Federica Di Sano

Endoplasmic Reticulum Stress Induces Apoptosis by an Apoptosome-dependent but Caspase 12-independent Mechanism

The endoplasmic reticulum (ER) is the cellular site of polypeptide folding and modification. When these processes are hampered, an unfolded protein response (UPR) is activated. If the damage is too broad, the mammalian UPR launches the apoptotic program. As a consequence, mobilization of ER calcium stores sensitizes mitochondria to direct proapoptotic stimuli. We make use of a mouse Apaf1-deficient cell system of proneural origin to understand the roles played in this context by the apoptosome, the most studied apoptotic machinery along the mitochondrial pathway of death. We show here that in the absence of the apoptosome ER stress induces cytochrome c release from the mitochondria but that apoptosis cannot occur. Under these circumstances, Grp78/BiP and GADD153/CHOP, both hallmarks of UPR, are canonically up-regulated, and calcium is properly released from ER stores. We also demonstrate that caspase 12, a protease until now believed to play a central role in the initiation of ER stress-induced cell death in the mouse system, is dispensable for the mitochondrial pathway of death to take place.

Cell death and autophagy: Cytokines, drugs, and nutritional factors

Cells may use multiple pathways to commit suicide. In certain contexts, dying cells generate large amounts of autophagic vacuoles and clear large proportions of their cytoplasm, before they finally die, as exemplified by the treatment of human mammary carcinoma cells with the anti-estrogen tamoxifen (TAM, < or = 1 microM). Protein analysis during autophagic cell death revealed distinct proteins of the nuclear fraction including GST-pi and some proteasomal subunit constituents to be affected during autophagic cell death. Depending on the functional status of caspase-3, MCF-7 cells may switch between autophagic and apoptotic features of cell death [Fazi, B., Bursch, W., Fimia, G.M., Nardacci R., Piacentini, M., Di Sano, F., Piredda, L., 2008. Fenretinide induces autophagic cell death in caspase-defective breast cancer cells. Autophagy 4(4), 435-441]. Furthermore, the self-destruction of MCF-7 cells was found to be completed by phagocytosis of cell residues [Petrovski, G., Zahuczky, G., Katona, K., Vereb, G., Martinet, W., Nemes, Z., Bursch, W., Fésüs, L., 2007. Clearance of dying autophagic cells of different origin by professional and non-professional phagocytes. Cell Death Diff. 14 (6), 1117-1128]. Autophagy also constitutes a cells strategy of defense upon cell damage by eliminating damaged bulk proteins/organelles. This biological condition may be exemplified by the treatment of MCF-7 cells with a necrogenic TAM-dose (10 microM), resulting in the lysis of almost all cells within 24h. However, a transient (1h) challenge of MCF-7 cells with the same dose allowed the recovery of cells involving autophagy. Enrichment of chaperones in the insoluble cytoplasmic protein fraction indicated the formation of aggresomes, a potential trigger for autophagy. In a further experimental model HL60 cells were treated with TAM, causing dose-dependent distinct responses: 1-5 microM TAM, autophagy predominant; 7-9 microM, apoptosis predominant; 15 microM, necrosis. These phenomena might be attributed to the degree of cell damage caused by tamoxifen, either by generating ROS, increasing membrane fluidity or forming DNA-adducts. Finally, autophagy constitutes a cells major adaptive (survival) strategy in response to metabolic challenges such as glucose or amino acid deprivation, or starvation in general. Notably, the role of autophagy appears not to be restricted to nutrient recycling in order to maintain energy supply of cells and to adapt cell(organ) size to given physiological needs. For instance, using a newly established hepatoma cell line HCC-1.2, amino acid and glucose deprivation revealed a pro-apoptotic activity, additive to TGF-beta1. The pro-apoptotic action of glucose deprivation was antagonized by 2-deoxyglucose, possibly by stabilizing the mitochondrial membrane involving the action of hexokinase II. These observations suggest that signaling cascades steering autophagy appear to provide links to those regulating cell number. Taken together, our data exemplify that a given cell may flexibly respond to type and degree of (micro)environmental changes or cell death stimuli; a cells response may shift gradually from the elimination of damaged proteins by autophagy and the recovery to autophagic or apoptotic pathways of cell death, the failure of which eventually may result in necrosis.

Fenretinide induces autophagic cell death in caspase-defective breast cancer cells

The elimination of tumour cells by apoptosis is the main mechanism of action of chemotherapeutic drugs. More recently, autophagic cell death has been shown to trigger a nonapoptotic cell death program in cancer cells displying functional defects of caspases. Fenretinide (FenR), a synthetic derivative of retinoic acid, promotes growth inhibition and induces apoptosis in a wide range of tumour cell types. The present study was designed to evaluate the ability of fenretinide to induce caspase-independent cell death and to this aim we used the human mammary carcinoma cell line MCF-7, lacking functional caspase-3 activity. We demonstrated that in these cells fenretinide is able to trigger an autophagic cell death pathway. In particular we found that fenretinide treatment resulted in the increase in Beclin 1 expression, the conversion of the soluble form of LC3 to the autophagic vesicle-associated form LC3-II and its shift from diffuse to punctate staining and finally the increase in lysosomes/autophagosomes. By contrast, caspase-3 reconstituted MCF-7 cell line showed apoptotic cell death features in response to fenretinide treatment. These data strongly suggest that fenretinide does not invariably elicit an apoptotic response but it is able to induce autophagy when apoptotic pathway is deregulated. The understanding of the molecular mechanisms involved in fenretinide action is important for the future design of therapies employing this retinoid in breast cancer treatment.

Reticulon-1C acts as a molecular switch between endoplasmic reticulum stress and genotoxic cell death pathway in human neuroblastoma cells

Damage or stress in many organelles may trigger apoptosis by several not yet fully elucidated mechanisms. A cell death pathway is induced by endoplasmic reticulum (ER) stress elicited by the unfolded protein response and/or by aberrant Ca2+ signalling. Reticulon‐1C (RTN‐1C) belongs to the reticulon family, neuroendocrine‐specific proteins localized primarily on the ER membrane. In the present study, we demonstrate that RTN‐1C is able to modulate, in a mutually exclusive way, the cellular sensitivity to different apoptosis pathways in human neuroblastoma cells. In fact, the increase of RTN‐1C protein levels per se results in ER stress‐induced cell death, mediated by an increase of cytosolic Ca2+, and significantly sensitizes cells to different ER stress inducers. In line with these findings, the reduction of RTN‐1C, by antisense DNA expression, reduced the sensitivity to ER‐stressors. In the presence of high RTN‐1C levels, genotoxic drugs become ineffective as a consequence of the cytoplasm translocation of p53 protein, while the silencing of endogenous RTN‐1C results in the potentiation of the genotoxic drugs action. These data indicate that RTN‐1C is able to modulate the cellular sensitivity to different apoptotic pathways representing a promising molecular target for new drug development.

Apoptosis induced by doxorubicin in neurotumor cells is divorced from drug effects on ceramide accumulation and may involve cell cycle-dependent caspase activation

Doxorubicin (0.5 μg/ml) induced caspase‐dependent apoptosis in SH‐SY5Y neuroblastoma and CHP‐100 neuroepithelioma cells. The apoptotic response started to be evident ~15 h after drug administration and, as monitored over a 48‐h period, was more pronounced in CHP‐100 than in SH‐SY5Y cells. In both systems, apoptosis was accompanied by elevation of intracellular ceramide levels. Ceramide accumulation was blocked by the ceramide synthase inhibitor fumonisin B1 (25 μM) ; this compound, however, did not prevent drug‐induced apoptosis. Untreated cells from both lines expressed negligible p53 levels ; on the other hand, whereas p53 and p21Cip1/Waf1 were rapidly up‐regulated in doxorubicin‐treated SH‐SY5Y cells, such a response was not observed in CHP‐100 cells. Doxorubicin induced a G2/M phase block in both cell lines, but whereas the G1 phase was markedly depleted in CHP‐100 cells, it was substantially retained in SH‐SY5Y cells. In the latter system, double G1 and G2/M block largely preceded cell death ; however, as apoptosis underwent completion, it selectively targeted late S and G2/M cells. Moreover, apoptosis suppression by caspase inhibition did not result in a recovery of the G1 cell population. These results support the notion that doxorubicin‐induced apoptosis and ceramide elevation are divorced events in neuroectodermal tumors and that p53 function is at least dispensable for apoptosis completion. Indeed, as G1 cells appear to be refractory to doxorubicin‐induced apoptosis, p53 up‐regulation and p21Cip1/Waf1 expression may provide an unfavorable setting for the apoptotic action of the drug.

Calpain involvement in calphostin C-induced apoptosis

A major problem in assessing the role of calpains in apoptosis induction concerns the fact that calpain inhibitors can also impair the activity of the proteasome, also reported to be involved in apoptosis. Herein we showed that apoptosis induced by calphostin C in U937 human promonocytic leukemia cells was associated, at its onset, with enhanced protein (poly)ubiquitination. This observation prompted us to study whether protein degradation through the ubiquitin/proteasome pathway was involved in apoptosis induction. We found that N-acetyl-Leu-Leu-norleucinal (50 microM), a proteasome as well as a calpain inhibitor, was able to reduce calphostin C-induced apoptosis by approximately 60%, whereas lactacystin (10 microM), a specific proteasome inhibitor, was ineffective. These results suggest that calphostin C-induced apoptosis is partly calpain-mediated, but does not require protein degradation through the ubiquitin/proteasome pathway.

The reticulons: Guardians of the structure and function of the endoplasmic reticulum

The endoplasmic reticulum (ER) consists of the nuclear envelope and a peripheral network of tubules and membrane sheets. The tubules are shaped by a specific class of curvature stabilizing proteins, the reticulons and DP1; however it is still unclear how the sheets are assembled. The ER is the cellular compartment responsible for secretory and membrane protein synthesis. The reducing conditions of ER lead to the intra/inter-chain formation of new disulphide bonds into polypeptides during protein folding assessed by enzymatic or spontaneous reactions. Moreover, ER represents the main intracellular calcium storage site and it plays an important role in calcium signaling that impacts many cellular processes. Accordingly, the maintenance of ER function represents an essential condition for the cell, and ER morphology constitutes an important prerogative of it. Furthermore, it is well known that ER undergoes prominent shape transitions during events such as cell division and differentiation. Thus, maintaining the correct ER structure is an essential feature for cellular physiology. Now, it is known that proper ER-associated proteins play a fundamental role in ER tubules formation. Among these ER-shaping proteins are the reticulons (RTN), which are acquiring a relevant position. In fact, beyond the structural role of reticulons, in very recent years new and deeper functional implications of these proteins are emerging in relation to their involvement in several cellular processes.

Ceramide accumulation precedes caspase-dependent apoptosis in CHP-100 neuroepithelioma cells exposed to the protein phosphatase inhibitor okadaic acid

The protein phosphatase inhibitor okadaic acid (OA) dose-dependently induced apoptosis in CHP-100 neuroepithelioma cells when administered for 24 h at concentrations ranging from 10–100 nM. Apoptosis was largely, albeit not completely, dependent on cystein protease (caspase) activation. CPP32 processing and poly(ADP-ribose) polymerase (PARP) cleavage started to be observed only at 20 nM OA; moreover, the caspase inhibitor Z-Val-Ala-DL-Asp-fluoromethylketone (Z-VAD.fmk) (100 μM) had negligible effect on apoptosis induced by 10 nM OA, but rescued from death an increasing cell fraction as OA concentration was raised from 20–100 nM. Cell treatment for 24 h with OA induced ceramide accumulation; the phenomenon started to be evident at 20 nM OA and reached its maximum at 50–100 nM OA. In cells exposed to 50 nM OA, ceramide was already elevated by 5 h; at this time, however, PARP cleavage and apoptosis were not yet observed. Z-VAD.fmk (100 μM) had no effect on ceramide elevation induced by 50 nM OA within 5 h, but markedly reduced ceramide accumulation as the incubation was prolonged to 24 h. The latter phenomenon was accompanied by elevation of glucosylceramide levels, thus suggesting that a caspase-dependent reduction of glucosylceramide synthesis might contribute to late ceramide accumulation. Short-chain ceramide (30 μM) induced apoptosis in CHP-100 cells and its effect was additive with that evoked by OA (10–20 nM). These results suggest that ceramide generation might be an important mechanism through which sustained protein phosphatase inhibition induces caspase activation and apoptosis in CHP-100 cells.

Synthetic induction of immunogenic cell death by genetic stimulation of endoplasmic reticulum stress

Cis-diamminedichloridoplatinum(II) (CDDP), commonly referred to as cisplatin, is a chemotherapeutic drug used for the treatment of a wide range of solid cancers. CDDP is a relatively poor inducer of immunogenic cell death (ICD), a cell death modality that converts dying cells into a tumor vaccine, stimulating an immune response against residual cancer cells that permits long-lasting immunity and a corresponding reduction in tumor growth. The incapacity of CDDP to trigger ICD is at least partially due to its failure to stimulate the premortem endoplasmic reticulum (ER)-stress response required for the externalization of the “eat-me” signal calreticulin (CRT) on the surface of dying cancer cells. Here, we developed a murine cancer cell line genetically modified to express the ER resident protein reticulon-1c (Rtn-1c) by virtue of tetracycline induction and showed that enforced Rtn-1c expression combined with CDDP treatment promoted CRT externalization to the surface of cancer cells. In contrast to single agent treatments, the tetracycline-mediated Rtn-1c induction combined with CDDP chemotherapy stimulated ICD as measured by the capacity of dying tumor cells, inoculated into syngenic immunocompetent mice, to mount an immune response to tumor re-challenge 1 week later. More importantly, established tumors, forced to constitutively express Rtn-1c in vivo by continuous treatment with tetracycline, became responsive to CDDP and exhibited a corresponding reduction in the rate of tumor growth. The combined therapeutic effects of Rtn-1c induction with CDDP treatment was only detected in the context of an intact immune system and not in nu/nu mice lacking thymus-dependent T lymphocytes. Altogether, these results indicate that the artificial or “synthetic” induction of immunogenic cell death by genetic manipulation of the ER-stress response can improve the efficacy of chemotherapy with CDDP by stimulating anticancer immunity.

Characterization of gene expression induced by RTN-1C in human neuroblastoma cells and in mouse brain

The endoplasmic reticulum (ER) stress-mediated pathway is involved in a wide range of human neurodegenerative disorders. Hence, molecules that regulate the ER stress response represent potential candidates as drug targets to tackle these diseases. In previous studies we demonstrated that upon acetylation the reticulon-1C (RTN-1C) variant of the reticulon family leads to inhibition of histone deacetylase (HDAC) enzymatic activity and endoplasmic reticulum stress-dependent apoptosis. Here, by microarray analysis of the whole human genome we found that RTN-1C is able to specifically regulate gene expression, modulating transcript clusters which have been implicated in the onset of neurodegenerative disorders. Interestingly, we show that some of the identified genes were also modulated in vivo in a brain-specific mouse model overexpressing RTN-1C. These data provide a basis for further investigation of RTN-1C as a potential molecular target for use in therapy and as a specific marker for neurological diseases.