The effect of protein degradation on cellular growth characteristics

Polycaprolactone Electrospun Scaffolds Produce an Enrichment of Lung Cancer Stem Cells in Sensitive and Resistant EGFRm Lung Adenocarcinoma

Simple Summary

The culture of lung cancer stem cells (LCSCs) is not possible using traditional flat polystyrene surfaces. The study of these tumor-initiating cells is fundamental due to their key role in the resistance to anticancer therapies, tumor recurrence, and metastasis. Hence, we evaluated the use of polycaprolactone electrospun (PCL-ES) scaffolds for culturing LCSC population in sensitive and resistant EGFR-mutated lung adenocarcinoma models. Our findings revealed that both cell models seeded on PCL-ES structures showed a higher drug resistance, enhanced levels of several genes and proteins related to epithelial-to-mesenchymal process, stemness, and surface markers, and the activation of the Hedgehog pathway. We also determined that the non-expression of CD133 was associated with a low degree of histological differentiation, disease progression, distant metastasis, and worse overall survival in EGFR-mutated non-small cell lung cancer patients. Therefore, we confirmed PCL-ES scaffolds as a suitable three-dimensional cell culture model for the study of LCSC niche.

Abstract

The establishment of a three-dimensional (3D) cell culture model for lung cancer stem cells (LCSCs) is needed because the study of these stem cells is unable to be done using flat surfaces. The study of LCSCs is fundamental due to their key role in drug resistance, tumor recurrence, and metastasis. Hence, the purpose of this work is the evaluation of polycaprolactone electrospun (PCL-ES) scaffolds for culturing LCSCs in sensitive and resistant EGFR-mutated (EGFRm) lung adenocarcinoma cell models. We performed a thermal, physical, and biological characterization of 10% and 15%-PCL-ES structures. Several genes and proteins associated with LCSC features were analyzed by RT-qPCR and Western blot. Vimentin and CD133 tumor expression were evaluated in samples from 36 patients with EGFRm non-small cell lung cancer through immunohistochemistry. Our findings revealed that PC9 and PC9-GR3 models cultured on PCL-ES scaffolds showed higher resistance to osimertinib, upregulation of ABCB1, Vimentin, Snail, Twist, Sox2, Oct-4, and CD166, downregulation of E-cadherin and CD133, and the activation of Hedgehog pathway. Additionally, we determined that the non-expression of CD133 was significantly associated with a low degree of histological differentiation, disease progression, and distant metastasis. To sum up, we confirmed PCL-ES scaffolds as a suitable 3D cell culture model for the study of the LCSC niche.

Generalized Cellular Hypertrophy is Induced by a Dual-Acting PPAR Agonist in Rat Urinary Bladder Urothelium In Vivo

Some developmental dual-acting PPARα/γ agonists, such as ragaglitazar, have shown carcinogenic effects in the rodent urinary bladder urothelium after months-years of dosing. We examined early (precancerous) changes in the bladder urothelium of rats orally dosed with ragaglitazar, using a newly developed flow cytometric method. Following 3 weeks of oral ragaglitazar dosing, increases in physical size occurred in a generalized fashion in rat bladder urothelial cells, determined by flow cytometry. Protein/DNA measurements confirmed increased protein content of urothelial cells in the bladder, and hypertrophy was observed in the kidney pelvis urothelium by histopathology. In animals exhibiting urothelial hypertrophy, no cell cycle changes were detected in parallel samples of bladder urothelium. Interestingly, urothelial cells from normal rats were found to constitute a unique type of noncycling population, with high G2/M fractions. In summary, our findings showed that in the urothelium of ragaglitazar-treated animals, hypertrophy (increased size and protein content per cell) was an early change, that affected the whole bladder urothelial cell population. The urothelial hypertrophy was primary, i.e., occurred in the absence of similarly pronounced changes in cell cycle distributions. To our knowledge, this is the first report of a direct hypertrophic effect of a PPAR agonist. Urothelial hypertrophy might be a relevant early biological endpoint in mechanistic studies regarding the bladder-carcinogenic effect of PPAR agonists.

Therapeutic interventions to disrupt the protein synthetic machinery in melanoma

Control of the protein synthetic machinery is deregulated in many cancers, including melanoma, to increase the protein production. Tumor suppressors and oncogenes play key roles in protein synthesis from the transcription of rRNA and ribosome biogenesis to mRNA translation initiation and protein synthesis. Major signaling pathways are altered in melanoma to modulate the protein synthetic machinery, thereby promoting tumor development. However, despite the importance of this process in melanoma development, involvement of the protein synthetic machinery in this cancer type is an underdeveloped area of study. Here, we review the coupling of melanoma development to deregulation of the protein synthetic machinery. We examine existing knowledge regarding RNA polymerase I inhibition and mRNA translation focusing on their inhibition for therapeutic applications in melanoma. Furthermore, the contribution of amino acid biosynthesis and involvement of ribosomal proteins are also reviewed as future therapeutic strategies to target deregulated protein production in melanoma.

Disruption of Proline Synthesis in Melanoma Inhibits Protein Production Mediated by the GCN2 Pathway

Many processes are deregulated in melanoma cells and one of those is protein production. Although much is known about protein synthesis in cancer cells, effective ways of therapeutically targeting this process remain an understudied area of research. A process that is upregulated in melanoma compared with normal melanocytes is proline biosynthesis, which has been linked to both oncogene and tumor suppressor pathways, suggesting an important convergent point for therapeutic intervention. Therefore, an RNAi screen of a kinase library was undertaken, identifying aldehyde dehydrogenase 18 family, member A1 (ALDH18A1) as a critically important gene in regulating melanoma cell growth through proline biosynthesis. Inhibition of ALDH18A1, the gene encoding pyrroline-5-carboxylate synthase (P5CS), significantly decreased cultured melanoma cell viability and tumor growth. Knockdown of P5CS using siRNA had no effect on apoptosis, autophagy, or the cell cycle but cell-doubling time increased dramatically suggesting that there was a general slowdown in cellular metabolism. Mechanistically, targeting ALDH18A1 activated the serine/threonine protein kinase GCN2 (general control nonderepressible 2) to inhibit protein synthesis, which could be reversed with proline supplementation. Thus, targeting ALDH18A1 in melanoma can be used to disrupt proline biosynthesis to limit cell metabolism thereby increasing the cellular doubling time mediated through the GCN2 pathway.

IMPLICATIONS

This study demonstrates that melanoma cells are sensitive to disruption of proline synthesis and provides a proof-of-concept that the proline synthesis pathway can be therapeutically targeted in melanoma tumors for tumor inhibitory efficacy.

Depletion of Amyloid Precursor Protein (APP) causes G0 arrest in non-small cell lung cancer (NSCLC) cells

We recently reported that Amyloid Precursor Protein (APP) regulates global protein synthesis in a variety of human dividing cells, including non-small cell lung cancer (NSCLC) cells. More specifically, APP depletion causes an increase of both cap- and IRES-dependent translation. Since growth and proliferation are tightly coupled processes, here, we asked what effects artificial downregulation of APP could have elicited in NSCLC cells proliferation. APP depletion caused a G0/G1 arrest through destabilization of the cyclin-C protein and reduced pRb phosphorylation at residues Ser802/811. siRNA to cyclin-C mirrored the cell cycle distribution observed when silencing APP. Cells arrested in G0/G1 (and with augmented global protein synthesis) increased their size and underwent a necrotic cell death due to cell membrane permeabilization. These phenotypes were reversed by overexpression of the APP C-terminal domain, indicating a novel role for APP in regulating early cell cycle entry decisions. It is seems that APP moderates the rate of protein synthesis before the cell clears growth factors- and nutrients-dependent checkpoint in mid G1. Our results raise questions on how such processes interact in the context of (at least) dividing NSCLC cells. The data presented here suggest that APP, although required for G0/G1 transitions, moderates the rate of protein synthesis before the cell fully commits to cell cycle progression following mechanisms, which seem additional to concurrent signals deriving from the PI3-K/Akt/mTORC-1 axis. APP appears to play a central role in regulating cell cycle entry with the rate of protein synthesis; and its loss-of-function causes cell size abnormalities and death.

Growth inhibitory effects of large subunit ribosomal proteins in melanoma

Ribosome biogenesis can modulate protein synthesis, a process heavily relied upon for cancer cell proliferation. In this study, involvement of large subunit ribosomal proteins (RPLs) in melanoma has been dissected and RPLs categorized based on modulation of cell proliferation and therapeutic targeting potential. Based on these results, two categories of RPLs were identified: the first causing negligible effects on cell viability, p53 expression, and protein translation, while the second category decreased cell viability and inhibited protein synthesis mediated with or without p53 protein stabilization. RPL13 represents the second category, where siRNA-mediated targeting inhibited tumor development through decreased cellular proliferation. Mechanistically, decreased RPL13 levels increased p53 stability mediated by RPL5 and RPL11 binding to and preventing MDM2 from targeting p53 for degradation. The consequence was p53-dependent cell cycle arrest and decreased protein translation. Thus, targeting certain category 2 RPL proteins can inhibit melanoma tumor development mediated through the MDM2-p53 pathway.

Angiogenin-mediated ribosomal RNA transcription as a molecular target for treatment of head and neck squamous cell carcinoma

Squamous cell carcinoma of the head and neck (HNSCC) is the eighth most common disease, affecting approximately 640,000 patients worldwide each year. Despite recent advances in surgery, radiotherapy, and chemotherapy, the overall cure for patients with HNSCC has remained at less than 50% for many decades. Patients with recurrent and metastatic disease have a median survival of only 6-10 months. Systemic chemotherapy is the only treatment option for those patients. New treatment options are thus desperately needed to supplement, complement, or replace currently available therapies. New agents that target molecular and cellular pathways of the disease pathogenesis of HNSCC are promising candidates. One class of these new agents is angiogenesis inhibitors that have been proven effective in the treatment of advanced colorectal, breast, and non-small cell lung cancers. Similar to other solid tumors, angiogenesis plays an important role in the pathogenesis of HNSCC. A number of angiogenic factors including vascular endothelial growth factor (VEGF) and angiogenin (ANG) have been shown to be significantly upregulated in HNSCC. Among them, ANG is unique in which it is a ribonuclease that regulates ribosomal RNA (rRNA) transcription. ANG-stimulated rRNA transcription has been shown to be a general requirement for angiogenesis induced by other angiogenic factors. ANG inhibitors have been demonstrated to inhibit angiogenesis and tumor growth induced not only by ANG but also by other angiogenic factors. As the role of ANG in HNSCC is being unveiled, the therapeutic potential of ANG inhibitors in HNSCC is expected.

The regulation of protein synthesis in cancer

Translational control of cancer is a multifaceted process, involving alterations in translation factor levels and activities that are unique to the different types of cancers and the different stages of disease. Translational alterations in cancer include adaptations of the tumor itself, of the tumor microenvironment, an integral component in disease, and adaptations that occur as cancer progresses from development to local disease and ultimately to metastatic disease. Adaptations include the overexpression and increased activity of specific translation factors, the physical or functional loss of translation regulatory components, increased production of ribosomes, selective mRNA translation, and alteration of signal transduction pathways to permit unfettered activation of protein synthesis. There is intense clinical interest to capitalize on the emerging new understanding of translational control in cancer by targeting specific components of the translation apparatus that are altered in disease for the development of specific cancer therapeutics. Clinical trial data are nascent but encouraging, suggesting that translational control constitutes an important new area for drug development in human cancer.

Coordinate regulation of ribosome biogenesis and function by the ribosomal protein S6 kinase, a key mediator of mTOR function

Current understanding of the mechanisms by which cell growth is regulated lags significantly behind our knowledge of the complex processes controlling cell cycle progression. Recent studies suggest that the mammalian target of rapamycin (mTOR) pathway is a key regulator of cell growth via the regulation of protein synthesis. The key mTOR effectors of cell growth are eukaryotic initiation factor 4E-binding protein 1 (4EBP-1) and the ribosomal protein S6 kinase (S6K). Here we will review the current models for mTOR dependent regulation of ribosome function and biogenesis as well as its role in coordinating growth factor and nutrient signaling to facilitate homeostasis of cell growth and proliferation. We will place particular emphasis on the role of S6K1 signaling and will highlight the points of cross talk with other key growth control pathways. Finally, we will discuss the impact of S6K signaling and the consequent feedback regulation of the PI3K/Akt pathway on disease processes including cancer.

Control of the hypoxic response through regulation of mRNA translation

Hypoxia is a common feature of most solid tumors which negatively impacts their treatment response. This is due in part to the biological changes that result from a coordinated cellular response to hypoxia. A large part of this response is driven by a transcriptional program initiated via stabilization of HIF, promoting both angiogenesis and cell survival. However, hypoxia also results in a rapid inhibition of protein synthesis which occurs through the repression of the initiation step of mRNA translation. This inhibition is fully reversible and occurs in all cell lines tested to date. Inhibition of translation is mediated by two distinct mechanisms during hypoxia. The first is through phosphorylation and inhibition of an essential eukaryotic initiation factor, eIF2alpha. Phosphorylation of this factor occurs through activation of the PERK kinase as part of a coordinated ER stress response program known as the UPR. Activation of this program promotes cell survival during hypoxia and facilitates tumor growth. Translation during hypoxia can also be inhibited through the inactivation of a second eukaryotic initiation complex, eIF4F. At least part of this inhibition is mediated through a REDD1 and TSC1/TSC2 dependent inhibition of the mTOR kinase. Inhibition of mRNA translation is hypothesized to affect the cellular tolerance to hypoxia in part by promoting energy homeostasis. However, regulation of translation also results in a specific increase in the synthesis of a subset of hypoxia induced proteins. Consequently, both arms of translational control during hypoxia influence hypoxia induced gene expression and the hypoxic phenotype.

RNA polymerases I and III, growth control and cancer

Transcription of rRNA and tRNA genes by RNA polymerases I and III is essential for sustained protein synthesis and is therefore a fundamental determinant of the capacity of a cell to grow. When cell growth is not required, this transcription is repressed by retinoblastoma protein, p53 and ARF. However, inactivation of these tumour suppressors in cancers deregulates RNA polymerases I and III, and oncoproteins such as Myc can stimulate these systems further. Such events might have a significant impact on the growth potential of tumours.

Kinetic analysis and modeling of firefly luciferase as a quantitative reporter gene in live mammalian cells

Firefly luciferase has proven to be a highly sensitive and quantitative reporter gene for studying gene delivery and regulation, and its recent use in live cells and organisms promises to further expand its utility. However, the intracellular behavior and properties of the enzyme are not well characterized. Specifically, information on the intracellular kinetics and stability of luciferase activity is necessary for real-time luminescence counts from live cells to be quantitatively meaningful. Here, we report a dynamic analysis of luciferase activity in the context of living mammalian cells. We have determined the relative light units measured in living cells to be proportional to that found in cell lysate. We have also calculated the K(m) of luciferase in living cells to be approximately 1 mM, a value much higher than the 10 microM found for pure enzyme in vitro. In addition, a 2-hour half-life of luciferase activity in live cells was measured in real time. Finally, we have modeled luciferase activity in live cells for the purposes of understanding and translating the luciferase signal into a more effective metric of gene expression and cell behavior.

RNA polymerase III transcription--a battleground for tumour suppressors and oncogenes

This review provides a summary of the European Association for Cancer Research Award Lecture, presented at the ECCO12 meeting in Copenhagen in September 2003. It describes what we have learnt about the mechanisms responsible for deregulating RNA polymerase III transcription in transformed cells. A network has been discovered of unanticipated links to key tumour suppressors and oncogenes. Novel functions have been revealed for RB, p53 and c-Myc, that may help explain their profound biological effects.

Insulin-like growth factor I receptor signaling and nuclear translocation of insulin receptor substrates 1 and 2

The insulin receptor substrate 1 (IRS-1) can translocate to the nuclei and nucleoli of several types of cells. Nuclear translocation can be induced by an activated insulin-like growth factor 1 receptor (IGF-IR), and by certain oncogenes, such as the Simian virus 40 T antigen and v-src. We have asked whether IRS-2 could also translocate to the nuclei. In addition, we have studied the effects of functional mutations in the IGF-IR on nuclear translocation of IRS proteins. IRS-2 translocates to the nuclei of mouse embryo fibroblasts expressing the IGF-IR, but, at variance with IRS-1, does not translocate in cells expressing the Simian virus 40 T antigen. Mutations in the tyrosine kinase domain of the IGF-IR abrogate translocation of the IRS proteins. Other mutations in the IGF-IR, which do not interfere with its mitogenicity but inhibit its transforming capacity, result in a decrease in translocation, especially to the nucleoli. Nuclear IRS-1 and IRS-2 interact with the upstream binding factor, which is a key regulator of RNA polymerase I activity and, therefore, rRNA synthesis. In 32D cells, wild-type, but not mutant, IRS-1 causes a significant activation of the ribosomal DNA promoter. The interaction of nuclear IRS proteins with upstream binding factor 1 constitutes the first direct link of these proteins with the ribosomal DNA transcription machinery.

Does the ribosome translate cancer?

Ribosome biogenesis and translation control are essential cellular processes that are governed at numerous levels. Several tumour suppressors and proto-oncogenes have been found either to affect the formation of the mature ribosome or to regulate the activity of proteins known as translation factors. Disruption in one or more of the steps that control protein biosynthesis has been associated with alterations in the cell cycle and regulation of cell growth. Therefore, certain tumour suppressors and proto-oncogenes might regulate malignant progression by altering the protein synthesis machinery. Although many studies have correlated deregulation of protein biosynthesis with cancer, it remains to be established whether this translates directly into an increase in cancer susceptibility, and under what circumstances.

RNA polymerase III transcription factor TFIIIC2 is overexpressed in ovarian tumors

Most transformed cells display abnormally high levels of RNA polymerase (pol) III transcripts. Although the full significance of this is unclear, it may be fundamental because healthy cells use two key tumor suppressors to restrain pol III activity. We present the first evidence that a pol III transcription factor is overexpressed in tumors. This factor, TFIIIC2, is a histone acetyltransferase that is required for synthesis of most pol III products, including tRNA and 5S rRNA. TFIIIC2 is a complex of five polypeptides, and mRNAs encoding each of these subunits are overexpressed in human ovarian carcinomas; this may explain the elevated TFIIIC2 activity that is found consistently in the tumors. Deregulation in these cancers is unlikely to be a secondary response to rapid proliferation, because there is little or no change in TFIIIC2 mRNA levels when actively cycling cells are compared with growth-arrested cells in culture. Using purified factors, we show that raising the level of TFIIIC2 is sufficient to stimulate pol III transcription in ovarian cell extracts. The data suggest that overexpression of TFIIIC2 contributes to the abnormal abundance of pol III transcripts in ovarian tumors.

RNA polymerase III transcription: its control by tumor suppressors and its deregulation by transforming agents

The level of RNA polymerase (pol) III transcription is tightly linked to the rate of growth; it is low in resting cells and increases following mitogenic stimulation. When mammalian cells begin to proliferate, maximal pol III activity is reached shortly before the G1/S transition; it then remains high throughout S and G2 phases. Recent data suggest that the retinoblastoma protein RB and its relatives p107 and p130 may be largely responsible for this pattern of expression. During G0 and early G1 phase, RB and p130 bind and repress the pol III-specific factor TFIIIB; shortly before S phase they dissociate from TFIIIB, allowing transcription to increase. At the end of interphase, when cells enter mitosis, pol III transcription is again suppressed; this mitotic repression is achieved through direct phosphorylation of TFIIIB. Thus, pol III transcription levels fluctuate as mammalian cells cycle, being high in S and G2 phases and low during mitosis and early G1. In addition to this cyclic regulation, TFIIIB can be bound and repressed by the tumor suppressor p53. Conversely, it is a target for activation by several viruses, including SV40, HBV, and HTLV-1. Some viruses also increase the activity of a second pol III-specific factor called TFIIIC. A large proportion of transformed and tumor cell types express abnormally high levels of pol III products. This may be explained, at least in part, by the very high frequency with which RB and p53 become inactivated during neoplastic transformation; loss of function of these cardinal tumor suppressors may release TFIIIB from key restraints that operate in normal cells.

Upregulated expression of the genes encoding translation initiation factors eIF-4E and eIF-2alpha in transformed cells

Increased protein synthesis is necessary for the transition of cells from quiescence to proliferation. It is shown in this paper that the induction of expression of the translation initiation factor eIF-4E in normal cells requires serum growth factors, while this requirement is abrogated in tumor cells analyzed in this study. Further, the expression of eIF-4E and eIF-2alpha is increased in c-myc, v-src, and v-abl-transformed cells. It is demonstrated that an increase in c-myc function leads to elevated expression of eIF-4E and eIF-2alpha, increases in net protein synthesis and cell proliferation. It may be suggested that constitutive activation of translational machinery may be one common mechanism by which various oncogenes exert their transforming function.

Evaluation of antitumor effect of tumor necrosis factor in terms of protein metabolism and cell cycle kinetics

To determine the significance of protein breakdown in regulating tumor growth and to better understand the antitumor mechanism of tumor necrosis factor in vivo, we measured the effects of a 6-h constant intravenous infusion of human recombinant tumor necrosis factor-alpha (rHuTNF) on tumor protein metabolism and cell cycle kinetics in rats bearing the Walker-256 carcinosarcoma. Protein metabolism was investigated with the use of [14C]leucine infusion; estimates of tumor cell cycle kinetics were obtained in vivo by use of 5-bromo-2'-deoxyuridine (BrdUrd) pulse labeling and bivariate BrdUrd/DNA analysis by flow cytometry. Reduction in tumor growth by rHuTNF was associated with a dose-dependent increase in tumor proteolysis but no change in tumor protein synthesis. At the cellular level, rHuTNF had a significant cytostatic effect on G2/M cells and caused a marked decrease in the fraction of cells capable of BrdUrd uptake. Release of BrdUrd, an indicator of cell death, was noted in only 7.5% of tumor cells labeled at the beginning of rHuTNF infusion. These results suggest that either tumor protein breakdown may influence cell cycle activity by regulating cytoplasmic protein mass or that tumor proteolysis may be a compensatory mechanism for limiting cytoplasmic size when cellular division is interrupted suddenly.

Nutrition and tumor promotion: in vivo methods for measurement of cellular proliferation and protein metabolism

The notion that tumors act as "nitrogen traps" has led to the belief that nutrition support of the cancer-bearing patient can enhance tumor growth. Proponents of this theory consider the provision of energy and essential nutrients as well as the influence of hormones and growth factors as responsible for this effect. On the other hand, nutrition administration in the debilitated cancer patient may improve antitumor host defense mechanisms and reduce tumor growth. This paper reviews methodologic issues related to the study of nutrition and cancer growth with emphasis on in vivo methods for measuring tumor protein turnover and cytokinetics. Using this combined approach, we previously demonstrated that dietary fat may significantly regulate tumor growth during chronic feeding as well as with short-term intravenous nutrition support in experimental models. Although the mechanism of this effect remains unclear, we have reasoned that by altering arachidonic acid metabolism and prostaglandin synthesis, omega-3 fatty acids could change tumor protein breakdown rates and inhibit the proliferation potential of these tumors. Acknowledging alternative hypotheses, we now present cytokinetic evidence that intracellular protein degradation may regulate tumor cell proliferation. Additional studies relating dietary fat, tumor protein metabolism and tumor proliferation potential are currently in progress. We propose that the effect of nutrition administration on tumor growth is complex and involves several regulatory systems. Thus, based on available evidence, an a priori tumor-enhancing effect for nutrition support is clearly not warranted. Intracellular protein breakdown and host defense mechanisms, both of which are energy dependent, are important loci at which nutrition and tumor growth regulation could interact.(ABSTRACT TRUNCATED AT 250 WORDS)

Protein turnover, growth and proliferation in CHO cells. Variation within and between mutant classes for salvage pathway enzymes

We have examined the clonal variation in rates of amino acid transport, protein synthesis, protein degradation, growth and proliferation for CHO cells with mutations in the purine and pyrimidine salvage pathways. First we compared three clonal cell lines, each with a different mutation, with the heterozygous parental line AT3-2. Overall, the correlation between rates of protein turnover, growth and proliferation was excellent. The slower growth and proliferation of one mutant, AB3 (TK-, APRT-), is explained by a low intrinsic rate of protein synthesis coupled with a smaller response in rates of amino acid transport, protein synthesis and protein degradation to insulin, serum and dexamethasone. Secondly, we compared seven aza-adenine-resistant and 14 thioguanine-resistant mutants of AT3-2 and found significant differences in control and insulin-stimulated rates of protein turnover both within and between mutant populations. A significant difference between the populations was unexpected because each individual cell line was cloned from a spontaneous pre-existing mutation in AT3-2, and each population should have the same average rate. Remarkably, all 24 mutants had lower rates of protein synthesis than AT3-2. We cannot explain the data solely in terms of mutations in the salvage pathways. Rather, we propose that the mutant survivors have randomly down-regulated the intrinsically fixed growth factor-regulated pathways of protein turnover, resulting in a broad spectrum of lower metabolic rates.

Autophagy and other vacuolar protein degradation mechanisms

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

Population study of cell cycle in a continuous culture of Candida utilis

The mean lengths of G1, S, G2 and M phases of the cell cycle were determined on the basis of the population distribution of Candida utilis grown in a continuous culture under steady-state conditions by using an original mathematical method. The length of the G2 phase was proportional to that of G1; the length of M was effectively independent of the growth rate. The length of S was proportional to the mean number of mitochondria in the cell.

Protein Turnover During Aging of Cultured Human Fibroblasts

Les cellules qui vieillissentin vitroou bien celles qui sont prélevées de donneurs d'àge avancé ou de sujets manifestant certaines des particularités qui s'apparentent à un vieillissement accéléré (progérie ou le syndrome de Werner), peuvent être qualifiées de 'vieilles'. Celles-ci on des taux de croissance ralentis en milieu de culture si on les compare aux cellules nouvelles ou à mi-passage prélevées de jeunes donneurs normaux. Durant la croissance exponentielle, les taux de constantes pour la synthèse protéique dans les jeunes cellules ne sont pas significativement différents de ceux retrouvés dans les vieilles cellules (0.023 ± 0.002h1vs. 0.021 ± 0.002h1respectivement) et pourtant les taux de croissance (i.e. accrétion protéique) sont de seulement 0.013±0.003h1dans les vieilles cellules comparés à 0.022±0.002h1dans les jeunes cellules. Done, le taux ralenti d'accumulation protéique durant la croissance des vieilles cellules comparé aux jeunes cellules est associé à une dégradation protéique accélérée (0.01±0.002h1vs 0.001 ±0.002h1;P<0.05) plutôt qu'à des taux ralentis de synthèse protéique. Lorsque les cellules deviennent quiescentes suivant une période d'inhibition de croissance due à la densité, les taux de synthèse protéique diminuent dans les jeunes et les vieilles cellules pour aboutir à des niveaux comparables (0.013±0.002h1) où les taux de croissance (0.003±0.0003h1) et de dégradation (0.01±0.003h1) ne sont significativement pas differents dans les deux groupes. Done, ce n'est qu'en période de croissance exponentielle qu'une différence dans le turn-over protéique entre jeunes et vieilles cellules est observée, alors que la dégradation est accélérée dans les vieilles cellules. La relation causale entre la dégradation protéique accélérée et les taux de croissance ralentis dans les vieilles cellules demeure inconnue.

Effect of fetal growth on maternal protein metabolism in postabsorptive rat

Rates of protein synthesis were measured in whole fetuses and maternal tissues at 17 and 20 days of gestation in postabsorptive rats using continuous infusion of L-[1-14C]leucine. Fetal protein degradation rates were derived from the fractional rates of synthesis and growth. Whole-body (plasma) leucine kinetics in the mother showed a significant reduction of the fraction of plasma leucine oxidized in the mothers bearing older fetuses, a slight increase in the plasma flux, with total leucine oxidation and incorporation into protein remaining similar at the two gestational ages. Estimates of fractional protein synthesis in maternal tissues revealed an increase in placental and hepatic rates at 20 days of gestation, whereas the fractional synthetic rate in muscle remained unchanged. A model for estimation of the redistribution of leucine between plasma and tissues is described in detail. This model revealed a more efficient utilization of leucine in fetal protein synthesis in comparison with other maternal tissues, a greater dependency of the fetus on plasma supply of leucine, and a significant increase (2-fold) in the release of leucine from maternal muscle as the fetal requirements increased proportionately with its size. The latter conclusion, supported by nitrogen analysis and the ratio of bound-to-free leucine in maternal tissues, confirms the importance of maternal stores in maintaining the homeostasis of essential amino acids during late pregnancy.

Cell cycle modelling

Models able to describe the events of cellular growth and division and the dynamics of cell populations are useful for the understanding of functional control mechanisms and for the theoretical support for automated analysis of flow cytometric data and of cell volume distributions. This paper reports on models that we have developed with this aim for different kinds of cells. The models are composed by two subsystems: one describes the growth dynamics of RNA and protein, and the second accounts for DNA replication and cell division, and describe in a rather unitary frame the cell cycle of eukaryotic cells, like mammalian cells and yeast, and of prokaryotic cells. The model is also used to study the effects of various sources of variability on the statistical properties of cell populations, and we find that in microbial cells the main source of variability appears to be an inaccuracy of the molecular mechanism that monitors cell size. In normal mammalian cells another source of variability, that depends upon the interaction with growth factors which give competence, is apparent. An extended version of the model, which comprises also this additional variability, is presented and used to describe the properties of mammalian cell growth.

Cell growth and cell proliferation may be dissociated in the mouse uterine luminal epithelium treated with female sex steroids

The mouse uterine epithelium under various hormonal regimes is a good system to identify biochemical events associated with cell growth, DNA synthesis and cell division. This is because estradiol-17 beta stimulates the cells to undergo a synchronized wave of DNA synthesis and cell division. Estriol, on the other hand, also stimulates DNA synthesis but because of the rapid loss of this hormone from the tissue some of the cells abort, giving a constant epithelial cell number. Three days of progesterone pretreatment, however, completely suppresses the estradiol-17 beta-induced wave of DNA synthesis and cell proliferation. Using these hormonal treatments we have shown that both estradiol-17 beta and estriol stimulate protein and rRNA synthesis with the concomitant increase of protein and rRNA per mg of DNA. These macromolecules accumulated in direct proportion to the fraction of cell committed to DNA synthesis. Estriol, however, did not sustain the growth responses and at the peak of DNA synthesis both rRNA and protein synthesis had returned to control levels. Progesterone pretreatment, despite inhibiting the proliferative response, failed to inhibit any of the estradiol-17 beta-induced increases in protein and rRNA synthesis. Indeed 12 h after estradiol-17 beta injection the cells had identical protein and rRNA contents, regardless of whether they had been exposed to progesterone or not. The present data therefore suggests that in the uterine epithelium cell growth as defined by protein and rRNA accumulation and DNA synthesis represents two independently regulated pathways.

Reduced autophagic activity, improved protein balance and enhanced in vitro survival of hepatocytes isolated from carcinogen-treated rats

Sequential carcinogen treatment (diethylnitrosamine/partial hepatectomy followed by 2-acetylaminofluorene (2-AAF] induced multiple hepatocarcinomas in rats with 100% certainty within a year. Enzyme-altered lesions, i.e. gamma-glutamyltranspeptidase (GGT)-positive and/or ATPase-negative cell foci, were numerous already at 8 weeks, and suspensions of purified hepatocytes isolated (by collagenase perfusion) at this time contained 30-40% GGT-positive cells. These hepatocyte suspensions were markedly deficient with respect to autophagic protein degradation (in comparison with cell suspensions from normal rats), and the cells lost less protein and survived much better than normal hepatocytes in culture under conditions of amino acid deprivation (which activates the autophagic mechanism). The anabolic advantage of reduced autophagy may possibly contribute to the selective outgrowth of preneoplastic cells during the earliest stage of liver carcinogenesis. Inclusion of the autophagy inhibitor 3-methyladenine in the culture medium elevated the survival of normal hepatocytes up to the level seen with hepatocytes from carcinogen-treated animals, suggesting that protection of normal cells by autophagy suppression may be a potentially interesting therapeutic principle.

The regulation of mononuclear phagocyte entry into S phase by the colony stimulating factor CSF-1

CSF-1 is a hemopoietic growth factor that specifically regulates the survival, proliferation, and differentiation of mononuclear phagocytic cells. Populations of adherent bone marrow-derived macrophages (BMM) devoid of CSF-1 producing cells were used to study regulation by CSF-1 of macrophage entry into S phase. More than 95% of BMM possess the CSF-1 receptor. It was shown that 93-98% of BMM are cycling (S phase 8-9 hr, doubling time 24-28 hr) when cultured in the presence of CSF-1. BMM incubated with 15% FCS in the absence of CSF-1 or in the presence of CSF-1 concentrations inducing survival without proliferation enter a quiescent state. This state is characterized by a reduction in the synthesis of DNA (98%), total protein (35%), ribosomal protein (76%), and histone (96%) compared with the synthetic rate of these components in exponentially growing cells. Addition of CSF-1 to BMM rendered quiescent by removal of CSF-1 stimulated entry into S phase with a lag period of approximately 12 h. This lag period is reduced to 8 hr in BMM made quiescent at concentrations of CSF-1 inducing survival without proliferation, an effect which may be related to the expected higher protein content of these cells (Tushinski and Stanley, J. Cell. Physiol., 116:67-75). Neutralization of CSF-1 by antibody at different times during the lag period indicates that CSF-1 is required for almost the entire lag period for the entry of any cells into S phase. In BMM rendered quiescent by removal of both serum and CSF-1, purified CSF-1 without serum stimulated entry of cells into S phase, whereas serum alone was ineffective. The results are consistent with a primary regulatory role of CSF-1 in mononuclear phagocyte proliferation, survival, and function.

Doubling of cell mass is not necessary in order to achieve cell division in cultured human cells

When exponentially growing NHIK 3025 cells were shifted from medium containing 30% serum to medium containing 0.03% serum the rate of net protein accumulation was reduced due to both a reduction in the rate of protein synthesis and an increase in the rate of protein degradation. This change in growth conditions increased the protein doubling time from 18 to 140 h. The cell cycle duration of cells synchronized by mitotic selection was, however, only increased from 17 to 26 h by this treatment. Therefore, when the cells divide by the end of the first cell cycle following synchronization, the cells shifted to 0.03% serum contained far less protein than those growing continuously in 30% serum. Hence, the attainment of a critical cell mass is probably not controlling cell division for cells growing in a balanced state.

Distinct proteolytic mechanisms in serum-sufficient and serum-restricted fibroblasts. Transformed 3T3 cells fail to regulate proteolysis in relation to culture density only during serum-sufficiency

Thymidine incorporation (reflecting cell division), degradation of long-half-life proteins and protein synthesis were compared in normal Swiss mouse 3T3 fibroblasts and their counterparts transformed by simian virus 40 at both high and low culture densities (no. of cells/cm2). Normal cells maintained faster proteolysis at high culture density than at low. Degradation was in all conditions enhanced by serum deprivation (1% serum). In serum-sufficient (10%) conditions, there was an inverse correlation between degradation and cell division, but in serum-restricted conditions proteolysis increased substantially as culture density was increased, without change in cell division. Protein synthesis generally changed in a converse sense to protein degradation. In serum-sufficient conditions, transformed 3T3 cells failed to regulate proteolysis in response to culture density. However, in serum-restricted conditions they can regulate proteolysis as do normal cells. Transformed 3T3 cells regulate protein synthesis and thymidine incorporation very poorly in response to culture density in both conditions studied. The failure of regulation of both protein synthesis and degradation may contribute to the exaggerated growth of transformed cells in serum-sufficient conditions. The retention by such cells of regulation of proteolysis during serum restriction may also aid their survival. Studies with several lysosomotropic agents indicated that lysosomes contribute to proteolysis in all conditions studied, but also that its regulation in serum restriction is distinct from that in serum sufficiency, and may involve primarily a non-lysosomal mechanism.

Protein degradation in 3T3 cells and tumorigenic transformed 3T3 cells

To study the relation of overall rates of protein degradation in the control of cell growth, we determined if transformation of fibroblasts to tumorigenicity affected their rates of degradation of short- and long-lived proteins. Rates of protein degradation were measured in nontumorigenic mouse Balb/c 3T3 fibroblasts, and in tumorigenic 3T3 cells transformed by different agents. Growing 3T3 cells, and cells transformed with Moloney sarcoma virus (MA-3T3) or Rous sarcoma virus (RS-3T3), degraded short- and long-lived proteins at similar rates. Simian virus 40 (SV-3T3)- and benzo(a)pyrene (BP-3T3)-transformed cells had slightly lower rates of degradation of both short- and long-lived proteins. Reducing the serum concentration in the culture medium from 10% to 0.5%, immediately caused about a twofold increase in the rate of degradation of long-lived proteins in 3T3 cells. Transformed lines increased their rates of degradation of long-lived proteins only by different amounts upon serum deprivation, but none of them to the same extent as did 3T3. Greater differences in the degradation rates of proteins were seen among the transformed cells than between 3T3 cells and some transformed cells. Thus, there was no consistent change in any rate of protein degradation in 3T3 cells due to transformation to tumorigenicity.

The regulation of macrophage protein turnover by a colony stimulating factor (CSF-1)

CSF-1 is a hemopoietic growth factor that specifically regulates the survival, proliferation, and differentiation of mononuclear phagocytic cells. A homogeneous population of mononuclear phagocytes, bone marrow derived macrophages (BMM), were used to study the regulation of protein turnover by CSF-1. Removal of CSF-1 (approximately 0.4 nM) from exponentially growing BMM cultured in 15% fetal calf serum containing medium decreases the rate of DNA synthesis by more than 100-fold. Addition of CSF-1 to these cells causes them to resume DNA synthesis within 12 h. More immediate effects of CSF-1 were observed on BMM protein metabolism. BMM cultured for 24 h in the absence of CSF-1 reduce their protein synthetic rate by 50-60%. The protein synthetic rate commences to decrease at 2-3 h after CSF-1 removal. Readdition of CSF-1 to BMM previously incubated in its absence causes a return to the protein synthetic rate of exponentially growing cells within 2 h. In the presence of CSF-1, BMM synthesize protein at a rate of approximately 8.7%/h and degrade it at a rate of approximately 0.9%/h. Removal of CSF-1 results in a decrease in the protein synthetic rate to approximately 3.4%/h and an increase in the rate of protein degradation to approximately 3.4%/h. The rate of protein synthesis by BMM increases linearly with CSF-1 concentration over the range of concentrations stimulating both survival and proliferation, while the rate of protein degradation decreases exponentially over the range of concentrations stimulating survival without proliferation. Therefore, it appears that the stimulation of the rate of protein synthesis and inhibition of the rate of protein degradation are two distinct effects of CSF-1, both part of the pleiotropic response to this growth factor. The inhibition of the rate of protein degradation by CSF-1 may be most significant for its survival inducing effect.

Progress through G1 and S in relation to net protein accumulation in human NHIK 3025 cells

We have investigated whether human NHIK 3025 cells are dependent upon a net increase in cellular protein content in order to traverse G1 and S. The increase in DNA and protein content was studied by means of two-parameter flow cytometry using populations of cells synchronized by mitotic selection. By adding 1 microM cycloheximide to the medium protein synthesis was partially inhibited, resulting in negligible net accumulation of protein. The cells were able to enter S and progress through S under such conditions. The latter was the case whether the cells had been accumulating protein during G1 or not. The results further indicate that the larger cells enter S earlier and traverse S at a higher rate than the smaller cells. Our conclusion is that net accumulation of protein does not seem to be a prerequisite for traverse through G1 and S, i.e. DNA replication may be dissociated from the general growth of cell mass.

Protein turnover and proliferation. Turnover kinetics associated with the elevation of 3T3-cell acid-proteinase activity and cessation of net protein gain

1. At least 95% of the total protein of A31-3T3 cell cultures undergoes turnover. 2. First-order exponential kinetics were used to provide a crude approximation of averaged protein synthesis, Ks, degradation, Kd, and net accumulation, Ka, as cells ceased growth at near-confluent density in unchanged Dulbecco's medium containing 10% serum. The values of the relationship Ka = Ks - Kd were : 5%/h = 6%/h - 1%/h in growing cells, and 0%/h = 3%/h - 3%/h in steady-state resting cells. 3. As determined by comparison of the progress of protein synthesis and net protein accumulation, the time course of increase in protein degradation coincided with the onset of an increase in lysosomal proteinase activity and decrease in thymidine incorporation after approx. 2 days of exponential growth. 4. After acute serum deprivation, rapid increases in protein degradation of less than 1%/h could be superimposed on the prevailing degradation rate in either growing or resting cells. The results indicate that two proteolytic mechanisms can be distinguished on the basis of the kinetics of their alterations. A slow mechanism changes in relation to proliferative status and lysosomal enzyme elevation. A prompt mechanism, previously described by others, changes before changes in cell-cycle distribution or lysosomal proteinase activity. 5. When the serum concentration of growing cultures was decreased to 1% or 0.25%, then cessation of growth was accompanied by a lower steady-state protein turnover rate of 2.0%/h or 1.5%/h respectively. When growth ceased under conditions of overcrowded cultures, or severe nutrient insufficiency, protein turnover did not attain a final steady state, but declined continually into the death of the culture.

Protein turnover and proliferation. Failure of SV-3T3 cells to increase lysosomal proteinases, increase protein degradation and cease net protein accumulation

The contrasting control of lysosomal proteinases, protein turnover and proliferation was studied in 3T3 and SV-3T3 (SV-40-virus-transformed 3T3) cells. 1. In 3T3 cells, net protein accumulation proceeded from 5%/h (doubling time, T(d)=14h) in growing cells to 0%/h as cells became quiescent. SV-3T3 cells never ceased to gain protein, but rather decreased their protein accumulation rate from 6-7%/h (T(d)=10-12h) to 2%/h (T(d)=35-40h) just before culture death in unchanged medium. 2. In both cell types the rates of protein synthesis per unit of protein (a) were proportional to the initial serum concentration from 0 to 6%, and (b) declined under progressive depletion of undefined serum growth factors. In depleted growth medium, leucine incorporation per unit of protein in 3T3 and SV-3T3 cells declined to almost equal synthetic rates while the 3T3 cell existed in a steady state of zero net gain, and the SV-3T3 cell continued to gain protein at a rate of 2%/h. 3. Whereas a large fraction of the control of 3T3-cell net protein accumulation can be accounted for by an increase in degradation from 1%/h to 3%/h, the SV-3T3 cell did not exhibit a growth-related increase in degradation appreciably above 1%/h. 4. Thus, by using first-order kinetics, the continued net protein accumulation of the transformed cell can be accounted for by a failure to increase protein degradation, whereas fractional synthesis can be made to decline to a rate similar to that in the quiescent non-transformed cell. 5. Upon acute serum deprivation, both cell types similarly exhibited small rapid increases in proteolysis independent of cell growth state or lysosomal enzyme status. 6. The 3T3 cell increased its lysosomal proteinase activity in conjunction with increase in the growth-state-dependent proteolytic mechanism; however, the SV-3T3 cell failed to increase lysosomal proteinases or the growth-state-dependent proteolytic mechanism.

Ribosomal proteins are synthesized preferentially in cells commencing growth

Mouse 3T3 cells, in stationary phase because of serum deprivation, have only half the ribosome content of growing cells. Furthermore, the proportion of protein synthesis devoted to ribosomal proteins is only half that in growing cells. On addition of serum the synthesis of each ribosomal protein increases threefold, demonstrating the coordination of the synthesis of the ribosomal proteins. Half that increase is due to a general increase in total protein synthesis; half is due to a differential increase in ribosomal protein synthesis. The latter is abolished by a concentration of actinomycin D which blocks only ribosomal RNA transcription. The results are discussed with reference to a general hypothesis of growth regulation proposed by Stanners et al. (1979).

Derangement of regulation of protein degradation in transforming fibroblasts

Changes in endogenous protein degradation of stable proteins with growth state have been observed in a number of normal cell lines: increases in degradation of between 15 and 40% occurred in 10 cell lines at confluence. No such regulation of proteolysis was observed in 4 cell lines which were clonogenic in soft agar. We have evidence that this derangement of regulation coincides with the transformation of cells, because such regulation disappears in spontaneously transforming fibroblasts and is decreased by the tumour promoter 12-0-tetradecanoyl-phorbol-13-acetate. In addition, regulation reappears in a growth control revertant of a transformed line.

Effect of serum step-down on protein metabolism and proliferation kinetics of NHIK 3025 cells

Human NHIK 3025 cells growing exponentially in 30% or 3% serum had population doubling times of 19.1 and 27.6 hours, respectively. These values were equal to the calculated protein doubling times (17.6 and 26.5 hours, respectively), showing that the cells were in balanced growth at both serum concentrations. Stepdown from 30% to 3% serum reduced the rate of protein synthesis within 1--2 hours, from 5.7%/hour to 4.3%/hour, while the rate of protein degradation was unchanged (1.7%/hour). In cells synchronized by mitotic selection from an exponentially growing population, the median cell cycle durations in 30% and 3% serum were 17.2 and 23.6 hours, respectively, which were also in good agreement with the protein doubling times. The median G1 durations were 7.1 and 9.6 hours, respectively. Thus the duration of G1 relative to the total cell cycle duration was the same in the two cases. Complete removal of serum for a period of 3 hours resulted in a 3-hour prolongation of the cell cycle regardless of the time after mitotic selection at which the serum was removed. For synchronized cells, the rate of entry into both the S phase and into the subsequent cell cycle were reduced in 3% serum as compared to 30% serum, the former rate being significantly greater than the latter at both serum concentrations. Our results thus indicate that these cells are continuously dependent upon serum throughout the entire cell cycle.

Effects of antibiotics on protein synthesis and degradation in primary cultures of rat hepatocytes

In primary hepatocyte cultures, maintained in a protein-free medium, streptomycin, penicillin, and Garamycin (gentamicin) all inhibited protein synthesis at concentrations above 0.1 mM. Some inhibition was also observed with the fungicide Mycostatin at 100 U/ml. Hepatocytic protein degradation was markedly inhibited by penicillin fat concentrations above 0.1 mM, whereas streptomycin and Garamycin only showed slight inhibition at concentrations in excess of 1 mM. None of the antibiotics had any detectable effect on the structural integrity (viability) of the cells.

Mammalian cells do not have a stringent response

A key attribute of the stringent response of bacteria is the rapid inhibition of ribosomal RNA synthesis mediated by unusual nucleotides in respnse to uncharged tRNA. The question as to whether mammalian cells show a stringent response analogous to that of bacteria was critically tested by the effective rapid amino acid starvation of both normal and transformed cells. Rapid starvation giving a high proportion of uncharged tRNA for leucine was produced within 7 minutes of expression of a nonleaky ts leucyl tRNA synthetase mutation in transformed CHO cells (tsH1) and in its normal growth control revertant (L-73). To control for the effect of temperature alone, ts revertants of tsH1 and L-73 were included in the study, and to control for effects due simply to the inhibition of protein synthesis, the translational elongation inhibitor cycloheximide was used. In addition, rapid starvation for histidine was effected by incubation of both the CHO cell lines and of freshly explanted normal Chinese hamster embryo fibroblasts in histidine-free medium containing high concentrations of histidinol. The rate of preribosomal RNA synthesis and the extent of its maturation to mature rRNA was measured using (3H-methyl) methionine as a donor of methyl groups during synthesis and methylation of pre-rRNA. There was no effect on pre-rRNA synthesis of the rapid generation of uncharged tRNA for 45 minutes for any of the cell types tested. A nonspecific inhibition of maturation of 18S rRNA and late (3 hour) inhibition of pre-rRNA synthesis was observed, but could be mimicked by the inhibition of protein synthesis to comparable levels with cycloheximide. Less severe amino acid starvation resulting in a more physiological inhibition of protein synthesis to 30% also had no specific effect on pre-rRNA synthesis and maturation. Intracellular nucleotide pools were also examined for the appearance of unusual nucleotides such as guanosine tetraphosphate or pentaphosphate and for changes in the levels of normal nucleotides after severe amino acid starvation. No such changes could be detected. We conclude that although mammalian cells may have some biochemical reactions which respond to uncharged tRNA, they do not possess a macromolecular control system analogous to the stringent response of bacteria.

Protein synthesis and degradation during expression of the temperature-sensitive defect in ts A1S9 mouse L-cells

The involvement of altered protein metabolism in the expression of the temperature-sensitive (ts) pleiotropic phenotype of ts A1S9 cells was investigated. Cells are ts in growth and DNA replication. They undergo decondensation of their heterochromatin, interruptions of chromatin synthesis, and changes in cell size and morphology at the non-permissive temperature (npt) of 38.5 degrees C. Whereas the rates of incorporation of 3H-leucine, 35S-methionine, and 3H-fucose into proteins were unaffected at 38.5 degrees C, net protein accumulation was greatly reduced. This imbalance resulted from a rapid increase in the rate of protein degradation at the npt. Enhancement of protein degradation was detected within 2-4 hours after temperature upshift and constitutes the earliest metabolic alteration thus far observed during expression of the temperature-sensitive phenotype. The average half-life of proteins performed in ts A1S9 cells at 34 degrees C was decreased four-fold at the npt, and all major cytoplasmic proteins were affected equally. Enhanced protein degradation at the npt was shown to be sensitive to cycloheximide, ammonia, chloroquine, and vinblastine at concentrations that did not affect the basal protein degradation of normally cycling cells. Increased protein degradation at 38.5 degrees C did not involve an equivalent increase in total cellular protease activity. The data obtained are compatible with a model that suggests that temperature inactivation of the ts A1S9 gene product results in activation of a lysosome-mediated mechanism for the rapid degradation of cytoplasmic proteins.

Protein degradation in skin fibroblasts from patients with Duchenne muscular dystrophy

The rates of degradation of [3H]leucine-labelled proteins have been measured in cultures of skin fibroblasts obtained from normal controls (five subjects) and patients with Duchenne muscular dystrophy (six subjects). Cultures were incubated with [3H]leucine (10 microCi/ml) for 60 min to label "short-lived" proteins, and with [3H]leucine (5 microCi/ml) for 60 h to label "long-lived" proteins. Optimal wash procedures were devised for removal of [3H]leucine from the extracellular space and from cell pools before beginning degradation measurements. Re-utilization of [3H]leucine released from degraded labelled proteins was prevented by supplementing the medium with 4mM-leucine. Rates of degradation did not depend on the growth state of the cells or on cell age over the range used (passages eight-20). Degradation of long-lived proteins was approximately linear over a 24h period, at a rate of 1.0% per h. 30% of short-lived protein was degraded within 6h. No differences were observed between protein degradation in normal fibroblasts and in those from patients with Duchenne muscular dystrophy.

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.