Insulin/IGF-I rescues immortalized brown adipocytes from apoptosis down-regulating Bcl-xS expression, in a PI 3-kinase- and map kinase-dependent manner

Adropin stimulates proliferation but suppresses differentiation in rat primary brown preadipocytes

Adropin is a peptide hormone encoded by Energy Homeostasis Associated (Enho) gene. Adropin modulates glucose and lipid metabolism, and adiposity. Recently, we found that adropin suppresses differentiation of rodent white preadipocytes into mature fat cells. By contrast, the role of adropin in controlling brown adipogenesis is largely unknown. Therefore, in the present study we evaluated the effects of adropin on proliferation and differentiation of adipocyte precursor cells in rats. Brown adipocyte precursor cells were isolated from male Wistar rats. Cell replication was measured by BrdU incorporation. Gene expression was studied using real time PCR. Protein phosphorylation and production was assessed by Western blot. Lipid accumulation was evaluated by Oil Red O staining. Colorimetric kits were used to evaluate glycerol and free fatty acids release. We report here that adropin stimulates proliferation of brown preadipocytes. Moreover, in brown preadipocytes, adropin suppresses mRNA expression of adipogenic genes (C/ebpα, C/ebpβ, Pgc1α, Pparγ and Prdm16) during differentiation process. In addition, adropin suppresses UCP1 protein production in brown adipocytes. Finally, adropin reduces intracellular lipid content in brown adipocytes. These results indicate that adropin stimulates proliferation of brown preadipocytes and suppresses their differentiation into mature adipocytes.

Disruption of tumor necrosis factor alpha receptor 1 signaling accelerates NAFLD progression in mice upon a high-fat diet

Obesity is accompanied by a low-grade inflammation state, characterized by increased proinflammatory cytokines levels such as tumor necrosis factor alpha (TNFα) and interleukin-1 beta (IL-1β). In this regard, there exists a lack of studies in hepatic tissue about the role of TNFα receptor 1 (TNFR1) in the context of obesity and insulin resistance during the progression of nonalcoholic fatty liver disease (NAFLD). The aim of this work was to evaluate the effects of high-caloric feeding (HFD) (40% fat, for 16 weeks) on liver inflammation-induced apoptosis, insulin resistance, hepatic lipid accumulation and its progression toward nonalcoholic steatohepatitis (NASH) in TNFR1 knock-out and wild-type mice. Mechanisms involved in HFD-derived IL-1β release and impairment of insulin signaling are still unknown, so we determined whether IL-1β affects liver insulin sensitivity and apoptosis through TNFα receptor 1 (TNFR1)-dependent pathways. We showed that knocking out TNFR1 induces an enhanced IL-1β plasmatic release upon HFD feed. This was correlated with higher hepatic and epididymal white adipose tissue mRNA levels. In vivo and in vitro assays confirmed an impairment in hepatic insulin signaling, in part due to IL-1β-induced decrease of AKT activation and diminution of IRS1 levels, followed by an increase in inflammation, macrophage (resident and recruited) accumulation, hepatocyte apoptotic process and finally hepatic damage. In addition, TNFR1 KO mice displayed higher levels of pro-fibrogenic markers. TNFR1 signaling disruption upon an HFD leads to an accelerated progression from simple steatosis to a more severe phenotype with many NASH features, pointing out a key role of TNFR1 in NAFLD progression.

High concentration of insulin promotes apoptosis of primary cultured rat ovarian granulosa cells via its increase in extracellular HMGB1

Polycystic ovary syndrome (PCOS) is a common endocrine disorder affecting women of reproductive age. Insulin resistance/hyperinsulinemia is a prevalent finding in women with PCOS, which indicates that insulin resistance/hyperinsulinemia may be an important player in the pathogenesis of the PCOS. However, the underlying mechanism of insulin resistance/hyperinsulinemia on the pathogenesis of the PCOS remains elusive. In this study, we found an increased high-mobility group box 1 (HMGB1) in the serum from women with PCOS having insulin resistance/hyperinsulinemia. Furthermore, we discovered that high concentration of insulin, which mimics insulin resistance model, promoted apoptosis in primary cultured rat ovarian granulosa cells (GCs) via its effect on the increase in extracellular HMGB1. Our data presented the first evidence that increased HMGB1 induced by insulin resistance/hyperinsulinemia promoted apoptosis of ovarian GCs, which provided new molecular basis for the PCOS pathogenesis.

Up-regulation of Bcl-2 during adipogenesis mediates apoptosis resistance in human adipocytes

Targeting apoptotic pathways in adipocytes has been suggested as a pharmacological approach to treat obesity. However, adipocyte apoptosis was identified as a cause for macrophage infiltration into adipose tissue. Previous studies suggest that mature adipocytes are less sensitive to apoptotic stimuli as compared to preadipocytes. Here, we aimed to identify proteins mediating apoptosis resistance in adipocytes. Our data revealed that the anti-apoptotic protein Bcl-2 (B-cell lymphoma 2) is up-regulated during adipogenic differentiation. Bcl-2 overexpression in preadipocytes lowers their apoptosis sensitivity to the level of mature adipocytes. Vice versa Bcl-2 knockdown in adipocytes sensitizes these cells to CD95-induced apoptosis. Taken together, our findings suggest a shift in the balance of pro-apoptotic and anti-apoptotic molecules during adipogenesis resulting in a higher apoptosis resistance. This study sheds new light on the apoptotic process in human fat cells and may constitute a new possible target for the specific regulation of adipose tissue mass.

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.

An inflammatory micro-environment promotes human adipocyte apoptosis

Obesity-associated macrophage infiltration into adipose tissue is responsible for both local and systemic inflammation. Recent findings suggest fat cell apoptosis as an initiator of macrophage recruitment. Here, we investigated the effects of an inflammatory micro-environment on fat cells using human THP-1 macrophages and SGBS adipocytes. Macrophage-secreted factors induced insulin resistance, inhibited insulin-stimulated Akt phosphorylation, and induced apoptosis of adipocytes. The apoptosis-inducing effect was even more pronounced in direct co-cultures of adipocytes and macrophages. Our data suggest a link between insulin resistance and apoptosis sensitivity. Accordingly, pharmacological and genetic inhibition of insulin signaling at the level of Akt2 sensitized adipocytes to apoptosis induction by macrophage-secreted factors. In conclusion, we describe here a novel interaction of macrophages and fat cells, i.e. induction of apoptosis. Our data suggest a feed-forward cycle in which macrophages further drive the inflammatory process by inducing insulin resistance and concomitant apoptosis of adipocytes.

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.

A kinase-independent role for unoccupied insulin and IGF-1 receptors in the control of apoptosis

Insulin and insulin-like growth factor 1 (IGF-1) act as antiapoptotic hormones. We found that, unexpectedly, double-knockout (DKO) cells that lacked both insulin and IGF-1 receptors (IR and IGF1R, respectively) were resistant to apoptosis induced through either the intrinsic or the extrinsic pathway. This resistance to apoptosis was associated with decreased abundance of the proapoptotic protein Bax and increases in abundance of the antiapoptotic proteins Bcl-2, Bcl-xL, XIAP, and Flip. These changes in protein abundance involved primarily posttranscriptional mechanisms. Restoration of IR or IGF1R to DKO cells also restored their sensitivity to apoptosis. Notably, expression of a catalytically inactive mutant form of the IR also restored susceptibility to apoptosis. Thus, IR and IGF1R have bidirectional roles in the control of cell survival and can be viewed as dependence receptors. Insulin and IGF-1 binding stimulates receptor tyrosine kinase activity and blocks apoptosis, whereas unliganded IR and IGF1R, acting through a mechanism independent of their catalytic activity, exert a permissive effect on cell death.

8,9-Epoxyeicosatrienoic acid analog protects pulmonary artery smooth muscle cells from apoptosis via ROCK pathway

Epoxyeicosatrienoic acids (EETs), metabolites of arachidonic acid (AA) catalyzed by cytochrome P450 (CYP), have many essential biologic roles in the cardiovascular system including inhibition of apoptosis in cardiomyocytes. In the present study, we tested the potential of 8,9-EET and derivatives to protect pulmonary artery smooth muscle cells (PASMCs) from starvation induced apoptosis. We found 8,9-epoxy-eicos-11(Z)-enoic acid (8,9-EET analog (214)), but not 8,9-EET, increased cell viability, decreased activation of caspase-3 and caspase-9, and decreased TUNEL-positive cells or nuclear condensation induced by serum deprivation (SD) in PASMCs. These effects were reversed after blocking the Rho-kinase (ROCK) pathway with Y-27632 or HA-1077. Therefore, 8,9-EET analog (214) protects PASMC from serum deprivation-induced apoptosis, mediated at least in part via the ROCK pathway. Serum deprivation of PASMCs resulted in mitochondrial membrane depolarization, decreased expression of Bcl-2 and enhanced expression of Bax, all effects were reversed by 8,9-EET analog (214) in a ROCK dependent manner. Because 8,9-EET and not the 8,9-EET analog (214) protects pulmonary artery endothelial cells (PAECs), these observations suggest the potential to differentially promote apoptosis or survival with 8,9-EET or analogs in pulmonary arteries.

The role of hyperglycemia in burned patients: evidence-based studies

Severely burned patients typically experience a systemic response expressed as increased metabolism, inflammation, alteration of cardiac and immune function, and associated hyperglycemia. Hyperglycemia has been associated with an increased risk of morbidity and mortality in critically ill patients. Until recently and for many years, hyperglycemia has been expectantly managed and considered a normal and desired response of an organism to stress. However, findings reported from recent studies now suggest beneficial effects of intensive insulin treatment of critically ill patients. The literature on the management of hyperglycemia in severely burned patients is sparse, with most of the available studies involving only small numbers of burned patients. The purpose of this article is to describe the pathophysiology of hyperglycemia after severe burns and to review the available literature on the outcome of intensive insulin treatment and other anti-hyperglycemic modalities in burned patients in an evidence-based medical approach.

Developmental consequences of alternative Bcl-x splicing during preimplantation embryo development

Elevated cell death in human preimplantation embryos is one of the cellular events compromising pregnancy rates after assisted reproductive technology treatments. We therefore explored the molecular pathways regulating cell death at the blastocyst stage in human embryos cultured in vitro. Owing to limited availability of human embryos, these pathways were further characterized in mouse blastocysts. Gene expression studies revealed a positive correlation between the cell death index and the expression of Bcl-x transcript. Cell death activation in human blastocysts was accompanied by changes in Bcl-x splicing, favoring production of Bcl-xS, an activator of cell death. Expression of Bcl-xS was detected in a subset of human blastocysts that show particular clustering in dying and/or dead cells. Altering the Bcl-xL/Bcl-xS ratio in mouse embryos, in antisense experiments, confirmed that upregulation of Bcl-xS, with concomitant downregulation of Bcl-xL, compromised developmental potential and committed a subset of cells to undergoing cell death. This was accompanied by increased accumulation of reactive oxygen species levels without any impact on mtDNA content. In addition, altered Bcl-x splicing in favor of Bcl-xS was stimulated by culture in HTF medium or by addition of excessive glucose, leading to compromised embryo development. Thus, we conclude that inappropriate culture conditions affect Bcl-x isoform expression, contributing to compromised preimplantation embryo development.

Hyperglycemia induces apoptosis and p53 mobilization to mitochondria in RINm5F cells

The mechanisms related to hyperglycemia-induced pancreatic beta-cell apoptosis are poorly defined. Rat insulin-producing cells (RINm5F) cultured in high glucose concentrations (30 mM) showed increased apoptosis and protein p53 translocation to mitochondria. In addition, hyperglycemia induced both the disruption of mitochondrial membrane potential (Delta psi (m)), and an increase in reactive oxygen species (ROS), as shown by fluorescence changes of JC-1 and dichlorodihydrofluorescein-diacetate (DCDHF-DA), respectively. The increased intracellular ROS by high glucose exposure was blunted by mitochondrial-function and NADPH-oxidase inhibitors. We postulate that the concomitant mobilization of p53 protein to the mitochondria and the subsequent changes on the Delta psi (m), lead to an important pancreatic beta-cell apoptosis mechanism induced by oxidative stress caused by hyperglycemia.

The role of the type I insulin-like growth factor receptor (IGF-IR) in glomerular integrity

Insulin-like growth factors (IGFs) have been implicated in normal mammalian kidney development. To confirm a role for the IGF system in podocyte and glomerular integrity, we generated a transgenic mouse that expresses a dominant-negative type 1 IGF receptor (IGF-IR) and determined the structural and functional consequences. Using a 4.25kb fragment of the murine nephrin promoter, the dominant-negative construct was expressed exclusively in the kidney, confirmed by Southern blot and RT-PCR analysis. IGF-Ir486(FLAGstop) protein localized specifically to the glomerular podocyte based on FLAG immunohistochemistry and on co-localization with nephrin and podocin. Wild type and transgenic glomeruli expressed both the alpha- and beta-subunits of the endogenous IGF-IR, with normal expression of both nephrin and podocin. Although the animals were viable and phenotypically normal, histological analysis of the kidneys revealed abnormal and small glomeruli with dilated glomerular capillaries and condensed podocyte nuclei, while ultra-structural examination revealed diffuse but segmental podocyte foot process broadening, fusion, and effacement. Explanted glomeruli from transgenic animals demonstrated a significant inhibition of podocyte cell outgrowth when compared to controls. These studies suggest that IGF signaling is essential for maintaining the integrity of the podocyte and that alterations of IGF signaling may play a role in progressive glomerular disease.

Interleukin-1β enhances the effect of serum deprivation on rat annular cell apoptosis

Excessive apoptosis of disc cells is believed to play an important role in intervertebral disc (IVD) degeneration. It has been shown that interleukin-1beta (IL-1beta) is involved in the failure of disc matrix by suppressing the synthesis of matrix components and stimulating the expression of matrix metalloproteinases. However, whether IL-1beta induces disc cell apoptosis is still unclear. The objective of this study was to investigate the effect of IL-1beta on the apoptosis of rat annular cells cultured with or without serum supplement. First-passage rat annular cells were cultured with 0% or 10% fetal bovine serum (FBS) supplement and stimulated with 0, 10, 20 or 50 ng/ml IL-1beta for 12, 24 or 48 h. Apoptotic incidences were quantified by flow cytometry, morphologic changes in apoptotic cells were visualized by Hoechst 33258 staining and phase-contrast microscopy, and caspase-3 activity was also determined. When rat annular cells were cultured with 10% FBS supplement, no significant changes in apoptotic incidences, apoptotic morphology and caspase-3 activity were observed even when cells were stimulated with 50 ng/ml IL-1beta for 48 h. In contrast, serum deprivation for 24 h led to an increase in apoptotic incidences, the number of apoptotic nuclei and caspase-3 activity, and IL-1beta significantly increased the effects of serum deprivation in a dose-dependent manner. Our results indicate that IL-1beta alone is not a sufficient stimulus to induce disc cell apoptosis and that in order to suppress disc cell apoptosis, improving the nutrient supply to the disc may be more effective than antagonizing the adverse effects of IL-1beta.

Insulin-like growth factors inhibit podocyte apoptosis through the PI3 kinase pathway

BACKGROUND

Abnormal podocyte development and progressive podocyte injury have been implicated in a number of human kidney diseases. Factors necessary for regulating development and maintenance of this cell type are only beginning to emerge.

METHODS

To study the role of the insulin-like growth factor (IGF) system in regulating podocyte survival, we induced human fetal podocytes to undergo apoptosis. We demonstrated a significant increase in apoptosis when these cells were incubated in the presence of etoposide, as measured by DNA fragmentation and nuclear membrane condensation and blebbing.

RESULTS

Podocyte apoptosis was reduced to control levels when the cells were coincubated in the presence of IGF-1. We showed that the protective effect of IGFs in this cell type was mediated through the activation of the phosphatidylinositol 3'-kinase (PI3K) pathway. IGF-1 stimulation resulted in the formation of the insulin receptor substrate (IRS)-1-p85 complex, an increase in PI3 kinase activity, and activation of protein kinase B (AKT/PKB) and the bcl-2 family member bad. Incubation of the podocytes with inhibitors of the PI3 kinase pathway resulted in a loss of this IGF-1 protective effect.

CONCLUSION

These data demonstrate an important role for the IGF system in fetal podocyte survival in vitro, and suggest potential mediators to slow or alleviate the loss or damage of the podocyte in progressive renal disease.

Insulin receptor-protein kinase C-gamma signaling mediates inhibition of hypoxia-induced necrosis of cortical neurons

Ischemic stress causes neuronal death and functional impairment. Evidence has suggested that cells in the ischemic core first lose viability due to the decline in blood flow and cellular energy metabolism and then die by necrosis. Although inhibition of necrosis could be a potent therapeutic target for brain ischemia, known neurotrophic factors are ineffective for neuronal necrosis. We previously reported that insulin, but not brain-derived neurotrophic factor or insulin like-growth factor-1, inhibited neuronal necrosis under serum-free starvation stress. Although insulin receptors are abundant in the central nervous system as well as in peripheral tissues, neurons are not dependent upon insulin for their glucose supply, indicating that insulin receptors have other roles in the central nervous system. In the present study, by using hypoxia-reperfusion stress, we showed that cortical neurons rapidly died by necrosis as evaluated by propidium iodide staining and transmission electron microscopic analysis. As expected, insulin treatment significantly inhibited neuronal necrosis, although this effect was blocked by pretreatment with an antisense oligonucleotide for the insulin receptor. Furthermore, an inhibitor of protein kinase C (PKC) eliminated the insulin-induced antinecrotic effect. The addition of insulin induced significant translocation of only the PKC-gamma isoform, whereas antisense oligonucleotide treatment for this isoform abolished the insulin-induced inhibition of necrosis. Together, these results suggest that insulin mediates inhibition of neuronal necrosis through a novel mechanism involving PKC-gamma activation.

The brown adipose cell: a model for understanding the molecular mechanisms of insulin resistance

Type 2 diabetes mellitus is a complex metabolic disease that occurs when insulin secretion can no longer compensate insulin resistance in peripheral tissues. At the molecular level, insulin resistance correlates with impaired insulin signalling. This review provides new insights into the molecular mechanisms of insulin action and resistance in brown adipose tissue and pinpoints the role of this tissue in the control of glucose homeostasis. Brown adipocytes are target cells for insulin and IGF-I action, especially during late foetal development when insulin supports survival and promotes both adipogenic and thermogenic differentiation. The main pathway involved in insulin induction of adipogenic differentiation, monitored by fatty acid synthase expression, is the cascade insulin receptor substrate (IRS)-1/phosphatidylinositol 3-kinase (PI3K)/Akt. Glucose transport in these cells is maintained mainly by the activity of GLUT4. Acute insulin treatment stimulates glucose transport largely by mediating translocation of GLUT4 to the plasma membrane, involving the activation of IRS-2/PI3K, and the downstream targets Akt and protein kinase C zeta. Tumour necrosis factor (TNF-alpha) caused insulin resistance on glucose uptake by impairing insulin signalling at the level of IRS-2. Activation of stress kinases and phosphatases by this cytokine contribute to insulin resistance. Furthermore, brown adipocytes are also target cells for rosiglitazone action since they show a high expression of peroxisome proliferator activated receptor gamma, and rosiglitazone increased the expression of the thermogenic uncoupling protein 1. Rosiglitazone ameliorates insulin resistance provoked by TNF-alpha, completely restoring insulin-stimulated glucose uptake in parallel to the insulin signalling cascade. Accordingly, foetal brown adipocytes represent a model for investigating insulin action, as well as for the mechanism by which rosiglitazone increase insulin sensitivity under situations that mimic insulin resistance.

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

To assess the role of insulin action and inaction in the liver, immortalized hepatocyte cell lines have been generated from insulin receptor substrate (IRS)-2(-/-) and wild-type mice. Using this model, we have recently demonstrated that the lack of IRS-2 in neonatal hepatocytes resulted in insulin resistance. In the current study, we show that immortalized neonatal hepatocytes undergo apoptosis on serum withdrawal, with caspase-3 activation and DNA laddering occurring earlier in the absence of IRS-2. Insulin rescued wild-type hepatocytes from serum withdrawal-induced caspase-3 activation and DNA fragmentation in a dose-dependent manner, but it failed to rescue hepatocytes lacking IRS-2. In IRS-2(-/-) cells, insulin failed to phosphorylate Bad. Furthermore, in these cells, insulin was unable to translocate Foxo1 from the nucleus to the cytosol. Adenoviral infection of wild-type cells with constitutively active Foxo1 (ADA) induced caspase-8 and caspase-3 activities, proapoptotic gene expression, DNA laddering and apoptosis. Dominant negative Foxo1 regulated the whole pathway in an opposite manner. Prolonged insulin treatment (24 hours) increased expression of antiapoptotic genes (Bcl-xL), downregulated proapoptotic genes (Bim and nuclear Foxo1), and decreased caspase-3 activity in wild-type hepatocytes but not in IRS-2(-/-) cells. Infection of IRS-2(-/-) hepatocytes with adenovirus encoding IRS-2 reconstituted phosphatidylinositol 3-kinase (PI 3-kinase)/Akt/Foxo1 signaling, restored pro- and antiapoptotic gene expression, and decreased caspase-3 activity in response to insulin, thereby blocking apoptosis. In conclusion, IRS-2 signaling is specifically required through PIP3 generation to mediate the survival effects of insulin. Epidermal growth factor, via PIP3/Akt/Foxo1 phosphorylation, was able to rescue IRS-2(-/-) hepatocytes from serum withdrawal-induced apoptosis, modulating pro- and anti-apoptotic gene expression and downregulating caspase-3 activity.

Susceptibility to apoptosis in insulin-like growth factor-I receptor-deficient brown adipocytes

Fetal brown adipocytes are insulin-like growth factor-I (IGF-I) target cells. To assess the importance of the IGF-I receptor (IGF-IR) in brown adipocytes during fetal life, we have generated immortalized brown adipocyte cell lines from the IGF-IR(-/-) mice. Using this experimental model, we demonstrate that the lack of IGF-IR in fetal brown adipocytes increased the susceptibility to apoptosis induced by serum withdrawal. Culture of cells in the absence of serum and growth factors produced rapid DNA fragmentation (4 h) in IGF-IR(-/-) brown adipocytes, compared with the wild type (16 h). Consequently, cell viability was decreased more rapidly in fetal brown adipocytes in the absence of IGF-IR. Furthermore, caspase-3 activity was induced much earlier in cells lacking IGF-IR. At the molecular level, IGF-IR deficiency in fetal brown adipocytes altered the balance of the expression of several proapoptotic (Bcl-xS and Bim) and antiapoptotic (Bcl-2 and Bcl-xL) members of the Bcl-2 family. This imbalance was irreversible even though in IGF-IR-reconstituted cells. Likewise, cytosolic cytochrome c levels increased rapidly in IGF-IR-deficient cells compared with the wild type. A rapid entry of Foxo1 into the nucleus accompanied by a rapid exit from the cytosol and an earlier activation of caspase-8 were observed in brown adipocytes lacking IGF-IR upon serum deprivation. Activation of caspase-8 was inhibited by 50% in both cell types by neutralizing anti-Fas-ligand antibody. Adenoviral infection of wild-type brown adipocytes with constitutively active Foxol (ADA) increased the expression of antiapoptotic genes, decreased Bcl-xL and induced caspase-8 and -3 activities, with the final outcome of DNA fragmentation. Up-regulation of uncoupling protein-1 (UCP-1) expression in IGF-IR-deficient cells by transduction with PGC-1alpha or UCP-1 ameliorated caspase-3 activation, thereby retarding apoptosis. Finally, insulin treatment prevented apoptosis in both cell types. However, the survival effect of insulin on IGF-IR(-/-) brown adipocytes was elicited even in the absence of phosphatidylinositol 3-kinase/Akt signaling. Thus, our results demonstrate for the first time the unique role of IGF-IR in maintaining the balance of death and survival in fetal brown adipocytes.

Rosiglitazone ameliorates insulin resistance in brown adipocytes of Wistar rats by impairing TNF-alpha induction of p38 and p42/p44 mitogen-activated protein kinases

AIMS/HYPOTHESIS

TNF-alpha caused insulin resistance on glucose uptake and on insulin signalling in fetal brown adipocytes. Since treatment with TNF-alpha activates stress kinases, including c-jun NH2 terminal kinase (JNK), and p42/p44 and p38 mitogen-activated protein kinases (MAPK), we explored the contribution of these pathways to insulin resistance by TNF-alpha. Rosiglitazone is used to treat Type 2 diabetes as it improves insulin sensitivity in vivo. However, its ability to ameliorate TNF-alpha-induced insulin resistance in brown adipocytes remains to be explored.

METHODS

We used fetal rat primary brown adipocytes cultured with TNF-alpha, with or without stress kinase inhibitors or rosiglitazone, and further stimulated with insulin. Then, we measured glucose uptake and GLUT4 translocation. To determine the insulin signalling cascade, we submitted cells to lysis, immunoprecipitation and immunoblotting.

RESULTS

Exposure to TNF-alpha for 24 h impairs insulin stimulation of the phosphatidylinositol (PI) 3-kinase activity associated with IRS-2 and Akt activity. Pretreatment with PD98059 or PD169316, which inhibit p42/p44MAPK and p38MAPK respectively, restored insulin signalling and insulin-induced glucose uptake in the presence of TNF-alpha. However, in the presence of SP600125, an inhibitor of JNK, TNF-alpha still produced insulin resistance. Rosiglitazone ameliorated insulin resistance by TNF-alpha in brown adipocytes, restoring completely insulin-stimulated glucose uptake and insulin-induced GLUT4 translocation to plasma membrane in parallel to the insulin signalling cascade IRS-2/PI 3-kinase/Akt.

CONCLUSIONS/INTERPRETATION

Rosiglitazone treatment impaired TNF-alpha activation of p38 and p42/p44MAPK, restoring insulin signalling and leading to normalisation of glucose uptake.

Inhibition of death-receptor mediated apoptosis in human adipocytes by the insulin-like growth factor I (IGF-I)/IGF-I receptor autocrine circuit

Adipose tissue mass is reflected by the volume and the number of adipocytes and is subject to homeostatic regulation involving cell death mechanisms. In this study we have investigated the mechanisms of apoptosis in human preadipocytes and adipocytes that may play a role in the regulation of adipose tissue mass. We found that death receptors (CD95, TNF-related apoptosis-inducing ligand receptors 1 and 2, and TNF receptor 1) are expressed in human fat cells and that apoptosis can be induced by specific ligands. Sensitivity to apoptosis could be stimulated by an inhibitor of biosynthesis. In addition, inhibition of auto-/paracrine action of IGF-I dramatically sensitizes human adipocytes for death ligand-induced apoptosis. Phosphoinositide 3-kinase and, to a weaker extent, p38 MAPK are involved in IGF-I-mediated survival. IGF-I protects human fat cells from apoptosis by maintaining the expression of antiapoptotic proteins, Bcl-x(L) and Fas-associated death domain-like IL-1-converting enzyme inhibitory protein. In conclusion, we identified mechanisms of apoptosis induction in human fat cells. We furthermore demonstrate that human fat cells protect themselves from apoptosis by IGF-I in an auto-/paracrine manner.

Brown adipose tissue: function and physiological significance

The function of brown adipose tissue is to transfer energy from food into heat; physiologically, both the heat produced and the resulting decrease in metabolic efficiency can be of significance. Both the acute activity of the tissue, i.e., the heat production, and the recruitment process in the tissue (that results in a higher thermogenic capacity) are under the control of norepinephrine released from sympathetic nerves. In thermoregulatory thermogenesis, brown adipose tissue is essential for classical nonshivering thermogenesis (this phenomenon does not exist in the absence of functional brown adipose tissue), as well as for the cold acclimation-recruited norepinephrine-induced thermogenesis. Heat production from brown adipose tissue is activated whenever the organism is in need of extra heat, e.g., postnatally, during entry into a febrile state, and during arousal from hibernation, and the rate of thermogenesis is centrally controlled via a pathway initiated in the hypothalamus. Feeding as such also results in activation of brown adipose tissue; a series of diets, apparently all characterized by being low in protein, result in a leptin-dependent recruitment of the tissue; this metaboloregulatory thermogenesis is also under hypothalamic control. When the tissue is active, high amounts of lipids and glucose are combusted in the tissue. The development of brown adipose tissue with its characteristic protein, uncoupling protein-1 (UCP1), was probably determinative for the evolutionary success of mammals, as its thermogenesis enhances neonatal survival and allows for active life even in cold surroundings.

Long-term treatment with insulin induces apoptosis in brown adipocytes: role of oxidative stress

Trying to define the precise role played by insulin regulating the survival of brown adipocytes, we have used rat fetal brown adipocytes maintained in primary culture. The effect of insulin on apoptosis and the mechanisms involved were assessed. Different from the known effects of insulin as a survival factor, we have found that long-term treatment (72 h) with insulin induces apoptosis in rat fetal brown adipocytes. This process is dependent on the phosphatidylinositol 3-kinase/mammalian target of rapamycin/p70 S6 kinase pathway. Short-term treatment with the conditioned medium from brown adipocytes treated with insulin for 72 h mimicked the apoptotic effect of insulin. During the process, caspase 8 activation, Bid cleavage, cytochrome c release, and activation of caspases 9 and 3 are sequentially produced. Treatment with the caspase inhibitor, benzyloxycarbonyl-Val-Ala-Asp (Z-VAD), prevents activation of this apoptotic cascade. The antioxidants, ascorbic acid and superoxide dismutase, also impair this process of apoptosis. Moreover, generation of reactive oxygen species (ROS), probably through reduced nicotinamide adenine dinucleotide phosphate oxidases, and a late decrease in reduced glutathione content are produced. According to this, antioxidants prevent caspase 8 activation and Bid cleavage, suggesting that ROS production is an important event mediating this process of apoptosis. However, the participation of uncoupling protein-1, -2, and -3 regulating ROS is unclear because their levels remain unchanged upon insulin treatment for 72 h. Our data suggest that the prolonged hyperinsulinemia might cause insulin resistance through the loss of brown adipose tissue.

Expression of the bcl-2 family of pro- and anti-apoptotic genes in multiple myeloma and normal plasma cells: regulation during interleukin-6(IL-6)-induced growth and survival

Aberrant expression of genes regulating apoptosis/survival seems to be essential in the stepwise development of human multiple myeloma (MM). In this paper we have compared the expression of bcl-2 family pro- and anti-apoptotic genes in MM cell lines, primary MM cells and normal plasma cells. The Bcl-2, Mcl-1, Bcl-xL/S, Bcl-w, Bax, Bak, and Bad were shown to be expressed in both malignant and non-neoplastic, normal plasma cells. Quantitative analysis revealed that the malignant phenotype seemed to correlate with an elevated expression of Mcl-1, a decreased expression of Bax and, to a lesser extent, an increased Bcl-2/Bax expression ratio. The possible influence of interleukin-6 (IL-6) in regulating the expression of the bcl-2-related genes was also examined. Using the IL-6-dependent MM cell lines U-1958 and U-266-1970 it was clearly shown that IL-6 deprivation induced cell cycle arrest in both cell lines, whereas apoptosis was only detected in the U-1958 cells. Furthermore, the anti-apoptotic proteins Bcl-2, Mcl-1 and Bcl-xL were down-regulated, while the expression of the pro-apoptotic Bax protein was increased. To conclude, we suggest that the expression pattern of the Bcl-2 family of proteins separates the malignant phenotype of MM from normal plasma cells, and that the protecting effect of IL-6 may be conducted via an altered balance between these proteins.

Insulin restores differentiation of Ras-transformed C2C12 myoblasts by inducing NF-kappaB through an AKT/P70S6K/p38-MAPK pathway

v-H-ras transformed C2C12 (C2Ras) myoblasts, overexpressing p21-Ras protein in the Ras-GTP active form, showed a differentiation-defective phenotype when cultured in low serum as compared with C2C12 myoblasts. Accordingly, the purpose of the present study was to delineate the signaling pathways that restore C2Ras myoblasts differentiation. Inhibition of p42/p44-MAPK with the chemical inhibitor PD98059, and activation of AKT/P70S6K and p38-MAPK with insulin, produced growth arrest (precluding the expression of PCNA, cyclin-D1 and retinoblastoma at the hyperphosphorylated state and inducing the expression of the cell cycle inhibitor p21(Cip)) and myogenesis (multinucleated myotubes formation and induction of creatine kinase, caveolin-3 and alpha-actin). Both events were accompanied by down-regulation of AP-1 and up-regulation of NF-kappaB transcriptional activities. Furthermore, inhibition of NF-kappaB transcriptional activity by the use of the proteasome inhibitor MG132 totally precluded differentiation by insulin+PD98059, demonstrating a direct role for NF-kappaB on C2Ras myogenesis. C2Ras myoblasts failed to restore differentiation when rapamycin or PD169316 were added in the presence of insulin+PD98059, indicating that the activation of both P70S6K and p38-MAPK was necessary to reach a fully differentiated phenotype. Finally, transient transfection of a constitutively active Myr-EGFP-AKT-HA construct (in the presence of PD98059) restored C2Ras myogenesis by its ability to activate P70S6K and p38-MAPK. A crosstalk between P70S6K and p38-MAPK was observed under rapamycin treatment in both insulin or active AKT induced myogenesis. Our results are delineating an AKT/P70S6K/p38-MAPK pathway involved in skeletal muscle differentiation.

Insulin-like growth factor I receptor is downregulated after alveolarization in an apoptotic fibroblast subset

After alveolar formation, >20% of interstitial lung fibroblasts undergo apoptosis, a process that is of critical importance for normal lung maturation. The immature lung contains two morphologically distinct fibroblast populations, lipid-filled interstitial fibroblasts (LIF) and non-LIF (NLIF), which differ with respect to contractile protein content, proliferative capacity, and expression of mRNAs for fibronectin and types I and III collagen, but not tropoelastin. After alveolarization, apoptosis occurs in only one fibroblast population, the LIF. Using flow cytometry to analyze fibroblasts stained with a lipophilic, fluorescent dye, we identified a subset, designated LIF(-), that contained fewer lipid droplets. Unlike LIF that retain lipid, LIF(+), the LIF(-) do not undergo apoptosis after alveolarization. In LIF(+), apoptosis was correlated with downregulation of insulin-like growth factor I receptor (IGF-IR) mRNA and cell surface protein expression. Treatment with anti-IGF-IR decreased total lung fibroblast survival (P = 0.05) as did treatment with the phosphatidylinositol 3-kinase inhibitor LY-294002 and the ras-raf-mitogen-activated protein kinase inhibitor PD-98059 (P < 0.002), which block IGF-I/insulin receptor survival pathways. These observations implicate downregulation of IGF-IR expression in fibroblast apoptosis after alveolar formation.

Glial cell line-derived neurotrophic factor-stimulated phosphatidylinositol 3-kinase and Akt activities exert opposing effects on the ERK pathway: importance for the rescue of neuroectodermic cells

Glial cell line-derived neurotrophic factor (GDNF) plays a crucial role in rescuing neural crest cells from apoptosis during their migration in the foregut. This survival factor binds to the heterodimer GDNF family receptor alpha1/Ret, inducing the Ret tyrosine kinase activity. ret loss-of-function mutations result in Hirschsprung's disease, a frequent developmental defect of the enteric nervous system. Although critical to enteric nervous system development, the intracellular signaling cascades activated by GDNF and their importance in neuroectodermic cell survival still remain elusive. Using the neuroectodermic SK-N-MC cell line, we found that the Ret tyrosine kinase activity is essential for GDNF to induce phosphatidylinositol 3-kinase (PI3K)/Akt and ERK pathways as well as cell rescue. We demonstrate that activation of PI3K is mandatory for GDNF-induced cell survival. In addition, evidence is provided for a critical up-regulation of the ERK pathway by PI3K at the level of Raf-1. Conversely, Akt inhibits the ERK pathway. Thus, both PI3K and Akt act in concert to finely regulate the level of ERK. We found that Akt activation is indispensable for counteracting the apoptotic signal on mitochondria, whereas ERK is partially involved in precluding procaspase-3 cleavage. Altogether, these findings underscore the importance of the Ret/PI3K/Akt pathway in GDNF-induced neuroectodermic cell survival.

Ceramide mediates insulin resistance by tumor necrosis factor-alpha in brown adipocytes by maintaining Akt in an inactive dephosphorylated state

Tumor necrosis factor (TNF)-alpha causes insulin resistance on glucose uptake in fetal brown adipocytes. We explored the hypothesis that some effects of TNF-alpha could be mediated by the generation of ceramide, given that TNF-alpha treatment induced the production of ceramide in these primary cells. A short-chain ceramide analog, C2-ceramide, completely precluded insulin-stimulated glucose uptake and insulin-induced GLUT4 translocation to plasma membrane, as determined by Western blot or immunofluorescent localization of GLUT4. These effects were not produced in the presence of a biologically inactive ceramide analog, C2-dihydroceramide. Analysis of the phosphatidylinositol (PI) 3-kinase signaling pathway indicated that C2-ceramide precluded insulin stimulation of Akt kinase activity, but not of PI-3 kinase or protein kinase C-zeta activity. C2-ceramide completely abolished insulin-stimulated Akt/protein kinase B phosphorylation on regulatory residues Thr 308 and Ser 473, as did TNF-alpha, and inhibited insulin-induced mobility shift in Akt1 and Akt2 separated in PAGE. Moreover, C2-ceramide seemed to activate a protein phosphatase (PP) involved in dephosphorylating Akt because 1) PP2A activity was increased in C2-ceramide- and TNF-alpha-treated cells, 2) treatment with okadaic acid concomitantly with C2-ceramide completely restored Akt phosphorylation by insulin, and 3) transient transfection of a constitutively active form of Akt did not restore Akt activity. Our results indicate that ceramide produced by TNF-alpha induces insulin resistance in brown adipocytes by maintaining Akt in an inactive dephosphorylated state.

Akt mediates insulin induction of glucose uptake and up-regulation of GLUT4 gene expression in brown adipocytes

Insulin acutely stimulated glucose uptake in rat primary brown adipocytes in a PI3-kinase-dependent but p70S6-kinase-independent manner. Since Akt represents an intermediate step between these kinases, this study investigated the contribution of Akt to insulin-induced glucose uptake by the use of a chemical compound, ML-9, as well as by transfection with a dominant-negative form of Akt (DeltaAkt). Pretreatment with ML-9 for 10 min completely inhibited insulin stimulation of (1) Akt kinase activity, (2) Akt phosphorylation on the regulatory residue Ser473 but not on Thr308, and (3) mobility shift in Akt1 and Akt2. However, ML-9 did not affect insulin-stimulated PI3-kinase nor PKCzeta activities. In consequence, ML-9 precluded insulin stimulation of glucose uptake and GLUT4 translocation to plasma membrane (determined by Western blot), without any effect on the basal glucose uptake. Moreover, DeltaAkt impaired insulin stimulation of glucose uptake and GFP-tagged GLUT4 translocation to plasma membrane in transiently transfected immortalised brown adipocytes and HeLa cells, respectively. Furthermore, ML-9 treatment for 6 h down-regulated insulin-induced GLUT4 mRNA accumulation, without affecting GLUT1 expression, in a similar fashion as LY294002. Indeed, co-transfection of brown adipocytes with DeltaAkt precluded the transactivation of GLUT4-CAT promoter by insulin in a similar fashion as a dominant-negative form of PI3-kinase. Our results indicate that activation of Akt may be an essential requirement for insulin regulation of glucose uptake and GLUT4 gene expression in brown adipocytes.

Comparison of anti-apoptotic signalling by the insulin receptor and IGF-I receptor in preadipocytes and adipocytes

We compared the effectiveness of insulin receptor (IR) and type I insulin-like growth factor (IGF) receptor (IGFR) cytoplasmic domains in mediating anti-apoptotic effects in 3T3-L1 preadipocytes and adipocytes. We used TrkC/IR and TrkC/IGFR chimeras, stably expressed in these cells and stimulated with neurotrophin-3 (NT-3), so as to avoid interference from endogenous receptors. After 24-h serum deprivation, 10% of preadipocytes and 2% of adipocytes appeared apoptotic as determined by fluorescence-activated cell sorter (FACS) analysis of cells stained with propidium iodide (PI) and Annexin V. When NT-3 was added, the two chimeras inhibited apoptosis to the same extent by 80% in preadipocytes and 50% in adipocytes. Mutation of juxtamembrane tyrosines (IR Y960F, IGFR Y950F) abrogated these anti-apoptotic effects. Qualitatively similar results were obtained by determination of viable rather than apoptotic cells. We conclude that IR and IGFR have equal potential to inhibit apoptosis in cell backgrounds, which are normally responsive to either IGF-I or insulin.

Insulin signaling leading to proliferation, survival, and membrane ruffling in C2C12 myoblasts

We have recently shown that insulin induced myogenesis in the mouse C2C12 skeletal muscle cell line by activation of phosphatidylinositol (PI) 3-kinase/p70S6-kinase and p38-mitogen-activated protein kinase (MAPK) and downregulation of p42/p44-MAPK. This study investigated the insulin-signaling pathways involved in mitogenesis, survival, and membrane ruffling in C2C12 myoblasts, a cellular system that besides IGF-I receptors, expressed a high number of functional insulin receptors. Insulin (10 nM) rapidly stimulated beta-chain insulin receptor and IRS-1 tyrosine phosphorylation, IRS-2 being poorly and SHC not phosphorylated at all. However, an association of SHC with IRS-1 was found under insulin stimulation. Insulin stimulated IRS-1 association with p85alpha leading to the activation of PI3-kinase, and, subsequently AKT and p70S6-kinases. Moreover, both p42/p44- and p38-MAPKs resulted in phosphorylation after insulin stimulation. Insulin treatment for 24 h produced mitogenesis, as demonstrated by the increase in ((3)H)-thymidine incorporation, DNA content, the expression of PCNA and cyclin D1 proteins, and the proportion of cells in S + G2/M phases of the cell cycle. This mitogenic effect of insulin was precluded by inhibition of p70S6-kinase (either by rapamycin or by the PI3-kinase inhibitor LY294002) as well as by inhibition of p44/p42-MAPK with PD098059, but was not affected by inhibition of p38-MAPK. Serum deprivation of C2C12 myoblasts resulted in growth arrest at the GO/G1 phases of the cell cycle and apoptosis, as detected either by DNA laddering or by increase in the percentage of hypodiploid cells. Insulin rescued serum-deprived cells from apoptosis in an AKT-dependent manner, as demonstrated by the inhibition of AKT-activity by the use of LY294002 and ML-9, meanwhile neither inhibition of p70S6-kinase, nor MAPK affected insulin-induced survival. Finally, we evaluated the capacity of insulin to modulate actin cytoskeleton rearrangement. Insulin stimulation of myoblasts produced membrane ruffling and decreased actin stress fibers; this biological response being dependent of p38-MAPK, as demonstrated by the use of the p38-MAPK inhibitors SB203580 or PD169316, but independent of PI3-kinase and p42/p44-MAPK.

Constitutively activated Stat3 protects fibroblasts from serum withdrawal and UV-induced apoptosis and antagonizes the proapoptotic effects of activated Stat1

Stats1 and 3 (signal transducers and activators of transcription) can be activated simultaneously, although not necessarily to the same degree or duration, by the interaction of cells with the same polypeptide ligand (EGF, PDGF, or high concentrations of IL-6, for example). However, these two Stat proteins can mediate opposing effects on cell growth and survival. Stat1 activation slows growth and promotes apoptosis. In contrast, activated Stat3 can protect cells from apoptosis. Furthermore, a constitutively active form of Stat3, Stat3-C (bridged by S-S linkages between cysteines instead of phosphotyrosines) can induce cellular transformation of fibroblasts. We have determined that fibroblasts transformed by Stat3-C are more resistant to proapoptotic stimuli than nontransformed cells. Also, to examine the potential opposing roles in apoptosis of Stat1 and Stat3, we studied the cervical carcinoma-derived cell line, Me180, which undergoes Stat1-dependent, IFN gamma-induced apoptosis. Me180 cells that express Stat3-C are protected against IFN gamma-mediated apoptosis.

Constitutively activated Stat3 protects fibroblasts from serum withdrawal and UV-induced apoptosis and antagonizes the proapoptotic effects of activated Stat1

Stats1 and 3 (signal transducers and activators of transcription) can be activated simultaneously, although not necessarily to the same degree or duration, by the interaction of cells with the same polypeptide ligand (EGF, PDGF, or high concentrations of IL-6, for example). However, these two Stat proteins can mediate opposing effects on cell growth and survival. Stat1 activation slows growth and promotes apoptosis. In contrast, activated Stat3 can protect cells from apoptosis. Furthermore, a constitutively active form of Stat3, Stat3-C (bridged by S-S linkages between cysteines instead of phosphotyrosines) can induce cellular transformation of fibroblasts. We have determined that fibroblasts transformed by Stat3-C are more resistant to proapoptotic stimuli than nontransformed cells. Also, to examine the potential opposing roles in apoptosis of Stat1 and Stat3, we studied the cervical carcinoma-derived cell line, Me180, which undergoes Stat1-dependent, IFN gamma-induced apoptosis. Me180 cells that express Stat3-C are protected against IFN gamma-mediated apoptosis.

Hepatocyte growth factor, but not insulin-like growth factor I, protects podocytes against cyclosporin A-induced apoptosis

Cyclosporin A (CsA) nephropathy is associated with altered expression of apoptosis regulatory genes such as Fas-ligand and Bcl-2 family members in the glomerular, tubulointerstitial, and vascular compartments. Both hepatocyte growth factor (HGF) and insulin-like growth factor (IGF-I) protect against apoptosis, and HGF specifically up-regulates Bcl-xL, a protein that regulates apoptosis. We investigated whether Bcl-xL and Fas/Fas-ligand were regulated by CsA in cultured podocytes and whether CsA-induced apoptosis was prevented by HGF or IGF-I. A murine podocyte cell line was treated with CsA in the presence or absence of HGF or IGF-I. Apoptosis was quantitated by ELISA and by flow cytometry; Bcl-xL, Fas, and Fas-ligand were measured by Western blotting. Inhibitors of MAP kinase/ERK kinase (MEK)-1 and of phosphatidylinositol 3'-kinase (PI3'-K) were used to determine the signaling pathways involved in Bcl-xL regulation. Apoptosis was induced by CsA in a dose- and time-dependent fashion. CsA also decreased Bcl-xL levels. HGF, but not IGF-I, prevented apoptosis and restored Bcl-xL levels. The regulation of Bcl-xL by HGF was mediated by the PI3'-K but not by the MEK-1 pathway. In summary, we showed that CsA induces apoptosis in podocytes. Apoptosis was prevented by pretreatment with HGF but not IGF-I. Decreased apoptosis appeared to be mediated by regulation of Bcl-xL via the PI3'-K pathway. Our data suggest that the effect of CsA on podocytes may contribute to the glomerular damage and that HGF could provide protection.

Regulation of proliferation-survival decisions is controlled by FGF1 secretion in retinal pigmented epithelial cells

Fibroblast growth factor 1 (FGF1) induces proliferation and differentiation in a wide variety of cells of mesodermal and neuroectodermal origin. FGF1 has no 'classical' signal sequence to direct its secretion, and there has been considerable debate concerning FGF1 secretion and its role in the biological activities of FGF1. We investigated the effects of FGF1 secretion and the signalling induced by signal peptide (SP)-containing FGFI and SP-less FGF1, on the proliferation and the apoptosis in retinal pigmented epithelial (RPE) cells. Primary RPE cell cultures were transfected with FGF1 (FGF1 cells) and SP-FGF1 (SP-FGF1 cells) cDNAs. SP-FGF1 cells secreted large amount of FGF1 and actively proliferated, whereas FGF1 and control cells did not. Secreted FGF1 induced short-term activation of both FGFR1 and ERK2, which were required for cell proliferation. In contrast, SP-FGF1 cells stopped secreting FGF1 and died rapidly, if cultured in the absence of serum. Surprisingly, FGF1 cells, but not control cells, secreted FGF1 and were resistant to apoptosis induced by serum depletion. Secreted FGF1 induced long-term activation of FGFR1 and ERK2, which was necessary to induce a constant and high level of Bcl-x production, and to induce cell survival in FGFI cells. Downregulation of ERK2 and Bcl-x increased apoptosis. Thus, the proliferation and survival activities of FGF1 depend on the secretion of FGF1 which is determined by the cell culture conditions. Cell proliferation was SP-dependent, whereas cell survival was not. The signal peptide controls the level and duration, 'whispering or shouting', of ERK2 activation cells which determines FGF1 biological function and may have important implications for anti-degenerative and anti-proliferative treatments.

TNF down-regulation of receptor tyrosine kinase-dependent mitogenic signal pathways as an important step in cytostasis induction and commitment to apoptosis of Kym-1 rhabdomyosarcoma cells

Growth of Kym-1 rhabdomyosarcoma cells depends on endogenous receptor tyrosine kinase signals activated by insulin and insulin-like growth factors (IGF), as revealed from enhancement of proliferation by insulin and IGF-1 and cytostatic action of inhibitors of IR/IGFR kinases. Depending on the presence or absence of the caspase inhibitor z-VAD-fmk, TNF induced full growth arrest or apoptosis, respectively, indicating dominance of TNF over mitogenic signal pathways in Kym-1 cells. In accordance with a caspase-independent cytostatic action, TNF downregulated IR kinase activity and caused a profound inhibition of downstream mitogenic signals including the MAPK cascade and STAT5, key pathways of proliferation and cell survival. Removal of z-VAD-fmk after 24 h induced rapid cell death in the absence of TNF. The inhibition of survival signals concomitant with persisting proapoptotic signals may tip the balance towards an irreversible commitment of the cell to apoptosis that becomes apparent upon relief of suppression of effector caspases.

The autonomic nervous system, adipose tissue plasticity, and energy balance

In most mammals, two types of adipose tissue, white and brown, are present. Both are able to store energy in the form of triacylglycerols and to hydrolyze them into free fatty acids and glycerol. Whereas white adipose tissue can provide lipids as substrates for other tissues according to the needs of the organism, brown adipose tissue will use fatty acids for heat production. Over the long term, white fat mass reflects the net balance between energy expenditure and energy intake. Even though these two parameters are highly variable during the life of an individual, most adult subjects remain relatively constant in body weight throughout their lives. This observation suggests that appetite, energy expenditure, and basal metabolic rate are linked. An important characteristic of the adipose tissue is its enormous plasticity for volume and cell-number variations and an apparent change in phenotype between the brown and white adipose tissues. The present review focuses on the cellular mechanisms participating in the plasticity of adipose tissues and their regulation by the autonomic nervous system. There is compelling evidence with regard to the importance of the nervous system in the regulation of adipose tissue mass, either brown or white, by acting on the metabolic pathways and on the plasticity (proliferation, differentiation, transdifferentiation, apoptosis) of these tissues. A better comprehension of the different mechanisms involved in the feedback loop linking the brain and these two types of adipose tissue will lead to a better understanding of the pathophysiology of various disorders including obesity, cachexia, anorexia, and type II diabetes mellitus.

Akt mediates insulin rescue from apoptosis in brown adipocytes: effect of ceramide

We have recently shown that insulin can rescue serum deprived adipocytes from apoptosis in a PI 3 kinase and MAP kinase dependent manner. This study investigated the contribution of Akt and p70S6-kinase in insulin rescue from two different apoptotic triggers, serum deprivation and ceramide treatment. Insulin rescued serum-deprived immortalized brown adipocytes from apoptosis through phosphatidylinositol (PI) 3-kinase and Akt pathways, but independently of p70S6-kinase, as demonstrated by the use of inhibitors such as LY294002 or Rapamycin, and transfection experiments with dominant-negative constructs of Akt or p85 subunit of PI 3-kinase. A constitutively active Akt construct mimicked the insulin survival effect, decreasing the percentage of hypodiploid cells, the percentage of apoptopic cells and precluding the formation of apoptotic nuclei. We propose that the insulin survival effect on immortalized brown adipocytes is mediated through activation of Akt. However, insulin and EGF failed to rescue brown adipocytes from ceramide-induced apoptosis, as determined by DNA laddering, hypodiploid cells and apoptotic nuclei. Ceramide treatment blunted Akt activity but not PI 3-kinase activity, and insulin and EGF were unable to activate Akt. Ceramide also caused apoptosis in cells transfected with a constitutively active Akt construct, since phosphorylation of Akt was impaired under these experimental conditions. This study suggests that activation of Akt may be an absolute requirement for the survival of brown adipocytes.

Endogenous and exogenous fibroblast growth factor 2 support survival of chick retinal neurons by control of neuronal neuronal bcl-x(L) and bcl-2 expression through a fibroblast berowth factor receptor 1- and ERK-dependent pathway

Fibroblast growth factor (FGF) 2 is a survival factor for various cell types, including retinal neurons. However, little is understood about the molecular bases of the neuroprotective role of FGF2 in the retina. In this report, FGF2 survival activity was studied in chick retinal neurons subjected to apoptosis by serum deprivation. Exogenous FGF2 supported neuronal survival after serum deprivation and increased neuronal bcl-x(L) and bcl-2 expression, through binding to its receptor R1 (FGF-R1), and subsequent extracellular signal-regulated kinase (ERK) activation. Endogenous FGF2 was transiently overexpressed after serum deprivation. Its down-regulation by antisense oligonucleotides and blockade of its signaling pathway (binding to FGF-R1, tyrosine phosphorylation, and ERK inhibition) decreased bcl-x(L) and bcl-2 levels and and enhanced apoptosis, suggesting that endogenous FGF2 supported neuronal survival through a pathway similar to that of exogenous FGF2. This pathway may serve to up-regulate, or maintain, bcl-x(L) and bcl-2 levels that normally decrease during the onset of apoptosis. Indeed, long-term ERK activation and high bcl-x(L) levels are necessary for the survival activity of both exogenous and endogenous FGF2. Because FGF2 is upregulated following retinal injury in vivo, we suggest that an injury-stimulated autocrine/paracrine FGF2 loop may serve to maintain high levels of survival proteins, such as Bcl-x(L), through ERK activation in retinal neurons.

IGF-1 regulates cardiac fibroblast apoptosis induced by osmotic stress

In this study we have determined the ability of IGF-1 to protect cardiac fibroblasts against osmotic-induced apoptosis and investigated the potential mechanism(s) underlying this protection. Treatment with IGF-1 (1-100 ng/ml) promoted a dose dependent increase in cell survival against osmotic cell death. Both Akt and ERK1/2 were rapidly phosphorylated by IGF-1 and blocked by wortmannin and PD98059, inhibitors of their upstream activators respectively. However, IGF-1-induced protection was mediated via a wortmannin-dependent but PD98059-independent pathway as determined by cell survival assay suggesting a role of PI3-K/Akt. Furthermore, IGF-1 appeared to reduce the activation of a number of early components in the apoptotic pathway in a wortmannin dependent manner including the osmotic stress-induced perturbation in mitochondrial membrane potential, cleavage and activation of caspase-3 and DNA fragmentation. Thus, the results suggest that IGF-1 regulates osmotic stress-induced apoptosis via the activation of the PI3-K/Akt pathway at a point upstream of the mitochondria and caspase-3.

Two independent signaling pathways mediate the antiapoptotic action of macrophage-stimulating protein on epithelial cells

In addition to its effects on macrophage function, macrophage-stimulating protein (MSP) is a growth and motility factor for epithelial cells. The growth and survival of epithelial cells generally require two signals, one generated by interaction with extracellular matrix via integrins, the other initiated by a growth factor. Therefore we investigated the effect of MSP on epithelial cell survival. Survival of epithelial cells cultured overnight in serum-free medium was promoted by adhesion, which activated both the phosphatidylinositol 3'-kinase (PI3-K)/AKT and mitogen-activated protein kinase (MAPK) pathways, operating independently of one another. The number of apoptotic cells resulting from inhibition of either pathway alone was approximately doubled by simultaneous inhibition of both pathways. This shows that each pathway made a partial contribution to the prevention of apoptosis. In the presence of an inhibitor of either pathway, MSP increased the activity of the other pathway so that the single uninhibited pathway alone was sufficient to prevent apoptosis. In contrast to the results with adherent cells, although MSP also prevented apoptosis of cells in suspension (anoikis), its effect was mediated only by the PI3-K/AKT pathway. Despite activation of MAPK by MSP, anoikis was not prevented in suspended cells with a blocked PI3-K/AKT pathway. Thus, activation of MAPK alone is not sufficient to mediate MSP antiapoptotic effects. Cell adhesion generates an additional signal, which is essential for MSP to use MAPK in an antiapoptotic pathway. This may involve translocation of MSP-activated MAPK from the cytoplasm into the nucleus, which occurs only in adherent cells. Our results suggest that there is cross talk between cell matrix adhesion and growth factors in the regulation of cell survival via the MAPK pathway. Growth factors induce MAPK activation, and adhesion mediates MAPK translocation from the cytoplasm into the nucleus.

Both FGF1 and bcl-x synthesis are necessary for the reduction of apoptosis in retinal pigmented epithelial cells by FGF2: role of the extracellular signal-regulated kinase 2

Retinal pigmented epithelial (RPE) cells are of central importance in the maintenance of neural retinal function. Changes in the RPE cells associated with repair activities have been described as metaplasia, while RPE cell apoptosis is responsible for the development of a variety of retinal degenerations. We investigated the regulation of the anti-apoptotic properties of the fibroblast growth factors (FGF) 2 in serum-free cultures of RPE cells. In the absence of serum, confluent stationary RPE cells died by apoptosis via a caspase 3-dependent pathway. The addition of FGF2 greatly reduced apoptosis over a 7-day culture period. We demonstrated the involvement of an autocrine loop involving endogenous FGF1 in the mechanisms that govern FGF2-induced resistance to apoptosis by showing: (1) higher levels of apoptosis in cells treated with antisense FGF1 oligonucleotide or after neutralization of excreted FGF1; (2) the long-term activation of FGFR1 and of ERK2, (3) the inhibition of FGFR1 and ERK2 activation and an increase in apoptosis if excreted FGF1 was neutralized. FGF2 also increased the de novo synthesis and the production of Bcl-xl before the onset of apoptosis. Both inhibition of ERK2 activation, which decreased Bcl-xl synthesis, and downregulation of Bcl-x by antisense oligonucleotide treatment inhibited the survival-promoting activity of FGF2. Thus, FGF2-induced cell survival is a progressive adaptive phenomenon involving ERK2 activation by excreted FGF1 and ERK2-dependent Bcl-x production.

Activated Ha-ras induces apoptosis by association with phosphorylated Bcl-2 in a mitogen-activated protein kinase-independent manner

Serum deprivation of Ha-ras-transformed brown adipocyte cell line resulted in a dramatic apoptotic cell death, as detected either by DNA laddering or by an increase in the percentage of hypodiploid cells or by nuclei condensation and fragmentation, as compared with immortalized cell line or primary fetal brown adipocytes. Moreover, transient transfection of immortalized brown adipocytes with a constitutively active ras gene (Ha-raslys12) mimics the high rate of apoptosis detected in the transformed cell line. On the other hand, transient transfection of the dominant-negative construct of raf-1 rescued serum-deprived Ha-ras-transformed brown adipocytes from apoptosis, decreasing the percentage of hypodiploid cells, the external display of phosphatidylserine, and the DNA laddering. However, inhibition of mitogen-activated protein kinase with PD098059 did not preclude apoptosis and in fact increased the rate of apoptosis observed in serum-deprived Ha-ras-transformed cells, indicating that the Ras/Raf-1 pathway induced apoptosis throughout a mitogen-activated protein kinase kinase 1 (MEK-1)-independent pathway. Furthermore, apoptosis in Ha-ras-transformed brown adipocytes is concurrent with an up-regulation in the expression of the pro-apoptotic protein Bcl-xS, the expression of the anti-apoptotic protein Bcl-2 being down-regulated. Finally, an association of Ras and Raf with phosphorylated Bcl-2 protein was demonstrated in immunoprecipitates from apoptotic cells. Thus, we propose a mechanism of apoptosis in Ha-ras-transformed adipocytes under serum deprivation involving Raf-1 association with phosphorylated Bcl-2, down-regulation of Bcl-2 expression, and up-regulation of Bcl-xS expression.

The IGF axis and programmed cell death

Insulin-like growth factors (IGF) are mitogenic peptides that have been implicated as positive regulators of cellular proliferation. In recent years, several studies have suggested an additional role for the IGF axis in the regulation of apoptosis. Signalling through the IGF receptor has been shown to have a potent survival function and protect cells from a variety of apoptotic stimuli. The actions of IGF are regulated by a family of high-affinity IGF binding proteins (IGFBP), which sequester the IGF from the IGF receptor. However, there is some evidence that one of these binding proteins, IGFBP-3, may have its own pro-apoptotic effects that are independent of its ability to modulate IGF bioavailability. In addition, it has been suggested that the tumour suppressor p53, a crucial mediator of apoptosis in response to cellular stress, may elicit several of its apoptotic effects through manipulation of components of the IGF axis. This review summarizes what is currently known about the role of the IGF system in the regulation of apoptosis, highlighting its implications in the context of tumorigenesis.