Regulation of ubiquitin-proteasome system mediated degradation by cytosolic stress

Proteomics uncovering possible key players in F508del-CFTR processing and trafficking

The achievement and maintenance of a protein native conformation is a very complex cellular process involving a multitude of key factors whose contribution to a successful folding remains to be elucidated. On top of this, it is known that correct folding is crucial for proteins to play their normal role and, consequently, for the maintenance of cellular homeostasis or proteostasis. If the folding process is affected, the protein is unable to achieve its native conformation, compromising its life and function, and a pathological condition may arise. Protein-misfolding diseases are characterized by either formation of protein aggregates that are toxic to the cell (gain-of-toxic-function diseases) or by an incorrect processing of proteins, which leads to a deficiency in protein activity (loss-of-function diseases). In this article we have focused on proteomics advances in the molecular knowledge of protein-misfolding diseases with direct impact on possible key players in F508del-CFTR processing and trafficking.

Connexins, cell motility, and the cytoskeleton

Connexins (Cx) comprise a family of transmembrane proteins, which form intercellular channels between plasma membranes of two adjoining cells, commonly known as gap junctions. Recent reports revealed that Cx proteins interact with diverse cellular components to form a multiprotein complex, which has been termed "Nexus". Potential interaction partners include proteins such as cytoskeletal proteins, scaffolding proteins, protein kinases and phosphatases. These interactions allow correct subcellular localization of Cxs and functional regulation of gap junction-mediated intercellular communication. Evidence is accruing that Cxs might have channel-independent functions, which potentially include regulation of cell migration, cell polarization and growth control. In the current review, we summarize recent knowledge on Cx interactions with cytoskeletal proteins and highlight some aspects of their role in cellular motility.

Lens gap junctions in growth, differentiation, and homeostasis

The cells of most mammalian organs are connected by groups of cell-to-cell channels called gap junctions. Gap junction channels are made from the connexin (Cx) family of proteins. There are at least 20 isoforms of connexins, and most tissues express more than 1 isoform. The lens is no exception, as it expresses three isoforms: Cx43, Cx46, and Cx50. A common role for all gap junctions, regardless of their Cx composition, is to provide a conduit for ion flow between cells, thus creating a syncytial tissue with regard to intracellular voltage and ion concentrations. Given this rather simple role of gap junctions, a persistent question has been: Why are there so many Cx isoforms and why do tissues express more than one isoform? Recent studies of lens Cx knockout (KO) and knock in (KI) lenses have begun to answer these questions. To understand these roles, one must first understand the physiological requirements of the lens. We therefore first review the development and structure of the lens, its numerous transport systems, how these systems are integrated to generate the lens circulation, the roles of the circulation in lens homeostasis, and finally the roles of lens connexins in growth, development, and the lens circulation.

Nerve growth factor attenuates 2-deoxy-d-glucose-triggered endoplasmic reticulum stress-mediated apoptosis via enhanced expression of GRP78

The glucose analog 2-deoxy-d-glucose (2DG) depletes cells of glucose. Inhibition of glycosylation caused by glucose depletion induces endoplasmic reticulum (ER) stress with subsequent apoptosis. Glucose-regulated protein 78 (GRP78) is a molecular chaperone that acts within the ER. During ER stress, GRP78 expression is induced as part of the unfolded protein response (UPR). We found that nerve growth factor (NGF) prevented 2DG-triggered ER stress-mediated apoptosis, but not the induction of GRP78 expression, in PC12 cells. Surprisingly, GRP78 expression was further up-regulated when NGF was added to 2DG-treated PC12 cells. When a specific inhibitor of phosphatidylinositol 3-kinase (PI3-K), LY294002, was added to 2DG plus NGF-treated cells, both the effects of NGF on 2DG-induced apoptosis and GRP78 expression were significantly diminished. In addition, versipelostatin (VST), a specific inhibitor of GRP78 expression, and small interfering RNA (siRNA) against GRP78 mRNA also decreased both the effects of NGF on 2DG-induced apoptosis and GRP78 expression. RT-PCR and Western blot analyses revealed that enhanced production of nuclear p50 ATF6, but not spliced XBP1, mainly contributed to the NGF-induced enhancement of GRP78 expression in 2DG-treated cells. These results suggest that the NGF-activated PI3-K/Akt signaling pathway plays a protective role against ER stress-mediated apoptosis via enhanced expression of GRP78 in PC12 cells.

Functional alterations of cardiac proteasomes under physiological and pathological conditions

The cardiac proteasome is a complex, heterogeneous, and dynamic organelle. Its function is regulated by its molecular organization, post-translational modifications, and associated partner proteins. Pressure overload, ischaemic heart disease, or genetic mutations in contractile proteins can cause heart failure, during which misfolded protein levels are elevated. At the same time, numerous interconnected signal transduction pathways are activated that may modulate any of the three proteasomal regulatory mechanisms mentioned above, resulting in functional changes in cardiac proteasomes. Many lines of evidence support the important role of the ubiquitin-proteasome system (UPS) in the development of heart diseases. Many researchers have focused on the UPS, applying new drug discovery methods not only in the field of cancer research but also in cardiovascular fields such as cardiac hypertrophy and ischaemic heart diseases. More understanding of UPS in the pathophysiology of heart diseases will lead to new routes for therapy.

Stressing the ubiquitin-proteasome system

Unfolded and misfolded proteins are inherently toxic to cells and have to be quickly and efficiently eliminated before they intoxicate the intracellular environment. This is of particular importance during proteotoxic stress when, as a consequence of intrinsic or extrinsic factors, the levels of misfolded proteins are transiently or persistently elevated. To meet this demand, metazoan cells have developed specific protein quality control mechanisms that allow the identification and proper handling of non-native proteins. An important defence mechanism is the specific destruction of these proteins by the ubiquitin-proteasome system (UPS). A number of studies have shown that various proteotoxic stress conditions can cause functional impairment of the UPS resulting in cellular dysfunction and apoptosis. In this review, we will summarize our current understanding of proteotoxic stress-induced dysfunction of the UPS and some of its implications for human pathologies.

Ubiquitin-like and ubiquitin-associated domain proteins: significance in proteasomal degradation

The ubiquitin-proteasome pathway of protein degradation is one of the major mechanisms that are involved in the maintenance of the proper levels of cellular proteins. The regulation of proteasomal degradation thus ensures proper cell functions. The family of proteins containing ubiquitin-like (UbL) and ubiquitin-associated (UBA) domains has been implicated in proteasomal degradation. UbL-UBA domain containing proteins associate with substrates destined for degradation as well as with subunits of the proteasome, thus regulating the proper turnover of proteins.

Ischemia-reperfusion-inducible protein modulates cell sensitivity to anticancer drugs by regulating activity of efflux transporter

Human ischemia-reperfusion-inducible protein (hIRIP) or hYrdC belongs to the SUA5/YrdC/YciO protein family and affects activity of a variety of cellular transporters. We observed that overexpression of wild-type or dominant-negative mutant of hIRIP protein affects the cellular sensitivity to anticancer drugs with different mechanisms of toxicity. Here we investigated in detail the effect of hIRIP on cell sensitivity to doxorubicin and show that hIRIP inhibits the drug efflux. Multidrug-resistant P-glycoprotein was identified as one of the target transporters. IRIP does not influence P-glycoprotein biosynthesis but affects its processing and promotes degradation. We also show that P-glycoprotein is associated with COP-α, one of the proteins of the COPI complex. This interaction is sensitive to the level of hIRIP expression. These findings suggest that hIRIP expression can regulate cargo assembly and function of efflux transporters, including P-glycoprotein, which mediates one of the most common mechanisms of the multidrug resistance.

Influence of glutamine infusion on ubiquitin, caspase-3, cathepsins L and B, and m-calpain expression in sheep with nutritionally induced metabolic acidosis

Provision of AA has shown success in attenuating proteolytic activity in monogastrics suffering from metabolic acidosis. However, it is unknown whether AA supplementation can provide any beneficial effects to ruminants with nutritionally induced metabolic acidosis. The objective of the current study was to examine the effects of glutamine infusion on various protein degradation components across several tissues in sheep with induced metabolic acidosis. Sheep were assigned to a randomized complete block design with 2 x 2 factorial arrangement of treatments (n = 6 sheep/treatment) consisting of a control or acidosis diet, and receiving a saline or L-glutamine infusion. Sheep were fed diets for 10 d and slaughtered on d 11. Liver, kidney, and muscle samples were collected at slaughter and examined for relative messenger RNA (mRNA) expression of ubiquitin, C8, E2, cathepsin L, cathepsin B, caspase-3, and m-calpain, as well as protein expression of ubiquitin. Relative mRNA expression of C8 (P = 0.02), E2 (P = 0.06), and ubiquitin (P = 0.07) was less in kidney in acidotic vs. control sheep. Additionally, mRNA expression of m-calpain in kidney was greater (P = 0.01) as a result of glutamine infusion. There were no significant alterations (P > 0.10) in mRNA of any component as a result of acidosis in the liver or muscle. This study demonstrates the inability of metabolic acidosis to increase expression of the ubiquitin-mediated proteolytic pathway in skeletal muscle; however, downregulation of renal mRNA expression of these components is apparent during the induction of metabolic acidosis.

The proteasome and its role in the degradation of oxidized proteins

The generation of free radicals and the resulting oxidative modification of cell structures are omnipresent in mammalian cells. This includes the permanent oxidation of proteins leading to the disruption of the protein structure and an impaired functionality. In consequence, these oxidized proteins have to be removed in order to prevent serious metabolic disturbances. The most important cellular proteolytic system responsible for the removal of oxidized proteins is the proteasomal system. For normal functioning, the proteasomal system needs the coordinated interaction of numerous components. This review describes the fundamental functions of the 20S "core" proteasome, its regulators, and the roles of the proteasomal system beyond the removal of oxidized proteins in mammalian cells.

Upregulation and maintenance of gap junctional communication in lens cells

The cells of the lens are joined by an extensive network of gap junction intercellular channels consisting of connexins 43, 46, and 50. We have proposed, and experimentally supported, the hypothesis that fibroblast growth factor (FGF) signaling is required for upregulation of gap junction-mediated intercellular coupling (GJIC) at the lens equator. The ability of FGF to increase GJIC in cultured lens cells requires sustained activation of extracellular signal-regulated kinase (ERK). In other cell types, activation of ERK has been shown to block GJIC mediated by connexin43 (Cx43). Why ERK signaling does not block lens cell coupling is not known. Another unresolved issue in lens gap junction regulation is how connexins, synthesized before the loss of biosynthetic organelles in mature lens fiber cells, avoid degradation during formation of the organelle-free zone. We have addressed these questions using serum-free cultures (termed DCDMLs) of primary embryonic chick lens epithelial cells. We show that FGF stimulates ERK in DCDMLs via the canonical Ras/Raf1 pathway, and that the reason that neither basal nor growth factor-stimulated GJIC is blocked by activation of ERK is because it is not mediated by Cx43. In fibroblastic cells, the normally rapid rate of degradation of Cx43 after its transport to the plasma membrane is reduced by treatments that either directly (ALLN; epoxomicin) or indirectly (generation of oxidatively un/mis-folded proteins by arsenic compounds) prevent the ubiquitin/proteasome system (UPS) from acting on its normal substrates. We show here that Cx45.6 and Cx56, the chick orthologs of mammalian Cx50 and Cx46, behave similarly in DCDMLs. When organelles lyse during the maturation of fiber cells, they release into the cytosol a large amount of new proteins that have the potential to saturate the capacity, and/or compromise the function, of the UPS. This would serve to spare gap junctions from degradation during formation of the organelle-free zone, thereby preserving GJIC between mature fiber cells despite the lack of de novo connexin synthesis.

One step at a time: endoplasmic reticulum-associated degradation

Protein folding in the endoplasmic reticulum (ER) is monitored by ER quality control (ERQC) mechanisms. Proteins that pass ERQC criteria traffic to their final destinations through the secretory pathway, whereas non-native and unassembled subunits of multimeric proteins are degraded by the ER-associated degradation (ERAD) pathway. During ERAD, molecular chaperones and associated factors recognize and target substrates for retrotranslocation to the cytoplasm, where they are degraded by the ubiquitin-proteasome machinery. The discovery of diseases that are associated with ERAD substrates highlights the importance of this pathway. Here, we summarize our current understanding of each step during ERAD, with emphasis on the factors that catalyse distinct activities.

Lens fiber connexin turnover and caspase-3-mediated cleavage are regulated alternately by phosphorylation

Lens connexins are phosphorylated in vivo; however, the function and regulation of the phosphorylation remain largely unknown. We have previously identified an in vivo phosphorylation site, Ser(364), at the COOH terminus of lens connexin (Cx) Cx45.6 and phosphorylation appears to regulate connexin protein turnover. To assess the specific mechanism of Ser(364) phosphorylation in Cx45.6, exogenous wild type and Ser(364) mutant Cx45.6 were expressed in primary lens cultures through retroviral infection. Cx45.6 turnover was attenuated primarily by proteasomal inhibitors and to a lesser extent by lysosomal inhibitors. Furthermore, the level of Cx45.6 protein in ubiquitin co-expressed cells was significantly reduced as compared to the cells expressing Cx45.6 alone. Moreover, overexpression of ubiquitin led to a more significant decrease in wild type Cx45.6 than Cx45.6(S364A), a mutant deficient of phosphorylation site at Ser(364), although we did not detect any difference in the levels of ubiquitination between wild type and mutant Cx45.6. Interestingly, the mutant mimicking constitutive phosphorylation, Cx45.6(S364D), partially prevented the cleavage of Cx45.6 by caspase-3. Together, our data suggest that phosphorylation of Cx45.6 at Ser(364) appears to stimulate Cx45.6 turnover primarily through proteasome pathway and this phosphorylation inhibits the cleavage of Cx45.6 by caspase-3. These findings provide further insights into regulatory mechanism of the specific phosphorylation of connexins in the lens.

The N-terminal domain of MyoD is necessary and sufficient for its nuclear localization-dependent degradation by the ubiquitin system

A growing number of proteins, including the myogenic transcription factor MyoD, are targeted for proteasomal degradation after N-terminal ubiquitination (NTU) where the first ubiquitin moiety is conjugated to the N-terminal residue rather than to an internal lysine. NTU might be essential in targeting both lysine-containing and naturally occurring lysine-less proteins such as p16(INK4a) and p14(ARF); however, the mechanisms that underlie this process are largely unknown. Specifically, the recognition motif(s) in the target substrates and the ubiquitin ligase(s) that catalyze NTU are still obscure. Here we show that the N-terminal domain of MyoD is critical for its degradation and that its destabilizing effect depends on nuclear localization of the protein. Deletion of the first 15 aa of MyoD blocked completely its lysine-independent degradation. Importantly, transfer of the first 30 N-terminal residues of MyoD to GFP destabilized this otherwise stable protein, and, here too, targeting for degradation depended on localization of the protein to the nucleus. Deletion of the N-terminal domain of lysine-less MyoD did not abolish completely ubiquitination of the protein, suggesting that this domain may be required for targeting the protein also in a postubiquitination step. Interestingly, NTU is evolutionarily conserved: in the yeast Saccharomyces cerevisiae lysine-less (LL) MyoD is degraded in a ubiquitin-, N-terminal domain-, and nuclear localization-dependent manner. Taken together, our data suggest that a short N-terminal segment of MyoD is necessary and sufficient to render MyoD susceptible for ubiquitin- and nuclear-dependent degradation.

Heat shock inhibits caspase-1 activity while also preventing its inflammasome-mediated activation by anthrax lethal toxin

Anthrax lethal toxin (LT) rapidly kills macrophages from certain mouse strains in a mechanism dependent on the breakdown of unknown protein(s) by the proteasome, formation of the Nalp1b (NLRP1b) inflammasome and subsequent activation of caspase-1. We report that heat-shocking LT-sensitive macrophages rapidly protects them against cytolysis by inhibiting caspase-1 activation without upstream effects on LT endocytosis or cleavage of the toxin's known cytosolic substrates (mitogen-activated protein kinases). Heat shock protection against LT occurred through a mechanism independent of de novo protein synthesis, HSP90 activity, p38 activation or proteasome inhibition and was downstream of mitogen-activated protein kinase cleavage and degradation of an unknown substrate by the proteasome. The heat shock inhibition of LT-mediated caspase-1 activation was not specific to the Nalp1b (NLRP1b) inflammasome, as heat shock also inhibited Nalp3 (NLRP3) inflammasome-mediated caspase-1 activation in macrophages. We found that heat shock induced pro-caspase-1 association with a large cellular complex that could prevent its activation. Additionally, while heat-shocking recombinant caspase-1 did not affect its activity in vitro, lysates from heat-shocked cells completely inhibited recombinant active caspase-1 activity. Our results suggest that heat shock inhibition of active caspase-1 can occur independently of an inflammasome platform, through a titratable factor present within intact, functioning heat-shocked cells.

TRPC1 binds to caveolin-3 and is regulated by Src kinase - role in Duchenne muscular dystrophy

Transient receptor potential canonical 1 (TRPC1), a widely expressed calcium (Ca(2+))-permeable channel, is potentially involved in the pathogenesis of Duchenne muscular dystrophy (DMD). Ca(2+) influx through stretch-activated channels, possibly formed by TRPC1, induces muscle-cell damage in the mdx mouse, an animal model of DMD. In this study, we showed that TRPC1, caveolin-3 and Src-kinase protein levels are increased in mdx muscle compared with wild type. TRPC1 and caveolin-3 colocalised and co-immunoprecipitated. Direct binding of TRPC1-CFP to caveolin-3-YFP was confirmed in C2 myoblasts by fluorescence energy resonance transfer (FRET). Caveolin-3-YFP targeted TRPC1-CFP to the plasma membrane. Hydrogen peroxide, a reactive oxygen species (ROS), increased Src activity and enhanced Ca(2+) influx, but only in C2 myoblasts co-expressing TRPC1 and caveolin-3. In mdx muscle, Tiron, a ROS scavenger, and PP2, a Src inhibitor, reduced stretch-induced Ca(2+) entry and increased force recovery. Because ROS production is increased in mdx/DMD, these results suggest that a ROS-Src-TRPC1/caveolin-3 pathway contributes to the pathogenesis of mdx/DMD.

The recognition and retrotranslocation of misfolded proteins from the endoplasmic reticulum

Secretory and membrane proteins that fail to fold in the endoplasmic reticulum (ER) are retained and may be sorted for ER-associated degradation (ERAD). During ERAD, ER-associated components such as molecular chaperones and lectins recognize folding intermediates and specific oligosaccharyl modifications on ERAD substrates. Substrates selected for ERAD are then targeted for ubiquitin- and proteasome-mediated degradation. Because the catalytic steps of the ubiquitin-proteasome system reside in the cytoplasm, soluble ERAD substrates that reside in the ER lumen must be retrotranslocated back to the cytoplasm prior to degradation. In contrast, it has been less clear how polytopic, integral membrane substrates are delivered to enzymes required for ubiquitin conjugation and to the proteasome. In this review, we discuss recent studies addressing how ERAD substrates are recognized, ubiquitinated and delivered to the proteasome and then survey current views of how soluble and integral membrane substrates may be retrotranslocated.