Margaret A. McNurlan

Chronic metabolic acidosis decreases albumin synthesis and induces negative nitrogen balance in humans.

Chronic metabolic acidosis has been previously shown to stimulate protein degradation. To evaluate the effects of chronic metabolic acidosis on nitrogen balance and protein synthesis we measured albumin synthesis rates and urinary nitrogen excretion in eight male subjects on a constant metabolic diet before and during two different degrees of chronic metabolic acidosis (NH4Cl 2.1 mmol/kg body weight, low dose group, and 4.2 mmol/kg body weight, high dose group, orally for 7 d). Albumin synthesis rates were measured by intravenous injection of [2H5ring]phenylalanine (43 mg/kg body weight, 7.5 atom percent and 15 atom percent, respectively) after an overnight fast. In the low dose group, fractional synthesis rates of albumin decreased from 9.9 +/- 1.0% per day in the control period to 8.4 +/- 0.7 (n.s.) in the acidosis period, and from 8.3 +/- 1.3% per day to 6.3 +/- 1.1 (P < 0.001) in the high dose group. Urinary nitrogen excretion increased significantly in the acidosis period (sigma delta 634 mmol in the low dose group, 2,554 mmol in the high dose group). Plasma concentrations of insulin-like growth factor-I, free thyroxine and tri-iodothyronine were significantly lower during acidosis. In conclusion, chronic metabolic acidosis causes negative nitrogen balance and decreases albumin synthesis in humans. The effect on albumin synthesis may be mediated, at least in part, by a suppression of insulin-like growth factor-I, free thyroxine and tri-iodothyronine.

Association of severe insulin resistance with both loss of limb fat and elevated serum tumor necrosis factor receptor levels in HIV lipodystrophy.

&NA;HIV‐lipodystrophy (HIV‐LD) is characterized by the loss of body fat from the limbs and face, an increase in truncal fat, insulin resistance, and hyperlipidemia, factors placing affected patients at increased risk for vascular disease. This study evaluated insulin sensitivity and inflammatory status associated with HIV‐LD and provides suggestions about its etiology. Insulin sensitivity and immune activation markers were assessed in 12 control subjects and 2 HIV‐positive groups, 14 without and 15 with LD syndrome. Peripheral insulin sensitivity (mostly skeletal muscle) was determined with the hyperinsulinemic‐euglycemic clamp. Circulating insulin‐like growth factor (IGF) binding protein‐1 (IGFBP‐1) and free fatty acid (FFA) levels, and their response to insulin infusion were indicative of insulin responsiveness of liver and adipose tissue, respectively. Serum levels of soluble type 2 tumor necrosis factor‐&agr; (TNF‐&agr;) receptor (sTNFR2) were used as an indicator of immune activation. HIV‐LD study subjects had significantly reduced (twofold) peripheral insulin sensitivity, but normal levels of FFA and reduced levels of IGFBP‐1, relative to the nonlipodystrophy groups, indicating that the loss of insulin sensitivity was more pronounced in skeletal muscle than in liver or fat. The significant loss of peripheral fat in the HIV‐LD group (34%; p < .05) closely correlated with the reduced peripheral insulin sensitivity (p = .0001). Levels of sTNFR2 were elevated in all HIV‐infected study subjects, but they were significantly higher in those with lipodystrophy than without, and sTNFR2 levels strongly correlated with the reduction in insulin sensitivity (p = .0001). Loss of peripheral fat, normal levels of FFA, and reduced levels of IGFBP‐1 indicate that insulin resistance in HIV‐LD is distinct from type 2 diabetes and obesity. The relationship between the degree of insulin resistance and sTNFR2 levels suggests an inflammatory stimulus is contributing to the development of HIV‐associated lipodystrophy.

Increased protein synthesis after acute IGF-I or insulin infusion is localized to muscle in mice

The purpose of the present study was to determine the effect of acute administration of insulin-like growth factor I (IGF-I) or insulin on in vivo protein synthesis in muscle and other organs in fasted mice and to compare this response with that produced by feeding. Recombinant IGF-I (3.3 nmol prime, 3.33 nmol/h) or insulin (0.056 nmol/h) was infused intravenously for 60 min along with glucose to prevent hypoglycemia. Fractional rates of tissue protein synthesis (FSR) were determined by injection of [2H5]phenylalanine (25 mg/100 g body wt, 40% enriched). Both IGF-I and insulin caused a 25% increase in FSR of heart ( P < 0.001) and soleus muscle ( P < 0.05) and a 65% increase in gastrocnemius and plantaris muscle (both P < 0.001), thus restoring rates to those seen in fed animals. A fivefold lower dose of IGF-I also stimulated protein synthesis in gastrocnemius muscle and heart (both P < 0.05) but not in soleus muscle. No significant effects of IGF-I on FSR were detected in liver, kidney, spleen, proximal small intestine, colon, lung, or brain. The results indicate that the ability of an overnight fast to decrease protein synthesis and the acute effects of insulin and IGF-I to stimulate protein synthesis are restricted to skeletal and cardiac muscles.

Adiponectin and leptin levels in HIV-infected subjects with insulin resistance and body fat redistribution.

Summary: In this study, we sought to determine the relationship between serum levels of leptin and adiponectin (Acrp30) in patients with HIV‐associated lipodystrophy (HIV‐LD). Three groups of subjects were studied; HIV‐positive subjects with lipodystrophy (HIV‐LD; n = 22), HIV‐positive subjects without lipodystrophy (HIV; n = 17), and ethnicity‐ and body mass index‐matched healthy control subjects (n = 20). Although total body fat from dual energy x‐ray absorptiometry was similar in all three groups, the HIV‐LD group had a significantly lower mean proportion of body fat in the limbs ± SEM (37.2% ± 2.2%) than either controls (49.8% ± 1.5%) or HIV subjects (45.7% ± 2.0%). The HIV‐LD group also had the lowest mean insulin sensitivity ± SEM (5.11 ± 0.59 mg of glucose/[kg of lean body mass • min] vs. 10.2 ± 0.72 mg of glucose/[kg of lean body mass • min] in controls and 8.64 ± 0.69 mg of glucose/[kg of lean body mass • min] in the HIV group). Leptin levels were similar in all three groups and were significantly correlated to total body fat (r = 0.86; p < .001), but these levels did not correlate with either insulin sensitivity or limb fat. Mean Acrp30 levels ± SEM were lowest in the HIV‐LD group (5.43 ± 0.44 &mgr;g/mL vs. 11.2 ± 1.4 &mgr;g/mL in the HIV group and 14.9 ± 1.8 &mgr;g/mL in control subjects). Further, Acrp30 levels were positively correlated with insulin sensitivity (r = 0.610; p < .001) and limb fat (r = 0.483; p < .001). However, the correlation between limb fat and insulin sensitivity disappeared when Acrp30 level and other potential mediators were removed from the association, suggesting that a deficiency in Acrp30 in subjects with HIV‐LD may be part of the mechanism for the reduced insulin sensitivity.

Improved insulin sensitivity and body fat distribution in HIV-infected patients treated with rosiglitazone: A pilot study

Summary: The insulin‐sensitizing drugs thiazolidinediones (TZDs), such as rosiglitazone, improve insulin sensitivity and also promote adipocyte differentiation in vitro. The authors hypothesized that TZDs might be beneficial to patients with HIV disease to improve insulin sensitivity and the distribution of body fat by increasing peripheral fat. The ability of rosiglitazone (8 mg/d) to improve insulin sensitivity (from hyperinsulinemic‐euglycemic clamp) and to improve body fat distribution (determined from computed tomography measurements of visceral adipose tissue [VAT] and subcutaneous adipose tissue [SAT]) was determined in 8 HIV‐positive patients. Before treatment, the insulin sensitivity of the patients was reduced to approximately 34% of that in control subjects. The rate of glucose disposal during a hyperinsulinemic‐euglycemic clamp (Rd) was 3.8 ± .4 (SEM) mg glucose/kg lean body mass/min compared with 11.08 ± 1.1 (p < .001) in healthy age‐ and body mass index (BMI)‐matched control subjects. After rosiglitazone treatment of 6 to 12 weeks, Rd increased to 5.99 ± .9 (p = .02), an improvement of 59 ± 22%. SAT increased by 23 ± 10% (p = .05), and, surprisingly, VAT was decreased by 21 ± 8% (p = .04) with a trend for increased SAT/VAT that failed to reach statistical significance. There were no significant changes in blood counts, viral loads, or CD4 counts with rosiglitazone treatment. The study demonstrates that rosiglitazone therapy improves insulin resistance and body fat distribution in some patients with HIV disease.

Tissue protein synthesis rates in critically ill patients

OBJECTIVES The aims of this study were to simultaneously determine the in vivo rates of protein synthesis in skeletal muscle, peripheral blood lymphocytes, and serum albumin in critically ill patients; to establish whether a relationship between the responses of these tissues could be observed; and to demonstrate if a protein synthesis pattern characteristic of critical illness exists. DESIGN Descriptive study. SETTING Intensive care unit of a 1000-bed university hospital. PATIENTS Fifteen patients treated in the intensive care unit. INTERVENTIONS None. MEASUREMENTS AND MAIN RESULTS Rates of tissue protein synthesis were determined in vivo once during the course of critical illness, using the flooding method with L-(2H5)phenylalanine. Protein synthesis in muscle was 1.49 +/- 0.16%/day; in circulating lymphocytes (i.e., mononuclear cells), protein synthesis was 11.10 +/- 1.82%/day. Albumin synthesis was 12.81 +/- 1.23%/day when expressed as the fractional rate, and was 184 +/- 19 mg/kg/day when expressed as the absolute rate. CONCLUSIONS The individual tissues responded differently to trauma, and showed a wide range of values. The responses were not significantly correlated with each other and no pattern of tissue protein synthesis characteristic of critical illness was observed. However, both muscle protein and albumin synthesis rates correlated with metabolic status and clinical indices of the severity of illness.

Influence of Dietary Protein Intake on Whole-Body Protein Turnover in Humans

Methods for measuring rates of protein synthesis and degradation in the whole body of humans with isotopes of carbon and nitrogen are described and attention is drawn to their relative merits and drawbacks for studying the nutritional control of protein metabolism. A review of published work on dietary protein and protein metabolism leads to the conclusion that protein is the major dietary determinant of whole-body protein turnover rates, and that energy intake is comparatively unimportant. Dietary protein affects protein turnover at two levels: an immediate response to the intake of protein in meals and a longer-term adaptation after a change in protein intake. An increase in the level of dietary protein enhances the response to meals, which mainly consists of a decrease in the rate of protein degradation. The adaptation to higher protein intakes involves an increase in the basal (postabsorptive) rates of both synthesis and degradation. Suggestions for future investigation include more detailed studies of the acute and adaptive responses, to facilitate understanding of dietary protein requirements, and the effects of very-high-protein intakes with continued development of techniques for studying protein turnover in individual tissues in humans.

Responsiveness of muscle protein synthesis to growth hormone administration in HIV-infected individuals declines with severity of disease.

This study was undertaken to determine if human recombinant growth hormone (hrGH, 6 mg/d for 2 wk) would stimulate muscle protein synthesis in AIDS wasting. Healthy controls were compared with patients who were HIV+, had AIDS without weight loss, and had AIDS with > 10% weight loss. Before hrGH, rates of skeletal muscle protein synthesis, measured with l-[2H5]phenylalanine, were the same in controls and in all stages of disease. Rates of myofibrillar protein degradation, however, assessed from urinary excretion of 3-methyl histidine, were higher in AIDS and AIDS wasting than in HIV+ or healthy individuals. The group with weight loss had significantly higher TNFalpha levels but not higher HIV viral loads. Muscle function, as determined by isokinetic knee extension and shoulder flexion, was significantly higher in controls than all infected individuals. After GH, rates of protein synthesis were stimulated 27% in controls, with a smaller increase (11%) in HIV+, and a significant depression (42%) in AIDS with weight loss, despite fourfold elevation in insulin-like growth factor-I in all groups. There was a significant correlation of hrGH-induced changes in muscle protein synthesis with severity of disease (P = 0.002). The results indicate increased basal muscle protein degradation and decreased responsiveness of muscle protein synthesis to GH in the later stages of disease.

Role of glutamine depletion in directing tissue-specific nutrient stress responses to L-asparaginase

l-Asparaginase is important in the induction regimen for treating acute lymphoblastic leukemia. Cytotoxic complications are clinically significant problems lacking mechanistic insight. To reveal tissue-specific molecular responses to this drug, mice were administered asparaginase from either Escherichia coli (clinically used) or Wolinella succinogenes (novel, glutaminase-free form). Both enzymes abolished serum asparagine, but only the E. coli form reduced circulating glutamine. E. coli asparaginase reduced protein synthesis in liver and spleen but not pancreas via increased phosphorylation of the translation factor eIF2. In contrast, treatment with Wolinella caused no untoward changes in protein synthesis in any tissue examined. Treating mice deleted for the eIF2 kinase, GCN2, with the E. coli enzyme showed eIF2 phosphorylation to be GCN2-dependent, but only initially. Furthermore, although eIF2 phosphorylation was not increased in the pancreas or by Wolinella asparaginase, expression of the amino acid stress response genes, asparagine synthetase and CHOP/GADD153, increased as a result of both enzymes, even in tissues demonstrating no change in eIF2 phosphorylation. Finally, signaling downstream of the mammalian target of rapamycin kinase was repressed in liver and pancreas by E. coli but not Wolinella asparaginase. These data demonstrate that the nutrient stress response to asparaginase is tissue-specific and exacerbated by glutamine depletion. Importantly, increased expression of asparagine synthetase and CHOP does not require eIF2 phosphorylation, signifying alternate or auxiliary means of inducing gene expression under conditions of amino acid depletion in the whole animal.

Protein-sparing effect in skeletal muscle of growth hormone treatment in critically ill patients.

OBJECTIVE To investigate the effect of growth hormone (GH) treatment on skeletal muscle protein catabolism in patients with multiple organ failure in the intensive care unit (ICU). SUMMARY BACKGROUND DATA Skeletal muscle depletion affects the incidence of complications and the length of hospital stay. A protein-sparing effect of GH treatment in skeletal muscle of long-term ICU patients was hypothesized. METHODS Twenty critically ill ICU patients were randomized to treatment with GH (0.3 U/kg/day) or as controls. Percutaneous muscle biopsy samples were taken before and after a 5-day treatment period starting on day 3 to 42 of the patients ICU stay. Protein content, protein synthesis, water, nucleic acids, and free amino acids in muscle were analyzed. RESULTS The protein content decreased by 8% +/- 11% in the control patients, with no significant change in the GH group. The fractional synthesis rate of muscle proteins increased in the GH group by 33% +/- 48%, and muscle free glutamine increased by 207% +/- 327% in the GH group. Total intramuscular water increased by 12% +/- 14% in the control group as a result of an increase in extracellular water of 67% +/- 86%; these increases were not seen in the GH group. In contrast, the intracellular water increased by 6% +/- 8% in the GH group. CONCLUSION Treatment with GH for 5 days in patients with multiple organ failure stimulated muscle protein synthesis, increased muscle free glutamine, and increased intracellular muscle water.