Regulation of muscle cathepsin B proteolytic activity in protein-depleted patients with chronic diseases.

BACKGROUND & AIMS: The lysosomal cathepsin system contributes to degrading cellular skeletal muscle proteins in many catabolic diseases. We have assessed the relationships between cathepsin B mRNA levels and the enzyme activity for this protease in the skeletal muscle of acutely ill patients with severe trauma (n=7) and in patients with a variety of chronic disease states (hemodialysis, n=3; nervous anorexia, n=1; type 2 diabetes, n=2; prolonged immobilization, n=1). METHODS: Muscle biopsies were taken from the vastus lateralis muscle in patients and controls to assess tissue levels of cathepsin B mRNA by competitive-quantitative polymerase chain reaction, cathepsin B proteolytic activity and myofibrillar protein content as alkali-soluble protein to DNA ratio (ASP/DNA). In the trauma patients, muscle protein loss was assessed by the arteriovenous balance technique as rate of phenylalanine release from leg muscle. RESULTS: The acute trauma patients exhibited a significant net phenylalanine release from leg muscle (33+/-4 nmol phenylalanine/min/100 ml leg volume) despite a continuous nutritional support. The muscle ASP/DNA ratio was lower (P<0.05) in the patients with chronic diseases (383+/-33) than in groups of healthy controls (554+/-41) or of uncomplicated, moderately obese subjects (525+/-26). Cathepsin B mRNA levels were 6-10 times greater (P<0.05) in the patients with acute trauma or chronic catabolic diseases than in the healthy subjects. Cathepsin B enzymatic activity were 2-3 times greater (P<0.05) in the chronic and acute patients than in the group of uncomplicated, moderately obese subjects. Regression analysis between cathepsin B mRNA and cathepsin B enzymatic activity indicates a significant direct correlation (r=0.84; P<0.05) in the chronic catabolic conditions, but not in the acute trauma patients (r=-0.05). CONCLUSIONS: In skeletal muscle of patients with stable chronic catabolic diseases, cathepsin B activity is directly related to cathepsin B mRNA levels, suggesting that in these patients this enzyme could be mainly regulated at the level of gene transcription.

Growth hormone decreases muscle glutamine production and stimulates protein synthesis in hypercatabolic patients.

We determined the effects of 24-h recombinant human growth hormone (rhGH) infusion into a femoral artery on leg muscle protein kinetics, amino acid transport, and glutamine metabolism in eight adult hypercatabolic trauma patients. Metabolic pathways were assessed by leg arteriovenous catheterization and muscle biopsies with the use of stable amino acid isotopes. Muscle mRNA levels of selected enzymes were determined by competitive PCR. rhGH infusion significantly accelerated the inward transport rates of phenylalanine and leucine and protein synthesis, whereas the muscle protein degradation rate and cathepsin B and UbB polyubiquitin mRNA levels were not significantly modified by rhGH. rhGH infusion decreased the rate of glutamine de novo synthesis and glutamine precursor availability, total branched-chain amino acid catabolism, and nonprotein glutamate utilization. Thus net glutamine release from muscle into circulation significantly decreased after rhGH administration ( approximately 50%), whereas glutamine synthetase mRNA levels increased after rhGH infusion, possibly to compensate for reduced glutamine precursor availability. We conclude that, after trauma, the anticatabolic action of rhGH is associated with a potentially harmful decrease in muscle glutamine production.

Slower activation of insulin action in hypertension associated with obesity.

OBJECTIVE: To determine whether kinetic abnormalities in the onset of insulin action contribute to the insulin resistance in obesity-associated hypertension. DESIGN: We monitored the rate of increase in glucose infusion during 6 h of hyperinsulinemic (40 mU/m2 per min) euglycemic clamps in hypertensive and normotensive obese subjects. The two groups of hypertensive (n=9) and normotensive (n=9) subjects were matched for age (48+/-2 versus 45+/-5 years), sex (five males and four females versus four males and five females) and body mass index (42+/-3 versus 40+/-2 kg/m2). RESULTS: In all subjects, the glucose infusion rate required to maintain euglycemia increased progressively during the clamp studies to achieve maximal, steady-state values within the fifth hour. During the first 2 h of the clamp, mean glucose infusion rate, the traditional approach to assessing insulin sensitivity, was lower in the hypertensive than in the normotensive obese patients (2.04+/-0.13 versus 3.29+/-0.41 mg/kg per min, respectively; P < 0.05). In contrast, the maximal steady-state glucose infusion rate, calculated as the mean value during the sixth hour of clamping, was similar in the hypertensive and in the normotensive obese patients (4.48+/-0.43 versus 4.81+/-0.45 mg/kg per min, respectively; NS). The time required to reach the half-maximal glucose infusion rate was greater in the hypertensive than normotensive obese patients (91+/-12 versus 38+/-5 min, respectively; P< 0.05). CONCLUSION: In obesity, hypertension was associated with a slower rate of activation of the insulin effect on glucose metabolism, whereas the maximal steady-state insulin effects were not altered by elevated blood pressure. Thus, the link between obesity and hypertension may be associated with the kinetics of onset of insulin action.

Relationship between whole-body protein turnover and serum creatinine in chronically uremic patients.

To investigate the relationship between decline in renal function and alterations of protein metabolism we determined the rate of whole-body protein turnover in a group of 15 postabsorptive chronically uremic patients (9 males and 6 females) with different levels of serum creatinine concentrations (average 5.7 +/- 0.4 (SE) mg x dl(-1); range 3.3-9.1). Patients' age and body mass index were 53 +/- 4 years (range 26-73) and 24.7 +/- 0.6 kg/m2 (range 20.3-28.7), respectively. Nutritional status (plasma albumin 3.6 +/- 0.4 g x dl[-1]) and acid-base equilibrium (arterial pH 7.38 +/- 0.01) were fairly controlled by therapy. Whole-body leucine rate of appearance (Ra), an index of whole-body protein turnover, was assessed using a stable isotope technique. L-[1-(13)C]leucine was continuously infused and plasma [1-(13)C]alpha-ketoisocaproic acid enrichment was determined in steady-state conditions as a marker of the intracellular leucine enrichment. The average leucine Rawas 2.03 +/- 0.13 micromol x kg(-1) x min(-1) (range 1.29-3.19). Using simple linear regression analysis, the coefficient of correlation between the individual values of serum creatinine concentration and leucine Ra was 0.59 (n = 15; p = 0.02). Leucine Ra did not significantly correlate with blood pH or plasma albumin. In conclusion, we found a positive linear relationship between the values of plasma creatinine concentration and the rate of whole-body protein degradation. This correlation suggests that the progression of renal insufficiency is associated with accelerated rates of turnover of body proteins.

Mechanisms of malnutrition in uremia.

The pathogenesis of protein wasting in chronic renal failure is multifactorial. Potential mediators of protein catabolism in chronic uremia include anorexia, low protein-energy intake, increased cortisol and parathyroid hormone secretion, insulin resistance, metabolic acidosis and unidentified uremic toxins. In non-acidotic uremic patients the rate of protein turnover (that is, synthesis and degradation) has often been found to be decreased. Malnutrition also decreases both protein synthesis and degradation. In contrast, during acidosis protein degradation is primarily accelerated and results in rapid loss of body proteins. Cytokine concentrations have often been found increased in both dialyzed and undialyzed chronically uremic patients. Our study determined the circulating levels of TNF-alpha and of type I (60 kDa) and type II (80 kDa) soluble TNF-alpha receptors in undialyzed uremic patients, and found that their plasma levels were greatly increased. Serum creatinine correlated with TNF-alpha soluble receptors but not with the TNF-alpha. Thus, TNF-alpha is potentially an important mediator of protein wasting in chronically uremic patients. Pharmacological therapy of protein catabolism in chronic uremia may include the administration of pentoxifylline, which has been shown to decrease protein degradation by interfering with the TNF-alpha system (that is, TNF-alpha and its soluble receptors) in experimental models. Growth hormone and insulin-like growth factor-1 administration may also be beneficial in these patients, but further evaluation of the hormone effects on glucose and glutamine metabolism is called for.

Metabolic response to injury and sepsis: changes in protein metabolism.

The metabolic response to trauma and sepsis involves an increased loss of body proteins. Specific sites of changes of protein and amino acid metabolism have been identified. In skeletal muscle, the rate of proteolysis is accelerated greatly. The rate of protein synthesis also may be increased but not enough to match the increase in degradation. Intramuscular glutamine concentration is decreased because of increased efflux and possibly decreased de novo synthesis. In the liver, the rate of synthesis of selected proteins (i.e., albumin, transferrin, prealbumin, retinol-binding protein, and fibronectin) is decreased, whereas acute phase protein synthesis is accelerated. Tissues characterized by rapidly replicating cells, such as enterocytes, immune cells, granulation tissue, and keratinocytes, exhibit early alterations in the case of decreased protein synthesis capacity. In these tissues, glutamine use is accelerated. Increased stress hormone (cortisol and glucagon) and cytokine secretion, as well as intracellular glutamine depletion, are potential mediators of altered protein metabolism in trauma and sepsis. However, the relative importance of these factors has not been clarified. Therapy of acute protein catabolism may include the use of biosynthetic human growth hormone, possibly in combination with insulin-like growth factor-1, and the administration of metabolites at pharmacologic doses. We recently studied the effects of carnitine and alanyl-glutamine administration in severely traumatized patients. We found that both carnitine and the glutamine dipeptide restrained whole-body nitrogen loss without affecting selected indices of protein metabolism in the skeletal muscle.

Effects of growth hormone administration on skeletal muscle glutamine metabolism in severely traumatized patients: preliminary report.

We have investigated the effects of 24 h human recombinant growth hormone (hGH) administration on leg muscle glutamine exchange and protein kinetics in severely traumatized patients. Muscle amino acid exchange and protein balance were evaluated using the leg arteriovenous balance technique, whereas changes in skeletal muscle free amino acid concentrations were evaluated in biopsy specimens. hGH infusion decreased phenylalanine release from protein degradation by 56 +/- 14%, and the rate of branched chain amino acid catabolism by 51 +/- 10%. Glutamine release from leg muscle was suppressed by 58 +/- 12%. This latter effect was completely accounted for by a hGH-mediated suppression of glutamine synthesis in skeletal muscle. In conclusion, growth hormone administration in trauma patients may restrain protein and amino acid catabolism in skeletal muscle. However, the growth hormone-mediated suppression of glutamine production we have observed in this study could decrease the systemic availability of this amino acid. During growth hormone treatment, this potential side-effect could be prevented by an exogenous glutamine administration.

Modulation of protein kinetics in chronic renal failure.

Many factors are involved in inducing muscle wasting and derangements of protein metabolism in chronic renal failure. Anorexia, low protein intake, hormonal abnormalities (increased cortisol and parathyroid hormone secretion, and insulin resistance), acidosis, abnormalities of the cytokine system, and other unidentified uremic toxins create a negative nitrogen balance and stimulate protein catabolism. The protein turnover rate (i.e., synthesis and degradation) is generally decreased in non-acidotic uremic patients. However, in uncorrected acidosis, protein degradation is accelerated and a rapid loss of body proteins supervenes. Nutrition can be improved in chronically uremic patients through pharmacological therapy that can control protein catabolism. Administration of growth hormone and insulin-like growth factor-1 has been shown to be effective. These hormones influence both glucose and glutamine metabolism. Another approach is the administration of pentoxifylline, which may have an anticatabolic effect by interfering with the tumor necrosis factor-alpha system.