Stimulation of albumin gene transcription by insulin in primary cultures of rat hepatocytes

MALDI-TOF MS Characterisation of the Serum Proteomic Profile in Insulin-Resistant Normal-Weight Individuals

Insulin resistance (IR) is one of the most common metabolic disorders worldwide and is involved in the development of diseases, such as diabetes and cardiovascular diseases, affecting civilisations. The possibility of understanding the molecular mechanism and searching for new biomarkers useful in assessing IR can be achieved through modern research techniques such as proteomics. This study assessed the protein–peptide profile among normal-weight patients with IR to understand the mechanisms and to define new risk biomarkers. The research involved 21 IR and 43 healthy, normal-weight individuals, aged 19–65. Serum proteomic patterns were obtained using matrix-assisted laser desorption/ionisation time-of-flight mass spectrometry. The proposed methodology identified six proteins differentiating normal weight IR and insulin sensitive individuals. They were fibrinogen alpha chain, serum albumin, kininogen-1, complement C3, serotransferrin, and Ig gamma-1 chain, which could potentially be related to inflammation. However, further investigation is required to confirm their correlation with IR.

U-shaped association between serum albumin and development of chronic kidney disease in general hypertensive patients

BACKGROUND & AIMS

We aimed to examine the association between serum albumin (SAlb) and the development of chronic kidney disease (CKD), and examine any possible effect modifiers in general hypertensive patients with normal renal function and with no previous cardiovascular diseases (CVD).

METHODS

This is a post-hoc analysis (performed at May, 2018) of 12,621 hypertensive adults with estimated glomerular filtration rate (eGFR) ≥60 mL/min/1.73 m² and SAlb ≥35.0 g/L from the renal sub-study of the China Stroke Primary Prevention Trial (CSPPT), conducted from May 2008 to August 2013. The primary outcome was development of CKD, defined as a decrease in eGFR of ≥30% and to a level of <60 mL/min/1.73 m²; or end stage renal disease.

RESULTS

The median follow-up duration was 4.4 years. Overall, the association between SAlb levels and risk of the primary outcome followed a U-shape. The risk of CKD development significantly decreased with the increment of SAlb (per g/L: OR = 0.92; 95% CI: 0.88-0.96) in participants with SAlb <51.4 g/L, and increased with the increment of SAlb (per g/L: OR = 1.06; 95%CI: 1.01-1.11) in participants with SAlb ≥51.4 g/L. Moreover, in participants with SAlb <51.4 g/L, the association between SAlb and CKD development remained significant in participants without proteinuria (per g/L: OR = 0.93; 95% CI: 0.88-0.99). The association between SAlb and CKD development was not significantly modified by age, sex, folic acid treatment, proteinuria, systolic blood pressure (SBP) at baseline and time-averaged SBP during the treatment period (all P-interactions>0.05).

CONCLUSIONS

There was a U-shaped association between SAlb levels and risk of CKD development among general hypertensive patients with normal renal function and without CVD, with a turning point at about 51.4 g/L.

Insulin Is Required to Maintain Albumin Expression by Inhibiting Forkhead Box O1 Protein

Diabetes is accompanied by dysregulation of glucose, lipid, and protein metabolism. In recent years, much effort has been spent on understanding how insulin regulates glucose and lipid metabolism, whereas the effect of insulin on protein metabolism has received less attention. In diabetes, hepatic production of serum albumin decreases, and it has been long established that insulin positively controls albumin gene expression. In this study, we used a genetic approach in mice to identify the mechanism by which insulin regulates albumin gene transcription. Albumin expression was decreased significantly in livers with insulin signaling disrupted by ablation of the insulin receptor or Akt. Concomitant deletion of Forkhead Box O1 (Foxo1) in these livers rescued the decreased albumin secretion. Furthermore, activation of Foxo1 in the liver is sufficient to suppress albumin expression. These results suggest that Foxo1 acts as a repressor of albumin expression.

Identification of differentially expressed genes related to metabolic syndrome induced with high-fat diet in E3 rats

Understanding the genes differentially expressing in aberrant organs of metabolic syndrome (MetS) facilitates the uncovering of molecular mechanisms and the identification of novel therapeutic targets for the disease. This study aimed to identify differentially expressed genes related to MetS in livers of E3 rats with high-fat-diet-induced metabolic syndrome (HFD-MetS). E3 rats were fed with high-fat diet for 24 weeks to induce MetS. Then, suppression subtractive hybridization (SSH) technology was used to identify the genes differentially expressed between HFD-MetS and control E3 rat livers. Twenty positive recombinant clones were chosen randomly from forward subtractive library and sent to sequence. BLAST analysis in GenBank database was used to determine the property of each cDNA fragment. In total, 11 annotated genes, 3 ESTs, and 2 novel gene fragments were identified by SSH technology. The expression of four genes (Alb, Pip4k2a, Scd1, and Tf) known to be associated with MetS and other five genes (Eif1, Rnase4, Rps12, Rup2, and Tmsb4) unknown to be relevant to MetS was significantly up-regulated in the livers of HFD-MetS E3 rats compared with control rats using real-time quantitative PCR (RT-qPCR). By analyzing the correlations between the expression of these nine genes and serum concentrations of TG, Tch, HDL-C, and LDL-C, we found that there were significant positive correlations between TG and the expression of five genes (Alb, Eif1, Pip4k2a, Rps12, and Tmsb4x), Tch and three genes (Rnase4, Scd1, and Tmsb4x), and LDL-C and two genes (Rnase4 and Scd1), as well there were significant negative correlations between HDL-C and the expression of three genes (Rup2, Scd1, and Tf). This study provides important clues for unraveling the molecular mechanisms of MetS.

Medium-chain triacylglycerol suppresses the decrease of plasma albumin level through the insulin-Akt-mTOR pathway in the livers of malnourished rats

Recent studies have shown that medium-chain triacylglycerol (MCT) improved serum albumin concentration in elderly people with protein-energy malnutrition (PEM) and in malnourished rats. However, the mechanism for this effect has not been clarified. Dietary MCT promotes insulin secretion from the pancreas, and insulin activates mammalian target of rapamycin (mTOR) complex 1 (mTORC1) via the activation of phosphoinositide 3-kinase (PI3K) and its downstream effecter, Akt. mTORC1 promotes mRNA translation through S6K and 4E-BP1. Therefore, we hypothesized that dietary MCT elevates albumin synthesis through promotion of insulin-Akt-mTOR transduction in the liver. To test this hypothesis, we measured phosphorylated Akt, mTOR and albumin in the livers of malnourished rats. In the present study we examined rats fed low-protein diets containing either MCT or long-chain triacylglycerol (LCT) with energy restriction. The plasma and liver albumin levels were significantly higher in the MCT-fed group than in the LCT-fed group. In addition, plasma insulin concentration, liver phosphorylated Akt/Akt and phosphorylated mTOR/mTOR levels were significantly higher in the MCT-fed group than in the LCT-fed group. These results suggest that one of the mechanisms for the albumin improvement effect of dietary MCT is the promotion of albumin synthesis through the insulin-Akt-mTOR signaling pathway of the liver.

Association between Serum Albumin, Insulin Resistance, and Incident Diabetes in Nondiabetic Subjects

BACKGROUND

Serum albumin has been suggested to be associated with insulin resistance. We evaluated the association between serum albumin concentration and insulin resistance. We also investigated whether serum albumin level has an independent effect on the development of diabetes.

METHODS

In our study, 9,029 subjects without diabetes, who underwent comprehensive health check-ups annually for 5 years, were categorized into tertiles based on their serum albumin levels at baseline. The odds ratio (OR) for the prevalence of insulin resistance, defined as the top quartile of homeostasis model assessment of insulin resistance and the presence of impaired fasting glucose and nonalcoholic fatty liver disease, was evaluated cross-sectionally. Also, the hazard ratio (HR) for incident diabetes was estimated longitudinally, according to the baseline albumin tertiles using Cox proportional hazard analysis respectively.

RESULTS

From the lowest to the highest tertile of albumin, the multivariable-adjusted ORs of insulin resistance increased significantly in both men and women. During the mean follow-up period of nearly 4 years, 556 (6.1%) subjects progressed to diabetes. The multivariable-adjusted HR (95% confidence interval [CI]) of diabetes in men were 1, 1.09 (95% CI, 0.86 to 1.40), and 1.10 (95% CI, 0.86 to 1.41), respectively, from the lowest to the highest tertiles of baseline albumin. Corresponding values for women were 1, 1.21 (95% CI, 0.66 to 2.21), and 1.06 (95% CI, 0.56 to 2.02), respectively.

CONCLUSION

Our study showed that increased serum albumin level was associated with insulin resistance. However, serum albumin did not have an independent effect on the development of diabetes.

Albumin synthesis rates are not responsive to hyperglycemic hyperinsulinemia in postoperative patients

BACKGROUND

Insulin regulates albumin synthesis in vitro and in various experimental models. The current study was undertaken to determine the effects of a physiologic hyperinsulinemia on albumin synthesis in postoperative patients in whom plasma albumin concentrations are decreased.

METHODS

Studies were performed in postabsorptive patients after major abdominal operations. Mass spectrometry techniques were used to directly determine the incorporation rate of 1-[(13)C]-leucine into albumin. Consecutive blood samples were taken during a continuous isotope (D-Glc) infusion (0.16 µmol/kg/min). Isotopic enrichments were determined at baseline (period I) and after a 4-hour D-glucose (D-Glc) infusion at currently recommended rates (170 mg/kg/h, n = 10) or after infusion of saline (control group, n = 8) (period II).

RESULTS

After D-Glc infusion, plasma insulin concentrations increased significantly (period I, 6.6 ± 1.8 µU/mL; period II, 21.4 ± 2.1 µU/mL; P < .01). In contrast, plasma insulin concentration remained constant in control patients (period I, 3.8 ± 0.9 µU/mL(-1); period II, 5.9 ± 1.1 µU/mL; not significant vs period I, but P < .005 vs the corresponding value at the end of period II in the control group). Hyperinsulinemia was without effect on fractional albumin synthesis (period I, 12.8% ± 1.9%/d; period II, 11.9% ± 1.9%/d; not significant), and synthesis rates corresponded to those measured in controls (period I, 13.0% ± 1.2%/d; period II, 12.1% ± 0.1%/d; not significant vs period I and vs D-Glc infusion).

CONCLUSIONS

A standard D-Glc infusion is insufficient to increase albumin synthesis in postoperative patients.

Monolayer and spheroid culture of human liver hepatocellular carcinoma cell line cells demonstrate distinct global gene expression patterns and functional phenotypes

Understanding cell biology of three-dimensional (3D) biological structures is important for more complete appreciation of in vivo tissue function and advancing ex vivo organ engineering efforts. To elucidate how 3D structure may affect hepatocyte cellular responses, we compared global gene expression of human liver hepatocellular carcinoma cell line (HepG2) cells cultured as monolayers on tissue culture dishes (TCDs) or as spheroids within rotating wall vessel (RWV) bioreactors. HepG2 cells grown in RWVs form spheroids up to 100 mum in diameter within 72 h and up to 1 mm with long-term culture. The actin cytoskeleton in monolayer cells show stress fiber formation while spheroids have cortical actin organization. Global gene expression analysis demonstrates upregulation of structural genes such as extracellular matrix, cytoskeletal, and adhesion molecules in monolayers, whereas RWV spheroids show upregulation of metabolic and synthetic genes, suggesting functional differences. Indeed, liver-specific functions of cytochrome P450 activity and albumin production are higher in the spheroids. Enhanced liver functions require maintenance of 3D structure and environment, because transfer of spheroids to a TCD results in spheroid disintegration and subsequent loss of function. These findings illustrate the importance of physical environment on cellular organization and its effects on hepatocyte processes.

Loss of insulin signaling in hepatocytes leads to severe insulin resistance and progressive hepatic dysfunction

The liver plays a central role in the control of glucose homeostasis and is subject to complex regulation by substrates, insulin, and other hormones. To investigate the effect of the loss of direct insulin action in liver, we have used the Cre-loxP system to inactivate the insulin receptor gene in hepatocytes. Liver-specific insulin receptor knockout (LIRKO) mice exhibit dramatic insulin resistance, severe glucose intolerance, and a failure of insulin to suppress hepatic glucose production and to regulate hepatic gene expression. These alterations are paralleled by marked hyperinsulinemia due to a combination of increased insulin secretion and decreased insulin clearance. With aging, the LIRKO liver exhibits morphological and functional changes, and the metabolic phenotype becomes less severe. Thus, insulin signaling in liver is critical in regulating glucose homeostasis and maintaining normal hepatic function.

Insulin-mediated modulation of cytochrome P450 gene induction profiles in primary rat hepatocyte cultures

In this investigation, we examined the effects of insulin on gene induction responsiveness in primary rat hepatocytes. Cells were cultured for 72 hours either in the absence or presence of 1 microM insulin and then exposed to increasing concentrations of phenobarbital (PB; 0.01-3.5 mM). Culturing in the absence of insulin produced 1.5-2-fold increases in the induction magnitude of CYP2B1 and CYP2B2 mRNA expression resulting from PB exposures, without altering the bell-shaped dose-response curve characteristic of this agent. However, for the CYP3A1 gene, insulin removal led to a pronounced shift in both the PB-induction magnitude and dose-response relationships of the induction response, with higher levels of CYP3A1 expression resulting from exposures to lower concentrations of inducer. Insulin removal also reduced the time required to attain maximal induction of CYP2B1/2 and CYP3A1 gene expression. The insulin effects were not specific for PB induction, as insulin deprivation similarly enhanced both dexamethasone- and beta-naphthoflavone-inducible CYP3A1 and CYP1A1 expression profiles, respectively. In contrast, the level of albumin mRNA expression was reduced considerably in cells deprived of insulin. We conclude that insulin is an important regulator of inducible and liver-specific gene expression in primary rat hepatocytes.

Debate: Albumin administration should be avoided in the critically ill

The benefit of albumin administration in the critically ill patient is unproven. Epidemiological evidence suggests that there is an increase in death among patients with burns, hypoalbuminaemia, and hypotension treated with human albumin solution (HAS). In critical illness, hypoalbuminaemia is a result of transcapillary leak, decreased synthesis, large volume body fluid losses, and dilution caused by fluid resuscitation. When treating patients with hypoalbuminaemia, efforts must be centred around correction of the underlying disorder rather than reversal of hypoalbuminaemia. Problems with using albumin arise because it is an expensive blood product, and can result in systemic changes that include cardiovascular, haematological, renal, pulmonary, and immunological effects.

Influence of pancreatic islets on growth and differentiation of hepatocytes in co-culture

Improvement of cell culture conditions in hepatic tissue engineering may permit cell/tissue banking and the generation of liver tissue equivalents for transplantation. In these systems, continuous hepatotrophic stimulation is still necessary. We investigated the stimulatory effects of pancreatic islets on hepatocytes in co-culture and characterized the stimulatory mechanisms. Hepatocytes and pancreatic islets were harvested from Lewis rats. Cells were cultured on collagen dishes either with nonstimulated media (controls and co-cultures with low or high islet rate) or stimulated media (controls and co-cultures). To characterize stimulatory mechanisms, additional co-cultures with membrane separation, with antiinsulin, antiglucagon, and with both antibodies were examined. Hepatocyte numbers, albumin secretion rate by enzyme-linked immunoadsorbent assay, and monoethylglycinxylidid biotransformation values by fluorescence polarization immunoassay were assessed. A radioimmunoassay measured insulin and glucagon concentrations. In groups with nonstimulated media, cell number was higher in co-cultures with low islet rate, and albumin secretion rate was increased in co-cultures with high islet rate compared to controls. MEGX biotransformation was decreased in co-cultures. In groups with stimulated media, co-culture had no impact on cell number or albumin secretion rate. Hepatocyte numbers and albumin secretion rates were not changed in co-cultures after membrane separation. Islet effects on hepatocytes were reduced in co-cultures with antiinsulin, antiglucagon, or both antibodies. Pancreatic islets provide stimulation for hepatocytes in vitro. Islet effects were mediated by soluble factors, and are dependent on insulin and glucagon. These results permit further investigations towards three-dimensional transplantable hepatocyte-islet devices for continuous in vitro and in vivo stimulation.

Effects of insulin, dexamethasone and cytokines on alpha 1-acid glycoprotein gene expression in primary cultures of normal rat hepatocytes

While the effects of insulin, dexamethasone and cytokines on alpha 1-acid glycoprotein gene expression have been investigated in various hepatoma cell lines, the individual and combined effects of these components on the expression of this gene have been rarely studied in cultured normal rat hepatocytes. In this cell model, we have shown that mRNA levels of alpha 1-acid glycoprotein were not decreased at least during the first 24 h of culture under basal conditions. During these short-term cultures, the expression of alpha 1-acid glycoprotein in normal hepatocytes showed a high degree of responsiveness to dexamethasone alone (20-fold increase) and to dexamethasone associated with various cytokines (interleukin-1 beta, interleukin-6 and tumor necrosis factor alpha) with a 40 to 100-fold increase depending on the cytokine. Insulin alone did not modify alpha 1-acid glycoprotein mRNA; however, this hormone exerted a positive effect (about 50% increase) in the presence of dexamethasone or dexamethasone with cytokines. These results indicate that the regulation of alpha 1-acid glycoprotein in cultured normal rat hepatocytes presents major differences when compared to reported observations in rat hepatoma cell lines.

Translational and pretranslational regulation of protein synthesis by amino acid availability in primary cultures of rat hepatocytes

Protein synthesis is inhibited in both rat liver and isolated rat hepatocytes following deprivation of single essential amino acids. The aim of the present study was to define the time course of changes in peptide-chain initiation, albumin synthesis, and albumin mRNA following histidine deprivation and the reversal of these changes in response to readdition of the deprived amino acid. A further aim was to ascertain whether there was an accommodation of the inhibition of initiation following long-term amino acid deprivation. Primary cultures of rat hepatocytes were maintained in serum-free medium containing either all amino acids (complete medium) or all except histidine. Synthesis of total protein was reduced to 34% of control values following 48 hr of histidine deprivation and was restored to control values within 1 hr of addition of complete medium to histidine-deprived cells. These changes in protein synthesis were due to translational regulation involving initiation. No accommodation of the inhibition was observed following long-term deprivation of histidine as has been observed under other conditions of cellular stress. The synthesis of albumin was reduced to a greater extent than that of total protein, and required 72 hr to recover to control values following return to complete medium. These changes in albumin synthesis were due to a combination of altered initiation and a mechanism involving pretranslational regulation as evidenced by corresponding alterations in albumin mRNA. The results show that amino acid availability controls protein synthesis in liver cells through both translational and pretranslational mechanisms.

Metformin induces an agonist-specific increase in albumin production by primary cultured rat hepatocytes

Metformin (MET) is known to increase several biological effects of insulin (INS), but there is no information concerning its direct effects on protein synthesis. We studied the action of MET on albumin production by primary cultures of freshly isolated rat hepatocytes, alone or in combination with various agonists: INS, IGF-1, EGF, thyroxin, and dexamethasone. While having no effect alone, MET in vitro potentiates the effects of INS, IGF-1, and EGF. When this increasing effect toward INS was studied over a broad concentration range, MET appeared to improve low-acting INS levels and to intensify the maximal INS effects. In contrast, MET did not change the production of albumin stimulated by thyroxin or dexamethasone. Animals chronically pretreated with MET in vivo showed a higher yield of isolated hepatocytes, better attachment, and especially higher viability after liver perfusion and during cell culture. This may largely explain why basal albumin rates were higher than in in vitro-treated cells. The effect of MET in the presence of the agonists exhibited the same agonist-specificity as in vitro. Our data provide new insights into the pharmacology of MET by showing that hepatic protein synthesis is increased by MET and INS. From the specificity of action of MET towards INS, IGF-1, and EGF (but not thyroxin or dexamethasone), we hypothesize that this biguanide may act on intracellular pathways located between membrane receptors and sites of branching in the signaling cascades shared by these agonists.

Evidence for a role of insulin in hepatocytic differentiation of human hepatoma BC1 cells

To examine the effect of insulin on hepatocytic differentiation, we took advantage of the properties of the newly established human hepatoma BC1 cell line to maintain quiescence after confluency and to progressively acquire in culture (3 weeks after confluency) an hepatocytic phenotype, as assessed by expression of specific hepatic genes (Le Jossicet al., 1995). In BC1 cells cultured in the presence of insulin (1 μM: ), expression of albumin and transferrin mRNA and protein occurs earlier than in cells cultured in its absence (1 weekvs 2 weeks). Moreover, at any time considered, the level of the two hepatic markers was higher (2- to 3-fold) in the former than in untreated cells. The beneficial effect of insulin on hepatocytic differentiation of BC1 cells was paralleled by: i) modest increases in insulin receptor (IR) mRNA level and IR binding activity, and ii) a 6-fold increase in sensitivity to insulin for stimulation of glycogenesis. These results provide the first evidence for insulin's ability to exert a positive effect on hepatocytic differentiation. The beneficial effect of insulin probably results both from increased IR expression and binding activity and from alteration at post-receptor levels.

Rapid regulation of albumin transcription by insulin and phorbol esters in rat hepatoma cells

The short-term effects of insulin and phorbol esters on the regulation of the albumin gene in rat H4IIE (H4) hepatoma cells were investigated and compared to the expression of a gene known to be inhibited by these agents, phosphoenol pyruvate carboxykinase (PEPCK). Both insulin and phorbol esters inhibited transcription of the albumin gene in a rapid, dose-dependent manner. Within 15 min, albumin transcription was reduced by approx. 80%. The inhibitory effects of insulin were evident at concentrations of insulin as low as 5.10(-11)M, suggesting that these effects were mediated through insulin-specific pathways. The ability of both phorbol esters and insulin to inhibit albumin transcription suggests that the negative control of this gene is a stable feature in H4 cells. The effect of phorbol esters to mimic insulin action on the albumin gene, and on several other genes in this cell line, implies that a common pathway may be shared by both insulin and phorbol esters.

Regulation of albumin expression in fetal rat hepatocytes cultured under proliferative conditions: role of epidermal growth factor and hormones

Sustained production of plasma proteins, notably albumin, is a reliable indicator of the differentiated state of hepatocytes. In this work, we have developed a fetal hepatocyte culture system where studying the regulation of albumin expression in proliferating liver cells. Our results show that under proliferative conditions (i.e., in the presence of EGF) fetal hepatocytes maintain albumin production above control quiescent non-treated cells. Glucagon and noradrenaline have no effect on the proliferation induced by EGF in cultured fetal hepatocytes; however, they act synergistically with the growth factor, increasing intracellular albumin levels. The maximum response is obtained by treatment of cells with EGF and noradrenaline. The stimulatory noradrenergic effect is mimicked by agents that increase cyclic AMP levels (forskolin plus IBMX). However, vasopressin or phorbol esters have no effect on albumin production, neither alone nor in combination with EGF. Dexamethasone, which does not alter the proliferative induction of EGF, increases albumin content. This effect is independent of the proliferative status of the cells and is not enhanced by glucagon, noradrenaline, or cyclic AMP increasing agents. The hormonal changes observed in albumin production partially correlate with changes in mRNA levels. This is the first time that cyclic AMP increasing agents are shown to act synergistically with EGF, increasing the expression of this liver specific gene.

Blood serum characteristics of newborn pigs: comparison of unaffected pigs with pigs belonging to five mortality groups

The blood serum levels of glucose, hemoglobin, insulin, cortisol, albumin, alpha-fetoprotein, alpha 2-macroglobulin f and s, alpha 2-antitrypsin inhibitor and alpha 1-protease inhibitor were determined at birth in 5 clinically and morphologically identified mortality groups of pigs. These were compared with the levels observed in unaffected, apparently normal newborn unsuckled pigs. The blood serum profile of the pigs in the stillborn intra partum, weak, splayleg and trauma groups, respectively, as well as that of clinically normal splayleg littermates, differed significantly from that of the unaffected pigs. This was especially true for the levels of hemoglobin and the two macroglobulins. The importance of placental insufficiency causing chronic episodes of hypoxia which ultimately lead to a disturbance in organ development in the etiology of the mortality groups is discussed.

Regulation of gene expression by insulin

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Differential effects of insulin deficiency on albumin and fibrinogen synthesis in humans

Insulin deficiency decreases tissue protein synthesis, albumin mRNA concentration, and albumin synthesis in rats. In contrast, insulin deficiency does not change, or, paradoxically, increases estimates of whole body protein synthesis in humans. To determine if such estimates of whole body protein synthesis could obscure potential differential effects of insulin on the synthetic rates of individual proteins, we determined whole body protein synthesis and albumin and fibrinogen fractional synthetic rates using 5-h simultaneous infusions of [14C]leucine and [13C]bicarbonate, in six type 1 diabetics during a continuous i.v. insulin infusion (to maintain euglycemia) and after short-term insulin withdrawal (12 +/- 2 h). Insulin withdrawal increased (P less than 0.03) whole body proteolysis by approximately 35% and leucine oxidation by approximately 100%, but did not change 13CO2 recovery from NaH13CO3 or estimates of whole body protein synthesis (P = 0.21). Insulin deficiency was associated with a 29% decrease (P less than 0.03) in the albumin fractional synthetic rate but a 50% increase (P less than 0.03) in that of fibrinogen. These data provide strong evidence that albumin synthesis in humans is an insulin-sensitive process, a conclusion consistent with observations in rats. The increase in fibrinogen synthesis during insulin deficiency most likely reflects an acute phase protein response due to metabolic stress. These data suggest that the absence of changes in whole body protein synthesis after insulin withdrawal is the result of the summation of differential effects of insulin deficiency on the synthesis of specific body proteins.

Alteration of microtubule physiology in hepatocytes by insulin

Incubation of primary cultures of rat hepatocytes with insulin caused an alteration in several parameters of microtubule physiology. There was a transient decrease in total cellular tubulin levels and, at the same time, an apparent stabilization of microtubule polymers which was also transient. The level of microtubule polymer, however, was not altered, which indicated that an insulin-inducible decrease in the level of nonpolymerized tubulin was responsible for the decrease in total cellular tubulin. These changes were followed by an increase in tubulin mRNA levels and, shortly thereafter, by an increase in tubulin protein synthesis. This study demonstrates that microtubule physiology in hepatocytes is modulated by insulin and, further, suggests a model by which insulin-induced changes in microtubule physiology may play a role in regulating other insulin responses.