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Nephrotoxicity is the major limiting factor for the use of cisplatin in cancer therapy. Recent studies have demonstrated an important role for p53 in cisplatin-induced renal injury. Nevertheless, pharmacological and genetic blockade of p53 only provides partial renoprotective effects, suggesting the presence of p53-independent injury mechanisms. To understand the p53-independent mechanisms, we have now examined cisplatin-induced apoptosis in p53-deficient kidney cells. We show that cisplatin could induce Bax activation, cytochrome c release, and apoptosis in primary cultures of p53-deficient renal tubular cells, albeit at a level that was lower than in the wild-type cells. Cisplatin could also induce typical apoptosis in p53-deficient baby mouse kidney (BMK) cells. The apoptosis was caspase dependent and could be completely blocked by general caspase inhibitors. Bax and Bak, two key molecules in the mitochondrial pathway of apoptosis, were interdependently activated by cisplatin, with Bax translocation to and Bax/Bak oligomerization in mitochondria, leading to cytochrome c release. Importantly, cytochrome c release and apoptosis were diminished in Bax/Bak single or double-knockout BMK cells. Furthermore, overexpression of Bcl-2 could ameliorate cisplatin-induced cytochrome c release and apoptosis. Together, the results have demonstrated a p53-independent mechanism of cisplatin nephrotoxicity that involves the mitochondrial pathway of apoptosis. Bax; Bak; cytochrome c

The early stage of diabetic nephropathy (DN) is linked to proteinuria. Transforming growth factor (TGF)-[beta]1 increases glomerular permeability to albumin ([P.sub.alb]), whereas 20-HETE and EETs reduce [P.sub.alb]. To investigate the impact of hyperglycemia and hyperlipidemia on 20-HETE, EETs, and TGF-[beta]1 in the glomeruli, rats were divided into four groups: ND rats were fed a normal diet, HF rats were fed a high-fat diet, STZ rats were treated with 35 mg/kg of streptozotocin, and HF/STZ rats were fed a HF diet and treated with STZ. After 10 wk on these regimens, blood glucose, urinary albumin, serum cholesterol, serum triglyceride levels, and the kidney-to-body weight ratio were significantly elevated in STZ and HF/STZ rats compared with HF and ND rats. STZ and HF/STZ rats had histopathologic changes and abnormal renal hemodynamics. Expression of glomerular CYP4A, enzymes for 20-HETE production, was significantly decreased in STZ rats, whereas expression of glomerular CYP2C and CYP2J, enzymes for EETs production, was significantly decreased in both STZ and HF/STZ rats. Moreover, glomerular TGF-[beta]1 levels were significantly greater in STZ and HF/STZ rats than in HF and ND rats. Five-week treatment of STZ rats with clofibrate induced glomerular CYP4A expression and 20-HETE production, but reduced glomerular TGF-[beta]1 and urinary protein excretion. These results demonstrate that hyperglycemia increases TGF-[beta]1 but decreases 20-HETE and EETs production in the glomeruli, changes that may be important in causing glomerular damage in the early stage of DN. CYP-derived eicosanoids; streptozotocin; kidney

Cisplatin induces renal cell injury and death, resulting in nephrotoxicity that limits its use in cancer therapy. Using cell culture models, recent work has suggested the involvement of p53 in renal cell apoptosis during cisplatin treatment. However, the signals upstream of p53 remain elusive. ATM and ATR are critical regulators of p53 under various conditions of DNA damage. Here, we show that ATM, and not ATR, was proteolytically cleaved into specific fragments of ~210 and 150 kDa during cisplatin-induced tubular cell apoptosis. ATM cleavage was paralleled by the development of apoptosis. VAD, a broad-spectrum inhibitor of caspases, attenuated the cleavage of ATM, whereas the inhibitors of specific caspases were less effective. In caspase-3-deficient MCF-7 cells, ATM was cleaved, releasing the 210- but not the 150-kDa fragment. Recombinant caspase-3 was much more effective than caspase-7 in cleaving ATM that was immunoprecipitated from cell lysates. During cisplatin incubation, VAD protected ATM and enhanced p53 phosphorylation. In vitro assay of protein kinase activity further showed that ATM immunoprecipitated from cisplatin-treated cells had significantly lower kinase activity toward p53 than that from control ceils. Importantly, the protein kinase activity was restored in ATM that was protected by VAD during cisplatin incubation. ATM deficiency sensitized the cells to cisplatin-induced apoptosis, suggesting a cytoprotective role of ATM in this experimental model. Thus proteolysis of ATM by caspases may inactivate this regulatory molecule to facilitate the progression of apoptosis. ATR; proteolysis; DNA damage; kidney; nephrotoxicity

We previously reported that inhibition of nitric oxide (NO) synthesis by N-nitro-L-arginine methyl ester (L-NAME) during late pregnancy leads to increased production of renal vascular 20-hydroxyeicosatetraenoic acid (20-HETE), a cytochrome P-450 (CYP) 4A-derived vasoconstrictor, in pregnant rats. However, the effect of upregulation of vascular 20-HETE production on renal function after NO inhibition is not known. To test the hypothesis that increased gestational vascular 20-HETE synthesis after NO inhibition is involved in mediating blood pressure and renal functional changes, we first determined the I[C.sub.50] value of the effect of nitroprusside (SNP), a NO donor, on renal 20-HETE production in cortical microsomes. We then divided pregnant rats and age-matched virgin rats into a vehicle control group, an L-NAME treatment group (0.25 mg/ml in drinking water), and a group treated with L-NAME plus N-methylsulfonyl- 12,12-dibromododec-11-enamide (DDMS; CYP4A-selective inhibitor, l0 mg x [kg.sup.-1] x [day.sup.-1] iv). After 4 days of treatment, we measured blood pressure, renal blood flow (RBF), renal vascular resistance (RVR), and glomerular filtration rate (GFR) in each group. The addition of SNP (I[C.sub.50] = 22 [micro]M) decreased renal cortical 20-HETE production. In pregnant rats, L-NAME treatment led to significantly higher mean arterial pressure (MAP) and RVR, and lower RBF and GFR. Combined treatment with DDMS and L-NAME significantly attenuated the increases in MAP and RVR and the decrease in GFR, but not the reduction in RBF induced by L-NAME treatment. L-NAME and L-NAME plus DDMS had no significant impact on renal hemodynamics in virgin rats. In addition, chronic treatment with DDMS selectively inhibited cortical 20-HETE production without a significant effect on CYP4A expression in L-NAME-treated pregnant rats. In conclusion, NO effectively inhibits renal cortical microsomal 20-HETE production in female rats. In pregnant rats, the augmentation of renal 20-HETE production after NO inhibition is associated with increased MAP and RVR, whereas decreased GFR is negated by treatment of a selective and competitive CYP4A inhibitor. These results demonstrate that the interaction between renal 20-HETE and NO is important in the regulation of renal function and blood pressure in pregnant rats. pregnancy; cytochrome P-450; arachidonic acid; eicosanoid; kidney; nitric oxide; hypertension

Tubular cell apoptosis has been implicated in the development of ischemic renal failure. In in vitro models, ATP depletion-induced apoptosis of tubular cells is mediated by the intrinsic pathway involving Bax translocation, cytochrome c release, and caspase activation. While the apoptotic cascade has been delineated, much less is known about its regulation. The current study has examined the regulation of ATP depletion-induced tubular cell apoptosis by acidic pH, a common feature of tissue ischemia. Cultured renal tubular cells were subjected to 3 h of ATP depletion with azide and then recovered in full culture medium. The treatment led to apoptosis in ~40% of cells. Apoptosis was significantly reduced, if the pH of ATP depletion buffer was lowered from 7-7.4 to 6-6.5. This was accompanied by the inhibition of caspase activation. However, acidic pH did not prevent Bax translocation and oligomerization in mitochondria. Cytochrome c release from mitochondria was not blocked either, suggesting that acidic pH inhibited apoptosis at the postmitochondrial level. To determine the postmitochondrial events that were blocked by acidic pH, we conducted in vitro reconstitution experiments. Exogenous cytochrome c, when added into isolated cell cytosol, induced caspase activation. Such activation was abrogated, when pH during the reconstitution was lowered to 6 or 6.5. Nevertheless, acidic pH did not prevent the recruitment and association of caspase-9 by Apaf-1, as shown by coimmunoprecipitation. Together, this study demonstrated the inhibition of tubular cell apoptosis following ATP depletion by acidic pH. A critical step blocked by acidic pH seems to be caspase-9 activation in apoptosome. ATP depletion; ischemia

Epoxyeicosatrienoic acids (EETs), which belong to cytochrome P-450 (CYP)-derived eicosanoids, have been implicated to vasodilate renal arterioles, inhibit sodium transport in the nephron, and regulate blood pressure in several animal models. Because pregnancy is associated with changes of blood pressure, the aim of this study was to examine whether renal EET synthesis is altered and whether EETs are involved in blood pressure regulation during pregnancy in rats. Renal microsomal epoxygenase activity increased by 47, 97, and 63% on days 6, 12, and 19 of gestation, respectively. The elevation of epoxygenase activity during pregnancy was associated with an increase in CYP2C11, CYP2C23, and CYP2J2 protein expression on days 6, 12, and 19 of gestation. Moreover, immunohistochemical analysis showed that renal tubular CYP2C11, CYP2C23, and CYP2J2 expression was significantly increased in pregnant rats on days 6, 12, and 19 of gestation. Administration of 6-(2-propargyloxyphenyl)hexanoic acid (PPOH), a selective epoxygenase inhibitor, caused a dose-dependent inhibition of microsomal expoxygenase activity without a significant effect on [omega]-hydroxylase activity in female rats. Interestingly, administration of PPOH (20 mg x [kg.sup.-1] x [day.sup.-1] for 4 days starting on day 15 of pregnancy) increased blood pressure by 21 mmHg and caused a significant decrease in the body weight of fetal pups (1.3 [+ or -] 0.08 g in control vs. 1.1 [+ or -] 0.06 g in PPOH). Moreover, PPOH treatment significantly decreased renal microsomal epoxygenase activity and the expression of CYP2C11, CYP2C23, and CYP2J in pregnant rats. This study demonstrates that EET synthesis in the kidney is elevated during pregnancy, and CYP2C11, 2C23, and CYP2J2 are responsible for the change of renal EET synthesis. The inhibition results demonstrate that the downregulation of renal epoxygenase activity by PPOH causes hypertension in pregnant rats. This study suggests that EETs may contribute to the control of blood pressure during pregnancy. blood pressure; cytochrome P-450; epoxyeicosatrienoic acids

Tubular damage by cisplatin leads to acute renal failure, which limits its use in cancer therapy. In tubular cells, a primary target for cisplatin is presumably the genomic DNA. However, the pathway relaying the signals of DNA damage to tubular cell death is unclear. In response to DNA damage, the tumor suppressor gene p53 is induced and is implicated in subsequent DNA repair and cell death by apoptosis. The current study was designed to examine the role of p53 in cisplatin-induced apoptosis in cultured rat kidney proximal tubular cells. Cisplatin at 20 [micro]M induced apoptosis in ~70% of cells, which was partially suppressed by carbobenzoxy-Val-Ala-Asp-fluoromethyl ketone (VAD), a general caspase inhibitor. Of interest, cisplatin-induced apoptosis was also suppressed by pifithrin-[alpha], a pharmacological inhibitor of p53. Cisplatin-induced caspase activation was completely inhibited by VAD, but only partially by pifithrin-[alpha]. Early during cisplatin treatment, p53 was phosphorylated and upregulated. The p53 activation was blocked by pifithrin-[alpha], but not by VAD. Bcl-2 expression abolished cisplatin-induced apoptosis without blocking p53 phosphorylation or induction. The results suggest that p53 activation might be an early signal for apoptosis during cisplatin treatment. To further determine the role of p53, tubular cells were stably transfected with a dominant-negative mutant of p53 with diminished transcriptional activity. Expression of the mutant attenuated cisplatin-induced apoptosis and caspase activation. In conclusion, the results support an important role for p53 in cisplatin-induced apoptosis in renal tubular cells, p53 May regulate apoptosis through the transcription of apoptotic genes. caspase; renal tubule

Nephrotoxicity is the major limiting factor for the use of cisplatin in cancer therapy. Recent studies have demonstrated an important role for p53 in cisplatin-induced renal injury. Nevertheless, pharmacological and genetic blockade of p53 only provides partial renoprotective effects, suggesting the presence of p53-independent injury mechanisms. To understand the p53-independent mechanisms, we have now examined cisplatin-induced apoptosis in p53-deficient kidney cells. We show that cisplatin could induce Bax activation, cytochrome c release, and apoptosis in primary cultures of p53-deficient renal tubular cells, albeit at a level that was lower than in the wild-type cells. Cisplatin could also induce typical apoptosis in p53-deficient baby mouse kidney (BMK) cells. The apoptosis was caspase dependent and could be completely blocked by general caspase inhibitors. Bax and Bak, two key molecules in the mitochondrial pathway of apoptosis, were interdependently activated by cisplatin, with Bax translocation to and Bax/Bak oligomerization in mitochondria, leading to cytochrome c release. Importantly, cytochrome c release and apoptosis were diminished in Bax/Bak single or double-knockout BMK cells. Furthermore, overexpression of Bcl-2 could ameliorate cisplatin-induced cytochrome c release and apoptosis. Together, the results have demonstrated a p53-independent mechanism of cisplatin nephrotoxicity that involves the mitochondrial pathway of apoptosis. [ABSTRACT FROM AUTHOR]

The early stage of diabetic nephropathy (DN) is linked to proteinuria. Transforming growth factor (TGF)-β1 increases glomerular permeability to albumin (Palb), whereas 20-HETE and EETs reduce Palb. To investigate the impact of hyperglycemia and hyperlipidemia on 20-HETE, EETs, and TGF-β1 in the glomeruli, rats were divided into four groups: ND rats were fed a normal diet, HF rats were fed a high-fat diet, STZ rats were treated with 35 mg/kg of streptozotocin, and HF/STZ rats were fed a HF diet and treated with STZ. After 10 wk on these regimens, blood glucose, urinary albumin, serum cholesterol, serum triglyceride levels, and the kidney-to-body weight ratio were significantly elevated in STZ and HF/STZ rats compared with HF and ND rats. STZ and HF/STZ rats had histopathologic changes and abnormal renal hemodynamics. Expression of glomemlar CYP4A, enzymes for 20-HETE production, was significantly decreased in STZ rats, whereas expression of glomerular CYP2C and CYP2J, enzymes for EETs production, was significantly decreased in both STZ and HF/STZ rats. Moreover, glomerular TGF-β1 levels were significantly greater in STZ and HF/STZ rats than in HF and ND rats. Five-week treatment of STZ rats with clofibrate induced glomerular CYP4A expression and 20-HETE production, but reduced glomerular TGF-β1 and urinary protein excretion. These results demonstrate that hyperglycemia increases TGF-β1 but decreases 20-HETE and EETs production in the glomeruli, changes that may be important in causing glomerular damage in the early stage of DN. [ABSTRACT FROM AUTHOR]

Cisplatin induces renal cell injury and death, resulting in nephrotoxicity that limits its use in cancer therapy. Using cell culture models, recent work has suggested the involvement of p53 in renal cell apoptosis during cisplatin treatment. However, the signals upstream of p53 remain elusive. ATM and ATR are critical regulators of p53 under various conditions of DNA damage. Here, we show that ATM, and not ATR, was proteolytically cleaved into specific fragments of ~210 and 150 kDa during cisplatin-induced tubular cell apoptosis. ATM cleavage was paralleled by the development of apoptosis. VAD, a broad-spectrum inhibitor of caspases, attenuated the cleavage of ATM, whereas the inhibitors of specific caspases were less effective. In caspase-3-deficient MCF-7 cells, ATM was cleaved, releasing the 210- but not the 150-kDa fragment. Recombinant caspase-3 was much more effective than caspase-7 in cleaving ATM that was immunoprecipitated from cell lysates. During cisplatin incubation, VAD protected ATM and enhanced p53 phosphorylation. In vitro assay of protein kinase activity further showed that ATM immunoprecipitated from cisplatin-treated cells had significantly lower kinase activity toward p53 than that from control cells. Importantly, the protein kinase activity was restored in ATM that was protected by VAD during cisplatin incubation. ATM deficiency sensitized the cells to cisplatin-induced apoptosis, suggesting a cytoprotective role of ATM in this experimental model. Thus proteolysis of ATM by caspases may inactivate this regulatory molecule to facilitate the progression of apoptosis. [ABSTRACT FROM AUTHOR]

We previously reported that inhibition of nitric oxide (NO) synthesis by N-nitro-L-arginine methyl ester (L-NAME) during late pregnancy leads to increased production of renal vascular 20-hydroxyeicosatetraenoic acid (20-HETE), a cytochrome P-450 (CYP) 4A-derived vasoconstrictor, in pregnant rats. However, the effect of upregulation of vascular 20-HETE production on renal function after NO inhibition is not known, To test the hypothesis that increased gestational vascular 20-HETE synthesis after NO inhibition is involved in mediating blood pressure and renal functional changes, we first determined the IC50 value of the effect of nitroprusside (SNP), a NO donor, on renal 20-HETE production in cortical microsomes. We then divided pregnant rats and age-matched virgin rats into a vehicle control group, an L-NAME treatment group (0.25 mg/ml in drinking water), and a group treated with L-NAME plus N-methylsulfonyl-12,12-dibromododec-11 enamide (DDMS; CYP4A-selective inhibitor, 10 mg∙kg-1∙day-1 iv). After 4 days of treatment, we measured blood pressure, renal blood flow (RBF), renal vascular resistance (RVR), and glomerular filtration rate (GFR) in each group. The addition of SNP (IC50 = 22 μM) decreased renal cortical 20-H ETE production. In pregnant rats, L-NAME treatment led to significantly higher mean arterial pressure (MAP) and RVR, and lower RBF and GFR. Combined treatment with DDMS and L-NAME significantly attenuated the increases in MAP and RVR and the decrease in GFR, but not the reduction in RBF induced by L-NAME treatment. L-NAME and L-NAME plus DDMS had no significant impact on renal hemodynamics in virgin rats. In addition, chronic treatment with DDMS selectively inhibited cortical 20-HETE production without a significant effect on CYP4A expression in L-NAME-treated pregnant rats. In conclusion, NO effectively inhibits renal cortical microsomal 20-HETE production in female rats. In pregnant rats, the augmentation of renal 20-HETE production after NO inhibition is associated with increased MAP and RVR, whereas decreased GFR is negated by treatment of a selective and competitive CYP4A inhibitor. These results demonstrate that the interaction between renal 20-HETE and NO is important in the regulation of renal function and blood pressure in pregnant rats. [ABSTRACT FROM AUTHOR]

Epoxyeicosatrienoic acids (EETs), which belong to cytochrome P-450 (CYP)-derived eicosanoids, have been implicated to vasodilate renal arterioles, inhibit sodium transport in the nephron, and regulate blood pressure in several animal models. Because pregnancy is associated with changes of blood pressure, the aim of this study was to examine whether renal EET synthesis is altered and whether EETs are involved in blood pressure regulation during pregnancy in rats. Renal microsomal epoxygenase activity increased by 47, 97, and 63% on days 6, 12, and 19 of gestation, respectively. The elevation of epoxygenase activity during pregnancy was associated with an increase in CYP2C11, CYP2C23, and CYP2J2 protein expression on days 6, 12, and 19 of gestation. Moreover, immunohistochemical analysis showed that renal tubular CYP2C11, CYP2C23, and CYP2J2 expression was significantly increased in pregnant rats on days 6, 12, and 19 of gestation. Administration of 6-(2-propargyloxyphenyl)hexanoic acid (PPOH), a selective epoxygenase inhibitor, caused a dose-dependent inhibition of microsomal expoxygenase activity without a significant effect on ω-hydroxylase activity in female rats. Interestingly, administration of PPOH (20 mg·kg-1· day-1 for 4 days starting on day 15 of pregnancy) increased blood pressure by 21 mmHg and caused a significant decrease in the body weight of fetal pups (1.3 ± 0.08 g in control vs. 1.1 ± 0.06 g in PPOH). Moreover, PPOH treatment significantly decreased renal microsomal epoxygenase activity and the expression of CYP2C11, CYP2C23, and CYP2J in pregnant rats. This study demonstrates that EET synthesis in the kidney is elevated during pregnancy, and CYP2C11, 2C23, and CYP2J2 am responsible for the change of renal EET synthesis. The inhibition results demonstrate that the downregulation of renal epoxygenase activity by PPOH causes hypertension in pregnant rats. This study suggests that EETs may contribute to the control of blood pressure during pregnancy. [ABSTRACT FROM AUTHOR]

Tubular damage by cisplatin leads to acute renal failure, which limits its use in cancer therapy. In tubular cells, a primary target for cisplatin is presumably the genomic DNA. However, the pathway relaying the signals of DNA damage to tubular cell death is unclear. In response to DNA damage, the tumor suppressor gene p53 is induced and is implicated in subsequent DNA repair and cell death by apoptosis. The current study was designed to examine the role of p53 in cisplatin-induced apoptosis in cultured rat kidney proximal tubular cells. Cisplatin at 20 μM induced apoptosis in ∼70% of cells, which was partially suppressed by carbobenzoxy-Val-Ala-Asp-fluoromethyl ketone (VAD), a general caspase inhibitor. Of interest, cisplatin-induced apoptosis was also suppressed by pifithrin-α, a pharmacological inhibitor of p53. Cisplatin-induced caspase activation was completely inhibited by VAD, but only partially by pifithrin-α. Early during cisplatin treatment, p53 was phosphorylated and upregulated. The p53 activation was blocked by pifithrin-α, but not by VAD. Bc1-2 expression abolished cisplatin-induced apoptosis without blocking p53 phosphorylation or induction. The results suggest that p53 activation might be an early signal for apoptosis during cisplatin treatment. To further determine the role of p53, tubular cells were stably transfected with a dominant-negative mutant of p53 with diminished transcriptional activity. Expression of the mutant attenuated cisplatin-induced apoptosis and caspase activation. In conclusion, the results support an important role for p53 in cisplatin-induced apoptosis in renal tubular cells, p53 May regulate apoptosis through the transcription of apoptotic genes. [ABSTRACT FROM AUTHOR]

P2X receptors have been suggested to be expressed on the central terminals of Aδ-afferent fibers innervating dorsal horn lamina V and play a role in modulating sensory synaptic transmission. These P2X receptors have been widely thought to be P2X2+3 receptors. However, we have recently found that P2X receptor-mediated modulation of sensory transmission in lamina V is not inhibited by trinitrophenyl-adenosine triphosphate (TNP-ATP), a potent antagonist of P2X1, P2X3 homomers, and P2X2+3 heteromers. To provide direct evidence for the presence of TNP-ATP-resistant P2X receptors on primary afferent fibers, we examined α,β-methylene-ATP (αβmeATP)-evoked currents and their sensitivity to TNP-ATP in rat dorsal root ganglion (DRG) neurons. αβmeATP evoked fast currents, slow currents, and mixed currents that contained both fast and slow current-components. Fast currents and fast current components in the mixed currents were both completely inhibited by 0.1 μM TNP-ATP (n = 14). Both slow currents and slow-current components in the mixed currents showed broad spectrum of sensitivity to 1 μM TNP-ATP, ranging from complete block (TNP-ATP-sensitive) to little block (TNP-ATP-resistant). TNP-ATP-resistant currents evoked by 10 μM αβmeATP could be largely inhibited by 10 μM iso-pyridoxalphosphate-6-azophenyl-2′,4′-disulphonic acid. Cells with P2X currents that were highly resistant to TNP-ATP were found to be insensitive to capsaicin. These results suggest that TNP-ATP-resistant P2X receptor subtypes are expressed on capsaicin-insensitive Aδ-afferent fibers and play a role in modulating sensory transmission to lamina V neurons. [ABSTRACT FROM AUTHOR]

Recent studies have shown mitochondrial fragmentation during cell stress and have suggested a role for the morphological change in mitochondrial injury and ensuing apoptosis. However, the underlying mechanism remains elusive. Here we demonstrate that mitochondrial fragmentation facilitates Bax insertion and activation in mitochondria, resulting in the release of apoptogenic factors. In HeLa cells, overexpression of mitofusins attenuated mitochondrial fragmentation during cisplatin- and azide-induced cell injury, which was accompanied by less apoptosis and less cytochrome c release from mitochondria. Similar effects were shown by inhibiting the mitochondrial fission protein Drp1 with a dominant negative mutant (dn-Drp1). Mitofusins and dn-Drp1 did not seem to significantly affect Bax translocation/accumulation to mitochondria; however, they blocked Bax insertion and activation in mitochondrial membrane. Consistently, in rat kidney proximal tubular cells, small interfering RNA knockdown of Drp1 prevented mitochondrial fragmentation during azide-induced ATP depletion, which was accompanied by less Bax activation, insertion, and oligomerization in mitochondria. These cells released less cytochrome c and AIF from mitochondria and showed significantly lower apoptosis. Finally, mitofusin-null mouse embryonic fibroblasts (MEF) had fragmented mitochondria. These MEFs were more sensitive to cisplatin-induced Bax activation, release of cytochrome c, and apoptosis. Together, this study provides further support for a role of mitochondrial fragmentation in mitochondrial injury and apoptosis. Mechanistically, mitochondrial fragmentation may sensitize the cells to Bax insertion and activation in mitochondria, facilitating the release of apoptogenic factors and consequent apoptosis.