Lysosomal enzyme activity in muscle tissue from patients with malignant tumor

2-Deoxy-D-glucose Alleviates Cancer Cachexia-Induced Muscle Wasting by Enhancing Ketone Metabolism and Inhibiting the Cori Cycle

Cachexia is characterized by progressive weight loss accompanied by the loss of specific skeletal muscle and adipose tissue. Increased lactate production, either due to the Warburg effect from tumors or accelerated glycolysis effects from cachectic muscle, is the most dangerous factor for cancer cachexia. This study aimed to explore the efficiency of 2-deoxy-D-glucose (2-DG) in blocking Cori cycle activity and its therapeutic effect on cachexia-associated muscle wasting. A C26 adenocarcinoma xenograft model was used to study cancer cachectic metabolic derangements. Tumor-free lean mass, hindlimb muscle morphology, and fiber-type composition were measured after in vivo 2-DG administration. Activation of the ubiquitin-dependent proteasome pathway (UPS) and autophagic–lysosomal pathway (ALP) was further assessed. The cachectic skeletal muscles of tumor-bearing mice exhibited altered glucose and lipid metabolism, decreased carbohydrate utilization, and increased lipid β-oxidation. Significantly increased gluconeogenesis and decreased ketogenesis were observed in cachectic mouse livers. 2-DG significantly ameliorated cancer cachexia-associated muscle wasting and decreased cachectic-associated lean mass levels and fiber cross-sectional areas. 2-DG inhibited protein degradation-associated UPS and ALP, increased ketogenesis in the liver, and promoted ketone metabolism in skeletal muscle, thus enhancing mitochondrial bioenergetic capacity. 2-DG effectively prevents muscle wasting by increasing ATP synthesis efficiency via the ketone metabolic pathway and blocking the abnormal Cori cycle.

Impaired cardiac performance, protein synthesis, and mitochondrial function in tumor-bearing mice

Background

To understand the underlying mechanisms of cardiac dysfunction in cancer, we examined cardiac function, protein synthesis, mitochondrial function and gene expression in a model of heart failure in mice injected with Lewis lung carcinoma (LLC1) cells.

Experimental design

Seven week-old C57BL/J6 male and female mice were injected with LLC1 cells or vehicle. Cardiac ejection fraction, ventricular wall and septal thickness were reduced in male, but not female, tumor-bearing mice compared to vehicle-injected control mice. Cardiac protein synthesis was reduced in tumor-bearing male mice compared to control mice (p = 0.025). Aspect ratio and form factor of cardiac mitochondria from the tumor-bearing mice were increased compared control mice (p = 0.042 and p = 0.0032, respectively) indicating a more fused mitochondrial network in the hearts of tumor-bearing mice. In cultured cardiomyocytes maximal oxygen consumption and mitochondrial reserve capacity were reduced in cells exposed to tumor cell-conditioned medium compared to non-conditioned medium (p = 0.0059, p = 0.0010). Whole transcriptome sequencing of cardiac ventricular muscle from tumor-bearing vs. control mice showed altered expression of 1648 RNA transcripts with a false discovery rate of less than 0.05. Of these, 54 RNA transcripts were reduced ≤ 0.5 fold, and 3 RNA transcripts were increased by ≥1.5-fold in tumor-bearing mouse heart compared to control. Notably, the expression of mRNAs for apelin (Apln), the apelin receptor (Aplnr), the N-myc proto-oncogene, early growth protein (Egr1), and the transcription factor Sox9 were reduced by >50%, whereas the mRNA for growth arrest and DNA-damage-inducible, beta (Gadd45b) is increased >2-fold, in ventricular tissue from tumor-bearing mice compared to control mice.

Conclusions

Lung tumor cells induce heart failure in male mice in association with reduced protein synthesis, mitochondrial function, and the expression of the mRNAs for inotropic and growth factors. These data provide new mechanistic insights into cancer-associated heart failure that may help unlock treatment options for this condition.

1α,25-dihydroxyvitamin D3 mitigates cancer cell mediated mitochondrial dysfunction in human skeletal muscle cells

Cancer cachexia is associated with muscle weakness and atrophy. We investigated whether 1α,25-dihydroxyvitamin D₃ (1α,25(OH)₂D₃), which has previously been shown to increase skeletal myoblast oxygen consumption rate, could reverse the deleterious effects of tumor cell conditioned medium on myoblast function. Conditioned medium from Lewis lung carcinoma (LLC1) cells inhibits oxygen consumption, increases mitochondrial fragmentation, inhibits pyruvate dehydrogenase activity, and enhances proteasomal activity in human skeletal muscle myoblasts. 1α,25(OH)₂D₃ reverses the tumor cell-mediated changes in mitochondrial oxygen consumption and proteasomal activity, without changing pyruvate dehydrogenase activity. 1α,25(OH)₂D₃ might be useful in treatment of weakness seen in association with CC.

Metabolic Changes During Cancer Cachexia Pathogenesis

Wasting of adipose tissue and skeletal muscle is a hallmark of metastatic cancer and a major cause of death. Like patients with cachexia caused by other chronic infections or inflammatory diseases, the cancer subject manifests both malnutrition and metabolic stress. Both carbohydrate utilization and amino acid incorporation are decreased in the muscles of cancer cachexia patients. Cancer cells affect host metabolism in two ways: (a) their own metabolism of nutrients into other metabolites and (b) circulating factors they secrete or induce the host to secrete. Accelerated glycolysis and lactate production, i.e., the Warburg effect and the resultant increase in Cori cycle activity, are the most widely discussed metabolic effects. Meanwhile, although a large number of pro-cachexia circulating factors have been found, such as TNFa, IL-6, myostatin, and PTHrp, none have been shown to be a dominant factor that can be targeted singly to treat cancer cachexia in humans. It is possible that given the complex multifactorial nature of the cachexia secretome, and the personalized differences between cancer patients, targeting any single circulating factor would always be insufficient to treat cachexia for all patients. Here we review the metabolic changes that occur in response to tumor growth and tumor-secreted factors during cachexia.

Preoperative overnight parenteral nutrition (TPN) improves skeletal muscle protein metabolism indicated by microarray algorithm analyses in a randomized trial

Loss of muscle mass is associated with increased risk of morbidity and mortality in hospitalized patients. Uncertainties of treatment efficiency by short‐term artificial nutrition remain, specifically improvement of protein balance in skeletal muscles. In this study, algorithmic microarray analysis was applied to map cellular changes related to muscle protein metabolism in human skeletal muscle tissue during provision of overnight preoperative total parenteral nutrition (TPN). Twenty‐two patients (11/group) scheduled for upper GI surgery due to malignant or benign disease received a continuous peripheral all‐in‐one TPN infusion (30 kcal/kg/day, 0.16 gN/kg/day) or saline infusion for 12 h prior operation. Biopsies from the rectus abdominis muscle were taken at the start of operation for isolation of muscle RNA. RNA expression microarray analyses were performed with Agilent Sureprint G3, 8 × 60K arrays using one‐color labeling. 447 mRNAs were differently expressed between study and control patients (P < 0.1). mRNAs related to ribosomal biogenesis, mRNA processing, and translation were upregulated during overnight nutrition; particularly anabolic signaling S6K1 (P < 0.01–0.1). Transcripts of genes associated with lysosomal degradation showed consistently lower expression during TPN while mRNAs for ubiquitin‐mediated degradation of proteins as well as transcripts related to intracellular signaling pathways, PI3 kinase/MAPkinase, were either increased or decreased. In conclusion, muscle mRNA alterations during overnight standard TPN infusions at constant rate altered mRNAs associated with mTOR signaling; increased initiation of protein translation; and suppressed autophagy/lysosomal degradation of proteins. This indicates that overnight preoperative parenteral nutrition is effective to promote muscle protein metabolism.

Molecular pathways leading to loss of skeletal muscle mass in cancer cachexia--can findings from animal models be translated to humans?

Background

Cachexia is a multi-factorial, systemic syndrome that especially affects patients with cancer of the gastrointestinal tract, and leads to reduced treatment response, survival and quality of life. The most important clinical feature of cachexia is the excessive wasting of skeletal muscle mass. Currently, an effective treatment is still lacking and the search for therapeutic targets continues. Even though a substantial number of animal studies have contributed to a better understanding of the underlying mechanisms of the loss of skeletal muscle mass, subsequent clinical trials of potential new drugs have not yet yielded any effective treatment for cancer cachexia. Therefore, we questioned to which degree findings from animal studies can be translated to humans in clinical practice and research.

Discussion

A substantial amount of animal studies on the molecular mechanisms of muscle wasting in cancer cachexia has been conducted in recent years. This extensive review of the literature showed that most of their observations could not be consistently reproduced in studies on human skeletal muscle samples. However, studies on human material are scarce and limited in patient numbers and homogeneity. Therefore, their results have to be interpreted critically.

Summary

More research is needed on human tissue samples to clarify the signaling pathways that lead to skeletal muscle loss, and to confirm pre-selected drug targets from animal models in clinical trials. In addition, improved diagnostic tools and standardized clinical criteria for cancer cachexia are needed to conduct standardized, randomized controlled trials of potential drug candidates in the future.

The emerging role of skeletal muscle oxidative metabolism as a biological target and cellular regulator of cancer-induced muscle wasting

While skeletal muscle mass is an established primary outcome related to understanding cancer cachexia mechanisms, considerable gaps exist in our understanding of muscle biochemical and functional properties that have recognized roles in systemic health. Skeletal muscle quality is a classification beyond mass, and is aligned with muscle's metabolic capacity and substrate utilization flexibility. This supplies an additional role for the mitochondria in cancer-induced muscle wasting. While the historical assessment of mitochondria content and function during cancer-induced muscle loss was closely aligned with energy flux and wasting susceptibility, this understanding has expanded to link mitochondria dysfunction to cellular processes regulating myofiber wasting. The primary objective of this article is to highlight muscle mitochondria and oxidative metabolism as a biological target of cancer cachexia and also as a cellular regulator of cancer-induced muscle wasting. Initially, we examine the role of muscle metabolic phenotype and mitochondria content in cancer-induced wasting susceptibility. We then assess the evidence for cancer-induced regulation of skeletal muscle mitochondrial biogenesis, dynamics, mitophagy, and oxidative stress. In addition, we discuss environments associated with cancer cachexia that can impact the regulation of skeletal muscle oxidative metabolism. The article also examines the role of cytokine-mediated regulation of mitochondria function, followed by the potential role of cancer-induced hypogonadism. Lastly, a role for decreased muscle use in cancer-induced mitochondrial dysfunction is reviewed.

Autophagic-lysosomal pathway is the main proteolytic system modified in the skeletal muscle of esophageal cancer patients

BACKGROUND

In cancer cachexia, muscle depletion is related to morbidity and mortality. Muscle-wasting mechanisms in cancer patients are not fully understood.

OBJECTIVE

We investigated the involvement of the proteolytic systems (proteasome, autophagic-lysosomal, calpain, and caspase) in muscle wasting during cancer cachexia.

DESIGN

Esophageal cancer patients [n = 14; mean ± SD age: 64.1 ± 6.6 y] and weight-stable control patients undergoing reflux surgery (n = 8; age: 57.5 ± 5.8 y) were included. Enzymatic activities were measured in the vastus lateralis and diaphragm. Protein expressions were also measured in the vastus lateralis of control (n = 7) and cancer (n = 8) patients.

RESULTS

Proteasome, calpain, and caspase 3 activities in the vastus lateralis and diaphragm muscles did not differ between the 2 groups. Cathepsin B and L activities were 90% (± SD) [2.4 ± 0.2 compared with 1.3 ± 0.2 pmol 7-amido-4-methylcoumarin (AMC) · μg protein⁻¹ · min⁻¹; P < 0.001] and 115% (5.3 ± 0.4 compared with 2.5 ± 0.3 pmol AMC · μg protein⁻¹ · min⁻¹; P < 0.001) greater, respectively, in the vastus lateralis of cancer patients than in that of control subjects. We observed (in conjunction with increased lysosomal protease activities) higher microtubule-associated protein 1 light chain 3B-II/I ratios (0.14 ± 0.08 compared with 0.04 ± 0.04) and cathepsin B and L expressions in the vastus lateralis of cancer patients than in that of control subjects (P < 0.05). Protein expression of p62 in the vastus lateralis did not differ between the 2 groups.

CONCLUSIONS

The autophagic-lysosomal pathway in the skeletal muscle of cancer patients was modified, whereas other proteolytic systems were unchanged. These findings suggest involvement of the autophagic-lysosomal proteolytic system during cancer cachexia development in humans.

Muscle wasting in cancer

Skeletal muscle loss appears to be the most significant clinical event in cancer cachexia and is associated with a poor outcome. With regard to such muscle loss, despite extensive study in a range of models, there is ongoing debate as to whether a reduction in protein synthesis, an increase in degradation or a combination of both is the more relevant. Each model differs in terms of key mediators and the pathways activated in skeletal muscle. Certain models do suggest that decreased synthesis accompanied by enhanced protein degradation via the ubiquitin proteasome pathway (UPP) is important. Murine models tend to involve rapid development of cachexia and may represent more acute muscle atrophy rather than the chronic wasting observed in humans. There is a paucity of human data both at a basic descriptive level and at a molecular/mechanism level. Progress in treating the human form of cancer cachexia can only move forwards through carefully designed large randomised controlled clinical trials of specific therapies with validated biomarkers of relevance to underlying mechanisms. This article is part of a Directed Issue entitled: Molecular basis of muscle wasting.

Muscle protein kinetics in cancer cachexia

PURPOSE OF REVIEW

Skeletal muscle loss appears to be the most significant event in cancer cachexia and is associated with a poor outcome. The balance between mechanisms that control synthesis and degradation is fundamental when designing new therapies. This review aims to highlight the molecular mechanisms that are associated with protein kinetics.

RECENT FINDINGS

The mechanisms that promote muscle synthesis have been explored in detail recently but moreover they have been the mechanisms behind degradation. Specific advances in cellular signalling molecules related to autophagy pathways including signal transducer and activators of transcription-3, activin type-2 receptor, TRAF6, and transcriptomic research have been given special attention in this review to highlight their roles in degradation and as potential targets for therapeutics. Ways to quantify muscle loss are badly needed for outcome measures; recent research using radiolabelled amino acids has also been discussed in this review.

SUMMARY

Only by having an appreciation of the complex regulation of muscle protein synthesis and degradation, will potential new therapeutics be able to be developed. This review identifies known targets in molecular pathways of current interest, explores methods used to find novel genes which may be involved in muscle kinetics and also highlights ways in which muscle kinetics may be measured to assess the efficacy of such interventions.

The regulation of skeletal muscle protein turnover during the progression of cancer cachexia in the Apc(Min/+) mouse

Muscle wasting that occurs with cancer cachexia is caused by an imbalance in the rates of muscle protein synthesis and degradation. The Apc(Min/+) mouse is a model of colorectal cancer that develops cachexia that is dependent on circulating IL-6. However, the IL-6 regulation of muscle protein turnover during the initiation and progression of cachexia in the Apc(Min/+) mouse is not known. Cachexia progression was studied in Apc(Min/+) mice that were either weight stable (WS) or had initial (≤5%), intermediate (6-19%), or extreme (≥20%) body weight loss. The initiation of cachexia reduced %MPS 19% and a further ∼50% with additional weight loss. Muscle IGF-1 mRNA expression and mTOR targets were suppressed with the progression of body weight loss, while muscle AMPK phosphorylation (Thr 172), AMPK activity, and raptor phosphorylation (Ser 792) were not increased with the initiation of weight loss, but were induced as cachexia progressed. ATP dependent protein degradation increased during the initiation and progression of cachexia. However, ATP independent protein degradation was not increased until cachexia had progressed beyond the initial phase. IL-6 receptor antibody administration prevented body weight loss and suppressed muscle protein degradation, without any effect on muscle %MPS or IGF-1 associated signaling. In summary, the %MPS reduction during the initiation of cachexia is associated with IGF-1/mTOR signaling repression, while muscle AMPK activation and activation of ATP independent protein degradation occur later in the progression of cachexia. IL-6 receptor antibody treatment blocked cachexia progression through the suppression of muscle protein degradation, while not rescuing the suppression of muscle protein synthesis. Attenuation of IL-6 signaling was effective in blocking the progression of cachexia, but not sufficient to reverse the process.

Using transcriptomics to identify and validate novel biomarkers of human skeletal muscle cancer cachexia

Background

Cancer cachexia is a multi-organ tissue wasting syndrome that contributes to morbidity and mortality in many cancer patients. Skeletal muscle loss represents an established key feature yet there is no molecular understanding of the disease process. In fact, the postulated molecular regulators of cancer cachexia originate largely from pre-clinical models and it is unclear how these translate to the clinical environment.

Methods

Rectus abdominis muscle biopsies were obtained from 65 upper gastrointestinal (UGI) cancer patients during open surgery and RNA profiling was performed on a subset of this cohort (n = 21) using the Affymetrix U133+2 platform. Quantitative analysis revealed a gene signature, which underwent technical validation and independent confirmation in a separate clinical cohort.

Results

Quantitative significance analysis of microarrays produced an 83-gene signature that was able to identify patients with greater than 5% weight loss, while this molecular profile was unrelated to markers of systemic inflammation. Selected genes correlating with weight loss were validated using quantitative real-time PCR and independently studied as general cachexia biomarkers in diaphragm and vastus lateralis from a second cohort (n = 13; UGI cancer patients). CaMKIIβ correlated positively with weight loss in all muscle groups and CaMKII protein levels were elevated in rectus abdominis. TIE1 was also positively associated with weight loss in both rectus abdominis and vastus lateralis muscle groups while other biomarkers demonstrated tissue-specific expression patterns. Candidates selected from the pre-clinical literature, including FOXO protein and ubiquitin E3 ligases, were not related to weight loss in this human clinical study. Furthermore, promoter analysis identified that the 83 weight loss-associated genes had fewer FOXO binding sites than expected by chance.

Conclusion

We were able to discover and validate new molecular biomarkers of human cancer cachexia. The exercise activated genes CaMKIIβ and TIE1 related positively to weight-loss across muscle groups, indicating that this cachexia signature is not simply due to patient inactivity. Indeed, excessive CaMKIIβ activation is a potential mechanism for reduced muscle protein synthesis. Our genomics analysis also supports the view that the available preclinical models do not accurately reflect the molecular characteristics of human muscle from cancer cachexia patients.

Mechanisms of cancer cachexia

Up to 50% of cancer patients suffer from a progressive atrophy of adipose tissue and skeletal muscle, called cachexia, resulting in weight loss, a reduced quality of life, and a shortened survival time. Anorexia often accompanies cachexia, but appears not to be responsible for the tissue loss, particularly lean body mass. An increased resting energy expenditure is seen, possibly arising from an increased thermogenesis in skeletal muscle due to an increased expression of uncoupling protein, and increased operation of the Cori cycle. Loss of adipose tissue is due to an increased lipolysis by tumor or host products. Loss of skeletal muscle in cachexia results from a depression in protein synthesis combined with an increase in protein degradation. The increase in protein degradation may include both increased activity of the ubiquitin-proteasome pathway and lysosomes. The decrease in protein synthesis is due to a reduced level of the initiation factor 4F, decreased elongation, and decreased binding of methionyl-tRNA to the 40S ribosomal subunit through increased phosphorylation of eIF2 on the alpha-subunit by activation of the dsRNA-dependent protein kinase, which also increases expression of the ubiquitin-proteasome pathway through activation of NFkappaB. Tumor factors such as proteolysis-inducing factor and host factors such as tumor necrosis factor-alpha, angiotensin II, and glucocorticoids can all induce muscle atrophy. Knowledge of the mechanisms of tissue destruction in cachexia should improve methods of treatment.

Cathepsin D activity is increased in alveolar macrophages and bronchoalveolar lavage fluid of smokers

Cathepsin D activity was determined in alveolar macrophages (AM) and cell-free bronchoalveolar lavage fluid (BALF) from volunteers who were current cigarette smokers and compared with that found in lifetime nonsmokers. Enzyme activity was determined with a highly sensitive and specific substrate [D-Phe-Ser(0-CH2-C6H5)-Phe-Phe-Ala-Ala-pAB]. Specific activity was more than three times higher in AM from smokers than in cells from nonsmokers (37,880 +/- 2,090 versus 10,300 +/- 1,200; p less than 0.001) and approximately seven times higher in BALF from smokers than from nonsmokers (3,620 +/- 490 versus 515 +/- 165; p less than 0.001). This study demonstrated that cigarette smoke is a potent inducer of cathepsin D activity in AM in vivo. Because cathepsin D is capable of degrading a variety of proteins, the finding of high concentrations of the enzyme in AM and BALF from smokers, along with previous observations of elevated cathepsin B activity, suggests that lysosomal enzymes may cause or contribute to structural lung damage associated with cigarette smoking.

Enzymatic and ultrastructural study of lysosomes in rats bearing radiation-induced thyroid follicular carcinoma

Radiation-induced well-differentiated and poorly differentiated follicular thyroid cancers were transplanted into the intrascapular fat pads of male Fisher 144 rats. The tumors grew in the recipient rats and after a time interval were removed and studied along with normal rat thyroids for lysosomal activity and ultrastructural characteristics. Plasma from experimental and control rats was also studied for lysosomal activity. Rats with radiation-induced thyroid carcinoma had a decrease in growth rate compared with normal rats. There was no significant increase in plasma lysosomal enzymes in the experimental rats. Well-differentiated thyroid carcinomatous tissue showed increased total activities of lysosomal enzymes as well as a difference in subcellular distribution compared with normal and poorly differentiated carcinomatous tissue. Electron microscopy of normal and carcinomatous tissue demonstrated the greatest number of lysosomes in the well-differentiated carcinoma and the fewest in the poorly differentiated carcinoma.

Cancer cachexia

Cancer cachexia is characterized clinically by anorexia, early satiety, weight loss. anemia, and marked asthenia. The syndrome is not the result of semistarvation alone but it represents a complex metabolic problem. In the host there are abnormalities in metabolism of energy, carbohydrate, lipid and protein, in water content, in acid-base balance, in electrolyte, mineral and vitamin concentrations, alterations in the activity of host tissue enzymes and changes in endocrine homeostasis and immunologic mechanisms. The cancer initiates and contributes to the genesis of the syndrome but complications of the disease and the treatment may also play a role. Only the control of the cancer can reverse completely the syndrome. It was proposed that cancer peptides throw the host metabolism into a chaotic biochemical state by activating and inactivating host enzymes. This results in increased energy expenditure; the released host metabolites and trapped by the growing cancer.

A comparative study of the influence of malignant tumor on host metabolism in mice and man: evaluation of an experimental model

Metabolic alterations in skeletal muscles and liver tissue from cancer patients were compared with corresponding alterations in mice (C-57) with sarcoma (MCG-101). In tumor-bearing man and mice similar changes in enzyme activities and in protein turnover were found. Glycolytic and oxidative enzyme activities were decreased in skeletal muscle tissue. Tumor-associated increase in lysosomal enzyme activities was found in both species. Leucine was incorporated into skeletal muscle proteins at a lower rate and into hepatic proteins at a higher rate in both species with malignant tumor. In tumor-bearing mice ribosome profiles from skeletal muscle, heart muscle and liver showed a preponderance of slowly sedimenting units of polyribosomes suggesting that initiation of protein synthesis may be a rate limiting step. The metabolic host reactions in tumor-bearing mice were similar to those in cancer patients implying that experimental tumors are relevant to use for analysis of mechanisms behind the development of cancer cachexia in man.

Clinical enzymology in cancer

It is fair to say that so far, and with few exceptions, the application of enzymology to clinical oncology has been disappointing. This is certainly true with regard to cancer screening and diagnosis. It is unlikely that any single enzyme or isoenzyme will emerge as a sufficiently sensitive or specific indicator of cancer, and it would seem more profitable to focus on multivariate or pattern analysis of several enzymes and other measurable body fluid constituents. Another suggested approach would be to establish the normal enzyme levels for individuals and then follow them for changes which might signal the development of a neoplasm. Finally, Weber's concept of key enzymes as the phenotypic markers of neoplasia and targets of chemotherapy would appear to open a new avenue for enzymology in clinical oncology.

[Amino acid-p-nitroanilide-splitting activities in human serum (author's transl)]

Proteolytic, arylamidase-like activities of human serum are determined by using a sensitive micromethod, based upon the Bratton-Marshall-reaction, and amino acid-p-nitroanilides as substrates. Besides the alanine-aminopeptidase and the oxytocinase, the dipeptide-aminopeptidase II, a neutral peptidase, and 6 other peptidases, not yet identified by us, are differentiated with the aid of 19 substrates, by adding 8 effectors, and by incubating the enzyme tests within the pH range 6.0 to 8.4.

Isolation of a protease from the cell-free medium of in vitro cultured mammary carcinoma

Trypsin-like protease with wide spectrum of enzymatic activities have been isolated from cell-free medium from in vivo cultured human mammary carcinoma cells, and from peripheral blood lymphocytes of patients with mammary carcinoma cultured in presence of cell-membrane carcinoma-associated glycoprotein.

Determination in vitro of the rate of protein synthesis and degradation in human-skeletal-muscle tissue

The present studies were aimed to evaluate the possibility to use a system for estimation in vitro of the biosynthesis and degradation rates of human skeletal muscle protein. A previously characterized human skeletal muscle preparation was used. Amino acids and insulin stimulated significantly the incorporation rate of leucine into proteins. The effect of amino acids was more pronounced than that of insulin. The stimulatory effect of insulin could be decreased by amino acids. Insulin did not influence the tissue uptake or the oxidation rate of leucine. The release of [14C]leucine deriving from degradation of prelabelled skeletal muscle fibre proteins was linear for at least 2.5 h of incubation and optimal with leucine at concentrations beyond 12.5 mmol/1 or in the presence of puromycin in the incubation medium. The rate of the release of radioactivity was significantly inhibited by amino acids and at borderline significance by insulin but not by puromycin. The specific radioactivity in prelabelled proteins decreased significantly in the presence of puromycin suggesting that leucine derived from protein degradation was reutilized in vitro. This reutilization was found to be 9 +/- 1% of leucine released from degradation of proteins in 30 subjects. A statistically significant positive correlation between the cathepsin D activity in human skeletal muscle tissue and the degradative rate of prelabelled muscle proteins in vitro was observed. The results indicate that biosynthesis and degradation of skeletal muscle proteins in this system in vitro were subjected to control mechanisms. It is suggested that the release of radioactivity from prelabelled muscle fibre proteins during incubation probably only reflects the degradation of some rapidly-turning-over proteins.