Increase in the ratio of mitochondrial Bax/Bcl-XL induces Bax activation in human leukemic K562 cell line

The p53- and Bcl-2-negative leukemic K562 cell line showed resistant to DNA damage-induced Bax activation and apoptosis. The constitutive balanced ratio of Bax/Bcl-XL in K562 mitochondria allowed the formation of active Bax and cytochrome c release from mitochondria in the presence of a BH3-only protein, tBid, in a cell-free system. Bax transfection led to Bax undergoing a conformational change, translocation to mitochondria and homo-oligomerization but not apoptosis in the K562 cell line. After treatment with UV light, while Bcl-XL but not Bax translocated to mitochondria in K562, both Bax and Bcl-XL translocated to mitochondria in the Bax stable transfectant K/Bax cells. The increased ratio of Bax/Bcl-XL in K/Bax mitochondria led to an increased conformationally changed Bax, formation of the homo-multimer of Bax-Bax, and a reduced hetero-dimerization of Bax-Bcl-XL. Increased proportion of active Bax was accompanied with increased percentage of apoptosis. We therefore demonstrate that direct increase in the ratio of mitochondrial Bax/Bcl-XL can induce Bax activation in the p53- and Bcl-2-negative leukemic cells. Increased Bcl-XL translocation and failure in Bax translocation from cytosol to mitochondria play important roles in preventing Bax activation.     

PUMA-BH3结构域短肽的原核表达、纯化及促凋亡活性鉴定

Bcl-2家族蛋白质在线粒体途径凋亡的调控机制中起着重要的作用,p53正向细胞凋亡调控因子（p53 up-regulated modulator of apoptosis protein,PUMA)是该家族的一种只含有BH3同源区域的促凋亡蛋白。为得到PUMA的BH3结构域短肽并检测其生物学活性,将人工合成的编码PUMA-BH3肽的DNA片段克隆到质粒pTYB2上,构建出表达PUMA-BH3-内含肽-几丁质结合域融合蛋白的原核表达载体pTYB2-PUMA-BH3,转化大肠杆菌BL-21(DE3)中IPTG诱导表达。表达的融合蛋白经几丁质亲和层析、二硫苏糖醇（DTT）的柱内还原,直接获得可溶性PUMA-BH3肽。通过研究重组PUMA-BH3肽在体外条件下对线粒体活力、线粒体肿胀度以及细胞色素c释放的影响来鉴定其生物学活性。结果表明,获得的可溶性PUMA-BH3肽能作用于离体线粒体,引起线粒体活力降低,线粒体肿胀并能诱导细胞色素c释放。环孢菌素A对此有一定的抑制作用,提示PUMA-BH3肽对线粒体的上述作用是通过促进通透性转运孔( PTP)开放实现的。经原核表达及纯化,获得了具有促凋亡活性的PUMA-BH3肽,为进一步研制控制凋亡过程的药物奠定了基础。     

PUMA Dissociates Bax and Bcl-X(L) to induce apoptosis in colon cancer cells

PUMA is a BH3-only Bcl-2 family protein that plays an essential role in DNA damage-induced apoptosis. PUMA interacts with anti-apoptotic Bcl-2 and Bcl-X(L) and is dependent on Bax to induce apoptosis. In this study, we investigated how the interactions of PUMA with the antiapoptotic proteins coordinate with Bax to initiate apoptosis in HCT116 colon cancer cells. We found that Bcl-X(L) was most effective among several antiapoptotic proteins in suppressing PUMA-induced apoptosis and PUMA-dependent apoptosis induced by the DNA-damaging agent adriamycin. Mutant Bcl-X(L) that cannot interact with Bax was unable to protect cells from PUMA-mediated apoptosis. Knockdown of Bcl-X(L) by RNA interference significantly enhanced PUMA-mediated apoptosis in HCT116 cells but not in PUMA-knockout cells. Furthermore, Bax was found to be dissociated preferentially from Bcl-X(L) in HCT116 cells but not in the PUMA-knockout cells, in response to PUMA induction and adriamycin treatment. PUMA inhibited the association of Bax and Bcl-X(L) in vitro by directly binding to Bcl-X(L) through its BH3 domain. Finally, we found that wild-type Bax, but not mutant Bax deficient in either multimerization or mitochondrial localization, was able to restore PUMA-induced apoptosis in the BAX-knockout cells. Together, these results indicate that PUMA initiates apoptosis in part by dissociating Bax and Bcl-X(L), thereby promoting Bax multimerization and mitochondrial translocation.     

p73 Induces apoptosis via PUMA transactivation and Bax mitochondrial translocation

p73, an important developmental gene, shares a high sequence homology with p53 and induces both G(1) cell cycle arrest and apoptosis. However, the molecular mechanisms through which p73 induces apoptosis are unclear. We found that p73-induced apoptosis is mediated by PUMA (p53 up-regulated modulator of apoptosis) induction, which, in turn, causes Bax mitochondrial translocation and cytochrome c release. Overexpression of p73 isoforms promotes cell death and bax promoter transactivation in a time-dependent manner. However, the kinetics of apoptosis do not correlate with the increase of Bax protein levels. Instead, p73-induced mitochondrial translocation of Bax is kinetically compatible with the induction of cell death. p73 is localized in the nucleus and remains nuclear during the induction of cell death, indicating that the effect of p73 on Bax translocation is indirect. The ability of p73 to directly transactivate PUMA and the direct effect of PUMA on Bax conformation and mitochondrial relocalization suggest a molecular link between p73 and the mitochondrial apoptotic pathway. Our data therefore indicate that PUMA-mediated Bax mitochondrial translocation, rather than its direct transactivation, correlates with cell death. Finally, human DeltaNp73, an isoform lacking the amino-terminal transactivation domain, inhibits TAp73-induced as well as p53-induced apoptosis. The DeltaNp73 isoforms seem therefore to act as dominant negatives, repressing the PUMA/Bax system and, thus, finely tuning p73-induced apoptosis. Our findings demonstrate that p73 elicits apoptosis via the mitochondrial pathway using PUMA and Bax as mediators.     

Trail-induced apoptosis in Type I leukemic cells is not enhanced by overexpression of bax

We have previously shown that Bax translocation was crucial in TNFalpha or etoposide-induced apoptosis. Overexpression of Bax sensitized chronic myeloid leukemic K562 cells to etoposide-induced apoptosis. Treatment with TNF-related apoptosis-inducing ligand (TRAIL) induces a loss of mitochondrial membrane potential (DeltaPsim), cytochrome c release from mitochondria, activation of caspases-8, -9, and -3, and cleavage of Bid in the K562 cell line. Bax failed to sensitize K562 cells to TRAIL-induced apoptosis. TRAIL did not induce Bax expression and/or translocation from cytosol to mitochondria in the K562 cell line. However, 100 microM Z-VAD.fmk, a pan caspase inhibitor, completely blocked TRAIL-initiated mitochondrial alterations and cleavages of caspases and Bid. We propose that TRAIL-induced apoptosis in K562 cells is via Type I apoptotic signal pathway. Bax translocation is not essential for TRAIL-induced cytochrome c release and DeltaPsim collapse in the Type I cells.     

Bak compensated for Bax in p53-null cells to release cytochrome c for the initiation of mitochondrial signaling during Withanolide D-induced apoptosis

The goal of cancer chemotherapy to induce multi-directional apoptosis as targeting a single pathway is unable to decrease all the downstream effect arises from crosstalk. Present study reports that Withanolide D (WithaD), a steroidal lactone isolated from Withania somnifera, induced cellular apoptosis in which mitochondria and p53 were intricately involved. In MOLT-3 and HCT116p53+/+ cells, WithaD induced crosstalk between intrinsic and extrinsic signaling through Bid, whereas in K562 and HCT116p53-/- cells, only intrinsic pathway was activated where Bid remain unaltered. WithaD showed pronounced activation of p53 in cancer cells. Moreover, lowered apoptogenic effect of HCT116p53-/- over HCT116p53+/+ established a strong correlation between WithaD-mediated apoptosis and p53. WithaD induced Bax and Bak upregulation in HCT116p53+/+, whereas increase only Bak expression in HCT116p53-/- cells, which was coordinated with augmented p53 expression. p53 inhibition substantially reduced Bax level and failed to inhibit Bak upregulation in HCT116p53+/+ cells confirming p53-dependent Bax and p53-independent Bak activation. Additionally, in HCT116p53+/+ cells, combined loss of Bax and Bak (HCT116Bax-Bak-) reduced WithaD-induced apoptosis and completely blocked cytochrome c release whereas single loss of Bax or Bak (HCT116Bax-Bak+/HCT116Bax+Bak-) was only marginally effective after WithaD treatment. In HCT116p53-/- cells, though Bax translocation to mitochondria was abrogated, Bak oligomerization helped the cells to release cytochrome c even before the disruption of mitochondrial membrane potential. WithaD also showed in vitro growth-inhibitory activity against an array of p53 wild type and null cancer cells and K562 xenograft in vivo. Taken together, WithaD elicited apoptosis in malignant cells through Bax/Bak dependent pathway in p53-wild type cells, whereas Bak compensated against loss of Bax in p53-null cells.     

6-Hydroxydopamine activates the mitochondrial apoptosis pathway through p38 MAPK-mediated, p53-independent activation of Bax and PUMA

Mitochondrial alterations have been associated with the cytotoxic effect of 6-hydroxydopamine (6-OHDA), a widely used toxin to study Parkinson's disease. In previous work, we have demonstrated that 6-OHDA increases mitochondrial membrane permeability leading to cytochrome c release, but the precise mechanisms involved in this process remain unknown. Herein we studied the mechanism of increased mitochondrial permeability of SH-SY5Y neuroblastoma cells in response to 6-OHDA. Cytochrome c release induced by 6-OHDA occurred, in both SH-SY5Y cells and primary cultures, in the absence of mitochondrial swelling or a decrease in mitochondrial calcein fluorescence, suggesting little involvement of the mitochondrial permeability transition pore in this process. In contrast, 6-OHDA-induced cell death was associated with a significant translocation of the pro-apoptotic Bax protein from the cytosol to mitochondria and with a significant induction of the BH3-only protein PUMA. Experiments in mouse embryonic fibroblasts deficient in Bax or PUMA demonstrated a role for both proteins in 6-OHDA-induced apoptosis. Although 6-OHDA elevated both total and nuclear p53 protein levels, activation of p53 was not essential for subsequent cell death. In contrast, we found that p38 mitogen-activated protein kinase (MAPK) was activated early during 6-OHDA-induced apoptosis, and that treatment with the p38 MAPK inhibitor SKF86002 potently inhibited PUMA induction, green fluorescent protein-Bax redistribution and apoptosis in response to 6-OHDA. These data demonstrate a critical involvement of p38 MAPK, PUMA, and Bax in 6-OHDA-induced apoptosis.     

PUMA Promotes Bax Translocation by Both Directly Interacting with Bax and by Competitive Binding to Bcl-XL during UV-induced Apoptosis

Cell apoptosis induced by UV irradiation is a highly complex process in which different molecular signaling pathways are involved. p53 up-regulated modulator of apoptosis (PUMA) has been proposed as an important regulator in UV irradiation-induced apoptosis. However, the molecular mechanism through which PUMA regulates apoptosis, especially how PUMA activates Bcl-2-associated X protein (Bax) in response to UV irradiation is still controversial. In this study, by using real-time single-cell analysis and fluorescence resonance energy transfer, we investigated the tripartite nexus among PUMA, Bax, and Bcl-X_L in living human lung adenocarcinoma cells (ASTC-a-1) to illustrate how PUMA promotes Bax translocation to initiate apoptosis. Our results show that the interaction between PUMA and Bax increased gradually, with Bax translocating to mitochondria and colocalizing with PUMA after UV irradiation, indicating PUMA promotes Bax translocation directly. Simultaneously, the interaction increased markedly between PUMA and Bcl-X_L and decreased significantly between Bcl-X_L and Bax after UV treatment, suggesting PUMA competitively binds to Bcl-X_L to activate Bax indirectly. The above-mentioned results were further confirmed by coimmunoprecipitation experiments. In addition, pifithrin-α (a p53 inhibitor) and cycloheximide (a protein synthesis inhibitor) could inhibit PUMA-mediated Bax translocation and cell apoptosis. Together, these studies create an important conclusion that PUMA promotes Bax translocation by both by directly interacting with Bax and by competitive binding to Bcl-X_L in UV-induced apoptosis.     

ASC is a Bax adaptor and regulates the p53-Bax mitochondrial apoptosis pathway

The apoptosis-associated speck-like protein (ASC) is an unusual adaptor protein that contains the Pyrin/PAAD death domain in addition to the CARD protein-protein interaction domain. Here, we present evidence that ASC can function as an adaptor molecule for Bax and regulate a p53-Bax mitochondrial pathway of apoptosis. When ectopically expressed, ASC interacted directly with Bax, colocalized with Bax to the mitochondria, induced cytochrome c release with a significant reduction of mitochondrial membrane potential and resulted in the activation of caspase-9, -2 and -3. The rapid induction of apoptosis by ASC was not observed in Bax-deficient cells. We also show that induction of ASC after exposure to genotoxic stress is dependent on p53. Blocking of endogenous ASC expression by small-interfering RNA (siRNA) reduced the apoptotic response and inhibited translocation of Bax to mitochondria in response to p53 or genotoxic insult, suggesting that ASC is required to translocate Bax to the mitochondria. Our findings demonstrate that ASC has an essential role in the intrinsic mitochondrial pathway of apoptosis through a p53-Bax network.     

Stabilization and translocation of p53 to mitochondria is linked to Bax translocation to mitochondria in simvastatin-induced apoptosis

Statins are cholesterol-lowing drugs with pleiotropic effects including cytotoxicity to cancer cells. In this study, we investigated the signaling pathways leading to apoptosis by simvastatin. Simvastatin induced cardinal features of apoptosis including increased DNA fragmentation, disruption of mitochondrial membrane potential (MMP), and increased caspase-3 activity by depleting isoprenoids in MethA fibrosarcoma cells. Interestingly, the simvastatin-induced apoptosis was accompanied by p53 stabilization involving Mdm2 degradation. The apoptosis was ameliorated in p53 knockdown clones of MethA cells as well as p53^−/− HCT116 cells. The stabilized p53 protein translocated to mitochondria with Bax, and cytochrome c was released into cytosol. Moreover, knockdown or deficiency of p53 expression reduced both Bax translocation to mitochondria and MMP disruption in simvastatin-induced apoptosis. Taken together, these all indicate that stabilization and translocation of p53 to mitochondria is involved in Bax translocation to mitochondria in simvastatin-induced apoptosis.     

Pharmacological inhibition of GSK3 attenuates DNA damage-induced apoptosis via reduction of p53 mitochondrial translocation and Bax oligomerization in neuroblastoma SH-SY5Y CELLS

Glycogen synthase kinase-3 (GSK3) and p53 play crucial roles in the mitochondrial apoptotic pathway and are known to interact in the nucleus. However, it is not known if GSK3 has a regulatory role in the mitochondrial translocation of p53 that participates in apoptotic signaling following DNA damage. In this study, we demonstrated that lithium and SB216763, which are pharmacological inhibitors of GSK3, attenuated p53 accumulation and caspase-3 activation, as shown by PARP cleavage induced by the DNA-damaging agents doxorubicin, etoposide and camptothecin. Furthermore, each of these agents induced translocation of p53 to the mitochondria and activated the mitochondrial pathway of apoptosis, as evidenced by the release of cytochrome C from the mitochondria. Both mitochondrial translocation of p53 and mitochondrial release of cytochrome C were attenuated by inhibition of GSK3, indicating that GSK3 promotes the DNA damage-induced mitochondrial translocation of p53 and the mitochondrial apoptosis pathway. Interestingly, the regulation of p53 mitochondrial translocation by GSK3 was only evident with wild-type p53, not with mutated p53. GSK3 inhibition also reduced the phosphorylation of wild-type p53 at serine 33, which is induced by doxorubicin, etoposide and camptothecin in the mitochondria. Moreover, inhibition of GSK3 reduced etoposide-induced association of p53 with Bcl2 and Bax oligomerization. These findings show that GSK3 promotes the mitochondrial translocation of p53, enabling its interaction with Bcl2 to allow Bax oligomerization and the subsequent release of cytochrome C. This leads to caspase activation in the mitochondrial pathway of intrinsic apoptotic signaling.     

Sensitization of TRAIL-resistant LNCaP cells by resveratrol (3, 4', 5 tri-hydroxystilbene): molecular mechanisms and therapeutic potential

Background

We have previously shown that prostate cancer LNCaP cells are resistant to TRAIL, and downregulation of PI-3K/Akt pathway by molecular and pharmacological means sensitizes cells to undergo apoptosis by TRAIL and curcumin. The purpose of this study was to examine the molecular mechanisms by which resveratrol sensitized TRAIL-resistant LNCaP cells.

Results

Resveratrol inhibited growth and induced apoptosis in androgen-dependent LNCaP cells, but had no effect on normal human prostate epithelial cells. Resveratrol upregulated the expression of Bax, Bak, PUMA, Noxa, Bim, TRAIL-R1/DR4 and TRAIL-R2/DR5, and downregulated the expression of Bcl-2, Bcl-X_L, survivin and XIAP. Treatment of LNCaP cells with resveratrol resulted in generation of reactive oxygen species, translocation of Bax and p53 to mitochondria, subsequent drop in mitochondrial membrane potential, release of mitochondrial proteins (cytochrome c, AIF, Smac/DIABLO and Omi/HtrA2), activation of caspase-3 and caspase-9 and induction of apoptosis. The ability of resveratrol to sensitize TRAIL-resistant LNCaP cells was inhibited by dominant negative FADD, caspase-8 siRNA or N-acetyl cysteine. Smac siRNA inhibited resveratrol-induced apoptosis, whereas Smac N7 peptide induced apoptosis and enhanced the effectiveness of resveratrol.

Conclusion

Resveratrol either alone or in combination with TRAIL or Smac can be used for the prevention and/or treatment of human prostate cancer.     

Bcl-XL protects BimEL-induced Bax conformational change and cytochrome C release independent of interacting with Bax or BimEL

The Bcl-2 homology (BH) 3-only pro-apoptotic Bcl-2 family protein Bim plays an essential role in the mitochondrial pathway of apoptosis through activation of the BH1-3 multidomain protein Bax or Bak. To further understand how the BH3-only protein activates Bax, we provide evidence here that BimEL induces Bax conformational change and apoptosis through a Bcl-XL-suppressible but heterodimerization-independent mechanism. Substitution of the conserved leucine residue in the BH3 domain of BimEL for alanine (M1) inhibits the interaction of BimEL with Bcl-XL but does not abolish the ability of BimEL to induce Bax conformational change and apoptosis. However, removal of the C-terminal hydrophobic region from the M1 mutant (M1DeltaC) abolishes its ability to activate Bax and to induce apoptosis, although deletion of the C-terminal domain (DeltaC) alone has little if any effect on the pro-apoptotic activity of BimEL. Subcellular fractionation experiments show that the Bim mutant M1DeltaC is localized in the cytosol, indicating that both the C-terminal hydrophobic region and the BH3 domain are required for the mitochondrial targeting and pro-apoptotic activity of BimEL. Moreover, the Bcl-XL mutant (mt1), which is unable to interact with Bax and BimEL, blocks Bax conformational change and cytochrome c release induced by BimEL in intact cells and isolated mitochondria. BimEL or Bak-BH3 peptide induces Bax conformational change in vitro only under the presence of mitochondria, and the outer mitochondrial membrane fraction is sufficient for induction of Bax conformational change. Interestingly, native Bax is attached loosely on the surface of isolated mitochondria, which undergoes conformational change and insertion into mitochondrial membrane upon stimulation by BimEL, Bak-BH3 peptide, or freeze/thaw damage. Taken together, these findings indicate that BimEL may activate Bax by damaging the mitochondrial membrane structure directly, in addition to its binding and antagonizing Bcl-2/Bcl-XL function.     

Puma, a critical mediator of cell death — one decade on from its discovery

PUMA (p53 upregulated modulator of apoptosis) is a pro-apoptotic member of the BH3-only subgroup of the Bcl-2 family. It is a key mediator of p53-dependent and p53-independent apoptosis and was identified 10 years ago. The PUMA gene is mapped to the long arm of chromosome 19, a region that is frequently deleted in a large number of human cancers. PUMA mediates apoptosis thanks to its ability to directly bind known anti-apoptotic members of the Bcl-2 family. It mainly localizes to the mitochondria. The binding of PUMA to the inhibitory members of the Bcl-2 family (Bcl-2-like proteins) via its BH3 domain seems to be a critical regulatory step in the induction of apoptosis. It results in the displacement of the proteins Bax and/or Bak. This is followed by their activation and the formation of pore-like structures on the mitochondrial membrane, which permeabilizes the outer mitochondrial membrane, leading to mitochondrial dysfunction and caspase activation. PUMA is involved in a large number of physiological and pathological processes, including the immune response, cancer, neurodegenerative diseases and bacterial and viral infections.     

p53 triggers apoptosis in oncogene-expressing fibroblasts by the induction of Noxa and mitochondrial Bax translocation

The mechanism of p53-dependent apoptosis is still only partly defined. Using early-passage embryonic fibroblasts (MEF) from wild-type (wt), p53(-/-) and bax(-/-) mice, we observe a p53-dependent translocation of Bax to the mitochondria and a release of mitochondrial Cytochrome c during stress-induced apoptosis. These events proceed independent of zVAD-inhibitable caspase activation, are not prevented by dominant negative FADD (DN-FADD), but are negatively regulated by Mdm-2. Bcl-x(L) expression prevents the release of mitochondrial Cytochrome c and apoptosis, but not Bax translocation. At a single-cell level, enforced expression of p53 is sufficient to induce Bax translocation and Cytochrome c release. Real-time RT-PCR analysis reveals a significant induction of RNA expression of Noxa and Bax in p53(+/+), but not in p53(-/-) MEF. Noxa protein expression becomes detectable prior to Bax translocation, and downregulation of endogenous Noxa by RNA interference protects wt MEF against p53-dependent apoptosis. Hence, in oncogene-expressing MEF p53 induces apoptosis by BH3 protein-dependent caspase activation.     

Contribution of membrane localization to the apoptotic activity of PUMA

The BH3-only protein PUMA plays an important role in the activation of apoptosis in response to p53. In different studies, PUMA has been described to function by either directly activating the pro-apoptotic proteins Bax and Bak, or by neutralizing anti-apoptotic members of the Bcl2 family. We have examined the contribution of regions of PUMA other than the BH3 domain to its localization and function. Although the hydrophobic domain in the C-terminus of PUMA is necessary for efficient mitochondrial localization of PUMA itself, PUMA proteins lacking this region can still induce apoptosis and localize to the mitochondria through binding to Bcl2. Even a nuclear localization signal (NLS)-tagged PUMA protein retains apoptotic activity and can be efficiently relocalized from the nucleus after interaction with ectopically expressed Bcl2, underscoring the efficiency of this interaction. Interestingly, unlike the Bcl2 interaction, the binding of PUMA to Bax is severely compromised by the loss of the C-terminal domain of PUMA. However, since the loss of the C-terminus does not compromise the ability of PUMA to induce cell death, our results indicate that the key apoptotic function of PUMA is through interaction with anti-apoptotic proteins such as Bcl2.