Regulation of autophagy by CaSuperscript2+/Supe

Authors: Fang Sun Xia Xu Xiaohong Wang Bei Zhang

Publish Date: 2016/11/18

Volume: 37, Issue: 12, Pages: 15467-15476

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Abstract

Autophagy is an evolutionarily conserved lysosomal catabolic process used as an internal engine in response to nutrient starvation or metabolic stress A number of protein complexes and an intricate network of stress signaling cascades impinge on the regulation of autophagy the mammalian target of rapamycin serves as a canonical player Ca2+ as a major intracellular second messenger regulates multiple physiological and pathological functions Although significant information is already wellestablished about the role of Ca2+ in apoptosis its role in autophagy has been recently determined and is poorly understood Intracellular Ca2+ positively and negatively affects autophagy In this review evidence for both views and the interplay of Ca2+ between autophagy and apoptosis induction are discussed The available data revealed the bidirectional role of Ca2+ in the regulation of autophagy Moreover the data also indicated that this role probably depends on the context of time space Ca2+ source and cell state thus either preventing or enhancing autophagyAutophagy is a main catabolic process of degrading and recycling intracellular components in lysosomes that cannot be executed by the ubiquitin–proteasome system These components include longlived proteins damaged organelles and some other macromolecules 1 2 Under wellfed conditions autophagy in most cells is maintained at a low basal level whereas under stressful conditions such as nutrition deprivation high temperatures oxidative stress accumulation of damaged organelles and cytosolic Ca2+ upload autophagy is activated to play an essential role in sustaining cellular homeostasis and energy requirements thus facilitating the survival of cells 3 4 Insufficient or altered autophagic flux is responsible for various diseases such as cancer neurodegenerative disorders and cardiovascular disease because of the significance of autophagy in cellular life and death processes 5 6 According to the manner of material delivery to the lysosomes autophagy is divided into three main types namely macroautophagy chaperonemediated autophagy and microautophagy 7 The former type is the most common form of autophagy hereafter referred to as autophagy and contains the formation and elongation of a typical doublemembranous cistern phagophores into a whole vesicle autophagosomes which ultimately fuses with lysosomes to form autolysosomes leading to the breakdown and recycling of the enclosed content 8The molecular basis of autophagy is complex So far more than 30 autophagyrelated genes ATGs have been identified as crucial regulators of this complex process from the initial signal to the final fusion 9 10 11 In this family one quite important member is Beclin 1 the mammalian ortholog of Atg6 in yeast Beclin 1 plays a central role in the initial formation of autophagosome 12 13 Beclin 1 can dimerize and interact with Vps34 Vps15 Bif1 UVRAG Ambra1 and some other proteins to generate phosphatidylinositol3phosphate PI3P which catalyzes vesicle elongation and phagophore nucleation by promoting the recruitment of other Atg proteins 14 15 Therefore Beclin 1 depletion results in autophagic deficiency 16 Remarkably although the role of Beclin 1 in apoptosis has not been clearly elucidated this ortholog has been determined as a member of the proapoptotic BH3only protein family Its BH3 domain mediates an interaction among antiapoptotic Bcl2 protein family members Bcl2 BclXl Mcl1 and Bclw and thereby blocks the induction of the autophagic machinery under normal conditions 17 18 However during autophagyinducing conditions Beclin 1 is allowed to dissociate from the Bcl2–Beclin 1 protein complex and subsequently activates the PtdIns3K complex III to induce autophagy 19 The interaction between Beclin 1 and Bcl2 proteins can be dynamically regulated by various mechanisms For instance the phosphorylation of either Bcl2 by cJun NH2terminal kinase1 or Beclin 1 by deathassociated protein kinase DAPK antagonizes the interaction between the two proteins and then released Beclin 1 promotes autophagy 20 21 In addition other proteins including BNIP3 nutrientdeprivation autophagy factor 1 NAF1 and high motility group box 1 HMGB1 competitively disrupt the binding of Beclin 1 to Bcl2 and regulate the onset of autophagy 22 23 24 Furthermore recent studies have shown that the suppression of Beclin 1’s function in autophagy is mainly attributed to Bcl2 located at the endoplasmic reticulum ER with inhibitory effects on Ca2+induced autophagy 25 26 27The network of stress signaling cascades and a number of protein complexes highlight significant considerations for autophagy regulation 28 To date the canonical and central sensor for the availability of energy and nutrients is the mammalian target of rapamycin mTOR in particular complex1 mTORC1 mTOR functions as a negative regulator of autophagy 29 30 Although multiple signal pathways converge to modulate mTOR activity one of the most important pathways is the AMPactivated protein kinase AMPK a positive regulator of autophagy 31 Activated AMPK promotes the phosphorylation of the tuberous sclerosis complexes TSC 1 and 2 Subsequently activation of TSC1/TSC2 suppresses mTOR activity by deactivating the mTORC1interacting protein Rheb inducing autophagy 32 33 34 In addition AMPK can be phosphorylated and activated by Ca2+calmodulindependent protein kinase kinaseβ CaMKKβ which provides a close association among Ca2+ signaling mTOR and autophagy 35 Indeed recent studies have implicated the contributions of intracellular Ca2+ signaling and inositol 145trisphosphate receptors IP3Rs to autophagy regulation IP3Rs are tetrameric Ca2+ channels located at ER and these channels release Ca2+ from the ER to the cytosol 36 IP3Rs are major regulators of autophagy and apoptosis which are also modulated by the members of the Bcl2 family of proteins The IP3Rs have been identified recently as a Bcl2regulated repressor of autophagy 37Ca2+ as a major intracellular second messenger regulates multiple physiological functions in cells such as contraction secretion metabolism survival or death and gene transcription It also participates in some pathological processes 38 39 Previous studies showed that interference with calcium homeostasis can provoke cell death in many types of tumor cells 40 A variety of cellular Ca2+transporting and Ca2+binding proteins are located mainly at the plasma cytosol membrane ER and mitochondria namely Ca2+ toolkit 41 These correct spatiotemporal distributions of Ca2+ determine several of the most commonly recognized and wellstudied intracellular signals 42 43 Although significant information is already wellestablished about the role of Ca2+ in apoptosis the role of Ca2+ in autophagy regulation remains poorly understood The role of Ca2+ signals involved in autophagy regulation was noted in the early 1990s and the pioneering study already suggested a complicated role of Ca2+ as inhibitor and promoter of Ca2+ release from internal stores suppressed by autophagy 44 In recent decades a series of studies on the dual role of Ca2+ in autophagy regulation began to emerge but their results still differ with respect to the precise mechanisms and pathways involved On the one hand numerous reports have revealed that Ca2+ functions as a negative regulator of autophagy 37 45 46 47 48 49 50 On the other hand other studies indicated a stimulatory role of Ca2+ toward autophagy 25 51 52 53 54 55 56 The role of the ER as a physiologically important Ca2+ store is universally recognized and IP3Rs may also have an important role in the control of autophagy although the current data are at least partially contradictory 57 58 By releasing Ca2+ from the ER these ubiquitously expressed channels control diverse cellular processes including cell proliferation and death In addition they play an essential role for a constitutive IP3Rmediated Ca2+ release to mitochondria in maintaining mitochondrial bioenergetics 48 59 60 61In this study the available evidence on the role of Ca2+ signal is summarized in particular the IP3Rmediated Ca2+ release in the regulation of autophagy Furthermore the interplay of Ca2+ between autophagy and apoptosis induction is discussed The available data not only showed a bidirectional role for Ca2+ in the control of autophagy but also suggested a model showing that this role may rely on the specific context thus either preventing or enhancing autophagyAn initial study indicating the inhibitory effect of IP3R on autophagy was based on the use of lithium Li+ Li+stimulated autophagic response in turn results in declined IP3 levels and diminishing Ca2+ release from the ER which then triggers autophagy 45 The study revealed a novel mTORindependent manner that regulates autophagy Criollo et al subsequently confirmed IP3R in the control of autophagy 37 They showed that chemical blockade with xestospongin B XeB or the silence of the IP3R with small interfering RNAs is a strong stimulus for the induction of autophagy in HeLa cells They also reported an organellespecific interaction between IP3R and Bcl2 and proposed that IP3R may function as a Bcl2regulated inhibitor of autophagy which specifically targeted the ER but not the mitochondria However ER stressactivated autophagy is not suppressed by Bcl2 which implies a variety of signaling pathways involved in the regulation of autophagy In view of the uncertain link between Ca2+ signaling and IP3Rmodulated autophagy the same laboratory proceeded to investigate and propose that IP3Rmediated Ca2+ release may not be an entitative part of the mechanism whereas protein interactions with the channel may play a dominant role 46 Thus they presented evidence showing that IP3R may shut down autophagy by decreasing the release of Beclin 1 from Bcl2mediated sequestration therefore facilitating the formation of antiautophagic Bcl2–Beclin 1 complexes Moreover xestospongin B or nutrient starvation would disrupt the interaction between the IP3R and Beclin 1 and thus release Beclin 1 available for the induction of autophagy In this model although autophagy negatively modulated by IP3R may be attributed to the obligate contribution of Beclin 1 instead of the involvement of the IP3R Ca2+ channel function the IP3R was identified as an unsuspected regulator of the Beclin 1 complex and bridged an intriguing signal network that converges on the ER and initial phagophore formation

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