Degradation of microinjected proteins: effects of lysosomotropic agents and inhibitors of autophagy

Common Markers and Small Molecule Inhibitors in Golgi Studies

In this chapter, we provide a detailed guide for the application of commonly used small molecules to study Golgi structure and function in vitro. Furthermore, we have curated a concise, validated list of endomembrane markers typically used in downstream assays to examine the consequent effect on the Golgi via microscopy and western blot after drug treatment. This chapter will be useful for researchers beginning their foray into the field of intracellular trafficking and Golgi biology.

9,10-Phenanthrenequinone provokes dysfunction of brain endothelial barrier through down-regulating expression of claudin-5

Chronic exposure to diesel exhaust particle (DEP) is considered to provoke dysfunction of the blood-brain barrier, but the detailed molecular mechanism remains unclear. In this study, we investigated the toxic effects of five DEP components against human vascular cells and found that, among them, 9,10-phenanthrenequinone (9,10-PQ), a major tricyclic quinone in DEP, most potently elicits the cellular toxicities. Additionally, treatment with 9,10-PQ at its cytolethal concentrations (more than 2 μM) facilitated the production of reactive oxygen species (ROS), caspase activation, and DNA fragmentation in human brain microvascular endothelial (HBME) cells, inferring that high concentrations of 9,10-PQ elicit the cell apoptosis through the ROS-dependent mechanism. Measurement of trans-endothelial electrical resistance and paracellular permeability showed that treatment with sublethal concentrations (less than 1 μM) of 9,10-PQ elevates permeability across HBME cell monolayer. Immunofluorescence observation and Western blotting analysis also revealed that the 9,10-PQ treatment remarkably down-regulated the intercellular localization and expression of claudin-5 (CLDN5), a tight junctional protein that plays a key role in function of the blood-brain barrier, and the down-regulation was markedly recovered by pretreatment with a proteasome inhibitor Z-Leu-Leu-Leu-CHO. This result may indicate that sublethal concentrations of 9,10-PQ facilitate the dysfunction of the endothelial cell barrier through lowering in the expression and proteasomal proteolysis of CLDN5. The treatment with 9,10-PQ promoted nitric oxide (NO) production presumably through the induction of inducible NO synthase. In addition, the 9,10-PQ-mediated down-regulation of CLDN5 was ameliorated and deteriorated by pretreating with a scavenger and donor, respectively, of NO. Similarly to the 9,10-PQ treatment, treatment with a donor of peroxynitrite, a highly reactive oxidant formed by the reaction of NO and superoxide anion, resulted in the marked reduction of CLDN5 expression and elevation of 26S proteasome-based proteolytic activities. Thus, it is suggested that the formation of NO and peroxynitrite participates in the mechanism of brain endothelial cell barrier dysfunction elicited by 9,10-PQ.

Recent Insights into NCL Protein Function Using the Model Organism Dictyostelium discoideum

The neuronal ceroid lipofuscinoses (NCLs) are a group of devastating neurological disorders that have a global distribution and affect people of all ages. Commonly known as Batten disease, this form of neurodegeneration is linked to mutations in 13 genetically distinct genes. The precise mechanisms underlying the disease are unknown, in large part due to our poor understanding of the functions of NCL proteins. The social amoeba Dictyostelium discoideum has proven to be an exceptional model organism for studying a wide range of neurological disorders, including the NCLs. The Dictyostelium genome contains homologs of 11 of the 13 NCL genes. Its life cycle, comprised of both single-cell and multicellular phases, provides an excellent system for studying the effects of NCL gene deficiency on conserved cellular and developmental processes. In this review, we highlight recent advances in NCL research using Dictyostelium as a biomedical model.

A new meroterpenoid functions as an anti-tumor agent in hepatoma cells by downregulating mTOR activation and inhibiting EMT

Liver cancer, also known as primary liver cancer, is cancer that starts in the liver. JNU-144, a new meroterpenoid purified from Lithospermum erythrorhizon, has exhibited promising anticancer activity; however, the molecular mechanisms of action of JNU-144 on malignant cells remain unclear. Our studies revealed that JNU-144 suppressed cell viability and proliferation in hepatoma cells by downregulating mTOR activation. Meanwhile, JNU-144 activated the intrinsic apoptosis pathway and subsequently triggered apoptotic cell death in SMMC-7721 cells. We also found that JNU-144 inhibited the epithelial–mesenchymal transition in both SMMC-7721 and HepG2 cells through reprogramming of epithelial–mesenchymal transition (EMT)-related gene expression or regulating protein instability. These findings indicate that JNU-144 exerts potent anticancer activity in hepatoma cells and may be developed as a potential therapeutic drug.

Chloroquine inhibits autophagic flux by decreasing autophagosome-lysosome fusion

Macroautophagy/autophagy is a conserved transport pathway where targeted structures are sequestered by phagophores, which mature into autophagosomes, and then delivered into lysosomes for degradation. Autophagy is involved in the pathophysiology of numerous diseases and its modulation is beneficial for the outcome of numerous specific diseases. Several lysosomal inhibitors such as bafilomycin A1 (BafA1), protease inhibitors and chloroquine (CQ), have been used interchangeably to block autophagy in in vitro experiments assuming that they all primarily block lysosomal degradation. Among them, only CQ and its derivate hydroxychloroquine (HCQ) are FDA-approved drugs and are thus currently the principal compounds used in clinical trials aimed to treat tumors through autophagy inhibition. However, the precise mechanism of how CQ blocks autophagy remains to be firmly demonstrated. In this study, we focus on how CQ inhibits autophagy and directly compare its effects to those of BafA1. We show that CQ mainly inhibits autophagy by impairing autophagosome fusion with lysosomes rather than by affecting the acidity and/or degradative activity of this organelle. Furthermore, CQ induces an autophagy-independent severe disorganization of the Golgi and endo-lysosomal systems, which might contribute to the fusion impairment. Strikingly, HCQ-treated mice also show a Golgi disorganization in kidney and intestinal tissues. Altogether, our data reveal that CQ and HCQ are not bona fide surrogates for other types of late stage lysosomal inhibitors for in vivo experiments. Moreover, the multiple cellular alterations caused by CQ and HCQ call for caution when interpreting results obtained by blocking autophagy with this drug.

2BC Non-Structural Protein of Enterovirus A71 Interacts with SNARE Proteins to Trigger Autolysosome Formation

Viruses have evolved unique strategies to evade or subvert autophagy machinery. Enterovirus A71 (EV-A71) induces autophagy during infection in vitro and in vivo. In this study, we report that EV-A71 triggers autolysosome formation during infection in human rhabdomyosarcoma (RD) cells to facilitate its replication. Blocking autophagosome-lysosome fusion with chloroquine inhibited virus RNA replication, resulting in lower viral titres, viral RNA copies and viral proteins. Overexpression of the non-structural protein 2BC of EV-A71 induced autolysosome formation. Yeast 2-hybrid and co-affinity purification assays showed that 2BC physically and specifically interacted with a N-ethylmaleimide-sensitive factor attachment receptor (SNARE) protein, syntaxin-17 (STX17). Co-immunoprecipitation assay further showed that 2BC binds to SNARE proteins, STX17 and synaptosome associated protein 29 (SNAP29). Transient knockdown of STX17, SNAP29, and microtubule-associated protein 1 light chain 3B (LC3B), crucial proteins in the fusion between autophagosomes and lysosomes) as well as the lysosomal-associated membrane protein 1 (LAMP1) impaired production of infectious EV-A71 in RD cells. Collectively, these results demonstrate that the generation of autolysosomes triggered by the 2BC non-structural protein is important for EV-A71 replication, revealing a potential molecular pathway targeted by the virus to exploit autophagy. This study opens the possibility for the development of novel antivirals that specifically target 2BC to inhibit formation of autolysosomes during EV-A71 infection.

Autophagy in malignant transformation and cancer progression

Autophagy plays a key role in the maintenance of cellular homeostasis. In healthy cells, such a homeostatic activity constitutes a robust barrier against malignant transformation. Accordingly, many oncoproteins inhibit, and several oncosuppressor proteins promote, autophagy. Moreover, autophagy is required for optimal anticancer immunosurveillance. In neoplastic cells, however, autophagic responses constitute a means to cope with intracellular and environmental stress, thus favoring tumor progression. This implies that at least in some cases, oncogenesis proceeds along with a temporary inhibition of autophagy or a gain of molecular functions that antagonize its oncosuppressive activity. Here, we discuss the differential impact of autophagy on distinct phases of tumorigenesis and the implications of this concept for the use of autophagy modulators in cancer therapy.

Vacuolin-1 potently and reversibly inhibits autophagosome-lysosome fusion by activating RAB5A

Autophagy is a catabolic lysosomal degradation process essential for cellular homeostasis and cell survival. Dysfunctional autophagy has been associated with a wide range of human diseases, e.g., cancer and neurodegenerative diseases. A large number of small molecules that modulate autophagy have been widely used to dissect this process and some of them, e.g., chloroquine (CQ), might be ultimately applied to treat a variety of autophagy-associated human diseases. Here we found that vacuolin-1 potently and reversibly inhibited the fusion between autophagosomes and lysosomes in mammalian cells, thereby inducing the accumulation of autophagosomes. Interestingly, vacuolin-1 was less toxic but at least 10-fold more potent in inhibiting autophagy compared with CQ. Vacuolin-1 treatment also blocked the fusion between endosomes and lysosomes, resulting in a defect in general endosomal-lysosomal degradation. Treatment of cells with vacuolin-1 alkalinized lysosomal pH and decreased lysosomal Ca²⁺ content. Besides marginally inhibiting vacuolar ATPase activity, vacuolin-1 treatment markedly activated RAB5A GTPase activity. Expression of a dominant negative mutant of RAB5A or RAB5A knockdown significantly inhibited vacuolin-1-induced autophagosome-lysosome fusion blockage, whereas expression of a constitutive active form of RAB5A suppressed autophagosome-lysosome fusion. These data suggest that vacuolin-1 activates RAB5A to block autophagosome-lysosome fusion. Vacuolin-1 and its analogs present a novel class of drug that can potently and reversibly modulate autophagy.

Autophagy in blood cancers: biological role and therapeutic implications

Autophagy is a cell recycling process the molecular apparatus of which has been identified over the past decade. Autophagy allows cells to survive starvation and inhospitable conditions and plays a key role in numerous physiological functions, including hematopoiesis and immune responses. In hematologic malignancies, autophagy can either act as a chemo-resistance mechanism or have tumor suppressive functions, depending on the context. In addition, autophagy is involved in other important aspects of blood cancers as it promotes immune competence and anti-cancer immunity, and may even help enhance patient tolerance to standard treatments. Approaches exploiting autophagy, either to activate or inhibit it, could find broad application in hematologic malignancies and contribute to improved clinical outcomes. These aspects are discussed here together with a brief introduction to the molecular machinery of autophagy and to its role in blood cell physiology.

IPF fibroblasts are desensitized to type I collagen matrix-induced cell death by suppressing low autophagy via aberrant Akt/mTOR kinases

Idiopathic pulmonary fibrosis (IPF) is a chronic, lethal interstitial lung disease in which the aberrant PTEN/Akt axis plays a major role in conferring a survival phenotype in response to the cell death inducing properties of type I collagen matrix. The underlying mechanism by which IPF fibroblasts become desensitized to polymerized collagen, thereby eluding collagen matrix-induced cell death has not been fully elucidated. We hypothesized that the pathologically altered PTEN/Akt axis suppresses autophagy via high mTOR kinase activity, which subsequently desensitizes IPF fibroblasts to collagen matrix induced cell death. We found that the autophagosome marker LC3-2 expression is suppressed, while mTOR activity remains high when IPF fibroblasts are cultured on collagen. However, LC3-2 expression increased in response to IPF fibroblast attachment to collagen in the presence of rapamycin. In addition, PTEN over-expression or Akt inhibition suppressed mTOR activity, thereby increasing LC3-2 expression in IPF fibroblasts. Furthermore, the treatment of IPF fibroblasts over-expressing PTEN or dominant negative Akt with autophagy inhibitors increased IPF fibroblast cell death. Enhanced p-mTOR expression along with low LC3-2 expression was also found in myofibroblasts within the fibroblastic foci from IPF patients. Our data show that the aberrant PTEN/Akt/mTOR axis desensitizes IPF fibroblasts from polymerized collagen driven stress by suppressing autophagic activity, which produces a viable IPF fibroblast phenotype on collagen. This suggests that the aberrantly regulated autophagic pathway may play an important role in maintaining a pathological IPF fibroblast phenotype in response to collagen rich environment.

Degradation of M(r) 25,000 protein by cathepsin L-like protease in Xenopus laevis oocytes

A phosphorylated protein with a molecular mass of 25,000 (pp25) is involved in Xenopus laevis vitellogenin B1 and partially overlaps with phosvitin and lipovitellin 2. The protease responsible for pp25 degradation was studied in vitro since this occurs during embryogenesis. Initially, a protease thought to be a contaminant of the purified pp25 preparation was analyzed and an antipain-sensitive protease presumed to be involved. When commercially available proteases were examined, pp25 was not degraded by calpain I or 20S proteasome, but it was degraded by cathepsin L in vitro. A survey of the protease responsible for pp25 degradation in the cytoplasm of Xenopus oocytes found partially purified pp25 was degraded in partly antipain-sensitive manner. These results suggest that an antipain-sensitive protease or cathepsin L (or a related protease) is a candidate for pp25 degradation.

Mst1 inhibits autophagy by promoting the interaction between Beclin1 and Bcl-2

Here we show that Mst1, a proapoptotic kinase, impairs protein quality control mechanisms in the heart through inhibition of autophagy. Stress-induced activation of Mst1 in cardiomyocytes promoted accumulation of p62 and aggresome formation, accompanied by the disappearance of autophagosomes. Mst1 phosphorylated the Thr108 residue in the BH3 domain of Beclin1, which enhanced the interaction between Beclin1 and Bcl-2 and/or Bcl-xL, stabilized the Beclin1 homodimer, inhibited the phosphatidylinositide 3-kinase activity of the Atg14L-Beclin1-Vps34 complex and suppressed autophagy. Furthermore, Mst1-induced sequestration of Bcl-2 and Bcl-xL by Beclin1 allows Bax to become active, thereby stimulating apoptosis. Mst1 promoted cardiac dysfunction in mice subjected to myocardial infarction by inhibiting autophagy, associated with increased levels of Thr108-phosphorylated Beclin1. Moreover, dilated cardiomyopathy in humans was associated with increased levels of Thr108-phosphorylated Beclin1 and signs of autophagic suppression. These results suggest that Mst1 coordinately regulates autophagy and apoptosis by phosphorylating Beclin1 and consequently modulating a three-way interaction among Bcl-2 proteins, Beclin1 and Bax.

Intracellular vesicle acidification promotes maturation of infectious poliovirus particles

The autophagic pathway acts as part of the immune response against a variety of pathogens. However, several pathogens subvert autophagic signaling to promote their own replication. In many cases it has been demonstrated that these pathogens inhibit or delay the degradative aspect of autophagy. Here, using poliovirus as a model virus, we report for the first time bona fide autophagic degradation occurring during infection with a virus whose replication is promoted by autophagy. We found that this degradation is not required to promote poliovirus replication. However, vesicular acidification, which in the case of autophagy precedes delivery of cargo to lysosomes, is required for normal levels of virus production. We show that blocking autophagosome formation inhibits viral RNA synthesis and subsequent steps in the virus cycle, while inhibiting vesicle acidification only inhibits the final maturation cleavage of virus particles. We suggest that particle assembly, genome encapsidation, and virion maturation may occur in a cellular compartment, and we propose the acidic mature autophagosome as a candidate vesicle. We discuss the implications of our findings in understanding the late stages of poliovirus replication, including the formation and maturation of virions and egress of infectious virus from cells.

The autophagic degradation pathway is a well-known agent of innate immunity. Several pathogens, including poliovirus (PV), a model for several medically important RNA viruses, subvert this pathway for their own benefit. In doing so, pathogens often inhibit the degradative portion of the pathway, presumably to prevent their own destruction. We show here that, surprisingly, PV infection results in high levels of degradative autophagy. However, we find that autophagic degradation is dispensable for PV replication. Inhibiting the formation of autophagosomes inhibits virus RNA replication and subsequent steps in virus production. Inhibiting the acidification of vesicles, which in the case of autophagosomes precedes fusion with lysosomes and autophagic degradation, inhibits a much later step in virus production. Our data suggest an important role for an acidic compartment of the cell in the final maturation step, cleaving a capsid protein to generate infectious virus. Importantly, these data also call into question the long-standing hypothesis that all steps in the production of infectious poliovirus are cytosolic.

BAMBI is expressed in endothelial cells and is regulated by lysosomal/autolysosomal degradation

BACKGROUND

BAMBI (BMP and Activin Membrane Bound Inhibitor) is considered to influence TGFβ and Wnt signaling, and thereby fibrosis. Surprisingly data on cell type-specific expression of BAMBI are not available. We therefore examined the localization, gene regulation, and protein turnover of BAMBI in kidneys.

METHODOLOGY/PRINCIPAL FINDINGS

By immunofluorescence microscopy and by mRNA expression, BAMBI is restricted to endothelial cells of the glomerular and some peritubular capillaries and of arteries and veins in both murine and human kidneys. TGFβ upregulated mRNA of BAMBI in murine glomerular endothelial cells (mGEC). LPS did not downregulate mRNA for BAMBI in mGEC or in HUVECs. BAMBI mRNA had a half-life of only 60 minutes and was stabilized by cycloheximide, indicating post-transcriptional regulation due to AU-rich elements, which we identified in the 3' untranslated sequence of both the human and murine BAMBI gene. BAMBI protein turnover was studied in HUVECs with BAMBI overexpression using a lentiviral system. Serum starvation as an inducer of autophagy caused marked BAMBI degradation, which could be totally prevented by inhibition of lysosomal and autolysosomal degradation with bafilomycin, and partially by inhibition of autophagy with 3-methyladenine, but not by proteasomal inhibitors. Rapamycin activates autophagy by inhibiting TOR, and resulted in BAMBI protein degradation. Both serum starvation and rapamycin increased the conversion of the autophagy marker LC3 from LC3-I to LC3-II and also enhanced co-staining for BAMBI and LC3 in autolysosomal vesicles.

CONCLUSIONS/SIGNIFICANCE

1. BAMBI localizes to endothelial cells in the kidney and to HUVECs. 2. BAMBI mRNA is regulated by post-transcriptional mechanisms. 3. BAMBI protein is regulated by lysosomal and autolysosomal degradation. The endothelial localization and the quick turnover of BAMBI may indicate novel, yet to be defined functions of this modulator for TGFβ and Wnt protein actions in the renal vascular endothelium in health and disease.

Beclin1-binding UVRAG targets the class C Vps complex to coordinate autophagosome maturation and endocytic trafficking

Autophagic and endocytic pathways are tightly regulated membrane rearrangement processes that are crucial for homeostasis, development and disease. Autophagic cargo is delivered from autophagosomes to lysosomes for degradation through a complex process that topologically resembles endosomal maturation. Here, we report that a Beclin1-binding autophagic tumour suppressor, UVRAG, interacts with the class C Vps complex, a key component of the endosomal fusion machinery. This interaction stimulates Rab7 GTPase activity and autophagosome fusion with late endosomes/lysosomes, thereby enhancing delivery and degradation of autophagic cargo. Furthermore, the UVRAG-class-C-Vps complex accelerates endosome-endosome fusion, resulting in rapid degradation of endocytic cargo. Remarkably, autophagosome/endosome maturation mediated by the UVRAG-class-C-Vps complex is genetically separable from UVRAG-Beclin1-mediated autophagosome formation. This result indicates that UVRAG functions as a multivalent trafficking effector that regulates not only two important steps of autophagy - autophagosome formation and maturation - but also endosomal fusion, which concomitantly promotes transport of autophagic and endocytic cargo to the degradative compartments.

What determines the degradation rate of an injected protein?

The fusion of cultured mammalian cells to red blood cells loaded with specific proteins provides a powerful system for the study of intracellular proteolysis. During the past four years the degradation rates of more than 30 proteins have been examined after their injection into HeLa cells. Results from these studies support the legitimacy of the microinjection approach. They also provide insight into the mechanism of intracellular proteolysis.

Proteolytic and lipolytic responses to starvation

Mammals survive starvation by activating proteolysis and lipolysis in many different tissues. These responses are triggered, at least in part, by changing hormonal and neural statuses during starvation. Pathways of proteolysis that are activated during starvation are surprisingly diverse, depending on tissue type and duration of starvation. The ubiquitin-proteasome system is primarily responsible for increased skeletal muscle protein breakdown during starvation. However, in most other tissues, lysosomal pathways of proteolysis are stimulated during fasting. Short-term starvation activates macroautophagy, whereas long-term starvation activates chaperone-mediated autophagy. Lipolysis also increases in response to starvation, and the breakdown of triacylglycerols provides free fatty acids to be used as an energy source by skeletal muscle and other tissues. In addition, glycerol released from triacylglycerols can be converted to glucose by hepatic gluconeogenesis. During long-term starvation, oxidation of free fatty acids by the liver leads to the production of ketone bodies that can be used for energy by skeletal muscle and brain. Tissues that cannot use ketone bodies for energy respond to these small molecules by activating chaperone-mediated autophagy. This is one form of interaction between proteolytic and lipolytic responses to starvation.

Mechanisms of chaperone-mediated autophagy

Chaperone-mediated autophagy is one of several lysosomal pathways of proteolysis. This pathway is activated by physiological stresses such as prolonged starvation. Cytosolic proteins with particular peptide sequence motifs are recognized by a complex of molecular chaperones and delivered to lysosomes. No vesicular traffic is required for this protein degradation pathway, so it differs from microautophagy and macroautophagy. Protein substrates bind to a receptor in the lysosomal membrane, the lysosome-associated membrane protein (lamp) type 2a. Levels of lamp2a in the lysosomal membrane are controlled by alterations in the lamp2a half-life as well as by the dynamic distribution of the protein between the lysosomal membrane and the lumen. Substrate proteins are unfolded before transport into the lysosome lumen, and the transport of substrate proteins requires a molecular chaperone within the lysosomal lumen. The exact roles of this lysosomal chaperone remain to be defined. The mechanisms of chaperone-mediated autophagy are similar to mechanisms of protein import into mitochondria, chloroplasts, and the endoplasmic reticulum.

Ubiquitin-protein conjugates in different structures of the central nervous system of the rat

The capacity to form ubiquitin (Ub)-protein conjugates was investigated in the cytosol of different structures of the rat central nervous system (CNS) in order to confirm the presence of this extralysosomal, adenosine triphosphate (ATP)-dependent, protein degradation system as well as its structural localization. Using 125I-Ub, we found that in the presence of ATP, the cytosol obtained from whole brains was able to form high molecular weight Ub-protein conjugates. These conjugates could be detected after sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and radioautography. The formation of these conjugates was much higher in the cerebral cortex than in the brain stem, which is mainly constituted by white matter, being intermediate in the cytosol isolated from whole brain total homogenates. These results suggested to us that under normal conditions the capacity to form Ub-protein conjugates was mainly located in structures containing neuronal cell bodies. Strong support for this contention was obtained when the cytosol isolated from rat optic nerves or from oligodendroglial cells isolated from whole brain was found to be totally unable to form Ub-protein conjugates. The inability of certain CNS structures to form conjugates with Ub could be attributed, among other reasons, to the lack of enzymes catalyzing the various steps of the Ub degradation system, to the absence of short half-life (target) proteins in those structures, or to the lack of activity of the enzymes catalyzing the reaction due to regulatory control mechanisms operating under normal conditions.

Ubiquitin metabolism in HeLa cells starved of amino acids

Radio-iodinated ubiquitin (Ub) was introduced into HeLa cells by red blood cell-mediated microinjection. The half-life and solubility of Ub, as well as the molecular weight distributions of Ub conjugates, were then measured in HeLa cells grown in complete medium or in medium lacking amino acids and fetal calf serum. Ub metabolism was similar in the two sets of cells. Thus, the dramatic changes in Ub metabolism induced by thermal stress are not observed upon amino acid deprivation.

Identification of a ubiquitin- and ATP-dependent protein degradation pathway in rat cerebral cortex

To investigate the existence of a ubiquitin-dependent protein degradation system in the brain, the proteolytic activity of the cerebral cortex was examined. The soluble extract of rat cerebral cortex degraded 125I-radiolabeled lysozyme in an ATP-dependent manner. The ATP-dependent proteolysis was suppressed with iodoacetamide, which inhibits ubiquitin conjugation, and was abolished by blocking of the amino residues of lysozyme. These results suggest the participation of ubiquitination in the proteolytic activity. An ATP-dependent 125I-ubiquitin-conjugating activity was detected in fraction II from the cerebral cortex. The presence of ATP-dependent proteolytic activity which acted preferentially on ubiquitinated lysozyme was demonstrated, using ubiquitin-125I-lysozyme conjugates as a substrate. The proteinase had a molecular mass of 1500 kDa and displayed nucleotide dependence and sensitivity to various proteinase inhibitors similar to those of the 26S proteinase complex found in reticulocytes. Dialysis of the soluble fraction caused a decrease in the proteolytic activity of ATP-dependent and preferential for ubiquitin-lysozyme conjugates and a reciprocal increase in the ATP-independent free 125I-lysozyme-degrading activity which was scarcely detected before dialysis. The former ATP-dependent proteolytic activity may play a physiological role in the brain.

Peptide sequences that target cytosolic proteins for lysosomal proteolysis

Lysosomes take up and degrade intracellular proteins in cultured cells in response to serum deprivation, and in tissues of organisms in response to starvation. One mechanism by which proteins enter lysosomes for subsequent degradation requires that substrate proteins contain peptide sequences biochemically related to Lys-Phe-Glu-Arg-Gln (KFERQ).

Purification and characterization of a protease from Xenopus embryos

A proteolytic enzyme was purified from Xenopus embryos. The purification procedure consisted of fractionation of an extract of embryos with acetone, gel filtration of Sephadex G-75 and chromatography on carboxymethyl-cellulose and hydroxylapatite. The preparation of enzyme appeared to be homogeneous as judged by electrophoresis in polyacrylamide gels. This protease had a molecular mass of 43-44 kDa and was composed of two subunits with molecular masses of 30 kDa and 13 kDa. The optimal pH of the reaction catalysed by the protease was approximately 4.0. This proteolytic activity was inhibited by antipain, leupeptin and iodoacetic acid; it was not affected by phenylmethylsulfonyl fluoride and pepstatin; and it was enhanced by dithiothreitol. In the presence of RNA, the optimal pH was shifted from pH 4.0 to pH 4.5. The protease was activated by addition of total RNA from Xenopus embryos, by poly(rU) or poly(rG). In contrast, after addition of tRNA or poly(rC), no activation of the protease was observed.

Effects of inhibitors on aldolase breakdown after its microinjection into HeLa cells

1. The regulation of protein breakdown as well as the generation of intermediates in the pathway from intact protein to amino acids was investigated by using 3H-labelled N-ethylmaleimide-modified aldolase (NEM-aldolase) as an indicator protein after its microinjection into HeLa cells. 2. NEM-aldolase degradation to acid-soluble products proceeded at a slower rate than that of endogenously labelled total cell protein, and was inhibited to a greater extent by 3-methyladenine, leupeptin and NH4Cl. The combination of leupeptin plus NH4Cl was particularly effective, decreasing the NEM-aldolase breakdown rate by 90%. 3. Measurements of the loss of radioactivity from the aldolase band located from fluorograms after SDS/polyacrylamide-gel electrophoresis showed that NEM-aldolase breakdown was much more rapid when measured by this method. The effects of insulin, 3-methyladenine, leupeptin and NH4Cl on this breakdown were also substantial. 4. Substantial amounts of peptide intermediates in the breakdown pathway of NEM-aldolase accumulated in cells. The production of small intermediates (less than 30 kDa) accounted for approx. 40% of the NEM-aldolase degraded in control cultures. Addition of NH4Cl increased the proportion of these intermediates. Large intermediates, between 31 and 38 kDa, were particularly evident in the presence of the cysteine proteinase inhibitor leupeptin, but almost no small intermediates were detected. 5. The results are best explained by the degradation of NEM-aldolase being predominantly a lysosomal process, with cysteine proteinases involved in early proteolytic steps and other proteinases that have acid pH optima required for the complete catabolism of small intermediates.

Microinjection of endogenous and exogenous proteins into primary cultures of rat hepatocytes and the degradation of the injected proteins

1. A method to microinject proteins into cells through packaging proteins to erythrocyte ghosts (erythrocyte-mediated microinjection) was modified partially in order to apply the method to primary cultures of rat hepatocytes. 2. Degradation of the microinjected proteins was examined employing the improved method. The mean half-life of the injected endogenous liver protein was 20 hr. The data suggested that the injected proteins are degraded through both lysosomal and non-lysosomal proteolytic pathways probably depending on their structure. 3. The present method to microinject exogenous proteins into primary cultures of rat hepatocytes can be employed usefully for the investigations of protein metabolism in liver.

Selective degradation of T cell antigen receptor chains retained in a pre-Golgi compartment

We have examined the fate of newly synthesized T cell antigen receptor (TCR) subunits in a T cell hybridoma deficient in expression of the clonotypic beta chain. Synthesis and assembly of the remaining chains proceed normally but surface expression of TCR chains is undetectable in these cells. A variety of biochemical and morphological techniques has been used to show that the TCR chains in these cells fail to be transported to any of the Golgi cisternae. Instead, they are retained in a pre-Golgi compartment which is either part of or closely related to the endoplasmic reticulum. The CD3-delta chain is degraded by a non-lysosomal process that is inhibited at temperatures at or below 27 degrees C. By contrast, the remaining chains (CD3-epsilon, CD3-gamma, and zeta) are very stable over 7 h. We propose possible mechanisms that may explain the differential fate of TCR chains retained in a pre-Golgi compartment.

Degradation of native and modified forms of fructose-bisphosphate aldolase microinjected into HeLa cells

The uptake and degradation of radiolabelled rabbit muscle fructose-bisphosphate aldolase (EC 4.1.2.13) was studied in HeLa cells microinjected by the erythrocyte ghost fusion system. Labelled aldolase was progressively modified by treatment with GSSG or N-ethylmaleimide (NEM) before microinjection to determine whether these agents, which inactivate and destabilize the enzyme in vitro, affect the half-life of the enzyme in vivo. Increasing exposure of aldolase to GSSG or NEM before microinjection increased the extent of aldolase transfer into the HeLa cells and decreased the proportion of the protein that could be extracted from the cells after water lysis. Some degradation of the GSSG- and NEM-inactivated aldolases was observed in the ghosts before microinjection; thus a family of radiolabelled proteins was microinjected in these experiments. In spite of the above differences, the 40 kDa subunit of each aldolase form was degraded with a half-life of 30 h in the HeLa cells. In contrast, the progressively modified forms of aldolase were increasingly susceptible to proteolytic action in vitro by chymotrypsin or by cathepsin B and in ghosts. These studies indicate that the rate of aldolase degradation in cells is not determined by attack by cellular proteinases that recognize vulnerable protein substrates; the results are most easily explained by a random autophagic process involving the lysosomal system.

Regulation of the breakdown rates of biotin-containing proteins in Swiss 3T3-L1 cells

1. Degradation rate constants for individual biotin-labelled proteins were measured in Swiss 3T3-L1 adipocytes that had been incubated with inhibitors of autophagy or of lysosomal proteolysis. 2. Inhibitory effects produced by 10 mM-3-methyladenine and a combination of 5 mM-NH4Cl and leupeptin (50 micrograms/ml) were approximately equal. The inclusion of NH4Cl did not significantly enhance the responses to 3-methyladenine, suggesting that autophagy was already maximally inhibited. 3. The extent of inhibition by 3-methyladenine or by the NH4Cl/leupeptin mixture was similar for the cytosolic enzyme acetyl-CoA carboxylase and for the three mitochondrial carboxylases. This inhibition averaged 50%. The breakdown rate of a more-stable 38 kDa biotin-containing mitochondrial protein was more responsive to the inhibitory agents. These results are best explained by mitochondrial proteolysis occurring via a combination of the degradation of whole mitochondria within autophagic vacuoles, supplemented by the selective intramitochondrial breakdown of more labile proteins. 4. A number of intermediate products in the degradation of biotin-containing proteins were detected. Differences in the patterns of radioactivity between these peptides after incubation of cells in the presence of inhibitors of the breakdown process provided evidence that some peptides were produced before autophagy, others as a result of intralysosomal inhibition, while at least one was associated with intramitochondrial proteolysis.

Degradation of horseradish peroxidase after microinjection into mammalian cells

Horseradish peroxidase (HRP) has been microinjected into mammalian cells in tissue culture by the erythrocyte ghost-mediated technique. This protein was selected because it can be localized and quantified after injection by cytochemical and spectrophotometric methods. HRP labeled by reductive methylation retained full catalytic activity, was efficiently loaded into erythrocyte ghosts, and did not associate to a significant degree with ghost membranes. A combination of cytochemical staining and autoradiography established that HRP injected into rat L6 myoblasts, HE(39)L human diploid fibroblasts, or HeLa cells was intracellular and uniformly distributed throughout the cell, while cell lysis techniques showed that the catalytically active HRP was not membrane bound. Inactivation of labeled HRP after injection paralleled the disappearance of the 40-kDa polypeptide, and was always more rapid than its overall degradation. This difference was associated with a pool of water-insoluble radioactivity in the injected cells. This material was of smaller molecular size than the native protein: many labeled peptides were detected in the range of 10 to 38 kDa. By the use of inhibitors of autophagic proteolysis or lysosomal function it was established that HRP degradation was not subjected quantitatively to the same regulatory processes as the average endogenous protein labeled in the same cultures.

Enhanced degradation of oxidized glutamine synthetase in vitro and after microinjection into hepatoma cells

Mixed-function oxidation of Escherichia coli glutamine synthetase has previously been suggested to mark the enzyme for intracellular degradation, and in vitro studies have demonstrated that oxidation renders the enzyme susceptible to proteolytic attack. In this study, the susceptibility of glutamine synthetase to degradation by purified proteases has been compared with the rate of degradation after microinjection into hepatoma cells. Upon exposure to an ascorbate mixed-function oxidation system the enzyme rapidly loses most of its activity, but further oxidation is required to cause susceptibility to extensive proteolytic attack either by a high-molecular-weight liver cysteine proteinase or by trypsin. The rate of degradation of biosynthetically 14C-labeled native and oxidized glutamine synthetase preparations after injection into hepatoma cells parallels their susceptibility to proteolysis in vitro. Native enzyme preparations and enzyme oxidatively inactivated, but not susceptible to extensive degradation by purified proteases, had similar intracellular half-lives; however, oxidized enzyme preparations that were susceptible to proteolytic breakdown in vitro were degraded almost ten times faster than the native enzyme within the growing hepatoma cells. These results suggest that the same features of the oxidized enzyme that render it susceptible to proteolysis in vitro are also recognized by the intracellular degradation system. In addition, they show that loss of enzyme activity does not necessarily imply decreased metabolic stability.

Ornithine decarboxylase production in vitro by using mouse cDNA

Microgram quantities of ornithine decarboxylase (ODC, EC 4.1.1.17)-specific mRNA were synthesized by transcription techniques in vitro, by using a plasmid containing mouse cDNA coding for this enzyme. The homogeneous RNA preparation was then used for cell-free synthesis of ODC protein, in rabbit reticulocyte lysates. Analysis of products translated in vitro by polyacrylamide-gel electrophoresis revealed predominantly one protein produced, with Mr approx. 54,000, which was immunoprecipitable by anti-ODC serum. Two-dimensional gel-electrophoretic analysis showed that the protein ODC synthesized in vitro had a pI of approx. 5.4, similar to the native enzyme isolated from mouse tissues. In addition, quantification of activity and protein amount showed that the enzyme synthesized in vitro had a specific activity of approx. 63,000 units (nmol/min)/mg, consistent with the purified mouse kidney enzyme's specific activity of approx. 47,000 units/mg. An average of nearly 200 pg of ODC protein was produced in vitro from various RNA preparations. These data demonstrate that ODC-specific mRNA and active ODC protein can be produced by 'in vitro' technology, which should prove useful in studying functional and structural characteristics of these molecules.

Mechanisms of intracellular protein catabolism. Intracellular fate of microinjected polypeptides translated in vitro

Erythrocyte-mediated microinjection was used to introduce [35S]polypeptides translated in vitro into 3T3-L1 cells. Such [35S]polypeptides are not degraded after loading into erythrocytes and are stable for the first 2 h after microinjection into growing 3T3-L1 cells. Similarly, little or no degradation of microinjected [35S]polypeptides is observed in either growing or confluent 3T3-L1 cells over a 70 h period. Microinjection of reticulocyte lysate alone does not affect the rate of degradation of long-lived endogenous protein. Reductively [3H]methylated lysate haemoglobin is degraded after microinjection by a cytosolic mechanism. Microinjected 125I-labelled bovine serum albumin is rapidly degraded by a cytosolic mechanism at the same rate in the absence or presence of reticulocyte lysate. The data do not support the notion that the observed lack of degradation of microinjected [35S]polypeptides translated in vitro is due to the presence of proteolytic inhibitors in reticulocyte lysates which can inhibit the degradation of microinjected or cellular proteins.

A putative protein-sequestration site involving intermediate filaments for protein degradation by autophagy. Studies with microinjected purified glycolytic enzymes in 3T3-L1 cells

Several glycolytic enzymes (lactate dehydrogenase, pyruvate kinase, glyceraldehyde-3-phosphate dehydrogenase) were radiolabelled by [125I]iodination, conjugation with 125I-labelled Bolton & Hunter reagent and reductive [3H]methylation, and their degradative rates after microinjection into 3T3-L1 cells compared with that of the extracellular protein bovine serum albumin. Although the albumin remains largely cytosolic in recipient cells, the glycolytic enzymes rapidly (less than 30 min) become insoluble, as measured by detergent and salt extractions. The microinjected glycolytic enzymes appear to form disulphide-linked aggregates, are found in a cell fraction rich in vimentin-containing intermediate filaments and histones (nuclear-intermediate-filament fraction), and are degraded slowly by a lysosomal mechanism, as judged by the effects of inhibitors (NH4Cl, leupeptin, 3-methyladenine). 125I-labelled bovine serum albumin appears to be degraded rapidly and non-lysosomally. Prolonged treatment (96 h) of cultured cells with leupeptin results in the accumulation of pulse-labelled ([35S]methionine for 24 h) endogenous cell proteins in the detergent-and salt-non-extractable residue, but NH4Cl and 3-methyladenine do not have this effect. The findings are in terms of the interpretation of experiments involving microinjection of proteins to study intracellular protein protein degradation by autophagy.

Spermidine mediates degradation of ornithine decarboxylase by a non-lysosomal, ubiquitin-independent mechanism

The mechanism of spermidine-induced ornithine decarboxylase (ODC, E.C. 4.1.1.17) inactivation was investigated using Chinese hamster ovary (CHO) cells, maintained in serum-free medium, which display a stabilization of ODC owing to the lack of accumulation of putrescine and spermidine (Glass and Gerner: Biochem. J., 236:351-357, 1986; Sertich et al.: J. Cell Physiol., 127:114-120, 1986). Treatment of cells with 10 microM exogenous spermidine leads to rapid decay of ODC activity accompanied by a parallel decrease in enzyme protein. Analysis of the decay of [35S]methionine-labeled ODC and separation by two-dimensional electrophoresis revealed no detectable modification in ODC structure during enhanced degradation. Spermidine-mediated inactivation of ODC occurred in a temperature-dependent manner exhibiting pseudo-first-order kinetics over a temperature range of 22-37 degrees C. In cultures treated continuously, an initial lag was observed after treatment with spermidine, followed by a rapid decline in activity as an apparent critical concentration of intracellular spermidine was achieved. Treating cells at 22 degrees C for 3 hours with 10 microM spermidine, followed by removal of exogenous polyamine, and then shifting to varying temperatures, resulted in rates of ODC inactivation identical with that determined with a continuous treatment. Arrhenius analysis showed that polyamine mediated inactivation of ODC occurred with an activation energy of approximately 16 kcal/mol. Treatment of cells with lysosomotrophic agents (NH4Cl, chloroquine, antipain, leupeptin, chymostatin) had no effect on ODC degradation. ODC turnover was not dependent on ubiquitin-dependent proteolysis. Shift of ts85 cells, a temperature-sensitive mutant for ubiquitin conjugation, to 39 degrees C (nonpermissive for ubiquitin-dependent proteolysis) followed by addition of spermidine led to a rapid decline in ODC activity, with a rate similar to that seen at 32 degrees C (the permissive temperature). In contrast, spermidine-mediated ODC degradation was substantially decreased by inhibitors of protein synthesis (cycloheximide, emetine, and puromycin). These data support the hypothesis that spermidine regulates ODC degradation via a mechanism requiring new protein synthesis, and that this occurs via a non-lysosomal, ubiquitin-independent pathway.

Purification and characterization of a high molecular weight proteinase (macropain) from human erythrocytes

An alkaline proteinase, previously identified in rat liver and heart, has been purified from the soluble fraction of human erythrocytes. The proteinase has an apparent molecular weight of 600 000 and is composed of eight subunits with molecular weights ranging from 32 000 to 21 000. The proteinase degrades both protein and synthetic peptide substrates with a broad pH optimum of 7.5-11.0. Among the synthetic peptides tested, tripeptides with arginine at the P1 position (e.g. Z-Val-Leu-Arg-4-methoxy-2-napthylamine and Boc-Leu-Gly-Arg-4-methylcoumarin-7-amide) are particularly good substrates. The proteinase appears to be sulfhydryl-dependent and is inhibited completely by mersalyl acid and by hemin; inhibitors of serine and metallo-type proteinases have no effect on proteinase activity. Interestingly, a variety of other proteinase inhibitors such as leupeptin, chymostatin and N-ethylmaleimide failed to completely inhibit protein-hydrolyzing activities of the enzyme. These results indicate that these activities may be accounted for by at least two different catalytic sites. Proteinase activity is stable in the presence of 1 M urea, 0.5% Triton X-100 or 0.03% SDS and is not affected by ATP. Based on the high molecular weight and sulfhydryl-dependence, we have named this proteinase macropain.

Intracellular protein degradation in serum-deprived human fibroblasts

IMR90 human fibroblasts were labelled by incubation of cells for 48 h in medium containing 10% serum and [3H]leucine. The labelled protein was degraded at a rate of 1%/h during a subsequent incubation in medium with 10% serum. Incubation in medium without serum caused a transient enhancement of the degradation of endogenous protein, which was also found in cells labelled in medium without serum. The degradation of micro-injected haemoglobin was enhanced by serum deprivation in a non-transient manner. These results suggest that enhanced degradation in serum-free medium occurs only for a subpopulation of cell proteins and that it appears transient because the major part of the pool of susceptible endogenous proteins is being degraded during the first 20-30 h in serum-free unlabelled medium. Protein turnover in various cell compartments was measured by a double-labelling technique. Most of the enhanced degradation in serum-deprived cultures (73-83%) was due to breakdown of cytosolic proteins. The enhanced degradation of cytosolic proteins seemed to affect several proteins irrespective of their molecular mass or metabolic stability.

Lysosomal uptake of isolated cell organelles microinjected into HeLa cells

Lysosomes and microsomes were isolated from rat liver and microinjected into the cytoplasm of HeLa cells. The fate of the transplanted organelles and their effects on the recipient cells were followed in the electron microscope at various time intervals after administration. Needle injection with buffer or sucrose did not seem to evoke any ultrastructural alterations, such as induced autophagy or other signs of sublethal cell injury. Recipients of microinjected cell organelles elicited a rapid and conspicuous increase in membrane-bounded cytoplasmic vacuoles, concomitant with the disappearance of the injected material. Golgi complexes became abundant with many small vesicles clustering around their cisternae. The volume density of the lysosomal compartment increased 2-3-fold after organelle injection as compared with control-injected (0.3 M sucrose) or noninjected cells. Our preliminary results show that isolated cell organelles can be microinjected into cells n culture and indicate that the microinjected organelles were segregated from the cytoplasm into membrane-bounded vacuoles probably through autophagolysosome formation. Thus, this technique offers an additional approach for studies on the segregation and degradation of cell organelles in somatic cells and may enable more detailed analyses on the mechanisms of autophagic sequestration of specific cell organelles.

Autophagy and lysosomal proteolysis in the liver

Autophagy is defined as any process whereby cellular macromolecules destined for degradation gain access to the lysosomes. A review is presented on the physiological significance, mechanisms and regulation of autophagy in hepatocytes, concentrating on the issue of regulation. The article ends by discussing techniques available for future research.

Differential proliferative responses of cultured Schwann cells to axolemma and myelin-enriched fractions. II. Morphological studies

Axolemma-enriched and myelin-enriched fractions were prepared from bovine CNS white matter and conjugated to fluorescein isothiocyanate (FITC). Both unlabelled and FITC-labelled axolemma and myelin were mitogenic for cultured rat Schwann cells. Treatment of Schwann cells with the FITC-labelled mitogens for up to 24 h resulted in two distinct morphological appearances. FITC-myelin-treated cells were filled with numerous round, fluorescent-labelled intracellular vesicles, while FITC-axolemma-treated cells appeared to be coated with a patchy, ill-defined fluorescence, primarily concentrated around the cell body but extending onto the cell processes. These observations were corroborated under phase microscopy. Electron microscopy revealed multiple, membrane-bound, membrane-containing phagosomes within myelin-treated cells and to a far lesser extent in axolemma-treated cells. The effect on the expression of the myelin-mediated and axolemma-mediated mitogenic signal when Schwann cells were treated with the lysosomal inhibitors, ammonium chloride and chloroquine, was evaluated. The mitogenicity of myelin was reduced 70-80% by these agents whereas the mitogenicity of axolemma was not significantly altered under these conditions. These results suggest that axolemma and myelin stimulate the proliferation of cultured Schwann cells by different mechanisms. Myelin requires endocytosis and lysosomal processing for expression of its mitogenic signal; in contrast, the mitogenicity of axolemma may be transduced at the Schwann cell surface.