Mitofusin 2 Triggers Vascular Smooth Muscle Cell Apoptosis via Mitochondrial Death Pathway

Role of mitofusin 2 in cardiovascular oxidative injury

Mitochondria are highly dynamic organelles with constant shape changes regulated by fusion and fission events. In addition to regulating mitochondrial morphology, mitochondrial fusion/fission is involved in fundamental mitochondrial biological processes, including mitochondrial metabolism, energization, respiration, mitochondrial membrane potential, and mtDNA stability. Dysfunction of mitochondrial dynamics has been implicated in various human diseases, especially in neurodegenerative diseases. Emerging evidence indicates that impaired expression of mitochondrial fusion proteins or their malfunction participates in oxidative stress-induced cardiovascular injury. This review will focus on recent advances of mitochondrial fusion in regulating various cellular processes in cardiovascular system.

Anti-tumour efficacy of mitofusin-2 in urinary bladder carcinoma

Mitochondrial GTPase mitofusin-2 gene (Mfn2) is a novel gene characterised as a cell proliferation inhibitor. Mfn2 protein over-expression, mediated by an adenovirus, has a significant anti-tumour effect in A548 and HT-29 cells. However, there is no report on the effect of Mfn2 on urinary bladder carcinoma (UBCC). In this study, we sought to investigate the function of Mfn2 in UBCC. Mfn2 expression in 36 paired UBCC samples was investigated by reverse transcription-polymerase chain reaction and Western blot analyses. An adenovirus encoding the complete Mfn2 open reading frame (Ad-Mfn2) was used to infect UBCC cells, and an adenoviral vector encoding green fluorescent protein (Ad-GFP) was used as a control. The effects of Mfn2 on cell-cycle distribution and apoptosis were assessed by flow cytometry and Western blot analyses. The Mfn2 protein showed significantly lower expression in UBCC tissues than nearby non-tumourous tissues. Ad-Mfn2 exhibited a significant anti-tumour effect in T24 and 5,637 cells. Mfn2 overexpression in T24 cells significantly inhibited cell proliferation, by arresting the transition of the cell cycle from the G(1) to S phase, and induced apoptosis by upregulating active caspase-3 and cleaved PARP levels. Mfn2 also induced increased p21 and p27 expression levels, but down-regulated PCNA levels. These findings indicate that Mfn2 is a potential UBCC tumour suppressor gene, which showed significantly lower expression in tumour tissues than adjacent non-tumourous tissues and could promote apoptosis and inhibit the proliferation of UBCC cells. Mfn2 may become an important therapeutic target for treating UBCC.

Mitochondria as integrators of signal transduction and energy production in cardiac physiology and disease

Fascinating links are beginning to be discovered between mitochondrial function and cardiac physiology and disease in the context of diverse signaling mechanisms, energy production, and intersection with pathways producing reactive oxygen species. Proteins long known to drive mitochondrial fusion and fission are now reported to have emergent functions in intracellular calcium homeostasis, apoptosis, and vascular smooth muscle cell proliferation, all key issues in cardiac disease. Moreover, mitochondrial fusion has been demonstrated to be required for normal myofibril organization in skeletal muscle, and decreasing fission may confer protection against ischemic heart disease. These processes broaden the traditional role in energy production undertaken by mitochondria and provide new directions for potential therapeutic leads.

The changing shape of mitochondrial apoptosis

Mitochondria are key organelles in conversion of energy, regulation of cellular signaling and amplification of programmed cell death. The anatomy of the organelle matches this functional versatility in complexity and is modulated by the concerted action of proteins that impinge on its fusion-fission equilibrium. A growing body of evidence implicates changes in mitochondrial shape in the progression of apoptosis and, therefore, proteins governing such changes are likely candidates for involvement in pathogenetic mechanisms in neurodegeneration and cancer. Here, we discuss the recent advancements in our knowledge about the machinery that regulates mitochondrial shape and on the role of molecular mechanisms controlling mitochondrial morphology during cell death.

MicroRNA-330 acts as tumor suppressor and induces apoptosis of prostate cancer cells through E2F1-mediated suppression of Akt phosphorylation

MicroRNAs (miRNAs) make up a novel class of gene regulators; they function as oncogenes or tumor suppressors by targeting tumor-suppressor genes or oncogenes. A recent study that analysed a large number of human cancer cell lines showed that miR-330 is a potential tumor-suppressor gene. However, the function and molecular mechanism of miR-330 in determining the aggressiveness of human prostate cancer has not been studied. Here, we show that miR-330 is significantly lower expressed in human prostate cancer cell lines than in nontumorigenic prostate epithelial cells. Bioinformatics analyses reveal a conserved target site for miR-330 in the 3'-untranslated region (UTR) of E2F1 at nucleotides 1018-1024. MiR-330 significantly suppressed the activity of a luciferase reporter containing the E2F1-3'-UTR in the cells. This activity could be abolished with the transfection of anti-miR-330 or mutated E2F1-3'-UTR. In addition, the expression level of miR-330 and E2F1 was inversely correlated in cell lines and prostate cancer specimens. After overexpressing of miR-330 in PC-3 cells, cell growth was suppressed by reducing E2F1-mediated Akt phosphorylation and thereby inducing apoptosis. Collectively, this is the first study to show that E2F1 is negatively regulated by miR-330 and also show that miR-330 induces apoptosis in prostate cancer cells through E2F1-mediated suppression of Akt phosphorylation.

Mineralocorticoid receptor antagonism attenuates vascular apoptosis and injury via rescuing protein kinase B activation

Emerging evidence indicates that mineralocorticoid receptor (MR) blockade reduces the risk of cardiovascular events beyond those predicted by its blood pressure-lowering actions; however, the underlying mechanisms remain unclear. To investigate whether protection elicited by MR blockade is through attenuation of vascular apoptosis and injury, independently of blood pressure lowering, we administered a low dose of the MR antagonist spironolactone or vehicle for 21 days to hypertensive transgenic Ren2 rats with elevated plasma aldosterone levels. Although Ren2 rats developed higher systolic blood pressures compared with Sprague-Dawley littermates, low-dose spironolactone treatment did not reduce systolic blood pressure compared with untreated Ren2 rats. Ren2 rats exhibited vascular injury as evidenced by increased apoptosis, hemidesmosome-like structure loss, mitochondrial abnormalities, and lipid accumulation compared with Sprague-Dawley rats, and these abnormalities were attenuated by MR antagonism. Protein kinase B activation is critical to vascular homeostasis via regulation of cell survival and expression of apoptotic genes. Protein kinase B serine(473) phosphorylation was impaired in Ren2 aortas and restored with MR antagonism. In vivo MR antagonist treatment promoted antiapoptotic effects by increasing phosphorylation of BAD serine(136) and expression of Bcl-2 and Bcl-xL, decreasing cytochrome c release and BAD expression, and suppressing caspase-3 activation. Furthermore, MR antagonism substantially reduced the elevated NADPH oxidase activity and lipid peroxidation, expression of angiotensin II, angiotensin type 1 receptor, and MR in Ren2 vasculature. These results demonstrate that MR antagonism protects the vasculature from aldosterone-induced vascular apoptosis and structural injury via rescuing protein kinase B activation, independent of blood pressure effects.

Mitofusin 2 tethers endoplasmic reticulum to mitochondria

Juxtaposition between endoplasmic reticulum (ER) and mitochondria is a common structural feature, providing the physical basis for intercommunication during Ca(2+) signalling; yet, the molecular mechanisms controlling this interaction are unknown. Here we show that mitofusin 2, a mitochondrial dynamin-related protein mutated in the inherited motor neuropathy Charcot-Marie-Tooth type IIa, is enriched at the ER-mitochondria interface. Ablation or silencing of mitofusin 2 in mouse embryonic fibroblasts and HeLa cells disrupts ER morphology and loosens ER-mitochondria interactions, thereby reducing the efficiency of mitochondrial Ca(2+) uptake in response to stimuli that generate inositol-1,4,5-trisphosphate. An in vitro assay as well as genetic and biochemical evidences support a model in which mitofusin 2 on the ER bridges the two organelles by engaging in homotypic and heterotypic complexes with mitofusin 1 or 2 on the surface of mitochondria. Thus, mitofusin 2 tethers ER to mitochondria, a juxtaposition required for efficient mitochondrial Ca(2+) uptake.

Emodin inhibits TNF-alpha-induced human aortic smooth-muscle cell proliferation via caspase- and mitochondrial-dependent apoptosis

Vascular smooth-muscle cell (VSMC) proliferation plays a vital role in hypertension, atherosclerosis and restenosis. It has been reported that emodin, an active component extracted from rhubarb, can stop the growth of cancer cells; however, it is not known if emodin exerts similar anti-atherogenic effects in TNF-alpha treated human aortic smooth-muscle cells (HASMC). In this study, emodin treatment showed potent inhibitory effects in TNF-alpha-induced HASMC proliferation that were associated with induced apoptosis, including the cleavage of poly ADP-ribose polymerase (PARP). Moreover, inhibitors of caspase-3, -8 and -9 (Ac-DEVD-CHO, Z-IETD-FMK and Z-LEHD-FMK) efficiently blocked emodin-induced apoptosis in TNF-alpha treated HASMC. Therefore, emodin-induced cell death occurred via caspase-dependent apoptosis. Emodin treatment resulted in the release of cytochrome c into cytosol and a loss of mitochondrial membrane potential (DeltaPsi(m)), as well as a decrease in the expression of an anti-apoptotic protein (Bcl-2) and an increase in the expression of an a pro-apoptotic protein (Bax). Emodin-mediated apoptosis was also blocked by a mitochondrial membrane depolarization inhibitor, which indicates that emodin-induced apoptosis occurred via a mitochondrial pathway. Taken together, the results of this study showed that emodin inhibits TNF-alpha-induced HASMC proliferation via caspase- and a mitochondrial-dependent apoptotic pathway. In addition, these results indicate that emodin has potential as an anti-atherosclerosis agent.

Mitochondrial dynamics and apoptosis

In healthy cells, mitochondria continually divide and fuse to form a dynamic interconnecting network. The molecular machinery that mediates this organelle fission and fusion is necessary to maintain mitochondrial integrity, perhaps by facilitating DNA or protein quality control. This network disintegrates during apoptosis at the time of cytochrome c release and prior to caspase activation, yielding more numerous and smaller mitochondria. Recent work shows that proteins involved in mitochondrial fission and fusion also actively participate in apoptosis induction. This review will cover the recent advances and presents competing models on how the mitochondrial fission and fusion machinery may intersect apoptosis pathways.