Suppression of myocardial protein degradation in the rat during fasting. Effects of insulin, glucose, and leucine

Incompatibility of chemical protein synthesis inhibitors with accurate measurement of extended protein degradation rates

Protein synthesis inhibitors are commonly used for measuring protein degradation rates, but may cause cytotoxicity via direct or indirect mechanisms. This study aimed to identify concentrations providing optimal inhibition in the absence of overt cytotoxicity. Actinomycin D, cycloheximide, emetine, and puromycin were assessed individually, and in two-, three-, and four-drug combinations for protein synthesis inhibition (IC₅₀ ) and cytotoxicity (CC₅₀ ) over 72 h. Experiments were conducted in HepG2 cells and primary rat hepatocytes (PRH). IC₅₀ for actinomycin D, cycloheximide, emetine, and puromycin were 39 ± 7.4, 6600 ± 2500, 2200 ± 1400, and 1600 ± 1200 nmol/L; with corresponding CC₅₀ values of 6.2 ± 7.3, 570 ± 510, 81 ± 9, and 1300 ± 64 nmol/L, respectively, in HepG2 cells. The IC₅₀ were 1.7 ± 1.8, 290 ± 90, 620 ± 920, and 2000 ± 2000 nmol/L, with corresponding CC₅₀ values of 0.98 ± 1.8, 680 ± 1300, 180 ± 700, and 1600 ± 1000 (SD) nmol/L, respectively, in PRH. CC₅₀ were also lower than the IC₅₀ for all drug combinations in HepG2 cells. These data indicate that using pharmacological interference is inappropriate for measuring protein degradation over a protracted period, because inhibitory effects cannot be extricated from cytotoxicity.

Myocardial protein turnover in patients with coronary artery disease. Effect of branched chain amino acid infusion

The regulation of protein metabolism in the human heart has not previously been studied. In 10 postabsorptive patients with coronary artery disease, heart protein synthesis and degradation were estimated simultaneously from the extraction of intravenously infused L-[ring-2,6-3H]phenylalanine (PHE) and the dilution of its specific activity across the heart at isotopic steady state. We subsequently examined the effect of branched chain amino acid (BCAA) infusion on heart protein turnover and on the myocardial balance of amino acids and branched chain ketoacids (BCKA) in these patients. In the postabsorptive state, there was a net release of phenylalanine (arterial-cardiac venous [PHE] = -1.71 +/- 0.32 nmol/ml, P less than 0.001; balance = -116 +/- 21 nmol PHE/min, P less than 0.001), reflecting protein degradation (142 +/- 40 nmol PHE/min) in excess of synthesis (24 +/- 42 nmol PHE/min) and net myocardial protein catabolism. During BCAA infusion, protein synthesis increased to equal the degradation rate (106 +/- 24 and 106 +/- 28 nmol PHE/min, respectively) and the phenylalanine balance shifted (P = 0.01) from negative to neutral (arterial-cardiac venous [PHE] = 0.07 +/- 0.36 nmol/ml; balance = 2 +/- 25 nmol PHE/min). BCAA infusion stimulated the myocardial uptake of both BCAA (P less than 0.005) and their ketoacid conjugates (P less than 0.001) in proportion to their circulating concentrations. Net uptake of the BCAA greatly exceeded that of other essential amino acids suggesting a role for BCAA and BCKA as metabolic fuels. Plasma insulin levels, cardiac double product, coronary blood flow, and myocardial oxygen consumption were unchanged. These results demonstrate that the myocardium of postabsorptive humans is in negative protein balance and indicate a primary anabolic effect of BCAA on the human heart.

Effects of insulin, biguanide antihyperglycaemic agents and beta-adrenergic agonists on pathways of myocardial proteolysis

Pathways of bulk protein degradation controlled by insulin and isoprenaline (isoproterenol) were distinguished in Langendorff-perfused rat hearts. Proteins were biosynthetically labelled in vitro with [3H]leucine, followed by addition of 2 mM non-radioactive leucine to competitively prevent reincorporation. Rapidly degraded proteins were eliminated during a 3 h preliminary perfusion period without insulin. One third of bulk myocardial protein degradation was inhibited by isoprenaline as described previously. An insulin concentration of 5 nM maximally inhibited proteolysis, beginning within 2 min. Inhibition reached 32% within 1.25 h and 35% after 1.5 h. The minimum effective insulin concentration was approx. 10-50 pM, which caused 10-20% inhibition. Following 3 h of perfusion without insulin, the lysosomal inhibitor, chloroquine (30 microM), inhibited 38% of bulk degradation. The 35% proteolytic inhibition caused by insulin was followed by very little further inhibition on subsequent concurrent infusion of chloroquine, i.e. the inhibitory effects of insulin and chloroquine were not additive. In contrast, prior inhibition of lysosomal proteolysis by insulin or chloroquine did not prevent the subsequent additive inhibition caused by isoprenaline. Insulin and beta-agonists additively inhibited approx. two-thirds of bulk degradation. The biguanide antihyperglycaemic agent phenformin (2 microM) inhibited 35% of bulk degradation, beginning at 2 min and reaching a near maximum at approx. 1.25-1.5 h. Following inhibition of proteolysis with phenformin (20 microM), subsequent infusion of chloroquine (30 microM) produced only a slight additional inhibition. Following inhibition of 35% of degradation by 1.5 h of perfusion with insulin (5 nM), subsequent exposure to phenformin (2 microM) produced only a slight additional inhibition which did not exceed 38% of basal proteolysis. Thus insulin and phenformin both inhibit lysosomal proteolysis; however, the adrenergic-responsive pathway is distinct.

Protein metabolism during nutrient deprivation and refeeding of neonatal heart cells

Pathological conditions or nutrient deprivation in the heart cause an imbalance between rates of protein synthesis and degradation, often resulting in a severe depletion of cardiac protein. We used cultured neonatal rat heart cells, a model system exhibiting positive nitrogen balance, to examine the effects of 10 h of starvation on myocardial glucose and protein metabolism. Cellular capacity for glucose utilization was depressed after starvation, as evidenced by lower hexokinase and other glycolytic enzyme activities and a 21% decrease in glucose usage. A 21.0% decrease in protein synthetic rate and an increase in protein degradation rate combined to yield a 29.5% decrease in total cellular protein during starvation. Degradation rates increased 29.0, 46.7, and 59.6% in 2-, 24-, and 96-h prelabeled cells, respectively, indicating that lability increased with half-life of proteins. During refeeding of starved, cultured cells, at least three proteins were synthesized at a lower rate. At the same time, proteins with approximate molecular masses of 45, 84, 92, and 174 kDa exhibited increased synthesis.

Effects of oxygen deprivation on incubated rat soleus muscle

Isolated soleus muscle deprived of oxygen produces more lactate and alanine than oxygen-supplied muscle. Oxygenated muscle synthesized glutamine, while anoxic muscle used this amino acid. Oxygen deprivation decreased adenine nucleotides leading to the efflux of nucleosides. Protein synthesis and degradation responded differently to anoxia. Synthesis almost completely ceased, while proteolysis increased. Therefore, protein degradation in soleus muscle is enhanced when energy supplies and oxygen tension are low.

Failure of a branched chain amino acid-enriched diet to reverse ethanol inhibition of cardiac protein synthesis in the rat

The fractional rate of protein synthesis was determined in the hearts of rats in vivo fed on diets containing 27% of energy as ethanol or on this diet supplemented with 5% of equimolar amounts of branched chain amino acids (BCAA). Administration of ethanol significantly decreased the fractional synthetic rate of mixed cardiac proteins and this depression was not ameliorated by concomitant feeding of BCAA. These data are discussed in relation to the stimulation of cardiac protein synthesis by BCAA observed in vitro.

Effects of dietary intake on myocardial glycogen in rats

Hearts with higher myocardial glycogen levels (MG) have improved tolerance to ischemia. The nutritional status of patients may influence MG levels and so its manipulation may be one way to delay myocardial damage during regional or global ischemia. In this study, the effects of a variety of dietary intakes were examined and correlated with MG levels in rats, grouped and fed in the following manner; Control; rat chow fed ad libitum, fasting of varying periods: 12 hr, 36 hr, 3 days, 5 days, and 7 days, diet manipulation; 4 groups pair-fed equicalorically with rat chow, chow and safflower oil, chow and safflower oil and dextrose, chow and dextrose. Daily weight change was recorded. At sacrifice, MG, myocardial DNA content, liver glycogen (LG), serum free fatty acid (FFA) were measured. It was found that fasting causes rapid elevation of MG within 12 hr, and during fasting the FFA level changes parallel with those of MG. Overall, weight gain had a positive correlation to LG and negative correlation to MG. With diet manipulation, certain substrates (dextrose), although equicaloric, resulted in greater weight gains and higher MG levels. Development of an optimum diet to augment MG and maintain good nutritional condition and to buy more time should be a useful clinical adjunct in patients suffering from unstable angina with high risk of imminent myocardial infarction and for preparing poor risk patients for cardiac surgery.

Contrasting response of protein degradation to starvation and insulin as measured by release of N tau-methylhistidine or phenylalanine from the perfused rat heart

An isotope-dilution method is described for the measurement of N tau-methylhistidine release from the perfused rat heart. We argue that release of N tau-methylhistidine is indicative of cardiac actin degradation. N tau-Methylhistidine release is compared with phenylalanine release in the presence of cycloheximide (phenylalanine release being a measure of degradation of mixed proteins). In hearts perfused with glucose plus acetate, the rate of actin degradation was increased by starvation and was not inhibited by insulin. In contrast, the rate of mixed-protein degradation was decreased by starvation and was inhibited by insulin. The fractional rate of degradation of mixed proteins in hearts from fed or starved rats was greater than that for actin. It is suggested that there are at least two pools of intracellular protein, the degradation rates of which differ in terms of their response to insulin and starvation.

Effects of glucocorticoid treatment on cardiac protein synthesis and degradation

We treated rats with dexamethasone (DEX, 1 mg . kg-1 . day-1) and examined the effects of this glucocorticoid on heart protein metabolism using atrial explant and Langendorff perfusion preparations. Fasted rats treated with DEX for 2 days had significantly lower body weights (92% of control, P less than 0.001) and larger hearts (106% of control, P less than 0.005) than fasted control animals. Protein and RNA concentrations remained constant. In atrial explants, DEX treatment produced a 19% increase in protein synthesis (P less than 0.001) and a 13% increase in protein degradation (P less than 0.002). In Langendorff-perfused hearts, DEX treatment caused a 36% increase in protein synthesis (P less than 0.02), while protein degradation was 8% above control (P greater than 0.05). Thus, in contrast to their catabolic effects on skeletal muscle, glucocorticoids are anabolic on the heart. The increased accumulation of total cardiac protein during early glucocorticoid administration is mediated entirely via increased rates of synthesis.

The effect of insulin and glucagon on systolic properties of the normal and septic isolated rat heart

Controversy exists in the literature concerning the effects of insulin and glucagon on cardiac muscle contractility, in particular during anoxia, ischemia or sepsis. The purpose of the present study was to determine the effects of insulin and glucagon on the systolic function of the normal and the dysfunctioning septic rat myocardium in the Langendorff preparation. In the normal isolated rat heart, neither insulin nor glucagon exhibited any lasting inotropic effect on systolic function or coronary flow. Sepsis (cecal ligation and puncture) resulted in a dramatic reduction of systolic function to 44% of control animals. All insulin-containing formulations tested improved systolic function in septic hearts by a mean of 85% compared to Krebs and glucose only. However, this improvement did not reach statistical significance compared to the use of Krebs and glucose only. Glucagon at 100 micrograms/l was doing as well as Krebs and glucose alone while at 1 mg/l glucagon was only able to maintain pre-perfusion contractility. Our results suggest that neither insulin nor glucagon seem to possess special inotropic properties for the isolated perfused normal or septic rat heart.

Regulation of protein turnover by glucose, insulin, and amino acids in adipose tissue

Protein synthesis and degradation were measured simultaneously in epididymal fat pads of rats by use of the incorporation of [14C]phenylalanine into protein and the sum of net protein breakdown and protein synthesis, respectively. Neither glucose nor insulin altered protein synthesis, but together they promoted this process; pyruvate could be substituted for glucose. Separately, glucose or insulin diminished proteolysis, and these effects were additive. In the presence of glucose and insulin, leucine, alanine, glutamine, glutamate, and aspartate lowered protein degradation to varying degrees but did not alter protein synthesis. Glutamate, but not leucine or alanine, was inhibitory without glucose and insulin present. When aminooxyacetic acid was provided to decrease the rate of transamination of amino acids, the inhibitory effects of leucine, alanine, and aspartate, but not of glutamate, appeared to be diminished. alpha-Ketoglutarate, but neither alpha-ketoisocaproate nor pyruvate, could diminish proteolysis. Inhibition of proteolysis was associated with a higher tissue content of glutamate and a greater production of glutamate and glutamine. These results suggest that glutamate itself may inhibit proteolysis in adipose tissue and mediate, at least in part, the effects of other amino acids.

Rates of protein turnover in vivo and in vitro in ventricular muscle of hearts from fed and starved rats

Starvation of 300 g rats for 3 days decreased ventricular-muscle total protein content and total RNA content by 15 and 22% respectively. Loss of body weight was about 15%. In glucose-perfused working rat hearts in vitro, 3 days of starvation inhibited rates of protein synthesis in ventricles by about 40-50% compared with fed controls. Although the RNA/protein ratio was decreased by about 10%, the major effect of starvation was to decrease the efficiency of protein synthesis (rate of protein synthesis relative to RNA). Insulin stimulated protein synthesis in ventricles of perfused hearts from fed rats by increasing the efficiency of protein synthesis. In vivo, protein-synthesis rates and efficiencies in ventricles from 3-day-starved rats were decreased by about 40% compared with fed controls. Protein-synthesis rates and efficiencies in ventricles from fed rats in vivo were similar to values in vitro when insulin was present in perfusates. In vivo, starvation increased the rate of protein degradation, but decreased it in the glucose-perfused heart in vitro. This contradiction can be rationalized when the effects of insulin are considered. Rates of protein degradation are similar in hearts of fed animals in vivo and in glucose/insulin-perfused hearts. Degradation rates are similar in hearts of starved animals in vivo and in hearts perfused with glucose alone. We conclude that the rates of protein turnover in the anterogradely perfused rat heart in vitro closely approximate to the rates in vivo in absolute terms, and that the effects of starvation in vivo are mirrored in vitro.

Metabolism of protein, amino acids, and glucose and their response to insulin in atria and cardiac myocytes of traumatized rats

Soft tissue injury to one hindlimb of rats was used to test the metabolic response of atrial and ventricular muscle to trauma. Effects of insulin on muscle metabolism were also studied. In myocytes and atria from normal animals, insulin increased protein synthesis and decreased protein degradation. For myocytes of rats at one and two days after trauma, this effect of insulin on proteolysis could not be detected. Over the next two days, the inhibitory effect returned to normal. Insulin also did not increase protein synthesis on day 1, but did thereafter. In atria, in contrast to heart cells, the inhibitory effect of insulin on proteolysis was enhanced at two and three days after trauma, and its stimulation of protein synthesis was unaltered. Insulin increased carbohydrate metabolism in both myocytes and atria of normal rats and traumatized rats after 2 days, and trauma did not alter this response. In myocytes, but not atria, trauma led to a faster oxidation of leucine and a significant rise in the production of alanine. Production of glutamine and glutamate was not affected in either tissue. These results show that the metabolic responses to trauma of atrial and ventricular muscle differ considerably.

The effect of starvation on branched-chain 2-oxo acid oxidation in rat muscle

Oxidative-decarboxylation rates of branched-chain amino acids in rat hemidiaphragm and of branched-chain 2-oxo acids in hemidiaphragm, soleus muscle and heart slices of 110-120 g rats were increased considerably by 3-4 days of starvation, when they were calculated from the specific radioactivity in the medium. When the supply from endogenous protein degradation to the oxidation-precursor pool was severely limited by transaminase inhibitors, oxidative-decarboxylation rates of branched-chain 2-oxo acids rose significantly. Since this apparent increase was relatively larger in preparations from fed rats than from 3-days-starved rats, the differences in oxidation rates with nutritional state became less or even not significant. With rat heart the smaller dilution of the oxidation precursor pool after starvation is in accordance with the reported decrease in protein breakdown. Since protein degradation increases with starvation in skeletal muscles, we suggest that the amino acid pool arising from protein degradation is more segregated from the oxidation precursor pool in muscles from starved than from fed rats. We conclude that starvation increases branched-chain amino acid and 2-oxo acid oxidation in skeletal and cardiac muscle considerably less than has been suggested by previous studies.

Role of cellular proteinases in acute myocardial infarction. I. Proteolysis in nonischemic and ischemic rat myocardium and the effects of antipain, leupeptin, pepstatin and chymostatin administered in vivo

To test the hypothesis that cellular proteinases contribute to ischemic myocellular death, measurements were made of tyrosine release (an index of overall proteolysis) from incubated slices of nonischemic and ischemic myocardium obtained at various times after coronary artery occlusion in rats. Proteolysis failed to increase in ischemic myocardium throughout the first 24 hours of occlusion, when irreversible damage develops, indicating that cellular proteinases do not undergo generalized activation in this phase. These data represent the first assessment of myocardial proteolysis throughout the development of ischemic death, and suggest that cellular proteinases do not play a causal role in this process. However, the possibility remains that ischemia selectively accelerates the breakdown of vital proteins, a phenomenon that may not be detected by measuring overall proteolysis. To determine whether future studies on the effects of proteolytic inhibition on infarct size are feasible, the ability of the proteinase inhibitors antipain, leupeptin, pepstatin and chymostatin, given in vivo, to interfere with proteolysis in ischemic myocardium was also evaluated. Leupeptin (10 or 40 mg/kg) inhibited proteolysis in a dose-related fashion (-49 and -72%, respectively, p less than 0.001). Antipain (20 mg/kg) decreased protein breakdown by 60% (p less than 0.001). The combination of antipain (20 mg/kg), leupeptin (40 mg/kg) and pepstatin (5 mg/kg) suppressed proteolysis almost completely at both 15 minutes (-88%, p less than 0.001) and at 6 hours (-72%, p less than 0.05) of ischemia, that is, throughout the development of irreversible injury. These results demonstrate that whatever proteolysis is occurring during acute myocardial infarction is largely mediated by cathepsins A, B, D, L and H and by calcium-activated neutral protease (that is, the enzymes sensitive to the inhibitors used). Because antipain, leupeptin and pepstatin significantly suppress such proteolysis, these agents might be useful in further assessing any potential contribution of cellular proteinases to the production of ischemic myocellular death. In addition, this study provides a new experimental model that affords serial assessments of regional myocardial proteolysis during the evolution of myocardial infarction.

Regulation of cathepsin D metabolism in rabbit heart: evidence for a role for precursor processing in the control of enzyme activity

Production of active lysosomal enzymes may involve limited proteolysis of inactive high molecular weight precursors. Precursor processing potentially regulates lysosomal enzyme activity. To test whether rabbit cardiac cathepsin D is first synthesized as a precursor and whether prolonged fasting (a condition affecting both cathepsin D and total cardiac protein turnover) influences precursor processing, rates of cathepsin D synthesis and processing were compared in left ventricular slices of control and 3-d-fasted rabbits incubated in vitro with [(35)S]methionine. (35)S-labeled cathepsin D was isolated by butanol-Triton X-100 extraction, immunoprecipitation, and dodecyl sulfate-polyacrylamide gel electrophoresis. Total cardiac protein synthesis was measured by tracer incorporation and normalized for differences in precursor pool size by direct measurement of [(35)S]aminoacyl-tRNA-specific radioactivity. Relative cathepsin D synthetic rates were obtained by comparing (35)S incorporation into cathepsin D with (35)S incorporation into all cardiac proteins. Enzyme processing was assessed in pulse-chase experiments and assayed by autoradiography. The results indicate that (a) rabbit cardiac cathepsin D is synthesized as a precursor (53,000 mol wt) that is processed to a 48,000-mol wt form, (b) rates of both cathepsin D and total cardiac protein synthesis are similar in control and fasted rabbits, suggesting that decreased enzyme degradation rather than increased synthesis is responsible for the elevated levels of cardiac cathepsin D in starvation, and (c) cathepsin D processing in hearts of fasted animals is incomplete, with accumulation of the precursor during pulse-chase experiments of 6 h duration. Based upon these results, a three-stage model for the regulation of cathepsin D activity in rabbit heart is proposed.

Effects of amino acid analogues on protein degradation in isolated rat hepatocytes

Analogues and derivatives of six of the amino acids which most effectively inhibit protein degradation in isolated rat hepatocytes (leucine, asparagine, glutamine, histidine, phenylalanine and tryptophan) were investigated to see if they could antagonize or mimic the effect of the parent compound. No antagonists were found. Amino alcohols and amino acid amides tended to inhibit protein degradation strongly, apparently by direct lysosomotropic effect as indicated by their ability to cause lysosomal vacuolation. Amino acid alkyl esters and dipeptides inhibited degradation to approximately the same extent as did their parent amino acids, possibly by being converted to free amino acids intracellularly. Of several leucine analogues tested, four (L-norleucine, L-norvaline, D-norleucine and L-allo-isoleucine) were found to be as effective as leucine in inhibiting protein degradation. None of the analogues had any effect on protein synthesis. Since leucine appears to play a unique role as a regulator of bulk autophagy in hepatocytes, the availability of active leucine agonists may help to elucidate the biochemical mechanisms for control of this important process.

Intracellular disruption of rat heart lysosomes by leucine methyl ester: effects on protein degradation

Perfusion of rat hearts with Krebs--Henseleit medium containing 10 mM L-leucine methyl ester leads to swelling of lysosomes and loss of lysosomal integrity within 30-60 min. No morphological changes can be detected in the nuclei, mitochondria, sarcoplasmic reticulum, or Golgi complex as a result of the treatment with leucine methyl ester, and the hearts continue to beat normally during the treatment period. Homogenates of rat hearts perfused with the methyl ester exhibit a decrease in the sedimentability of cathepsin D activity compared to controls, thus providing additional evidence for a loss of lysosomal integrity. Swelling and disruption of the lysosomes presumably occurs because of the extensive accumulation of leucine within the organelles resulting from the intralysosomal hydrolysis of the freely permeating methyl ester. The lysosomal dysfunction that occurs with exposure to leucine methyl ester produces a 30% decrease in cardiac protein degradation. These results provide an estimate of the contribution of lysosomes to total protein degradation in the rat heart, and they also suggest that the enzymes released as a result of lysosomal disruption are relatively inactive in hydrolyzing cellular constituents under the perfusion conditions used here. The use of amino acid methyl esters to produce rapid, specific loss of lysosomal integrity in situ provides an approach to the study of lysosomal function in intact cells.