In this study, we report a novel mechanism of action for a cytotoxic derivative of betulinic acid (BA). B10 is a semi-synthetic glycosylated derivative of BA selected for its enhanced cytotoxic activity. Interestingly, although B10 induces apoptosis, caspase-3 downregulation incompletely prevents B10-induced cell death, Bcl-2 overexpression fails to protect cells and DNA fragmentation rates do not reflect cell death rates in contrast to cytoplasmic membrane permeabilization. These results implicate that apoptotic and non-apoptotic cell death coexist upon B10 treatment. Unexpectedly, we found that B10 induces autophagy and also abrogates the autophagic flux. B10 destabilizes lysosomes as shown by Lysotracker Red staining and by cathepsin Z and B release from lysosomes into the cytoplasm. Consistently, the cathepsin inhibitor Ca074Me significantly decreases B10-induced cell death, further supporting the fact that the release of lysosomal enzymes contributes to B10-triggered cell death. Downregulation of ATG7, ATG5 or BECN1 by RNAi significantly decreases caspase-3 activation, lysosomal permeabilization and cell death. Thus, by concomitant induction of autophagy and inhibition of the autophagic flux, B10 turns autophagy into a cell death mechanism. These findings have important implications for the therapeutic exploitation of BA derivatives, particularly in apoptosis-resistant cancers. Bioactive natural compounds or their semi-synthetic derivatives offer a considerable therapeutic potential against cancer.1 Among these, betulinic acid (BA), a triterpenoid initially derived from white birch tree, has received attention because of its multiple biological activities in mammalian cells. The pluripotency of BA is of great interest, as this compound is active against HIV, parasitic diseases and cancer.2-5 Nevertheless, the therapeutic application of this drug is currently limited because of its poor solubility in aqueous solvents and an ill-defined mode of action. To overcome these limitations, a wide variety of semi-synthetic derivatives of BA has been generated to improve its pharmacological properties for in vivo studies. In addition, detailed studies are being performed to elucidate the mechanisms of action underlying their cytotoxic activity. To date, BA and its derivatives have been reported to induce cell death in cancer cells via the activation of the intrinsic pathway of apoptosis.6 Accordingly, BA triggers the release of pro-apoptotic factors, such as cytochrome-c, from the mitochondria supposedly by interacting with the permeability pore transition complex and by modifying the balance of pro-and anti-apoptotic proteins of the Bcl-2 protein family. 6 In addition, BA and its derivatives appear to interfere with multiple signaling pathways involved in survival. PI3K, NF-kB and lipid metabolism are modulated by BA-related compounds, but whether and how these effects contribute to cell death is unknown.7-10 A better understanding of the mechanism of action of these compounds is critical to promote their therapeutic...

Cancer stem cells (CSC) have been postulated to be responsible for the key features of a malignancy and its maintenances, as well as therapy resistance, while differentiated cells are believed to make up the rapidly growing tumour bulk. It is therefore important to understand the characteristics of those two distinct cell populations in order to devise treatment strategies which effectively target both cohorts, in particular with respect to cancers, such as glioblastoma. Glioblastoma is the most common primary brain tumour in adults, with a mean patient survival of 12-15 months. Importantly, therapeutic improvements have not been forthcoming in the last decade. In this study we compare key features of three pairs of glioblastoma cell populations, each pair consisting of stem cell-like and differentiated cells derived from an individual patient. Our data suggest that while growth rates and expression of key survival-and apoptosis-mediating proteins are more similar according to differentiation status than genetic similarity, we found no intrinsic differences in response to standard therapeutic interventions, namely exposure to radiation or the alkylating agent temozolomide. Interestingly, we could demonstrate that both stem celllike and differentiated cells possess the ability to form stem cell-containing tumours in immunocompromised mice and that differentiated cells could potentially be dedifferentiated to potential stem cells. Taken together our data suggest that the differences between tumour stem cell and differentiated cell are particular fluent in glioblastoma.

Members of the PI3K/Akt/mTor signaling cascade are among the most frequently altered proteins in cancer, yet the therapeutic application of pharmacological inhibitors of this signaling network, either as monotherapy or in combination therapy (CT) has so far not been particularly successful. In this review we will focus on the role of PI3K/Akt/mTOR in two distinct tumors, Glioblastoma multiforme (GBM), an adult brain tumor which frequently exhibits PTEN inactivation, and Neuroblastoma (NB), a childhood malignancy that affects the central nervous system and does not harbor any classic alterations in PI3K/Akt signaling. We will argue that inhibitors of PI3K/Akt signaling can be components for potentially promising new CTs in both tumor entities, but further understanding of the signal cascade’s complexity is essential for successful implementation of these CTs. Importantly, failure to do this might lead to severe adverse effects, such as treatment failure and enhanced therapy resistance.

Breaking resistance to chemotherapy is a major goal of combination therapy in many tumors, including advanced neuroblastoma. We recently demonstrated that increased activity of the PI3K/Akt network is associated with poor prognosis, thus providing an ideal target for chemosensitization. Here we show that targeted therapy using the PI3K/mTOR inhibitor NVP-BEZ235 significantly enhances doxorubicin-induced apoptosis in neuroblastoma cells. Importantly, this increase in apoptosis was dependent on scheduling: while pretreatment with the inhibitor reduced doxorubicin-induced apoptosis, the sensitizing effect in co-treatment could further be increased by delayed addition of the inhibitor post chemotherapy. Desensitization for doxorubicin-induced apoptosis seemed to be mediated by a combination of cell cycle-arrest and autophagy induction, whereas sensitization was found to occur at the level of mitochondria within one hour of NVP-BEZ235 posttreatment, leading to a rapid loss of mitochondrial membrane potential with subsequent cytochrome c release and caspase-3 activation. Within the relevant time span we observed marked alterations in a ∼30 kDa protein associated with mitochondrial proteins and identified it as VDAC1/Porin protein, an integral part of the mitochondrial permeability transition pore complex. VDAC1 is negatively regulated by the PI3K/Akt pathway via GSK3β and inhibition of GSK3β, which is activated when Akt is blocked, ablated the sensitizing effect of NVP-BEZ235 posttreatment. Our findings show that cancer cells can be sensitized for chemotherapy induced cell death – at least in part – by NVP-BEZ235-mediated modulation of VDAC1. More generally, we show data that suggest that sequential dosing, in particular when multiple inhibitors of a single pathway are used in the optimal sequence, has important implications for the general design of combination therapies involving molecular targeted approaches towards the PI3K/Akt/mTOR signaling network.

The binary C2 toxin from Clostridium botulinum mono-ADP-ribosylates G-actin in the cytosol of eukaryotic cells. This modification leads to depolymerization of actin filaments accompanied by cell rounding within 3 h of incubation but does not immediately induce cell death. Here we investigated the long-term responses of mammalian cell lines (HeLa and Vero) following C2 toxin treatment. Cells stayed round even though the toxin was removed from the medium after its internalization into the cells. No unmodified actin reappeared in the C2 toxin-treated cells within 48 h. Despite actin being completely ADP-ribosylated after about 7 h, no obvious decrease in the overall amount of actin was observed for at least 48 h. Therefore, ADP-ribosylation was not a signal for an accelerated degradation of actin in the tested cell lines. C2 toxin treatment resulted in delayed apoptotic cell death that became detectable about 15 to 24 h after toxin application in a portion of the cells. Poly(ADP)-ribosyltransferase 1 (PARP-1) was cleaved in C2 toxin-treated cells, an indication of caspase 3 activation and a hallmark of apoptosis. Furthermore, specific caspase inhibitors prevented C2 toxin-induced apoptosis, implying that caspases 8 and 9 were activated in C2 toxin-treated cells. C2I, the ADP-ribosyltransferase component of the C2 toxin, remained active in the cytosol for at least 48 h, and no extensive degradation of C2I was observed. From our data, we conclude that the long-lived nature of C2I in the host cell cytosol was essential for the nonreversible cytotoxic effect of C2 toxin, resulting in delayed apoptosis of the tested mammalian cells.Clostridium botulinum C2 toxin, the prototype of the family of binary actin ADP-ribosylating toxins, mono-ADP-ribosylates G-actin at Arg-177 (1). C2 toxin consists of the enzyme component C2I (431 amino acid residues, 49.3 kDa) and the binding/translocation component C2II (721 amino acid residues, 80.8 kDa). The ADP-ribosyltransferase activity of C2I is located in its C-terminal domain (5), while the enzymatically inactive N-terminal domain (C2IN, amino acid residues 1 to 225) mediates interaction with C2IIa and translocation of C2I into the cytosol (6). Following limited proteolysis, the active C2IIa protein (ϳ60 kDa) forms ring-shaped heptamers that assemble with C2I and mediate binding of the toxin complex to the cellular receptor, a carbohydrate structure (2, 11, 19). During cellular uptake, the heptamers form pores in endosomal membranes, thereby facilitating translocation of C2I into the cytosol (7). Translocation of C2I requires unfolding (14) and subsequent refolding of the protein in the cytosol by the host cell chaperone Hsp90 (13).In the cytosol, C2I catalyzes covalent transfer of the ADPribose moiety from NAD (NAD ϩ ) to Arg-177 of G-actin (1). Mono-ADP-ribosylation of actin induces the depolymerization of F-actin and thereby a complete loss of actin filaments, resulting in rounding of cultured monolayer cells (1). The ADPribosylation at Arg-177 turns G-actin monomers into "cappin...