IRS-2 mediates the antiapoptotic effect of insulin in neonatal hepatocytes

"MiR-7 controls cholesterol biosynthesis through posttranscriptional regulation of DHCR24 expression"

Dysregulation of cholesterol homeostasis is associated with several pathologies including cardiovascular diseases and neurological disorders such as Alzheimer's disease (AD). MicroRNAs (miRNAs) have emerged as key post-transcriptional regulators of cholesterol metabolism. We previously established the role of miR-7 in regulating insulin resistance and amyloidosis, which represents a common pathological feature between type 2 diabetes and AD. We show here an additional metabolic function of miR-7 in cholesterol biosynthesis. We found that miR-7 blocks the last steps of the cholesterol biosynthetic pathway in vitro by targeting relevant genes including DHCR24 and SC5D posttranscriptionally. Intracranial infusion of miR-7 on an adeno-associated viral vector reduced the expression of DHCR24 in the brain of wild-type mice, supporting in vivo miR-7 targeting. We also found that cholesterol regulates endogenous levels of miR-7 in vitro, correlating with transcriptional regulation through SREBP2 binding to its promoter region. In parallel to SREBP2 inhibition, the levels of miR-7 and hnRNPK (the host gene of miR-7) were concomitantly reduced in brain in a mouse model of Niemann Pick type C1 disease and in murine fatty liver, which are both characterized by intracellular cholesterol accumulation. Taken together, the results establish a novel regulatory feedback loop by which miR-7 modulates cholesterol homeostasis at the posttranscriptional level, an effect that could be exploited for therapeutic interventions against prevalent human diseases.

Ethnopharmacological, Phytochemical, Pharmacological, and Toxicological Review on Senna auriculata (L.) Roxb.: A Special Insight to Antidiabetic Property

Avartaki (Senna auriculata (L.) Roxb. syn. Cassia auriculata L.; Family- Fabaceae ) is a traditional medicinal plant, widely used for the treatment of various ailments in Ayurveda and Siddha system of medicine in India. Almost all the parts of the plant, such as flowers, leaves, seeds, barks, and roots have been reported for their medicinal uses. Traditionally, it has been used in the treatment of diabetes, asthma, rheumatism, dysentery, skin disease, and metabolic disorders. The principle phytochemicals in Senna auriculata (L.) Roxb. are alkaloids, anthraquinone, flavone glycosides, sugar, saponins, phenols, terpenoids, flavonoids, tannins, steroids, palmitic acid, linoleic acid, benzoic acid 2-hydroxyl methyl ester, 1-methyl butyl ester, resorcinol, α-tocopherol-β-D-mannosidase, epicatechin, ferulic acid, quercetin-3-O-rutinoside, quercetin, proanthocyanidin B1. The extracts from its different parts and their isolated compounds possess a wide range of pharmacological activities such as antidiabetic, antioxidant, anti-inflammatory, antihyperlipidemic, hepatoprotective, nephroprotective, cardioprotective, anti-atherosclerotic, anticancer, antimutagenic, antimicrobial, antiulcer, antipyretic, anthelmintic, immunomodulatory, antifertility, anti-venom, and anti-melanogenesis. The toxicological findings from preclinical studies ensured the safety of the plant, but comprehensive clinical studies are required for the safety and efficacy of the plant in humans. The current review article aimed to provide up-to-date information about Senna auriculata (L.) Roxb. covering its ethnomedicinal, phytochemical, pharmacological, and toxicological aspects with special emphasis on its clinical implications in diabetes.

Insulin receptor substrate 2 (IRS2) deficiency delays liver fibrosis associated with cholestatic injury

Insulin receptor substrate 2 (IRS2) is a key downstream mediator of insulin and insulin-like growth factor 1 (IGF1) signalling pathways and plays a major role in liver metabolism. The aim of this study was to investigate whether IRS2 had an impact on the hepatic fibrotic process associated with cholestatic injury. Bile duct ligation (BDL) was performed in wild-type (WT) and Irs2-deficient (IRS2KO) female mice. Histological and biochemical analyses, together with fibrogenic and inflammatory responses were evaluated in livers from mice at 3, 7 and 28 days following BDL. We also explored whether activation of human hepatic stellate cells (HSCs) induced by IGF1 was modulated by IRS2. IRS2KO mice displayed reduced disruption of liver histology, such hepatocyte damage and excess deposition of extracellular matrix components, compared with WT mice at 3 and 7 days post-BDL. However, no histological differences between genotypes were found at 28 days post-BDL. The less pro-inflammatory profile of bile acids accumulated in the gallbladder of IRS2KO mice after BDL corresponded with the reduced expression of pro-inflammatory markers in these mice. Stable silencing of IRS2 or inhibition of ERK1/2 reduced the activation of human LX2 cells and also reduced induction of MMP9 upon IGF1 stimulation. Furthermore, hepatic MMP9 expression was strongly induced after BDL in WT mice, but only a slight increase was found in mice lacking IRS2. Our results have unravelled the signalling pathway mediated by IGF1R–IRS2–ERK1/2–MMP9 as a key axis in regulating HSC activation, which might be therapeutically relevant for targeting liver fibrosis.

Summary: IRS2 is a key mediator of IGF1R signalling in hepatic stellate cell activation in cholestatic liver injury.

Sequential testicular atrophy involves changes in cellular proliferation and apoptosis associated with variations in aromatase P450 expression levels in Irs-2-deficient mice

Insulin receptor substrate 2 (Irs-2) is an intracellular protein susceptible to phosphorylation after activation of the insulin receptor. Its suppression affects testis development and its absence induces peripheral resistance to insulin. The aim of this study was to identify changes induced by the deletion of Irs-2 in the testicular structure and by the altered expression of cytochrome P450 aromatase, a protein necessary for the development and maturation of germ cells. Adult knockout (KO) mice (Irs-2^-/- , 6 and 12 weeks old) and age-matched wild-type (WT) mice were used in this study. Immunohistochemistry and Western blot analyses were performed to study proliferation (PCNA), apoptosis (active caspase-3) and P450 aromatase expression in testicular histological sections. Deletion of Irs-2 decreased the number of epithelial cells in the seminiferous tubule and rete testis. Aberrant cells were frequently detected in the epithelia of Irs-2^-/- mice, accompanied by variations in spermatogonia, which were shown to exhibit small hyperchromatic nuclei as well as polynuclear and anuclear structures. The amount of cell proliferation was significantly lower in Irs-2^-/- mice than in WT mice, whereas apoptotic processes were more common in Irs-2^-/- mice. Aromatase P450 reactivity was higher in 6-week-old KO mice than in WT mice of the same age and was even higher at 12 weeks. Our results suggest that Irs-2 is a key element in spermatogenesis because silencing Irs-2 induces the sequential development of testicular atrophy. The effects are observed mainly in germ cells present in the seminiferous tubule, which may be due to changes in cytochrome P450 aromatase expression.

IL-4 and IL-13 Receptor Signaling From 4PS to Insulin Receptor Substrate 2: There and Back Again, a Historical View

In this historical perspective, written in honor of Dr. William E. Paul, we describe the initial discovery of one of the dominant substrates for tyrosine phosphorylation stimulated by IL-4. We further describe how this “IL-4-induced phosphorylated substrate” (4PS) was characterized as a member of the insulin receptor substrate (IRS) family of large adaptor proteins that link IL-4 and insulin receptors to activation of the phosphatidyl-inositol 3′ kinase pathway as well as other downstream signaling pathways. The relative contribution of the 4PS/IRS pathway to the early models of IL-4-induced proliferation and suppression of apoptosis are compared to our more recent understanding of the complex interplay between positive and negative regulatory pathways emanating from members of the IRS family that impact allergic responses.

Differential involvement of the microtubule cytoskeleton in insulin receptor substrate 1 (IRS-1) and IRS-2 signaling to AKT determines the response to microtubule disruption in breast carcinoma cells

The insulin receptor substrate (IRS) proteins serve as essential signaling intermediates for the activation of PI3K by both the insulin-like growth factor 1 receptor (IGF-1R) and its close family member, the insulin receptor (IR). Although IRS-1 and IRS-2 share significant homology, they regulate distinct cellular responses downstream of these receptors and play divergent roles in breast cancer. To investigate the mechanism by which signaling through IRS-1 and IRS-2 results in differential outcomes, we assessed the involvement of the microtubule cytoskeleton in IRS-dependent signaling. Treatment with drugs that either stabilize or disrupt microtubules reveal that an intact microtubule cytoskeleton contributes to IRS-2- but not IRS-1-mediated activation of AKT by IGF-1. Proximal IGF-1R signaling events, including IRS tyrosine phosphorylation and recruitment of PI3K, are not inhibited by microtubule disruption, indicating that IRS-2 requires the microtubule cytoskeleton at the level of downstream effector activation. IRS-2 colocalization with tubulin is enhanced upon Taxol-mediated microtubule stabilization, which, together with the signaling data, suggests that the microtubule cytoskeleton may facilitate access of IRS-2 to downstream effectors such as AKT. Of clinical relevance is that our data reveal that expression of IRS-2 sensitizes breast carcinoma cells to apoptosis in response to treatment with microtubule-disrupting drugs, identifying IRS-2 as a potential biomarker for the response of breast cancer patients to Vinca alkaloid drug treatment.

The role of insulin resistance in nonalcoholic steatohepatitis and liver disease development--a potential therapeutic target?

Insulin resistance (IR) is defined by the inability of insulin to exert its metabolic actions, due to impaired activation of intracellular insulin signaling. This condition is caused by genetic defects or by environmental conditions, among which the most common is obesity. Systemic IR determines the development of hepatic fat accumulation, which can progress to nonalcoholic steatohepatitis, cirrhosis and hepatocellular carcinoma, and is a major determinant of liver disease independently of coexisting factors. Therefore, insulin-sensitizing drugs are currently under evaluation to improve steatohepatitis. Indeed, manipulation of nuclear hormone receptors is already under scrutiny for liver disease prevention by amelioration of IR, whereas NOTCH signaling inhibition represents a novel approach. Nevertheless, further research is warranted to better understand the mechanism linking IR to progressive fibrogenesis in the absence of inflammation and to identify novel drug targets.

Potential Contributions of the Tobacco Nicotine-Derived Nitrosamine Ketone to White Matter Molecular Pathology in Fetal Alcohol Spectrum Disorder

Background

Fetal alcohol spectrum disorder (FASD) is associated with long-term deficits in cognitive and motor functions. Previous studies linked neurodevelopmental abnormalities to increased oxidative stress and white matter hypotrophy. However, similar effects occur with low-dose nitrosamine exposures, alcohol abuse correlates with cigarette smoking, and tobacco smoke contains tobacco-specific nitrosamines, including NNK.

Hypothesis

Tobacco smoke exposure is a co-factor in FASD.

Design

Long Evans rat pups were i.p. administered ethanol (2 g/kg) on postnatal days (P) 2, 4, 6 and/or NNK (2 mg/kg) on P3, P5, and P7 to simulate third trimester human exposures. Oligodendroglial myelin-associated, neuroglial, and relevant transcription factor mRNA transcripts were measured using targeted PCR arrays.

Results

Ethanol and NNK differentially altered the expression of immature and mature oligodendroglial, neuronal and astrocytic structural and plasticity-associated, and various transcription factor genes. NNK’s effects were broader and more pronounced than ethanol’s, and additive or synergistic effects of dual exposures impacted expression of all four categories of genes investigated.

Conclusion

Developmental exposures to alcohol and NNK (via tobacco smoke) contribute to sustained abnormalities in brain white matter structure and function via distinct but overlapping alterations in the expression of genes that regulate oligodendrocyte survival, maturation and function, neuroglial structural integrity, and synaptic plasticity. The results support the hypothesis that smoking may contribute to brain abnormalities associated with FASD.

Increased oxidative stress and apoptosis in the hypothalamus of diabetic male mice in the insulin receptor substrate-2 knockout model

Insulin receptor substrate-2-deficient (IRS2(-/-)) mice are considered a good model to study the development of diabetes because IRS proteins mediate the pleiotropic effects of insulin-like growth factor-I (IGF-I) and insulin on metabolism, mitogenesis and cell survival. The hypothalamus might play a key role in the early onset of diabetes, owing to its involvement in the control of glucose homeostasis and energy balance. Because some inflammatory markers are elevated in the hypothalamus of diabetic IRS2(-/-) mice, our aim was to analyze whether the diabetes associated with the absence of IRS2 results in hypothalamic injury and to analyze the intracellular mechanisms involved. Only diabetic IRS2(-/-) mice showed increased cell death and activation of caspase-8 and -3 in the hypothalamus. Regulators of apoptosis such as FADD, Bcl-2, Bcl-xL and p53 were also increased, whereas p-IκB and c-FLIPL were decreased. This was accompanied by increased levels of Nox-4 and catalase, enzymes involved in oxidative stress. In summary, the hypothalamus of diabetic IRS2(-/-) mice showed an increase in oxidative stress and inflammatory markers that finally resulted in cell death via substantial activation of the extrinsic apoptotic pathway. Conversely, non-diabetic IRS2(-/-) mice did not show cell death in the hypothalamus, possibly owing to an increase in the levels of circulating IGF-I and in the enhanced hypothalamic IGF-IR phosphorylation that would lead to the stimulation of survival pathways. In conclusion, diabetes in IRS2-deficient male mice is associated with increased oxidative stress and apoptosis in the hypothalamus.

Essential role of Nrf2 in the protective effect of lipoic acid against lipoapoptosis in hepatocytes

Excess of saturated free fatty acids, such as palmitic acid (PA), in hepatocytes has been implicated in nonalcoholic fatty liver disease. α-Lipoic acid (LA) is an antioxidant that protects against oxidative stress conditions. We have investigated the effects of LA in the early activation of oxidative and endoplasmic reticulum stress, lipid accumulation, and Nrf2-mediated antioxidant defenses in hepatocytes treated with PA or in rats fed a high-fat diet. In primary human hepatocytes, a lipotoxic concentration of PA triggered endoplasmic reticulum stress, induced the apoptotic transcription factor CHOP, and increased the percentage of apoptotic cells. Cotreatment with LA prevented these effects. Similar results were found in mouse hepatocytes in which LA attenuated PA-mediated activation of caspase 3 and reduced lipid accumulation by decreasing PA uptake and increasing fatty acid oxidation and lipophagy, thereby preventing lipoapoptosis. Moreover, LA augmented the proliferation capacity of hepatocytes after PA challenge. Antioxidant effects of LA ameliorated reactive oxygen species production and endoplasmic reticulum stress and protected against mitochondrial apoptosis in hepatocytes treated with PA. Cotreatment with PA and LA induced an early nuclear translocation of Nrf2 and activated antioxidant enzymes, whereas reduction of Nrf2 by siRNA abolished the benefit of LA on PA-induced lipoapoptosis. Importantly, posttreatment with LA reversed the established damage induced by PA in hepatocytes, as well as preventing obesity-induced oxidative stress and lipoapoptosis in rat liver. In conclusion, our work has revealed that in hepatocytes, Nrf2 is an essential early player in the rescue of oxidative stress by LA leading to protection against PA-mediated lipoapoptosis.

Diabetic liver injury from streptozotocin is regulated through the caspase-8 homolog cFLIP involving activation of JNK2 and intrahepatic immunocompetent cells

The endemic occurrence of obesity and the associated risk factors that constitute the metabolic syndrome have been predicted to lead to a dramatic increase in chronic liver disease. Non-alcoholic steatohepatitis (NASH) has become the most frequent liver disease in countries with a high prevalence of obesity. In addition, hepatic steatosis and insulin resistance have been implicated in disease progression of other liver diseases, including chronic viral hepatitis and hepatocellular carcinoma. The molecular mechanisms underlying the link between insulin signaling and hepatocellular injury are only partly understood. We have explored the role of the antiapoptotic caspase-8 homolog cellular FLICE-inhibitory protein (cFLIP) on liver cell survival in a diabetic model with hypoinsulinemic diabetes in order to delineate the role of insulin signaling on hepatocellular survival. cFLIP regulates cellular injury from apoptosis signaling pathways, and loss of cFLIP was previously shown to promote injury from activated TNF and CD95/Apo-1 receptors. In mice lacking cFLIP in hepatocytes (flip^−/−), loss of insulin following streptozotocin treatment resulted in caspase- and c-Jun N-terminal kinase (JNK)-dependent liver injury after 21 days. Substitution of insulin, inhibition of JNK using the SP600125 compound in vivo or genetic deletion of the mitogen-activated protein kinase (MAPK)9 (JNK2) in all tissues abolished the injurious effect. Strikingly, the difference in injury between wild-type and cFLIP-deficient mice occurred only in vivo and was accompanied by liver-infiltrating inflammatory cells with a trend toward increased amounts of NK1.1-positive cells and secretion of proinflammatory cytokines. Transfer of bone marrow from rag-1-deficient mice that are depleted from B and T lymphocytes prevented liver injury in flip^−/− mice. These findings support a direct role of insulin on cellular survival by alternating the activation of injurious MAPK, caspases and the recruitment of inflammatory cells to the liver. Thus, increasing resistance to insulin signaling pathways in hepatocytes appears to be an important factor in the initiation and progression of chronic liver disease.

Protein tyrosine phosphatase 1B modulates GSK3β/Nrf2 and IGFIR signaling pathways in acetaminophen-induced hepatotoxicity

Acute hepatic failure secondary to acetaminophen (APAP) poisoning is associated with high mortality. Protein tyrosine phosphatase 1B (PTP1B) is a negative regulator of tyrosine kinase growth factor signaling. In the liver, this pathway confers protection against injury. However, the involvement of PTP1B in the intracellular networks activated by APAP is unknown. We have assessed PTP1B expression in APAP-induced liver failure in humans and its role in the molecular mechanisms that regulate the balance between cell death and survival in human and mouse hepatocytes, as well as in a mouse model of APAP-induced hepatotoxicity. PTP1B expression was increased in human liver tissue removed during liver transplant from patients for APAP overdose. PTP1B was upregulated by APAP in primary human and mouse hepatocytes together with the activation of c-jun (NH2) terminal kinase (JNK) and p38 mitogen-activated protein kinase (p38 MAPK), resulting in cell death. Conversely, Akt phosphorylation and the antiapoptotic Bcl2 family members BclxL and Mcl1 were decreased. PTP1B deficiency in mouse protects hepatocytes against APAP-induced cell death, preventing glutathione depletion, reactive oxygen species (ROS) generation and activation of JNK and p38 MAPK. APAP-treated PTP1B^−/− hepatocytes showed enhanced antioxidant defense through the glycogen synthase kinase 3 (GSK3)β/Src kinase family (SKF) axis, delaying tyrosine phosphorylation of the transcription factor nuclear factor-erythroid 2-related factor (Nrf2) and its nuclear exclusion, ubiquitination and degradation. Insulin-like growth factor-I receptor-mediated signaling decreased in APAP-treated wild-type hepatocytes, but was maintained in PTP1B^−/− cells or in wild-type hepatocytes with reduced PTP1B levels by RNA interference. Likewise, both signaling cascades were modulated in mice, resulting in less severe APAP hepatotoxicity in PTP1B^−/− mice. Our results demonstrated that PTP1B is a central player of the mechanisms triggered by APAP in hepatotoxicity, suggesting a novel therapeutic target against APAP-induced liver failure.

Insulin-receptor substrate-2 (irs-2) is required for maintaining glucokinase and glucokinase regulatory protein expression in mouse liver

Insulin receptor substrate (IRS) proteins play important roles in hepatic nutrient homeostasis. Since glucokinase (GK) and glucokinase regulatory protein (GKRP) function as key glucose sensors, we have investigated the expression of GK and GKRP in liver of Irs-2 deficient mice and Irs2(−/−) mice where Irs2 was reintroduced specifically into pancreatic β-cells [RIP-Irs-2/IRS-2(−/−)]. We observed that liver GK activity was significantly lower (p<0.0001) in IRS-2(−/−) mice. However, in RIP-Irs-2/IRS-2(−/−) mice, GK activity was similar to the values observed in wild-type animals. GK activity in hypothalamus was not altered in IRS-2(−/−) mice. GK and GKRP mRNA levels in liver of IRS-2(−/−) were significantly lower, whereas in RIP-Irs-2/IRS-2(−/−) mice, both GK and GKRP mRNAs levels were comparable to wild-type animals. At the protein level, the liver content of GK was reduced in IRS-2(−/−) mice as compared with controls, although GKRP levels were similar between these experimental models. Both GK and GKRP levels were lower in RIP-Irs-2/IRS-2(−/−) mice. These results suggest that IRS-2 signalling is important for maintaining the activity of liver GK. Moreover, the differences between liver and brain GK may be explained by the fact that expression of hepatic, but not brain, GK is controlled by insulin. GK activity was restored by the β-cell compensation in the RIP-Irs-2/IRS-2 mice. Interestingly, GK and GKRP protein expression remained low in RIP-Irs-2/IRS-2(−/−) mice, perhaps reflecting different mRNA half-lives or alterations in the process of translation and post-translational regulation.

Randomized clinical trial of the impact of insulin therapy on liver function in patients undergoing major liver resection

BACKGROUND

Postoperative liver dysfunction is the major source of morbidity and mortality in patients undergoing partial hepatectomy. This study tested the benefits of a metabolic support protocol based on insulin infusion, for reducing liver dysfunction following hepatic resection.

METHODS

Consecutive consenting patients scheduled for liver resection were randomized to receive preoperative dextrose infusion followed by insulin therapy using the hyperinsulinaemic normoglycaemic clamp protocol (n = 29) or standard therapy (control group, n = 27). Patients in the insulin therapy group followed a strict dietary regimen for 24 h before surgery. Intravenous dextrose was started at 2 mg per kg per min the night before and continued until surgery. Hyperinsulinaemic therapy for a total of 24 h was initiated at 2 munits per kg per min at induction of anaesthesia, and continued at 1 munit per kg per min after surgery. Normoglycaemia was maintained (3.5-6.0 mmol/l). Control subjects received no additional dietary supplement and a conventional insulin sliding scale during fasting. All patients were tested serially to evaluate liver function using the Schindl score. Liver tissue samples were collected at two time points during surgery to measure glycogen levels.

RESULTS

Demographics were similar in the two groups. More liver dysfunction occurred in the control cohort (liver dysfunction score range 0-8 versus 0-4 with insulin therapy; P = 0.031). Median (interquartile range) liver glycogen content was 278 (153-312) and 431 (334-459) µmol/g respectively (P = 0.011). The number of complications rose with increasing severity of postoperative liver dysfunction (P = 0.032) CONCLUSION: The glucose-insulin protocol reduced postoperative liver dysfunction and improved liver glycogen content.

REGISTRATION NUMBER

NCT00774098 (http://www.clinicaltrials.gov).

Studies of naturally occurring friedelane triterpenoids as insulin sensitizers in the treatment type 2 diabetes mellitus

Type 2 diabetes mellitus (T2DM) is a rapidly expanding public epidemic and frequently results in severe vascular complications. In an attempt to find anti-diabetic agents, we report herein on the isolation, structural elucidation and bioactivity of nine friedelane-type triterpenes (1-9) and twenty two known ones (10-31) from the root barks of Celastrus vulcanicola and Maytenus jelskii. Their structures were elucidated on the basis of spectroscopic analysis, including 1D and 2D NMR techniques. Two compounds from this series (1 and 3) exhibited increased insulin-mediated signalling, which suggests these friedelane triterpenes have potential therapeutic use in insulin resistant states.

IRS2 signalling is required for the development of a subset of sensory spinal neurons

Insulin and insulin-like growth factor-I play important roles in the development and maintenance of neurons and glial cells of the nervous system. Both factors activate tyrosine kinase receptors, which signal through adapter proteins of the insulin receptor substrate (IRS) family. Although insulin and insulin-like growth factor-I receptors are expressed in dorsal root ganglia (DRG), the function of IRS-mediated signalling in these structures has not been studied. Here we address the role of IRS2-mediated signalling in murine DRG. Studies in cultured DRG neurons from different embryonic stages indicated that a subset of nerve growth factor-responsive neurons is also dependent on insulin for survival at very early time points. Consistent with this, increased apoptosis during gangliogenesis resulted in a partial loss of trkA-positive neurons in DRG of Irs2 mutant embryos. Analyses in adult Irs2(-/-) mice revealed that unmyelinated fibre afferents, which express calcitonin gene-related peptide/substance P and isolectin B4, as well as some myelinated afferents to the skin were affected by the mutation. The diminished innervation of glabrous skin in adult Irs2(-/-) mice correlated with longer paw withdrawal latencies in the hot-plate assay. Collectively, these findings indicate that IRS2 signalling is required for the proper development of spinal sensory neurons involved in the perception of pain.

IRS2 and PTP1B: Two opposite modulators of hepatic insulin signalling

Type 2 Diabetes mellitus (T2D) is the most common endocrine disorder associated to metabolic syndrome (MS) and occurs when insulin secretion can no compensate peripheral insulin resistance. Among peripheral tissues, the liver controls glucose homeostasis due to its ability to consume and produce glucose. The molecular mechanism underlying hepatic insulin resistance is not completely understood; however, it involves the impairment of the insulin signalling network. Among the critical nodes of hepatic insulin signalling, insulin receptor substrate 2 (IRS2) and protein tyrosine phosphatase 1B (PTP1B) modulate the phosphatidylinositol (PI) 3-kinase/Akt/Foxo1 pathway that controls the suppression of gluconeogenic genes. In this review, we will focus on recent findings regarding the molecular mechanism by which IRS2 and PTP1B elicit opposite effects on carbohydrate metabolism in the liver in response to insulin. Finally, we will discuss the involvement of the critical nodes of insulin signalling in non-alcoholic fatty liver disease (NAFLD) in humans.

Protein Tyrosine Phosphatase 1B (PTP1B) deficiency accelerates hepatic regeneration in mice

Protein tyrosine phosphatase 1B (PTP1B) is a key regulator of metabolism and cell growth by its ability to dephosphorylate tyrosine kinase receptors and modulate the intensity of their signaling cascades. Because liver regeneration involves tyrosine phosphorylation-mediated signaling, we investigated the role of PTP1B in this process by performing partial hepatectomy in wild-type (PTP1B(+/+)) and PTP1B-deficient (PTP1B(-/-)) mice. The expression of PCNA and cyclins D1 and E (cell proliferation markers) was enhanced in PTP1B(-/-) regenerating livers, in parallel with 5'-bromo-2'-deoxyuridine incorporation. Phosphorylation of JNK1/2 and STAT3, early triggers of hepatic regeneration in response to TNF-α and IL-6, was accelerated in PTP1B(-/-) mice compared with PTP1B(+/+) mice. These phosphorylations were increased in PTP1B(-/-) hepatocytes or by silencing PTP1B in wild-type cells and decreased further after the addition of recombinant PTP1B. Enhanced EGF- and HGF receptor-mediated signaling was observed in regenerating livers lacking PTP1B and in EGF- or HGF-stimulated PTP1B(-/-) hepatocytes. Moreover, PTP1B(-/-) mice displayed a more rapid increase in intrahepatic lipid accumulation than PTP1B(+/+) control mice. Late responses to partial hepatectomy revealed additional divergences because stress-mediated signaling was attenuated at 24 to 96 hours in PTP1B(-/-) mice compared with PTP1B(+/+) mice. Finally, PTP1B deficiency also improves hepatic regeneration in mice fed a high-fat diet. These results suggest that pharmacological inhibition of PTP1B would improve liver regeneration in patients with acute or chronic liver injury.

Hepatic insulin resistance is associated with increased apoptosis and fibrogenesis in nonalcoholic steatohepatitis and chronic hepatitis C

BACKGROUND & AIMS

We aimed to elucidate whether hepatic insulin resistance may contribute to hepatocyte apoptosis and fibrogenesis in nonalcoholic fatty liver disease (NAFLD) and in chronic hepatitis C virus (HCV) infection.

METHODS

Twenty-seven nonalcoholic steatosis (NAST), 24 nonalcoholic steatohepatitis (NASH), 71 HCV, and 29 patients with histological normal liver (NL) were studied. Real-time PCR, the TUNEL assay, and Western blots were used to assess insulin-signaling molecules, hepatocyte apoptosis, antiapoptotic mediators, active caspase 3, and type I collagen in liver biopsies. HCV core-transfected human hepatocytes were used as an in vitro model.

RESULTS

In NAFLD patients, hepatic levels of insulin receptor substrate (IRS) 1, IRS2 2, the p85α subunit of phosphatidylinositol 3-kinase (p85α), phosphorylated protein kinase B (pAkt), phosphorylated forkhead box-containing protein O subfamily-1 (FoxO), and phosphorylated 5' adenosine monophosphate-activated protein kinase (pAMPK) as well as the antiapoptotic mediators B-cell lymphoma 2 protein (Bcl-2) and myeloid cell leukemia protein-1 (Mcl-1) were significantly lower in NASH than in NAST and NL. Furthermore, hepatocyte apoptosis and increased active caspase 3 were only present in NASH. In HCV patients, hepatic insulin signaling was markedly impaired, regardless of viral genotype and the presence of steatosis paralleled with enhanced apoptosis. In cultured human hepatocytes, HCV core protein decreased pAkt and increased phosphorylation of c-Jun N-terminal kinase (JNK). This effect was more pronounced in lipid-loaded hepatocytes.

CONCLUSIONS

Hepatic insulin signaling is impaired in NASH and HCV patients, and downregulation of insulin-sensitive targets is associated with increased apoptosis and fibrogenesis in both conditions. JNK might be a target for HCV-induced insulin resistance.

Beneficial effects of PTP1B deficiency on brown adipocyte differentiation and protection against apoptosis induced by pro- and anti-inflammatory stimuli

Insulin is an inducer of brown fat adipogenesis through the activation of a signalling network that involves positive/negative modulators. Given the importance of brown adipose tissue (BAT) for basal thermogenic energy expenditure, we investigated the role of PTP1B in the acquisition of terminal differentiated phenotype and in the apoptotic responses of brown adipocytes. Immortalized brown preadipocytes lacking (PTP1B(-/-)) or expressing (PTP1B(+/+)) PTP1B have been generated. PTP1B deficiency accelerated a full program of brown adipogenesis including induction of transcription factors, coactivators, adipogenic markers and signalling molecules. Fully differentiated PTP1B(-/-) brown adipocytes were resistant to tumor necrosis factor (TNFalpha)-induced apoptosis as these cells were protected against caspase-8 activation, FLIP degradation, Bid cleavage and caspase-3 activation compared to wild-type controls. These events were recovered by PTP1B rescue. Survival signalling including phosphorylation of IRS-1 and Akt/PKB and BclxL expression were decreased in TNFalpha-treated PTP1B(-/-) cells but not in the wild-type. Similarly, PTP1B(-/-) brown adipocytes were protected against resveratrol-induced apoptosis. Phosphorylation of Akt/PKB and Foxo1 phosphorylation/acetylation decreased exclusively in resveratrol-treated wild-type cells, leading to nuclear localization of Foxo1 and up-regulation of Bim. Thus, PTP1B inhibition could be of benefit against obesity by counteracting TNFalpha-induced brown fat atrophy, and combined with resveratrol might improve low-grade inflammation.

Insulin receptor substrate (IRS)-2, not IRS-1, protects human neuroblastoma cells against apoptosis

Insulin receptor substrates (IRS)-1 and -2 are major substrates of insulin and type I insulin-like growth factor (IGF-I) receptor (IGF-IR) signaling. In this study, SH-EP human neuroblastoma cells are used as a model system to examine the differential roles of IRS-1 and IRS-2 on glucose-mediated apoptosis. In the presence of high glucose, IRS-1 underwent caspase-mediated degradation, followed by focal adhesion kinase (FAK) and Akt degradation and apoptosis. IRS-2 expression blocked all these changes whereas IRS-1 overexpression had no effect. In parallel, IRS-2, but not IRS-1, overexpression enhanced IGF-I-mediated Akt activation without affecting extracellular regulated kinase signaling. While IRS-1 was readily degraded by caspases, hyperglycemia-mediated IRS-2 degradation was unaffected by caspase inhibitors but blocked by proteasome and calpain inhibitors. Our data suggest that the differential degradation of IRS-1 and IRS-2 contributes to their distinct modes of action and the increased neuroprotective effects of IRS-2 in this report are due, in part, to its resistance to caspase-mediated degradation.

The liver-brain axis of alcohol-mediated neurodegeneration: role of toxic lipids

Alcohol abuse causes progressive toxicity and degeneration in liver and brain due to insulin resistance, which exacerbates oxidative stress and pro-inflammatory cytokine activation. Alcohol-induced steatohepatitis promotes synthesis and accumulation of ceramides and other toxic lipids that cause insulin resistance. Ceramides can readily cross the blood-brain barrier, and ceramide exposure causes neurodegeneration with insulin resistance and oxidative stress, similar to the effects of alcohol. Therefore, in addition to its direct neurotoxic effects, alcohol misuse establishes a liver-brain axis of neurodegeneration mediated by toxic lipid trafficking across the blood-brain barrier, leading to progressive white matter degeneration and cognitive impairment.

S6K1 deficiency protects against apoptosis in hepatocytes

The mammalian target of rapamycin (mTOR)/S6K1 signaling pathway controls cell growth and proliferation. To assess the importance of S6K1 in the balance between death and survival in the liver, we have generated immortalized hepatocyte cell lines from wild-type and S6K1-deficient (S6K1(-/-)) mice. In S6K1(-/-) hepatocytes, caspase-8 and the pro-apoptotic protein Bid were constitutively down-regulated as compared with wild-type. Moreover, S6K1(-/-) hepatocytes failed to respond to the apoptotic trigger of death receptor activation. Neither caspase-8 activation nor FLIP(L) degradation in response to tumor necrosis factor alpha (TNF-alpha) or anti-Fas antibody (Jo2) was observed in cells lacking S6K1. Downstream events such as Bid cleavage, cytochrome C release, caspase-3 activation, DNA laddering, as well as the percentage of apoptotic cells were attenuated as compared with wild-type. In addition, the anti-apoptotic protein Bclx(L) was down-regulated in TNF-alpha-treated or Jo2-treated wild-type hepatocytes, but this response was abolished in S6K1(-/-)cells. In vivo, S6K1-deficient mice were protected against concanavalin A-induced apoptosis. The withdrawal of growth factors strongly induced apoptosis in wild-type, but not in S6K1(-/-) hepatocytes. S6K1 deficiency did not decrease Bclx(L)/Bim ratio on serum withdrawal, thereby protecting cells from cytochrome C release and DNA fragmentation. At the molecular level, the lack of S6K1-mediated negative feedback decreased insulin receptor substrate-1 (IRS-1) serine phosphorylation, resulting in activation of survival pathways mediated by phosphatidylinositol 3-kinase/Akt and extracellular signal-regulated kinase (ERK). However, S6K1(-/-) hepatocytes underwent apoptosis on serum withdrawal in combination with phosphatidylinositol 3-kinase (PI3K) or ERK inhibitors.

CONCLUSION

This finding might explain the mechanism of resistance to mTOR inhibitors in cancer treatments and strongly suggests that the inhibition of S6K1 could protect against acute liver failure and, in combination with inhibitors that abrogate the sustained activation of Akt and ERK, could improve the efficacy of hepatocarcinoma (HCC) treatment.

Insulin-like growth factor-I receptor blockade improves outcome in mouse model of lung injury

RATIONALE

The insulin-like growth factor-I (IGF-I) pathway is an important determinant of survival and proliferation in many cells. However, little is known about the role of the IGF-I pathway in lung injury. We previously showed elevated levels of IGF-I in bronchoalveolar lavage fluid from patients with acute respiratory distress syndrome. Furthermore, immunodepletion of IGF from acute respiratory distress syndrome bronchoalveolar lavage increased fibroblast apoptosis.

OBJECTIVES

We examined the effect of blockade of type 1 IGF tyrosine kinase receptor (IGF-IR) in a murine model of bleomycin-induced lung injury and fibrosis.

METHODS

Mice were treated with a monoclonal antibody against the IGF-I receptor (A12) or vehicle after intratracheal bleomycin instillation.

MEASUREMENTS AND MAIN RESULTS

Mice treated with A12 antibody had significantly improved survival after bleomycin injury compared with control mice. Both groups of mice had a similar degree of fibrosis on days 7 and 14, but by Day 28 the A12-treated group had significantly less fibrosis. Delayed treatment with A12 also resulted in decreased fibrosis. A12-treated mice had significantly decreased apoptotic cells on Day 28 compared with control mice. We confirmed that A12 treatment induced mouse lung fibroblast apoptosis in vitro. In addition, IGF-I increased lung fibroblast migration. The primary pathway activated by IGF-I in lung fibroblasts was the insulin receptor substrate-2/phosphatidylinositol 3-kinase/Akt axis.

CONCLUSIONS

IGF-I regulated survival and migration of fibrogenic cells in the lung. Blockade of the IGF pathway increased fibroblast apoptosis and subsequent resolution of pulmonary fibrosis. Thus, IGF-IR may be a potential target for treatment of lung injury and fibrosis.

Synergistic premalignant effects of chronic ethanol exposure and insulin receptor substrate-1 overexpression in liver

AIM

Insulin receptor substrate, type 1 (IRS-1) transmits growth and survival signals, and is overexpressed in more than 90% of hepatocellular carcinomas (HCCs). However, experimental overexpression of IRS-1 in the liver was found not to be sufficient to cause HCC. Since chronic alcohol abuse is a risk factor for HCC, we evaluated potential interactions between IRS-1 overexpression and chronic ethanol exposure by assessing premalignant alterations in gene expression.

METHODS

Wild-type (wt) or IRS-1 transgenic (Tg) mice, constitutively overexpressing the human (h) transgene in the liver, were pair-fed isocaloric liquid diets containing 0% or 24% ethanol for 8 weeks. The livers were used for histopathologic study and gene expression analysis, focusing on insulin, insulin-like growth factor (IGF) and wingless (WNT)-Frizzled (FZD) pathways, given their known roles in HCC.

RESULTS

In wt mice, chronic ethanol exposure caused hepatocellular microsteatosis with focal chronic inflammation, reduced expression of proliferating cell nuclear antigen (PCNA) and increased expression of IGF-I and IGF-I receptor. In hIRS-1 Tg mice, chronic ethanol exposure caused hepatic micro- and macrosteatosis, focal chronic inflammation, apoptosis and disordered lobular architecture. These effects of ethanol in hIRS-1 Tg mice were associated with significantly increased expression of IGF-II, insulin, IRS-4, aspartyl-asparaginyl beta hydroxylase (AAH), WNT-1 and FZD 7, as occurs in HCC.

CONCLUSION

In otherwise normal liver, chronic ethanol exposure mainly causes liver injury and inflammation with impaired DNA synthesis. In contrast, in the context of hIRS-1 overexpression, chronic ethanol exposure may serve as a cofactor in the pathogenesis of HCC by promoting expression of growth factors, receptors and signaling molecules known to be associated with hepatocellular transformation.

Insulin resistance in experimental alcohol-induced liver disease

BACKGROUND AND AIM

Chronic ethanol consumption impairs liver regeneration due, in part, to inhibition of insulin signaling. This study characterizes the mechanisms and consequences of ethanol-impaired insulin signaling in relation to oxidative injury and altered gene expression.

METHODS

Long-Evans rats were fed for 8 weeks with isocaloric liquid diets containing 0% (control) or 37% ethanol (caloric content). Livers were used to examine histopathology, indices of oxidative stress, gene expression required for insulin and insulin-like growth factor (IGF) signaling, insulin-responsive gene expression, i.e. glyceraldehydes-3-phosphate dehydrogenase (GAPDH) and aspartyl-asparaginyl-beta-hydroxylase (AAH), and competitive equilibrium binding to the insulin, IGF-I, and IGF-II receptors.

RESULTS

Chronic ethanol exposure caused liver injury with increased hepatocellular steatosis, inflammation, apoptosis, and increased immunoreactivity for activated caspase-3, 8-hydroxy-2'-deoxyguanosine, and 4-hydroxy-2,3-nonenol. These effects were associated with increased expression of IGF-I receptor, IGF-II, and IGF-II receptor, and expression of IGF-I, AAH, and GAPDH, which mediate energy metabolism and cell motility/remodeling, and reduced binding to the insulin receptor.

CONCLUSIONS

Chronic ethanol-induced liver injury causes insulin resistance with inhibition of insulin-responsive genes needed for metabolism, remodeling, and regeneration. In contrast, the IGF-I and IGF-II signaling mechanisms remain relatively preserved, suggesting that insulin-regulated hepatic functions may be selectively vulnerable to the toxic effects of ethanol.

Long-term consumption of saponins derived from Platycodi radix (22 years old) enhances hepatic insulin sensitivity and glucose-stimulated insulin secretion in 90 % pancreatectomized diabetic rats fed a high-fat diet

Crude saponins derived from Chinese Platycodi radix have been reported to prevent increases in body weight and liver TAG in mice fed a high-fat diet. We investigated the effects of an extract (PR) taken from Korean Platycodi radix, which is cultivated for 22 years in the ground (Jangsaeng doraji), and its saponins (PRS) on insulin resistance and glucose-stimulated insulin secretion in 90 % pancreatectomized diabetic rats fed high-fat diets. Four groups were orally supplemented with 2 g PR, 0.2 g PRS, 20 mg rosiglitazone (positive control) or 0.5 g cellulose (negative control) per kg body weight on a daily basis for 8 weeks. We found that PRS lowered body weight, visceral fat mass and serum leptin levels in pancreatectomized rats in comparison to the control. PR enhanced first- and second-phase insulin secretion while PRS stimulated only first-phase insulin secretion. Glucose infusion rates to maintain euglycaemia at hyperinsulinaemic states decreased in a descending order of rosiglitazone, PRS, PR and control, but they increased hepatic glucose output in the same order. This reduction was associated with the storage of decreased TAG and increased glycogen, which was a result of enhanced tyrosine phosphorylation of anti-insulin receptor substrate-2 and serine473 phosphporylation of protein kinase B (PKB, Akt). Improved hepatic insulin signalling led to decreased phosphoenolpyruvate carboxykinase expression and reduced hepatic glucose output accordingly. In conclusion, PRS principally improves glucose homeostasis by enhancing hepatic insulin sensitivity as a consequence of reducing fat storage and stimulating insulin signalling in diabetic rats. In addition, PR contains components that promote glucose-stimulated insulin secretion.

Iron depletion by deferoxamine up-regulates glucose uptake and insulin signaling in hepatoma cells and in rat liver

Iron depletion improves insulin resistance in patients with nonalcoholic fatty liver disease and diabetes and also stabilizes the hypoxia-inducible factor (HIF)-1, resulting in increased glucose uptake in vitro. This study investigated the effect of iron depletion by deferoxamine on insulin signaling and glucose uptake in HepG2 hepatocytes and in rat liver. In HepG2 cells, deferoxamine stabilized HIF-1alpha and induced the constitutive glucose transporter Glut1 and the insulin receptor. Up-regulation of insulin receptor by deferoxamine was mimicked by the intracellular iron chelator deferasirox and the hypoxia inducer CoCl2 and required the HIF-1 obligate partner ARNT/HIF-1beta. Iron depletion increased insulin receptor activity, whereas iron supplementation had the opposite effect. Deferoxamine consistently increased the phosphorylation status of Akt/PKB and its targets FoxO1 and Gsk3beta, which mediate the effect of insulin on gluconeogenesis and glycogen synthesis, and up-regulated genes involved in glucose uptake and utilization. Iron depletion of Sprague-Dawley rats increased HIF-1alpha expression, improved glucose clearance, and was associated with up-regulation of insulin receptor and Akt/PKB levels and of glucose transport in hepatic tissue. Conversely, gluconeogenic genes were not affected. In rats with fatty liver because of a high-calorie and high-fat diet, glucose clearance was increased by iron depletion and decreased by iron supplementation. Thus, iron depletion by deferoxamine up-regulates glucose uptake, and increases insulin receptor activity and signaling in hepatocytes in vitro and in vivo.

Type 2 diabetes mellitus as risk factor for colorectal cancer

Colorectal cancer occurs more frequently in patients with type 2 diabetes mellitus. The hyperinsulinemia-hypothesis suggests that elevated levels of insulin and free IGF-1 promote proliferation of colon cells and lead to a survival benefit of transformed cells, ultimately resulting in colorectal cancer. In patients with type 2 diabetes mellitus, epidemiological studies show an increased risk for colorectal cancer and an even higher risk if patients are treated with sulphonylureas or insulin. Moreover, tumour progression at hyperinsulinemia is more rapid and tumour-associated mortality is increased. Colorectal cancer can be avoided by screening. Recommendations for colorectal cancer screening should employ the recent epidemiologic evidence. All patients with type 2 diabetes mellitus should be recommended to undergo colonoscopy before starting insulin therapy, and screening intervals should not exceed 5 years. For this concept, a review of the evidence is presented, and a screening algorithm for colorectal cancer in patients with type 2 diabetes mellitus is proposed.

Role of insulin receptor and balance in insulin receptor isoforms A and B in regulation of apoptosis in simian virus 40-immortalized neonatal hepatocytes

We have investigated the unique role of the insulin receptor (IR) and the balance of its isoforms A and B in the regulation of apoptosis in simian virus 40 (SV40)-immortalized neonatal hepatocytes. Immortalized hepatocytes lacking (HIR KO) or expressing the entire IR (HIR LoxP), and cells expressing either IRA (HIR RecA) or IRB (HIR RecB) have been generated. IR deficiency in hepatocytes increases sensitivity to the withdrawal of growth factors, because these cells display an increase in reactive oxygen species, a decrease in Bcl-x(L), a rapid accumulation of nuclear Foxo1, and up-regulation of Bim. These events resulted in acceleration of caspase-3 activation, DNA laddering, and cell death. The single expression of either IRA or IRB produced a stronger apoptotic phenotype. In these cells, protein complexes containing IRA or IRB and Fas/Fas-associating protein with death domain activated caspase-8, and, ultimately, caspase-3. In hepatocytes expressing IRA, Bid cleavage and cytochrome C release were increased whereas direct activation of caspase-3 by caspase-8 and a more rapid apoptotic process occurred in hepatocytes expressing IRB. Conversely, coexpression of IRA and IRB in IR-deficient hepatocytes rescued from apoptosis. Our results suggest that balance alteration of IRA and IRB may serve as a ligand-independent apoptotic trigger in hepatocytes, which may regulate liver development.

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.

Differential activation of insulin receptor substrates 1 and 2 by insulin-like growth factor-activated insulin receptors

The insulin-like growth factors (insulin-like growth factor I [IGF-I] and IGF-II) exert important effects on growth, development, and differentiation through the IGF-I receptor (IGF-IR) transmembrane tyrosine kinase. The insulin receptor (IR) is structurally related to the IGF-IR, and at high concentrations, the IGFs can also activate the IR, in spite of their generally low affinity for the latter. Two mechanisms that facilitate cross talk between the IGF ligands and the IR at physiological concentrations have been described. The first of these is the existence of an alternatively spliced IR variant that exhibits high affinity for IGF-II as well as for insulin. A second phenomenon is the ability of hybrid receptors comprised of IGF-IR and IR hemireceptors to bind IGFs, but not insulin. To date, however, direct activation of an IR holoreceptor by IGF-I at physiological levels has not been demonstrated. We have now found that IGF-I can function through both splice variants of the IR, in spite of low affinity, to specifically activate IRS-2 to levels similar to those seen with equivalent concentrations of insulin or IGF-II. The specific activation of IRS-2 by IGF-I through the IR does not result in activation of the extracellular signal-regulated kinase pathway but does induce delayed low-level activation of the phosphatidylinositol 3-kinase pathway and biological effects such as enhanced cell viability and protection from apoptosis. These findings suggest that IGF-I can function directly through the IR and that the observed effects of IGF-I on insulin sensitivity may be the result of direct facilitation of insulin action by IGF-I costimulation of the IR in insulin target tissues.

Amphiregulin Contributes to the Transformed Phenotype of Human Hepatocellular Carcinoma Cells

Hepatocellular carcinoma is a major cause of cancer-related deaths. Current treatments are not effective, and the identification of relevant pathways and novel therapeutic targets are much needed. Increasing evidences point to the activation of the epidermal growth factor receptor (EGFR) as an important mechanism in the development of hepatocarcinoma. We previously described that amphiregulin (AR), a ligand of the EGFR, is not expressed in healthy liver but is up-regulated during chronic liver injury, the background on which most liver tumors develop. Now, we have studied the expression and role of AR in human hepatocarcinoma. AR expression and function was studied in human liver tumors and cell lines. AR is expressed in human hepatocellular carcinoma tissues and cell lines and behaves as a mitogenic and antiapoptotic growth factor for hepatocarcinoma cells. We provide several lines of evidence, including AR silencing by small interfering RNAs and inhibition of amphiregulin by neutralizing antibodies, showing the existence of an AR-mediated autocrine loop that contributes to the transformed phenotype. Indeed, interference with endogenous AR production resulted in reduced constitutive EGFR signaling, inhibition of cell proliferation, anchorage-independent growth, and enhanced apoptosis. Moreover, knockdown of AR potentiated transforming growth factor-beta and doxorubicin-induced apoptosis. Conversely, overexpression of AR in SK-Hep1 cells enhanced their proliferation rate, anchorage-independent growth, drug resistance, and in vivo tumorigenic potential. These observations suggest that AR is involved in the acquisition of neoplastic traits in the liver and thus constitutes a novel therapeutic target in human hepatocarcinoma.

Irs1 and Irs2 signaling is essential for hepatic glucose homeostasis and systemic growth

Insulin receptor substrates, including Irs1 and Irs2, integrate insulin and IGF receptor signals with heterologous pathways to coordinate growth and metabolism. Since Irs2 is thought to be especially important in hepatic nutrient homeostasis, we deleted Irs2 [corrected] from hepatocytes of WT mice (called LKO) or genetically insulin-resistant Irs1-/- mice (called LKO::Irs1-/-). Viable LKO::Irs1-/- mice were 70% smaller than WT or LKO mice, and 40% smaller than Irs1-/- mice. Hepatic insulin receptors were functional in all the mice, but insulin signaling via the Akt-FoxO1 pathway was reduced in Irs1-/- and LKO liver, and undetected in LKO::Irs1-/- liver; however, Gsk3beta phosphorylation (Ser9) and hepatic glycogen stores were nearly normal in all of the mice. LKO and Irs1-/- mice developed insulin resistance and glucose intolerance that never progressed to diabetes, whereas LKO::Irs1-/- mice developed hyperglycemia and hyperinsulinemia immediately after birth. Regardless, few hepatic genes changed expression significantly in Irs1-/- or LKO mice, whereas hundreds of genes changed in LKO::Irs1-/- mice--including elevated levels of Pck1, G6pc, Ppargc1, Pparg, and Igfbp1. Thus, signals delivered by Irs1 or Irs2 regulate hepatic gene expression that coordinates glucose homeostasis and systemic growth.

Death by association: BH3 domain-only proteins and liver injury

Apoptosis, a prominent form of cell death, is a prime feature of many acute and chronic liver diseases. Apoptosis requires mitochondrial dysfunction, which is regulated by proteins of the Bcl-2 family. Whether or not a cell should live or die is controlled by the interaction of multidomain Bcl-2 proteins with proapoptotic BH3 domain-only proteins of this family. Current models suggest multidomain, antiapoptotic Bcl-2 proteins prevent mitochondrial dysfunction by sequestering and/or preventing activation of its proapoptotic relatives. BH3-only proteins initiate cell death by neutralizing and or ligating multidomain prosurvival Bcl-2 proteins. Thus BH3 domain-only proteins are paramount in the apoptotic process as exemplified by the role of the BH3 domain-only protein Bid in liver injury. In this concise review, we will focus on how these BH3 domain-only proteins are regulated in the cell, their association with the Bcl-2 family of proteins, and finally, current information regarding their involvement in liver cell apoptosis and injury.