Inhibition of basal protein degradation in rat embryo fibroblasts by cycloheximide: correlation with activities of lysosomal proteases

Quantifying the Contribution of Defective Ribosomal Products to Antigen Production: A Model-Based Computational Analysis

Antigenic peptides (epitopes) presented on the cell surface by MHC class I molecules derive from proteolytic degradation of endogenous proteins. Some recent studies have proposed that the majority of epitopes stem from so-called defective ribosomal products (DRiPs), i.e., freshly synthesized proteins that are unable to adopt the native conformation and thus undergo immediate degradation. However, a reliable computational analysis of the data underlying this hypothesis was lacking so far. Therefore, we have applied kinetic modeling to derive from existing kinetic data (Princiotta et al. 2003, Immunity 18, 343-354) the rates of the major processes involved in the cellular protein turnover and MHC class I-mediated Ag presentation. From our modeling approach, we conclude that in these experiments 1) the relative share of DRiPs in the total protein synthesis amounted to approximately 10% thus being much lower than reported so far, 2) DRiPs may become the decisive source of epitopes within an early phase after onset of the synthesis of a long-lived (e.g., virus derived) protein, and 3) inhibition of protein synthesis by the translation inhibitor cycloheximide appears to be paralleled with an instantaneous decrease of protein degradation down to approximately 1/3 of the normal value.

Fasting-induced apoptosis in rat liver is blocked by cycloheximide

The effect of cycloheximide (CH) on the fasting-induced changes of rat liver cell and protein turnover has been investigated. Late starvation phase (3-4-day-fasting period) was characterised by a decrease in liver weight and protein and DNA content. The loss of DNA was not related to liver cell necrosis but due not only to depression of cell proliferation as shown by the drop in the labelling index but also induction of apoptosis. This type of apoptosis was documented by the increase in the apoptotic index (cells labelled by TUNEL) and transglutaminase activity as well as by DNA fragmentation. The liver cells of fasted rats appeared smaller as shown by the higher cell density and DNA/protein ratio than in controls. Females were more resistant to fasting-induced apoptosis than males. A single dose of CH, a drug primary known as inhibitor of protein synthesis, induced or enhanced apoptosis in fed and 2-days fasted male rats, respectively, without any sign of cell necrosis. On the contrary, the administration of repeated doses of CH blocked apoptosis induced by fasting. CH "froze" protein and DNA content as well as apoptotic process at the level of 2 days-fasted rats. While fasting-induced liver protein loss resulted from a marked reduction in protein synthesis with a slight decrease in degradation, repeated treatment with CH virtually blocked protein loss by abolishing protein catabolism. These data suggest a direct relationship between the catabolic side of protein turnover and the apoptotic process.

Implication of proteasome in estrogen receptor degradation

In MCF-7 breast cancer cells, estradiol (E2) and pure antiestrogen RU 58668 down-regulate the estrogen receptor (ER). Interestingly, the protein synthesis inhibitor cycloheximide (CHX) abrogated solely the effect of E2 suggesting a selective difference in the degradation of the receptor induced by estrogenic and antiestrogenic stimulations. A panel of lysosome inhibitors (i.e. bafilomycin, chloroquine, NH4Cl, and monensin), calpain inhibitors (calpastatin and PD 150606) and proteasome inhibitors (lactacystin and proteasome inhibitor I) were tested to assess this hypothesis. Among all inhibitors tested, lactacystin and proteasome inhibitor I were the sole inhibitors to abrogate the elimination of the receptor induced by both E2 and RU 58668; this selective effect was also recorded in cells prelabeled with [3H]tamoxifen aziridine before exposure to these ligands. Hence, differential sensitivity to CHX seems to be linked to the different mechanisms which target proteins for proteasome-mediated destruction. Moreover, the two tested proteasome inhibitors produced a slight increase of ER concentration in cells not exposed to any ligand, suggesting also the involvement of proteasome in receptor turnover.

Tumor growth influences skeletal muscle protein turnover in the pregnant rat

The implantation of a fast growing tumor (the Yoshida AH-130 ascites hepatoma) to mid-pregnant rats resulted in no changes in fetus weight, in spite of an important body weight decrease observed in the mother. Tumor-bearing pregnant rats showed an accelerated muscle protein degradation that resulted in decreases in both gastrocnemius and soleus muscle weight and protein content. Although very slight changes were observed in liver protein turnover after tumor implantation, muscle protein degradation and ubiquitin gene expression were increased (in relation with the non-tumor-bearing pregnant rats) in the first postimplantation period (0-4 d), whereas it remained lower in the second studied period (4-7 d), compensating for the initial differences when the whole period (0-7 d) was considered. Similar results were observed when muscle protein synthesis was studied. On the whole, tumor growth resulted in a slightly decreased protein accumulation rate. The results presented suggest that the implantation of this tumor in the pregnant rat has little or no consequences in fetal growth but results in an important muscle waste in the mother.

Tumor necrosis factor-alpha mediates changes in tissue protein turnover in a rat cancer cachexia model

Rats bearing the Yoshida AH-130 ascites hepatoma showed enhanced fractional rates of protein degradation in gastrocnemius muscle, heart, and liver, while fractional synthesis rates were similar to those in non-tumor bearing rats. This hypercatabolic pattern was associated with marked perturbations of the hormonal homeostasis and presence of tumor necrosis factor in the circulation. The daily administration of a goat anti-murine TNF IgG to tumor-bearing rats decreased protein degradation rates in skeletal muscle, heart, and liver as compared with tumor-bearing rats receiving a nonimmune goat IgG. The anti-TNF treatment was also effective in attenuating early perturbations in insulin and corticosterone homeostasis. Although these results suggest that tumor necrosis factor plays a significant role in mediating the changes in protein turnover and hormone levels elicited by tumor growth, the inability of such treatment to prevent a reduction in body weight implies that other mediators or tumor-related events were also involved.

Cell death induced in L-cells by treatment with thymidine: staging of the process and relationship to apoptosis

The purpose of this study was to characterize the stages in the development of thymidine-induced cell death. L-cells were characterized by both morphologic and quantitative techniques and evaluated at 24, 48, and 72 h of treatment. Cells first enlarged (stage I); about 50% of these enlarged cells then decreased in size with blebbing and compacting (stage II). This residual cell body transformed into a smooth eosinophilic hyaline body (stage III) by 72 h, many of which could be identified within the vacuolar system of viable cells. These changes were reflected in morphologic counts and Coulter sizing. Cell death (loss of labeled DNA) began in stage II and was most prominent in stage III. No cleavage of DNA into oligonucleosomal fragments was detected by agarose gel electrophoresis at any stage. The similarity of these changes to the complete spectrum of apoptosis in vivo is discussed.

The effect of protein intake on the potential activity of the lysosomal vacuolar system in the cat

1. Experiments were conducted to examine the relationship between protein intake and protein degradation in the liver of cats. 2. The cats were fed either a low protein/high carbohydrate diet (LP) or a high protein diet devoid of carbohydrate (HP). 3. The potential proteolytic activity of the lysosomal vacuolar system in the liver was assessed by both indirect (osmotic fragility of hepatic lysosomes) and direct (stereological measurement of lysosomal volume) methods. 4. The results from both tests indicated a significantly lower autophagic activity of the lysosomal system in the LP fed animals than in the HP fed cats. 5. This suppression of lysosomal protein degradation may represent an important mechanism for the conservation of proteins by the cat when low protein diets are fed.

Regulation of protein turnover versus growth state. Studies on the mechanism(s) of initiation of acidic vacuolar proteolysis in cells of stationary ascites hepatoma

1. After transplantation, the rat AH-130 Yoshida ascites hepatoma enters a phase of exponential (log) growth, followed by a quasi-stationary (sta) state. Combining measurements made in vivo and in vitro, cessation of protein accumulation (growth) in sta phase has previously been shown to result from convergent reduction of protein synthesis and enhancement of protein breakdown [Tessitore, Bonelli, Cecchini, Amenta & Baccino (1987) Arch. Biochem. Biophys. 255, 372-384]. 2. One day after labelling in the animal with [3H]leucine, AH-130 cells were processed for short-term assays in vitro to measure rates of endogenous protein breakdown. 3. Exposure of AH-130 cells to inhibitors interfering with different steps of the acidic vacuolar pathway (AVP) showed that: (i) in log tumour cells the AVP was extensively suppressed; (ii) in sta tumour cells virtually all of the proteolytic acceleration was accounted for by activation of the AVP. 4. Treating log tumour cells with glucagon, cyclic AMP, or nutritional deprivation failed to elevate substantially the proteolytic rates. Nor could the elevation in proteolysis be explained by changes in free amino acids, which were more concentrated in the ascitic fluid of sta tumours. 5. The enhanced proteolysis in sta tumour cells was not associated with any increase in the intracellular activity levels of lysosomal cathepsins B, D, H, and L. 6. The above growth-related modulation of protein breakdown in AH-130 cells was probably a reflection of the tumour growth state rather than the direct effect of environmental stimuli.

Regulation of protein turnover versus growth state: ascites hepatoma as a model for studies both in the animal and in vitro

Cell protein turnover states as related to growth phase have been analyzed in a rat ascites hepatoma (Yoshida AH-130), which after transplantation entered a period of exponential growth, followed by a quasi-stationary state. Evaluation of AH-130 cell protein turnover in the animal (slow-turnover protein pool) was combined with rapid assays of proteolytic rates of cells transferred in vitro. Protein accumulation in the exponential phase reflected the balance between sustained synthetic rates and relatively low degradative rates. Cessation of growth resulted from convergent reduction of synthesis (from 3.10 to 1.49%/h) and enhancement of protein breakdown (from 0.61 to 1.43%/h). Endogenous proteolytic rates in vitro were very close to the above degradation rates. As shown by incubation with ammonia or other lysosomal inhibitors, the acidic vacuolar pathway for protein degradation, while totally suppressed in exponential tumor cells, was activated in cells from stationary tumors to such an extent that it fully accounted for the enhanced proteolysis. In contrast, energy metabolism inhibitors were effective on cells in either growth state, the residual ongoing proteolysis being similar in both cells. The possible contribution of cell death to activation of the acidic vacuolar proteolysis in stationary tumors is discussed.

Early development of protein metabolic perturbations in the liver and skeletal muscle of tumour-bearing rats. A model system for cancer cachexia

In rats into which a fast-growing ascites hepatoma (Yoshida AH-130) had been transplanted, tumour growth elicited a marked loss of body weight until the animal's death in about 2 weeks. Overall tissue protein metabolism was simultaneously studied in vivo in the gastrocnemius muscle and liver after labelling with [14C]bicarbonate. Early and progressive atrophy developed in the gastrocnemius muscle, the underlying metabolic imbalance being expressed by an elevation in the apparent protein-degradation rate, with no changes in the apparent synthesis rate. A transient hyperplastic response preceded waste in the liver, both states being associated with alterations in protein-degradation rate: an initial decrease during liver growth, then an acceleration as liver regressed. Protein-synthesis rates, virtually unchanged during liver growth, were elevated in the subsequent phase, although not sufficient to balance the enhanced breakdown. Thus, in the tumour host tissues examined, altered states of protein turnover appeared to result mostly from changes in rates of protein breakdown. In sharp contrast with the negative protein balance in the host, the ascites hepatoma cells had the ability to grow or at least, in advanced stages, to maintain a stationary state.

Cellular distribution of lysosomal hydrolase activities in the regenerating rat liver

Cathepsins B and D, beta-galactosidase, and acid phosphatase activities were found to be decreased in the regenerating rat liver, the reduction being maximal around the peak of hepatocyte mitoses (30 h). To investigate whether these changes could be heterogeneously distributed among hepatic cells, total cell populations from control or two-thirds hepatectomized rat livers were dissociated by the collagenase perfusion technique and analysed by different procedures. Isopycnic centrifugation in a Metrizamide gradient satisfactorily resolved hepatocytes and non-parenchymal cells from control animals but was not adequate when applied to 30-h regenerating liver cells. Colchicine treatment of the hepatectomized animals, resulted in substantial accumulation of phase M-hepatocytes. Subpopulations considerably enriched in fast-sedimenting phase M-cells were obtained by sedimentation at 1 g of the total liver cell population, and subsequently analysed by isopycnic equilibration. Phase M-hepatocytes were shown to have markedly reduced levels of beta-galactosidase, acid phosphatase, and cathepsin B activities in comparison, not only with control hepatocytes, but also with those parenchymal cells which were not metaphase-arrested in the same regenerating livers. Therefore, in partially-hepatectomized rats, hepatocytes progressing up to metaphase in the first mitotic cycle exhibited a selective depletion of lysosomal enzyme activities. The mechanism(s) underlying this change remain(s) presently unknown.

Evidence for a special relationship between proteolysis and single cell necrosis

A high rate of single cell necrosis is a common phenomenon in neoplastic and preneoplastic lesions, accounting for growth rates that are significantly less than the cell birth rate. We present data relating the process of protein turnover to single cell necrosis. Cells were labeled with 3H-leucine and 14C-thymidine; the loss of radioactivity from the cell protein and DNA was then measured for 3-6 days. Preliminary experiments showed that cell necrosis by freeze-thawing cells did not significantly contribute to the degradation of cell proteins. Similar results were observed with dying 3T3-SV40 cells at high density. L-cells, however, showed a progressive increase in cell loss as higher cell densities were attained on the monolayer. Although proteolysis remained constant in the culture, analysis of the cells recovered from the high density monolayers showed little loss of labeled protein after adjustment for loss of label in the DNA. Three possible explanations are proposed: DNA turns over with cell protein (unlikely), single cell necrosis involves a special mechanism that facilitates reutilization of amino acids, or single cell necrosis includes only cells that are selectively involved in protein turnover. A unique relationship between single cell necrosis and proteolysis is suggested.

Effect of ethanol on proteolysis in isolated liver cells

Ethanol, when tested alone, inhibited proteolysis by about 20%; however, no effect was detected when it was combined with exogenous oxidizable fuels which inhibited proteolysis by themselves. Ethanol was effective in inhibiting proteolysis in the presence of protease inhibitors like ammonia, leupeptin or methylamine, indicating that its mechanism of action involves a non-lysosomal pathway of degradation. Ethanol oxidation is mandatory for it to have effect on proteolysis, however, its action is not related to an increased state of reduction of the NAD system. In contrast to other reductants of the NAD system, ethanol effect is accompanied by a rise in the phosphorylation state of the adenine nucleotides, suggesting that its action might be related to the cellular energy state.

Regulation of the androgen receptor by androgen in normal and androgen-resistant genital skin fibroblasts

Normal genital skin fibroblast (GSF) monolayers incubated with serum-free medium containing 3 nM [3H]-5 alpha-dihydrotestosterone (DHT) at 37 degrees C for 20 h have about 35% more specific DHT-binding than replicates incubated in serum-free medium with [3H]-DHT for only 1 h to saturate basal specific androgen-receptor activity. If, after 19 h, spent medium is replaced by fresh medium with 3 nM [3H]-DHT for 1 h, specific DHT binding is 85% more than basal. The acquisition of increased binding is temperature dependent (37 greater than 27 degrees C) and cycloheximide (2 microM) suppressible. The increased binding activity is considered to represent an augmentation of androgen receptor concentration because it has the same equilibrium dissociation constant (KD approximately 0.5 nM), rate constant of dissociation (k-1 approximately 6 x 10(-3) min-1) and ligand specificity as basal androgen-receptor activity, and because basal DHT-binding activity is stable in cells preincubated in androgen-free or serum-free medium alone for up to 72 h before assay. Prolonged incubation with methyltrienolone (R1881), a nonmetabolizable synthetic androgen, causes a greater, more persistent increment of androgen receptor activity than does equimolar DHT. The fibroblasts from two subjects with receptor-positive, partial androgen resistance lose their basal receptor activity during prolonged incubation with DHT, but augment it normally with R1881. This suggests that defective DHT metabolism is somehow involved in the pathogenesis of their androgen resistance.

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

1. Exposure to [3H]biotin during the differentiation of 3T3-L1 cells to adipocytes selectively labelled pyruvate carboxylase (EC 6.4.1.1). A subsequent incubation of labelled cells permitted the measurement of the degradation rate constant of this mitochondrial enzyme. 2. In medium without serum, pyruvate carboxylase was degraded with a half-life of 64 h, considerably longer than that found for average cell protein. The long half-life is commensurate with the enzyme being catabolized when whole mitochondria are destroyed. 3. The breakdown of pyruvate carboxylase was inhibited to a greater extent than the breakdown of total cell protein by insulin, NH4Cl and inhibitors of lysosomal proteinases, suggesting that the enzyme is degraded by the autophagic lysosomal system of the cell. 4. The above evidence implies that whole mitochondria are degraded in lysosomes, a conclusion that agrees with earlier electron-microscopic evidence showing mitochondria within autophagic vacuoles. 5. A second degradative pathway must be invoked to account for the breakdown of mitochondrial proteins of short half-life.

Inhibition of protein synthesis blocks the response to 25-hydroxycholesterol by inhibiting degradation of hydroxymethylglutaryl-CoA reductase

The activity of the rate-limiting enzyme of the cholesterol biosynthetic pathway, 3-hydroxy-3-methylglutaryl coenzyme A reductase in Chinese hamster ovary (CHO) cells decreased more rapidly in cells treated with 25-hydroxycholesterol alone (t 1/2 = 1.5 h) than in those incubated with cycloheximide alone (t 1/2 = 5 h). The inhibitory action of 25-hydroxycholesterol on reductase activity was reduced when the sterol and cycloheximide were added together, and was totally abolished when cells were preincubated with cycloheximide for 30 min before the addition of 25-hydroxycholesterol. The effect of puromycin was similar to that of cycloheximide. Treatment of cells with an inhibitor of RNA synthesis, i.e., actinomycin D or cordycepin, had little effect on hydroxymethylglutaryl-CoA reductase activity; however, preincubation of cells with these reagents also decreased the ability of 25-hydroxycholesterol to suppress the reductase activity. These data are consistent with a model which suggests (a) that 25-hydroxycholesterol inhibits the activity of hydroxymethylglutaryl-CoA reductase by repressing its synthesis, (b) that cycloheximide and puromycin affect hydroxymethylglutaryl-CoA reductase activity by blocking the de novo synthesis of the enzyme and by reducing the degradation of the preexisting enzyme, (c) that actinomycin D and cordycepin affect the supply of message for the continuous synthesis of at least one component of a system which degrades hydroxymethylglutaryl-CoA reductase, and (d) that one component of the degradative system has a half-life shorter than 0.5 h.

Control of cell protein catabolism in rat liver. Effects of starvation and administration of cycloheximide

1. The loss of liver protein occurring in rats starved for 24 h was largely prevented by the administration of repeated doses of cycloheximide, an inhibitor of protein synthesis. Similar effects were produced on tubulin, a 'fixed' liver protein. 2. Starvation accelerated, whereas cycloheximide markedly lowered, the rate of protein radioactivity decay after labelling with [3H]valine or [14C]bicarbonate, indicating that changes in catabolic rates played an important role in the above regulations of liver protein mass. 3. The total activity of several lysosomal hydrolases showed little change in livers of starved rats, but a marked progressive decline developed after the administration of cycloheximide, particularly in the activities of cathepsins B, D and L as well as acid ribonuclease. There was no evidence that these changes might be due to endogenous inhibitors (at least for cathepsin B activity, which fell to less than 30% of the control values) or enzyme leakage into the bloodstream; rather, plasma beta-galactosidase and beta-N-acetylglucosaminidase activities fell progressively during the cycloheximide treatment. 4. Endogenous proteolytic rates, measured in vitro by incubating subcellular preparations from livers prelabelled in vivo with [3H]valine, were markedly decreased in cycloheximide-treated animals. 5. The osmotic fragility of hepatic lysosomes, appreciably enhanced in starved animals, after cycloheximide treatment was found to be even lower than in fed controls. 6. The present data are consistent with the view that in starved animals the loss of liver protein is mostly accounted for by increased breakdown, due, in part at least, to enhanced autophagocytosis. 7. Cycloheximide largely counteracted these effects of starvation, altering the liver from being 'poised' in a proteolytic direction to a protein-sparing condition. The present data suggest that, besides suppression of the autophagic processes, a decrease in the lysosomal proteolytic enzyme system may also play a role in this regulation, and they seem to provide further circumstantial evidence for the existence of co-ordinating mechanisms between protein synthesis and degradation.

Evidence for endogenous polypeptide-mediated inhibition of cell-cycle transit in human diploid cells

Previous studies have shown that the senescent phenotype is dominant with respect to DNA synthesis in fusions between late passage and actively replicating human diploid fibroblasts. Brief postfusion treatments with the protein synthesis inhibitor cycloheximide (CHX) or puromycin have been found to significantly delay (by 24-48 h) the inhibition of entry into DNA synthesis of young nuclei in heterokaryons after fusion with senescent cells. A significant fraction of the senescent nuclei incorporated tritiated thymidine in CHX-treated heterokaryons. The optimal duration of exposure to CHX was 1-3 h immediately after fusion, although treatments beginning as late as 9 h after fusion elevated the heterokaryon labeling index. Prefusion treatments with CHX were without a significant effect. These results are consistent with the interpretation that regulatory cell cycle inhibitor(s) which are dependent upon protein synthesis may be present in heterokaryons between senescent and actively replicating cells.

Protein synthesis and degradation in growth regulation in rat embryo fibroblasts: role of fast-turnover and slow-turnover protein

Cultured rat embryo fibroblasts, when stimulated to grow by the addition of fresh medium containing 10% serum, showed an increase in synthesis of slow-turnover proteins while maintaining a uniform degradation rate for these proteins. Slow-turnover proteins with a half-life of 2.4 days accounted for approximately 95% of the cell protein, while the remaining protein could be described in terms of two fast-turnover pools. When we labeled cells to limiting levels over a period of 4 days, the fast-turnover pools became undetectable; with 2-hour labeling periods, however, 25% of the label entered the fast-turnover pools. Fibroblasts, stimulated to grow by fresh growth medium, showed proportionate and coordinate increases in synthesis of both fast-turnover and slow-turnover proteins during the growth period, both returning to baseline levels on reaching the new steady state. No changes could be detected in degradation of either pool during growth. Fibroblasts placed in a serum-free medium showed a decrease in cellular protein and an increased degradation of slow-turnover proteins, while degradation of fast-turnover proteins remained unchanged. We conclude that the slow-turnover protein pool forms the bulk of the cell proteins and turns over at a fairly constant rate. Growth stimulation is effected almost entirely by stimulation of protein synthesis in this pool, while decreasing cellular protein growth is a result of enhanced degradation within this pool.

Evidence of heterogeneity of protein-turnover states in cultured cells

Previous studies on L-cell cultures [Amenta & Sargus (1979) Biochem. J. 182, 847--859] have suggested: (a) that degradation of slow-turnover proteins occurs in a distinct cell state (D-state); (b) that cells randomly enter the D-state with a first-order transition constant, rapidly degrade cell protein, and return to a quiescent G0-state. In the present study we have tested the hypothesis that the putative D-state exists as a substate within A-state (non-replicating) fibroblasts. Rat-embryo fibroblasts were prelabelled with [14C]leucine and [3H]thymidine, 'chased' for 24 h, and then placed in fresh growth medium containing either vinblastine (10 microM) or colchicine (25 microM) for three successive 24 h periods. Cells trapped in mitosis were separated from the residual non-replicating cells and rates of protein synthesis, degradation and net accumulation were measured in both populations. We observed that significant protein degradation occurred only in the non-replicating population, although both populations showed equally high rates of protein synthesis induced by fresh growth medium. These data support the hypothesis that degradation of slow-turnover protein is heterogeneous, occurring only in A-state cells. A model that proposes a separate D-state within G0-phase successfully accounts for these observations and previous reports on this cell line [Amenta, Sargus & Baccino (1978) J. Cell. Physiol. 97, 267--283] showing no differences in degradation of the slow-turnover protein pool in growth-stimulated and stationary-phase fibroblast cultures.

Effect of tunicamycin and cycloheximide on the secretion of acid hydrolases from I-cell cultured fibroblasts

I-cell cultures fibroblasts secrete excessive amounts of N-acetyl-beta-D-hexosaminidase and alpha-L-fucosidase into the culture media as compared with normal fibroblasts. Addition of tunicamycin or cyd [14C]leucine (40--50%) into trichloroacetic acid-precipitable material decreased the secretion of these I-cell hydrolases to normal values within 24 h, but had no effect on the secretion of acid hydrolases from normal fibroblasts. These results indicate that I-cell cultured fibroblasts secrete at least two types of acid hydrolases: one is tunicamycin- and cycloheximide-sensitive and constitutes the greater proportion of the secreted hydrolases, and a smaller proportion is insensitive to tunicamycin and cycloheximide, similar t9 the acid hydrolases secreted by normal cultured fibroblasts.

The inhibition of macrophage protein turnover by a selective inhibitor of thiol proteinases

1. A new inhibitor of thiol proteinases, benzyloxycarbonylphenylalanylalanine diazomethyl ketone (benzyloxycarbonylphenylalanylalanyldiazomethane, Z-Phe-Ala-CHN2) was added to cultured mouse peritoneal macrophages prelabelled with [14C]leucine. The degradation of protein was studied under conditions of basal proteolysis in the presence of 10% pig serum. After a lag of about 6 h a time- and dose-dependent inhibition of protein degradation was observed, up to a maximum of about 40%. 2. The inhibitor entered the cells with kinetics consistent with entry by pinocytosis, giving access to the lysosomal system. 3. Intracellular cathepsin B was almost completely inactivated after 90 min of exposure of the culture to 0.1 mm-inhibitor. 4. The inhibition of proteolysis and of cathepsin B was reversed virtually completely within 24 h, when the inhibitor was removed from the medium. Since the inhibitor forms a covalent bond with the enzyme, the recovery of cathepsin B activity presumably reflects production of new molecules of active enzyme. 5. The inhibitory effects of pepstatin, the carboxyl proteinase inhibitor, were under some circumstances additive with those Z-Phe-Ala-CHN2, and were also largely reversible. 6. It is concluded that thiol proteinases play a major role in lysosomal proteolysis in cultured macrophages.

Role of lysosomes in protein turnover: catch-up proteolysis after release from NH4Cl inhibition

Cultured rat embryo fibroblasts, when placed in media with 10% serum containing 20 mM NH4Cl, show an inhibition of protein degradation and, concurrently, an accumulation of numerous, large vacuoles, partially filled with cellular debris. Cells placed in a serum-free media exhibit an enhanced degradation of cell protein, which is also inhibited by NH4Cl. When these cells are removed from media containing NH4Cl and placed in fresh media, the material accumulated in these vacuoles is rapidly and quantitatively released to the media in both an acid-soluble and acid-insoluble from. NH4Cl inhibits rapidly and specifically the lysosomal proteolytic mechanism, and is without effect on the basal turnover mechanism. The lysosomal proteolytic mechanism accounts for approximately 25% of protein turnover, and, at least in low density cultures, can be stimulated to levels which account for more than half of the protein turnover in the cell. The major pathway for the degradation of fast turnover proteins appears to be separate from lysosomal mechanism.