Mitochondrial Dysfunction Contributes to Oncogene-Induced Senescence

The expression of oncogenic ras in normal human cells quickly induces an aberrant proliferation response that later is curtailed by a cell cycle arrest known as cellular senescence. Here, we show that cells expressing oncogenic ras display an increase in the mitochondrial mass, the mitochondrial DNA, and the mitochondrial production of reactive oxygen species (ROS) prior to the senescent cell cycle arrest. By the time the cells entered senescence, dysfunctional mitochondria accumulated around the nucleus. The mitochondrial dysfunction was accompanied by oxidative DNA damage, a drop in ATP levels, and the activation of AMPK. The increase in mitochondrial mass and ROS in response to oncogenic ras depended on intact p53 and Rb tumor suppression pathways. In addition, direct interference with mitochondrial functions by inhibiting the expression of the Rieske iron sulfur protein of complex III or the use of pharmacological inhibitors of the electron transport chain and oxidative phosphorylation was sufficient to trigger senescence. Taking these results together, this work suggests that mitochondrial dysfunction is an effector pathway of oncogene-induced senescence.Mitochondria are central to cell metabolism and energy production. High-energy electrons coming from the oxidation of different carbon sources such as glucose and fatty acids enter the mitochondrial electron transport chain as reduced equivalents, and their energy gradually is converted into a proton gradient. Mitochondria use this gradient to synthesize ATP that later is used for biosynthetic reactions (9, 30). Mitochondria also control decisions for life and death. Changes in mitochondrial membrane permeability lead to the release of proapoptotic mediators that can kill cells with DNA damage or activated oncogenes (16). In this way, mitochondria control one of the major tumor suppressor responses: apoptosis (27). Some oncogenes, such as RasV12, STAT5, and Bcl2, have antiapoptotic activity, and some cell types have a high apoptosis threshold. Another tumor suppressor response, called cellular senescence, serves as a fail-safe mechanism against the transforming activity of antiapoptotic oncogenes (29, 40, 43). However, currently it is unknown whether mitochondria also can play a role in oncogene-induced senescence (OIS).OIS is phenotypically similar to the senescence response triggered by short telomeres, also known as replicative senescence (6). Replicative senescence is, in essence, the consequence of a DNA damage response triggered by short telomeres (11). OIS also involves the DNA damage response (2, 15, 28), but the mechanism of DNA damage and the contribution of mitochondria to it are unclear. It has been demonstrated that mitochondria play a critical role in replicative senescence, and several mitochondrial changes, including an increase in the production of reactive oxygen species (ROS), were reported in cells with short telomeres (34, 35). Mitochondrion-derived ROS contribute to the senescent phenotype by damaging the DNA (35) and therefore amplifying the DNA damage signals originally caused by short telomeres. We reasoned that a similar amplifying mechanism involving the mitochondria could operate in cells expressing oncogenes.Here, we use Ha-RasV12, an oncogenic allele of Ha-Ras, to study the role of mitochondria in OIS. RasV12 is a very important human oncogene and was the first linked to the senescence program (43). We report that oncogenic ras induces an increase in mitochondrial mass, mitochondrial DNA, and mitochondrial superoxide production before any sign of senescent cell cycle arrest. With time, these mitochondrial changes evolved into a severe mitochondrial dysfunction characterized by a further increase in ROS production, the accumulation of depolarized mitochondria around the cell nucleus, a decrease in ATP, and the activation of AMPK. The mechanism of the increase in mitochondrial mass and ROS in response to oncogenic ras was found to be dependent on either p53 or Rb. In addition, direct interference with mitochondrial functions by downregulating the mitochondrial Rieske iron sulfur protein (RISP) or by using pharmacological inhibitors of oxidative phosphorylation induced senescence. We suggest that the senescence effector mechanism acting downstream of p53 and Rb involves mitochondrial dysfunction.