Mitochondria and Cancer: Is There a Morphological Connection?
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Oncogene (2006) 25, 4706–4716 & 2006 Nature Publishing Group All rights reserved 0950-9232/06 $30.00 www.nature.com/onc REVIEW Mitochondria and cancer: is there a morphological connection? E Alirol1 and JC Martinou Department of Cell Biology, University of Geneva, Gene`ve, Switzerland Mitochondria are key players in several cellular functions mitochondrial remodelling (Olichon et al., 2003). including growth, division, energy metabolism, and Mitofilin is another protein that has recently been apoptosis.The mitochondrial network undergoes constant involved in cristae morphology (John et al., 2005). remodelling and these morphological changes are of direct Although models for mitochondrial fusion are still relevance for the role of this organelle in cell physiology. incomplete, the mechanism proceeds through three Mitochondrial dysfunction contributes to a number of steps: docking, fusion of the outer mitochondrial human disorders and may aid cancer progression.Here, membrane (OMM) and fusion of the IMM. All three we summarize the recent contributions made in the field of steps involve GTP hydrolysis, and the IMM fusion mitochondrial dynamics and discuss their impact on our requires an intact membrane potential (Mattenberger understanding of cell function and tumorigenesis. et al., 2003). Other players have been identified in yeast Oncogene (2006) 25, 4706–4716. doi:10.1038/sj.onc.1209600 but no mammalian homologs to them have been described so far (Tieu and Nunnari, 2000). Keywords: mitochondrial morphology; apoptosis; The mitochondrial fission pathway involves at least energy metabolism; cancer two proteins: dynamin related protein 1 (Drp1) and Fis1. Drp1 is a cytosolic GTPase, which has been proposed to couple GTP hydrolysis to membrane constriction and fission (Smirnova et al., 2001). Fis1 is an outer membrane protein evenly distributed on the Introduction surface of mitochondria (James et al., 2003). It is thought to recruit Drp1 to punctuate structures on Mitochondria play essential and diverse roles in the mitochondria and is thus considered as the limiting physiology of eukaryotic cells. Not only do they provide factor in the fission process (Stojanovski et al., 2004). energy, but they also participate in numerous metabolic The mechanism of constriction is not understood reactions and play central roles in apoptosis (Desagher but the current model postulates that Drp1 homo- and Martinou, 2000). Impairments of mitochondrial oligomerizes and forms a ring around the mitochon- functions have been implicated in a wide variety of drial tubule (Okamoto and Shaw, 2005). Drp1 complexes human pathologies, among which cancer and age- might generate mechanical force via conformational related diseases (Wallace, 2005). changes, leading to membrane constriction, similar to In the past years, a number of groups showed that dynamin (Danino et al., 2004). Modification of lipid mitochondria are dynamic structures that undergo composition is likely to be involved, and additional fusion and fission events continuously throughout the proteins may participate in mitochondrial fission. For life of a cell (Okamoto and Shaw, 2005). Mounting example, endophilin B1, a putative fatty acyl transfer- evidence indicate that mitochondrial dynamics have ase, has been shown to act downstream of Drp1 in the roles beyond maintenance of morphology, and impact control of mitochondrial morphology (Karbowski et al., on both cell death and cell metabolism (Chen and Chan, 2004) and MTP18 is a novel mitochondrial protein that 2005). In mammals, mitochondrial fusion is driven by is able to trigger mitochondrial fragmentation in a two GTPases of the outer membrane named Mitofusins Drp1-dependent fashion (Tondera et al., 2004). 1 and 2 (Mfn1, Mfn2) (Santel and Fuller, 2001; Chen GDPAP1, which is expressed in Schwann cells and et al., 2003; Eura et al., 2003). The Mitofusins appear to neurons, has also been reported to mediate mitochon- play distinct roles in fusion and the Mfn1 presumably drial fission (Niemann et al., 2005). works together with Opa1, a GTPase of the inner Membrane fusion and fission engage rupture and mitochondrial membrane (IMM) to promote fusion deformation of lipid bilayers, and involve destabilized (Cipolat et al., 2004). While its exact function remains monolayer intermediates (Chernomordik and Kozlov, unclear, Opa1 seems also involved in further intra- 2003). The two membranes are first brought in close contact, then some hydrophobic monolayer discontinu- Correspondence: Dr J-C Martinou, Department of Cell Biology, ity appears that enables the rupture of the bilayer and University of Geneva, Quai Ernest-Ansermet 30, 1211 Geneva 4, the formation of a fusion stalk intermediate. Successive Switzerland. E-mail: [email protected] steps include the opening of a pore and its expansion, 1Current address: Venetian Institute of Molecular Medecine, Via Orus connecting the two aqueous volumes (Kozlovsky et al., 2, 35 129Padova, Italy Mitochondria and cancer E Alirol and JC Martinou 4707 2002). These processes involve specific proteins that and a hallmark of most, if not all, types of cancer establish local intermembrane contacts and generate (Hanahan and Weinberg, 2000; Green and Evan, 2002). local membrane stresses. Absence or excess of one of The inactivation of the p53 protein is the most common these proteins could in theory result in failure of specific strategy developed by tumour cells to evade apoptosis. steps and accumulation of intermediates. p53 is central in the DNA damage response and triggers As summarized in Table 1, disruption of the fission/ cell death by both transcription-dependent and fusion machinery strongly alters organelle function -independent mechanisms (Slee et al., 2004; Yee and affecting both programmed cell death and energy Vousden, 2005). Other examples of apoptosis evasion production. Several lines of evidence suggest that include disruption of Fas death receptor pathway, mitochondria contribute to neoplastic transformation upregulation of the widespread antiapoptotic family of by changing cellular energy capacities, increasing inhibitors of apoptosis proteins (IAPs) or downregula- mitochondrial oxidative stress and modulating apop- tion of caspases (Muschen et al., 2000; Teitz et al., 2000; tosis. A shift in the rate of mitochondrial fission or Yang et al., 2003). The PI3K/Akt/ PTEN pathway was fusion may provide new mechanistic explanation for the also shown to play a role in abrogating apoptosis. Akt/ aetiology of cancer, and may offer additional strategies PKB is activated in a wide variety of cancers and this for therapeutic intervention. activation leads to the phosphorylation of a number of Here, we discuss a possible role played by mitochon- key apoptotic proteins thereby inactivating them (Testa dria in oncogenesis and explore how misregulation of and Bellacosa, 2001). fission and fusion machinery may influence apoptosis, cell metabolism and tumour formation. Mitochondrial membrane permeabilization Many death signals converge to mitochondria which respond by releasing numerous proteins in the cytosol Mitochondrial apoptosis and its subversion in oncogenesis (Patterson et al., 2000), among which several have apoptogenic properties (e.g. cytochrome c, SMAC/ The possibility that apoptosis is implicated in cancer DIABLO, AIF, Endo G and Omi/HtrA2). In many cell formation was first raised in 1972 when Kerr et al. types mitochondrial membrane permeabilization (1972) described massive apoptosis in tumoural cell (MMP) is considered as the point of nonreturn in the populations. In the following years, the antiapoptotic cell suicide (Ferri and Kroemer, 2001). However, in protein Bcl2 was shown to be a tumour-specific marker sympathetic neurons for example, this event appears to which expression was enhanced in many lymphoma be reversible and death can be circumvented down- (Tsujimoto et al., 1984; Bakhshi et al., 1985; Crescenzi stream of cytochromic c release (Martinou et al., 1999; et al., 1988; Vaux et al., 1988; Nunez et al., 1990), Deshmukh et al., 2000; Tolkovsky et al., 2002). Whether opening the investigation of apoptosis in cancer at the it is also the case in cancer cells remains to be molecular level. Suppression of apoptosis is among the determined. Upon release, cytochrome c triggers the minimal requirements for a cell to become cancerous, apoptosome formation and initiates the caspase cascade Table 1 Proteins involved in mitochondrial fusion and fission and their effect on apoptosis and energy metabolism Protein Function Localization Apoptosis Energy metabolism References DRP-1 Fission cytosol Overexpression sensitizes cells Breckenridge et al. (2003), and OMM to apoptosis; silencing and Frank et al. (2001), dominant negative mutant Lee et al. (2004), reduce MMP induced by several Szabadkai et al. (2004) apoptotic stimuli Fis1 Fission OMM Overexpression triggers MMP Overexpression James et al. (2003), and silencing protects from affects Dcm Lee et al. (2004), several apoptotic stimuli Niemann et al. (2005); Yu et al. (2005) Endophilin B1 Fission Cytosol Silencing and dominant negative mutant Karbowski et al. (2004) and OMM cause separation of OMM and IMM MTP18 Fission Mitochondria Silencing sensitizes cells to UV or Tondera et al. (2004) TNFa treatment. Overexpression triggers MMP GDPAP1 Fission OMM Niemann et al. (2005) Mfns Fusion OMM Overexpression inhibits MMP Silencing affects Dcm and Chen et al. (2005), and Bax/Bak activation respiration. Overexpression Pich et al. (2005), of Mfn2 stimulates OXPHOS