Inhibition of pyruvate carboxylase degradation and total protein breakdown by lysosomotropic agents in 3T3-L1 cells

NDRG2 inhibits pyruvate carboxylase-mediated anaplerosis and combines with glutamine blockade to inhibit the proliferation of glioma cells

Due to the rapid proliferation, cancer cells have increased anabolic biosynthesis, which requires anaplerosis to replenish precursor intermediates. The major anaplerotic sources are pyruvate and glutamine, which require the catalysis of pyruvate carboxylase (PC) and glutaminase (GLS) respectively. In GLS-suppressed cancer cells, the PC-mediated pathway for anaplerosis is crucial to maintain cell growth and proliferation. Here, we investigated the regulatory role and molecular mechanism of N-myc downstream-regulated gene 2 (NDRG2) in PC and PC-mediated anaplerosis. NDRG2 interacted with PC and induced the degradation of PC in glutamine-deprived cells. NDRG2 also inhibited the activity of PC and PC-mediated anaplerosis. As a result, NDRG2 significantly inhibited the malignant growth and proliferation of glioma cells in combination with a glutamine antagonist. In addition, NDRG2 more significantly inhibited the protein level of PC in isocitrate dehydrogenase 1 (R132H)-mutant glioma cells than in wild-type glioma cells. These findings indicate that the molecular mechanism of NDRG2 inhibits PC-mediated anaplerosis and collaborates with glutamine antagonist to inhibit the malignant proliferation of glioma cells, thus providing a theoretical and experimental basis for targeting anaplerosis in glioma therapy.

Comparative proteomics study reveals that bacterial CpG motifs induce tumor cell autophagy in vitro and in vivo

Unmethylated CpG dinucleotides, present in bacterial DNA, are recognized in vertebrates via the Toll-like receptor 9 (TLR9) and are known to act as an anticancer agent by stimulating immune cells to induce a proinflammatory response. Although the effects of CpG-oligodeoxynucleotides (CpG-ODNs) in immune cells have been widely studied, little is known regarding their molecular effects in TLR9-positive tumor cells. To better understand the role of these bacterial motifs in cancer cells, we analyzed proteome modifications induced in TLR9-positive tumor cells in vitro and in vivo after CpG-ODN treatment in a rat colon carcinoma model. Proteomics analysis of tumor cells by two-dimensional gel electrophoresis followed by mass spectrometry identified several proteins modulated by bacterial CpG motifs. Among them, several are related to autophagy including potential autophagic substrates. In addition, we observed an increased glyceraldehyde-3-phosphate dehydrogenase expression, which has been shown to be sufficient to trigger an autophagic process. Autophagy is a self-digestion pathway whereby cytoplasmic material is sequestered by a structure termed the autophagosome for subsequent degradation and recycling. As bacteria are known to trigger autophagy, we assessed whether bacterial CpG motifs might induce autophagy in TLR9-positive tumor cells. We showed that CpG-ODN can induce autophagy in rodent and human tumor cell lines and was TLR9-dependent. In addition, an increase in the number of autophagosomes can also be observed in vivo after CpG motif intratumoral injection. Our findings bring new insights on the effect of bacterial CpG motifs in tumor cells and may be relevant for cancer treatment and more generally for gene therapy approaches in TLR9-positive tissues.

Identification of Sites of Mannose 6-Phosphorylation on Lysosomal Proteins

Most newly synthesized soluble lysosomal proteins contain mannose 6-phosphate (Man-6-P), a specific carbohydrate modification that is recognized by Man-6-P receptors (MPRs) that direct targeting to the lysosome. A number of proteomic studies have focused on lysosomal proteins, exploiting the fact that Man-6-P-containing forms can be purified by affinity chromatography on immobilized MPRs. These studies have identified many known lysosomal proteins as well as many proteins not previously classified as lysosomal. The latter are of considerable biological interest with potential implications for lysosomal function and as candidates for lysosomal storage diseases of unknown etiology. However, a significant problem in interpreting the biological relevance of such proteins has been in distinguishing true Man-6-P glycoproteins from simple contaminants and from proteins associated with true Man-6-P glycoproteins (e.g. protease inhibitors and lectins). In this report, we describe a mass spectrometric approach to the verification of Man-6-phosphorylation based upon LC-MS of MPR-purified proteolytic glycopeptides. This provided a useful tool in validating novel MPR-purified proteins as true Man-6-P glycoproteins and also allowed identification of low abundance components not observed in the analysis of the total Man-6-P glycoprotein mixture. In addition, this approach allowed the global mapping of 99 Man-6-phosphorylation sites from 44 known lysosomal proteins purified from mouse and human brain. This information is likely to provide useful insights into protein determinants for this modification and may be of significant value in protein engineering approaches designed to optimize protein delivery to the lysosome in therapeutic applications such as gene and enzyme replacement therapies.

Human biotinidase isn't just for recycling biotin

For years, the major role of biotin has been as the coenzyme for four carboxylases in humans. Although there has been evidence that biotin might have other functions, none has been firmly established. The discovery that human serum biotinidase has biotinyl-transferase activity, in addition to biotinidase hydrolase activity, presents new possibilities for the role of biotinidase in biotin metabolism. Specific transfer of biotin to histones by biotinidase provides a possible explanation for why biotin is found in the nucleus and the nature of its role in the regulation of protein transcription. Future studies will help to determine the functions of biotinidase in biotin metabolism and in disease states.

Regulation of pyruvate carboxylase in 3T3-L1 cells

When 3T3-L1 fibroblasts differentiate to adipocytes, the specific activity of pyruvate carboxylase (PC) increases about 25-fold in parallel with its intracellular protein concentration. The increase in PC protein concentration is accompanied by a 9-10-fold increase in the relative abundance of 4.2 kb PC mRNA measured by Northern-blot analysis using a cDNA probe encoding a segment of the PC gene of 3T3-L1 adipocytes. The effects of cyclic AMP (cAMP) alone and together with insulin on levels of cellular protein, PC activity, PC protein and on the relative abundance of PC mRNA were examined in mature 3T3-L1 adipocytes. Adipocytes exposed to cAMP for 24 h exhibited a 25% decrease in cellular protein and marked decreases in enzyme activity (88%) and PC mRNA abundance (98%) compared with untreated adipocyte controls. After 48 h of exposure to cAMP, PC activity and PC mRNA diminished to levels approaching their detection limits. When exposed to medium containing cAMP plus insulin, adipocyte enzyme activity and PC mRNA declined more slowly during the first 24 h exposure (about 20% decrease) but after 48 h fell to values comparable with those of adipocytes exposed to cAMP alone. Despite these decreases in enzyme activity, the PC protein content of adipocytes treated with cAMP alone or cAMP plus insulin are nearly identical with that of control adipocytes. The inactivation of PC in cAMP-treated adipocytes does not involve loss of the prosthetic group from the holoenzyme. Cross-linking experiments suggest that the spatial arrangement of protomers in inactive PC may differ from that in the active tetrameric enzyme. Data presented suggest that, in addition to inducing inactivation, cAMP may also regulate adipocyte PC by decreasing transcription of the PC gene and/or enhancing the rate of degradation of PC mRNA.

Transblot identification of biotin-containing proteins in rat liver

Peroxidase-conjugated avidin was used to detect biotin-containing carboxylases in rat liver. By a transblot method, avidin-peroxidase interacted with liver proteins with estimated molecular masses of 120 and 74 kDa. The proteins were identified as pyruvate carboxylase (120 kDa, 6.4 pI) and methylcrotonyl-CoA carboxylase (74 kDa, 7.2 pI) by two-dimensional gel electrophoresis and transblot method. An additional band with estimated molecular mass of 220 kDa was detected in the cytosol fraction of rat liver, compatible with acetyl-CoA carboxylase. Rat liver proteins were prepared and treated with avidin and analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and transblot with avidin-peroxidase. A 190-kDa band was found with a parallel decrease in the 120-kDa band determined by Coomassie blue staining; however, these proteins did not stain by the transblot avidin-peroxidase method. When the transblot of parallel proteins was incubated with biotin and subsequently with avidin-peroxidase, two additional bands, namely 190 and 145 kDa, were detected while the 74-kDa band disappeared correlated with decreased staining of the 120-kDa band. The present procedure is a simple, rapid, and inexpensive method for detecting biotin-containing proteins in various tissues and organs and in determining the occurrence of nonspecific staining with the avidin-biotin complex method of immunoblot.

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.

Fate of the monoamine oxidase inhibitor pargyline in cultured hepatocytes

The interaction of the monoamine oxidase inhibitor pargyline with cultured hepatocytes has been studied. [Phenyl-3, benzyl-3H] pargyline (38 nM) rapidly enters the cells and a plateau of incorporation into a trichloroacetic acid insoluble form (monoamine oxidase) is reached after 2 hr. The level of labelling is lower in freshly isolated cells than in those in later culture. The maximum incorporation accounts for only 6% of the added radioactivity and produces a 9% inhibition of monoamine oxidase activity. The remaining [3H] pargyline is metabolized and quickly accumulates in the cell culture medium in a form which cannot label exogenous mitochondria. The metabolism of pargyline varies both qualitatively and quantitatively with culture age. In 0 hr and 20 hr-cultured cells one metabolite preferentially appears whilst in 140 hr cultured hepatocytes at least three metabolites are formed. The metabolism of [3H] pargyline in early culture is consistent with a cytochrome P-450 involvement. The use of [3H] pargyline to label monoamine oxidase in cultured hepatocytes offers several attractive features for studying the turnover of this enzyme. These include speed of interaction, non-reutilization, application to normal cells, controlled inhibition of monoamine oxidase and metabolism of non-specific label.

Protein turnover and cellular autophagy in growing and growth-inhibited 3T3 cells

The relationship between growth, protein degradation, and cellular autophagy was tested in growing and in growth-inhibited 3T3 cell monolayers. For the biochemical evaluation of DNA and protein metabolism, growth-inhibited 3T3 cell monolayers with high cell density and growing 3T3 cell monolayers with low cell density were labeled simultaneously with [14C]thymidine and [3H]leucine. The evaluation of the DNA turnover and additional [3H]thymidine autoradiography showed that 24 to 5% of 3T3 cells continue to replicate even in the growth-inhibited state, where no accumulation of protein and DNA can be observed. Cell loss, therefore, has to be assumed to compensate for the ongoing cell proliferation. When the data of protein turnover were corrected for cell loss, it was found that the rate constant of protein synthesis in nongrowing monolayers was reduced to half the value found in growing monolayers. Simultaneously, the rate constant of protein degradation in nongrowing monolayers was increased to about 1.5-fold the value of growing monolayers. In parallel to the increased rate constant of protein degradation, the cytoplasmic volume fraction of early autophagic vacuoles (AVs) as determined by electron microscopic morphometry was found to be increased twofold in nongrowing 3T3 cell monolayers when compared with the volume fraction of early AVs in growing 3T3 cell monolayers. These data are in agreement with the assumption that cellular autophagy represents a major pathway of regulating protein degradation in 3T3 cells and that the regulation of autophagic protein degradation is of relevance for the transition from a growing to a nongrowing state.

Multiple biotin-containing proteins in 3T3-L1 cells

Extracts of 3T3-L1 cells prepared after labelling the monolayer cultures with [3H]biotin contained numerous protein bands that were detected by fluorography of dried SDS/polyacrylamide electrophoresis gels. All labelled proteins in the extracts could be removed by avidin affinity chromatography. The biotin-containing subunits of acetyl-CoA carboxylase, pyruvate carboxylase, methylcrotonyl-CoA carboxylase and propionyl-CoA carboxylase, with molecular masses of approx. 220, 120, 75 and 72 kDa respectively, were detected together with minor bands at 100, 85 and 37 kDa that did not appear to be partial degradation products. Additional labelled bands increased in amount during incubation of cell extracts or did not occur in extracts prepared with trichloroacetic acid, 9.5 M-urea or proteolytic inhibitors, and were tentatively classified as partial degradation products. The unknown bands were not removed by incubation of cell monolayers for 24 h, a treatment that gave degradation rate constants of 0.47 day-1 for acetyl-CoA carboxylase and 0.28 day-1 for pyruvate carboxylase. Upon two-dimensional electrophoresis, pyruvate carboxylase, methylcrotonyl-CoA carboxylase and propionyl-CoA carboxylase had isoelectric points of 6.4, 7.2 and 6.4 respectively. Several additional discrete spots with isoelectric points below 6.2 were also present. All the unknown biotin-containing proteins banded with intact mitochondria during density-gradient centrifugation. We conclude that several unknown biotin-containing proteins are present in the mitochondria of 3T3-L1 cells, whereas others are partial breakdown products of mitochondrial proteolysis.

Distribution and degradation of biotin-containing carboxylases in human cell lines

Incubation of cultured cells with [3H]biotin leads to the labelling of acetyl-CoA carboxylase, pyruvate carboxylase, propionyl-CoA carboxylase and methylcrotonyl-CoA carboxylase. The biotin-containing subunits of the last two enzymes from rat cell lines are not separated by sodium dodecyl sulphate/polyacrylamide-gel electrophoresis, but adequate separation is achieved with the enzymes from human cells. Since incorporated biotin is only released upon complete protein breakdown, the loss of radioactivity from gel slices coinciding with fluorograph bands was used to quantify degradation rates for each protein. In HE(39)L diploid human fibroblasts, the degradation rate constants are 0.55, 0.40, 0.31 and 0.19 day-1 for acetyl-CoA carboxylase, pyruvate carboxylase, methylcrotonyl-CoA carboxylase and propionyl-CoA carboxylase respectively. A similar series of rate constants is found for AG2804 transformed fibroblasts. The degradation rate constants are decreased by 31-67% in the presence of 50 micrograms of leupeptin/ml plus 5 mM-NH4Cl. Although the largest percentage effect was noted with the most stable enzyme, propionyl-CoA carboxylase, the absolute change in rate constant produced by the lysosomotropic inhibitors was similar for the three mitochondrial carboxylases, but greater for the cytosolic enzyme acetyl-CoA carboxylase. The heterogeneity in degradation rate constants for the mitochondrial carboxylases indicates that only part of their catabolism can occur via the autophagy-mediated unit destruction of mitochondria. Calculations showed that the autophagy-linked process had degradation rate constants of 0.084 and 0.102 day-1 respectively in HE(39)L and AG2804 cells. It accounted for two-thirds of the catabolic rate of propionyl-CoA carboxylase and a lesser proportion for the other enzymes.

Proteolysis of liver acetyl coenzyme A carboxylase by cathepsin B

Proteolysis of acetyl-CoA carboxylase was examined with cathepsin B. When chicken liver acetyl-CoA carboxylase was incubated with cathepsin B at pH 6.3, the native 220-kDa polypeptide was primarily cleaved into two polypeptides of 125 and 115 kDa, and further degraded to polypeptides of 100-50 kDa.

Biotinidase deficiency: a novel vitamin recycling defect

The recent finding that biotinidase deficiency is the primary biochemical defect in late-onset multiple carboxylase deficiency was stimulated new interest in the inherited disorders of biotin-dependent carboxylases. The clinical and biochemical features of biotinidase deficiency are discussed. We also speculate about two exciting areas currently being investigated: the localization of action biotinidase, and the possible role of the enzyme as a binding or carrier protein for biotin.

Amino acid control of autophagic sequestration and protein degradation in isolated rat hepatocytes

Sequestration of the inert cytosolic marker [14C]sucrose by sedimentable organelles was measured in isolated rat hepatocytes made transiently permeable to sucrose by means of electropermeabilization. Lysosomal integrity, protein degradation, autophagic sequestration, and other cellular functions were not significantly impaired by the electric treatment. Hepatocytes sequestered sucrose at an initial rate of approximately 10%/h, which is threefold higher than the estimated rate of autophagic-lysosomal protein degradation. Almost one-third would appear to represent mitochondrial fluid uptake; the rest was nearly completely and specifically inhibited by 3-methyladenine (3MA) and can be regarded as autophagic sequestration. A complete amino acid mixture was somewhat less inhibitory than 3MA, and partially antagonized the effect of the latter. This paradoxical effect, taken together with the high sequestration rate, may suggest heterogeneity as well as selectivity in autophagic sequestration. There was no detectable recycling of sequestered [14C]sucrose between organelles and cytosol. Studies of individual amino acids revealed histidine as the most effective sequestration inhibitor. Leucine may have a regulatory function, as indicated by its unique additive/synergistic effect, and a combination of Leu + His was as effective as the complete amino acid mixture. Asparagine inhibited sequestration only 20%, i.e., its very strong effect on overall (long-lived) protein degradation must partially be due to post-sequestrational inhibition. The lysosomal (amine-sensitive) degradation of short-lived protein was incompletely inhibited by 3MA, indicating a contribution from nonautophagic processes like crinophagy and endocytic membrane influx. The ability of an amino acid mixture to specifically antagonize the inhibition of short-lived protein degradation by AsN + GIN (but not by 3MA) may suggest complex amino acid interactions at the level of fusion between lysosomes and other vesicles in addition to the equally complex interactions at the level of autophagic sequestration.

Degradation of proteins in rat liver mitochondrial outer membrane transplanted into different cell types. Evidence for alternative processing

The degradation of proteins in reductively [3H]methylated mitochondrial outer membrane (MOM) transplanted into cells by a poly(ethylene glycol)-mediated process has been studied. The average rate of degradation (t1/2 24-28 h) of MOM proteins transplanted into HTC cells was not the same as for endogenous MOM proteins (t1/2 56 h), mitoplast proteins (t1/2 120 h), plasma membrane proteins (t1/2 approx. 90 h) or cytosol proteins (t1/2 75 h). The degradation of transplanted MOM proteins was inhibited to the same extent (30-45%) as that of endogenous mitochondrial and plasma membrane proteins by leupeptin and NH4Cl. No inhibition of HTC cell cytosol protein degradation by NH4Cl was observed. NH4Cl differentially inhibited the degradation of endogenous MOM and mitoplast protein subunits as shown after sodium dodecyl sulphate/polyacrylamide-gel electrophoresis. Proteins in MOM transplanted into tissue culture cells were degraded either with t1/2 24-28 h (MRC-5, B82 and A549 cells) or with t1/2 55-70 h (CHO-K1 and 3T3-L1 cells) similar to that of proteins in MOM transplanted into rat hepatocytes [Evans & Mayer (1983) Biochem. J. 216, 151-161]. The data suggest that membrane protein destruction is but the end part of a fundamental intracellular membrane recognition process.

Degradative fate of transplanted proteins

The majority of cell proteins are non-cytosolic and are found in specific extracytosolic cytomorphological sites. Rat liver mitochondria and outer mitochondrial membrane (OMM) vesicles were transplanted homologously into rat hepatocytes and heterologously into rat hepatoma (HTC) cells by polyethylene glycol-mediated organelle--cell or OMM vesicle-cell fusion. The subsequent destructive fate of these non-cytosolic proteins was studied. During culture of hepatocyte monolayers in conditions which give in vivo catabolic rates, the transplanted organelle proteins and monoamine oxidase were degraded at rates similar to in vivo rates, although the transplanted material was not found in the hepatocyte mitochondria. Degradation was preceded by internalization (1-6 h) of the transplanted material and its translocation to a perinuclear, vesicular cytoplasmic position. Prevention of translocation by the disruption of the cytoskeleton inhibited subsequent degradation. In contrast, rat OMM heterologously transplanted into HTC cells was patched, capped and internalized into 'unique' vesicles and degraded 2.5 times faster than in hepatocytes. In both hepatocytes and HTC cells mitochondrial protein degradation was partially susceptible to lysosomotropic agents. The results are discussed in terms of a protein turnover cycle which attempts to coordinate the biochemistry and cell biology of protein synthesis and degradation in eukaryotic cells.