Bax and Bak coalesce into novel mitochondria-associated clusters during apoptosis.

Bax is a member of the Bcl-2 family of proteins known to regulate mitochondria-dependent programmed cell death. Early in apoptosis, Bax translocates from the cytosol to the mitochondrial membrane. We have identified by confocal and electron microscopy a novel step in the Bax proapoptotic mechanism immediately subsequent to mitochondrial translocation. Bax leaves the mitochondrial membranes and coalesces into large clusters containing thousands of Bax molecules that remain adjacent to mitochondria. Bak, a close homologue of Bax, colocalizes in these apoptotic clusters in contrast to other family members, Bid and Bad, which circumscribe the outer mitochondrial membrane throughout cell death progression. We found the formation of Bax and Bak apoptotic clusters to be caspase independent and inhibited completely and specifically by Bcl-X(L), correlating cluster formation with cytotoxic activity. Our results reveal the importance of a novel structure formed by certain Bcl-2 family members during the process of cell death.

Acidic dopamine metabolites are actively extruded from PC12 cells by a novel sulfonylurea-sensitive transporter.

Incubation of PC 12 cells with the sulfonylurea drug, glipizide (1-100 microM), increased intracellular levels of the acidic metabolites of dopamine, 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA). The levels of these acids in the medium were decreased, indicating the presence of a sulfonylurea-sensitive organic anion transporter. In the present study, we demonstrate that the sulfonylurea-sensitive transport of acidic dopamine metabolites is unidirectional, ATP dependent, unaffected by ouabain or by tetrodotoxin and blocked by drugs that interact with the multidrug-resistance protein-1 (MRP1). However, over-expression of MRP1 did not affect transport of the acid metabolites. The pharmacological profile and ion dependence of the transporter also differs from that of known ATP-binding cassette (ABC) family members. Using microdialysis, we also demonstrated a sulfonylurea-sensitive transport process in the striatum of freely moving rats. These results show that acidic dopamine metabolites are actively secreted from dopaminergic cells into surrounding extracellular fluid by a previously undescribed transporter.

Effect of glipizide on dopamine synthesis, release and metabolism in PC12 cells.

Sulfonylureas block ATP-dependent K(+) channels (K/ATP channels) in pancreatic beta cells and brain gamma-aminobutyric acid (GABA) containing neurons causing depolarization-evoked insulin or GABA release. In high concentrations, sulfonylureas also inhibit catecholamine release from bovine adrenal chromaffin cells and isolated guinea pig aorta. In this study, we examined the effect of glipizide, a sulfonylurea, on dopamine release from PC12 cells and found that neither basal nor K(+)-stimulated dopamine release was affected. Although PC12 cells expressed mRNA for the K/ATP channel, functional K/ATP channels could not be demonstrated electrophysiologically, consistent with the lack of effect of glipizide on dopamine release. Glipizide did, however, increase cytoplasmic retention of the acidic dopamine metabolites, 3, 4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA), indicating blockade of their outward transport. The cellular accumulation of DOPAC was accompanied by reduced tyrosine hydroxylase activity and reduced formation of dopamine and its metabolites presumably by a negative feedback effect of the increased cytoplasmic concentrations of DOPAC.

Linker-based GnRH-PE chimeric proteins inhibit cancer growth in nude mice.

Since the number of cancer-related deaths has not decreased in recent years, major efforts are being made to find new drugs for cancer treatment. In this report we introduce the gonadotropin releasing hormone-Pseudomonas exotoxin (GnRH-PE) based chimeric proteins L-GnRH-PE66 and L-GnRH-PE40. These proteins are composed of a GnRH moiety attached to modified forms of Pseudomonas exotoxin via a polylinker (gly4ser)2. The chimeric proteins L-GnRH-PE66 and L-GnRH-PE40 have the ability to target and kill adenocarcinoma cell lines in vitro, whereas non-adenocarcinoma cell lines are not affected. We demonstrate that L-GnRH-PE66 and L-GnRH-PE40 efficiently inhibit cancer growth. Nude mice were injected subcutaneously with the SW-48 adenocarcinoma cell line to produce xenograft tumours. When the tumours were established and visible, the animals were injected with chimeric proteins for 10 days. At the end of this period, a reduction of up to 3-fold in tumor size was obtained in the treated mice, as compared with the control group, which received equivalent amounts of GnRH; the difference was even greater 13 days after termination of treatment. Thus, the chimeric proteins L-GnRH-PE66 and L-GnRH-PE40 are promising candidates for treatment of a variety of adenocarcinomas and their use in humans should be considered.

Bax, but not Bcl-xL, decreases the lifetime of planar phospholipid bilayer membranes at subnanomolar concentrations.

Release of proteins through the outer mitochondrial membrane can be a critical step in apoptosis, and the localization of apoptosis-regulating Bcl-2 family members there suggests they control this process. We used planar phospholipid membranes to test the effect of full-length Bax and Bcl-xL synthesized in vitro and native Bax purified from bovine thymocytes. Instead of forming pores with reproducible conductance levels expected for ionic channels, Bax, but not Bcl-xL, created arbitrary and continuously variable changes in membrane permeability and decreased the stability of the membrane, regardless of whether the source of the protein was synthetic or native. This breakdown of the membrane permeability barrier and destabilization of the bilayer was quantified by using membrane lifetime measurements. Bax decreased membrane lifetime in a voltage- and concentration-dependent manner. Bcl-xL did not protect against Bax-induced membrane destabilization, supporting the idea that these two proteins function independently. Corresponding to a physical theory for lipidic pore formation, Bax potently diminished the linear tension of the membrane (i.e., the energy required to form the edge of a new pore). We suggest that Bax acts directly by destabilizing the lipid bilayer structure of the outer mitochondrial membrane, promoting the formation of a pore-the apoptotic pore-large enough to allow mitochondrial proteins such as cytochrome c to be released into the cytosol. Bax could then enter and permeabilize the inner mitochondrial membrane through the same hole.

Conformation of the Bax C-terminus regulates subcellular location and cell death.

Bax, a pro-apoptotic member of the Bcl-2 family, translocates from the cytosol to the mitochondria during programmed cell death. We report here that both gain-of-function and loss-of-function mutations can be achieved by altering a single amino acid in the Bax hydrophobic C-terminus. The properly mutated C-terminus of Bax can target a non-relevant protein to the mitochondria, showing that specific conformations of this domain alone allow mitochondrial docking. These data along with N-terminus epitope exposure experiments suggest that the C- and the N-termini interact and that upon triggering of apoptosis, Bax changes conformation, exposing these two domains to insert into the mitochondria and regulate the cell death machinery.

Adenocarcinoma cells are targeted by the new GnRH-PE66 chimeric toxin through specific gonadotropin-releasing hormone binding sites.

Luteinizing hormone-releasing hormone, also termed gonadotropin-releasing hormone (GnRH), accounts for the hypothalamic-pituitary gonadal control of human reproduction. The involvement of GnRH has been demonstrated in several carcinomas of hormone-responsive tissues. Exploiting this common feature, we constructed a Pseudomonas exotoxin (PE)-based chimeric toxin (GnRH-PE66) aimed at targeting those cancer cells bearing GnRH binding sites. We report here the strong growth inhibition and killing of a surprisingly wide variety of cancers, confined to the adenocarcinoma type. These cancer cells arising from hormone-responsive tissues, as well as non-responsive ones, express specific GnRH binding sites as indicated by the marked killing of ovarian, breast, endometrial, cervical, colon, lung, hepatic, and renal adenocarcinoma. This cytotoxicity is specific as it could be blocked upon addition of excess GnRH. The specificity of GnRH-PE66 chimeric toxin was also confirmed by GnRH binding assays, and its ability to prevent the formation of colon cancer xenografts in nude mice is presented. Although the functional role of specific GnRH binding sites in human carcinomas remains obscure, GnRH-PE66 displays considerable targeting potential and its use as a therapeutic agent for cancer should be considered.

Movement of Bax from the cytosol to mitochondria during apoptosis.

Bax, a member of the Bcl-2 protein family, accelerates apoptosis by an unknown mechanism. Bax has been recently reported to be an integral membrane protein associated with organelles or bound to organelles by Bcl-2 or a soluble protein found in the cytosol. To explore Bcl-2 family member localization in living cells, the green fluorescent protein (GFP) was fused to the NH2 termini of Bax, Bcl-2, and Bcl-XL. Confocal microscopy performed on living Cos-7 kidney epithelial cells and L929 fibroblasts revealed that GFP-Bcl-2 and GFP-Bcl-XL had a punctate distribution and colocalized with a mitochondrial marker, whereas GFP-Bax was found diffusely throughout the cytosol. Photobleaching analysis confirmed that GFP-Bax is a soluble protein, in contrast to organelle-bound GFP-Bcl-2. The diffuse localization of GFP-Bax did not change with coexpression of high levels of Bcl-2 or Bcl-XL. However, upon induction of apoptosis, GFP-Bax moved intracellularly to a punctate distribution that partially colocalized with mitochondria. Once initiated, this Bax movement was complete within 30 min, before cellular shrinkage or nuclear condensation. Removal of a COOH-terminal hydrophobic domain from GFP-Bax inhibited redistribution during apoptosis and inhibited the death-promoting activity of both Bax and GFP-Bax. These results demonstrate that in cells undergoing apoptosis, an early, dramatic change occurs in the intracellular localization of Bax, and this redistribution of soluble Bax to organelles appears important for Bax to promote cell death.

ABL and BCR genes are not imprinted in androgenetic and gynogenetic human tissues.

In the translocation leading to the formation of the Philadelphia chromosome, the hallmark of chronic myeloid leukemia (CML), the translocated chromosome 9 (ABL), is of paternal descent whereas chromosome 22 (BCR) is of maternal origin (1). To study possible imprinting of the human ABL and BCR genes, we used human tissues exclusively endowed with their maternally (benign teratoma) or paternally (complete hydatidiform mole) inherited chromosomes. Using the sensitive PCR technique followed by northern blotting, we demonstrate here that ABL and BCR are expressed to a similar extent in androgenetic and gynogenetic human tissues, thus suggesting that ABL and BCR genes are not imprinted in these human tissues.