Prolyl-tRNA-based rates of protein and collagen synthesis in human lung fibroblasts

The role of metabolic reprogramming and de novo amino acid synthesis in collagen protein production by myofibroblasts: implications for organ fibrosis and cancer

Fibrosis is a pathologic condition resulting from aberrant wound healing responses that lead to excessive accumulation of extracellular matrix components, distortion of organ architecture, and loss of organ function. Fibrotic disease can affect every organ system; moreover, fibrosis is an important microenvironmental component of many cancers, including pancreatic, cervical, and hepatocellular cancers. Fibrosis is also an independent risk factor for cancer. Taken together, organ fibrosis contributes to up to 45% of all deaths worldwide. There are no approved therapies that halt or reverse fibrotic disease, highlighting the great need for novel therapeutic targets. At the heart of almost all fibrotic disease is the TGF-β-mediated differentiation of fibroblasts into myofibroblasts, the primary cell type responsible for the production of collagen and other matrix proteins and distortion of tissue architecture. Recent advances, particularly in the field of lung fibrosis, have highlighted the role that metabolic reprogramming plays in the pathogenic phenotype of myofibroblasts, particularly the induction of de novo amino acid synthesis pathways that are required to support collagen matrix production by these cells. In this review, we will discuss the metabolic changes associated with myofibroblast differentiation, focusing on the de novo production of glycine and proline, two amino acids which compose over half of the primary structure of collagen protein. We will also discuss the important role that synthesis of these amino acids plays in regulating cellular redox balance and epigenetic state.

Surfaceome dynamics reveal proteostasis-independent reorganization of neuronal surface proteins during development and synaptic plasticity

Neurons are highly compartmentalized cells with tightly controlled subcellular protein organization. While brain transcriptome, connectome and global proteome maps are being generated, system-wide analysis of temporal protein dynamics at the subcellular level are currently lacking. Here, we perform a temporally-resolved surfaceome analysis of primary neuron cultures and reveal dynamic surface protein clusters that reflect the functional requirements during distinct stages of neuronal development. Direct comparison of surface and total protein pools during development and homeostatic synaptic scaling demonstrates system-wide proteostasis-independent remodeling of the neuronal surface, illustrating widespread regulation on the level of surface trafficking. Finally, quantitative analysis of the neuronal surface during chemical long-term potentiation (cLTP) reveals fast externalization of diverse classes of surface proteins beyond the AMPA receptor, providing avenues to investigate the requirement of exocytosis for LTP. Our resource (neurosurfaceome.ethz.ch) highlights the importance of subcellular resolution for systems-level understanding of cellular processes.

Cell surface proteins contribute to neuronal development and activity-dependent synaptic plasticity. Here, the authors perform a time-resolved surfaceome analysis of developing primary neurons and in response to homeostatic synaptic scaling and chemical long-term potentiation (cLTP), revealing surface proteome remodeling largely independent of global proteostasis.

New approaches and recent results concerning human-tissue collagen synthesis

PURPOSE OF REVIEW

Knowledge of the physiological regulation of human-tissue collagen metabolism in vivo is poor, due to the lack of appropriately robust methods. Recent application of stable isotope tracer techniques to measure human collagen synthesis has provided some insights into the role of nutrition and exercise on collagen turnover in the extracellular matrix of the musculoskeletal system.

RECENT FINDINGS

Collagen turnover in the musculoskeletal system is faster than previously thought. Bone collagen synthesis is increased by feeding, whereas both muscle collagen and tendon are unresponsive. Exercise stimulates collagen synthesis in both muscle and tendon in an apparently coordinated manner. There are also sex differences and normal aging is associated with increased muscle collagen synthesis and reductions in bone collagen synthesis, particularly in mature bone collagen.

SUMMARY

Collagen turnover appears to be faster than previously thought and is regulated by feeding and exercise, in a tissue-specific manner. Further application of these approaches, coupled with measures of gene and protein expression, to measure the acute regulation of collagen, will lead to a better understanding of the physiology and pathophysiology of human collagen turnover. This is particularly important for developing new therapies to improve bone health and minimize tissue fibrosis.

Extracellular matrix remodeling--methods to quantify cell-matrix interactions

Tissue turnover during wound healing, regeneration or integration of biomedical materials depends on the rate and extent of materials trafficking into and out of cells involved in extracellular matrix (ECM) remodeling. To exploit these processes, we report the first model for matrix trafficking in which these issues are quantitatively assessed for cells grown on both native collagen (normal tissue) and denatured collagen (wound state) substrates. Human fibroblasts more rapidly remodeled denatured versus normal collagen type I to form new ECM. Fluxes to and from the cells from the collagen substrates and the formation of new ECM were quantified using radioactively labeled substrates. The model can be employed for the systematic and quantitative study of the impact of a broad range of physiological factors and disease states on tissue remodeling, integrating extracellular matrix structures and cell biology.

In vivo muscle amino acid transport involves two distinct processes

We have tested the hypothesis that transit through the interstitial fluid, rather than across cell membranes, is rate limiting for amino acid uptake from blood into muscle in human subjects. To quantify muscle transmembrane transport of naturally occurring amino acids, we developed a novel 4-pool model that distinguishes between the interstitial and intracellular fluid compartments. Transport kinetics of phenylalanine, leucine, lysine, and alanine were quantified using tracers labeled with stable isotopes. The results indicate that interstitial fluid is a functional compartment insofar as amino acid kinetics are concerned. In the case of leucine and alanine, transit between blood and interstitial fluid was potentially rate limiting for muscle amino acid uptake and release in the postabsorptive state. For example, in the case of leucine, the rate of transport between blood and interstitial fluid compared with the corresponding rate between interstitial fluid and muscle was 247 +/- 36 vs. 610 +/- 95 nmol.min(-1).100 ml leg(-1), respectively (P < 0.05). Our results are consistent with the process of diffusion governing transit from blood to interstitial fluid without selectivity, and of specific amino acid transport systems with varying degrees of efficiency governing transit from interstitial fluid to muscle. These results imply that changes in factors that affect the transit of amino acids from blood through interstitial fluid, such as muscle blood flow or edema, could play a major role in controlling the rate of muscle amino acid uptake.

Long- and short-term D-alpha-tocopherol supplementation inhibits liver collagen alpha1(I) gene expression

We analyzed the role of oxidative stress on liver collagen gene expression in vivo. Long- and short-term supplementation with the lipophilic antioxidant D-alpha-tocopherol (40 IU/day for 8 wk or 450 IU for 48 h) to normal C57BL/6 mice selectively decreased liver collagen mRNA by approximately 70 and approximately 60%, respectively. In transgenic mice, the -0.44 kb of the promoter and the first intron of the human collagen alpha1(I) gene were sufficient to confer responsiveness to D-alpha-tocopherol. Inhibition of collagen alpha1(I) transactivation in primary cultures of quiescent stellate cells from these transgenic animals by D-alpha-tocopherol required only -0.44 kb of the 5' regulatory region. This regulation resembled that of the intact animal following D-alpha-tocopherol treatment and indicates that D-alpha-tocopherol may act directly on stellate cells. Transfection of stellate cells with collagen-LUC chimeric genes allowed localization of an "antioxidant"-responsive element to the -0.22 kb of the 5' region excluding the first intron. These findings suggest that oxidative stress, independently of confounding variables such as tissue necrosis, inflammation, cell activation, or cell proliferation, modulates in vivo collagen gene expression.

Measurement of dermal collagen synthesis rate in vivo in humans

Accumulation of collagen produces organ dysfunction in many pathological conditions. We measured the fractional synthesis rate (FSR) of dermal collagen in five human volunteers from the increment of [13C]proline in detergent-soluble dermal collagen hydroxylated to hydroxyproline during a continuous infusion of L-[1-13C]proline. In these and eight other volunteers, we measured [13C]proline enrichment in skin aminoacyl-tRNA, skin tissue fluid amino acid, and plasma. The prolyl-[13C]tRNA enrichment was one-half that in tissue fluid proline and more than threefold less than in plasma. The FSR of dermal collagen was 0.076 +/- 0.063%/h (mean +/- SD), similar to previously reported rates for skeletal muscle contractile proteins and substantially slower than hepatically derived circulating proteins such as albumin or fibrinogen. We conclude that the FSR of human dermal collagen resembles that of other human proteins considered to display slow turnover. The current method for its measurement may be used to determine the regulation of collagen synthesis in other organs and disease states.

A specific quantitative assay for collagen synthesis by cells seeded in collagen-based biomaterials using sirius red F3B precipitation

The measurement of collagen synthesis by seeded cells in vitro is a prerequisite for the assessment of biocompatibility of many biomaterials. Existing methods are either complicated or not applicable to systems utilizing collagen-based materials, and the development of a rapid and simple technique would be an advantage. In the current paper, a method is described which relies on the radiolabelling of newly synthesized protein with [3H]-proline followed by specific precipitation of collagen using 1% sirius red dissolved in water. The results indicate that collagen binding to sirius red is unaffected by using water rather than picric acid as a solvent and the dye binds in a similar fashion to collagen type I, II and III. Cycloheximide treatment of the gels indicated that precipitated [3H]-proline was restricted to macromolecular protein. Collagenase treatment eradicated labelled precipitation formation when using 1% sirius red in water, indicating a high degree of specificity for collagen whilst specificity was poor when using 1% sirius red in picric acid. The method described is both simple and rapid and shows a high degree of specificity and sensitivity. For these reasons it is highly suited for the assessment of collagen synthesis by cells in collagen-based materials.

Glutamine increases collagen gene transcription in cultured human fibroblasts

We have previously shown that glutamine stimulates the synthesis of collagen in human dermal confluent fibroblast cultures (Bellon, G. et al. [1987] Biochim. Biophys. Acta, 930, 39-47). In this paper, we examine the effects of glutamine on collagen gene expression. A dose-dependent effect of glutamine on collagen synthesis was demonstrated from 0 to 0.25 mM followed by a plateau up to 10 mM glutamine. Depending on the cell population, collagen synthesis was increased by 1.3-to 2.3-fold. The mean increase in collagen and non-collagen protein synthesis was 63% and 18% respectively. Steady-state levels of alpha 1(I) and alpha 1(III) mRNAs, were measured by hybridizing total RNA to specific cDNA probes at high stringency. Glutamine increased the steady-state level of collagen alpha 1(I) and alpha 1(III) mRNAs in a dose-dependent manner. At 0.15 mM glutamine, collagen mRNAs were increased by 1.7-and 2.3-fold respectively. Nuclear run-off experiments at this concentration of glutamine indicated that the transcriptional activity was increased by 3.4-fold for the pro alpha 1(I) collagen gene. The effect of glutamine on gene transcription was also supported by the measurement of pro alpha 1(I) collagen mRNA half-life since glutamine did not affect its stability. Protein synthesis seemed to be required for the glutamine-dependent induction of collagen gene expression since cycloheximide suppressed the activation. The effect of glutamine appeared specific because analogues and/or derivatives of glutamine, such as acivicin, 6-diazo-5-oxo-L-norleucine, homoglutamine, ammonium chloride and glutamate did not replace glutamine. The influence of amino acid transport systems through plasma membrane was assessed by the use of 2(methylamino)-isobutyric acid and beta 2-aminobicyclo-(2.2.1)-heptane-2-carboxylic acid. The glutamine-dependent induction of collagen gene expression was found to be independent of transport system A but dependent on transport system L whose inhibition induced a decrease in pro alpha 1(I) collagen gene transcription by an unknown mechanism. Thus, glutamine, at physiological concentrations, indirectly regulates collagen gene expression.

Age-related alterations in collagen and total protein metabolism determined in cultured rat dermal fibroblasts: age-related trends parallel those observed in rat skin in vivo

The cultured fibroblast has been extensively used as a model system to study aging. However, few studies have examined the veracity of observations obtained in cultured fibroblasts aged in vitro to those made in animal tissues in vivo. This paper compares age-related alterations in collagen metabolism measured in cultured cells with previously reported results in the aging rat (Mays et al. (1991) Biochem. J. 276, 307-313). Age-related changes in collagen synthesis in rat skin fibroblasts in vitro over 30 population doublings were determined based on the production of hydroxy-[14C]proline. Degradation of newly synthesized collagen was based on the appearance of free hydroxy-[14C]proline in the culture system. Total protein synthesis rates were based on the incorporation of [14C]proline into proteins. In vitro rates of collagen synthesis decreased 5-fold over 30 population doublings (P < 0.05). Degradation of newly synthesized collagen increased from 33.0 +/- 0.8% (n = 4, SEM) to 45.2 +/- 1.1% (n = 4; P < 0.05) over the same period, with a maximum after 25 population doublings of 55.8 +/- 1.1% (n = 4). Total protein synthesis rates decreased by one-half over 30 population doublings (P < 0.05). The results indicated that collagen production decreased as cells aged in vitro and that this was due to both changes in synthesis and degradation. The results demonstrate that age-related alterations in collagen and total protein metabolism of skin fibroblasts in culture were similar to those reported previously for skin in vivo, suggesting that for studies of these processes, fibroblasts in culture provide an appropriate model.

Source of amino acids for tRNA acylation. Implications for measurement of protein synthesis

Estimates of protein-synthesis rates using radioisotopes require accurate measurement of the specific radioactivity of the label in protein and in the precursor pool over time. Although the extracellular and intracellular pools of amino acids are easiest to sample, the tRNA pool is the direct precursor and is the appropriate pool for sampling. To test if the intracellular or extracellular pools reflect the tRNA specific radioactivity, a chicken macrophage cell line was incubated in medium containing either 0.23 mM-leucine and 14.5 microCi of [3H]leucine (tracer dose) or 2.3 microM-leucine plus 145.0 microCi of [3H]leucine (flooding dose). At both leucine levels, the tRNA specific radioactivity reached a plateau quickly, but did not equilibrate with either the extracellular or intracellular specific radioactivity within 30 min, and remained closer to that of protein. In a second experiment, proteins in chicken macrophages were labelled with [3H]leucine for 2 days. Labelling medium was removed, and the cells were washed free of residual free [3H]leucine and incubated with medium containing either 0.23 mM- or 2.3 mM-leucine (unlabelled). The specific radioactivity of leucyl-tRNA leucine reached a plateau within 2 min and remained considerably closer to that in the protein than that in intracellular or extracellular pools for at least 60 min. These results suggest that amino acids from protein degradation are a primary source for charging tRNA. When protein-synthesis rates are estimated by label incorporation, use of extracellular or intracellular specific-radioactivity values result in a marked underestimation.

Isolation of aminoacyl-tRNA and its labeling with stable-isotope tracers: Use in studies of human tissue protein synthesis

We isolated aminoacyl-tRNA (60-70% yield) from human and rat tissues and measured, by GC/MS, its labeling in vivo by [15N]- and [13C]leucine. Tracer dilution artifacts seemed unlikely since, after infusion of L-[1-13C,15N]leucine into rats, (i) muscle leucyl-tRNA labeling exceeded tissue free leucine labeling, (ii) values were largely unaffected by storing over 5 min at 22 degrees C, and (iii) L-[2,4,5-methyl-13C]leucine was not incorporated into leucyl-tRNA during homogenization. Leucyl-tRNA labeling in liver and muscle suggested charging from extra- and intracellular pools: e.g., after infusing L-[1-13C,15N]leucine, rat muscle tissue free leucine 13C labeling (8.97 +/- 0.30 atom % excess) exceeded that by 15N (3.37 +/- 0.33 atom % excess), and both were significantly lower (P less than 0.02) than venous plasma (13C, 12.1 +/- 1.8; 15N, 5.54 +/- 0.6 atom % excess) indicating tracer dilution by transamination and by proteolysis; however, leucyl-tRNA labeling by either isotope (13C, 10.26 +/- 0.50; 15N, 4.72 +/- 0.72 atom % excess) was significantly above mixed tissue free leucine (P less than 0.05). Labeling of leucyl-tRNA in human erector spinae muscle (obtained after preoperative L-[1-13C]leucine infusion) was, at 4.98 +/- 0.43 atom % excess, lower (27%) than venous plasma leucine (P less than 0.05) and intermediate between muscle free leucine (9% lower; P less than 0.01) and venous alpha-ketoisocaproate (11% higher; P less than 0.02). Human placental leucyl-tRNA labeling (after predelivery tracer infusion) was 37% lower (P less than 0.05) than maternal uterine vein labeling but not significantly different from placental free leucine or umbilical arterial leucine.

d-alpha-tocopherol inhibits collagen alpha 1(I) gene expression in cultured human fibroblasts. Modulation of constitutive collagen gene expression by lipid peroxidation

Ascorbic acid stimulates collagen gene transcription in cultured fibroblasts, and this effect is mediated through the induction of lipid peroxidation by ascorbic acid. Quiescent cultured fibroblasts in the absence of ascorbic acid have a high constitutive level of collagen production, but the mechanisms of collagen gene regulation in this unstimulated state are not known. Because lipid peroxidation also occurs in normal cells, we wondered if lipid peroxidation plays a role in the regulation of basal collagen gene expression. Inhibition of lipid peroxidation in cultured human fibroblasts with d-alpha-tocopherol or methylene blue decreased the synthesis of collagen, the steady-state levels of procollagen alpha 1(I) mRNA and the transcription of the procollagen alpha 1(I) gene. This effect on collagen gene expression was selective and not associated with cellular toxicity. Thus, these experiments suggest a role for lipid peroxidation in the modulation of constitutive collagen gene expression.

Age-related changes in collagen synthesis and degradation in rat tissues. Importance of degradation of newly synthesized collagen in regulating collagen production

During developmental growth, collagens are believed to be continuously deposited into an extracellular matrix which is increasingly stabilized by the formation of covalent cross-links throughout life. However, the age-related changes in rates of synthetic and degradative processes are less well understood. In the present study we measured rates of collagen synthesis in vivo using a flooding dose of unlabelled proline given with [14C]proline and determining production of hydroxy[14C]proline. Degradation of newly synthesized collagen was estimated from the amount of free hydroxy [14C]proline in tissues 30 min after injection. Collagen fractional synthesis rates ranged from about 5%/day in skeletal muscle to 20%/day in hearts of rats aged 1 month. At 15 months of age, collagen fractional synthesis rates had decreased markedly in lung and skin, but in skeletal muscle and heart, rates were unchanged. At 24 months of age, synthesis rates had decreased by at least 10-fold in all tissues, compared with rates at 1 month. The proportion of newly synthesized collagen degraded ranged from 6.4 +/- 0.4% in skin to 61.6 +/- 5.0% in heart at 1 month of age. During aging the proportion degraded increased in all tissues to maximal values at 15 months, ranging from 56 +/- 7% in skin to 96 +/- 1% in heart. These data suggest that there are marked age-related changes in rates of collagen metabolism. They also indicate that synthesis is active even in old animals, where the bulk of collagens produced are destined to be degraded.

In vivo collagen turnover during development of thyroxine-induced left ventricular hypertrophy

Cardiac fibroblasts synthesize large amounts of procollagens, yet only a small fraction of mature collagens accumulate in the extracellular matrix. To determine the roles of intracellular degradation of newly synthesized procollagens and extracellular degradation of mature collagens during normal growth and during thyroxine-induced left ventricular hypertrophy, in vivo left ventricular procollagen synthetic rates were assessed in control rats and rats treated with L-thyroxine for 1, 2, 4, and 8 wk (1 mg.kg-1.day-1). A modification of the flooding infusion method was developed using measurements of cardiac prolyl-tRNA, and tissue-free and protein-bound hydroxyproline specific radioactivities 60 min after intravenous administration of a massive dose of [3H]proline. Degradative rates of newly synthesized procollagens and mature collagens were then derived as the difference between rates of procollagen synthesis and collagen accumulation. Left ventricular procollagen synthetic rates were markedly increased after 1 wk of hormone administration (256 +/- 16 and 166 +/- 13 micrograms/day per left ventricle for thyroxine-treated and control animals, respectively; P less than 0.01). An even greater increase in procollagen synthetic rates was observed after 8 wk (438 +/- 46 and 202 +/- 18 micrograms/day for thyroxine-treated and control animals, respectively; P less than 0.01). Despite increased procollagen synthesis, disproportionate accumulation of fibrillar collagens (assessed as the relative concentration of protein-bound hydroxyproline in left ventricular tissue) did not occur. Derived left ventricular degradative rates for newly synthesized procollagens as well as for mature collagens were increased in thyroxine-treated animals. Increased procollagen synthesis, enhanced flux of newly synthesized procollagens through intracellular degradative pathways, and extensive extracellular matrix remodeling without disproportionate collagen accumulation are characteristics of this form of "physiological" left ventricular hypertrophy.

The effect of transforming growth factor beta on rates of procollagen synthesis and degradation in vitro

Transforming growth factor beta (TGF beta) is known to stimulate procollagen production and steady-state levels of procollagen mRNAs, but its ability to affect post-translational processing of procollagen has been little studied. This paper demonstrates the application of recently developed ultrasensitive methods for measuring hydroxyproline to assess rates of procollagen synthesis and degradation in vitro with and without TGF beta. Foetal rat fibroblasts synthesized 8.63 +/- 0.21 pmol hydroxyproline/micrograms DNA per h, which corresponds to approx. 40 molecules of procollagen/cell per s. Addition of TGF beta to cultures increased total amounts of procollagen synthesized and degraded by 112% and 82%, respectively, but there was a significant decrease in the proportion of procollagen degraded (control, 38.0 +/- 1.1%; TGF beta, 32.3 +/- 0.9%; P less than 0.005). This study demonstrates a novel mechanism which may contribute to the TGF beta-induced increase in procollagen production by fibroblasts.

Application of high-pressure liquid chromatography to studies of collagen production by isolated cells in culture

Techniques for assessing collagen production by cells in culture are usually based on evaluation of uptake of radiolabeled proline into collagen. Although simple in theory, this approach is often flawed because of uncertainties concerning the specific activity of labeled proline in the precursor pool for collagen synthesis. An alternative approach is to assess collagen production directly by measuring hydroxyproline in proteins secreted by cultured cells, although this has been difficult, due to the insensitivity of the methods available. Here we apply high-pressure liquid chromatography using reverse-phase elution of 7-chloro-4-nitrobenzo-2-oxa-1,3-diazole derivatives of hydroxyproline to measure collagen production by fibroblasts. The method is easy to perform and allows quantitation of hydroxyproline down to 5 pmol, making it applicable to fibroblasts in 12-well culture plates. Collagen production was shown to be constant over a period of 24 h, with a mean rate of 391 +/- 18 (SE n = 14) ng collagen/10(6) cells/h. Similar values were obtained using thin-layer chromatography and an enzyme-linked immunosorbent assay for type I collagen, but these techniques were judged to be less convenient and required additional assumptions compared with the technique described here in full.

Minocycline-treatment of diabetic rats normalizes skin collagen production and mass: possible causative mechanisms

Daily minocycline-treatment of streptozotocin-induced diabetic rats not only prevented a diabetes-caused atrophy of skin collagen mass (10-mos old rats), but also normalized skin collagen mass to match that of growing (ca. 1%/d) non-diabetic controls (4- and 5-mos old rats). The causative mechanism by which minocycline-treatment normalizes skin collagen mass must, in part, be related to a general anabolic effect on growth (body weight) because the effect on skin collagen mass correlates strongly to that on body weight. Consequently, a minocycline-stimulated increase of a systemic factor (such as insulin-like growth factor) is not unlikely. The anabolic effect of minocycline-treatment of diabetic rats is also expressed as a normalized cellular ribosome mass (an index of total protein synthetic capacity) and a normalized absolute rate of collagen production. (Calculation of an absolute rate was justified by an apparent maximum saturation of the prolyl-tRNA pool(s) of skin, maximum saturation obtained by the pool-flooding approach). The normalized skin ribosome amount does not, however, explain a selective effect of minocycline-treatment on collagen production as opposed to that for non-collagen protein, this selective effect measured as relative collagen production. To explain such selectivity, the inhibition of diabetes-induced excess skin collagenase activity seems unlikely. (This inference is based on results from a preliminary study indicating that recently [less than 2 h] synthesized collagen is not degraded by the excess collagenase in skin of diabetic rats). Thus, the principal collagen fraction acted on by pathologically excess collagenase might be collagen at a later stage (greater than 2 h after synthesis) in its life cycle. (Another possibility for the selective effect of minocycline on collagen production, as yet untested, is reduced intracellular procollagen degradation.) Overall, this is the first study aimed at discerning the mechanism(s) by which minocycline-treatment enhances the rate of collagen production in tissues of a diabetic rat. For future studies, the extent to which the positive effect on growth, ribosome mass, and rate of collagen production contributes to the change of collagen mass must, along with the known minocycline-inhibition of collagenase activity, be quantified. Such quantification is a prerequisite for evaluating the chemotherapeutic efficacy of minocycline-treatment on collagen-degradative diseases.

Parathyroid hormone alteration of free and tRNA-bound proline specific activities in cultured mouse osteoblast-like cells

The effects of the synthetic amino-terminal fragment of parathyroid hormone [bPTH-(1-34)] on proline uptake and on the specific activities of intracellular free proline and tRNA-bound proline were studied in confluent primary cultures of osteoblast-like cells isolated from neonatal mouse calvaria. Pretreatment of cells for 4 hours with 24 nM bPTH-(1-34) increased subsequent proline uptake by approximately 50-60%; also increased were the specific activities of both intracellular free proline and tRNA-bound proline when [3H]proline was included in the extracellular uptake solution. Specific activities of the free and tRNA-bound proline pools remained elevated after proline uptake times of as long as 30 minutes and 120 minutes, respectively. These results indicate that experiments in which radiolabeled proline is used to evaluate PTH-induced protein synthesis in bone cells must be interpreted cautiously, since apparent changes in protein synthesis might actually reflect, at least in part, PTH-induced changes in the specific activities of precursor pools.

Effects of puromycin and hydroxynorvaline on net production and intracellular degradation of collagen in human fetal lung fibroblasts

Amino acid substitutions in collagen that impair folding of the triple helix result in significant increases in intracellular degradation of newly synthesized collagen. We have studied the effects of agents that cause other kinds of defects in collagen: hydroxynorvaline, a threonine analog that interferes with association of pro-alpha chains; and puromycin, an antibiotic that causes premature release of nascent polypeptides. cis-Hydroxyproline and cycloheximide, whose effects on collagen synthesis and degradation have already been studied and reported, were employed as reference compounds. Human fetal lung fibroblasts were used in these experiments. All the agents inhibited total protein production, and all except cycloheximide inhibited percentage collagen production. Intracellular collagen degradation was increased in cultures exposed to puromycin, hydroxynorvaline, and cis-hydroxyproline, but not in cultures exposed to cycloheximide. These results suggest that pro-alpha chains that were either unassociated (due to hydroxynorvaline) or shortened (due to puromycin) were recognized as abnormal and degraded to the same extent as chains that contained cis-hydroxyproline. However, the increases in degradation could not account completely for the decreases in collagen production (except when cis-hydroxyproline was used at low concentrations). These findings indicate that, in addition to rendering newly synthesized procollagen molecules or partial polypeptide chains more susceptible to intracellular degradation, puromycin, hydroxynorvaline, and cis-hydroxyproline significantly inhibited collagen synthesis.

Biosynthesis of collagen crosslinks. II. In vivo labelling and stability of lung collagen in rats

Rat lung collagen was labelled in vivo by a single intraperitoneal injection of [3H]lysine at several key timepoints in lung development: days 11 (alveolar proliferation), 26 (start of equilibrated growth), 42 (end of equilibrated growth), and 100 (adult lung structure present). The rates of deposition of labelled hydroxylysine and the difunctional, Schiff base-derived crosslinks hydroxylysinonorleucine (HLNL) and dihydroxylysinonorleucine (DHLNL) were quantified. We also measured total lung content of the trifunctional, mature crosslink hydroxypyridinium (OHP) in these same animals. While the relative rates of accumulation of labelled collagen [3H]hydroxylysine differed by a factor of about 6 at the different times of injection of labelled precursor, quantitative and qualitative patterns of collagen crosslinking were very similar at all of the lung developmental stages studied. Furthermore, there was little or no breakdown of the lung collagen pool as defined by the presence of labelled crosslinks; changes in lung DHLNL content could be completely accounted for by its maturation to OHP, regardless of the age of the rats when injected with the radioactive precursor. We conclude that mature, crosslinked collagen in the lungs of rats, which is obligatorily an extracellular pool, is not being degraded at a measurable rate. Therefore, studies of others that have shown apparent high rates of breakdown of newly synthesized collagen in lungs of whole animals using different methods are probably not reflective of the metabolic fate of total lung collagen, and may indicate that degradation of normal lung collagen occurs predominantly or exclusively intracellularly.

Parathyroid hormone regulation of proline uptake by cultured neonatal mouse osteoblastlike cells

Regulation of proline uptake by the synthetic amino-terminal fragment of bovine parathyroid hormone [bPTH-(1-34)] has been studied in confluent primary cultures of osteoblastlike cells isolated from neonatal mouse calvaria. The initial velocity of proline transport was increased by 85% in cultures treated with 24 nM bPTH-(1-34) for 6 h. Cycloheximide, at a concentration that inhibited protein synthesis by 97%, did not prevent this effect. However, adding the inhibitor during the first 1-2 h of hormone treatment did significantly reduce its magnitude. Exposure of cells to the inhibitor alone caused a time-dependent decrease in the basal rate of proline uptake. In the absence of protein synthesis, the maximal velocity (Vmax) of transport was 60% greater in cultures treated with 24 nM bPTH-(1-34) than in controls. The concentration of proline at which half-maximal transport occurred (Km) was unchanged. In cultures treated with cycloheximide alone, proline transport decreased as a first-order exponential with a half-life of 250-280 min. Parathyroid hormone significantly reduced this decline, increasing the half-life of proline transport activity about fourfold. These effects were duplicated by 1 mM DBcAMP. It is concluded that bPTH-(1-34) increases proline transport in osteoblastlike cells by decreasing the degradation of amino acid transport system A proteins. The hormone may also affect the synthesis of these molecules. These effects appear to be mediated by cAMP.

Nonisotopic evaluation of collagen in fibroblasts cultures

A method for the evaluation of collagen concentrations in the medium of fibroblasts in culture was developed. Collagen was precipitated with other proteins by addition of ethanol and hydrolyzed by 6 M HCl. The primary amino acids of the hydrolyzate were reacted with o-phthalaldehyde (OPA) and secondary amino acids (Pro, Hyp) were derivatized with 9-fluorenylmethyl-chloroformate (FMOC-Cl). The mixture was separated by isocratic HPLC on a reverse-phase column. FMOC-derivatives were detected by fluorometry, whereas OPA-derivatives were not. This method is suitable for the monitoring of collagen metabolism in fibroblast cultures exposed to various effectors.

Effects of prostaglandin E1 on collagen production and degradation in human fetal lung fibroblasts

We examined the effects of prostaglandin E1 on the production and degradation of collagen in human fetal lung fibroblasts. Percentage collagen production was determined by incubating confluent cultures for 6 h with [3H]proline and either [14C]glycine or [14C]leucine and measuring the relative amounts of radioactivity incorporated into collagenase-sensitive and collagenase-insensitive material. Percentage collagen degradation was determined by measuring hydroxy[14C]proline in a low-molecular-weight fraction relative to total hydroxy[14C]proline. Prostaglandin E1, when present at a concentration as low as 0.25 micrograms/ml, reduced net collagen production by a factor of one-half, from 8 +/- 2 to 4 +/- 1% (P less than 0.05). In contrast, the change in percentage degradation was relatively gradual, rising steadily from the control value of 15 +/- 2 to 33 +/- 2% at 4 micrograms/ml (P less than 0.05). The increase in degradation, while significant, could not account for the total decrease in collagen production. We conclude that prostaglandin E1 exerts its inhibitory effect on collagen production in two essentially independent ways: lowering the rate of synthesis and increasing intracellular degradation. However, the decrease in synthesis is greater than the increase in degradation.

Increased production of collagen in vivo by hepatocytes and nonparenchymal cells in rats with carbon tetrachloride-induced hepatic fibrosis

We have shown, using the proline:ornithine dual label method, that in normal rats, hepatocytes contribute in vivo about 80 to 90% of the newly synthesized hepatic collagen. In order to quantify the contribution of hepatocytes and nonparenchymal cells to collagen synthesis in vivo in hepatic fibrogenesis, rats with CCl4-induced liver fibrosis were given [5(3H)]proline and [14C]ornithine intraperitoneally. About 80% of the 14C in albumin and transferrin was present as arginine, following conversion of [14C]ornithine via the urea cycle. In contrast to hepatocyte proteins, in nonparenchymal cells and serum a negligible percentage of the radioactivity was present as [14C]arginine. These combined findings indicate that, in spite of the hepatocellular damage, the labeling of hepatocyte proteins was efficient and specific, validating the use of the proline:ornithine method in this experimental model of hepatic fibrosis. We calculated the [3H]proline/[14C]arginine ratio in hepatic collagen (after correcting for the relative frequencies of amino acids) as a percentage of the same ratio in either albumin or transferrin, the index hepatocyte proteins. In this experimental model, during active fibrogenesis, both hepatocytes and nonparenchymal cells increase their production of collagen 2-fold when compared to normal animals, and hepatocytes produce the majority of the newly synthesized hepatic collagen.

Alterations in type II pneumocytes cultured after partial pneumonectomy

Type II pulmonary epithelial cells prepared from the lungs of normal rats were compared in primary culture to cells derived from the right lung of animals subjected previously to left pneumonectomy (PNX). Studies were initiated on the sixth post-PNX day, during the rapid phase of compensatory right lung growth. After 24 h in vitro, PNX cells were 30-40% larger than controls and contained 20-50% more DNA. The magnitude of these differences was dependent on serum concentration (fetal calf serum; 1 and 10%, respectively) and, under most conditions, decreased as culture time was extended to 48 or 72 h. Incorporation of [3H]thymidine into DNA was also elevated (greater than 50%) on the first culture day in the PNX group at both serum levels, and remained so through day 3 at low serum, as thymidine incorporation became more rapid in all cells. Similarly, rates of spermidine uptake were elevated in cells prepared from lungs of PNX animals on culture day 1, but this effect too was lost by day 3. Thus type II pneumocytes isolated from the lungs of PNX rats exhibit metabolic changes typical of accelerated cell growth at early intervals of primary culture in vitro. Although these changes are lost as culture time is extended and the cells lose differentiated characteristics, the results suggest that such pneumocytes may provide useful information regarding factors which regulate compensatory growth of lung tissue.

Matrix deposition and extracellular processing of newly synthesized collagens in the isolated perfused rat lung

We have examined the matrix deposition and proteolytic processing of newly synthesized interstitial and basement membrane collagens in the isolated perfused adult rat lung. Isolated, perfused, and ventilated lungs were labeled for up to 4 h with radiolabeled proline. Collagens were partially purified from homogenates by salt fractionation and ion exchange chromatography and examined by sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE). The major collagenous species were identified as types I, III, and IV collagen by peptide mapping and indirect immunoprecipitation assays. Whereas extraction with neutral salts recovered radiolabeled types I and III collagen, extraction of the neutral salt residue with 2 M guanidine-HCl preferentially recovered types III and IV collagen. Reextraction of the guanidine-HCl residue in the presence of dithiothreitol selectively recovered type IV procollagen (PC) and covalently cross-linked aggregates of type IV chains. In pulse-chase experiments we observed extensive conversion of type I PC to collagen during a 4-h chase. Although type III PC was efficiently converted to p-collagen, only small amounts of fully processed chains were identified. Type IV PC did not undergo detectable proteolytic processing. The isolated perfused rat lung should prove useful for further studies of lung collagen metabolism.

Collagen synthesis in cultured myoblasts and myotubes from patients with Duchenne muscular dystrophy

Collagen synthesis was studied in cultured myoblasts and myotubes from 4 patients with Duchenne muscular dystrophy (DMD) and 4 control persons. Incorporation of [3H]proline into collagen of DMD cells and control cells was not significantly different. The same types of collagen, i.e., type I and type III were synthesized by myogenic cells from DMD patients and controls. In contrast to others, we could not obtain evidence for an increase in the degree of prolyl hydroxylation in collagen of the DMD muscle cells.

Role of laminin in maintenance of type II pneumocyte morphology and function

Loss of differentiated function by type II pneumocytes plated on plastic surfaces was demonstrated by decreased lamellar body content, increased cellular protein, and rapid cellular flattening, changes that were retarded modestly by plating cells on laminin-coated surfaces. Laminin surfaces also inhibited [3H]thymidine (THM) incorporation into cellular DNA by 40% compared with plastic at 40 h, but did not alter an additional mitogenic effect of rat serum over fetal calf serum. In contrast, cells plated on the laminin-rich basement membrane-like gel formed from an extract of EHS mouse sarcoma, matrix gel (MG), maintained a high content of intracellular lipids in lamellar inclusions and retained a rounded morphology for at least 3 days. MG markedly inhibited THM incorporation and morphological changes when cells were cultured on this surface or when MG was formed over cells initially plated on plastic for various intervals. The importance of the laminin component of MG was demonstrated when these surfaces were pretreated with a highly specific antilaminin serum. Type II cells commenced flattening on the treated MG surface, and THM incorporation increased with the same time course as did control cells on plastic. The data suggest that short-term culture and study of differentiated type II pneumocytes may require a laminin-rich substratum. THM incorporation into type II cell DNA provides an important early and sensitive index of cell-basement membrane interaction and subsequent maintenance of function.

Compartmentalization of proline pools and apparent rates of collagen and non-collagen protein synthesis in arterial smooth muscle cells in culture

Rates of collagen and non-collagen protein synthesis in rabbit arterial smooth muscle cells (SMC) were determined by using the specific (radio)activity of [3H]proline in the extracellular, intracellular, and prolyl-tRNA pools. The intracellular free proline specific activity was only 25% of the extracellular value in cultures incubated for 12 h in 0.25 mM-proline. The specific activity of prolyl-tRNA was less than 10% of the extracellular specific activity. Increasing the extracellular proline concentration 10-fold (to 2.5 mM), while keeping the extracellular specific activity of proline constant, resulted in equilibration of the specific activities of intracellular and extracellular free proline, but the specific activity of prolyl-tRNA remained at less than 10% of the extracellular specific activity. Therefore, calculated rates of collagen and non-collagen protein synthesis were greatly underestimated using the intracellular or extracellular specific activity of proline. SMC were also incubated with 0.1 mM-[14C]ornithine in 0.25 nM or 2.5 mM non-labelled proline to examine synthesis de novo of proline and prolyl-tRNA from ornithine. In SMC cultures containing 0.25 mM unlabelled proline, the specific activity of intracellular ornithine was approx. 45% of the extracellular specific activity, due to the production of unlabelled ornithine. The specific activity of ornithine-derived intracellular free proline in SMC incubated with 2.5 mM-proline was significantly lower than in SMC incubated in 0.25 mM-proline, due to the influx of unlabelled proline. However, a corresponding difference in the specific activity of [14C]prolyl-tRNA between SMC in 0.25 mM- or 2.5 mM-proline was not observed. Ornithine-derived [14C]proline was incorporated into proteins in a manner different from that of exogenously added radiolabelled proline. A much higher proportion of the proline synthesized de novo was channelled into collagen synthesis relative to total protein synthesis. Together, these results show that intracellular proline pools are highly compartmentalized in arterial SMC. They also suggest that proline synthesized from ornithine may enter a prolyl-tRNA pool separate from that of proline entering from the extracellular medium.

Proteolysis in cultured cells during prolonged serum deprivation and replacement

Cells in culture show a series of changes in intracellular protein degradation in response to serum deprivation and replacement that are similar to alterations in degradation in tissues of starved and refed animals. Rates of intracellular protein degradation are increased in confluent cultures of IMR-90 human diploid fibroblasts when deprived of serum, but this enhanced proteolysis is transient. By 24-48 h, rates of protein degradation decline to values comparable to or below those for cells incubated in the presence of serum. Longer serum deprivation leads to further reductions in proteolysis. The reduced proteolysis after long-term deprivation cannot be explained by experimental artifacts or by gradual depletion of glucocorticoids or thyroid hormones from cells. Readdition of serum to deprived cells that are still in the enhanced phase of proteolysis restores degradation rates to values comparable to those in nondeprived cells. However, in cells deprived of serum for 24-48 h or longer, readdition of serum to the medium results in a marked reduction in proteolysis to rates below those observed in nondeprived cells. These responses of cultured cells to long-term serum deprivation and readdition may be of considerable physiological importance in that the proteolytic responses of tissues in starved and refed animals may be at least partially due to mechanisms operating at the cellular level.

Inhibition of cell proliferation by interferons. Relative contributions of changes in protein synthesis and breakdown to growth control of human lymphoblastoid cells

Treatment of the Daudi line of human lymphoblastoid cells with concentrations of human interferons within the physiological range progressively inhibits cell proliferation over 1-4 days. Rigorous measurement of the overall rate of protein synthesis during this period, using a concentration of [3H]phenylalanine sufficient to equalize the specific radioactivity of intracellular and extracellular precursor pools, shows that protein synthesis becomes progressively inhibited as the growth inhibition develops. There is a strong correlation between inhibition of amino acid incorporation and inhibition of cell proliferation. In contrast, we find no evidence for any increase in protein degradation rate under these conditions. These results suggest that interferon treatment of susceptible cells can inhibit protein synthesis even in the absence of virus infection and that this inhibition is of a sufficient magnitude to account for the anti-proliferative effect.

Hepatocyte collagen production in vivo in normal rats

Although hepatocytes produce collagen in vitro, their contribution to hepatic collagen synthesis in vivo is unknown. To answer this question, we injected rats intraperitoneally with [3H]proline and [14C]ornithine. [3H]Proline labeled prolyl-t-RNA in both hepatocytes and nonparenchymal cells. In contrast, [14C]ornithine was rapidly converted to [14C]arginine via the urea cycle only in hepatocytes, labeling arginyl-t-RNA. Approximately 60% of the 14C in albumin and transferrin was present as arginine while the remainder was found in proline and related amino acids. As expected for proteins that have the same proline/arginine ratio and that are produced solely by the hepatocyte, the [3H]proline/[14C]arginine ratio was very similar in albumin and transferrin. Conversely, in nonparenchymal cells a negligible percentage of 14C was present as arginine. A sizeable percentage of the 14C in hepatic collagen was present as arginine; given the greater proline(+hydroxyproline)/arginine ratio in hepatic collagen, our data indicate that in normal rats, hepatocytes contribute most of newly synthesized hepatic collagen.

Cell culture approaches to the analysis of glomerular inflammation

In this paper, we have attempted to provide an overview of the methods and findings of a large number of investigators who have dealt with an analysis of the glomerular inflammatory response using tissue culture techniques. These observations represent only a beginning. With the growing interest in this aspect of kidney disease, it is to anticipated that many further advancements in the understanding of the cell biology of the glomerulus are forthcoming. The translation of this fundamental information into new diagnostic and therapeutic modalities is an exciting challenge to investigative nephrology.

Evidence for a special relationship between proteolysis and single cell necrosis

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

The control of fibrogenesis: stimulation and suppression of collagen synthesis in the chick chorioallantoic membrane with fibrin degradation products, wound extracts and proteases

The chick chorioallantoic membrane model (CAM) has previously been used to demonstrate cell proliferation, characteristic of both angiogenesis and fibrogenesis, after exposure to fibrin degradation products. This model has now been adapted for quantitative in vivo assay of collagen polypeptide synthesis and prolyl hydroxylase activity. The CAM exhibits oscillations in the level of labelled collagen, a pattern attributable to rapid intracellular degradation and proline recycling following a brief labelling period. Both collagen synthesis and prolyl hydroxylase activity are stimulated by fibrin degradation products (less than 50 000 MW). Such stimulation occurs by 3 h and precedes the rise in general protein synthesis. Extracts of healing mouse skin wounds, rich in proteases, inhibited collagen synthesis, as did pure plasmin. Conversely, stimulation was achieved when proteolytic activity was neutralized by soybean trypsin inhibitor. These findings help to explain the observation that fibroblasts and endothelial cells proliferate and migrate centrally in an inflammatory lesion without depositing collagen, whilst in a milieu of high proteolytic activity. More peripherally, where proteases are inactivated by antiproteases in inflammatory exudate, such cell movement ceases and collagen deposition is observed.

Comparison of in vitro and in vivo rates of collagen synthesis in normal and damaged lung tissue

The rate of collagen synthesis was measured in vivo and in vitro in both normal and damaged mouse lung tissue. Acute lung damage was induced by the administration of butylated hydroxytoluene (BHT). The production of labeled hydroxyproline, following the administration of labeled proline, was used as an index of collagen production. Total and labeled hydroxyproline in normal and damaged lung tissue were solubilized equally following digestion with purified collagenase. Assuming that the extent of hydroxylation was not altered, this indicated that hydroxyproline was an accurate index of collagen content and production in damaged as well as normal lung tissue. The quantities of hydroxyproline formed at various times both in vivo and in vitro were calculated from the specific activity of free proline in lung tissue. The specific activity of free proline in normal and damaged lung tissue remained constant in vivo for at least 90 minutes after the intravenous injection of labeled proline. Hydroxyproline production was a linear function of time for up to 90 minutes in vivo and three hours in vitro. The in vivo rate of hydroxyproline production was significantly greater than the in vitro rate in lung tissue from similarly treated mice. The difference ranged from five-fold in normal lung tissue to eight-fold in lung tissue damaged by the administration of BHT. Comparable differences were seen between the in vivo and in vitro rates of non-collagen protein synthesis. Despite these differences in rates, the percentage of total protein synthesis committed to collagen in vivo was the same as in vitro in normal lung, and identical increases were seen in damaged lung. These data show that in vivo rates of both collagen and non-collagen protein synthesis are significantly higher than those measured in mouse lung tissue in vitro. Although the relative increases in collagen synthesis that occur in response to lung damage are larger in vivo, measurements of collagen synthesis in vitro do accurately reflect the general changes that accompany acute lung damage.

Protein synthesis in perfused rat lungs: determinations based on incorporation of radioactive proline

Functional compartmentation and metabolism of radioactive proline was evaluated to define conditions under which synthesis of lung proteins could be measured accurately based on proline incorporation. Rat lungs were perfused with Krebs-Henseleit bicarbonate buffer equilibrated with O2/N2/CO2 (20:75:5) and containing 4.5% (w/v) bovine serum albumin, 5.6 mM glucose and amino acids at plasma levels. Intracellular proline increased linearly as perfusate proline concentration was increased from 108 microM, the plasma level, to 540 or 1080 microM. At each concentration, the pool of proline which provided precursors to protein synthesis rapidly reached a steady-state specific radioactivity, but when extracellular proline was 108 microM, this pool was diluted significantly by proline from endogenous sources. At 540 or 1080 microM extracellular proline, the specific radioactivities of perfusate and intracellular proline approached equality and rates of protein synthesis calculated based on the specific radioactivity of extracellular proline compared favorably with those calculated from the specific radioactivity of phenylalanyl-tRNA. Similar results were obtained in lungs of two groups of rats in which intracellular proline concentration differed 3-fold. Thus, the contribution of endogenous proline to the pathway of protein synthesis was minimized when extracellular proline was present at high concentration. Under this condition, calculations of protein synthesis based on proline incorporation were most accurate.

Comparison of the use of isotopic proline vs leucine to measure protein synthesis in cultured fibroblasts

Compartmentation of the amino acid precursor pools for protein synthesis in cultured cells can substantially complicate measurements of synthesis rates. This is particularly true for nonessential amino acids such as proline, an amino acid often used in isotopic form to measure collagen synthesis. We have made a comparative study of this problem in cultured IMR-90 fibroblasts using isotopic proline and leucine to measure total protein and collagen synthesis. 3H-leucine in the extracellular (EC) medium equilibrates with tRNA-leucine at an EC concentration of 0.4 mM in both dividing and stationary cells. Thus, under these experimental conditions there is no complicating compartmentation of leucine for protein synthesis. Equilibration of EC and tRNA-bound 3H-proline, however, does not occur even when the EC concentration is in the mM range, based upon simultaneous measurements of synthesis rates using 3H-proline and 3H-leucine together. Furthermore, significant changes in EC proline concentration and specific activity occur over short time intervals (2 hr) if the initial EC proline concentration is below 0.2 mM. Thus, the use of isotopic proline to measure protein synthesis introduces substantial interpretive problems. Serum deprivation causes changes in both total collagen synthesis and the percent of protein synthesis devoted to collagen when measured with either 14C-leucine or 3H-proline. At the same time, isotopic proline remains the better choice for measuring percent collagen synthesis.

A study of collagen metabolism in cell cultures by fluorometric determination of proline and hydroxyproline

A technique of derivatization of proline (Pro) and 4-hydroxyproline (Hyp) by 7-chloro-4-nitrobenzo-2-oxa-1,3-diazole permitted the measurement of Pro and Hyp radioactivities, concentrations, and specific activities in the main fractions separated from cultures of fibroblast cells (extracellular collagen and non-collagen proteins, intracellular free Pro and Hyp, Pro- and Hyp-containing peptides, procollagen, and non-collagen proteins). The evaluation of collagen in the medium was obtained from as few as 10(4) cells. The method might advantageously replace [14C] Pro or [3H] Pro incorporation studies. It permits measurement of the size of the Pro pool and the amount of peptides formed by intracellular catabolism of collagen. It demonstrates that the time necessary for a full equilibration of intracellular Pro and intracellular collagen is longer than is generally believed. It avoids the uncertainties of protein labelling, which may vary with uncontrolled variations of the intracellular Pro specific activity.

Protein synthesis in 3T3 cells stimulated to proliferate at various rates

Reproducible conditions were defined for using rates of leucine incorporation as a valid measure of rates of de novo protein synthesis in mouse 3T3 cells. Upon stimulation of quiescent cultures, rates of de novo synthesis of proteins increased and pool levels of amino acids decreased in proportion to the concentration of serum in the stimulating medium. Rates of de novo protein synthesis (per cell) exhibited a biphasic pattern of increase. These rates approached a plateau value at the end of the lag phase and increased again as cells entered S phase. This pattern of behaviour helps to explain the observed relationships between cell growth (increase in mass) and cell proliferation (increase in cell number).

Bleomycin selectively elevates mRNA levels for procollagen and fibronectin following acute lung injury

We employed the technique of dot blot hydridization of radiolabeled cDNA probes to examine the role of specific mRNA content in the control of extracellular matrix turnover in the remodeling rat lung. Following bleomycin instillation, total RNA content gradually doubled during the first 5 days following the initial lung injury, then rose much more rapidly during the ensuing 9 days. Individual mRNAs for procollagens I and III and for fibronectin were selectively enriched 2- to 4-fold above total RNA during the first week after bleomycin instillation. No comparable increases were observed in specific RNAs from liver, indicating that the response observed in the lung was not generalized to other organs. Moreover, the increases in mRNA species for procollagen types I or III in the lung could not be related to the influx of inflammatory cells which migrate into the lungs during acute injury, as cells obtained by alveolar lavage contained no mRNAs for procollagens.

Synthesis of cytoskeletal and contractile proteins by cultured IMR-90 fibroblasts

Models of the assembly of cytoskeletal and contractile proteins of eukaryotic cells require quantitative information about the rates of synthesis of individual component proteins. We applied the dual isotope technique of Clark and Zak (1981, J. Biol. Chem., 256:4863-4870) to measure the synthesis rates of cytoskeletal and contractile proteins in stationary and growing cultures of IMR-90 fibroblasts. Fibroblast proteins were labeled to equilibrium with [14C]leucine over several days, at the end of which there was a 4-h pulse with [3H]leucine. Fractional synthesis rates (percent per hour) were calculated from the 3H/14C ratio of cell protein extracts or protein purified by one- or two-dimensional polyacrylamide gel electrophoresis and the 3H/14C ratio of medium-free leucine. The average fractional synthesis rate for total, SDS- or urea-soluble; Triton-soluble; and cytoskeletal protein extracts in stationary cells each was approximately 4.0%/h. The range of values for the synthesis of individual proteins from total cell extracts or cytoskeletal extracts sliced from one-dimensional gels was similar, though this range was greater than that for major proteins of Triton-soluble protein extracts. Three specific cytoskeletal proteins--actin, vimentin, and tubulin--were synthesized at similar rates that were significantly slower than the average fractional synthesis rate for total protein. Myosin, on the other hand, was synthesized faster than average. Synthesis rates were the same for beta-and gamma-actin and polymerized (cytoskeletal extract) vs. Triton-soluble actin. The same was true for alpha- and beta-tubulin and two different forms of vimentin. Synthesis rates were uniformly higher in growing cells, though the same pattern of differential rates was observed as for stationary cells. Synthesis rates in growing cells were higher than the rate necessary to maintain the growth rate, even for those cytoskeletal proteins being synthesized slowly. Therefore, there appears to be some turnover of these cytoskeletal elements even during growth. We conclude that proteins in cytoskeletal extracts may have nonuniform rates of synthesis, but at least one important subclass of cytoskeletal proteins that comprise filament subunits have the same synthesis rates.

Analytical errors in measuring radioactivity in cell proteins and their effect on estimates of protein turnover in L cells

Previous studies from this laboratory on protein turnover in 3H-labelled L-cell cultures have shown recovery of total 3H at the end of a 3-day experiment to be always significantly in excess of the 3H recovered at the beginning of the experiment. In this study we have critically reviewed a number of possible sources for this error in measuring radioactivity in cell proteins. 3H-labelled proteins, when dissolved in 0.3 M-NaOH and counted for radioactivity in a liquid-scintillation spectrometer, showed losses of 30-40% of the radioactivity; neither external or internal standardization compensated for this loss. Hydrolysis of these proteins with either Pronase or concentrated HCl significantly increased the measured radioactivity. In addition, approx. 5-10% of the cell protein is left on the plastic culture dish when cells are recovered in phosphate-buffered saline. To aggravate this latter loss further, this surface-adherent protein, after pulse labelling, contains proteins of high radioactivity that turn over rapidly and make a major contribution to the accumulating radioactivity in the medium. These combined errors can account for up to 60% of the total radioactivity in the cell culture. Similar analytical errors have been found in studies of other cell cultures. The effect of these analytical errors on estimates of protein turnover in cell cultures is discussed.

Protein synthesis in rat lung. Measurements in vivo based on leucyl-tRNA and rapidly turning-over procollagen I

The relationships of the specific radioactivities of leucine in serum, leucine acylated to tRNA and leucine in procollagen I, procollagen III and total protein in lungs of unanaesthetized young male rats in vivo were assessed as a function of time during constant intravenous infusion of radiolabelled leucine. The specific radioactivity of free leucine in plasma reached a steady-state plateau value within 30 min of initiation of [3H]leucine infusion. Leucine acylated to tRNA isolated from lungs had the same specific radioactivity as free serum leucine. Leucine in procollagen I rapidly achieved a specific radioactivity equal to that of serum leucine and leucyl-tRNA, indicating that serum leucine and leucyl-tRNA isolated from total lung were in rapid equilibrium with the precursor leucine pool for procollagen I synthesis. On the basis of leucyl-tRNA or free serum leucine as the precursor, half-times of fractional conversion of procollagen I and III were calculated as 9 and 38 min respectively. The incorporation of leucine into mixed lung proteins calculated from the tracer studies was 6.8 mumol/day for the first 30 min of the infusion, after which the calculated rate increased to 15.0 mumol/day. This apparent increase correlated with the appearance of rapidly labelled plasma proteins trapped in the lungs. On the basis of short infusions lasting 30 min or less, followed by vascular perfusion of the lung, the average fractional synthesis rate of mixed pulmonary proteins in young male rats was 20%/day.