Glucocorticoids downregulate Fyn and inhibit IP(3)-mediated calcium signaling to promote autophagy in T lymphocytes

Corticosteroids block autophagy protein recruitment in Aspergillus fumigatus phagosomes via targeting dectin-1/Syk kinase signaling

Aspergillus fumigatus is the predominant airborne fungal pathogen in immunocompromised patients. Genetic defects in NADPH oxidase (chronic granulomatous disease [CGD]) and corticosteroid-induced immunosupression lead to impaired killing of A. fumigatus and unique susceptibility to invasive aspergillosis via incompletely characterized mechanisms. Recent studies link TLR activation with phagosome maturation via the engagement of autophagy proteins. In this study, we found that infection of human monocytes with A. fumigatus spores triggered selective recruitment of the autophagy protein LC3 II in phagosomes upon fungal cell wall swelling. This response was induced by surface exposure of immunostimulatory β-glucans and was mediated by activation of the Dectin-1 receptor. LC3 II recruitment in A. fumigatus phagosomes required spleen tyrosine kinase (Syk) kinase-dependent production of reactive oxygen species and was nearly absent in monocytes of patients with CGD. This pathway was important for control of intracellular fungal growth, as silencing of Atg5 resulted in impaired phagosome maturation and killing of A. fumigatus. In vivo and ex vivo administration of corticosteroids blocked LC3 II recruitment in A. fumigatus phagosomes via rapid inhibition of phosphorylation of Src and Syk kinases and downstream production of reactive oxygen species. Our studies link Dectin-1/Syk kinase signaling with autophagy-dependent maturation of A. fumigatus phagosomes and uncover a potential mechanism for development of invasive aspergillosis in the setting of CGD and corticosteroid-induced immunosupression.

Suppression of autophagy by CUB domain-containing protein 1 signaling is essential for anchorage-independent survival of lung cancer cells

CUB (C1r/C1s, urchin embryonic growth factor, BMP1) domain-containing protein 1 (CDCP1) has been implicated in promoting metastasis of cancer cells through several mechanisms, including the inhibition of anoikis, which is cell death triggered by the loss of extracellular matrix interactions. However, the mechanism inhibiting cell death regulated by CDCP1 remains elusive. Inhibition of CDCP1 expression using small interfering RNA (siRNA) induced the cell death of suspended cancer cells without cleaving caspase-3, a marker of apoptosis; cell death was not inhibited by a general caspase inhibitor, suggesting that the loss of CDCP1 induces caspase-independent cell death. In contrast, knockdown of CDCP1 as well as protein kinase Cδ (PKCδ), a downstream effector of CDCP1, in a suspension culture of lung cancer cells resulted in marked induction of membranous microtubule-associated protein 1 light chain 3 (LC3)-II protein, a hallmark of autophagy, and caused the formation of an autophagosome structure visualized using green fluorescent protein-tagged LC3-II. Expression and phosphorylation of exogenous CDCP1 by Fyn kinase reduced the formation of autophagosomes and inhibited phosphorylation of CDCP1 by PP2, a Src kinase inhibitor or inhibited PKCδ by rottlerin, stimulating autophagosome formation. Moreover, death of suspended lung cancer cells induced by CDCP1 siRNA or by PKCδ siRNA was reduced by the autophagy inhibitor 3-methyladenine. These results indicate that CDCP1-PKCδ signaling plays a critical role in inhibiting autophagy, which is responsible for anoikis resistance of lung cancer cells.

mTOR-Controlled Autophagy Requires Intracellular Ca(2+) Signaling

Autophagy is a lysosomal degradation pathway important for cellular homeostasis and survival. Inhibition of the mammalian target of rapamycin (mTOR) is the best known trigger for autophagy stimulation. In addition, intracellular Ca²⁺ regulates autophagy, but its exact role remains ambiguous. Here, we report that the mTOR inhibitor rapamycin, while enhancing autophagy, also remodeled the intracellular Ca²⁺-signaling machinery. These alterations include a) an increase in the endoplasmic-reticulum (ER) Ca²⁺-store content, b) a decrease in the ER Ca²⁺-leak rate, and c) an increased Ca²⁺ release through the inositol 1,4,5-trisphosphate receptors (IP₃Rs), the main ER-resident Ca²⁺-release channels. Importantly, buffering cytosolic Ca²⁺ with BAPTA impeded rapamycin-induced autophagy. These results reveal intracellular Ca²⁺ signaling as a crucial component in the canonical mTOR-dependent autophagy pathway.

Hyperactivation of the mammalian degenerin MDEG promotes caspase-8 activation and apoptosis

Intracellular calcium overload plays a critical role in numerous pathological syndromes such as heart failure, brain ischemia, and stroke. Hyperactivation of the acid-sensing ion channels including degenerin/epithelial amiloride-sensitive sodium (DEG/ENaC) channels has been shown to elevate intracellular calcium and cause subsequent neuronal cell death that is independent of the canonical Egl-1/Ced-9/Ced-4/Ced-3 apoptotic pathway in Caenorhabditis elegans. In mammalian cells, hyperactivation of the DEG/ENaC channels can also lead to cell death, although the underlying mechanism remains largely unknown. Here, we use a tetracycline-inducible system to express the hyperactivation mutant of a mammalian DEG/ENaC channel protein, MDEG G430F, in murine kidney epithelial cells deficient in the key mitochondrial apoptotic proteins Bax and Bak. Remarkably, expression of MDEG G430F induces increased intracellular calcium, reactive oxygen species (ROS) production, and cell death. The MDEG G430F-induced cell death is blocked by the intracellular calcium chelator 1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (acetoxymethyl ester), ROS scavengers, and the caspase inhibitor z-VAD-fmk (where z and fmk are benzyloxycarbonyl and fluoromethyl ketone). Mechanistically, the intracellular calcium overload and ROS increase lead to the inhibition of proteasomal and autophagic protein degradation, which promotes the accumulation of protein aggregates containing caspase-8 and subsequent caspase-8 activation. As protein aggregation upon the inhibition of proteasomal and autophagic degradation pathways is mediated by the ubiquitin-binding protein SQSTM1/p62 and the autophagy-related protein LC3, silencing of p62 and LC3 protects cells from MDEG G430F-induced cell death. Our results uncover a new mechanism of caspase-8-mediated apoptosis induced by intracellular calcium overload that is dependent on the autophagy-related proteins LC3 and p62 upon hyperactivation of DEG/ENaC channels.

Metabolic regulation of organelle homeostasis in lupus T cells

Abnormal T-cell signaling and activation are characteristic features in systemic lupus erythematosus (SLE). Lupus T cells are shifted toward an over-activated state, important signaling pathways are rewired, and signaling molecules are replaced. Disturbances in metabolic and organelle homeostasis, importantly within the mitochondrial, endosomal, and autophagosomal compartments, underlie the changes in signal transduction. Mitochondrial hyperpolarization, enhanced endosomal recycling, and dysregulated autophagy are hallmarks of pathologic organelle homeostasis in SLE. This review is focused on the metabolic checkpoints of endosomal traffic that control immunological synapse formation and mitophagy and may thus serve as targets for treatment in SLE.

High-content RNAi screening identifies the Type 1 inositol triphosphate receptor as a modifier of TDP-43 localization and neurotoxicity

Cytosolic aggregation of the nuclear RNA-binding protein (RBP) TDP-43 (43 kDa TAR DNA-binding domain protein) is a suspected direct or indirect cause of motor neuron deterioration in amyotrophic lateral sclerosis (ALS). In this study, we implemented a high-content, genome-wide RNAi screen to identify pathways controlling TDP-43 nucleocytoplasmic shuttling. We identified ∼60 genes whose silencing increased the cytosolic localization of TDP-43, including nuclear pore complex components and regulators of G2/M cell cycle transition. In addition, we identified the type 1 inositol-1,4,5-trisphosphate (IP3) receptor (ITPR1), an IP3-gated, endoplasmic reticulum (ER)-resident Ca(2+) channel, as a strong modulator of TDP-43 nucleocytoplasmic shuttling. Knockdown or chemical inhibition of ITPR1 induced TDP-43 nuclear export in immortalized cells and primary neurons and strongly potentiated the recruitment of TDP-43 to Ubiquilin-positive autophagosomes, suggesting that diminished ITPR1 function leads to autophagosomal clearance of TDP-43. The functional significance of the TDP-43-ITPR1 genetic interaction was tested in Drosophila, where mutant alleles of ITPR1 were found to significantly extended lifespan and mobility of flies expressing TDP-43 under a motor neuron driver. These combined findings implicate IP3-gated Ca(2+) as a key regulator of TDP-43 nucleoplasmic shuttling and proteostasis and suggest pharmacologic inhibition of ITPR1 as a strategy to combat TDP-43-induced neurodegeneration in vivo.

Autophagy Contributes to the Death/Survival Balance in Cancer PhotoDynamic Therapy

Autophagy is an important cellular program with a “double face” role, since it promotes either cell survival or cell death, also in cancer therapies. Its survival role occurs by recycling cell components during starvation or removing stressed organelles; when damage becomes extensive, autophagy provides another programmed cell death pathway, known as Autophagic Cell Death (ACD). The induction of autophagy is a common outcome in PhotoDynamic Therapy (PDT), a two-step process involving the irradiation of photosensitizer (PS)-loaded cancer cells. Upon tissue oxygen interaction, PS provokes immediate and direct Reactive Oxygen Species (ROS)-induced damage to Endoplasmic Reticulum (ER), mitochondria, plasma membrane, and/or lysosomes. The main biological effects carried out in cancer PDT are direct cytotoxicity to tumor cells, vasculature damage and induction of inflammatory reactions stimulating immunological responses. The question about the role of autophagy in PDT and its putative immunological impact is hotly controversial and largely studied in recent times. This review deals with the induction of autophagy in PDT protocols and its dual role, also considering its interrelationship with apoptosis, the preferential cell death program triggered in the photodynamic process.

T lymphocytes from patients with systemic lupus erythematosus are resistant to induction of autophagy

Autophagy, the cytoprotection mechanism that takes place under metabolic impairment, has been implicated in the pathogenesis of autoimmunity. Here, we investigated the spontaneous and induced autophagic behavior of T lymphocytes from patients with systemic lupus erythematosus (SLE) compared with that of T lymphocytes from healthy donors by measuring the autophagy marker microtubule-associated protein 1 light chain 3 (LC3)-II. No significant differences in spontaneous autophagy were found between T lymphocytes from patients with SLE and from healthy donors, apart from CD4(+) naive T cells from patients with SLE in which constitutively higher levels of autophagy (P<0.001) were detected. At variance, whereas treatment of T lymphocytes from healthy donors with serum IgG from patients with SLE resulted in a 2-fold increase in LC3-II levels (P<0.001), T lymphocytes from SLE patients were resistant to autophagic induction and also displayed an up-regulation of genes negatively regulating autophagy, e.g., α-synuclein. These findings could open new perspectives in the search for pathogenetic determinants of SLE progression and in the development of therapeutic strategies aimed to recover T-cell compartment homeostasis by restoring autophagic susceptibility.

Mitochondrial Ca(2+) signals in autophagy

Macroautophagy (autophagy) is a lysosomal degradation pathway that is conserved from yeast to humans that plays an important role in recycling cellular constituents in all cells. A number of protein complexes and signaling pathways impinge on the regulation of autophagy, with the mammalian target of rapamycin (mTOR) as the central player in the canonical pathway. Cytoplasmic Ca(2+) signaling also regulates autophagy, with both activating and inhibitory effects, mediated by the canonical as well as non-canonical pathways. Here we review this regulation, with a focus on the role of an mTOR-independent pathway that involves the inositol trisphosphate receptor (InsP(3)R) Ca(2+) release channel and Ca(2+) signaling to mitochondria. Constitutive InsP(3)R Ca(2+) transfer to mitochondria is required for autophagy suppression in cells in nutrient-replete media. In its absence, cells become metabolically compromised due to insufficient production of reducing equivalents to support oxidative phosphorylation. Absence of this Ca(2+) transfer to mitochondria results in activation of AMPK, which activates mTOR-independent pro-survival autophagy. Constitutive InsP(3)R Ca(2+) release to mitochondria is an essential cellular process that is required for efficient mitochondrial respiration, maintenance of normal cell bioenergetics and suppression of autophagy.

Role of autophagy in immunity and autoimmunity, with a special focus on systemic lupus erythematosus

Autophagy is a lysosome-mediated catabolic process that allows cells to degrade unwanted cytoplasmic constituents and to recycle nutrients. Autophagy is also involved in innate and adaptive immune responses, playing a key role in interactions against microbes, in antigen processing for major histocompatibility complex (MHC) presentation, and in lymphocyte development, survival, and proliferation. Over recent years, perturbations in autophagy have been implicated in a number of diseases, including autoimmunity. Systemic lupus erythematosus (SLE) is a multifactorial disease characterized by autoimmune responses against self-antigens generated by dying cells. Genome-wide association studies have linked several single-nucleotide polymorphisms (SNPs) in the autophagy-related gene Atg5 to SLE susceptibility. Loss of Atg5-dependent effects, including clearance of dying cells and cell antigen presentation, might contribute to the autoimmunity and inflammation associated with SLE. Moreover, activation of the mammalian target of rapamycin (mTOR), a key player in the autophagy regulation, has recently been demonstrated in SLE, confirming an altered autophagy pathway in this disease. In the present review, we summarize the autophagy mechanisms, their molecular regulation, and their relevance in immunity and autoimmunity. The potential of targeting autophagy pathway in SLE, by developing innovative therapeutic approaches, has finally been discussed.

Glucocorticoid elevation of dexamethasone-induced gene 2 (Dig2/RTP801/REDD1) protein mediates autophagy in lymphocytes

Glucocorticoid hormones, including dexamethasone, induce apoptosis in lymphocytes and consequently are used clinically as chemotherapeutic agents in many hematologic malignancies. Dexamethasone also induces autophagy in lymphocytes, although the mechanism is not fully elucidated. Through gene expression analysis, we found that dexamethasone induces the expression of a gene encoding a stress response protein variously referred to as Dig2, RTP801, or REDD1. This protein is reported to inhibit mammalian target of rapamycin (mTOR) signaling. Because autophagy is one outcome of mTOR inhibition, we investigated the hypothesis that Dig2/RTP801/REDD1 elevation contributes to autophagy induction in dexamethasone-treated lymphocytes. In support of this hypothesis, RNAi-mediated suppression of Dig2/RTP801/REDD1 reduces mTOR inhibition and autophagy in glucocorticoid-treated lymphocytes. We observed similar results in Dig2/Rtp801/Redd1 knock-out murine thymocytes treated with dexamethasone. Dig2/RTP801/REDD1 knockdown also leads to increased levels of dexamethasone-induced cell death, suggesting that Dig2/RTP801/REDD1-mediated autophagy promotes cell survival. Collectively, these findings demonstrate for the first time that elevation of Dig2/RTP801/REDD1 contributes to the induction of autophagy.