Effect of diabetes on the ultrastructure of the hepatocyte and on the distribution and activity of ribosomes in the free and membrane-bound populations

Morphology and protein content of hepatocytes in type I diabetic rats submitted to physical exercises

The importance of physical exercise practice in the treatment of diabetes has been reported in many studies recently, but only limited data can be found regarding its benefits on liver morphology and protein content of hepatocytes. In order to assess the changes arising from the development of type I diabetes and the benefits of a training protocol, Wistar rats were divided into four groups: sedentary control (SC), trained control (TC), sedentary diabetic (SD) and trained diabetic (TD). The training protocol consisted of swimming for 60 min a day, 5 days/week, during 8 weeks. Liver samples were collected, processed and analyzed by histochemical and ultrastructural techniques. Biochemical tests were also conducted to examine the protein content and quantity of DNA in the liver. In morphological assessment, the presence of areas of cytoplasmic basophilia observed in control subjects was not visualized in sedentary diabetics. It was related to differences in the amount of mitochondria in the cytosol. The mitochondrial structure has not undergone relevant changes, and the number of rough endoplasmic reticulum cisterns was clearly inferior in sedentary diabetics, suggesting lower protein production. However, the biochemical analysis of protein content indicated no statistical differences between groups. The exercise, in turn, was not responsible for major changes in these characteristics. On the whole, the morphological damages arising from type I diabetes were noteworthy. Nevertheless, regular physical training was not responsible for significant improvements in some respects, making evident the need for combined application of a distinct form of treatment.

Effects of streptozotocin-induced diabetes on rough endoplasmic reticulum and lysosomes of rat liver

In the absence of amino acids and insulin, ribosome-free regions of the rough endoplasmic reticulum (RER) invaginate to form an autophagosome, which matures into an autolysosome (W. A. Dunn, Jr., J. Cell Biol. 110: 1923-1933, 1990). In this study, biochemical and morphological methods were used to examine the structure and integrity of the RER and the lysosome-vacuolar system in livers of untreated (normal serum insulin) and streptozotocin (STZ)-treated (depressed serum insulin) fed and fasted rats. Degradation of endogenous proteins was increased by 70% in STZ-treated animals. Proteolysis was further enhanced when these animals were deprived of food for 24 h. These alterations in protein turnover were accompanied by increases in the fractional volume of autophagic vacuoles and in the hepatic amounts of three lysosomal hydrolases. These effects of STZ were prevented on administration of insulin. In addition, there was an insulin-dependent 50% loss of RER surface area in livers from STZ-treated rats. This loss of structural RER was accompanied by comparable decreases in the cellular amounts of two RER membrane proteins and one luminal protein, suggesting that the RER was degraded as a unit. Additional losses of RER were observed when STZ-treated rats were fasted. Furthermore, the hepatic amounts of two serum proteins decreased, suggesting the functional capacity of the RER was reduced. Combined, the data suggest that in STZ-induced diabetes the losses in RER are related to enhanced autophagy.

Effect of monensin and diabetes on asialoglycoprotein degradation in rat hepatocytes

We have studied the effects of two modulations--streptozotocin-induced diabetes in vivo, and the presence of the carboxylic proton ionophore monensin in vitro--on the degradation of 3H-asialoorosomucoid ligand in isolated rat hepatocytes. The ligand was internalized by means of a synchronous wave procedure. Diabetes was associated with a marked decrease in the amount of total degraded radioactive ligand compared to that in normal cells (3.6% and 37.3% of internalized ligand respectively, at 60 min), together with increased secretion of degradation products into the incubation medium (87% and 46.3% of the total degraded ligand was secreted by diabetic and normal cells, respectively). Monensin induced similar effects in normal cells, but had no apparent effect in diabetic cells.

Disposition of fluorescein-labelled dextran (150 kD) in isolated perfused livers from control and diabetic rats

Isolated perfused livers were used to study the hepatic disposition of fluorescein-labelled dextran with a MW of 150 kD (FD-150), in control and streptozotocin-induced diabetic rats. A 100-microliter volume of FD-150 solution (10%, w/v) was injected as a rapid bolus dose into the inlet catheter of the isolated livers, and samples were collected from the outlet catheter in 2-sec intervals for 80 sec. Statistical moment theory was used to calculate the distribution and elimination parameters of the tracer based on the concentrations of FD-150 in the outflow. The values (mean +/- SD) of mean transit time, volume of distribution, and extraction ratio of FD-150 in the isolated livers from control rats were 16.3 +/- 2.00 sec, 0.298 +/- 0.054 ml/g liver, and 0.24, respectively. Similar values were obtained in diabetic livers, suggesting that streptozotocin-induced diabetes does not affect the hepatic disposition of FD-150.

Effects of streptozotocin diabetes in the rat on blood levels of ten specific plasma proteins and on their net biosynthesis by the isolated perfused liver

Adult male Sprague-Dawley rats with streptozotocin-induced diabetes (6 to 8 wk duration), treated or untreated with insulin, were studied with two aims: (a) to ascertain whether protracted diabetes in the rat is associated with changes in circulating plasma protein levels analogous to those reported in human diabetic patients with clinical evidence of complications; (b) to evaluate the effects of experimental diabetes on the net cumulative biosynthesis of 10 specific plasma proteins by the isolated liver, perfused for 24 hr. Samples of liver donor plasma and samples of perfusate were analyzed by single radial immunodiffusion or by rocket immunoelectrophoresis for albumin, alpha 1-macroglobulin and the acute phase glycoproteins: fibrinogen, alpha 1-acid glycoprotein (Darcy), alpha 1-acid glycoprotein (Kawasaki), haptoglobin, alpha 2-(acute phase) globulin, hemopexin, C3-complement and ceruloplasmin. Diabetes (6 to 8 wk), untreated with insulin, resulted in significantly increased liver donor plasma levels of alpha 1-acid glycoprotein (Darcy) and alpha 1-acid glycoprotein (Kawasaki); plasma levels of hemopexin and of C3 decreased to 75% and 30% of normal, respectively. Insulin treatment of diabetic liver donors for 6 to 8 wk prevented the increase in alpha 1-acid glycoprotein (Darcy) and alpha 1-acid glycoprotein (Kawasaki) and minimized the decrease in C3 to 75% of normal. Perfused livers from untreated diabetic rats (6 to 8 wk) showed slightly decreased cumulative synthesis and secretion of alpha 1-acid glycoprotein (Darcy); however, synthesis of albumin was reduced to 35% of normal and that of eight glycoproteins ranged from 25% of normal (fibrinogen) to 12% of normal (C3). The striking in vitro induction of increased synthesis of acute-phase proteins by cortisol plus insulin in the isolated perfused normal liver was in contrast to the severely attenuated induction in perfused livers of untreated diabetic rats, which ranges from 50% of normal for alpha 1-acid glycoprotein (Darcy) to 5% of normal (C3). Severely negative perfusate nitrogen balance and impaired glucose utilization by perfused untreated diabetic livers contrasted with positive nitrogen balance and good glucose utilization of normal livers in response to insulin plus cortisol. The plasma protein synthetic capacity and the in vitro response to insulin plus cortisol of perfused livers from insulin-treated diabetic rats were normal for seven of the proteins but moderately decreased for albumin, haptoglobin and C3.

Time course of changes in albumin synthesis and mRNA in diabetic and insulin-treated diabetic rats

Albumin synthesis in rat liver in vivo decreased from 12.7 to 2.2% of total protein synthesis during the first 3 days after the induction of diabetes and then remained relatively constant at this depressed rate for another 3 days. Insulin treatment begun on the 3rd day after the induction of diabetes restored albumin synthesis to control values within 3 days. Hybridization of total polyadenylate-containing RNA with a specific albumin cDNA probe revealed a close correspondence between the relative abundance of albumin mRNA and the relative rate of albumin synthesis after induction of diabetes and in response to insulin treatment. The apparent half-life of albumin mRNA, based on the rate of change of the message from one steady-state level to another, was approximately 22 h in both diabetic and insulin-treated diabetic rats. Diabetes of 3-day duration had no effect on the average sizes of total and albumin-synthesizing polysomes or on the ribosomal half-transit time for total protein and albumin. However, the number of albumin-synthesizing polysomes decreased as a result of diabetes to approximately one-third the number found in control livers. Taken together the results indicate that albumin synthesis was regulated by the availability of albumin mRNA and not by alterations in degradation, sequestration, or translation of message.

Changes in glucose and lipid metabolism in starved or starved-refed Japanese quail (coturnix coturnix japonica) in relation to fine structure of liver cells

1. Metabolic response of adult quail to fasting or refeeding was studied by measuring the main blood and hepatic metabolites. Moreover, the fine structure of hepatocytes in these physiological conditions was described. 2. Starvation or refeeding did not affect glycemia in male as in female quails. 3. Fasting had no effect on plasma free fatty acids in female quails, whereas plasma triglycerides were markedly decreased. 4. In fasted quails, there was an active ketogenesis with a high 3-hydroxybutyrate/acetoacetate ratio. 5. Ultrastructural aspect of liver parenchymal cells from fasted quails revealed alterations in the quantity of glycogen, smooth endoplasmic reticulum, lysosomes and in the form of the rough endoplasmic reticulum. 6. The significance of these morphological changes was discussed in relation to an hormonal stimulation.

Rapid alterations induced by insulin in hepatocyte ultrastructure and glycogen levels

The speed with which insulin alters hepatocyte ultrastructure and glycogen levels in insulin-deficient rats has been studied. Insulin deficiency was induced with alloxan, followed by insulin treatment with regular and NPH insulin. Rats were killed at various times after the insulin injection, blood samples were obtained, plasma glucose levels were determined, and liver samples were prepared for electron microscopy and glycogen determinations. Plasma glucose levels in insulin-deficient rats declined to normal values by 4 hours post insulin, returning to insulin-deficient levels by 8 hours post insulin. Hepatic glycogen was considerably reduced in the insulin-deficient rats. By 1 hour post insulin hepatic glycogen increased, reached maximal levels by 8 hours, then declined to insulin-deficient levels by 36 hours. The ultrastructural appearance of both centrilobular and periportal hepatocytes from insulin-deficient rats showed abundant vesicular smooth endoplasmic reticulum (SER), decreased rough endoplasmic reticulum (RER), and enlarged RER intracisternal spaces. One-half hour post insulin, centrilobular hepatocytes were unchanged. In periportal hepatocytes, however, vesicular SER was no longer visible, the RER intracisternal spaces appeared normal, and the amount of RER had increased. By 1 hour post insulin the centrilobular hepatocytes showed similar ultrastructural changes. These changes became more pronounced in the next few hours and remained through 24 hours. By 36 hours both centrilobular and periportal hepatocytes appeared similar to those in the insulin-deficient rat. These results demonstrate the rapid and lobular-specific effects insulin has on the hepatocyte.

Genetics of receptors for bioactive polypeptides: a variant of Swiss/3T3 fibroblasts resistant to a cytotoxic insulin accumulates lysosome-like vesicles

Recently we have isolated six variants of Swiss/3T3 mouse fibroblasts that are resistant to the cytotoxic insulin-diphtheria toxin A fragment. All of the variants proved to have greatly reduced or no insulin binding capacity, and several variants showed altered morphologies and growth characteristics. We now report on the further characterization of one of these variants, CI-3, which displays a massive accumulation of membranous vesicles in its cytoplasm. By electron microscopy these vesicles resemble lysosomes. They also appear to fluoresce bright orange after treatment of viable cells with acridine orange. However, the specific activity of several lysosomal enzymes is depressed in CI-3. Additionally, there is an apparent shift in the density of vesicles containing lysosomal enzymes in this variant. These alterations may be directly related to CI-3's resistance to the cytotoxic insulin and have some important bearings on the mechanism of insulin action.

Polyamine and amino acid content, and activity of polyamine-synthesizing decarboxylases, in liver of streptozotocin-induced diabetic and insulin-treated diabetic rats

1. Concentrations of polyamines, amino acids, glycogen, nucleic acids and protein, and activities of ornithine decarboxylase and S-adenosylmethionine decarboxylase, were measured in livers from control, streptozotocin-diabetic and insulin-treated diabetic rats. 2. Total DNA per liver and protein per mg of DNA were unaffected by diabetes, whereas RNA per mg of DNA and glycogen per g of liver were decreased. Insulin treatment of diabetic rats induced both hypertrophy and hyperplasia, as indicated by an increase in all four of these constituents to or above control values. 3. Spermidine content was increased in the livers of diabetic rats, despite the decrease in RNA, but it was further increased by insulin treatment. Spermine content was decreased by diabetes, but was unchanged by insulin treatment. Thus the ratio spermidine/spermine in the adult diabetic rat was more typical of that seen in younger rats, whereas insulin treatment resulted in a ratio similar to that seen in rapidly growing tissues. 4. Ornithine decarboxylase activity was variable in the diabetic rat, showing a positive correlation with endogenous ornithine concentrations. This correlation was not seen in control or insulin-treated rats. Insulin caused a significant increase in ornithine decarboxylase activity relative to control or diabetic rats. 5. S-Adenosylmethionine decarboxylase activity was increased approx. 2-fold by diabetes and was not further affected by insulin. 6. Hepatic concentrations of the glucogenic amino acids, alanine, glutamine and glycine were decreased by diabetes. Their concentrations and that of glutamate were increased by injection of insulin. Concentrations of ornithine, proline, leucine, isoleucine and valine were increased in livers of diabetic rats and were decreased by insulin. Diabetes caused a decrease in hepatic concentration of serine, threonine, lysine and histidine. Insulin had no effect on serine, lysine and histidine, but caused a further fall in the concentration of threonine.

Phenobarbitone-induced stimulation of protein synthesis in liver of diabetic rat

The effect of phenobarbitone on liver weight, on the rate of protein synthesis and on the sedimentation profiles of polyribosomes from livers was studied in diabetic rats. The rate of protein synthesis by isolated postmitochondrial supernatants from diabetic rats is lower than that from normal animals. The analysis of polyribosome profiles and the effect of Sephadex chromatography on protein synthesis demonstrated that the reduction was dependent in part on polyribosomal disaggregation and in part on the presence in the cytosol of low molecular weight inhibitor(s). Phenobarbitone administration had the same effect in either diabetic or normal rats in that it increased, (a) the degree of polyribosomal aggregation, (b) the rate of protein synthesis by the isolated postmitochondrial supernatants, (c) liver weight and (d) the activity of the inducible enzyme, NADPH-cytochrome c reductase. Both polyribosomal and soluble factors appear to be involved in the phenobarbitone effect. As the diabetic rats do not secret insulin the results suggest that insulin is not involved in the control of protein synthesis by phenobarbitone. It is suggested that the intracellular redox state has a major influence on the rate of protein synthesis.

Studies of hormonal regulation of metabolism using isolated hepatocytes

A number of enzymatic methods have been developed to prepare hepatocytes using collagenase and hyaluronidase. However, best cell preparations are obtained by using only low concentrations of collagenase and exposing the liver to the enzyme for a very short period of time. These isolated cells with intact cell membranes and large numbers of microvilli on the cell surface respond to hormones at physiological concentrations suggesting that these microvilli contain hormone receptors. In addition, high glycogen content is essential to maintain the in vivo metabolic characteristics of the hepatocytes suggesting that intracellular glycogen plays an important role in the hormonal regulation of metabolism in hepatocytes. Studies with glucagon and insulin on carbohydrate metabolism show that the molar ratios of these hormones control gluconeogenesis and glycogenolysis. Furthermore, in vitro addition of insulin stimulates glycogen synthesis and activates glycogen synthase. Insulin also stimulates protein synthesis in cells containing high glycogen and maintains more normal parallel strands of polyribosomes. Studies with isolated hepatocytes from diabetic, hypophysectomized and adrenalectomized animals show a reduced glucagon response to glycogenolysis. This lack of glucagon response was not due to reduction in glycogen levels. Other hormones such as somatostatin and parathyroid also give rise to alterations in carbohydrate metabolism in isolated hepatocytes.

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

Parenchymal rat liver cells were isolated by a modification of the collagenase method of Quistorff, Bondesen and Grunnet. The cells secreted albumin into the medium and incorporated 14C-leucine both into cell proteins and proteins secreted into the medium. Albumin production measured from the immunologically determined increment in the incubation medium was 1.7 +/- 0.2 mug albumin/min per g liver wet wt. This is about 30% of the rate of production in the perfused liver. Addition of insulin (10(-6)-10(-10) M) enhanced albumin production (50-17%), and incorporation of 14C-leucine both into albumin (50-8%), secreted proteins (40-9%) and cell proteins (20-8%). Insulin does not increase the production of albumin by depleting the cells. The effect of insulin on albumin production is compatible with an effect on the rate of synthesis as the specific activity of albumin is unaffected by addition of insulin.

Studies on gluconeogenesis and stimulation of glycogen and protein synthesis in isolated hepatocytes in alloxan diabetic, normal fed and fasted animals

Hepatocytes were isolated by collagenase in vitro perfusion technique. Net glucose production in isolated hepatocytes obtained from fed, fasted and alloxan diabetic rats was studied. Net glucose production from alanine, pyruvate and fructose was increased by 2-5 fold in isolated hepatocytes obtained from fasted and alloxan diabetic rats. Similar increases in the incorporation of 14C-bicarbonate into glucose was also observed. Net glucose production in isolated hepatocytes was also compared to other in vitro preparations. Net glucose production was much higher (2-5 fold) in isolated hepatocytes than that reported previously for liver slices or perfused liver. Studies on glycogen and protein synthesis show a 2 fold stimulation in the incorporation of 1-14C-glucoase into glycogen and U-14C-leucine into protein by the addition of 100 muU of insulin to isolated hepatocytes.