Degradation of proteins microinjected into cultured mammalian cells

A Practical Review of Proteasome Pharmacology

The ubiquitin proteasome system (UPS) degrades individual proteins in a highly regulated fashion and is responsible for the degradation of misfolded, damaged, or unneeded cellular proteins. During the past 20 years, investigators have established a critical role for the UPS in essentially every cellular process, including cell cycle progression, transcriptional regulation, genome integrity, apoptosis, immune responses, and neuronal plasticity. At the center of the UPS is the proteasome, a large and complex molecular machine containing a multicatalytic protease complex. When the efficiency of this proteostasis system is perturbed, misfolded and damaged protein aggregates can accumulate to toxic levels and cause neuronal dysfunction, which may underlie many neurodegenerative diseases. In addition, many cancers rely on robust proteasome activity for degrading tumor suppressors and cell cycle checkpoint inhibitors necessary for rapid cell division. Thus, proteasome inhibitors have proven clinically useful to treat some types of cancer, especially multiple myeloma. Numerous cellular processes rely on finely tuned proteasome function, making it a crucial target for future therapeutic intervention in many diseases, including neurodegenerative diseases, cystic fibrosis, atherosclerosis, autoimmune diseases, diabetes, and cancer. In this review, we discuss the structure and function of the proteasome, the mechanisms of action of different proteasome inhibitors, various techniques to evaluate proteasome function in vitro and in vivo, proteasome inhibitors in preclinical and clinical development, and the feasibility for pharmacological activation of the proteasome to potentially treat neurodegenerative disease.

Liposomes for gene transfer and expression in vivo

A recombinant plasmid encoding rat preproinsulin I was encapsulated in large liposomes and injected intravenously into rats. Glycaemia and blood, splenic and hepatic insulin were assayed from 6 h after inoculation. Control animals received (i) empty liposomes, (ii) liposomes carrying the Escherichia coli pBR 322 plasmid, (iii) the free rat insulin I gene, or (iv) no injection. All controls showed unchanged glucose and insulin levels. Six hours after inoculation the treated rats had 72 +/- 5 mg glucose/100 ml of blood, compared with 107 +/- 2 mg/ml for controls. Radioimmunoassay of blood insulin gave 61 +/- 8 microunits/ml (43 +/- 5 microunits/ml for controls). Spleen and liver values were 242 +/- 22 and 204 +/- 20 microunits/g of tissue, respectively (112 +/- 20 and 87 +/- 15 microunits/g in controls). The kinetics and extent of uptake of liposomes by spleen and liver were studied by external gamma-camera imaging after injection of 111In-labelled liposomes. The results paralleled insulin synthesis in the two organs. The insulin gene was localized in liver cells after injection of liposomes containing the plasmid encoding the gene. Livers were processed 4 h after inoculation for isolation of hepatocytes, Kupffer cells and endothelial cells. DNA was purified and exogenous DNA detected by Southern blotting. Kupffer cells were the primary target for gene incorporation with liposomes consisting of phospholipids and cholesterol. Targeting of liposomes to other liver cells was attempted by including lactosylceramide in the liposomes. This increased the amount of the exogenous gene in hepatocytes and particularly in endothelial cells. The efficiency of liposome-mediated gene transfer in vivo is high, since a few per cent of the transferred DNA is taken up by the liver cells and detected.

What determines the degradation rate of an injected protein?

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

Can Myoglobin Expression in Pancreatic Beta Cells Improve Insulin Secretion Under Hypoxia? An Exploratory Study With Transgenic Porcine Islets

The feasibility of myoglobin (Mb)-facilitated oxygen transport in improving porcine islet survival under hypoxia was investigated. Discrete groups of islets were transfected with replication-defective adenoviral vector Ad5 respiratory syncitial virus (RSV) to induce expression of Mb or green fluorescent protein (GFP). Native islets served as the controls. In vitro studies at 37 degrees C assessed islet insulin secretion efficacy: (i) to a glucose challenge from 30 to 300 mg/dL at fixed pO2; and (ii) at variable oxygen tensions ranging from 5 to 40 mm Hg over 12 h. The transfection was effective in initiating islet expression of Mb or GFP. Low Mb-expression levels equivalent to 2% the Mb concentration in a muscle cell (0.25 ng of Mb per islet) were documented, with no statistical improvement in insulin secretion. A surprising side note is that insulin secretion was impaired in islets expressing GFP. Improved Mb expression is essential to determine the feasibility of enhancing islet survival under hypoxia.

Age-related changes in barrier function in mouse brain I. Accelerated age-related increase of brain transfer of serum albumin in accelerated senescence prone SAM-P/8 mice with deficits in learning and memory

The time course of brain accumulation of radiolabelled human serum albumin ((125)I-HSA) injected intravenously and the transfer of (125)I-HSA from blood to brain were evaluated in DDD mice using a double isotope technique. The brain accumulation of (125)I-HSA at 3 and 9 h but not at 24 h postinjection and the brain transfer rates were significantly higher in 22-month-old DDD mice than in 4-month-old ones. The brain transfer rates of (125)I-HSA were measured also in senescence accelerated prone mice (SAM-P/8) with age-related deficits in learning and memory, and in senescence accelerated resistant mice (SAM-R/I) without these deficits. The brain transfer rates were significantly higher in 13-month-old SAM-P/8 and 22-month-old SAM-R/1 than in 3-month-old mice of the same strains, respectively. The mean brain transfer rates in five regions observed in 22-month-old DDD mice, 22-month-old SAM-R/1 and 13-month-old SAM-P/8 increased by 31%, 41% and 51% compared with corresponding values in 3- or 4-month-old mice of the same strains. DDD mice and SAM-R/1 mice with normal characteristics of aging showed similar age-related significant changes in brain transfer rates. Age-related increase in the brain transfer rate was manifested at the youngest age in SAM-P/8 among the three strains examined. These findings show that the transfer of human serum albumin into the mouse brain increases with aging and suggest that the barrier function in the mouse brain against macromolecules changes with aging.

Alteration in the organ distribution of iron by truncated transferrin: implications for iron chelation therapy

The ability of the partial molecule of transferrin, truncated transferrin (t-Tf), to act as an excretable biologic iron chelator was examined. We confirmed the observations of Zak and Aisen (Zak O, Aisen P. Biochem Biophys Acta 1985;1952:24-8) that thermolysin treatment of human transferrin produces half molecules that retain iron-binding capacity. These molecules are poorly recognized by surface receptors on either human or murine cells. Although the plasma half-life of human transferrin in mice is moderately long (40 hours), injection of t-Tf into mice results in its rapid clearance (half-life = 10 minutes). Injection of iron 59-labeled transferrin results in the deposition of iron in the major hematopoetic organs of mice such as the spleen, bone marrow, and liver. Injection of 59Fe-labeled t-Tf results in the quantitative recovery of iron in the kidneys: 59Fe is retained in the kidney for substantial periods of time with little evidence of its excretion into urine. Injection of iodine 125-labeled t-Tf also results in the deposition of radioactivity in the kidneys, but 125I is rapidly excreted into the urine, where it is detected as free iodine. These results indicate that although t-Tf is directed to the kidney and filtered by the glomerulus, the molecule is reabsorbed and degraded, and iron is retained. These results have implications in the design of iron chelators.

Efficiency of cytoplasmic delivery by pH-sensitive liposomes to cells in culture

The intracellular processing of pH-sensitive liposomes composed of cholesterylhemisuccinate (CHEMS) and dioleoylphosphatidylethanolamine (DOPE) by eukaryotic cell lines has been compared to non-pH-sensitive liposomes made of CHEMS and dioleoylphosphatidylcholine (DOPC). The pH-sensitive liposomes can deliver encapsulated fluorescent molecules [calcein, fluoresceinated dextran, fluoresceinated polypeptide, and diphtheria toxin A chain (DTA)] into the cytoplasm. Cytoplasmic delivery can be blocked in the presence of ammonium chloride or EDTA, indicating that the process requires a low-pH environment and the presence of divalent cations. Inhibition of cellular protein synthesis by DTA delivery from the pH-sensitive liposome is orders of magnitude greater than from the non-pH-sensitive liposome composition. The delivery of DTA into the cytoplasm by pH-sensitive liposomes is at least 0.01% of cell-associated liposomal DTA. There is no significant difference in the degradation rate of bovine serum albumin (BSA) or the rate of acidification of pH-sensitive dye, 8-hydroxy-1,3,6-pyrene-trisulfonate (HPTS), when delivered to cells in pH-sensitive and non-pH-sensitive liposomes. Thus the efficiency of cytoplasmic delivery is less than 10% of the cell-associated liposome contents, which is the smallest difference that can be detected by these two assays. Based upon the various assays used to measure liposome content disposition in the cell, we conclude that the efficiency of cytoplasmic delivery by the CHEMS/DOPE liposomes is greater than 0.01% and less than 10% of the cell-associated liposomal contents.

Will heat-stable proteins nonspecifically protect cells from thermal stress?

In cell-free systems, stress-resistant proteins nonspecifically stabilize stress-susceptible proteins. This mechanism has been suggested to contribute to thermotolerance in cells (Minton et al.: Proc. Natl. Acad. Sci. USA, 79: 7107-7117, 1982). To test this hypothesis, red-blood-cell-mediated microinjection was used to transfer macromolecules into monolayers of CHO cells. We introduced the heat-stable proteins fetuin and ovomucoid into RBCs during hypotonic hemolysis and then fused the RBCs to CHO cells with polyethylene glycol as fusogen. Fetuin and ovomucoid were successfully transferred into 36-55% of the CHO cells as demonstrated by fluorescence of FITC-conjugated proteins. The plating efficiency of these CHO cells after fusion ranged from 35% to 60%. Three hours after fusion, CHO cells microinjected with fetuin or ovomucoid were exposed to 43 degrees C for 0-180 min or 45 degrees C for 0-40 min, and thermal survival was determined. There was no difference in cell survival between control untreated cells, control cells fused with nonloaded RBCs, and cells fused with RBCs loaded with fetuin or ovomucoid. While our results do not support the hypothesis that heat-stable proteins nonspecifically protect cells from thermal stress, several possible explanations are provided for this observation.

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

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

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

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

Degradation of sperm histones in vitro by cytoplasm of the sea urchin egg

Sperm-specific histone variants in the sea urchin Paracentrotus lividus are replaced early after fertilization with a specific embryonic set of histone variants. A possible in vitro model for the involvement of a degradation mechanism in the replacement of sperm-specific histones is presented. Soluble sperm histones are shown to be degraded quickly by egg cytoplasm. The proteolytic activity is maximal at pH 3.0; H1 and H2A histones are the most sensitive while H3 and H4 are the most resistant. H2B histones have an intermediate sensitivity. Histone degradation by egg cytoplasm or by purified fractions of it can be inhibited by chymostatin and leupeptin and, to a lesser degree, by pepstatin.

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

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

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

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

Covalent linkage of ribonuclease S-peptide to microinjected proteins causes their intracellular degradation to be enhanced during serum withdrawal

The amino-terminal 20 amino acids are required for microinjected ribonuclease A (RNase A) to be taken up by lysosomes and degraded at an enhanced rate during serum withdrawal. We used water-soluble carbodiimides to covalently attach the RNase S-peptide (residues 1-20) to [3H]RNase S-protein (residues 21-124) at unspecified locations. We then measured catabolism of the [3H]S-protein-S-peptide conjugate after its microinjection into human diploid fibroblasts. The attached S-peptide caused the degradation of S-protein to be enhanced 2-fold in the absence of serum. Control experiments showed that degradation of [3H]RNase S-protein remained unresponsive to serum after conjugation with the inactive fragment, RNase S-peptide (residues 1-10). Covalent attachment of RNase S-peptide had a similar effect on the catabolism of two other proteins. Degradation rates of microinjected 125I-labeled lysozyme and 125I-labeled insulin A chain are normally unresponsive to serum withdrawal. However, breakdown rates of microinjected 125I-labeled lysozyme-S-peptide and 125I-labeled insulin A chain-S-peptide conjugates were increased 2-fold during serum deprivation. We suggest that RNase S-peptide acts as a "single sequence" that directs cytosolic proteins to lysosomes through a pathway that is activated by deprivation conditions.

Autophagic-lysosomal and mitochondrial sequestration of [14C]sucrose. Density gradient distribution of sequestered radioactivity

[14C]Sucrose, introduced into the cytosol of isolated rat hepatocytes by means of electropermeabilization, was sequestered by sedimentable subcellular particles during incubation of the cells at 37 degrees C. The sedimentation characteristics of particle-associated [14C]sucrose were different from the lysosomal marker enzyme acid phosphatase, suggesting an involvement of organelles of greater size than the average lysosome. Isopycnic banding in isotonic metrizamide/sucrose density gradients resolved two major peaks of radioactivity: a light peak (1.08-1.10 g/ml) coinciding with lysosomal marker enzymes, and a dense peak (1.15 g/ml), coinciding with a mitochondrial marker enzyme. The dense peak was preferentially associated with large-size particles having the sedimentation properties of mitochondria, and it was resistant to the detergent digitonin at a concentration which extracted all of the radioactivity in the light peak. Similarly the autophagy inhibitor 3-methyladenine prevented accumulation of [14C]sucrose in the light peak, while the radioactivity in the dense peak was unaffected. We therefore tentatively conclude that the light peak represents autophagic sequestration of [14C]sucrose into lysosomes (and probably autophagosomes) while the dense peak represents a mitochondrial uptake unrelated to autophagy.

Red blood cell-mediated microinjection: methodological considerations

Red blood cell-mediated microinjection is a powerful approach to introducing proteins into the cytoplasm of cultured cells. In the course of our microinjection studies of intracellular protein degradation, we have encountered several potential problems with certain proteins. The microinjection procedure may be accompanied by denaturation of protein by radiolabeling procedures, binding of protein to red cell ghosts during loading, degradation of protein by the red cell ghost prior to microinjection, and adsorption of protein that leaks from red cell ghosts in the presence of fusogen to the fibroblast monolayer. We conclude with a list of points that must be considered prior to use of red cell-mediated microinjection to study a particular protein.

Degradation of erythrocyte-microinjected and scrape-loaded homologous cytosolic proteins by 3T3-L1 cells

Homologous cytosol was introduced into 3T3-L1 cells by two different methods. Erythrocytes loaded with radiolabelled cytosolic proteins extracted from 3T3-L1 cells were fused with the aid of Sendai virus to 3T3-L1 cells, which were then seeded to confluent and non-confluent cultures. Cytosolic proteins were also introduced into cells by the technique of scrape-loading. In confluent cells, injected cytosolic proteins were recovered largely (54-93%) in a sedimentable (6 X 10(6) gav.-min) fraction from recipient cells irrespective of the method of introduction or of radiolabelling of the injected proteins [( 125I]iodination, reductive methylation with NaB3H4 and backbone labelling with L-[4,5-3H]leucine). The degradation of microinjected cytosolic proteins was in all cases inhibited by the lysosomotropic agent NH4Cl to a greater extent (32-75%) than that observed for endogenous cytosolic (less than or equal to 19%) proteins (labelled with L-[4,5-3H]leucine). In growing cells both endogenous total cell proteins and microinjected proteins were degraded at a slower rate than in confluent cell monolayers. The inhibition by NH4Cl of the degradation of both the endogenous and microinjected proteins is decreased compared with the inhibition observed in confluent monolayers. The results are discussed in terms of the cytoplasmic capacity to segregate microinjected homologous proteins before protein degradation can take place.

Microinjection of somatic cells with micropipettes: comparison with other transfer techniques

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Analysis of BHK cell growth kinetics after microinjection of catalytic subunit of cyclic AMP-dependent protein kinase

The effect of catalytic subunit (C) of cyclic AMP-dependent protein kinase on cell growth kinetics of BHK cells was assessed by microinjection with chicken erythrocyte ghosts as vehicles for introduction of the protein into the cytosol of large populations of cells. The advantage in using chicken erythrocytes for microinjection is that the inactive erythrocyte nuclei serve as a probe for identifying and analyzing microinjection events. By utilizing this procedure, BHK cells were microinjected with an amount of C that was 5- to 10-fold greater than their endogenous levels. Growth kinetics were analyzed by [3H]thymidine incorporation and autoradiography. Cells were stained after autoradiography to more clearly reveal the chicken nuclei, and at each time point, cells were categorized into four groups: (i) not microinjected, not in S phase, (ii) not microinjected, in S phase, (iii) microinjected, not in S phase, (iv) microinjected, in S phase. Those cells not microinjected served as internal controls. Two experimental protocols were used to test the notion that C is involved in blocking cell progression through G1 phase of the cell cycle. First, cells were arrested in G0 phase by serum deprivation, microinjected with C or control proteins, and stimulated to proceed to S phase by the addition of serum or purified growth factors. Second, cells were collected in mitosis, microinjected with C or control proteins, and stimulated to proceed to S phase by the addition of serum. The results of these studies indicate that a 5- to 10-fold increase in the intracellular concentration of C is not a sufficient signal to arrest cell growth in G1 phase. Thus, growth-inhibitory effects of cyclic AMP on BHK cells are unlikely to be the result of activation of cyclic AMP-dependent protein kinase.

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.

Analysis of BHK cell growth kinetics after microinjection of catalytic subunit of cyclic AMP-dependent protein kinase

The effect of catalytic subunit (C) of cyclic AMP-dependent protein kinase on cell growth kinetics of BHK cells was assessed by microinjection with chicken erythrocyte ghosts as vehicles for introduction of the protein into the cytosol of large populations of cells. The advantage in using chicken erythrocytes for microinjection is that the inactive erythrocyte nuclei serve as a probe for identifying and analyzing microinjection events. By utilizing this procedure, BHK cells were microinjected with an amount of C that was 5- to 10-fold greater than their endogenous levels. Growth kinetics were analyzed by [3H]thymidine incorporation and autoradiography. Cells were stained after autoradiography to more clearly reveal the chicken nuclei, and at each time point, cells were categorized into four groups: (i) not microinjected, not in S phase, (ii) not microinjected, in S phase, (iii) microinjected, not in S phase, (iv) microinjected, in S phase. Those cells not microinjected served as internal controls. Two experimental protocols were used to test the notion that C is involved in blocking cell progression through G1 phase of the cell cycle. First, cells were arrested in G0 phase by serum deprivation, microinjected with C or control proteins, and stimulated to proceed to S phase by the addition of serum or purified growth factors. Second, cells were collected in mitosis, microinjected with C or control proteins, and stimulated to proceed to S phase by the addition of serum. The results of these studies indicate that a 5- to 10-fold increase in the intracellular concentration of C is not a sufficient signal to arrest cell growth in G1 phase. Thus, growth-inhibitory effects of cyclic AMP on BHK cells are unlikely to be the result of activation of cyclic AMP-dependent protein kinase.

Microinjection of cultured cells using red-cell-mediated fusion and osmotic lysis of pinosomes: a review of methods and applications

Proteins and other macromolecules can be injected into cultured cells by several different methods. Here we review the strengths and limitations of two of these methods, red-cell-mediated microinjection and osmotic lysis of pinosomes, and indicate how they may be successfully applied to the study of cultured cells.

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.

Role of the p53 protein in cell proliferation as studied by microinjection of monoclonal antibodies

Two monoclonal antibodies against the p53 protein, PAb 122 and 200-47, were microinjected into mammalian cells as a probe to determine the role of the p53 protein in cell proliferation. PAb 122 recognizes the p53 proteins of mouse and human cells but not of hamster cells, whereas 200-47 recognizes the p53 proteins of mouse and hamster cells but not of human cells. The ability of these antibodies to inhibit serum-stimulated DNA synthesis of cells in culture correlates with their ability to recognize the species-specific antigenic determinants. More important, however, is the observation that microinjected PAb 122 inhibits the transition of Swiss 3T3 cells from G0 to S phase, but has no effect on the progression of these cells from mitosis to the S phase.

Role of the p53 protein in cell proliferation as studied by microinjection of monoclonal antibodies

Two monoclonal antibodies against the p53 protein, PAb 122 and 200-47, were microinjected into mammalian cells as a probe to determine the role of the p53 protein in cell proliferation. PAb 122 recognizes the p53 proteins of mouse and human cells but not of hamster cells, whereas 200-47 recognizes the p53 proteins of mouse and hamster cells but not of human cells. The ability of these antibodies to inhibit serum-stimulated DNA synthesis of cells in culture correlates with their ability to recognize the species-specific antigenic determinants. More important, however, is the observation that microinjected PAb 122 inhibits the transition of Swiss 3T3 cells from G0 to S phase, but has no effect on the progression of these cells from mitosis to the S phase.

Cellular DNA replication is independent of the synthesis or accumulation of ribosomal RNA

We have used an antibody against RNA polymerase I to investigate the role of rRNA synthesis and/or accumulation in the control of cell proliferation. The antibody was microinjected directly into the nuclei of quiescent Swiss 3T3 cells that were subsequently stimulated with serum. Under the experimental conditions used, the microinjection of the antibody against RNA polymerase I (RNA pol I) caused a 50-70% decrease in nucleolar RNA synthesis that lasted for at least 17 h, a greater than 90% inhibition in the accumulation of nucleolar RNA, and a 70% inhibition in the accumulation of total cellular RNA. A control IgG, similarly microinjected into Swiss 3T3 cells had no inhibitory effect on either the synthesis or accumulation of nucleolar and cellular RNA. Despite the dramatic effect on the synthesis and accumulation of ribosomal RNA (rRNA) the antibody against RNA (rRNA) the antibody against RNA pol I was totally ineffective in inhibiting the entry into S phase of serum-stimulated Swiss 3T3 cells. Cells depleted of cellular RNA by metaphase arrest also entered S phase with subnormal amounts of cellular RNA. The results of these experiments clearly indicate that a normal rate of nucleolar RNA synthesis, and a normal rate of accumulation of total cellular RNA are not a prerequisite for the entry of cells into S phase.

Erythrocyte-mediated microinjection, a technique to study protein degradation in muscle cells

The technique of erythrocyte-mediated microinjection has been successfully adapted for use with cultured muscle cells. Erythrocytes were fused with primary chick myotube cultures with poly(ethylene glycol), and fluorescent antibodies to haemoglobin demonstrated that this protein was injected into the sarcoplasm of myotubes. The microinjection treatment did not significantly alter protein metabolism in the muscle cells as monitored by rates of synthesis and degradation of muscle proteins. 125I-labelled ribonuclease A and bovine serum albumin were degraded with the expected exponential decay kinetics after microinjection into muscle cells, and the half-life of ribonuclease A (40 h) was approximately twice that of bovine serum albumin (17 h). The degradation of ribonuclease A in the muscle cells was enhanced 1.6-fold in the absence of horse serum and chick-embryo extract, whereas the degradation of bovine serum albumin was not altered during deprivation. These results are characteristic of the breakdown of microinjected ribonuclease A and bovine serum albumin in other cell types. Therefore, our experiments indicate the erythrocyte-mediated microinjection is a valid technique to study protein degradation in primary chick muscle cultures.

Degradation of transplanted mitochondrial proteins by hepatocyte monolayers

Reductively [3H]methylated rat mitochondria and mitochondrial-outer-membrane vesicles and mitochondrial-outer-membrane vesicles where monoamine oxidase is irreversibly labelled by [3H]pargyline have been transplanted into hepatocytes by poly(ethylene glycol)-mediated organelle or organelle-vesicle cell fusion. During subsequent culture of hepatocyte monolayers for 4-5 days, under conditions which mimic endogenous catabolic rates in vivo the transplanted organelle proteins retain their degradation characteristics observed in vivo (e.g. mitochondria: average t 1/2 72.5 h; monoamine oxidase: t1/2 55 h). In all cases protein degradation with first-order kinetics is only observed after an initial lag period (i.e. 24-30 h after fusion). Transplantation of fluorescein-conjugated organelles showed that the fluorescent material is rapidly internalized (average t1/2 1-6 h) and uniformly distributed in the cytoplasm. During a subsequent 18-24 h period (which corresponds to the lag period for intracellular destruction of transplanted mitochondrial material) the transplanted material is translocated to assume a perinuclear distribution. The destruction of transplanted mitochondrial proteins is compared with endogenous mitoribosomally synthesized proteins (average t1/2 52.5 h). Percoll fractionation of cell homogenates containing transplanted mitochondrial outer membranes where the enzyme monoamine oxidase is irreversibly labelled with [3H]pargyline shows a distribution of enzyme similar to lysosomal acid phosphatase. After transplantation of reductively methylated 3H-labelled mitochondrial-outer-membrane vesicles the cells were treated with leupeptin to alter lysosomal density. This treatment leads to the predominant association of acid phosphatase with dense structures, whereas the 3H-labelled transplanted material predominantly does not change density. Therefore transplanted mitochondrial-outer-membrane proteins are found in intracellular vesicular structures from which the proteins are donated for destruction, at least in part, by a lysosomal mechanism.

Degradation of transplanted rat liver mitochondrial-outer-membrane proteins in hepatoma cells

Reductively [3H]methylated 3H mitochondrial-outer-membrane vesicles from rat liver and vesicles where monoamine oxidase has been derivatized irreversibly by [3H]-pargyline have been deliberately miscompartmentalized by heterologous transplantation into hepatoma (HTC) cells by poly(ethylene glycol)-mediated vesicle-cell fusion. Fluorescein-conjugated mitochondrial-outer-membrane vesicles have also been used to show that transplanted material is patched, capped and internalized. Reductively methylated outer-membrane proteins and monoamine oxidase are destroyed at the same rate (t1/2 24 h). Mitochondrial-outer-membrane proteins are not degraded at the same rate as HTC plasma-membrane proteins, endogenous cell protein, or endocytosed protein. Transplanted radiolabelled mitochondrial-outer-membrane proteins accumulate intracellularly in structures that are distinct from plasma membrane and lysosomes. However, when mitochondrial-outer-membrane vesicles derivatized with [14C]sucrose are transplanted, the acid-soluble degradation products accumulate in the lysosomal fraction. [14C]Sucrose-conjugated HTC cell plasma membrane accumulates in intracellular structures that are again distinct from plasma membrane and lysosomes. In contrast with the above observations, homologously transplanted mitochondrial-outer-membrane proteins from rat liver are destroyed in hepatocytes at rates that are remarkably similar (t1/2 60-70 h) to the rates in rat liver in vivo [Evans & Mayer (1982) Biochem. Biophys. Res. Commun. 107, 51-58].

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

HeLa cells, injected with radioiodinated proteins by fusion with RBC ghosts, were exposed to inhibitors of lysosomal proteolysis and autophagy. The degradation of injected [125I]bovine serum albumin (BSA) was unaffected by chloroquine, NH4Cl, nocodazole, colcemid, puromycin, cycloheximide, or enucleation. Although degradation of [125I]lactate dehydrogenase (LDH) and [125I]pyruvate kinase (PK) was inhibited one-third by chloroquine or ammonia, their degradation was unaffected by the other compounds. In contrast, enhanced degradation of 125I-PK resulting from depriving injected HeLa cells of amino acids and serum was inhibited 70% by colcemid and abolished by chloroquine or ammonia. Similarly, degradation of [14C]sucrose-labeled BSA-polylysine conjugates that entered HeLa cells by endocytosis was inhibited as much as 80% by chloroquine and ammonia. Sensitivity of both enhanced proteolysis and degradation of exogenous proteins to ammonia or chloroquine indicates they are effective inhibitors of lysosomal proteolysis in HeLa cells. Failure of ammonia or chloroquine to inhibit degradation of injected 125I-BSA and the modest inhibition of degradation of injected 125I-LDH or 125I-PK indicates that virtually all BSA molecules and most PK or LDH molecules are degraded by a nonlysosomal proteolytic system. Components of this degradative system are present in vast excess or are long lived, since inhibition of protein synthesis for 20 hr had no effect on the degradation of injected proteins.

Insulin-cholera toxin binding unit conjugate: a hybrid molecule with insulin biological activity and cholera toxin binding specificity

The polypeptide hormone insulin and the binding unit of cholera toxin (CTB) were coupled via a disulfide bond. This hybrid molecule had 1/30 the ability of native insulin to bind to the insulin receptor and 1/30 the biological activity of native insulin in H35 rat hepatoma cells and rat adipocytes. Thus, in these two cell types that are very sensitive to insulin, the biological activity of the hybrid molecule was as predicted on the basis of the ability of the molecule to interact with the insulin receptor. In contrast, in HTC rat hepatoma cells and rat thymocytes, two poorly responsive cell types, the insulin-CTB conjugate had 1/3 the biological activity of native insulin, a value 10 times greater than its insulin receptor binding potency. This increased activity of the conjugate did not appear to be due to cholera toxin in the preparation, since a control of uncoupled CTB had no biological activity. Furthermore, native cholera toxin increased intracellular levels of cAMP by 20-fold, whereas the conjugate had no effect on cAMP levels. The CTB moiety did, however, contribute to the biological activity of the conjugate, since the activity of the hybrid molecule, like cholera toxin, was inhibited by gangliosides, whereas the activity of native insulin was not. Finally, the binding to thymocytes of insulin-CTB conjugate, but not insulin, was inhibited by gangliosides. Thus, a hybrid hormone molecule has been constructed which has insulin-like biological activity with the receptor specificity of cholera toxin in poorly responsive cells.

Intracellular distribution and degradation of immunoglobulin G and immunoglobulin G fragments injected into HeLa cells

Intact rabbit immunoglobulin G molecules (IgGs) and their papain or pepsin fragments were radio-iodinated and injected into HeLa cells. Whole IgGs, Fab2, and Fc fragments were degraded with half-lives of 60-90 h, whereas half-lives of Fab fragments were 110 h. These results indicate that proteolytic cleavage in the hinge region of the IgG molecule is not the rate-limiting step in its intracellular degradation. The hingeless human myeloma protein, Mcg, was degraded at the same rate as bulk human IgG, providing further evidence that the proteolytically susceptible hinge region is not important for intracellular degradation of IgG molecules. SDS acrylamide gel analysis of injected rabbit IgG molecules revealed that heavy and light chains were degraded at the same rate. Injected rabbit IgGs and rabbit IgG fragments were also examined on isoelectric focusing gels. Fab, Fab2, and Fc fragments were degraded without any correlation with respect to isoelectric point. Positively charged rabbit IgGs disappeared more rapidly than their negative counterparts, contrary to the trend reported for normal intracellular proteins. The isoelectric points of two mouse monoclonal antibodies were essentially unchanged after injection into HeLa cells, suggesting that the altered isoelectric profile observed for intact rabbit IgG resulted from degradation and not protein modification. The intracellular distributions of IgG fragments and intact rabbit IgG molecules were determined by autoradiography of thin sections through injected cells. Intact IgG molecules were excluded from HeLa nuclei whereas both Fab and Fc fragments readily entered them. Thus, for some proteins, entry into the nuclear compartment is determined primarily by size.

Conjugation of ubiquitin to denatured hemoglobin is proportional to the rate of hemoglobin degradation in HeLa cells

Ubiquitin was radioiodinated and introduced into HeLa cells by the erythrocyte-mediated fusion procedure. Fractionation of injected HeLa cells and subsequent NaDodSO4/polyacrylamide gel electrophoresis showed that HeLa nuclei contained two major labeled proteins: ubiquitin and the histone H2A-ubiquitin conjugate, protein A24. HeLa cytosol contained ubiquitin and a series of ubiquitin-protein conjugates of diverse molecular weights. When injected HeLa cells were treated with phenylhydrazine to denature the cotransferred hemoglobin, a series of prominent ubiquitin-globin conjugates appeared. The identity of these conjugates was established by microinjection experiments in which both proteins were labeled. At low doses of phenylhydrazine, the intracellular concentration of globin-ubiquitin conjugates was proportional to the rate of hemoglobin degradation. This result, together with the observation that ubiquitin conjugation to globin is markedly enhanced by phenylhydrazine-induced denaturation of hemoglobin, provides support for the hypothesis that the covalent attachment of ubiquitin to proteins signals proteolysis.

Microinjection of monoclonal antibody to protein p53 inhibits serum-induced DNA synthesis in 3T3 cells

Monoclonal antibody directed against the transformation-related protein p53 was microinjected manually into the nuclei of quiescent Swiss 3T3 mouse cells. The cells were subsequently stimulated with 10% fetal calf serum. Microinjection of p53 antibody at or around the time of serum stimulation clearly inhibited the subsequent entry of Swiss 3T3 cells into the S phase of the cell cycle. p53 antibody had no effect on serum-stimulated DNA synthesis when it was microinjected 4 hr or later after serum stimulation. Monoclonal antibody to an unrelated antigen, Lyt-2.2, had no effect on serum-stimulated DNA synthesis regardless of the time it was microinjected. Under similar experimental conditions, p53 antibody had no effect on simian virus 40- or adenovirus 2-induced DNA synthesis. These experiments add strength to the suggestion that p53 is involved in the regulation of cell proliferation.

Intracellular stability of diphtheria toxin fragment A in the presence and absence of anti-fragment A antibody

The erythrocyte ghost function method was used to introduce 125I-labeled diphtheria toxin, its fragments A and B, and two labeled crossreacting material (CRM), mutant proteins CRM176 and CRM197, into cultured mouse cells. Fragment A was relatively stable in mouse cytoplasm at 37 degrees C and at least 80% was recovered from cells after 24 hr of incubation. In contrast, wild-type fragment B and A fragments from CRM176 and CRM197 were unstable and were degraded with half-lives of about 2.5 hr under similar conditions. When a rabbit anti-fragment A IgG fraction was introduced with wild-type A, the rate of degradation of A was accelerated, whereas the rates of degradation of A176 and A197 were retarded by the same antibody. In every instance the degradation rate appeared to be that of the IgG fraction itself with a half-life of about 7.5 hr.

Comparative studies on microinjected high-mobility-group chromosomal proteins, HMG1 and HMG2

The nonhistone chromosomal proteins, HMG1 and HMG2, were iodinated and introduced into HeLa cells, bovine fibroblasts, or mouse 3T3 cells by erythrocyte-mediated microinjection. Autoradiographic analysis of injected cells fixed with glutaraldehyde consistently showed both molecules concentrated within nuclei. Fixation with methanol, on the other hand, resulted in some leakage of the microinjected proteins from the nuclei so that more autoradiographic grains appeared over the cytoplasm or outside the cells. Both injected and endogenous HMG1 and HMG2 partitioned unexpectedly upon fractionation of bovine fibroblasts, HeLa, or 3T3 cells, appearing in the cytoplasmic fractions. However, in calf thymus, HMG1 and HMG2 molecules appeared in the 0.35 M NaCl extract of isolated nuclei, as expected. These observations show that the binding of HMG1 and HMG2 to chromatin differs among cell types or that other tissue-specific components can influence their binding. Coinjection of [125I]HMG1 and [131I]HMG2 into HeLa cells revealed that the two molecules display virtually equivalent distributions upon cell fractionation, identical stability, identical intracellular distributions, and equal rates of equilibration between nuclei. In addition, HMG1 and HMG2 did not differ in their partitioning upon fractionation nor in their stability in growing vs. nongrowing 3T3 cells. Thus, we have not detected any significant differences in the intracellular behavior of HMG1 and HMG2 after microinjection into human, bovine, or murine cells.