Although mitochondrial impairment has often been implicated in carcinogenesis, the mechanisms of its development in cancer remain unknown. We report here that autophagy triggered by oncogenic K-Ras mediates functional loss of mitochondria during cell transformation to overcome an energy deficit resulting from glucose deficiency. When Rat2 cells were infected with a retrovirus harboring constitutively active K-RasV12, mitochondrial respiration significantly declined in parallel with the acquisition of transformation characteristics. Decreased respiration was not related to mitochondrial biogenesis but was inversely associated with the increased formation of acidic vesicles enclosing mitochondria, during which autophagy-related proteins such as Beclin 1, Atg5, LC3-II and vacuolar ATPases were induced. Interestingly, blocking autophagy with conventional inhibitors (bafilomycin A, 3-methyladenin) and siRNA-mediated knockdown of autophagy-related genes recovered respiratory protein expression and respiratory activity; JNK was involved in these phenomena as an upstream regulator. The cells transformed by K-RasV12 maintained cellular ATP level mainly through glycolytic ATP production without induction of GLUT1, the low K m glucose transporter. Finally, K-RasV12-triggered LC3-II formation was modulated by extracellular glucose levels, and LC3-II formation increased only in hepatocellular carcinoma tissues exhibiting low glucose uptake and increased K-Ras expression. Taken together, our observations suggest that mitochondrial functional loss may be mediated by oncogenic K-Ras-induced mitophagy during early tumorigenesis even in the absence of hypoxia, and that this mitophagic process may be an important strategy to overcome the cellular energy deficit triggered by insufficient glucose.
Bibliographical noteFunding Information:
media for 15 min or 1 h at 37°C. After cells were washed with This work was supported by the Korean Science and Engineering ice-cold PBS, the radioactivity of cell lysates was measured by Foundation (KOSEF) funded by a grant from the Korean liquid scintillation counter (Tri-Carb2100TR, Perkin Elmer Life government (MEST) (R13-2003-019-01007-0), by the Mid-Science, Inc.). career Research Program through an NRF grant funded
by the MEST (2009-0079076), and by the Korea Research Foundation Grant funded by the Korean Government (KRF-2008-314-C00286).
- Cell transformation
- Energy deficit
- Mitochondrial loss
ASJC Scopus subject areas
- Molecular Biology
- Cell Biology