Constitutive p53 heightens mitochondrial apoptotic priming and favors cell death induction by BH3 mimetic inhibitors of BCL-xL.

Proapoptotic molecules directly targeting the BCL-2 family network are promising anticancer therapeutics, but an understanding of the cellular stress signals that render them effective is still elusive. We show here that the tumor suppressor p53, at least in part by transcription independent mechanisms, contributes to cell death induction and full activation of BAX by BH3 mimetic inhibitors of BCL-xL. In addition to mildly facilitating the ability of compounds to derepress BAX from BCL-xL, p53 also provides a death signal downstream of anti-apoptotic proteins inhibition. This death signal cooperates with BH3-induced activation of BAX and it is independent from PUMA, as enhanced p53 can substitute for PUMA to promote BAX activation in response to BH3 mimetics. The acute sensitivity of mitochondrial priming to p53 revealed here is likely to be critical for the clinical use of BH3 mimetics.

Cardiolipin or MTCH2 can serve as tBID receptors during apoptosis.

During apoptosis, proapoptotic BAX and BAK trigger mitochondrial outer membrane (MOM) permeabilization by a mechanism that is not yet fully understood. BH3-only proteins such as tBID, together with lipids of the MOM, are thought to play a key role in BAX and BAK activation. In particular, cardiolipin (CL) has been shown to stimulate tBID-induced BAX activation in vitro. However, it is still unclear whether this process also relies on CL in the cell, or whether it is more dependent on MTCH2, a proposed receptor for tBID present in the MOM. To address this issue, we deleted both alleles of cardiolipin synthase in human HCT116 cells by homologous recombination, which resulted in a complete absence of CL. The CL-deficient cells were fully viable in glucose but displayed impaired oxidative phosphorylation and an inability to grow in galactose. Using these cells, we found that CL was not required for either tBID-induced BAX activation, or for apoptosis in response to treatment with TRAIL. Downregulation of MTCH2 in HCT116 cells also failed to prevent recruitment of tBID to mitochondria in apoptotic conditions. However, when both CL and MTCH2 were depleted, a significant reduction in tBID recruitment was observed, suggesting that in HCT116 cells, CL and MTCH2 can have redundant functions in this process.

Contributions to Bax insertion and oligomerization of lipids of the mitochondrial outer membrane.

Under many apoptotic conditions, Bax undergoes conformational rearrangements, leading to its insertion in the mitochondrial outer membrane as a transmembrane oligomer. At the same time, mitochondria undergo fragmentation and activated Bax was reported to localize to fission sites. We studied how lipid composition and membrane curvature regulate Bax activation. When isolated mitochondria were incubated with phospholipase A2, which led to phosphatidylethanolamine and cardiolipin hydrolysis, tBid and Bax insertion were hindered. We thus studied in liposomes how phosphatidylethanolamine, cardiolipin, and its hydrolysis products affect Bax activation. Whereas phosphatidylethanolamine, a lipid with negative curvature, did not affect Bax insertion, it inhibited Bax oligomerization. Conversely, Bax insertion required cardiolipin, and was not blocked by cardiolipin hydrolysis products. These experiments support a direct role for cardiolipin in the recruitment and activation of Bax. To examine if the increase in membrane curvature that accompanies mitochondrial fission participates in Bax activation, we studied how liposome size affects the process, and observed that it was inhibited in small liposomes (

Bax activation and stress-induced apoptosis delayed by the accumulation of cholesterol in mitochondrial membranes.

Activation of Bax or Bak is essential for the completion of many apoptotic programmes. Under cytotoxic conditions, these proteins undergo a series of conformational rearrangements that end up with their oligomerization. We found that unlike inactive monomeric Bax, active oligomerized Bax is partially resistant to trypsin digestion, providing a convenient read out to monitor Bax activation. Using this assay, we studied how the lipid composition of membranes affects tBid-induced Bax activation in vitro with pure liposomes. We report that Bax activation is inhibited by cholesterol and by decreases in membrane fluidity. This observation was further tested in vivo using the drug U18666A, which we found increases mitochondrial cholesterol levels. When incubated with tBid, mitochondria isolated from U18666A-treated cells showed a delay in the release of Smac/Diablo and Cytochrome c, as well as in Bax oligomerization. Moreover, pre-incubation with U18666A partially protected cells from stress-induced apoptosis. As many tumours display high mitochondrial cholesterol content, inefficient Bax oligomerization might contribute to their resistance to apoptosis-inducing agents.

Phosphorylation of bid by casein kinases I and II regulates its cleavage by caspase 8.

Bid plays an essential role in Fas-mediated apoptosis of the so-called type II cells. In these cells, following cleavage by caspase 8, the C-terminal fragment of Bid translocates to mitochondria and triggers the release of apoptogenic factors, thereby inducing cell death. Here we report that Bid is phosphorylated by casein kinase I (CKI) and casein kinase II (CKII). Inhibition of CKI and CKII accelerated Fas-mediated apoptosis and Bid cleavage, whereas hyperactivity of the kinases delayed apoptosis. When phosphorylated, Bid was insensitive to caspase 8 cleavage in vitro. Moreover, a mutant of Bid that cannot be phosphorylated was found to be more toxic than wild-type Bid. Together, these data indicate that phosphorylation of Bid represents a new mechanism whereby cells control apoptosis.

Bax is present as a high molecular weight oligomer/complex in the mitochondrial membrane of apoptotic cells.

Bax is a Bcl-2 family protein with proapoptotic activity, which has been shown to trigger cytochrome c release from mitochondria both in vitro and in vivo. In control HeLa cells, Bax is present in the cytosol and weakly associated with mitochondria as a monomer with an apparent molecular mass of 20,000 Da. After treatment of the HeLa cells with the apoptosis inducer staurosporine or UV irradiation, Bax associated with mitochondria is present as two large molecular weight oligomers/complexes of 96,000 and 260,000 Da, which are integrated into the mitochondrial membrane. Bcl-2 prevents Bax oligomerization and insertion into the mitochondrial membrane. The outer mitochondrial membrane protein voltage-dependent anion channel and the inner mitochondrial membrane protein adenosine nucleotide translocator do not coelute with the large molecular weight Bax oligomers/complexes on gel filtration. Bax oligomerization appears to be required for its proapoptotic activity, and the Bax oligomer/complex might constitute the structural entirety of the cytochrome c-conducting channel in the outer mitochondrial membrane.

The destabilization of lipid membranes induced by the C-terminal fragment of caspase 8-cleaved bid is inhibited by the N-terminal fragment.

Bid is a proapoptotic, BH3-domain-only member of the Bcl-2 family. In Fas-induced apoptosis, Bid is activated through cleavage by caspase 8 into a 15.5-kDa C-terminal fragment (t(c)Bid) and a 6.5 kDa N-terminal fragment (t(n)Bid). Following the cleavage, t(c)Bid translocates to the mitochondria and promotes the release of cytochrome c into the cytosol by a mechanism that is not understood. Here we report that recombinant t(c)Bid can act as a membrane destabilizing agent. t(c)Bid induces destabilization and breaking of planar lipid bilayers without appearance of ionic channels; its destabilizing activity is comparable with that of Bax and at least 30-fold higher than that of full-length Bid. Consistently, t(c)Bid, but not full-length Bid, permeabilizes liposomes at physiological pH. The destabilizing effect of t(c)Bid on liposomes and planar bilayers is independent of the BH3 domain. In contrast, mutations in the BH3 domain impair t(c)Bid ability to induce cytochrome c release from mitochondria. The permeabilizing effect of t(c)Bid on planar bilayers, liposomes, and mitochondria can be inhibited by t(n)Bid. In conclusion, our results suggest a dual role for Bid: BH3-independent membrane destabilization and BH3-dependent interaction with other proteins. Moreover, the dissociation of Bid after cleavage by caspase 8 represents an additional step at which apoptosis may be regulated.

Bax oligomerization is required for channel-forming activity in liposomes and to trigger cytochrome c release from mitochondria.

Bax is a Bcl-2-family protein with pro-apoptotic activity that can form channels in lipid membranes. The protein has been shown to trigger cytochrome c release from mitochondria both in vitro and in vivo. Recombinant human Bax isolated in the presence of detergent was found to be present as an oligomer with an apparent molecular mass of approx. 160000 Da on gel filtration. When Bax was isolated in the absence of detergent the purified protein was monomeric with an apparent molecular mass of 22000 Da. Bax oligomers formed channels in liposomes and triggered cytochrome c release from isolated mitochondria, whereas monomeric Bax was inactive in both respects. Incubation of the monomeric Bax with 2% octyl glucoside induced formation of oligomers that displayed channel-forming activity in liposomes and triggered cytochrome c release from mitochondria. Triton X-100, Nonidet P-40 and n-dedecyl maltoside also activated monomeric Bax, whereas CHAPS had no activating effect. In cytosolic extracts from mouse liver, Bax migrated at a molecular mass of 24000 Da on gel filtration, whereas after incubation of the cytosol with 2% octyl glucoside Bax migrated at approximately 140000 Da. These results show that oligomeric Bax possesses channel-forming activity whereas monomeric Bax has no such activity.

Bid-induced conformational change of Bax is responsible for mitochondrial cytochrome c release during apoptosis.

Here we report that in staurosporine-induced apoptosis of HeLa cells, Bid, a BH3 domain containing protein, translocates from the cytosol to mitochondria. This event is associated with a change in conformation of Bax which leads to the unmasking of its NH2-terminal domain and is accompanied by the release of cytochrome c from mitochondria. A similar finding is reported for cerebellar granule cells undergoing apoptosis induced by serum and potassium deprivation. The Bax-conformational change is prevented by Bcl-2 and Bcl-xL but not by caspase inhibitors. Using isolated mitochondria and various BH3 mutants of Bid, we demonstrate that direct binding of Bid to Bax is a prerequisite for Bax structural change and cytochrome c release. Bcl-xL can inhibit the effect of Bid by interacting directly with Bax. Moreover, using mitochondria from Bax-deficient tumor cell lines, we show that Bid- induced release of cytochrome c is negligible when Bid is added alone, but dramatically increased when Bid and Bax are added together. Taken together, our results suggest that, during certain types of apoptosis, Bid translocates to mitochondria and binds to Bax, leading to a change in conformation of Bax and to cytochrome c release from mitochondria.

Expression and purification of full-length human Bax alpha.

Bax is a proapoptotic ion channel forming protein of the Bcl-2 family. In cells the protein is found in the cytosol and in the mitochondria membrane where it presumably is involved during apoptosis in disruption of the mitochondrial membrane potential and release of cytochrome c. The protein has a hydrophobic domain at the C-terminus, which renders it a limited solubility. Thus, all studies on recombinant Bax has so far been performed on C-terminal truncated protein. We have expressed and purified the full-length human Bax alpha. The protein was expressed with a His tag at the N-terminus and purified by affinity chromatography on Ni-NTA-agarose followed by ion-exchange chromatography on Q-Sepharose. The protein was more than 98% pure on SDS-PAGE and in the presence of 1% (w/v) octyl glucoside it could be concentrated up to 0.5 mg/ml. Full-length Bax was 25-fold more efficient, compared to C-terminal truncated Bax, in forming ion channels and trigger carboxyfluorescein release from liposomes.

An in vitro 96-well plate assay of the mitogen-activated protein kinase cascade.

Mitogen-activated protein (MAP) kinases of the extracellular signal-regulated kinase (ERK) family are activated in response to many growth and differentiation factors as well as some oncogenes. ERK activation follows phosphorylation by a class of specific upstream MAP kinase/ERK kinase (MEK) exemplified by MEK-1. Activated ERKs control many short- and long-term changes in cell function through phosphorylating a number of intracellular target substrates which include stathmin, a phosphoprotein regulating microtubule stability. We report here the development of a simple, 96-well plate, quantitative in vitro assay measuring purified ERK2 catalytic activation by a constitutive MEK-1 mutant (S218E S222E). Enzymatic activity was detected by 33P phosphorylation of purified biotinylated stathmin captured on streptavidin-coated scintillation proximity assay beads which eliminates the need for wash steps. The assay was optimized and the K0.5 value for ATP was found to be 0.9 microM and the Km for stathmin was determined to be 16 microM. The assay was also used to determine IC50 values for the protein kinase inhibitors PD98059 and staurosporine. This simple assay allows several hundred quantitative measurements of MEK1-dependent ERK2 activation to be performed in a day.

Bax-induced cytochrome C release from mitochondria is independent of the permeability transition pore but highly dependent on Mg2+ ions.

Bcl-2 family members either promote or repress programmed cell death. Bax, a death-promoting member, is a pore-forming, mitochondria-associated protein whose mechanism of action is still unknown. During apoptosis, cytochrome C is released from the mitochondria into the cytosol where it binds to APAF-1, a mammalian homologue of Ced-4, and participates in the activation of caspases. The release of cytochrome C has been postulated to be a consequence of the opening of the mitochondrial permeability transition pore (PTP). We now report that Bax is sufficient to trigger the release of cytochrome C from isolated mitochondria. This pathway is distinct from the previously described calcium-inducible, cyclosporin A-sensitive PTP. Rather, the cytochrome C release induced by Bax is facilitated by Mg2+ and cannot be blocked by PTP inhibitors. These results strongly suggest the existence of two distinct mechanisms leading to cytochrome C release: one stimulated by calcium and inhibited by cyclosporin A, the other Bax dependent, Mg2+ sensitive but cyclosporin insensitive.

Inhibition of Bax channel-forming activity by Bcl-2.

Proteins of the Bcl-2 family are intracellular membrane-associated proteins that regulate programmed cell death (apoptosis) either positively or negatively by as yet unknown mechanisms. Bax, a pro-apoptotic member of the Bcl-2 family, was shown to form channels in lipid membranes. Bax triggered the release of liposome-encapsulated carboxyfluorescein at both neutral and acidic pH. At physiological pH, release could be blocked by Bcl-2. Bcl-2, in contrast, triggered carboxyfluorescein release at acidic pH only. In planar lipid bilayers, Bax formed pH- and voltage-dependent ion-conducting channels. Thus, the pro-apoptotic effects of Bax may be elicited through an intrinsic pore-forming activity that can be antagonized by Bcl-2.

Expression, purification, and characterization of a highly soluble N-terminal-truncated form of the neuron-specific membrane-associated phosphoprotein SCG10.

SCG10 is a neuron-specific growth-associated protein with high sequence homology to the ubiquitous phosphoprotein stathmin/Op18. The main structural difference between the two proteins is the 34-amino-acid N-terminal extension of SCG10, which is responsible for the membrane attachment. Full length SCG10 has been purified and shows limited solubility, in contrast to stathmin, which is a highly soluble protein. In order to obtain a more soluble form of SCG10 which would be better suited for biochemical and structural studies, we deleted the N-terminal extension and expressed the C-terminal portion of the protein. Two forms of N-terminal-truncated SCG10 (delta SCG10 and delta SCG10r) were purified to homogeneity in a four-step purification procedure. delta SCG10 starts at amino acid 35 and delta SCG10r at amino acid 48 in the SCG10 sequence, giving proteins of 16,899 and 15,189 kDa, respectively. The truncated SCG10 was highly soluble up to concentrations of 20 mg/ml. The proteins were like the full length SCG10 substrate for serine/threonine protein kinases, including MAP kinase, PKA, and p34cdc2 kinase. With these highly soluble forms of SCG10 biochemical and structural studies of this multiphosphoprotein become feasible.

Protein kinase C in yeast. Characteristics of the Saccharomyces cerevisiae PKC1 gene product.

The Saccharomyces cerevisiae PKC1 gene encodes a homolog of mammalian protein kinase C (Levin, D. E., Fields, F.O., Kunisawa, R., Bishop, J.M., and Thorner, J. (1990) Cell 62, 213-224). A protein of 150 kDa is recognized by a polyclonal antiserum raised against a trpE-Pkc1 fusion protein. In subcellular fractionations, Pkc1p associates with the 100,000 x g particulate fraction. This association is resistant to extraction with high salt concentrations, alkali buffer, or nonionic detergents, suggesting that Pkc1p may be associated with a large protein complex. Pkc1p modified at its COOH terminus with two repeats of the Staphylococcus aureus protein A IgG-binding fragment (ZZ sequence tag) was able to fully restore the growth defects of a pkc1ts strain at restrictive temperature. ZZ-tagged Pkc1p was partially purified by chromatography on DEAE-Sepharose, followed by IgG-Sepharose. In vitro, Pkc1p phosphorylates the pseudosubstrate peptide and myelin basic protein, but not histones. Replacing an isoleucine with an arginine 2 amino acids COOH-terminal of the acceptor serine in the substrate peptide resulted in a 10-fold decrease of Km. Pkc1p activity was independent of cofactors such as phospholipids, diacylglycerol, and Ca2+, known to activate several mammalian protein kinase C isoenzymes, making it a rather distantly related member of the protein kinase C superfamily.