The atypical PKC-interacting protein p62 channels NF-kappaB activation by the IL-1-TRAF6 pathway

Decreased spinal synaptic inputs to phrenic motor neurons elicit localized inactivity-induced phrenic motor facilitation

Phrenic motor neurons receive rhythmic synaptic inputs throughout life. Since even brief disruption in phrenic neural activity is detrimental to life, on-going neural activity may play a key role in shaping phrenic motor output. To test the hypothesis that spinal mechanisms sense and respond to reduced phrenic activity, anesthetized, ventilated rats received micro-injections of procaine in the C2 ventrolateral funiculus (VLF) to transiently (~30min) block axon conduction in bulbospinal axons from medullary respiratory neurons that innervate one phrenic motor pool; during procaine injections, contralateral phrenic neural activity was maintained. Once axon conduction resumed, a prolonged increase in phrenic burst amplitude was observed in the ipsilateral phrenic nerve, demonstrating inactivity-induced phrenic motor facilitation (iPMF). Inhibition of tumor necrosis factor alpha (TNFα) and atypical PKC (aPKC) activity in spinal segments containing the phrenic motor nucleus impaired ipsilateral iPMF, suggesting a key role for spinal TNFα and aPKC in iPMF following unilateral axon conduction block. A small phrenic burst amplitude facilitation was also observed contralateral to axon conduction block, indicating crossed spinal phrenic motor facilitation (csPMF). csPMF was independent of spinal TNFα and aPKC. Ipsilateral iPMF and csPMF following unilateral withdrawal of phrenic synaptic inputs were associated with proportional increases in phrenic responses to chemoreceptor stimulation (hypercapnia), suggesting iPMF and csPMF increase phrenic dynamic range. These data suggest that local, spinal mechanisms sense and respond to reduced synaptic inputs to phrenic motor neurons. We hypothesize that iPMF and csPMF may represent compensatory mechanisms that assure adequate motor output is maintained in a physiological system in which prolonged inactivity ends life.

TRAF6-mediated ubiquitination of NEMO requires p62/sequestosome-1

The atypical protein kinase C-interacting protein p62/sequestosome-1 (p62) has emerged as a crucial molecule in a variety of cellular functions due to its involvement in various signaling mechanisms. p62 has been implicated in the activation of NF-κB in TNFα-stimulated cells and has been shown to be activated in response to interleukin-1β (IL-1β). Here we demonstrate that p62 interacts with NEMO, the regulatory subunit of the complex responsible for activation of NF-κB transcription factor. Depletion of p62 obtained through a short interfering RNA targeting p62 mRNA abrogated TRAF6 capacity to promote NEMO ubiquitination and severely impairs NF-κB activation following IL-1β stimulation. Together, these results indicate that p62 is an important intermediary in the NF-κB activation pathways implemented through non-degradative ubiquitination events.

Defective autophagy impairs ATF3 activity and worsens lung injury during endotoxemia

Autophagy has emerged as a key regulator of the inflammatory response. To examine the role of autophagy in the development of organ dysfunction during endotoxemia, wild-type and autophagy-deficient (Atg4b-null) mice were challenged with lipopolysaccharide. Animals lacking Atg4b showed increased mortality after endotoxemia. Among the different organs studied, only the lungs showed significant differences between genotypes, with increased damage in mutant animals. Autophagy was activated in lungs from wild-type, LPS-treated mice. Similarly, human bronchial cells show an increased autophagy when exposed to serum from septic patients. We found an increased inflammatory response (increased neutrophilic infiltration, higher levels of Il6, Il12p40, and Cxcl2) in the lungs from knockout mice and identified perinuclear sequestration of the anti-inflammatory transcription factor ATF3 as the putative mechanism responsible for the differences between genotypes. Finally, induction of autophagy by starvation before LPS exposure resulted in a dampened pulmonary response to LPS in wild-type, but not knockout, mice. Similar results were found in human bronchial cells exposed to LPS. Our results demonstrate the central role of autophagy in the regulation of the lung response to endotoxemia and sepsis and its potential modulation by nutrition.

KEY MESSAGES

Endotoxemia and sepsis trigger autophagy in lung tissue. Defective autophagy increases mortality and lung inflammation after endotoxemia. Impairment of autophagy results is perinuclear ATF3 sequestration. Starvation ameliorates lung injury by an autophagy-dependent mechanism.

SQSTM1/p62/A170 regulates the severity of Legionella pneumophila pneumonia by modulating inflammasome activity

Sequestosome1/A170/p62 (SQSTM1) is a scaffold multifunctional protein involved in several cellular events, such as signal transduction, cell survival, cell death, and inflammation. SQSTM1 expression by macrophages is induced in response to environmental stresses; however, its role in macrophage-mediated host responses to environmental stimuli, such as infectious pathogens, remains unclear. In this study, we investigated the role of SQSTM1 in host responses to Legionella pneumophila, an intra-cellular pathogen that infects macrophages, in both an SQSTM1-deficient (SQSTM1(-/-) ) mouse model and macrophages from these mice. Compared with wild-type (WT) macrophages, the production and secretion of the proinflammatory cytokine IL-1β was significantly enhanced in SQSTM1(-/-) macrophages after infection with L. pneumophila. Inflammasome activity, indicated by the level of IL-18 and caspase-1 activity, was also elevated in SQSTM1(-/-) macrophages after infection with L. pneumophila. SQSTM1 may interact with nucleotide-binding oligomerization domain-like receptor family, caspase recruitment domain-containing 4 and nucleotide-binding oligomerization domain like receptor family, pyrin domain containing 3 proteins to inhibit their self-dimerization. Acute pulmonary inflammation induced by L. pneumophila and silica was enhanced in SQSTM1(-/-) mice with an increase in IL-1β levels in the bronchoalveolar lavage fluids. These findings suggest that SQSTM1 is a negative regulator of acute pulmonary inflammation, possibly by regulating inflammasome activity and subsequent proinflammatory cytokine production.

Significant role of PB1 and UBA domains in multimerization of Joka2, a selective autophagy cargo receptor from tobacco

Tobacco Joka2 protein is a hybrid homolog of two mammalian selective autophagy cargo receptors, p62 and NBR1. These proteins can directly interact with the members of ATG8 family and the polyubiquitinated cargoes designed for degradation. Function of the selective autophagy cargo receptors relies on their ability to form protein aggregates. It has been shown that the N-terminal PB1 domain of p62 is involved in formation of aggregates, while the UBA domains of p62 and NBR1 have been associated mainly with cargo binding. Here we focus on roles of PB1 and UBA domains in localization and aggregation of Joka2 in plant cells. We show that Joka2 can homodimerize not only through its N-terminal PB1-PB1 interactions but also via interaction between N-terminal PB1 and C-terminal UBA domains. We also demonstrate that Joka2 co-localizes with recombinant ubiquitin and sequestrates it into aggregates and that C-terminal part (containing UBA domains) is sufficient for this effect. Our results indicate that Joka2 accumulates in cytoplasmic aggregates and suggest that in addition to these multimeric forms it also exists in the nucleus and cytoplasm in a monomeric form.

Monitoring mitophagy in mammalian cells

It is essential for a cell to maintain a proper mitochondrial quality and quantity for normal cellular functions. Damaged or unwanted mitochondria can be selectively removed through mitophagy. Mitophagy research has attracted great attention from life sciences and biomedical fields, it is thus important for the community to properly measure mitophagy. Here, we will focus on the current techniques that have been used to monitor mitophagy in mammalian cells, including morphological and biochemical parameters for monitoring the occurrence of mitophagy.

Structural and biochemical insights into the homotypic PB1-PB1 complex between PKCζ and p62

The atypical PKC isoforms (ζ and ı) play essential roles in regulating various cellular processes. Both the hetero-interaction between PKCζ and p62 through their N-terminal PB1 domains and the homo-oligomerization of p62 via its PB1 domain are critical for the activation of NF-κB signaling; however, the molecular mechanisms concerning the formation and regulation of these homotypic complexes remain unclear. Here we determined the crystal structure of PKCζ-PB1 in complex with a monomeric p62-PB1 mutant, where the massive electrostatic interactions between the acidic OPCA motif of PKCζ-PB1 and the basic surface of p62-PB1, as well as additional hydrogen bonds, ensure the formation of a stable and specific complex. The PKCζ-p62 interaction is interfered with the modification of a specific Cys of PKCζ by the antiarthritis drug aurothiomalate, though all four cysteine residues in the PKCζ-PB1 domain can be modified in in vitro assay. In addition, detailed structural and biochemical analyses demonstrate that the PB1 domains of aPKCs belong to the type I group, which can depolymerize the high-molecular-weight p62 aggregates into homo-oligomers of lower order. These data together unravel the molecular mechanisms of the homo-or hetero-interactions between p62 and PKCζ and provide the basis for designing inhibitors of NF-κB signaling.

Atypical protein kinase C couples cell sorting with primitive endoderm maturation in the mouse blastocyst

During mouse pre-implantation development, extra-embryonic primitive endoderm (PrE) and pluripotent epiblast precursors are specified in the inner cell mass (ICM) of the early blastocyst in a ‘salt and pepper’ manner, and are subsequently sorted into two distinct layers. Positional cues provided by the blastocyst cavity are thought to be instrumental for cell sorting; however, the sequence of events and the mechanisms that control this segregation remain unknown. Here, we show that atypical protein kinase C (aPKC), a protein associated with apicobasal polarity, is specifically enriched in PrE precursors in the ICM prior to cell sorting and prior to overt signs of cell polarisation. aPKC adopts a polarised localisation in PrE cells only after they reach the blastocyst cavity and form a mature epithelium, in a process that is dependent on FGF signalling. To assess the role of aPKC in PrE formation, we interfered with its activity using either chemical inhibition or RNAi knockdown. We show that inhibition of aPKC from the mid blastocyst stage not only prevents sorting of PrE precursors into a polarised monolayer but concomitantly affects the maturation of PrE precursors. Our results suggest that the processes of PrE and epiblast segregation, and cell fate progression are interdependent, and place aPKC as a central player in the segregation of epiblast and PrE progenitors in the mouse blastocyst.

Role of p62/SQSTM1 in liver physiology and pathogenesis

p62/sequestosome-1/A170/ZIP (hereafter referred to as p62) is a scaffold protein that has multiple functions, such as signal transduction, cell proliferation, cell survival, cell death, inflammation, tumourigenesis and oxidative stress response. While p62 is an autophagy substrate and is degraded by autophagy, p62 serves as an autophagy receptor for selective autophagic clearance of protein aggregates and organelles. Moreover, p62 functions as a signalling hub for various signalling pathways, including NF-κB, Nrf2 and mTOR. In this review, we discuss the pathophysiological role of p62 in the liver, including formation of hepatic inclusion bodies, cholestasis, obesity, insulin resistance, liver cell death and tumourigenesis.

Inhibition of mTOR pathway sensitizes acute myeloid leukemia cells to aurora inhibitors by suppression of glycolytic metabolism

Aurora kinases are overexpressed in large numbers of tumors and considered as potential therapeutic targets. In this study, we found that the Aurora kinases inhibitors MK-0457 (MK) and ZM447439 (ZM) induced polyploidization in acute myeloid leukemia (AML) cell lines. The level of glycolytic metabolism was significantly increased in the polyploidy cells, which were sensitive to glycolysis inhibitor 2-deoxy-D-glucose (2DG), suggesting that polyploidy cells might be eliminated by metabolism deprivation. Indeed, inhibition of mTOR pathway by mTOR inhibitors (rapamycin and PP242) or 2DG promoted not only apoptosis but also autophagy in the polyploidy cells induced by Aurora inhibitors. Mechanically, PP242 or2DGdecreased the level of glucose uptake and lactate production in polyploidy cells as well as the expression of p62/SQSTM1. Moreover, knockdown of p62/SQSTM1 sensitized cells to the Aurora inhibitor whereas overexpression of p62/SQSTM1 reduced drug efficacy. Thus, our results revealed that inhibition of mTOR pathway decreased the glycolytic metabolism of the polyploidy cells, and increased the efficacy of Aurora kinases inhibitors, providing a novel approach of combination treatment in AML.

NF-κB signaling and bone resorption

The transcription factor NF-κB is a family of proteins involved in signaling pathways essential for normal cellular functions and development. Deletion of various components of this pathway resulted with abnormal skeletal development. Research in the last decade has established that NF-κB signaling mediates RANK ligand-induced osteoclastogenesis. Consistently, it was shown that inhibition of NF-κB was an effective approach to inhibit osteoclast formation and bone resorptive activity. Identification of the molecular machinery underlying NF-κB activation permitted osteoclast-specific deletion of the major components of this pathway. As a result, it was clear that deletion of members of the proximal IKK kinase complex and the distal NF-κB subunits and downstream regulators affected skeletal development. These studies provided several targets of therapeutic intervention in osteolytic diseases. NF-κB activity has been also described as the centerpiece of inflammatory responses and is considered a potent mediator of inflammatory osteolysis. Indeed, inflammatory insults exacerbate physiologic RANKL-induced NF-κB signals leading to exaggerated responses and to inflammatory osteolysis. These superimposed NF-κB activities appear to underlie several bone pathologies. This review will describe the individual roles of NF-κB molecules in bone resorption and inflammatory osteolysis.

Cleavage of sequestosome 1/p62 by an enteroviral protease results in disrupted selective autophagy and impaired NFKB signaling

The adaptor protein, sequestosome 1 (SQSTM1)/p62, plays an essential role in mediating selective autophagy. It serves as an autophagy receptor targeting ubiquitinated proteins to autophagosomes for degradation. In addition, it functions as a scaffold protein to regulate signaling pathways. Here we explored the interplay between coxsackievirus B3 (CVB3) and SQSTM1-mediated selective autophagy. We reported that SQSTM1 was cleaved at glycine 241 following CVB3 infection through the activity of viral protease 2A(pro). The resulting cleavage fragments of SQSTM1 were no longer the substrates of autophagy, and their ability to form protein aggregates was greatly decreased. Although the C-terminal truncation sustained the binding activity of SQSTM1 to microtubule-associated protein 1 light chain (LC3), it failed to interact with ubiquitinated proteins. It was also found that colocalization between the C-terminal fragment of SQSTM1 (SQSTM1-C) and LC3 and ubiquitin within the punctate structures was markedly disrupted. Moreover, we observed that SQSTM1-C retained the ability of SQSTM1 to stabilize antioxidant transcription factor NFE2L2 [nuclear factor (erythroid-derived 2)-like 2]; however, both the N-terminal fragment of SQSTM1 (SQSTM1-N) and SQSTM1-C lost the function of SQSTM1 in activating NFKB (the nuclear factor of kappa light polypeptide gene enhancer in B-cells) pathway. Collectively, our results suggest a novel model by which cleavage of SQSTM1 as a result of CVB3 infection impairs the function of SQSTM1 in selective autophagy and host defense signaling.

New insights into the role of sequestosome 1/p62 mutant proteins in the pathogenesis of Paget's disease of bone

Paget's disease of bone (PDB) is characterized by focal areas of aberrant and excessive bone turnover, specifically increased bone resorption and disorganized bone formation. Germline mutations in the sequestosome 1/p62 (SQSTM1/p62) gene are common in PDB patients, with most mutations affecting the ubiquitin-associated domain of the protein. In vitro, osteoclast precursor cells expressing PDB-mutant SQSTM1/p62 protein are associated with increases in nuclear factor κB activation, osteoclast differentiation, and bone resorption. Although the precise mechanisms by which SQSTM1/p62 mutations contribute to disease pathogenesis and progression are not well defined, it is apparent that as well as affecting nuclear factor κB signaling, SQSTM1/p62 is a master regulator of ubiquitinated protein turnover via autophagy and the ubiquitin-proteasome system. Additional roles for SQSTM1/p62 in the oxidative stress-induced Keap1/Nrf2 pathway and in caspase-mediated apoptosis that were recently reported are potentially relevant to the pathogenesis of PDB. Thus, SQSTM1/p62 may serve as a molecular link or switch between autophagy, apoptosis, and cell survival signaling. The purpose of this review is to outline recent advances in understanding of the multiple pathophysiological roles of SQSTM1/p62 protein, with particular emphasis on their relationship to PDB, including challenges associated with translating SQSTM1/p62 research into clinical diagnosis and treatment.

K63 polyubiquitination and activation of mTOR by the p62-TRAF6 complex in nutrient-activated cells

The ability of cells to respond to changes in nutrient availability is critical for an adequate control of metabolic homeostasis. Mammalian target of rapamycin complex 1 (mTORC1) is a central complex kinase in these processes. The signaling adaptor p62 binds raptor, and integral component of the mTORC1 pathway. p62 interacts with TNF receptor associated factor 6 (TRAF6) and is required for mTORC1 translocation to the lysosome and its subsequent activation. Here we show that TRAF6 is recruited to and activates mTORC1 through p62 in amino acid-stimulated cells. We also show that TRAF6 is necessary for the translocation of mTORC1 to the lysosomes and that the TRAF6-catalyzed K63 ubiquitination of mTOR regulates mTORC1 activation by amino acids. TRAF6, through its interaction with p62 and activation of mTORC1, modulates autophagy and is an important mediator in cancer cell proliferation. Interfering with the p62-TRAF6 interaction serves to modulate autophagy and nutrient sensing.

Spinal TNF is necessary for inactivity-induced phrenic motor facilitation

A prolonged reduction in central neural respiratory activity elicits a form of plasticity known as inactivity-induced phrenic motor facilitation (iPMF), a 'rebound' increase in phrenic burst amplitude apparent once respiratory neural activity is restored. iPMF requires atypical protein kinase C (aPKC) activity within spinal segments containing the phrenic motor nucleus to stabilize an early transient increase in phrenic burst amplitude and to form long-lasting iPMF following reduced respiratory neural activity. Upstream signal(s) leading to spinal aPKC activation are unknown. We tested the hypothesis that spinal tumour necrosis factor-α (TNFα) is necessary for iPMF via an aPKC-dependent mechanism. Anaesthetized, ventilated rats were exposed to a 30 min neural apnoea; upon resumption of respiratory neural activity, a prolonged increase in phrenic burst amplitude (42 ± 9% baseline; P < 0.05) was apparent, indicating long-lasting iPMF. Pretreatment with recombinant human soluble TNF receptor 1 (sTNFR1) in the intrathecal space at the level of the phrenic motor nucleus prior to neural apnoea blocked long-lasting iPMF (2 ± 8% baseline; P > 0.05). Intrathecal TNFα without neural apnoea was sufficient to elicit long-lasting phrenic motor facilitation (pMF; 62 ± 7% baseline; P < 0.05). Similar to iPMF, TNFα-induced pMF required spinal aPKC activity, as intrathecal delivery of a ζ-pseudosubstrate inhibitory peptide (PKCζ-PS) 35 min following intrathecal TNFα arrested TNFα-induced pMF (28 ± 8% baseline; P < 0.05). These data demonstrate that: (1) spinal TNFα is necessary for iPMF; and (2) spinal TNFα is sufficient to elicit pMF via a similar aPKC-dependent mechanism. These data are consistent with the hypothesis that reduced respiratory neural activity elicits iPMF via a TNFα-dependent increase in spinal aPKC activity.

Bcl-2-dependent upregulation of autophagy by sequestosome 1/p62 in vitro

AIM

To investigate whether sequestosome 1/p62 (p62), a key cargo adaptor protein involved in both the ubiquitin-proteasome system and the autophagy-lysosome system, could directly regulate autophagy in vitro.

METHODS

HEK 293 cells or HeLa cells were transfected with p62-expressing plasmids or siRNA targeting p62. The cells or the cell lysates were subsequently subjected to immunofluorescence assay, immunoprecipitation assay, or immunoblot analysis. In vitro pulldown assay was used to study the interaction of p62 with Bcl-2.

RESULTS

Overexpression of p62 significantly increased the basal level of autophagy in both HEK 293 cells and HeLa cells, whereas knockdown of p62 significantly decreased the basal level of autophagy. In vitro pulldown assay showed that p62 directly interacted with Bcl-2. It was observed in HeLa cells that p62 co-localized with Bcl-2. Furthermore, knockdown of p62 in HEK 293 cells significantly increased the amount of Beclin 1 that co-immunoprecipitated with Bcl-2.

CONCLUSION

p62 induces autophagy by disrupting the association between Bcl-2 and Beclin 1.

p62 sequestosome 1/light chain 3b complex confers cytoprotection on lung epithelial cells after hyperoxia

Lung epithelial cell death is a prominent feature of hyperoxic lung injury, and has been considered a very important underlying mechanism of acute lung injury (ALI) and acute respiratory distress syndrome (ARDS). Here we report on a novel mechanism involved in epithelial cytoprotection and homeostasis after oxidative stress. p62 (sequestosome 1; SQSTM1) is a ubiquitously expressed cellular protein. It interacts with ubiquitinated proteins and autophagic marker light chain 3b (LC3b), thus mediating the degradation of selective targets. In this study, we explored the role of p62 in mitochondria-mediated cell death after hyperoxia. Lung alveolar epithelial cells demonstrate abundant p62 expression, and p62 concentrations are up-regulated by oxidative stress at both the protein and mRNA levels. The p62/LC3b complex interacts with Fas and truncated BID (tBID) physically. These interactions abruptly diminish after hyperoxia. The deletion of p62 robustly increases tBID and cleaved caspase-3, implying an antiapoptotic effect. This antiapoptotic effect of p62 is further confirmed by measuring caspase activities, cleaved poly ADP ribose polymerase, and cell viability. The deletion of the p62 PBI domain or the ubiquitin-associated domain both lead to elevated tBID, cleaved caspase-3, and significantly more cell death after hyperoxia. Moreover, p62 traffics in an opposite direction with LC3b after hyperoxia, leading to the dissociation of the p62/Cav-1/LC3b/BID complex. Subsequently, the LC3b-mediated lysosomal degradation of tBID is eliminated. Taken together, our data suggest that the p62/LC3b complex regulates lung alveolar epithelial cell homeostasis and cytoprotection after hyperoxia.

p62/SQSTM1 enhances NOD2-mediated signaling and cytokine production through stabilizing NOD2 oligomerization

NOD2 is a cytosolic pattern-recognition receptor that senses muramyl dipeptide of peptidoglycan that constitutes the bacterial cell wall, and plays an important role in maintaining immunological homeostasis in the intestine. To date, multiple molecules have shown to be involved in regulating NOD2 signaling cascades. p62 (sequestosome-1; SQSTM1) is a multifaceted scaffolding protein involved in trafficking molecules to autophagy, and regulating signal cascades activated by Toll-like receptors, inflammasomes and several cytokine receptors. Here, we show that p62 positively regulates NOD2-induced NF-κB activation and p38 MAPK, and subsequent production of cytokines IL-1β and TNF-α. p62 associated with the nucleotide binding domain of NOD2 through a bi-directional interaction mediated by either TRAF6-binding or ubiquitin-associated domains. NOD2 formed a large complex with p62 in an electron-dense area of the cytoplasm, which increased its signaling cascade likely through preventing its degradation. This study for the first time demonstrates a novel role of p62 in enhancing NOD2 signaling effects.

Effect of NF-κB inhibitor aurothiomalate on local inflammation in experimental Th1- and Th2-type immune response

We compared the effect of NF-κB inhibitor aurothiomalate and voltaren on local inflammation in different types of immune response. Both substances reduced edema caused by sheep erythrocytes (Th1-type immune response) and local immediate-type hypersensitivity response induced with ovalbumin (Th2-dependent response). The anti-inflammatory effects of aurothiomalate were similar to those of voltaren during Th1-type immune response.

Spinal atypical protein kinase C activity is necessary to stabilize inactivity-induced phrenic motor facilitation

The neural network controlling breathing must establish rhythmic motor output at a level adequate to sustain life. Reduced respiratory neural activity elicits a novel form of plasticity in circuits driving the diaphragm known as inactivity-induced phrenic motor facilitation (iPMF), a rebound increase in phrenic inspiratory output observed once respiratory neural drive is restored. The mechanisms underlying iPMF are unknown. Here, we demonstrate in anesthetized rats that spinal mechanisms give rise to iPMF and that iPMF consists of at least two mechanistically distinct phases: (1) an early, labile phase that requires atypical PKC (PKCζ and/or PKCι/λ) activity to transition to a (2) late, stable phase. Early (but not late) iPMF is associated with increased interactions between PKCζ/ι and the scaffolding protein ZIP (PKCζ-interacting protein)/p62 in spinal regions associated with the phrenic motor pool. Although PKCζ/ι activity is necessary for iPMF, spinal atypical PKC activity is not necessary for phrenic long-term facilitation (pLTF) following acute intermittent hypoxia, an activity-independent form of spinal respiratory plasticity. Thus, while iPMF and pLTF both manifest as prolonged increases in phrenic burst amplitude, they arise from distinct spinal cellular pathways. Our data are consistent with the hypotheses that (1) local mechanisms sense and respond to reduced respiratory-related activity in the phrenic motor pool and (2) inactivity-induced increases in phrenic inspiratory output require local PKCζ/ι activity to stabilize into a long-lasting iPMF. Although the physiological role of iPMF is unknown, we suspect that iPMF represents a compensatory mechanism, assuring adequate motor output in a physiological system in which prolonged inactivity ends life.

Protein kinase C-ζ mediates lung injury induced by diesel exhaust particles

Recently, we reported that diesel exhaust particles (DEPs) disrupt tight junctions (TJs) in alveolar epithelial cells (AECs) via an increase in reactive oxygen species (ROS). In this study, we investigated the role of protein kinase C (PKC)-ζ activation in DEP-induced lung injury. C57/bl6 mice were instilled intratracheally with 50 μl of saline containing 100 μg of DEPs or titanium dioxide (TiO2). Twenty-four hours later, bronchoalveolar lavage was performed to assess neutrophil counts and protein concentrations. In addition, in vitro experiments were performed in primary rat and human AECs exposed to DEPs (50 μg/cm(2)) for 3 hours. Transepithelial electrical conductance was measured, and TJ protein association was analyzed by immunoprecipitation. To determine whether the overexpression of antioxidants prevented DEP-induced lung injury, AECs and mice were infected with adenoviruses containing catalase and manganese superoxide dismutase (MnSOD) plasmids. In vivo, the overexpression of catalase and MnSOD prevented DEP-induced neutrophil recruitment. The inhibition of PKC-ζ activation also prevented DEP-induced neutrophil recruitment in vivo. In vitro, DEPs activated PKC-ζ in AECs, but not in alveolar macrophages. Using a specific myristolated PKC-ζ pseudosubstrate pepetide (PKC-ζ ps), we showed that PKC-ζ mediated the DEP-induced dissociation of occludin and zonula occludin-1 (ZO1) in rat and human AECs. In addition, the overexpression of constitutively active PKC-ζ induced the dissociation of occludin and ZO1 in AECs. DEP-induced TJ disruption occurs via PKC-ζ. TJ disruption seems to be in part responsible for DEP-induced lung injury.

TRAF6 and p62 inhibit amyloid β-induced neuronal death through p75 neurotrophin receptor

Amyloid β (Aβ) aggregates are the primary component of senile plaques in Alzheimer disease (AD) patient's brain. Aβ is known to bind p75 neurotrophin receptor (p75(NTR)) and mediates Aβ-induced neuronal death. Recently, we showed that NGF leads to p75(NTR) polyubiquitination, which promotes neuronal cell survival. Here, we demonstrate that Aβ stimulation impaired the p75(NTR) polyubiquitination. TRAF6 and p62 are required for polyubiquitination of p75(NTR) on NGF stimulation. Interestingly, we found that overexpression of TRAF6/p62 restored p75(NTR) polyubiquitination upon Aβ/NGF treatment. Aβ significantly reduced NF-κB activity by attenuating the interaction of p75(NTR) with IKKβ. p75(NTR) increased NF-κB activity by recruiting TRAF6/p62, which thereby mediated cell survival. These findings indicate that TRAF6/p62 abrogated the Aβ-mediated inhibition of p75(NTR) polyubiquitination and restored neuronal cell survival.

Prognostic impact of Beclin 1, p62/sequestosome 1 and LC3 protein expression in colon carcinomas from patients receiving 5-fluorouracil as adjuvant chemotherapy

Autophagy is a cellular degradation process that can be activated in tumor cells to confer stress tolerance. During autophagy initiation and autophagosome formation, Beclin 1 binds microtubule-associated protein-1 light chain 3 (LC3I) that is converted to its membrane-bound form (LC3II) and interacts with the ubiquitin-binding protein p62/sequestosome 1 (SQSTM1). We determined the association of Beclin 1, LC3 and p62 protein expression with clinical outcome in resected stage II and III colon carcinomas (n = 178) from participants in 5-fluororuacil (5-FU)-based adjuvant therapy trials. The immunopercentage for each marker was determined and dichotomized for analysis with overall survival (OS) using Cox models. We found that autophagy markers localized to the tumor cell cytoplasm and showed increased expression relative to normal epithelial cells. Overexpression of Beclin 1, LC3 and p62 proteins were detected in 69%, 79% and 85% of tumors, respectively. Expression levels were not significantly associated with clinicopathological variables. In a multivariable analysis adjusting for tumor grade, stage and patient age, Beclin 1 overexpression was independently associated with worse OS [hazard ratio (HR), 1.82; 95% confidence interval (CI), 1.0-3.3; p = 0.042] in patients who received 5-FU-based adjuvant therapy. Neither LC3 nor p62 overexpression was prognostic. In conclusion, Beclin 1 overexpression was associated with reduced survival in colon cancer patients treated with adjuvant 5-FU. These data are consistent with preclinical evidence indicating that autophagy can protect colon cancer cells from 5-FU and support the targeting of autophagy for therapeutic advantage in this malignancy.

Role of adipose and hepatic atypical protein kinase C lambda (PKCλ) in the development of obesity and glucose intolerance

PKCλ, an atypical member of the multifunctional protein kinase C family, has been implicated in the regulation of insulin-stimulated glucose transport and of the intracellular immune response. To further elucidate the role of this cellular regulator in diet-induced obesity and insulin resistance, we generated both liver (PKC-Alb) and adipose tissue (PKC-Ap2) specific knockout mice. Body weight, fat mass, food intake, glucose homeostasis and energy expenditure were evaluated in mice maintained on either chow or high fat diet (HFD). Ablation of PKCλ from the adipose tissue resulted in mice that were indistinguishable from their wild-type littermates. However, PKC-Alb mice were resistant to diet-induced obesity (DIO). Surprisingly this DIO resistance was not associated with either a reduction in caloric intake or an increase in energy expenditure as compared with their wild-type littermates. Furthermore, these mice displayed an improvement in glucose tolerance. When maintained on chow diet, these mice were similar to wild types in respect to body weight and fat mass, yet insulin sensitivity was impaired compared with wt littermates. Taken together these data suggest that hepatic PKCλ is modulating insulin-mediated glucose turnover and response to high fat diet feeding, thus offering a deeper understanding of an important target for anti-obesity therapeutics.

p62 binding to protein kinase C ζ regulates tumor necrosis factor α-induced apoptotic pathway in endothelial cells

OBJECTIVE

Protein kinase C (PKC) ζ is a key pathological mediator of endothelial cell apoptosis. p62 is a scaffold protein that regulates several cell signaling pathways by binding to target proteins. Because PKCζ and p62 contain Phox/Bem1p (PB1) modules that mediate protein-protein interactions, we hypothesized that an interaction between p62 and PKCζ is required for tumor necrosis factor α-induced PKCζ signaling in endothelial cells.

METHODS AND RESULTS

In human umbilical vein endothelial cell, tumor necrosis factor α (10 ng/mL) enhanced the interaction between p62 and PKCζ. Transfection with p62 small interfering RNA reduced the activation of both PKCζ and its downstream targets JNK and caspase 3, suggesting that p62 is necessary for PKCζ signaling. Overexpression of only the PB1 domain of p62 inhibited p62-PKCζ interaction, showing that binding of these 2 proteins is mediated by their PB1 domains. Furthermore, overexpression of the p62 PB1 domain suppressed tumor necrosis factor α-induced PKCζ activation and subsequent activation of JNK and caspase 3. Finally, transfection of either p62 small interfering RNA or the PB1 domain of p62 inhibited human umbilical vein endothelial cell apoptosis.

CONCLUSIONS

Our results suggest a novel function of p62 that regulates the activity of PKCζ by binding to PKCζ, thereby activating the PKCζ-JNK-caspase 3 apoptotic pathway in endothelial cells.

p62 at the interface of autophagy, oxidative stress signaling, and cancer

SIGNIFICANCE

Sequestosome 1 (p62/SQSTM1) is a multifunctional adapter protein implicated in selective autophagy, cell signaling pathways, and tumorigenesis.

RECENT ADVANCES

Recent evidence has revealed that p62/SQSTM1 has a critical role in an oxidative stress response pathway by its direct interaction with the ubiquitin ligase adaptor Kelch-like ECH-associated protein 1 (KEAP1), which results in constitutive activation of the transcription factor NF-E2-related factor 2 (NRF2).

CRITICAL ISSUES

Both NRF2 and KEAP1 are frequently mutated in cancer. The findings just cited uncover a link between p62/SQSTM1, autophagy, and the KEAP1-NRF2 stress response pathway in tumorigenesis and shed light on the interplay between autophagy and cancer.

FUTURE DIRECTIONS

Here, we review the mechanisms by which p62/SQSTM1 implements its multiple roles in the regulation of tumorigenesis with emphasis on the KEAP1-NRF2 stress response signaling pathway. Uncovering the molecular mechanisms of p62/SQSTM1 function in oxidative stress signaling might contribute to elucidating its role in tumorigenesis.

Tissue specific induction of p62/Sqstm1 by farnesoid X receptor

Background

Farnesoid X Receptor (FXR) is a member of the nuclear receptor superfamily and is a ligand-activated transcription factor essential for maintaining liver and intestinal homeostasis. FXR is protective against carcinogenesis and inflammation in liver and intestine as demonstrated by the development of inflammation and tumors in the liver and intestine of FXR knock-out mice. However, mechanisms for the protective effects of FXR are not completely understood. This study reports a novel role of FXR in regulating expression of Sqstm1, which encodes for p62 protein. p62 plays an important role in maintaining cellular homeostasis through selective autophagy and activating signal transduction pathways, such as NF-κB to support cell survival and caspase-8 to initiate apoptosis. FXR regulation of Sqstm1 may serve as a protective mechanism.

Methods and Results

This study showed that FXR bound to the Sqstm1 gene in both mouse livers and ileums as determined by chromatin immunoprecipitation. In addition, FXR activation enhanced transcriptional activation of Sqstm1 in vitro. However, wild-type mice treated with GW4064, a synthetic FXR ligand, showed that FXR activation induced mRNA and protein expression of Sqstm1/p62 in ileum, but not in liver. Interestingly, FXR-transgenic mice showed induced mRNA expression of Sqstm1 in both liver and ileum compared to wild-type mice.

Conclusions

Our current study has identified a novel role of FXR in regulating the expression of p62, a key factor in protein degradation and cell signaling. Regulation of p62 by FXR indicates tissue-specific and gene-dosage effects. Furthermore, FXR-mediated induction of p62 may implicate a protective mechanism of FXR.

Autophagy in the control and pathogenesis of viral infection

Autophagy is an evolutionary conserved cell process that plays a central role in eukaryotic cell metabolism. Constitutive autophagy allows cells to ensure their energy needs are met during times of starvation, degrade long-lived cellular proteins, and recycle organelles. In addition, autophagy and its machinery can also be utilized to degrade intracellular pathogens, and this function likely represents one of the earliest eukaryotic defense mechanisms against viral pathogens. Within the past decade, it has become clear that autophagy has not only retained its evolutionary ancient ability to degrade intracellular pathogens, but also has co-evolved with the vertebrate immune system to augment and fine tune antiviral immune responses. Herein, we aim to summarize these recent findings as well as to highlight key unanswered questions of the field.

Walnut diet reduces accumulation of polyubiquitinated proteins and inflammation in the brain of aged rats

An increase in the aggregation of misfolded/damaged polyubiquitinated proteins has been the hallmark of many age-related neurodegenerative diseases. The accumulation of these potentially toxic proteins in brain increases with age, in part due to increased oxidative and inflammatory stresses. Walnuts, rich in omega fatty acids, have been shown to improve memory, cognition and neuronal effects related to oxidative stress (OS) and inflammation (INF) in animals and human trials. The current study found that feeding 19-month-old rats with a 6% or 9% walnut diet significantly reduced the aggregation of polyubiquitinated proteins and activated autophagy, a neuronal housekeeping function, in the striatum and hippocampus. Walnut-fed animals exhibited up-regulation of autophagy through inhibiting phosphorylation of mTOR, up-regulating ATG7 and Beclin 1, and turnover of MAP1BLC3 proteins. The clearance of polyubiquitinated protein aggregates such as p62/SQSTM1 was more profound in hippocampus, a critical region in the brain involved in memory and cognitive performance, than striatum. The clearance of ubiquitinated aggregates was in tandem with significant reductions in OS/INF, as indicated by the levels of P38-MAP kinase and phosphorylations of nuclear factor kappa B and cyclic AMP response element binding protein. The results demonstrate the effectiveness of a walnut-supplemented diet in activating the autophagy function in brain beyond its traditionally known antioxidant and anti-inflammatory benefits.

The atypical PKCs in inflammation: NF-κB and beyond

From the very early days of nuclear factor-κB (NF-κB) research, it was recognized that different protein kinase C (PKC) isoforms might be involved in the activation of NF-κB. Pharmacological tools and pseudosubstrate inhibitors suggested that these kinases play a role in this important inflammatory and survival pathway; however, it was the analysis of several genetic mouse knockout models that revealed the complexity and interrelations between the different components of the PB1 network in several cellular functions, including T-cell biology, bone homeostasis, inflammation associated with the metabolic syndrome, and cancer. These studies unveiled, for example, the critical role of PKCζ as a positive regulator of NF-κB through the regulation of RelA but also its inflammatory suppressor activities through the regulation of the interleukin-4 signaling cascade. This observation is of relevance in T cells, where p62, PKCζ, PKCλ/ι, and NBR1 establish a mesh of interactions that culminate in the regulation of T-cell effector responses through the modulation of T-cell polarity. Many questions remain to be answered, not just from the point of view of the implication for NF-κB activation but also with regard to the in vivo interplay between these pathways in pathophysiological processes like obesity and cancer.

Structural insights into specificity and diversity in mechanisms of ubiquitin recognition by ubiquitin-binding domains

UBDs [Ub (ubiquitin)-binding domains], which are typically small protein motifs of <50 residues, are used by receptor proteins to transduce post-translational Ub modifications in a wide range of biological processes, including NF-κB (nuclear factor κB) signalling and proteasomal degradation pathways. More than 20 families of UBDs have now been characterized in structural detail and, although many recognize the canonical Ile44/Val70-binding patch on Ub, a smaller number have alternative Ub-recognition sites. The A20 Znf (A20-like zinc finger) of the ZNF216 protein is one of the latter and binds with high affinity to a polar site on Ub centred around Asp58/Gln62. ZNF216 shares some biological function with p62, with both linked to NF-κB signal activation and as shuttle proteins in proteasomal degradation pathways. The UBA domain (Ub-associated domain) of p62, although binding to Ub through the Ile44/Val70 patch, is unique in forming a stable dimer that negatively regulates Ub recognition. We show that the A20 Znf and UBA domain are able to form a ternary complex through independent interactions with a single Ub molecule, supporting functional models for Ub as a 'hub' for mediating multi-protein complex assembly and for enhancing signalling specificity.

Sequestosome 1/p62, a scaffolding protein, is a newly identified partner of IRS-1 protein

Defects in the insulin-signaling pathway may lead to the development of skeletal muscle insulin resistance, which is one of the earliest abnormalities detected in individuals with the metabolic syndrome and predisposes them to develop type 2 diabetes. Previous studies have shown that deletion of the mouse sequestosome 1/p62 gene results in mature-onset obesity that progresses to insulin and leptin resistance and, ultimately, type 2 diabetes. Sequestosome 1/p62 is involved in receptor-mediated signal transduction and functions as an intracellular signal modulator or adaptor protein. Insulin receptor substrate-1 (IRS-1) plays a central role in transducing the insulin signal via phosphorylation, protein-protein interactions, and protein modifications. Mapping studies demonstrated that the SH(2) domain at the amino terminus of sequestosome 1/p62 interacts with IRS-1 upon insulin stimulation. Further, IRS-1 interacts with p62 through its YMXM motifs at Tyr-608, Tyr-628, and/or Tyr-658 in a manner similar to its interaction with p85 of phosphoinositol 3-kinase. Overexpression of p62 increased phosphorylation of Akt, GLUT4 translocation, and glucose uptake, providing evidence that p62 participates in the insulin-signaling pathway through its interactions with IRS-1.

When autophagy meets cancer through p62/SQSTM1

Although p62/SQSTM1 was initially identified as an essential mediator of NFκB signaling, several recent studies have also highlighted its important role at the crossroad between the mTOR or MAPK signaling pathways and selective autophagy. The p62 structure containing important interaction domains attests to the ability of this protein to regulate and modulate the activation of these signaling pathways during tumor formation and propagation. The second very important function of this protein is to act as a molecular adaptor between the autophagic machinery and its substrates. Consequently, p62 is degraded following an increase in autophagic flux for which this protein currently serves as an indicator. However, the measurement of p62 expression strictly as a marker of autophagic flux is still controversial and can be misinterpreted mainly because this protein is subject to complex regulation at both the transcriptional and post-translational levels. Finally, because p62 is an autophagic substrate, it acts as a molecular link between cancer and autophagy by conferring a high level of selectivity through the degradation of important signaling molecules.

Interferon-γ ablation exacerbates myocardial hypertrophy in diastolic heart failure

Diastolic heart failure (HF) accounts for up to 50% of all HF admissions, with hypertension being the major cause of diastolic HF. Hypertension is characterized by left ventricular (LV) hypertrophy (LVH). Proinflammatory cytokines are increased in LVH and hypertension, but it is unknown if they mediate the progression of hypertension-induced diastolic HF. We sought to determine if interferon-γ (IFNγ) plays a role in mediating the transition from hypertension-induced LVH to diastolic HF. Twelve-week old BALB/c (WT) and IFNγ-deficient (IFNγKO) mice underwent either saline (n = 12) or aldosterone (n = 16) infusion, uninephrectomy, and fed 1% salt water for 4 wk. Tail-cuff blood pressure, echocardiography, and gene/protein analyses were performed. Isolated adult rat ventricular myocytes were treated with IFNγ (250 U/ml) and/or aldosterone (1 μM). Hypertension was less marked in IFNγKO-aldosterone mice than in WT-aldosterone mice (127 ± 5 vs. 136 ± 4 mmHg; P < 0.01), despite more LVH (LV/body wt ratio: 4.9 ± 0.1 vs. 4.3 ± 0.1 mg/g) and worse diastolic dysfunction (peak early-to-late mitral inflow velocity ratio: 3.1 ± 0.1 vs. 2.8 ± 0.1). LV ejection fraction was no different between IFNγKO-aldosterone vs. WT-aldosterone mice. LV end systolic dimensions were decreased significantly in IFNγKO-aldosterone vs. WT-aldosterone hearts (1.12 ± 0.1 vs. 2.1 ± 0.3 mm). Myocardial fibrosis and collagen expression were increased in both IFNγKO-aldosterone and WT-aldosterone hearts. Myocardial autophagy was greater in IFNγKO-aldosterone than WT-aldosterone mice. Conversely, tumor necrosis factor-α and interleukin-10 expressions were increased only in WT-aldosterone hearts. Recombinant IFNγ attenuated cardiac hypertrophy in vivo and modulated aldosterone-induced hypertrophy and autophagy in cultured cardiomyocytes. Thus IFNγ is a regulator of cardiac hypertrophy in diastolic HF and modulates cardiomyocyte size possibly by regulating autophagy. These findings suggest that IFNγ may mediate adaptive downstream responses and challenge the concept that inflammatory cytokines mediate only adverse effects.

Intermolecular binding between TIFA-FHA and TIFA-pT mediates tumor necrosis factor alpha stimulation and NF-κB activation

The forkhead-associated (FHA) domain recognizes phosphothreonine (pT) with high specificity and functional diversity. TIFA (TRAF-interacting protein with an FHA domain) is the smallest FHA-containing human protein. Its overexpression was previously suggested to provoke NF-κB activation, yet its exact roles in this signaling pathway and the underlying molecular mechanism remain unclear. Here we identify a novel threonine phosphorylation site on TIFA and show that this phosphorylated threonine (pT) binds with the FHA domain of TIFA, leading to TIFA oligomerization and TIFA-mediated NF-κB activation. Detailed analysis indicated that unphosphorylated TIFA exists as an intrinsic dimer and that the FHA-pT9 binding occurs between different dimers of TIFA. In addition, silencing of endogenous TIFA resulted in attenuation of tumor necrosis factor alpha (TNF-α)-mediated downstream signaling. We therefore propose that the TIFA FHA-pT9 binding provides a previously unidentified link between TNF-α stimulation and NF-κB activation. The intermolecular FHA-pT9 binding between dimers also represents a new mechanism for the FHA domain.

Rags connect mTOR and autophagy

mTOR, the master regulator of protein metabolism, is activated by growth factor signaling, amino acids and other nutrients. Emerging evidence indicates that an unexpected physical association between mTOR and lysosomes plays a critical role in amino acid mediated mTOR activation. Rag GTPases, together with a multi-protein complex called Ragulator, mediate amino acid-mediated mTOR recruitment to the lysosome surface where mTOR becomes activated. Furthermore, mTOR is also recruited to a unique cytoplasmic compartment composed of autolysosomes, which is observed in oncogenic Ras-induced senescent (RIS) cells. Formation of this TOR-autophagy spatial coupling compartment (TASCC) seems to allow activation of mTOR and autophagy in a mutually reinforcing manner. Proper formation of the TASCC also requires active Rag proteins. Interestingly, inhibition of activity of Rag proteins also suppresses acute induction of secretory protein synthesis during RIS. Thus, the TASCC provides evidence for the functional relevance of the Rag-mediated association between lysosomes and mTOR, and provides a mechanism for the simultaneous activation of anabolic and catabolic processes.

p62: a versatile multitasker takes on cancer

Since its initial discovery as an atypical protein kinase C (PKC)-interacting protein, p62 has emerged as a crucial molecule in a myriad of cellular functions. This multifunctional role of p62 is explained by its ability to interact with several key components of various signaling mechanisms. Not surprisingly, p62 is required for tumor transformation owing to its roles as a key molecule in nutrient sensing, as a regulator and substrate of autophagy, as an inducer of oxidative detoxifying proteins, and as a modulator of mitotic transit and genomic stability; all crucial events in the control of cell growth and cancer.

(-)-Epigallocatechin-3-gallate stimulates both AMP-activated protein kinase and nuclear factor-kappa B signaling pathways

We previously reported that (-)-epigallocatechin-3-gallate (EGCG) increased the level of SHARP-1 mRNA via a phosphoinositide 3-kinase/atypical protein kinase C lambda signaling pathway in rat H4IIE hepatoma cells. In the present study, we investigated other signaling pathway(s). Treating with either compound-C, BAY11-7082, or both, partially blocked the up-regulation of the SHARP-1 gene by EGCG. This suggests that AMP-activated protein kinase (AMPK)- and nuclear factor-kappa B (NF-κB)-signaling pathways were additively involved in the induction mediated by EGCG. Indeed, an AMPK activator induced a level of SHARP-1 mRNA. Although actinomycin D partially blocked the EGCG-induction of that SHARP-1 mRNA level, the nucleotide sequence between -1501 and -1 in the rat SHARP-1 gene did not positively respond to EGCG and NF-κB, respectively. Thus, we conclude that EGCG stimulates multiple signaling pathways in the SHARP-1 gene expression at the transcriptional and post-transcriptional levels and that there is no regulatory region susceptible to EGCG and NF-κB in the examined region.

Autophagy in regulation of Toll-like receptor signaling

Toll-like receptors (TLRs) serve as the major innate immune sensors for detection of specific molecular patterns on various pathogens. TLRs activate signaling events mainly by utilizing ubiquitin-dependent mechanisms. Recent research advances have provided evidence that TLR signaling is linked to induction of autophagy. Autophagy is currently known to affect both of the immune defense and suppression of inflammatory responses. In TLR-associated immune responses, autophagic lysis of intracellular microbes (called xenophagy) contributes to the former mechanism, while the latter seems to be mediated by the control of the mitochondrial integrity or selective autophagic clearance of aggregated signaling proteins (called aggrephagy). Several autophagy-related ubiquitin-binding proteins, such as SQSTM1/p62 and NDP52, mediate xenophagy and aggrephagy. In this review, we summarize the expanded knowledge regarding TLR signaling and autophagy signaling. After that, we will focus on autophagy-associated signaling downstream of TLRs and the effect of autophagy on TLR signaling, thus highlighting the signaling crosstalk between the TLR-associated innate immune responses and the regulation of innate immunity by xenophagy and aggrephagy.

Autophagy mechanism of right ventricular remodeling in murine model of pulmonary artery constriction

Although right ventricular failure (RVF) is the hallmark of pulmonary arterial hypertension (PAH), the mechanism of RVF is unclear. Development of PAH-induced RVF is associated with an increased reactive oxygen species (ROS) production. Increases in oxidative stress lead to generation of nitro-tyrosine residues in tissue inhibitor of metalloproteinase (TIMPs) and liberate active matrix metalloproteinase (MMPs). To test the hypothesis that an imbalance in MMP-to-TIMP ratio leads to interstitial fibrosis and RVF and whether the treatment with folic acid (FA) alleviates ROS generation, maintains MMP/TIMP balance, and regresses interstitial fibrosis, we used a mouse model of pulmonary artery constriction (PAC). After surgery mice were given FA in their drinking water (0.03 g/l) for 4 wk. Production of ROS in the right ventricle (RV) was measured using oxidative fluorescent dye. The level of MMP-2, -9, and -13 and TIMP-4, autophagy marker (p62), mitophagy marker (LC3A/B), collagen interstitial fibrosis, and ROS in the RV wall was measured. RV function was measured by Millar catheter. Treatment with FA decreased the pressure to 35 mmHg from 50 mmHg in PAC mice. Similarly, RV volume in PAC mice was increased compared with the Sham group. A robust increase of ROS was observed in RV of PAC mice, which was decreased by treatment with FA. The protein level of MMP-2, -9, and -13 was increased in RV of PAC mice in comparison with that in the sham-operated mice, whereas supplementation with FA abolished this effect and mitigated MMPs levels. The protein level of TIMP-4 was decreased in RV of PAC mice compared with the Sham group. Treatment with FA helped PAC mice to improve the level of TIMP-4. To further support the claim of mitophagy occurrence during RVF, the levels of LC3A/B and p62 were measured by Western blot and immunohistochemistry. LC3A/B was increased in RV of PAC mice. Similarly, increased p62 protein level was observed in RV of PAC mice. Treatment with FA abolished this effect in PAC mice. These results suggest that FA treatment improves MMP/TIMP balance and ameliorates mitochondrial dysfunction that results in protection of RV failure during pulmonary hypertension.

Activation of autophagy by inflammatory signals limits IL-1β production by targeting ubiquitinated inflammasomes for destruction

Autophagosomes delivers cytoplasmic constituents to lysosomes for degradation, whereas inflammasomes are molecular platforms activated by infection or stress that regulate the activity of caspase-1 and the maturation of interleukin 1β (IL-1β) and IL-18. Here we show that the induction of AIM2 or NLRP3 inflammasomes in macrophages triggered activation of the G protein RalB and autophagosome formation. The induction of autophagy did not depend on the adaptor ASC or capase-1 but was dependent on the presence of the inflammasome sensor. Blocking autophagy potentiated inflammasome activity, whereas stimulating autophagy limited it. Assembled inflammasomes underwent ubiquitination and recruited the autophagic adaptor p62, which assisted their delivery to autophagosomes. Our data indicate that autophagy accompanies inflammasome activation to temper inflammation by eliminating active inflammasomes.

KrasG12D-induced IKK2/β/NF-κB activation by IL-1α and p62 feedforward loops is required for development of pancreatic ductal adenocarcinoma

Constitutive Kras and NF-κB activation is identified as signature alterations in pancreatic ductal adenocarcinoma (PDAC). However, how NF-κB is activated in PDAC is not yet understood. Here, we report that pancreas-targeted IKK2/β inactivation inhibited NF-κB activation and PDAC development in Kras(G12D) and Kras(G12D);Ink4a/Arf(F/F) mice, demonstrating a mechanistic link between IKK2/β and Kras(G12D) in PDAC inception. Our findings reveal that Kras(G12D)-activated AP-1 induces IL-1α, which, in turn, activates NF-κB and its target genes IL-1α and p62, to initiate IL-1α/p62 feedforward loops for inducing and sustaining NF-κB activity. Furthermore, IL-1α overexpression correlates with Kras mutation, NF-κB activity, and poor survival in PDAC patients. Therefore, our findings demonstrate the mechanism by which IKK2/β/NF-κB is activated by Kras(G12D) through dual feedforward loops of IL-1α/p62.

The adaptor protein p62/SQSTM1 in osteoclast signaling pathways

Paget's disease of bone (PDB) is a skeletal disorder characterized by focal and disorganized increases in bone turnover and overactive osteoclasts. The discovery of mutations in the SQSTM1/p62 gene in numerous patients has identified protein p62 as an important modulator of bone turnover. In both precursors and mature osteoclasts, the interaction between receptor activator of NF-κB ligand (RANKL) and its receptor RANK results in signaling cascades that ultimately activate transcription factors, particularly NF-κB and NFATc1, promoting and regulating the osteoclast differentiation, activity, and survival. As a scaffold with multiple protein-protein interaction motifs, p62 is involved in virtually all the RANKL-activated osteoclast signaling pathways, along with being implicated in numerous other cellular processes. The p62 adaptor protein is one of the functional links reported between RANKL and TRAF6-mediated NF-κB activation, and also plays a major role as a shuttling factor that targets polyubiquitinated proteins for degradation by either the autophagy or proteasome pathways. The dysregulated expression and/or activity of p62 in bone disease up-regulates osteoclast functions. This review aims to outline and summarize the role of p62 in RANKL-induced signaling pathways and in ubiquitin-mediated signaling in osteoclasts, and the impact of PDB-associated p62 mutations on these processes.

Serine 403 phosphorylation of p62/SQSTM1 regulates selective autophagic clearance of ubiquitinated proteins

Selective macroautophagy (autophagy) of ubiquitinated protein is implicated as a compensatory mechanism of the ubiquitin-proteasome system. p62/SQSTM1 is a key molecule managing autophagic clearance of polyubiquitinated proteins. However, little is known about mechanisms controlling autophagic degradation of polyubiquitinated proteins. Here, we show that the specific phosphorylation of p62 at serine 403 (S403) in its ubiquitin-associated (UBA) domain increases the affinity between UBA and polyubiquitin chain, resulting in efficiently targeting polyubiquitinated proteins in "sequestosomes" and stabilizing sequestosome structure as a cargo of ubiquitinated proteins for autophagosome entry. Casein kinase 2 (CK2) phosphorylates S403 of p62 directly. Furthermore, CK2 overexpression or phosphatase inhibition reduces the formation of inclusion bodies of the polyglutamine-expanded huntingtin exon1 fragment in a p62-dependent manner. We propose that phosphorylation of p62 at S403 regulates autophagic clearance of ubiquitinated proteins and protein aggregates that are poorly degraded by proteasomes.

Selective autophagy mediated by autophagic adapter proteins

Mounting evidence suggests that autophagy is a more selective process than originally anticipated. The discovery and characterization of autophagic adapters, like p62 and NBR1, has provided mechanistic insight into this process. p62 and NBR1 are both selectively degraded by autophagy and able to act as cargo receptors for degradation of ubiquitinated substrates. A direct interaction between these autophagic adapters and the autophagosomal marker protein LC3, mediated by a so-called LIR (LC3-interacting region) motif, their inherent ability to polymerize or aggregate as well as their ability to specifically recognize substrates are required for efficient selective autophagy. These three required features of autophagic cargo receptors are evolutionarily conserved and also employed in the yeast cytoplasm-to-vacuole targeting (Cvt) pathway and in the degradation of P granules in C. elegans. Here, we review the mechanistic basis of selective autophagy in mammalian cells discussing the degradation of misfolded proteins, p62 bodies, aggresomes, mitochondria and invading bacteria. The emerging picture of selective autophagy affecting the regulation of cell signaling with consequences for oxidative stress responses, tumorigenesis and innate immunity is also addressed.