate glycolysis in cancer cells but its role in mitochondrial respiration is unknown. Using PTEN-knockout mouse embryonic fibroblasts (MEFPTEN2/2) with hyper-activated Akt as a cell model, we observed a higher respiratory capacity in MEFPTEN2/2 compared to the wildtype (MEFWT). The respiratory phenotype observed in MEFPTEN2/2 was reproduced in MEFWT by gene silencing of PTEN which substantiated its role in regulating mitochondrial function. The increased activities of the respiratory complexes (RCs) I, III and IV were retained in the same relative proportions as those present in MEFWT, alluding to a possible co-ordinated regulation by PTEN/Akt. Using LY294002 (a PI3K inhibitor) and Akt inhibitor IV, we showed that the regulation of enzyme activities and protein expressions of the RCs was dependent on PI3K/Akt. There was insignificant difference in the protein expressions of mitochondrial transcription factor: peroxisome proliferator-activated receptor gamma coactivator 1-alpha (PGC-1a) and its downstream targets, the nuclear respiratory factor 1 (NRF-1) and mitochondrial transcription factor A (mtTFA) between MEFPTEN2/2 and MEFWT. Similarly, mRNA levels of the same subunits of the RCs detected in Western blots were not significantly different between MEFPTEN2/2 and MEFWT suggesting that the regulation by Akt on mitochondrial function was probably not via gene transcription. On the other hand, a decrease of total 4E-BP1 with a higher expression of its phosphorylated form relative to total 4E-BP1 was found in MEFPTEN2/2, which inferred that the regulation of mitochondrial respiratory activities by Akt wa