Degradation of structurally characterized proteins injected into HeLa cells. Effects of intracellular location and the involvement of lysosomes

Coordinated time-dependent modulation of AMPK/Akt/mTOR signaling and autophagy controls osteogenic differentiation of human mesenchymal stem cells

We investigated the role of AMP-activated protein kinase (AMPK), Akt, mammalian target of rapamycin (mTOR), autophagy and their interplay in osteogenic differentiation of human dental pulp mesenchymal stem cells. The activation of various members of AMPK, Akt and mTOR signaling pathways and autophagy was analyzed by immunoblotting, while osteogenic differentiation was assessed by alkaline phosphatase staining and real-time RT-PCR/immunoblot quantification of osteocalcin, Runt-related transcription factor 2 and bone morphogenetic protein 2 mRNA and/or protein levels. Osteogenic differentiation of mesenchymal stem cells was associated with early (day 1) activation of AMPK and its target Raptor, coinciding with the inhibition of mTOR and its substrate p70S6 kinase. The early induction of autophagy was demonstrated by accumulation of autophagosome-bound LC3-II, upregulation of proautophagic beclin-1 and a decrease in the selective autophagic target p62. This was followed by the late activation of Akt/mTOR at days 3-7 of differentiation. The RNA interference-mediated silencing of AMPK, mTOR or autophagy-essential LC3β, as well as the pharmacological inhibitors of AMPK (compound C), Akt (10-DEBC hydrochloride), mTOR (rapamycin) and autophagy (bafilomycin A1, chloroquine and ammonium chloride), each suppressed mesenchymal stem cell differentiation to osteoblasts. AMPK knockdown prevented early mTOR inhibition and autophagy induction, as well as late activation of Akt/mTOR signaling, while Akt inhibition suppressed mTOR activation without affecting AMPK phosphorylation. Our data indicate that AMPK controls osteogenic differentiation of human mesenchymal stem cells through both early mTOR inhibition-mediated autophagy and late activation of Akt/mTOR signaling axis.

Chloroquine-mediated lysosomal dysfunction enhances the anticancer effect of nutrient deprivation

PURPOSE

To investigate the ability of chloroquine, a lysosomotropic autophagy inhibitor, to enhance the anticancer effect of nutrient deprivation.

METHODS

Serum-deprived U251 glioma, B16 melanoma and L929 fibrosarcoma cells were treated with chloroquine in vitro. Cell viability was measured by crystal violet and MTT assay. Oxidative stress, apoptosis/necrosis and intracellular acidification were analyzed by flow cytometry. Cell morphology was examined by light and electron microscopy. Activation of AMP-activated protein kinase (AMPK) and autophagy were monitored by immunoblotting. RNA interference was used for AMPK and LC3b knockdown. The anticancer efficiency of intraperitoneal chloroquine in calorie-restricted mice was assessed using a B16 mouse melanoma model.

RESULTS

Chloroquine rapidly killed serum-starved cancer cells in vitro. This effect was not mimicked by autophagy inhibitors or LC3b shRNA, indicating autophagy-independent mechanism. Chloroquine-induced lysosomal accumulation and oxidative stress, leading to mitochondrial depolarization, caspase activation and mixed apoptotic/necrotic cell death, were prevented by lysosomal acidification inhibitor bafilomycin. AMPK downregulation participated in chloroquine action, as AMPK activation reduced, and AMPK shRNA mimicked chloroquine toxicity. Chloroquine inhibited melanoma growth in calorie-restricted mice, causing lysosomal accumulation, mitochondrial disintegration and selective necrosis of tumor cells.

CONCLUSION

Combined treatment with chloroquine and calorie restriction might be useful in cancer therapy.

Sodium overload and water influx activate the NALP3 inflammasome

The NALP3 inflammasome is activated by low intracellular potassium concentrations [K(+)](i), leading to the secretion of the proinflammatory cytokine IL-1β. However, the mechanism of [K(+)](i) lowering after phagocytosis of monosodium urate crystals is still elusive. Here, we propose that endosomes containing monosodium urate crystals fuse with acidic lysosomes. The low pH in the phagolysosome causes a massive release of sodium and raises the intracellular osmolarity. This process is balanced by passive water influx through aquaporins leading to cell swelling. This process dilutes [K(+)](i) to values below the threshold of 90 mm known to activate NALP3 inflammasomes without net loss of cytoplasmic potassium ions. In vitro, the inhibitors of lysosomal acidification (ammonium chloride, chloroquine) and of aquaporins (mercury chloride, phloretin) all significantly decreased the production of IL-1β. In vivo, only the pharmacological inhibitor of lysosome acidification chloroquine could be used which again significantly reduced the IL-1β production. As a translational aspect one may consider the use of chloroquine for the anti-inflammatory treatment of refractory gout.

Age-dependent deamidation of asparagine residues in proteins

Nonenzymatic deamidation of peptides and proteins represents an important degradation reaction occurring in vitro in the course of isolation or storage and in vivo during development and/or aging of cells. This review first presents a synopsis of the influence of structure on deamidation reaction proceeding via a five-membered succinimide intermediate, followed by an outline of procedures for separation and detection of deamidated forms. Selected examples for in vitro and in vivo deamidation are reviewed including the possible biological consequences of this protein degradation. Finally, the reaction of protein methyltransferase with L-isoaspartyl- and D-aspartyl residues and its possible role in protein repair is elucidated.

Ubiquitin and the enigma of intracellular protein degradation

Contrary to widespread belief, the regulation and mechanism of degradation for the mass of intracellular proteins (i.e. differential, selective protein turnover) in vertebrate tissues is still a major biological enigma. There is no evidence for the conclusion that ubiquitin plays any role in these processes. The primary function of the ubiquitin-dependent protein degradation pathway appears to lie in the removal of abnormal, misfolded, denatured or foreign proteins in some eukaryotic cells. ATP/ubiquitin-dependent proteolysis probably also plays a role in the degradation of some so-called 'short-lived' proteins. Evidence obtained from the covalent modification of such natural substrates as calmodulin, histones (H2A, H2B) and some cell membrane receptors with ubiquitin indicates that the reversible interconversion of proteins with ubiquitin followed by concomitant functional changes may be of prime importance.

A heterogeneous set of urea-insoluble proteins in dividing PC12 pheochromocytoma cells is passed on to at least the generation of great-granddaughter cells

About 5% of the total cellular protein synthesized in exponentially dividing PC12 pheochromocytoma cells remains insoluble after extractions with aqueous buffer, nonionic detergent, and a strong denaturant, 6M urea. Single- and double-radiolabel pulse-chase labeling experiments with radioactive leucine indicate that for much of the 6 M urea-insoluble protein there is either a lag between its synthesis and deposition in a urea-insoluble compartment and/or the urea-insoluble protein is comparatively stabilized from destruction. Given the doubling time of PC12 cells, much of the long-lived and urea-insoluble protein of PC12 cells is passed on for at least three generations. Electrophoretic analyses show there are many species of long-lived proteins in the 6M urea-insoluble fraction, displayed as a near continuum of subunit molecular weights.

Structure and modifications of the junior chaperone alpha-crystallin. From lens transparency to molecular pathology

alpha-Crystallin is a high-molecular-mass protein that for many decades was thought to be one of the rare real organ-specific proteins. This protein exists as an aggregate of about 800 kDa, but its composition is simple. Only two closely related subunits termed alpha A- and alpha B-crystallin, with molecular masses of approximately 20 kDa, form the building blocks of the aggregate. The idea of organ-specificity had to be abandoned when it was discovered that alpha-crystallin occurs in a great variety of nonlenticular tissues, notably heart, kidney, striated muscle and several tumors. Moreover alpha B-crystallin is a major component of ubiquinated inclusion bodies in human degenerative diseases. An earlier excitement arose when it was found that alpha B-crystallin, due to its very similar structural and functional properties, belongs to the heat-shock protein family. Eventually the chaperone nature of alpha-crystallin could be demonstrated unequivocally. All these unexpected findings make alpha-crystallin a subject of great interest far beyond the lens research field. A survey of structural data about alpha-crystallin is presented here. Since alpha-crystallin has resisted crystallization, only theoretical models of its three-dimensional structure are available. Due to its long life in the eye lens, alpha-crystallin is one of the best studied proteins with respect to post-translational modifications, including age-induced alterations. Because of its similarities with the small heat-shock proteins, the findings about alpha-crystallin are illuminative for the latter proteins as well. This review deals with: structural aspects, post-translational modifications (including deamidation, racemization, phosphorylation, acetylation, glycation, age-dependent truncation), the occurrence outside of the eye lens, the heat-shock relation and the chaperone activity of alpha-crystallin.

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.

Autophagy and other vacuolar protein degradation mechanisms

Autophagic degradation of cytoplasm (including protein, RNA etc.) is a non-selective bulk process, as indicated by ultrastructural evidence and by the similarity in autophagic sequestration rates of various cytosolic enzymes with different half-lives. The initial autophagic sequestration step, performed by a poorly-characterized organelle called a phagophore, is subject to feedback inhibition by purines and amino acids, the effect of the latter being potentiated by insulin and antagonized by glucagon. Epinephrine and other adrenergic agonists inhibit autophagic sequestration through a prazosin-sensitive alpha 1-adrenergic mechanism. The sequestration is also inhibited by cAMP and by protein phosphorylation as indicated by the effects of cyclic nucleotide analogues, phosphodiesterase inhibitors and okadaic acid. Asparagine specifically inhibits autophagic-lysosomal fusion without having any significant effects on autophagic sequestration, on intralysosomal degradation or on the endocytic pathway. Autophaged material that accumulates in prelysosomal vacuoles in the presence of asparagine is accessible to endocytosed enzymes, revealing the existence of an amphifunctional organelle, the amphisome. Evidence from several cell types suggests that endocytosis may be coupled to autophagy to a variable extent, and that the amphisome may play a central role as a collecting station for material destined for lysosomal degradation. Protein degradation can also take place in a 'salvage compartment' closely associated with the endoplasmic reticulum (ER). In this compartment unassembled protein chains are degraded by uncharacterized proteinases, while resident proteins return to the ER and assembled secretory and membrane proteins proceed through the Golgi apparatus. In the trans-Golgi network some proteins are proteolytically processed by Ca(2+)-dependent proteinases; furthermore, this compartment sorts proteins to lysosomes, various membrane domains, endosomes or secretory vesicles/granules. Processing of both endogenous and exogenous proteins can occur in endosomes, which may play a particularly important role in antigen processing and presentation. Proteins in endosomes or secretory compartments can either be exocytosed, or channeled to lysosomes for degradation. The switch mechanisms which decide between these options are subject to bioregulation by external agents (hormones and growth factors), and may play an important role in the control of protein uptake and secretion.

Age-related change in the degradation rate of ovalbumin microinjected into mouse liver parenchymal cells

Change in the degradation rate of ovalbumin (OVA) microinjected into liver parenchymal cells isolated from mice of various ages was studied. OVA was injected by osmotic lysis of pinosomes and the amount of OVA was determined by immunoblotting using purified antibody to OVA. Cellular activity as judged by rates of protein synthesis and degradation of pulse-labeled proteins was not affected by the injection. To localize injected OVA in the cells, cell extracts were fractionated by differential centrifugation and the amount of OVA and the activity of beta-D-galactosidase as a lysosomal marker enzyme of each fraction were determined. The ovalbumin was mostly found in the soluble fraction, while the beta-D-galactosidase activity was found in the particulate fraction, indicating that the ovalbumin was spread in the cytosol but was not present in the pinosomes or lysosomes. The average half-lives of OVA were 106, 113, and 164 h in the cells from young (3.5-6.5 months), middle-aged (13.5-20.0 months), and old (24.5-30.5 months) mice, respectively. Thus, the half-life of ovalbumin in the cells of senescent mice was about 50% longer than that in the cells of young or middle-aged mice. These results are in good agreement with those of our previous investigation, which showed that the half-life of inactivation of horseradish peroxidase was extended by about 50% in the hepatocytes from old mice (Ishigami and Goto, 1988, Mech. Ageing Dev., 46, 125-133).