Effect of insulin on albumin production and incorporation of 14C-leucine into proteins in isolated parenchymal liver cells from normal rats

Systemic inflammation, adipose tissue tumor necrosis factor, and leptin expression

OBJECTIVES

To explore the links between tumor necrosis factor alpha (TNFalpha) and leptin adipose tissue expression and low-grade systemic inflammation and to determine the relationship between inflammation and the degree of adiposity, the presence of type 2 diabetes, and other cardiovascular risk factors.

RESEARCH METHODS AND PROCEDURES

Ninety-one women (BMI 19 to 65 kg/m(2)) were divided into tertiles of CRP. Insulin resistance was calculated using the HOMA method. Albumin, fibrinogen, C-reactive protein (CRP), interleukin-6, sTNFR1, sTNFR2, and leptin levels were measured in serum and plasma samples. TNFalpha and leptin expression were measured by reverse transcription-polymerase chain reaction in abdominal subcutaneous adipose tissue samples.

RESULTS

CRP was positively related to BMI and upper distribution of adiposity. TNFalpha and leptin adipose tissue expression were higher in the upper tertile of CRP. Also, peripheral levels of both soluble TNFRs and leptin were higher in patients with the greatest inflammation degree. Diabetes, dislipidemia, and hypertension were most prevalent in patients in the upper CRP tertile. Inflammatory markers of diabetic women were significantly different from those of nondiabetic women, even after adjusting for differences in body fat. BMI, type 2 diabetes, and adipose TNFalpha mRNA levels were significant predictors of serum CRP levels (r(2) = 0.28, p < 0.001).

DISCUSSION

These results are in agreement with the hypothesis that the synthesis of adipose tissue TNFalpha and leptin could induce the production of interleukin-6, CRP, and other acute-phase reactants, thus contributing to the maintenance of chronic low-grade inflammation state involved in the progression of obesity and its associated comorbidities.

Quantification of protein turnover in primary cultures of rat hepatocytes

1. A radioactive tracer method, based on [3H]valine, for determination in parallel experiments of degradation of cellular proteins and synthesis of both cellular and secreted proteins in cultured rat hepatocytes was developed with special emphasis on methods of calculation, the number of protein pools and maintenance of nitrogen balance. 2. An extracellular concentration of 2 mM valine ensured a specific activity of the precursor pool for protein synthesis, which was constant during the experimental period and practically identical to that of extracellular valine. 3. Amino acid concentrations in the culture medium used were not rate-limiting for the synthesis of proteins. 4. The rate of labelling of the cellular protein pool during 8 days was in accordance with first-order saturation kinetics, which together with a constant ratio between labelling of soluble and membrane bound proteins, is compatible with a single pool of cellular proteins. 5. Protein degradation can be accurately measured by the release from prelabelled proteins of [3H]valine in the presence of 2 mM extracellular valine, if a 1 h chasing period is included in the experimental design. 6. The constancy of the degradation constant (kd) during protein labelling for up to 8 days, is in accordance with the existence of only one pool of cellular protein. 7. The cultured hepatocytes were in nitrogen balance during the experimental period of 8 days as reflected in a constant protein content per cell. The absolute rates of degradation and synthesis of cellular protein were identical, which confirm the validity of the method described.

Amphibolic role of the Krebs cycle in the insulin-stimulated protein synthesis

It has been a generally held view that insulin does not significantly affect the incorporation of amino acids into liver protein. This interpretation was based on data obtained from studies using the branched chain amino acids, which are poorly metabolized by the hepatic tissue. The effect of insulin on 14CO2 formation and protein incorporation of several 1-14C-labeled or U-14C-labeled amino acids was studied in isolated rat hepatocytes and diaphragm pieces. It was shown that insulin enhanced 14CO2 formation and protein incorporation primarily of those carbons of amino acids which are metabolized through the mitochondrial Krebs cycle. Using aminooxyacetic acid (0.5 mM), a potent inhibitor of the transamination reaction, it was shown that there exists an "insulin-sensitive" pool of glutamate which is preferentially utilized for protein synthesis in the presence of insulin. The insulin effect on protein incorporation of 14C-labeled glutamate generated in the Krebs cycle was abolished in the presence of aminooxyacetic acid. We interpret these results to signify that mitochondrial transamination of alpha-ketoglutarate to glutamate is essential for insulin stimulation of 14C incorporation into hepatocyte protein.

Alterations in circulating thyroid hormones and thyroxine-binding globulin levels during diabetic ketoacidosis

During diabetic ketoacidosis, in 17 adult patients, significant decreases in serum TBG and total T4 levels were observed, without significant alteration of the T4 to TBG binding property. In addition, serum free T4 (FT4) was moderately elevated and the TSH response to TRH was markedly blunted. No correlation, however, was found between TSH blunting and FT4 elevation. Correlation of these serum anomalies required at least 5 days of adequate control of the diabetes. Thus, diabetic ketoacidosis in euthyroid patients is characterized by multiple alterations in thyroid function parameters and caution is recommended in the interpretation of thyroid tests during and in the days following this severe metabolic disorder.

Stimulation of protein synthesis in isolated hepatocytes by somatomedin

Insulin (1 mU/ml) stimulated the incorporation of 14C-leucine into trichloroacetic acid-precipitable material by isolated hepatocytes from normal and hypophysectomized adult rats and 12-day-old rabbits. Somatomedin (200 ng/ml) purified from human plasma had an insulinlike effect in hepatocytes from hypophysectomized rats and baby rabbits but not from normal rats. This study suggests that, as well as being a site of somatomedin synthesis, the liver may be a target organ for this hormone.

Ammonia inhibits protein secretion in isolated rat hepatocytes

The general secretion of proteins by isolated rat hepatocytes in suspension is inhibited by colchicine, anoxia, or ammonia (NH4Cl). The inhibition by ammonia is accompanied by the cytoplasmic retention and swelling of protein secretory vesicles, suggesting that ammonia accumulates in the vesicles and thereby prevents their translocation to the cell periphery. General protein synthesis appears to be relatively unaffected by ammonia.

Secretion of lecithin: cholesterol acyltransferase from isolated rat hepatocytes

1. Lecithin:cholesterol acyltransferase is secreted from isolated rat heptocytes. 2. The secretion is stimulated when serum is added to the incubation medium. 3. Optimal conditions for secretion are: 5-10(6) hepatocytes per ml, 5 h incubation, pH 7.3-7.4 and 25% serum in the incubation medium. 4. Concomitantly with the secretion of lecithin:cholesterol acyltransferase there is a secretion of unesterified cholesterol and triacylglycerol. 5. Colchicine or cycloheximide in the incubation medium inhibits secretion of lecithin:cholesterol acyltransferase.

Incorporation of radioactive amino acids into protein in isolated rat hepatocytes

The incorporation of radioactivity from a 14C-labelled amino acid mixture (algal protein hydrolysate) into protein in isolated rat hepatocytes has been studied. The incorporation rate declined with increasing cell concentration, an effect which could be explained partly by isotope consumption, partly (and largely) by isotope dilution due to the formation of non-labelled amino acids by the cells. At a high extracellular amino acid concentration, the rate of incorporation into protein became independent of cell concentration, because the isotope dilution effect was now quantitatively insignificant. The time course of protein labelling at various cells concentrations correlated better with the intracellular than with the extracellular amino acid specific activity, suggesting that amino acids for protein synthesis were taken from an intracellular pool. With increasing extracellular amino acid concentrations both the intracellular amino acid concentration, the intracellular radioactivity and the rate of incorporation into protein increased. Protein labelling exhibited a distinct time lag at high amino acid concentrations, presumably reflecting the time-dependent expansion of the intracellular amino acid pool. The gradual increase in the rate of protein labelling could be due either to an increased intracellular specific activity, or to a real stimulation of protein synthesis by amino acids, depending on whether the total intracellular amino acid pool or just the expandable compartment is the precursor pool for protein synthesis.

Effect of glucagon on cyclic AMP, albumin metabolism and incorporation of 14C-leucine into proteins in isolated parenchymal rat liver cells

Parenchymal rat liver cells were isolated by the collagenase method and incubated in Krebs-Henseleit buffer containing 0.5% gelatin. The basal level of cyclic AMP in isolated cells was 0.52 nmol per g liver wet wt. Glucagon (10(-10)-10(-6) M) caused a significant increase in the level of cyclic AMP. Maximum levels were obtained 2-15 min after addition of glucagon. Repeated administration of glucagon caused a new increase in cyclic AMP, but the response was lesser than after the first addition of glucagon, indicating refractoriness to glucagon. The rate of albumin secretion was 4.6 mug/min per g liver wet wt. This is about the rate found in the perfused liver, Glucagon (10(-8-10(-6) M) inhibited albumin secretion and the incorporation of 14C-leucine into albumin, into total proteins in the medium and into total proteins in the cell suspension. The effect of glucagon on albumin secretion is compatible with an effect on the rate of synthesis. A positive correlation existed between the maximal level of cyclic AMP after glucagon administration and the inhibition of both albumin secretion and the incorporation of 149leucine.