Increased let-7d-5p in non-alcoholic fatty liver promotes insulin resistance and is a potential blood biomarker for diagnosis

BACKGROUND AND AIMS

The molecular mechanisms driving non-alcoholic fatty liver disease (NAFLD) are poorly understood; however, microRNAs might play a key role in these processes. We hypothesize that let-7d-5p could contribute to the pathophysiology of NAFLD and serve as a potential diagnostic biomarker.

METHODS

We evaluated let-7d-5p levels and its targets in liver biopsies from a cross-sectional study including patients with NAFLD and healthy donors, and from a mouse model of NAFLD. Moreover, the induction of let-7d-5p expression by fatty acids was evaluated in vitro. Further, we overexpressed let-7d-5p in vitro to corroborate the results observed in vivo. Circulating let-7d-5p and its potential as a NAFLD biomarker was determined in isolated extracellular vesicles from human plasma by RT-qPCR.

RESULTS

Our results demonstrate that hepatic let-7d-5p was significantly up-regulated in patients with steatosis, and this increase correlated with obesity and a decreased expression of AKT serine/threonine kinase (AKT), insulin-like growth factor 1 (IGF1), IGF-I receptor (IGF1R) and insulin receptor (INSR). These alterations were corroborated in a NAFLD mouse model. In vitro, fatty acids increased let-7d-5p expression, and its overexpression decreased AKT, IGF-IR and IR protein expression. Furthermore, let-7d-5p hindered AKT phosphorylation in vitro after insulin stimulation. Finally, circulating let-7d-5p significantly decreased in steatosis patients and receiver operating characteristic (ROC) analyses confirmed its utility as a diagnostic biomarker.

CONCLUSIONS

Our results highlight the emerging role of let-7d-5p as a potential therapeutic target for NAFLD since its overexpression impairs hepatic insulin signalling, and also, as a novel non-invasive biomarker for NAFLD diagnosis.

Specific knockout of p85α in brown adipose tissue induces resistance to high-fat diet-induced obesity and its metabolic complications in male mice

Objective

An increase in mass and/or brown adipose tissue (BAT) functionality leads to an increase in energy expenditure, which may be beneficial for the prevention and treatment of obesity. Moreover, distinct class I PI3K isoforms can participate in metabolic control as well as in systemic dysfunctions associated with obesity. In this regard, we analyzed in vivo whether the lack of p85α in BAT (BATp85αKO) could modulate the activity and insulin signaling of this tissue, thereby improving diet-induced obesity and its associated metabolic complications.

Methods

We generated BATp85αKO mice using Cre-LoxP technology, specifically deleting p85α in a conditional manner. To characterize this new mouse model, we used mice of 6 and 12 months of age. In addition, BATp85αKO mice were submitted to a high-fat diet (HFD) to challenge BAT functionality.

Results

Our results suggest that the loss of p85α in BAT improves its thermogenic functionality, high-fat diet–induced adiposity and body weight, insulin resistance, and liver steatosis. The potential mechanisms involved in the improvement of obesity include (1) increased insulin signaling and lower activation of JNK in BAT, (2) enhanced insulin receptor isoform B (IRB) expression and association with IRS-1 in BAT, (3) lower production of proinflammatory cytokines by the adipose organ, (4) increased iWAT browning, and (5) improved liver steatosis.

Conclusions

Our results provide new mechanisms involved in the resistance to obesity development, supporting the hypothesis that the gain of BAT activity induced by the lack of p85α has a direct impact on the prevention of diet-induced obesity and its associated metabolic complications.

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The lack of p85α in brown adipose tissue confers obesity resistance.

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BATp85αKO mice show improved thermogenic function, fatty liver and insulin resistance.

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High IRB levels in BAT and iWAT browning might explain the improvement of obesity.

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Increase in BAT functionality has a direct impact on the prevention of obesity.

Liver-specific insulin receptor isoform A expression enhances hepatic glucose uptake and ameliorates liver steatosis in a mouse model of diet-induced obesity

Among the main complications associated with obesity are insulin resistance and altered glucose and lipid metabolism within the liver. It has previously been described that insulin receptor isoform A (IRA) favors glucose uptake and glycogen storage in hepatocytes compared with isoform B (IRB), improving glucose homeostasis in mice lacking liver insulin receptor. Thus, we hypothesized that IRA could also improve glucose and lipid metabolism in a mouse model of high-fat-diet-induced obesity. We addressed the role of insulin receptor isoforms in glucose and lipid metabolism in vivo. We expressed IRA or IRB specifically in the liver by using adeno-associated viruses (AAVs) in a mouse model of diet-induced insulin resistance and obesity. IRA, but not IRB, expression induced increased glucose uptake in the liver and muscle, improving insulin tolerance. Regarding lipid metabolism, we found that AAV-mediated IRA expression also ameliorated hepatic steatosis by decreasing the expression of Fasn, Pgc1a, Acaca and Dgat2 and increasing Scd-1 expression. Taken together, our results further unravel the role of insulin receptor isoforms in hepatic glucose and lipid metabolism in an insulin-resistant scenario. Our data strongly suggest that IRA is more efficient than IRB at favoring hepatic glucose uptake, improving insulin tolerance and ameliorating hepatic steatosis. Therefore, we conclude that a gene therapy approach for hepatic IRA expression could be a safe and promising tool for the regulation of hepatic glucose consumption and lipid metabolism, two key processes in the development of non-alcoholic fatty liver disease associated with obesity.

This article has an associated First Person interview with the first author of the paper.

Summary: Adeno-associated-virus-mediated gene therapy for insulin receptor isoform A expression in the liver improves glucose disposal and alleviates lipid accumulation in wild-type mice under a high-fat diet.

The Role of Insulin Receptor Isoforms in Diabetes and Its Metabolic and Vascular Complications

The insulin receptor (IR) presents by alternative splicing two isoforms: IRA and IRB. The differential physiological and pathological role of both isoforms is not completely known, and it is determinant the different binding affinity for insulin-like growth factor. IRB is more abundant in adult tissues and it exerts mainly the metabolic actions of insulin, whereas IRA is mainly expressed in fetal and prenatal period and exerts mitogenic actions. However, the change in the expression profile of both IR isoforms and its dysregulation are associated with the development of different pathologies, such as cancer, insulin resistance, diabetes, obesity, and atherosclerosis. In some of them, there is a significant increase of IRA/IRB ratio conferring a proliferative and migratory advantage to different cell types and favouring IGF-II actions with a sustained detriment in the metabolic effects of insulin. This review discussed specifically the role of IR isoforms as well as IGF-IR in diabetes and its associated complications as obesity and atherosclerosis. Future research with new IR modulators might be considered as possible targets to improve the treatment of diabetes and its associated complications.

Prevalent role of the insulin receptor isoform A in the regulation of hepatic glycogen metabolism in hepatocytes and in mice

AIMS/HYPOTHESIS

In the postprandial state, the liver regulates glucose homeostasis by glucose uptake and conversion to glycogen and lipids. Glucose and insulin signalling finely regulate glycogen synthesis through several mechanisms. Glucose uptake in hepatocytes is favoured by the insulin receptor isoform A (IRA), rather than isoform B (IRB). Thus, we hypothesised that, in hepatocytes, IRA would increase glycogen synthesis by promoting glucose uptake and glycogen storage.

METHODS

We addressed the role of insulin receptor isoforms on glycogen metabolism in vitro in immortalised neonatal hepatocytes. In vivo, IRA or IRB were specifically expressed in the liver using adeno-associated virus vectors in inducible liver insulin receptor knockout (iLIRKO) mice, a model of type 2 diabetes. The role of IR isoforms in glycogen synthesis and storage in iLIRKO was subsequently investigated.

RESULTS

In immortalised hepatocytes, IRA, but not IRB expression induced an increase in insulin signalling that was associated with elevated glycogen synthesis, glycogen synthase activity and glycogen storage. Similarly, elevated IRA, but not IRB expression in the livers of iLIRKO mice induced an increase in glycogen content.

CONCLUSIONS/INTERPRETATION

We provide new insight into the role of IRA in the regulation of glycogen metabolism in cultured hepatocytes and in the livers of a mouse model of type 2 diabetes. Our data strongly suggest that IRA is more efficient than IRB at promoting glycogen synthesis and storage. Therefore, we suggest that IRA expression in the liver could provide an interesting therapeutic approach for the regulation of hepatic glucose content and glycogen storage.

Insulin receptor isoform A ameliorates long-term glucose intolerance in diabetic mice

Type 2 diabetes mellitus is a complex metabolic disease and its pathogenesis involves abnormalities in both peripheral insulin action and insulin secretion. Previous in vitro data showed that insulin receptor isoform A, but not B, favours basal glucose uptake through its specific association with endogenous GLUT1/2 in murine hepatocytes and beta cells. With this background, we hypothesized that hepatic expression of insulin receptor isoform A in a mouse model of type 2 diabetes could potentially increase the glucose uptake of these cells, decreasing the hyperglycaemia and therefore ameliorating the diabetic phenotype. To assure this hypothesis, we have developed recombinant adeno-associated viral vectors expressing insulin receptor isoform A (IRA) or isoform B (IRB) under the control of a hepatocyte­-specific promoter. Our results demonstrate that in the long term, hepatic expression of IRA in diabetic mice is more efficient than IRB in ameliorating glucose intolerance. Consequently, it impairs the induction of compensatory mechanisms through beta cell hyperplasia and/or hypertrophy that finally lead to beta cell failure, reverting the diabetic phenotype in about 8 weeks. Our data suggest that long-term hepatic expression of IRA could be a promising therapeutic approach for the treatment of type 2 diabetes mellitus.

Summary: The specific hepatic expression of insulin receptor isoform A, but not isoform B, is able to revert, in the long term, the global glucose intolerance observed in diabetic mice.

Insulin receptor isoform A confers a higher proliferative capability to pancreatic beta cells enabling glucose availability and IGF-I signaling

The main compensatory response to insulin resistance is the pancreatic beta cell hyperplasia to account for increased insulin secretion. In fact, in a previous work we proposed a liver-pancreas endocrine axis with IGF-I (insulin-like growth factor type I) secreted by the liver acting on IRA insulin receptor in beta cells from iLIRKO mice (inducible Liver Insulin Receptor KnockOut) that showed a high IRA/IRB ratio. However, the role of insulin receptor isoforms in the IGF-I-induced beta cell proliferation as well as the underlying molecular mechanisms remain poorly understood. For this purpose, we have used four immortalized mouse beta cell lines: bearing IR (IRLoxP), lacking IR (IRKO), expressing exclusively IRA (IRA), or alternatively expressing IRB (IRB). Pancreatic beta cell proliferation studies showed that IRA cells are more sensitive than those expressing IRB to the mitogenic response induced by IGF-I, acting through the pathway IRA/IRS-1/2/αp85/Akt/mTORC1/p70S6K. More importantly, IRA beta cells, but not IRB, showed an increased glucose uptake as compared with IRLoxP cells, this effect being likely owing to an enhanced association between Glut-1 and Glut-2 with IRA. Overall, our results strongly suggest a prevalent role of IRA in glucose availability and IGF-I-induced beta cell proliferation mainly through mTORC1. These results could explain, at least partially, the role played by the liver-secreted IGF-I in the compensatory beta cell hyperplasia observed in response to severe hepatic insulin resistance in iLIRKO mice.

Antagonistic effect of TNF-alpha and insulin on uncoupling protein 2 (UCP-2) expression and vascular damage

Background

It has been reported that increased expression of UCP-2 in the vasculature may prevent the development of atherosclerosis in patients with increased production of reactive oxygen species, as in the diabetes, obesity or hypertension. Thus, a greater understanding in the modulation of UCP-2 could improve the atherosclerotic process. However, the effect of TNF-α or insulin modulating UCP-2 in the vascular wall is completely unknown. In this context, we propose to study new molecular mechanisms that help to explain whether the moderate hyperinsulinemia or lowering TNF-α levels might have a protective role against vascular damage mediated by UCP-2 expression levels.

Methods

We analyzed the effect of insulin or oleic acid in presence or not of TNF-α on UCP-2 expression in murine endothelial and vascular smooth muscle cells. At this step, we wondered if some mechanisms studied in vitro could be of any relevance in vivo. We used the following experimental models: ApoE^−/− mice under Western type diet for 2, 6, 12 or 18 weeks, BATIRKO mice under high-fat diet for 16 weeks and 52-week-old BATIRKO mice with o without anti-TNF-α antibody pre-treatment.

Results

Firstly, we found that TNF-α pre-treatment reduced UCP-2 expression induced by insulin in vascular cells. Secondly, we observed a progressive reduction of UCP-2 levels together with an increase of lipid depots and lesion area in aorta from ApoE^−/− mice. In vivo, we also observed that moderate hyperinsulinemic obese BATIRKO mice have lower TNF-α and ROS levels and increased UCP-2 expression levels within the aorta, lower lipid accumulation, vascular dysfunction and macrovascular damage. We also observed that the anti-TNF-α antibody pre-treatment impaired the loss of UCP-2 expression within the aorta and relieved vascular damage observed in 52-week-old BATIRKO mice. Finally, we observed that the pretreatment with iNOS inhibitor prevented UCP-2 reduction induced by TNF-α in vascular cells. Moreover, iNOS levels are augmented in aorta from mice with lower UCP-2 levels and higher TNF-α levels.

Conclusions

Our data suggest that moderate hyperinsulinemia in response to insulin resistance or lowering of TNF-α levels within the aorta attenuates vascular damage, this protective effect being mediated by UCP-2 expression levels through iNOS.

Electronic supplementary material

The online version of this article (doi:10.1186/s12933-014-0108-9) contains supplementary material, which is available to authorized users.

Brown fat lipoatrophy and increased visceral adiposity through a concerted adipocytokines overexpression induces vascular insulin resistance and dysfunction

In this study, we analyzed the role played by concerted expression of adipocytokines associated with brown fat lipoatrophy and increased visceral adiposity on triggering vascular insulin resistance and dysfunction in brown adipose tissue (BAT) insulin receptor knockout (BATIRKO) mice. In addition, we assessed whether vascular insulin resistance may aggravate vascular damage. The 52-wk-old, but not 33-wk-old, BATIRKO mice had a significant decrease of BAT mass associated with a significant increase of visceral white adipose tissue (WAT) mass, without changes in body weight. Brown fat lipoatrophy and increased visceral adiposity enhanced the concerted expression of adipocytokines (TNF-α, leptin, and plasminogen activator inhibitor 1) and nuclear factor-κB binding activity in BAT and visceral WAT, mainly in the gonadal depot, and aorta. Although those mice showed insulin sensitivity in the liver and skeletal muscle, insulin signaling in WAT (gonadal depot) and aorta was markedly impaired. Treatment with anti-TNF-α antibody impaired the inflammatory activity in visceral adipose tissue, attenuated insulin resistance in WAT and aorta and induced glucose tolerance. Finally, 52-wk-old BATIRKO mice showed vascular dysfunction, macrophage infiltration, oxidative stress, and a significant increase of gene markers of endothelial activation and inflammation, the latter effect being totally reverted by anti-TNF-α antibody treatment. Our results suggest that brown fat lipoatrophy and increased visceral adiposity through the concerted overexpression of cytoadipokines induces nuclear factor-κB-mediated inflammatory signaling, vascular insulin resistance, and vascular dysfunction. Inhibition of inflammatory activity by anti-TNF-α antibody treatment attenuates vascular insulin resistance and impairs gene expression of vascular dysfunction markers.

RNAi-mediated silencing of insulin receptor substrate-4 enhances actinomycin D- and tumor necrosis factor-alpha-induced cell death in hepatocarcinoma cancer cell lines

Insulin receptor substrate-4 (IRS-4) transmits signals from the insulin-like growth factor receptor (IGF-IR) and the insulin receptor (IR) to the PI3K/AKT and the ERK1/2 pathways. IRS-4 expression increases dramatically after partial hepatectomy and plays an important role in HepG2 hepatoblastoma cell line proliferation/differentiation. In human hepatocarcinoma, IRS-4 overexpression has been associated with tumor development. Herein, we describe the mechanism whereby IRS-4 depletion induced by RNA interference (siRNA) sensitizes HepG2 cells to treatment with actinomycin D (Act D) and combined treatment with Act D plus tumor necrosis factor-alpha (TNF-alpha). Similar results have been obtained in HuH 7 and Chang cell lines. Act D therapy drove the cells to a mitochondrial-dependent apoptotic program involving cytochrome c release, caspase 3 activation, PARP fragmentation and DNA laddering. TNF-alpha amplifies the effect of Act D on HepG2 cell apoptosis increasing c-jun N-terminal kinase (JNK) activity, IkappaB-alpha proteolysis and glutathione depletion. IRS-4 depleted cells that were treated with Act D showed an increase in cytochrome c release and procaspase 3 and PARP proteolysis with respect to control cells. The mechanism involved in IRS-4 action is independent of Akt, IkappaB kinase and JNK. IRS-4 down regulation, however, decreased gamma-glutamylcysteine synthetase content and cell glutathione level in the presence of Act D plus TNF-alpha. These results suggest that IRS-4 protects HepG2 cells from oxidative stress induced by drug treatment.

Beta-Cell hyperplasia induced by hepatic insulin resistance: role of a liver-pancreas endocrine axis through insulin receptor A isoform

OBJECTIVE

Type 2 diabetes results from a combination of insulin resistance and impaired insulin secretion. To directly address the effects of hepatic insulin resistance in adult animals, we developed an inducible liver-specific insulin receptor knockout mouse (iLIRKO).

RESEARCH DESIGN AND METHODS

Using this approach, we were able to induce variable insulin receptor (IR) deficiency in a tissue-specific manner (liver mosaicism).

RESULTS

iLIRKO mice presented progressive hepatic and extrahepatic insulin resistance without liver dysfunction. Initially, iLIRKO mice displayed hyperinsulinemia and increased beta-cell mass, the extent of which was proportional to the deletion of hepatic IR. Our studies of iLIRKO suggest a cause-and-effect relationship between progressive insulin resistance and the fold increase of plasma insulin levels and beta-cell mass. Ultimately, the beta-cells failed to secrete sufficient insulin, leading to uncontrolled diabetes. We observed that hepatic IGF-1 expression was enhanced in iLIRKO mice, resulting in an increase of circulating IGF-1. Concurrently, the IR-A isoform was upregulated in hyperplastic beta-cells of iLIRKO mice and IGF-1-induced proliferation was higher than in the controls. In mouse beta-cell lines, IR-A, but not IR-B, conferred a proliferative capacity in response to insulin or IGF-1, providing a potential explanation for the beta-cell hyperplasia induced by liver insulin resistance in iLIRKO mice.

CONCLUSIONS

Our studies of iLIRKO mice suggest a liver-pancreas endocrine axis in which IGF-1 functions as a liver-derived growth factor to promote compensatory pancreatic islet hyperplasia through IR-A.

Levels of protein tyrosine phosphatase 1B determine susceptibility to apoptosis in serum-deprived hepatocytes

Protein tyrosine phosphatase 1B (PTP1B) is a negative regulator of tyrosine kinase growth factor signaling. To assess the importance of PTP1B in the balance between death and survival in the liver, we have developed immortalized neonatal hepatocyte cell lines lacking (PTP1B(-/-)) or overexpressing (PTP1B(+/+PTP1B)) PTP1B. Early activation of caspase-3 occurred in PTP1B(+/+PTP1B) hepatocytes but was nearly abolished in PTP1B(-/-) cells. At the molecular level, PTP1B overexpression/deficiency altered the balance of pro-(Bim) and anti-(Bcl-x(L)) apoptotic members of the Bcl-2 family upon serum withdrawal. Likewise, cytosolic cytochrome C increased rapidly in PTP1B(+/+PTP1B) hepatocytes whereas it was retained in the mitochondria of PTP1B(-/-) cells. DNA fragmentation and the increase of apoptotic cells induced by serum withdrawal in wild-type (PTP1B(+/+)) hepatocytes were absent in PTP1B(-/-) cells. Conversely, overexpression of PTP1B accelerated DNA laddering and increased the number of apoptotic cells. In serum-deprived PTP1B(+/+PTP1B) hepatocytes, a rapid entry of Foxo1 into the nucleus and an earlier activation of caspase-8 was observed. However, both events were suppressed in PTP1B(-/-) hepatocytes. Moreover, PTP1B deficiency conferred resistance to apoptosis induced by activation of Fas and constitutively active Foxo1. Rescue of PTP 1B in deficient hepatocytes recovered the phenotype of wild-type cells whereas reduction of PTP1B by siRNA suppressed apoptosis. Our results reveal a unique role for PTP1B as a mediator of the apoptotic pathways triggered by trophic factors withdrawal in hepatocytes. This novel mechanism may represent an important target in the design of therapeutic strategies for human liver regeneration after pathological damage as well as for treatment of hepatocarcinomas.

Role of insulin receptor substrate-4 in IGF-I-stimulated HEPG2 proliferation

BACKGROUNDS/AIMS

Insulin receptor substrate-4 (IRS-4) is a scaffold protein that mediates the actions of insulin-like growth factor-I (IGF-I). Its expression increases dramatically after partial hepatectomy (a liver regeneration model). Herein, we report IRS-4 expression in a human hepatoblastoma cell line (HepG2) and IGF-I-dependent IRS-4 tyrosine phosphorylation.

METHODS

The role of IRS-4 in HepG2 proliferation was established by RNA interference (siRNA). After 72h of transfection with IRS-4 siRNA, we observed a specific reduction in IRS-4 expression.

RESULTS

Depletion of IRS-4 levels decreased ERK phosphorylation, p70S6K phosphorylation and IGF-I-stimulated cell proliferation. Changes in ERK phosphorylation in IRS-4-depleted cells were independent of ras/raf/MEK1/2- and PI3K/Akt-cascades. IRS-4 down-regulation abolished IGF-I-, TPA- and IGF-I plus TPA-stimulated ERK and p70S6K activities. Our results suggest that PKC-epsilon mediates the effect of IRS-4 on ERK activity. Moreover, decreased IRS-4 levels diminished FBS- and IGF-I-stimulated HepG2 growth and cause stress fiber disruption in HepG2 cell line.

CONCLUSIONS

Collectively, our data suggest that IRS-4 plays an important role in HepG2 proliferation/differentiation and exerts its actions through ERK and p70S6K activation in a ras/raf/MEK1/2- and PI3Kinase/Akt-independent manner and in a PKC-dependent way.

IRS-3 mediates insulin-induced glucose uptake in differentiated IRS-2(-/-) brown adipocytes

IRS-2 mediates insulin-induced glucose uptake in brown preadipocytes. Upon differentiation, basal IRS-3 expression increased concurrently with an enhancement in the IRS-3-associated phosphatidylinositol (PI) 3-kinase activity in the Triton-insoluble fraction in wild-type and IRS-2-deficient brown adipocytes stimulated with insulin. Moreover, insulin induced protein kinase B (Akt) and protein kinase C (PKC) zeta phosphorylation in both kinds of cells. More importantly, insulin induced glucose uptake in differentiated IRS-2-deficient brown adipocytes in a wortmannin-dependent manner. However, while insulin induced Akt phosphorylation occurred mainly in the cytosolic fraction, PKC zeta activation was constrained to the Triton-insoluble fraction. The reduction of IRS-3 expression by siRNA inhibited insulin-induced glucose uptake and also PKC zeta activation in differentiated IRS-2(-/-) brown adipocytes. In addition, inhibition of PKC zeta totally blunted insulin-induced glucose uptake in those cells. Our results provide evidences suggesting that IRS-3/PI 3-kinase/PKC zeta signaling is the main responsible for the insulin-induced glucose uptake observed upon differentiation of brown adipocytes lacking IRS-2.

Developmental switch from prolonged insulin action to increased insulin sensitivity in protein tyrosine phosphatase 1B-deficient hepatocytes

Protein tyrosine phosphatase 1B (PTP1B) is a negative regulator of insulin signaling and a therapeutic target for type 2 diabetes. The purpose of this study was to evaluate the differences in insulin sensitivity between neonate and adult hepatocytes lacking PTP1B. Immortalized neonatal hepatocytes and primary neonatal and adult hepatocytes have been generated from PTP1B(-/-) and wild-type mice. PTP1B deficiency in immortalized neonatal hepatocytes prolonged insulin-induced tyrosine phosphorylation of the insulin receptor (IR) and IR substrates (IRS) -1, -2 compared with wild-type control cells. Endogenous IR and IRS-2 were down-regulated, whereas IRS-1 was up-regulated in PTP1B(-/-) neonatal hepatocytes and livers of PTP1B(-/-) neonates. Insulin-induced activation of phosphatidylinositol 3-kinase/Akt pathway was prolonged in PTP1B(-/-) immortalized neonatal hepatocytes. However, insulin sensitivity was comparable to wild-type hepatocytes. Rescue of PTP1B in deficient cells suppressed the prolonged insulin signaling, whereas RNA interference in wild-type cells promoted prolonged signaling. In primary neonatal PTP1B(-/-) hepatocytes, insulin prolonged the inhibition of gluconeogenic mRNAs, but the sensitivity to this inhibition was similar to wild-type cells. By contrast, in adult PTP1B-deficient livers, p85alpha was down-regulated compared with the wild type. Moreover, primary hepatocytes from adult PTP1B(-/-) mice displayed enhanced Akt phosphorylation and a more pronounced inhibition of gluconeogenic mRNAs than wild-type cells. Hepatic insulin sensitivity due to PTP1B deficiency is acquired through postnatal development. Thus, changes in IR and IRS-2 expression and in the balance between regulatory and catalytic subunits of phosphatidylinositol 3-kinase are necessary to achieve insulin sensitization in adult PTP1B(-/-) hepatocytes.

Azathioprine acts upon rat hepatocyte mitochondria and stress-activated protein kinases leading to necrosis: protective role of N-acetyl-L-cysteine

Azathioprine is an immunosuppressant drug widely used. Our purpose was to 1) determine whether its associated hepatotoxicity could be attributable to the induction of a necrotic or apoptotic effect in hepatocytes, and 2) elucidate the mechanism involved. To evaluate cellular responses to azathioprine, we used primary culture of isolated rat hepatocytes. Cell metabolic activity, reduced glutathione, cell proliferation, and lactate dehydrogenase release were assessed. Mitochondria were isolated from rat livers, and swelling and oxygen consumption were measured. Mitogen-activated protein kinase pathways and proteins implicated in cell death were analyzed. Azathioprine decreased the viability of hepatocytes and induced the following events: intracellular reduced glutathione (GSH) depletion, metabolic activity reduction, and lactate dehydrogenase release. However, the cell death was not accompanied by DNA laddering, procaspase-3 cleavage, and cytochrome c release. The negative effects of azathioprine on the viability of hepatocytes were prevented by cotreatment with N-acetyl-L-cysteine. In contrast, 6-mercaptopurine showed no effects on GSH content and metabolic activity. Azathioprine effect on hepatocytes was associated with swelling and increased oxygen consumption of intact isolated rat liver mitochondria. Both effects were cyclosporine A-sensitive, suggesting an involvement of the mitochondrial permeability transition pore in the response to azathioprine. In addition, the drug's effects on hepatocyte viability were partially abrogated by c-Jun N-terminal kinase and p38 kinase inhibitors. In conclusion, our findings suggest that azathioprine effects correlate to mitochondrial dysfunction and activation of stress-activated protein kinase pathways leading to necrotic cell death. These negative effects of the drug could be prevented by coincubation with N-acetyl-L-cysteine.

Insulin receptor substrate-4 signaling in quiescent rat hepatocytes and in regenerating rat liver

This study was designed to characterize insulin receptor substrate-4 (IRS-4) in isolated rat hepatocytes and to examine its role in liver regeneration. Subcellular fractionation revealed that 85% of IRS-4 is located at isolated hepatocyte plasma membranes. The distribution of IRS-4 among intracellular compartments remained unchanged in insulin-stimulated cells. Two bands corresponding to 145 and 138 kd were observed in immunoblotting experiments. Immunoprecipitation of hepatocyte lysates with a highly specific antibody against IRS-4 led to an insulin and insulin-like growth factor 1 (IGF-1)-dependent increase in phosphotyrosine residues of the 145-kd band. IRS-4 was found to be associated with Src homology 2 (SH2) domain-containing proteins (phosphatidylinositol 3-kinase [PI 3-kinase] and Src homology phosphatase [SHP-2]) and with protein kinase C zeta (PKC zeta). Insulin and IGF-1 elicited a rapid and dose-dependent binding of these 3 proteins to IRS-4. These data suggest that IRS-4 is insulin-/IGF-1-activated by phosphorylation and not by translocation, inducing the recruitment of SH2 domain-containing proteins and PKC zeta to the membrane. To evaluate the possible role of IRS-4 in liver regeneration, we also examined this system after partial hepatectomy (PH). One day after PH, IRS-1 expression increased, consistent with a stimulatory role in the regenerative process, whereas it decreased 7 days after liver resection. This drastic IRS-1 depletion occurred at the expense of increased IRS-2 and IRS-4 expression 7 days after PH. In addition, at this period of time after surgery, the in vivo insulin stimulation of remnant rat livers showed an increase in IRS-4/PI 3-kinase association. Given that 1 and 7 days after PH isolated hepatocytes responded similarly to insulin in terms of induced cell proliferation, a compensatory role is proposed for IRS-2/4 induction. In conclusion, IRS-4 is activated by insulin and IGF-1-like IRS-1 in rat hepatocytes, and the induced expression of IRS-4 is a compensatory mechanism that plays a role in conditions of liver regeneration.

Pretreatment with FK506 up-regulates insulin receptors in regenerating rat liver

This report examines the effect of FK506 pretreatment on liver insulin receptor expression in partially (70%) hepatectomized rats. FK506 pretreatment led to an increased insulin receptor number 24 hours after hepatectomy, detected by means of insulin binding and cross-linking procedures. This increase was related to enhanced insulin receptor expression determined by in vitro mRNA translation and Western blot techniques. We also tested the functionality of the expressed insulin receptors by [(3)H] thymidine incorporation into DNA in insulin-stimulated hepatocytes. The results show that FK506 pretreatment elicits an increase in the amount of insulin receptor alpha-subunits as measured by Western blot. Maximum alpha-subunit expression recorded 24 hours after surgery was preceded by increased insulin receptor mRNA levels, which were detected 6 hours after hepatectomy. Moreover, in FK506-pretreated rat hepatocytes, obtained from remnant livers 24 hours after partial hepatectomy (PH), the increase in insulin receptor number was associated with improved sensitivity to the hormone. However, in both experimental groups (FK506-pretreated and nonpretreated rats), the sensitivity of hepatocytes toward epidermal growth factor (EGF) showed no significant change, which suggests a specific effect of FK506 on insulin receptor expression. In conclusion, our findings suggest that FK506 pretreatment induces insulin receptor expression in regenerating rat liver and promotes liver regeneration in hepatectomized rats.

Thiopurine methyltransferase activity in a Spanish population sample: decrease of enzymatic activity in multiple sclerosis patients

The present study was performed in order to obtain the thiopurine methyltransferase (TPMT) activity frequency distribution histogram in a Spanish population. A total of 3640 Spanish clinical laboratory samples were evaluated, which included 1249 patients with Crohn's disease, 589 with ulcerative colitis, 348 with multiple sclerosis (MS), 487 with several autoimmune diseases different from the above-mentioned diseases and 967 a donor group. We have measured the TPMT activity in red blood cells (RBCs) by a radiochemical method, using S-adenosyl-L-[methyl-3H]methionine as methyl donor. The different groups present in their entirety a normal distribution histogram and a wide range of TPMT activity from 0 to 41 U/ml RBCs. The differences found between the Spanish population TPMT activity frequency distribution histogram and the pattern previously described in a North American population were not due to azathioprine treatment or gender. The effect of autoimmune diseases on TPMT activity was evaluated: the enzymatic activity was similar in the donor group (19.9 +/- 6.3 U/ml RBCs) and in the patients with Crohn's disease (20.0 +/- 5.8 U/ml RBCs) and ulcerative colitis (19.7 +/- 6.1 U/ml RBCs); however, it decreased significantly (p<0.0001) in MS patients (17.1 +/- 6.1 U/ml RBCs) with respect to the donor group. In conclusion, our results show that the Spanish population TPMT distribution is closer to that of the Jewish population of Israel than to North American populations, and that in MS the enzymatic activity of TPMT decreases significantly. This observation may take into account the usage of azathioprine as therapeutic agent in Spanish MS patients.

Determination of thiopurine methyltransferase activity in human erythrocytes by high-performance liquid chromatography: comparison with the radiochemical method

The current article describes a new assay to measure thiopurine methyltransferase (TPMT) activity from red blood cells. This method is based on the measurement of the reaction product 6-methylmercaptopurine (6-MMP) by high-performance liquid chromatography (HPLC). 6-MMP is extracted by ethyl acetate with recoveries of 85%, 80%, 80%, and 92% for 50, 250, 500, and 1,000 ng/100 microL packed red blood cells, respectively. 6-MMP was identified and measured by a Zorbax CN column installed in an HPLC system. The chromatograms were resolved using a mobile phase consisting of 40 mmol/L sodium phosphate buffer (pH 3) and methanol in a gradient from 1% to 20% of methanol. Under these conditions 6-MMP is well resolved from substrates (6-mercaptopurine and S-adenosyl-L-methionine) and endogenous peaks. When the TPMT activity from 20 patients was measured by the HPLC-linked assay and the classic radiochemical method, a linear correlation was obtained between both procedures ( y = 0.99x + 0.33; x-axis, radiochemical assay; y-axis, HPLC-linked assay; r = 0.98). In conclusion, the current report describes a new, reliable, safe, and nonradioactive method to measure TPMT activity that is shorter and simpler than the previously described ones.