Degradation of abnormal proteins in HeLa cells

Growth inhibition of A549 human lung adenocarcinoma cells by L-canavanine is associated with p21/WAF1 induction

L-Canavanine (CAV) is a higher plant nonprotein amino acid and a potent L-arginine antimetabolite. CAV can inhibit the proliferation of tumor cells in vitro and in vivo, but little is known regarding the molecular mechanisms mediating these effects. We demonstrated that the treatment of human lung adenocarcinoma A549 cells with CAV caused growth inhibition; G1 phase arrest is accompanied by accumulation of an incompletely phosphorylated form of the retinoblastoma protein, whose phosphorylation is necessary for cell cycle progression from G1 to S phase. In addition, CAV induces the expression of p53 and subsequent expression of a cyclin-dependent kinase inhibitor, p21/WAF1. The p53-dependent induction of p21/WAF1 and the following dephosphorylation of the retinoblastoma protein by CAV could account for the observed CAV-mediated G1 phase arrest.

Degradation of ornithine decarboxylase in mammalian cells is ATP dependent but ubiquitin independent

Ornithine decarboxylase (ODC), a key enzyme in the biosynthesis of polyamines in mammalian cells is characterized by an extremely short half-life. In the present study, ODC degradation was investigated in 653-1 mouse myeloma cells that overproduce ODC and in ts85 cells that are thermosensitive for conjunction of ubiquitin to target proteins. Addition of 2-deoxyglucose and dinitrophenol (agents that efficiently deplete cellular ATP) to the growth medium of these cells inhibited ODC degradation. In contrast, chloroquine and leupeptin, inhibitors of intralysosomal proteolysis, did not affect ODC degradation. Shifting ts85 cells to 42 degrees C (a non-permissive temperature that inhibited conjugation of ubiquitin to target proteins) did not prevent ODC degradation. The ATP-dependent degradation of ODC in 653-1 cells was inhibited substantially by N alpha-tosyl-L-lysine chloromethane (TosPheMeCl), iodoacetamide and o-phenanthroline. These results suggest that ODC degradation occurs via a non-lysosomal. ATP-requiring and ubiquitin-independent cellular proteolytic mechanism, and that serine proteases and enzymes containing sulphydryl groups and metalloenzyme(s) may be involved in this process.

Selective cytotoxicity of L-canavanine in tumorigenic Madin-Darby canine kidney T1 cells

L-canavanine, an analog of L-arginine, was examined for toxicity in a normal canine kidney epithelial cell line, Madin-Darby canine kidney (MDCK), and in its chemically transformed derivative MDCK-T1. Under conditions where the analog reversibly arrested growth of MDCK cells, more than 90% of the tumorigenic cells were killed. This selective cytotoxicity was not due to any difference in growth rate (both lines doubled every 24 h). Nor was it caused by the inhibition of protein synthesis or DNA replication, since amino acid deprivation and drugs which inhibited replication directly and indirectly did not kill the transformed cells preferentially. To the contrary, cycloheximide killed MDCK cells preferentially. Although the mechanism for the selective cytotoxicity of canavanine in the tumorigenic cells remains obscure, one clue was afforded by the observation that the analog caused a loss in plating efficiency of the T1 cells prior to their detachment from plates. Selective sensitivity to canavanine may therefore reside in cell surface proteins, which are known to differ both qualitatively and quantitatively between normal and transformed cells. The present results support our previous findings suggesting the possible value of using canavanine as an agent for cancer chemotherapy.

Effects of canavanine on the secretion of plasma proteins by Hep G2 cells

Many secretory proteins contain an amino-terminal propeptide extension which is removed prior to secretion. The point of cleavage is usually marked by a basic pair of amino acids containing arginine. Canavanine, an analogue of arginine, is incorporated into protein and has been shown to inhibit the proteolytic processing of several of these prosecretory proteins. The addition of 3 mM canavanine to Hep G2 cells incubated with L-[35S]methionine inhibited the secretion of 11 plasma proteins studied. Of the secretory proteins studied only albumin is thought to contain a propeptide, which is marked by a pair of arginine residues at its point of proteolytic processing. Canavanine had varying effects on the secretion of plasma proteins; ranging from a 43-53% inhibition of secretion of alpha 1 antitrypsin and alpha 1 anti-chrymotrypsin to nearly abolishing (93% inhibition) secretion of transferrin. Canavanine also caused most of the proteins studied to migrate slower on sodium dodecyl sulfate polyacrylamide gel electrophoresis. Two of the canavanine-treated proteins (albumin and transferrin) which underwent marked changes in electrophoretic mobility were more sensitive than untreated proteins to proteolysis by Staphylococcus Aureus V8 proteinase. The slower electrophoretic migration and the greater sensitivity to proteolysis of these proteins may be attributed to marked structural changes caused by the incorporation of canavanine. This suggests that the inhibition of plasma protein secretion by canavanine is not only due to an inhibition of the processing of proteins but may be caused by structural distortions of the secretory proteins.

Comparison of the inhibitory effects of diverse amino acids and amino acid analogs on 12-O-tetradecanoylphorbol-13-acetate-induced ornithine decarboxylase activity in isolated epidermal cells

At a concentration of 1.25 mM, 14 amino acids were capable of inhibiting the induction of ornithine decarboxylase (L-ornithine carboxy-lyase, EC 4.1.1.17) activity by the tumor promoter 12-O-tetradecanoylphorbol-13-acetate (TPA) in isolated epidermal cells. The greatest percentages of inhibition of TPA-induced epidermal ornithine decarboxylase activity were as follows: cysteine, 98%; tryptophan, 74%; methionine, 64%; phenylalanine, 51%; glycine, 44%; asparagine, 43%; glutamic acid, 42%; leucine, 40%; and arginine, 39%. These amino acid treatments did not alter the time- and concentration-response curves for induction of ornithine decarboxylase activity by TPA. Moreover, there was no difference between the rates at which [3H]arginine, [3H]leucine, [3H]phenylalanine, [3H]methionine, [3H]tryptophan and [14C]cysteine were taken up by freshly isolated epidermal cells or incorporated into epidermal proteins. Arginine, phenylalanine and methionine inhibited the induction of ornithine decarboxylase activity by the tumor promoter to degrees comparable to those elicited by their analogs canavanine and homoarginine, beta-2-thienyl-DL-alanine, and ethionine, respectively. These amino acids and amino acid analogs did not alter the overall rate of protein synthesis. In contrast, both the amino acids and their analogs increased the rates of proteolysis in isolated epidermal cells, an effect which correlated well with the abilities of these different compounds to inhibit TPA-induced ornithine decarboxylase activity. Moreover, both methionine and phenylalanine decreased the half-life and increased the rate of heat denaturation of the TPA-induced enzyme, a result identical to that obtained after treatment with the analogs ethionine and beta-2-thienyl-DL-alanine, respectively. Taken together, these results suggest that millimolar concentrations of exogenous amino acids might induce the synthesis of abnormal proteins and nonfunctional enzymes. Therefore, it is speculated that the uptake of unbalanced amounts of amino acids into the epidermal target cells might alter the stability and the ultrastructure of the TPA-stimulated enzyme just as the amino acid analogs do.

Energy requirement for degradation of tumor-associated protein p53

A 53,000-dalton protein (p53) present in large amounts in several types of tumorigenic cells was rapidly degraded in nontumorigenic BALB/c 3T3 fibroblasts (t 1/2, approximately 0.5 h) but not in tumorigenic methylcholanthrene-induced mouse sarcoma cells (t 1/2, greater than 2 h). In 3T3 cells, dinitrophenol and 2-deoxyglucose, agents which reduce ATP production, inhibited the rapid degradation of p53 and the slower breakdown of total cell protein. After removal of these agents, the degradation of both p53 and total cell proteins resumed at their normal rates. Inhibitors of intralysosomal proteolysis (Ep475 and chloroquine) did not reduce the rate of degradation of p53. Thus, in 3T3 cells, p53 appears to be degraded by a nonlysosomal, ATP-dependent proteolytic system similar to that previously shown to degrade short- and long-lived proteins in growing fibroblasts. The immunoreactive p53 which remained in ATP-depleted cells had the same molecular weight as the p53 in the control cells. No intermediate products of p53 degradation were detected by immunoprecipitation in either ATP-depleted or control cells. Hence, ATP seems to be required for an initial step in the degradation of p53. Although the amount of labeled p53 was increased in simian virus 40-transformed and methylcholanthrene-induced mouse sarcoma cells, the amount of p53 labeled during a 3-h pulse in Moloney virus- and Rous sarcoma virus-transformed cells and untransformed 3T3 cells was similar. Thus, an increased net rate of p53 accumulation is not a common feature of transformed tumorigenic cells.

Energy requirement for degradation of tumor-associated protein p53

A 53,000-dalton protein (p53) present in large amounts in several types of tumorigenic cells was rapidly degraded in nontumorigenic BALB/c 3T3 fibroblasts (t 1/2, approximately 0.5 h) but not in tumorigenic methylcholanthrene-induced mouse sarcoma cells (t 1/2, greater than 2 h). In 3T3 cells, dinitrophenol and 2-deoxyglucose, agents which reduce ATP production, inhibited the rapid degradation of p53 and the slower breakdown of total cell protein. After removal of these agents, the degradation of both p53 and total cell proteins resumed at their normal rates. Inhibitors of intralysosomal proteolysis (Ep475 and chloroquine) did not reduce the rate of degradation of p53. Thus, in 3T3 cells, p53 appears to be degraded by a nonlysosomal, ATP-dependent proteolytic system similar to that previously shown to degrade short- and long-lived proteins in growing fibroblasts. The immunoreactive p53 which remained in ATP-depleted cells had the same molecular weight as the p53 in the control cells. No intermediate products of p53 degradation were detected by immunoprecipitation in either ATP-depleted or control cells. Hence, ATP seems to be required for an initial step in the degradation of p53. Although the amount of labeled p53 was increased in simian virus 40-transformed and methylcholanthrene-induced mouse sarcoma cells, the amount of p53 labeled during a 3-h pulse in Moloney virus- and Rous sarcoma virus-transformed cells and untransformed 3T3 cells was similar. Thus, an increased net rate of p53 accumulation is not a common feature of transformed tumorigenic cells.

Effects of amino acid treatments on 12-O-tetradecanoylphorbol-13-acetate-induced ornithine decarboxylase activity in mouse epidermis in vivo and in vitro

We have compared the effects of several amino acid treatments on the induction of ornithine decarboxylase activity and the accumulation of putrescine, spermidine, and spermine by 12-O-tetradecanoylphorbol-13-acetate (TPA) in mouse epidermis in vivo and in vitro. Incubation of isolated epidermal cells with mM concentrations of glycine, asparagine, glutamic acid, canavanine, arginine, and/or lysine inhibited dramatically the induction of ornithine decarboxylase activity by the tumor promoter. These remarkable inhibitory effects were concentration-dependent and additive. Arginine and its analog, canavanine, inhibited to the same degree TPA-induced ornithine decarboxylase activity, and potentiated to the same extent the inhibitory effects of glutamic acid, asparagine, and glycine on this enzyme. However, the inhibitory effects of arginine and canavanine were not additive. Similar alterations of tumor promoter-induced epidermal ornithine decarboxylase activity were observed in vivo when 62.5 mumol of the amino acids were injected i.p. 2 h before the topical application of 8.5 nmol of TPA to mouse skin. The results suggest the possibility that treatments with glycine, asparagine, glutamic acid, and arginine, the amino acids that were the most effective in inhibiting the tumor promoter-induced accumulation of polyamines in vivo, may reduce the tumor-promoting ability of TPA.

A comparison of lysosomal involvements in the degradation of normal and abnormal endogenous proteins of differing half-lives in MRC5 cells

Protein degradation by diploid human-embryo lung fibroblasts (MRC5 cells) in monolayer culture was studied. 1. Varying the labelling period of proteins was found to alter the half-lives of labelled abnormal canavanine-containing proteins to an extent very similar to that obtained with normal proteins. 2. By manipulating the times of labelling it was possible to generate a species of abnormal protein with a greater half-life than that of a species of normal protein. A comparison of the lysosomal involvement in their degradation as determined both by inhibition by methylamine, a lysosomotropic agent, and by the degree of increase in protein degradation in step-down conditions, indicated that the degree of lysosomal involvement was not entirely dependent upon the half-life of the protein, but that abnormal proteins are preferentially degraded non-lysosomally. 3. The microtubule inhibitors colchicine and vinblastine were found to stimulate statistically basal protein degradation of normal long-labelled protein, whereas they had less effect upon the basal degradation of the other species of proteins studied and very little effect upon step-down degradation of all proteins studied. The stimulation in protein degradation found did not seem to involve the acid proteinases of lysosomes.

The effect of canavanine on protein synthesis and protein degradation in IMR-90 fibroblasts

Proteins of IMR-90 fibroblasts incorporating [35S]methionine during a 1 h labelling period in the presence of the arginine analogue canavanine were degraded twice as rapidly in the cells as were proteins similarly made in the presence of arginine. Using both isoelectric focusing and SDS-polyacrylamide gel electrophoretic analyses, the banding patterns of proteins labelled in the presence of canavanine and arginine were found to differ. This banding difference was detected as early as 15 min after canavanine treatment. With the exception of one minor band in isoelectric focusing gel, the relative intensity of labelled protein bands for the control samples remained unchanged during the 2 h period of protein degradation being investigated. This was also true for the proteins labelled in the presence of canavanine, despite the increase in their rate of degradation. Banding difference between canavanine and arginine treatment was also detected in an in vitro reticulocyte lysate translation system dependent on fibroblast mRNA. Proteins labelled in the presence of a different analogue, p-fluorophenylalanine instead of phenylalanine, however, had similar banding patterns as the control both in the lysate system and in intact cells.

Turnover and regulation of Na-K-ATPase in HeLa cells

HeLa cells in log growth have 10(6) surface Na-K-ATPase molecules as estimated by the specific binding of [3H]-ouabain. Studies utilizing ouabain as a label show that the ligand is internalized at a rate corresponding to the turnover of three sets of Na-K-ATPase enzymes per generation. The label is taken up exclusively into a particulate cell compartment where it is codistributed with beta-hexosaminidase, identifying the internal compartment as lysosomal. Turnover is an important parameter in the recovery of the cells from glycoside intoxication. The unmetabolized glycoside is subsequently released by exocytosis. 13C-density-labeled Na-K-ATPase has been identified by specific phosphorylation of its catalytic subunit with [32P]ATP or [33P]ATP, and the rate of turnover of the density label is shown to be the same as the internalization of the ouabain-labeled site. There is a transit time of about 4 h from the onset of synthesis of the catalytic subunit to its insertion in the surface membrane; 2,800 catalytic subunits are synthesized per minute per cell, and 2,100 are turned over K+-starved cells respond to the stress in 24-30 h with modulation of the surface density of Na-K-ATPase the synthetic rate remains constant; the number of functional enzymes per cell is controlled by change in the rate constant for turnover.

Cultured animal cells exposed to amino acid analogues or puromycin rapidly synthesize several polypeptides

Four major acidic polypeptides, with molecular weights of 88, 72, 71, and 23 thousand, and minor polypeptides with molecular weights of 110, 50, 38, and 30 thousand rapidly accumulated in cultured chick embryo (CE) cells which were exposed for three hours to the arginine analogue canavanine. P110, P88, P71,72, and P23 had unique peptide maps. Evidence of a 27,000 dalton precursor to P23 was obtained. The analogue-stimulated proteins were not related to another set of inducible avian polypeptides known as the glucose-regulated proteins. In mammalian cells, the rate of accumulation of several polypeptides, which were similar in size to the avian proteins, sharply increased after canavanine treatment. Proteins with the same electrophoretic mobilities, isoelectric points, and peptide maps as the analogue-stimulated proteins were expressed at low levels in untreated cultures. To determine the time courses of the canavanine-mediated increases in protein accumulation and the recovery of protein metabolism after analogue treatment, radioactively labeled proteins were extracted from CE cells and analyzed on SDS-polyacrylamide gels. In cultures exposed to canavanine, the rates of accumulation of P88 and P71,72 increased from basal to new plateau levels in about 1.5 hours, while P23 required about 2.5 hours. When added with the analogue, actinomycin D and cordycepin blocked the increases in protein accumulation. These inhibitors also blocked the rapid decline in the rates of accumulation of the enhanced proteins which occurred after removal of canavanine. Studies of the matabolic stability of the enhanced proteins indicated that the changes in their accumulation were caused by alterations in their rates of synthesis. Thus, the analogue-mediated response fulfilled several of the criteria for inducible eucaryotic gene expression. The amino acid analogue p-fluorophenylalanine and the chain-terminating analogue of amino acyl-tRNA puromycin stimulated the synthesis of the same set of proteins induced by canavanine. The enhanced synthesis of these proteins appeared to be a cellular response to either the presence or catabolism of abnormal proteins and puromycyl peptides.

The effect of age on the protein degradation system in the nematode Turbatrix aceti

The administration of 0.2% of the amino acid analogues canavanine and 6-fluorotryptophan to young nematode cultures caused the appearance of inactive but antigenically reactive aldolase molecules at levels comparable to those found in old untreated cultures. Concomitant with the rapid appearance of inactive enzyme molecules a rapid rate of mortality could be detected. Mortality, however, ceased when a massive disappearance of inactive molecules could be observed. Subsequently, mortality resumed at a rate similar to that found in untreated cultures. The NaH14CO3 method of Swick and Ip (J. Biol. Chem., 249 (1974) 6836-6841) was used to estimate the half-life of proteins. The half-life of total soluble proteins in old and intermediate age animals was 4 times and 2.5 times longer, respectively, than that of young nematodes. No differences could be discerned in the half-lives of total proteins between analogue-treated and control animals of young and intermediate ages. However, the rate of disposal of inactive molecules was slower in intermediate age as compared to young animals as it was closely related to the general rate of protein degradation in the different age groups. The results suggest that (a) the proportion of inactive enzyme molecules encountered in old animals is potentially detrimental, (b) the decline in efficiency of the degradation system in older animals may account for the accumulation of altered protein molecules in aging organisms.

Phenotypic lag and mutation to 6-thioguanine resistance in diploid human lymphoblasts

Mutants of a diploid human lymphoblast line resistant to 6-thioguanine (6TG) appear 6--16 generations after treatment with any of a diverse group of mutagents: methylnitrosourea (MNU), methylnitrosoguanidine (MNNG), ICR-191, 5-bromodeoxyuridine (BUdR). A hypothesis is advanced that expression of the 6-thioguanine-resistant state may require the removal of essentially all pre-existing hypoxanthine--guanine phosphoribosyl transferase (HGPRT) molecules via division, dilution, and protein turnover. Design of protocols for quantitative mutation assays requires attention to this phenomenon.

Studies on phosphoglucose isomerase from cultured human fibroblasts: absence of detectable ageing effects on the enzyme

Some properties of the enzyme phosphoglucose isomerase (PGI) from the foetal lung fibroblast strain MRC-5 have been investigated throughout the in vitro lifespan of this cell strain. No significant age-related alterations in specific activity or thermostability of PGI could be detected. Titration of enzymatic activity with antibody directed against purified PGI showed no detectable differences in PGI from extracts of early passage cells compared with enzyme from senescent cells. The effect of p-fluorophenylalanine incorporation on PGI was examined in early passage fibroblasts. Thermostability studies showed increased heat lability of PGI from analogue treated cells when compared with enzyme from control cells at the same passage. However, no inactive PGI protein could be detected by anti-serum titration in extracts from analogue-treated cells. The results indicate that no significant amount of altered or inactive PGI is produced in ageing fibroblasts.

Responses of protein synthesis and degradation in growth control of WI-38 cells

The overall rates of protein synthesis and degradation in perfusion-grown WI-38 cells were followed in the three days after a stepdown in the serum concentration of the culture medium, from 10% to 0.3%. Within three hours after the stepdown, the rate of protein synthesis had decreased and the rate of protein degradation had increased, the combined result being the cessation of protein accumulation. The degradation rate returned over the next three days to its original value, but a zero rate of accumulation was retained because the synthesis rate continued to decline. The rate of DNA synthesis remained constant for six hours after the stepdown. It then declined steadily until reaching a minimum about eight hours later. The results show that extracellular control of protein accumulation depends on adjustments in both protein synthesis and protein degradation, and that the adjustments take place rapidly. This behavior suggests that the cell cycle is arrested after a stepdown because post-mitotic cells are unable to accumulate additional protein. However, an alternative interpretation of the data is that at least part of the changed accumulation is the result, rather than the cause, of the cycle arrest, and that the arrest is caused by other, more specific, reactions than those of general protein metabolism.

Effect of a nutritional shift on the degradation of abnormal proteins in the mouse liver. Decreased degradation during rapid liver growth

1. The intravenous injection of puromycin to mice 0.5 min after administration of radioactive leucine resulted in the release of labelled ribosome-bound nascent protein chains with the next 0.5 min. 2. During the subsequent 13 min, 40% of the liver protein radioactivity disappeared. The rate of this process was already maximal 0.5 min after the injection of puromycin, with no apparent lag. 3. Evidence is presented that this phenomenon represents the selective degradation of puromycinyl-peptides: (a) the magnitude of this fraction corresponded to the calculated proportion of protein radioactivity in nascent chains at the time of the puromycin effect; (b) the size distribution of the proteins disappearing between 2 and 14 min was smaller than that of those retained at 14 min; and (c) when the injection of puromycin was delayed for 5 min, or when the leucine pulse was interrupted by the injection of cycloheximide (rather than puromycin), the fraction disappearing within 14 min was much smaller. 4. The degradation of puromycinyl-peptides was much slower in the rapidly growing livers of animals recovering from a protein depletion than in the protein-depleted controls. It is concluded that the large decrease in the overall rates of total liver protein degradation previously described during liver growth is a general phenomenon, also affecting the rate of scavenging of abnormal proteins.

Ornithine decarboxylase inactivation in HeLa cells

Ornithine decarboxylase (ODC) can be induced up to 100-fold over basal levels four hours after addition of glutamine to the medium of HeLa cells growing in suspension culture. As demonstrated in several other cell types, ODC is inactivated very rapidly in HeLa cells, and the rate of inactivation is seen to vary with a half life of 9-15 minutes in uninduced cells and rises to ca. 60 minutes at the peak of induction. Quantitatively, the change in rate of inactivation cannot completely account for the observed rise in activity, thus synthesis or activation of ODC must also be involved in the induction process. The inactivation process requires metabolic energy and it can be sustained by glycolytic derived energy. Other factors which are known to inhibit protein breakdown in mammalian cells, such as sodium fluoride, insulin, or tosyl phenylalanyl chloromethyl ketone, had no effect on the rate of inactivation of ODC. Attempts to demonstrate ODC inactivation in a cell free system at neutral pH were unsuccessful.