Mitochondrial fusion is regulated by Reaper to modulate Drosophila programmed cell death

In most multicellular organisms, the decision to undergo programmed cell death in response to cellular damage or developmental cues is typically transmitted through mitochondria. It has been suggested that an exception is the apoptotic pathway of Drosophila melanogaster, in which the role of mitochondria remains unclear. Although IAP antagonists in Drosophila such as Reaper, Hid and Grim may induce cell death without mitochondrial membrane permeabilization, it is surprising that all three localize to mitochondria. Moreover, induction of Reaper and Hid appears to result in mitochondrial fragmentation during Drosophila cell death. Most importantly, disruption of mitochondrial fission can inhibit Reaper and Hid-induced cell death, suggesting that alterations in mitochondrial dynamics can modulate cell death in fly cells. We report here that Drosophila Reaper can induce mitochondrial fragmentation by binding to and inhibiting the pro-fusion protein MFN2 and its Drosophila counterpart dMFN/Marf. Our in vitro and in vivo analyses reveal that dMFN overexpression can inhibit cell death induced by Reaper or γ-irradiation. In addition, knockdown of dMFN causes a striking loss of adult wing tissue and significant apoptosis in the developing wing discs. Our findings are consistent with a growing body of work describing a role for mitochondrial fission and fusion machinery in the decision of cells to die.     

Mechanism of action of Drosophila Reaper in mammalian cells: Reaper globally inhibits protein synthesis and induces apoptosis independent of mitochondrial permeability

Drosophila Reaper can bind inhibitor of apoptosis proteins (IAP) and thereby rescue caspases from proteasomal degradation. In insect cells, this is sufficient to induce apoptosis. Reaper can also induce apoptosis in mammalian cells, in which caspases need to be activated, usually via the mitochondrial pathway. Nevertheless, we find that Reaper efficiently induces apoptosis in mammalian cells in the absence of mitochondrial permeabilisation and cytochrome c release. Moreover, this capacity was only marginally affected by deletion of Reaper's amino-terminal IAP-binding motif. Independent of this motif, Reaper could globally suppress protein synthesis. Deletion of 20 amino acids from the carboxy-terminus of Reaper fully abrogated its potential to inhibit protein synthesis and to induce apoptosis in the absence of IAP-binding. Our findings indicate that the newly identified capacity of Reaper to suppress protein translation can operate in mammalian cells and may be key to its pro-apoptotic activity.     

A chemical strategy to promote helical peptide-protein interactions involved in apoptosis

Protein-protein interactions often involve secondary structural elements, such as helices. Protein-protein interactions within the Bcl-2 family are mediated by the helical BH3 domain of proapoptotic family members, such as Bad, Bak, Bax, or Bid. Here, we report that two 5-residue fragments located at the N- and C-termini of the 16-residue BH3 domain of Bad, respectively, serve as affinity-enhancing motifs (AEMs) for the BH3 domain. When added to the BH3 domain derived from other proapoptotic proteins such as Bak, Bax, or Bid, these AEMs significantly increased the Bcl-2-binding affinity of these BH3 peptides by promoting the helical structure. This finding may point to a new strategy for studying and mimicking helical peptide-protein interactions involved in apoptosis.     

Dissection of membrane protein degradation mechanisms by reversible inhibitors

The degradation of slowly turning over 125I-lactoperoxidase-labeled plasma membrane polypeptides in response to reversible temperature and lysosomotropic inhibitors was studied in rat hepatoma cultures. Cells were radiolabeled and left for 24 h to allow the removal of rapidly degraded proteins. Remaining trichloroacetic acid-precipitable protein was degraded (t 1/2 = 40-68 h) by an apparent first order process 60-86% sensitive to 10 mM NH4Cl or 5 mM methylamine and greater than 95% inhibited by temperature reduction to 18 degrees C. Thus, membrane proteins are selected for degradation in a time-dependent manner by a system which is sensitive to both 18 degrees C and to lysosomotropic amines. When inhibitory conditions were removed after 40-48 h, degradation of 125I-labeled protein resumed at the same rate as that seen in their absence. Since membrane proteins do not exhibit accelerated degradation after removal of inhibitory conditions, there can be no marking or sorting of those proteins destined for degradation during the 40-h exposure to inhibitory conditions. Exposure to amines or 18 degrees C did not affect the position of two-dimensionally resolved labeled polypeptides. Fractionation of labeled cells on Percoll gradients after 40 h of exposure to low temperature or amines showed that labeled protein remained in the plasma membrane fractions of the gradient although shifted to a slightly lower buoyant density in the presence of amines. These results support the notion that selection of plasma membrane proteins for degradation requires their internalization into acidic vesicles. Lysosomotropic amines and reduced temperature interfere with the selection process by preventing membrane fusion events.     

Mitochondrial dismissal in mammals,from protein degradation to mitophagy

Mitochondria are double-membraned highly dynamic organelles; the shape, location and function of which are determined by a constant balance between opposing fusion and fission events. A fine modulation of mitochondrial structure is crucial for their correct functionality and for many physiological cell processes, the status of these organelles, being thus a key aspect in a cell's fate. Indeed, the homeostasis of mitochondria needs to be highly regulated for the above mentioned reasons, and since a) they are the major source of energy; b) they participate in various signaling pathways; albeit at the same time c) they are also the major source of reactive oxygen species (ROS, the main damaging detrimental players for all cell components). Elaborate mechanisms of mitochondrial quality control have evolved for maintaining a functional mitochondrial network and avoiding cell damage. The first mechanism is the removal of damaged mitochondrial proteins within the organelle via chaperones and protease; the second is the cytosolic ubiquitin–proteasome system (UPS), able to eliminate proteins embedded in the outer mitochondrial membrane; the third is the removal of the entire mitochondria through mitophagy, in the case of extensive organelle damage and dysfunction. In this review, we provide an overview of these mitochondria stability and quality control mechanisms, highlighting mitophagy, and emphasizing the central role of mitochondrial dynamics in this context. This article is part of a Special Issue entitled: Dynamic and ultrastructure of bioenergetic membranes and their components.     

Inhibition of hepatocytic protein degradation by methylaminopurines and inhibitors of protein synthesis

Nutritional control of protein degradation in isolated rat hepatocytes can take place in the absence of protein synthesis. Suppression of degradation by amino acids (step-up) is unaffected and the enhanced degradation seen upon amino acid deprivation (step-down) is only partially inhibited by cycloheximide at a concentration (10^?3 M) which inhibits protein synthesis virtually completely. Protein degradation per se is, however, inhibited by cycloheximide as well as by puromycin, apparently at least in part by mechanisms additional or unrelated to their effect on protein synthesis. Several puromycin analogues (methylaminopurines) are stronger inhibitors of protein degradation than of protein synthesis, most notably puromycin aminonucleoside and 6-dimethylaminopurine riboside (N⁶, N⁶-dimethyladenosine). The latter compounds appear to specifically inhibit cellular autophagy, since neither the degradation of endocytosed protein (asialofetuin) nor the extralysosoma (amino acid-, propylamine- and leupeptin-resistant) degradation are affected.     

A GH3-like domain in reaper is required for mitochondrial localization and induction of IAP degradation

Reaper is a potent pro-apoptotic protein originally identified in a screen for Drosophila mutants defective in apoptotic induction. Multiple functions have been ascribed to this protein, including inhibition of IAPs (inhibitors of apoptosis); induction of IAP degradation; inhibition of protein translation; and when expressed in vertebrate cells, induction of mitochondrial cytochrome c release. Structure/function analysis of Reaper has identified an extreme N-terminal motif that appears to be sufficient for inhibition of IAP function. We report here that this domain, although required for IAP destabilization, is not sufficient. Moreover, we have identified a small region of Reaper, similar to the GH3 domain of Grim, that is required for localization of Reaper to mitochondria, induction of IAP degradation, and potent cell killing. Although a mutant Reaper protein lacking the GH3 domain was deficient in these properties, these defects could be fully rectified by appending either the C-terminal mitochondrial targeting sequence from Bcl-xL or a homologous region from the pro-apoptotic protein HID. Together, these data strongly suggest that IAP destabilization by Reaper in intact cells requires Reaper localization to mitochondria and that induction of IAP instability by Reaper is important for the potent induction of apoptosis in Drosophila cells.     

Peptide-based covalent inhibitors of protein–protein interactions

Protein–protein interactions (PPI) are involved in all cellular processes and many represent attractive therapeutic targets. However, the frequently rather flat and large interaction areas render the identification of small molecular PPI inhibitors very challenging. As an alternative, peptide interaction motifs derived from a PPI interface can serve as starting points for the development of inhibitors. However, certain proteins remain challenging targets when applying inhibitors with a competitive mode of action. For that reason, peptide-based ligands with an irreversible binding mode have gained attention in recent years. This review summarizes examples of covalent inhibitors that employ peptidic binders and have been tested in a biological context.     

Modulation of mitochondrial apoptosis by PI3K inhibitors

Most anticancer therapies exert their action by triggering programmed cell death (apoptosis) in cancer cells. The mitochondrial pathway of apoptosis is initiated by mitochondrial outer membrane permeabilization, leading to the release of apoptogenic factors such as cytochrome c or Smac from the mitochondrial intermembrane space into the cytosol. Mitochondrial outer membrane permeabilization is tightly controlled, for example by pro- and anti-apoptotic proteins of the Bcl-2 family. Recent evidence indicates that inhibition of the PI3K/Akt/mTOR pathway by small-molecule PI3K inhibitors primes cancer cells to mitochondrial apoptosis by tipping the balance towards pro-apoptotic Bcl-2 proteins, resulting in increased mitochondrial outer membrane permeabilization. Thus, mitochondrial apoptotic events play an important role in PI3K inhibitor-mediated sensitization for apoptosis.     

Kaposi's sarcoma-associated herpesvirus K7 protein targets a ubiquitin-like/ubiquitin-associated domain-containing protein to promote protein degradation

Pathogens exploit host machinery to establish an environment that favors their propagation. Because of their pivotal roles in cellular physiology, protein degradation pathways are common targets for viral proteins. Protein-linking integrin-associated protein and cytoskeleton 1 (PLIC1), also called ubiquilin, contains an amino-terminal ubiquitin-like (UBL) domain and a carboxy-terminal ubiquitin-associated (UBA) domain. PLIC1 is proposed to function as a regulator of the ubiquitination complex and proteasome machinery. Kaposi's sarcoma-associated herpesvirus (KSHV) contains a small membrane protein, K7, that protects cells from apoptosis induced by various stimuli. We report here that cellular PLIC1 is a K7-interacting protein and that the central hydrophobic region of K7 and the carboxy-terminal UBA domain of PLIC1 are responsible for their interaction. Cellular PLIC1 formed a dimer and bound efficiently to polyubiquitinated proteins through its carboxy-terminal UBA domain, and this activity correlated with its ability to stabilize cellular I kappa B protein. In contrast, K7 interaction prevented PLIC1 from forming a dimer and binding to polyubiquitinated proteins, leading to the rapid degradation of I kappa B. Furthermore, K7 expression promoted efficient degradation of the p53 tumor suppressor, resulting in inhibition of p53-mediated apoptosis. These results indicate that KSHV K7 targets a regulator of the ubiquitin- and proteasome-mediated degradation machinery to deregulate cellular protein turnover, which potentially provides a favorable environment for viral reproduction.     

Natural product inhibitors of protein–protein interactions mediated by Src-family SH2 domains

In this Letter, we report the natural products salvianolic acid A, salvianolic acid B, and caftaric acid as inhibitors of the protein–protein interactions mediated by the SH2 domains of the Src-family kinases Src and Lck, two established disease targets. Moreover, we propose a binding mode for the inhibitors based on molecular modeling, which will facilitate chemical optimization efforts of these important lead structures for drug discovery.     

Involvement of histone phosphorylation in thymocyte apoptosis by protein phosphatase inhibitors

Incubation of rat thymocytes with the inhibitors of protein phosphatase such as calyculin A and okadaic acid resulted in an increase in DNA fragmentation. These effects were dependent on the concentration of the inhibitors and the incubation time. Analyses of the fragmented DNA revealed the production of approximately 50 kbp of DNA and a 180 bp DNA ladder. In addition, a laser scanning-microscopic analysis showed that these compounds caused nuclear condensation. Thus, these results demonstrated that protein phosphatase inhibitors induced thymocyte apoptosis. The inhibitors of protein phosphatase increased the phosphorylation of proteins of approximately 15 kDa. The phosphorylation of proteins preceded the DNA fragmentation induced by these inhibitors. Judging from acetic acid-urea-Triton X-100 gel electrophoresis, the phosphorylated proteins were histone H1 and H2A/H3. Therefore, these results suggest that phosphorylation of histones triggers the DNA fragmentation of thymocytes undergoing apoptosis.     

Cathepsin D-mediated yolk protein degradation is blocked by acid phosphatase inhibitors

Vitellin (VT) is a lipoglycophosphoprotein stored inside the eggs of every oviparous organism during oogenesis. In the blood-sucking bug Rhodnius prolixus, VT is deposited inside growing oocytes together with two acid hydrolases: acid phosphatase (AP) and cathepsin D (CD). Egg fertilization triggers AP activity and VT proteolysis in vivo [Insect Biochem. Mol. Biol. 2002 (32) 847]. Here, we show that CD is the main protease targeting VT proteolysis during egg development. CD activity in total egg homogenates is blocked by the classical aspartyl protease inhibitor, pepstatin A. Surprisingly, AP inhibitors such as NaF, Na+/K+ tartrate, and inorganic phosphate also block VT proteolysis, whereas this effect is not observed when tyrosine phosphatase inhibitors such as vanadate and phenylarsine oxide or an inhibitor of alkaline phosphatases such as levamisole are used in a VT proteolysis assay. NaF concentrations that block isolated AP activity do not affect the activity of partially purified CD. Therefore, a specific repressor of VT proteolysis must be dephosphorylated by AP in vivo. In conclusion, these results demonstrate for the first time that acid hydrolases act cooperatively to promote yolk degradation during egg development in arthropods.     

Coronin7 forms a novel E3 ubiquitin ligase complex to promote the degradation of the anti-proliferative protein Tob

Tob belongs to the anti-proliferative Tob/BTG family. The level of Tob throughout the cell cycle is regulated by the SCF (Skp1/Cullin/F-box protein)^Skp2 ubiquitin ligase (E3) complex. Here, we show that Coronin7 (CRN7) is also involved in Tob degradation. We identified CRN7 as a Tob-interacting molecule. A sequence containing two of the six WD motifs in the middle of CRN7 was responsible for the interaction. CRN7 enhanced the polyubiquitination of Tob in vitro, and overexpression of CRN7 promoted proteasome-dependent degradation of Tob. Furthermore, CRN7 interacted with Cullin1 and Roc1 to form a novel SCF-like E3 complex, suggesting that Tob protein is regulated by multiple ubiquitination machineries.

Structured summary

Cullin1physically interacts with CRN7: shown by anti tag coimmunoprecipitation (view interaction)Roc1physically interacts with CRN7: shown by anti tag coimmunoprecipitation (view interaction)CRN7physically interacts with Tob1: shown by anti tag coimmunoprecipitation (view interaction)CDC34physically interacts with CRN7: shown by anti tag coimmunoprecipitation (view interaction)Tob1 and CRN7colocalize: shown by fluorescence microscopy (view interaction)Elongin Bphysically interacts with CRN7: shown by anti tag coimmunoprecipitation (view interaction)Elongin Cphysically interacts with CRN7: shown by anti tag coimmunoprecipitation (view interaction)Tob1physically interacts with CRN7: shown by two hybrid (view interaction)     

Caveolin-1 sensitizes rat pituitary adenoma GH3 cells to bromocriptine induced apoptosis

Background  

Prolactinoma is the most frequent pituitary tumor in humans. The dopamine D₂ receptor agonist bromocriptine has been widely used clinically to treat human breast tumor and prolactinoma through inhibition of hyperprolactinemia and induction of tumor cell apoptosis, respectively, but the molecular mechanism of bromocriptine induction of pituitary tumor apoptosis remains unclear. Caveolin-1 is a membrane-anchored protein enriched on caveolae, inverted flask-shaped invaginations on plasma membranes where signal transduction molecules are concentrated. Currently, caveolin-1 is thought to be a negative regulator of cellular proliferation and an enhancer of apoptosis by blocking signal transduction between cell surface membrane receptors and intracellular signaling protein cascades. Rat pituitary adenoma GH3 cells, which express endogenous caveolin-1, exhibit increased apoptosis and shrinkage after exposure to bromocriptine. Hence, the GH3 cell line is an ideal model for studying the molecular action of bromocriptine on prolactinoma.     

Intracellular antibody capture: A molecular biology approach to inhibitors of protein–protein interactions

Many proteins of interest in basic biology, translational research studies and for clinical targeting in diseases reside inside the cell and function by interacting with other macromolecules. Protein complexes control basic processes such as development and cell division but also abnormal cell growth when mutations occur such as found in cancer. Interfering with protein–protein interactions is an important aspiration in both basic and disease biology but small molecule inhibitors have been difficult and expensive to isolate. Recently, we have adapted molecular biology techniques to develop a simple set of protocols for isolation of high affinity antibody fragments (in the form of single VH domains) that function within the reducing environment of higher organism cells and can bind to their target molecules. The method called Intracellular Antibody Capture (IAC) has been used to develop inhibitory anti-RAS and anti-LMO2 single domains that have been used for target validation of these antigens in pre-clinical cancer models and illustrate the efficacy of the IAC approach to generation of drug surrogates. Future use of inhibitory VH antibody fragments as drugs in their own right (we term these macrodrugs to distinguish them from small molecule drugs) requires their delivery to target cells in vivo but they can also be templates for small molecule drug development that emulate the binding sites of the antibody fragments. This article is part of a Special Issue entitled: Recent advances in molecular engineering of antibody.