The ER Stress Sensor PERK Coordinates ER-Plasma Membrane Contact Site Formation through Interaction with Filamin-A and F-Actin Remodeling.

Loss of ER Ca2+ homeostasis triggers endoplasmic reticulum (ER) stress and drives ER-PM contact sites formation in order to refill ER-luminal Ca2+. Recent studies suggest that the ER stress sensor and mediator of the unfolded protein response (UPR) PERK regulates intracellular Ca2+ fluxes, but the mechanisms remain elusive. Here, using proximity-dependent biotin identification (BioID), we identified the actin-binding protein Filamin A (FLNA) as a key PERK interactor. Cells lacking PERK accumulate F-actin at the cell edges and display reduced ER-PM contacts. Following ER-Ca2+ store depletion, the PERK-FLNA interaction drives the expansion of ER-PM juxtapositions by regulating F-actin-assisted relocation of the ER-associated tethering proteins Stromal Interaction Molecule 1 (STIM1) and Extended Synaptotagmin-1 (E-Syt1) to the PM. Cytosolic Ca2+ elevation elicits rapid and UPR-independent PERK dimerization, which enforces PERK-FLNA-mediated ER-PM juxtapositions. Collectively, our data unravel an unprecedented role of PERK in the regulation of ER-PM appositions through the modulation of the actin cytoskeleton.

A novel pathway combining calreticulin exposure and ATP secretion in immunogenic cancer cell death.

Surface-exposed calreticulin (ecto-CRT) and secreted ATP are crucial damage-associated molecular patterns (DAMPs) for immunogenic apoptosis. Inducers of immunogenic apoptosis rely on an endoplasmic reticulum (ER)-based (reactive oxygen species (ROS)-regulated) pathway for ecto-CRT induction, but the ATP secretion pathway is unknown. We found that after photodynamic therapy (PDT), which generates ROS-mediated ER stress, dying cancer cells undergo immunogenic apoptosis characterized by phenotypic maturation (CD80(high), CD83(high), CD86(high), MHC-II(high)) and functional stimulation (NO(high), IL-10(absent), IL-1ß(high)) of dendritic cells as well as induction of a protective antitumour immune response. Intriguingly, early after PDT the cancer cells displayed ecto-CRT and secreted ATP before exhibiting biochemical signatures of apoptosis, through overlapping PERK-orchestrated pathways that require a functional secretory pathway and phosphoinositide 3-kinase (PI3K)-mediated plasma membrane/extracellular trafficking. Interestingly, eIF2α phosphorylation and caspase-8 signalling are dispensable for this ecto-CRT exposure. We also identified LRP1/CD91 as the surface docking site for ecto-CRT and found that depletion of PERK, PI3K p110α and LRP1 but not caspase-8 reduced the immunogenicity of the cancer cells. These results unravel a novel PERK-dependent subroutine for the early and simultaneous emission of two critical DAMPs following ROS-mediated ER stress.

New functions of mitochondria associated membranes in cellular signaling.

In all eukaryotic cells, the endoplasmic reticulum (ER) and the mitochondria establish a tight interplay, which is structurally and functionally modulated through a proteinaceous tether formed at specific subdomains of the ER membrane, designated mitochondria-associated membranes or MAMs. The tethering function of the MAMs allows the regulation of lipid synthesis and rapid transmission of calcium (Ca(2+)) signals between the ER and mitochondria, which is crucial to shape intracellular Ca(2+) signaling and regulate mitochondrial bioenergetics. Research on the molecular characterization and function of MAMs has boomed in the last few years and the list of signaling and structural proteins dynamically associated with the ER-mitochondria contact sites in physiological and pathological conditions, is rapidly increasing along with the realization of an unprecedented complexity underlying the functional role of MAMs. Besides their established role as a signaling hub for Ca(2+) and lipid transfer between ER and mitochondria, MAMs have been recently shown to regulate mitochondrial shape and motility, energy metabolism and redox status and to be central to the modulation of various key processes like ER stress, autophagy and inflammasome signaling. In this review we will discuss some emerging cell-autonomous and cell non-autonomous roles of the MAMs in mammalian cells and their relevance for important human diseases. This article is part of a Special Issue entitled: Calcium signaling in health and disease. Guest Editors: Geert Bultynck, Jacques Haiech, Claus W. Heizmann, Joachim Krebs, and Marc Moreau.

Dynamic interplay between autophagic flux and Akt during melanoma progression in vitro.

Despite advances in cancer diagnosis and therapy, metastatic melanoma remains untreatable, due to its notorious resistance to apoptosis, deeming traditional therapies obsolete. Deregulated PI3K/Akt signalling is a common oncogenic event enabling melanocyte transformation and represents a significant and 'druggable' pathway in melanoma. Emerging data show that the ability of cancer cells to survive is also facilitated by alteration of vital homoeostatic mechanisms, such as autophagy. Although the role of autophagy in melanoma is still controversial, recent studies suggest that basal autophagy is down-modulated in primary melanomas. However, the dynamic connection between pro-tumorigenic PI3K/Akt and autophagy during melanoma progression has not been systematically studied. By using human primary melanocytes, incipient melanoma and metastatic melanoma cell lines, we show that early in melanomagenesis, increased Akt activity is associated with a low baseline autophagic flux. However, during melanoma progression, metastatic melanoma cells regain the ability to stimulate autophagic flux, supporting survival. Heightened autophagy is associated with an attenuated Akt activation status and can be suppressed by overexpressing a constitutive active mutant of Akt. On the other hand, blocking the higher Akt activity of primary melanoma is sufficient to incite autophagy. Interestingly, we found that although Akt supports survival of melanocytes and all melanoma cell lines, autophagy inhibition specifically targeted the metastatic melanoma cells, thus indicating a stage-specific requirement for Akt and autophagic flux, throughout melanoma progression. Therefore, this study highlights a dynamic interplay between Akt signalling and autophagic rescue in melanoma, which should be considered in the design of therapeutic strategies targeting these pathways.

Pro-apoptotic signaling induced by photo-oxidative ER stress is amplified by Noxa, not Bim.

Pro-apoptotic signaling instigated by endoplasmic reticulum (ER) stress is tightly governed by the BH3-only proteins like Noxa and Bim, which help trigger apoptosis, in part by inactivating mitochondria protecting proteins like Mcl-1. Bim/Noxa-based pro-apoptotic signaling has been implicated for various ER stressors but not yet for those causing "ER-focused" production of severe oxidative stress. In the present study we found that photo-oxidative (phox)-ER stress induced by hypericin-based photodynamic therapy is associated with activation of PERK (an ER sessile, stress sensor), robust induction of CHOP (a pro-apoptotic transcription factor) and induction of Bim and Noxa (accompanied by an eventual drop in Mcl-1 levels). Interestingly Noxa, but not Bim, contributed toward phox-ER stress induced apoptosis, regulated by PERK in a CHOP-independent, temporally-defined manner. These observations shed further light on complex signaling pathways elicited byphox-ER stress and vouch for directing more investigation toward the role of PERK in cell death governance.

Autophagy pathways activated in response to PDT contribute to cell resistance against ROS damage.

Reactive oxygen species (ROS) concurrently instigate apoptosis and autophagy pathways, but the link between these processes remains unclear. Because cytotoxic ROS formation is exploited in anticancer therapy, such as in photodynamic therapy (PDT), a better understanding of the complex interplay between autophagy and apoptosis is urgently required. Previously, we reported that ROS generated by PDT with an endoplasmic reticulum (ER)-associated sensitizer leads to loss of ER-Ca(2+) homeostasis, ER stress and apoptosis. Here we show that PDT prompted Akt-mTOR (mammalian target of rapamycin) pathway down-regulation and stimulated macroautophagy (MA) in cancer and normal cells. Overexpression of the antioxidant enzyme glutathione peroxidase-4 reversed mTOR down-regulation and blocked MA progression and apoptosis. Attenuating MA using Atg5 knockdown or 3-methyladenine, reduced clearance of oxidatively damaged proteins and increased apoptosis, thus revealing a cytoprotective role of MA in PDT. Paradoxically, genetic loss of MA improved clearance of oxidized proteins and reduced photokilling. We found that up-regulation of chaperone-mediated autophagy (CMA) in unstressed Atg(-/-) cells compensated for MA loss and increased cellular resistance to PDT. CMA-deficient cells were significantly sensitized to photokilling but were protected against the ER stressor thapsigargin. These results disclose a stress-specific recruitment of autophagy pathways with cytoprotective function and unravel CMA as the dominant defence mechanism against PDT.

p38(MAPK)-regulated induction of p62 and NBR1 after photodynamic therapy promotes autophagic clearance of ubiquitin aggregates and reduces reactive oxygen species levels by supporting Nrf2-antioxidant signaling.

Emerging evidence indicates that oxidative stress instigates the formation of ubiquitin (Ub) aggregates, substrates of autophagy, through a process requiring the ubiquitin binding adaptors p62/SQSTM1 and NBR1. Here, we have investigated the role of p62 and NBR1 in cell survival after hypericin-mediated photodynamic therapy (Hyp-PDT), a procedure known to incite robust reactive oxygen species (ROS)-based endoplasmic reticulum stress and autophagy pathways. We found that Hyp-PDT stimulated the formation of p62- and NBR1-associated Ub aggregates in normal and cancer cells, which were ultimately removed by autophagy, through a mechanism partially regulated by p38(MAPK). In line with this, genetic or pharmacological p38(MAPK) inhibition reduced p62 and NBR1 levels and aggregate formation and impaired Nrf2 activation, thus increasing photo-oxidative stress and cell death. p62-deficient cells, or cells lacking p62 and with reduced levels of NBR1 (through siRNA knockdown), also displayed reduced aggregate formation but exhibited attenuated ROS levels, reduced caspase activation, and improved survival after Hyp-PDT. The increased resistance to photo-oxidative stress exhibited by cells lacking p62 and/or NBR1 was overruled by the inhibition of p38(MAPK), which restored cytotoxic ROS levels, thus indicating the relevance of this signal in the control of cell viability. Taken together these findings provide evidence that in photodynamically treated cells a p38(MAPK)-regulated pathway coordinates the p62/NBR1-mediated clearance of cytosolic aggregates and mitigates PDT-induced proteotoxicity. They also reveal that a functional p38(MAPK)-Nrf2 signal is required to keep ROS levels in check and protect against PDT-induced proteotoxicity, independent of aggregate formation.

Targeting ER stress induced apoptosis and inflammation in cancer.

Disturbance in the folding capacity of the endoplasmic reticulum (ER), caused by a variety of endogenous and exogenous insults, prompts a cellular stress condition known as ER stress. ER stress is initially shaped to re-establish ER homeostasis through the activation of an integrated intracellular signal transduction pathway termed as unfolded protein response (UPR). However, when ER stress is too severe or prolonged, the pro-survival function of the UPR turns into a toxic signal, which is predominantly executed by mitochondrial apoptosis. Moreover, accumulating evidence implicates ER stress pathways in the activation of various 'classical' inflammatory processes in and around the tumour microenvironment. In fact, ER stress pathways evoked by certain conventional or experimental anticancer modalities have been found to promote anti-tumour immunity by enhancing immunogenicity of dying cancer cells. Thus, the ER functions as an essential sensing organelle capable of coordinating stress pathways crucially involved in maintaining the cross-talk between the cancer cell's intracellular and extracellular environment. In this review we discuss the emerging link between ER stress, cell fate decisions and immunomodulation and the potential therapeutic benefit of targeting this multifaceted signaling pathway in anticancer therapy.

ROS-induced autophagy in cancer cells assists in evasion from determinants of immunogenic cell death.

Calreticulin surface exposure (ecto-CALR), ATP secretion, maturation of dendritic cells (DCs) and stimulation of T cells are prerequisites for anticancer therapy-induced immunogenic cell death (ICD). Recent evidence suggests that chemotherapy-induced autophagy may positively regulate ICD by favoring ATP secretion. We have recently shown that reactive oxygen species (ROS)-based endoplasmic reticulum (ER) stress triggered by hypericin-mediated photodynamic therapy (Hyp-PDT) induces bona fide ICD. However, whether Hyp-PDT-induced autophagy regulates ICD was not explored. Here we showed that, in contrast to expectations, reducing autophagy (by ATG5 knockdown) in cancer cells did not alter ATP secretion after Hyp-PDT. Autophagy-attenuated cancer cells displayed enhanced ecto-CALR induction following Hyp-PDT, which strongly correlated with their inability to clear oxidatively damaged proteins. Furthermore, autophagy-attenuation in Hyp-PDT-treated cancer cells increased their ability to induce DC maturation, IL6 production and proliferation of CD4(+) or CD8(+) T cells, which was accompanied by IFNG production. Thus, our study unravels a role for ROS-induced autophagy in weakening functional interaction between dying cancer cells and the immune system thereby helping in evasion from ICD prerequisites or determinants.

Death and survival signals in photodynamic therapy.

Photodynamic therapy (PDT) is an anticancer modality utilizing the generation of singlet oxygen and other reactive oxygen species through visible light irradiation of a photosensitive dye accumulated in the cancerous tissue. Upon exposure of cancer cells to the photodynamic stress, multiple signaling cascades are concomitantly activated and depending on the subcellular location of the generated ROS and the intensity of the oxidative damage, they dictate whether cells will cope with the stress and survive or succumb and die. Different methodologies have been developed to allow the discrimination of cell death subroutines at the morphological, ultrastructural, and biochemical levels and to scrutinize signaling cascades in response to PDT. Here we describe a selection of useful techniques to characterize apoptosis and autophagy and to monitor the activation status of the MAPK- and Akt-mTOR pathways after PDT.

Linking ER Stress to Autophagy: Potential Implications for Cancer Therapy.

Different physiological and pathological conditions can perturb protein folding in the endoplasmic reticulum, leading to a condition known as ER stress. ER stress activates a complex intracellular signal transduction pathway, called unfolded protein response (UPR). The UPR is tailored essentially to reestablish ER homeostasis also through adaptive mechanisms involving the stimulation of autophagy. However, when persistent, ER stress can switch the cytoprotective functions of UPR and autophagy into cell death promoting mechanisms. Recently, a variety of anticancer therapies have been linked to the induction of ER stress in cancer cells, suggesting that strategies devised to stimulate its prodeath function or block its prosurvival function, could be envisaged to improve their tumoricidial action. A better understanding of the molecular mechanisms that determine the final outcome of UPR and autophagy activation by chemotherapeutic agents, will offer new opportunities to improve existing cancer therapies as well as unravel novel targets for cancer treatment.

Proteasome inhibition potentiates antitumor effects of photodynamic therapy in mice through induction of endoplasmic reticulum stress and unfolded protein response.

Photodynamic therapy (PDT) is an approved therapeutic procedure that exerts cytotoxic activity toward tumor cells by inducing production of reactive oxygen species such as singlet oxygen. PDT leads to oxidative damage of cellular macromolecules, including proteins that undergo multiple modifications such as fragmentation, cross-linking, and carbonylation that result in protein unfolding and aggregation. Because the major mechanism for elimination of carbonylated proteins is their degradation by proteasomes, we hypothesized that a combination of PDT with proteasome inhibitors might lead to accumulation of carbonylated proteins in endoplasmic reticulum (ER), aggravated ER stress, and potentiated cytotoxicity toward tumor cells. We observed that Photofrin-mediated PDT leads to robust carbonylation of cellular proteins and induction of unfolded protein response. Pretreatment of tumor cells with three different proteasome inhibitors, including bortezomib, MG132, and PSI, gave increased accumulation of carbonylated and ubiquitinated proteins in PDT-treated cells. Proteasome inhibitors effectively sensitized tumor cells of murine (EMT6 and C-26) as well as human (HeLa) origin to PDT-mediated cytotoxicity. Significant retardation of tumor growth with 60% to 100% complete responses was observed in vivo in two different murine tumor models (EMT6 and C-26) when PDT was combined with either bortezomib or PSI. Altogether, these observations indicate that combination of PDT with proteasome inhibitors leads to potentiated antitumor effects. The results of these studies are of immediate clinical application because bortezomib is a clinically approved drug that undergoes extensive clinical evaluations for the treatment of solid tumors.