Antiapoptotic signaling by the insulin receptor in Chinese hamster ovary cells

The Effects of Lycopene and Insulin on Histological Changes and the Expression Level of Bcl-2 Family Genes in the Hippocampus of Streptozotocin-Induced Diabetic Rats

The aim of this study was to evaluate the effects of antioxidants lycopene and insulin on histological changes and expression of Bcl-2 family genes in the hippocampus of streptozotocin-induced type 1 diabetic rats. Forty-eight Wistar rats were divided into six groups of control (C), control treated with lycopene (CL), diabetic (D), diabetic treated with insulin (DI), diabetic treated with lycopene (DL), and diabetic treated with insulin and lycopene (DIL). Diabetes was induced by an injection of streptozotocin (60 mg/kg, IP), lycopene (4 mg/kg/day) was given to the lycopene treated groups as gavages, and insulin (Sc, 1-2 U/kg/day) was injected to the groups treated with insulin. The number of hippocampus neurons undergoing cell death in group D had significant differences with groups C and DIL (p < 0.001). Furthermore, insulin and lycopene alone or together reduced the expression of Bax, but increased Bcl-2 and Bcl-x_L levels in DI, DL, and DIL rats, especially when compared to group D (p < 0.001). The ratios of Bax/Bcl-2 and Bax/Bcl-xL in DI, DL, and DIL rats were also reduced (p < 0.001). Our results indicate that treatment with insulin and/or lycopene contribute to the prevention of cell death by reducing the expression of proapoptotic genes and increasing the expression of antiapoptotic genes in the hippocampus.

Dissecting PUGNAc-mediated inhibition of the pro-survival action of insulin

Previous studies utilizing PUGNAc, the most widely used β-N-acetylglucosaminidase (OGA) inhibitor to increase global O-N-acetylglucosamine (GlcNAc) levels, have reported a variety of effects including insulin resistance as a direct result of elevated O-GlcNAc levels. The notion of OGA inhibition causing insulin resistance was not replicated in studies in which elevated global O-GlcNAc levels were achieved using two other OGA inhibitors. Related to insulin action, work by others has suggested that O-GlcNAc elevation may inhibit the anti-apoptotic action of insulin. Thus, we examined the pro-survival action of insulin upon serum deprivation in the presence of PUGNAc as well as two selective OGA inhibitors (GlcNAcstatin-g and Thiamet-G), and a selective lysosomal hexosaminidase inhibitor (INJ2). We established that PUGNAc inhibits the pro-survival action of insulin but this effect is not recapitulated by the selective OGA inhibitors suggesting that elevation in O-GlcNAc levels alone is not responsible for PUGNAc's effect on the anti-apoptotic action of insulin. Further, we demonstrate that a selective hexosaminidase A/B (HexA/B) inhibitor does not impact insulin action suggesting that PUGNAc's effect is not due to inhibition of lysosomal hexosaminidase. Finally, we tested a combination of selective OGA and lysosomal hexosaminidase inhibitors but were not able to recapitulate the inhibition of insulin action generated by PUGNAc alone. These results strongly suggest that the defect in insulin action upon PUGNAc treatment does not derive from its inhibition of OGA or HexA/B, and that there is an unknown target of PUGNAc that is the likely culprit in inhibiting the protective effect of insulin from apoptosis.

Insulin catalyzes the curcumin-induced wound healing: an in vitro model for gingival repair

OBJECTIVES

Human gingival fibroblasts (hGFs) play a major role in the maintenance and repair of gingival connective tissue. The mitogen insulin with IGFs etc. synergizes in facilitating wound repair. Although curcumin (CUR) and insulin regulate apoptosis, their impact as a combination on hGF in wound repair remains unknown. Our study consists of: 1) analysis of insulin-mediated mitogenesis on CUR-treated hGF cells, and 2) development of an in vitro model of wound healing.

MATERIALS AND METHODS

Apoptotic rate in CUR-treated hGF cells with and without insulin was observed by AnnexinV/PI staining, nuclear morphological analysis, FACS and DNA fragmentation studies. Using hGF confluent cultures, wounds were mechanically created in vitro and incubated with the ligands for 48 h in 0.2% fetal bovine serum DMEM.

RESULTS

CUR alone showed dose-dependent (1-50 μM) effects on hGF. Insulin (1 μg/ml) supplementation substantially enhanced cell survival through up-regulation of mitogenesis/anti-apoptotic elements.

CONCLUSIONS

The in vitro model for gingival wound healing establishes that insulin significantly enhanced wound filling faster than CUR-treated hGF cells over 48 h. This reinforces the pivotal role of insulin in supporting CUR-mediated wound repair. The findings have significant bearing in metabolic dysfunctions, e.g. diabetes, atherosclerosis, etc., especially under Indian situations.

cAMP antagonizes ERK-dependent antiapoptotic action of insulin

Insulin has antiapoptotic activity in various cell types. However, the signaling pathways underlying the antiapoptotic activity of insulin is not yet known. This study was conducted to determine if cAMP affects the antiapoptotic activity of insulin and the activity of PI3K and ERK in CHO cells expressing human insulin receptors (CHO-IR). Insulin-stimulated ERK activity was completely suppressed by cAMP-elevating agents like as pertussis toxin (Ptx) and cholera toxin (Ctx) after 4 h treatment. Insulin-stimulated PKB/Akt activity was not affected at all. Ptx treatment together with insulin increased the number of apoptotic cells and the degree of DNA fragmentation. Ctx or 8-brcAMP treatment also increased the number of apoptotic cells and stimulated the cleavage of caspase-3 and the hydrolysis of PARP. Taken together, cAMP antagonizes the antiapoptotic activity of insulin and the main target molecule of cAMP in this process is likely ERK, not PI3K-dependent PKB/Akt.

Insulin and growth hormone-releasing peptide-6 (GHRP-6) have differential beneficial effects on cell turnover in the pituitary, hypothalamus and cerebellum of streptozotocin (STZ)-induced diabetic rats

Poorly controlled type1 diabetes is associated with hormonal imbalances and increased cell death in different tissues, including the pituitary, hypothalamus and cerebellum. In the pituitary, lactotrophs are the cell population with the greatest increase in cell death, whereas in the hypothalamus and cerebellum astrocytes are most highly affected. Insulin treatment can delay, but does not prevent, diabetic complications. As ghrelin and growth hormone (GH) secretagogues are reported to prevent apoptosis in different tissues, and to modulate glucose homeostasis, a combined hormonal treatment may be beneficial. Hence, we analyzed the effect of insulin and GH-releasing peptide 6 (GHRP-6) on diabetes-induced apoptosis in the pituitary, hypothalamus and cerebellum of diabetic rats. Adult male Wistar rats were made diabetic by streptozotocin injection (65 mg/kg ip) and divided into four groups from diabetes onset: those receiving a daily sc injection of saline (1 ml/kg/day), GHRP-6 (150 μg/kg/day), insulin (1-8U/day) or insulin plus GHRP-6 for 8 weeks. Control non-diabetic rats received saline (1 ml/kg/day). Diabetes increased cell death in the pituitary, hypothalamus and cerebellum (P<0.05). In the pituitary, insulin treatment prevented diabetes-induced apoptosis (P<0.01), as well as the decline in prolactin and GH mRNA levels (P<0.05). In the hypothalamus, neither insulin nor GHRP-6 decreased diabetes-induced cell death. However, the combined treatment of insulin+GHRP-6 prevented the diabetes induced-decrease in glial fibrillary acidic protein (GFAP) levels (P<0.05). In the cerebellum, although insulin treatment increased GFAP levels (P<0.01), only the combined treatment of insulin+ GHRP-6 decreased diabetes-induced apoptosis (P<0.05). In conclusion, insulin and GHRP-6 exert tissue specific effects in STZ-diabetic rats and act synergistically on some processes. Indeed, insulin treatment does not seem to be effective on preventing some of the diabetes-induced alterations in the central nervous system.

The α-isoform of class II phosphoinositide 3-kinase is necessary for the activation of ERK but not Akt/PKB

Phosphoinositide 3-kinases (PI3Ks) are key enzymes that activate intracellular signaling molecules when a number of different growth factors bind to cell surface receptors. PI3Ks are divided into three classes (I, II, III), and enzymes of each class have different tissue specificities and physiological functions. The α-isoform (PI3K-C2α) of class II PI3Ks is considered ubiquitous and preferentially activated by insulin. Our previous study showed that suppression of PI3K-C2α leads to apoptotic cell death. The aim of this study is to determine whether depletion of PI3K-C2α affects ERK or PKB/Akt activity following stimulation with serum and insulin growth factors in Chinese hamster ovary cells expressing human insulin receptors (CHO-IR) and human HepG2 liver cells. Different antisense oligonucleotides (ODNs), which were designed based on the sequence of the C2 domain of the human PI3K-C2α gene, were transfected into cells to inhibit PI3K-C2α expression. Insulin- or serum-induced stimulation of ERK was significantly suppressed by depletion of PI3K-C2α, whereas phosphorylation of IRS-1 and the stimulation of PKB/Akt by insulin were not affected. The number of apoptotic cells was also increased by depletion of PI3K-C2α protein levels. Taken together, our data indicate that PI3K-C2α may be a crucial factor in the stimulation of ERK activity in response to serum or insulin, whereas it is less important for the stimulation of PKB/Akt activity in response to insulin.

Vascular risk factors and Alzheimer's disease

Vascular cognitive impairment risk factors include stroke, hypertension, diabetes and atherosclerosis. In the elderly, vascular risk factors occur in the presence of high levels of amyloid in the aging brain. Stroke alters the clinical expression of a given load of Alzheimer's disease (AD) pathology. Experimentally, large vessel infarcts or small striatal infarcts are larger in the presence of amyloid. Patients with minor cerebral infarcts and moderate AD lesions will develop the clinical manifestations of dementia. Moreover, there is also an association between other vascular risk factors and the clinical expression of cognitive decline and dementia. The risk of AD is increased in subjects with adult-onset diabetes mellitus, hypertension, atherosclerotic disease and atrial fibrillation. Experimentally, small striatal infarcts in the presence of high levels of amyloid in the brain exhibit a progression in infarct size over time with enhanced degree of cognitive impairment, AD-type pathology and neuroinflammation compared with striatal infarcts or high amyloid levels alone.

The effect of C-peptide on cognitive dysfunction and hippocampal apoptosis in type 1 diabetic rats

Primary diabetic encephalopathy is a recently recognized late complication of diabetes resulting in a progressive decline in cognitive faculties. In the spontaneously type 1 diabetic BB/Wor rat, we recently demonstrated that cognitive impairment was associated with hippocampal apoptotic neuronal loss. Here, we demonstrate that replacement of proinsulin C-peptide in this insulinopenic model significantly prevented spatial learning and memory deficits and hippocampal neuronal loss. C-peptide replacement prevented oxidative stress-, endoplasmic reticulum-, nerve growth factor receptor p75-, and poly(ADP-ribose) polymerase-related apoptotic activities. It partially ameliorated apoptotic stresses mediated via impaired insulin and IGF activities. These findings were associated with the prevention of increased expression of Bax and active caspase 3 and the frequency of caspase 3-positive neurons. The results show that several partially interrelated apoptotic mechanisms are involved in primary encephalopathy and suggest that impaired insulinomimetic action by C-peptide plays a prominent role in cognitive dysfunction and hippocampal apoptosis in type 1 diabetes. Although these abnormalities were not fully prevented by C-peptide replacement, the findings suggest that this regime will substantially prevent cognitive decline in the type 1 diabetic population.

Suppression of the alpha-isoform of class II phosphoinositide 3-kinase gene expression leads to apoptotic cell death

Phosphoinositide 3-kinases (PI3Ks) have known to be key enzymes activating intracellular signaling molecules when a number of growth factors bind to their cell surface receptors. PI3Ks are divided into three classes (I, II, and III) and enzymes of each class have different tissue-specificities and physiological functions. Class II PI3Ks consist of three isoforms (alpha,beta,gamma). Although the alpha-isoform (PI3K-C2alpha) is considered ubiquitous and preferentially activated by insulin rather than the beta-isoform, the physiological significance of PI3K-C2alpha is poorly understood. The present study aimed to determine whether PI3K-C2alpha is associated with the suppression of apoptotic cell death. Different sense- and antisense oligonucleotides (ODNs) were synthesized based on the sequence of C2 domain of PI3K-C2alpha gene. Transfection of CHO-IR cells with two different antisense ODNs clearly reduced the protein content as well as mRNA levels of PI3K-C2alpha whereas neither the nonspecific mock- nor sense ODNs affected. The decrease of PI3K-C2alpha gene expression was paralleled by cellular changes indicating apoptotic cell death such as nuclear condensation, formation of apoptotic bodies, and DNA fragmentation. PI3K-C2alpha mRNA levels were also reduced when cells were incubated in growth factor-deficient medium. Supplementing growth factors (serum or insulin) into medium lead to an increase of PI3K-C2alpha mRNA levels. This finding strongly suggests that PI3K-C2alpha is a crucial survival factor.

The role of impaired insulin/IGF action in primary diabetic encephalopathy

We have previously shown that hippocampal neuronal apoptosis accompanied by impaired cognitive functions occurs in type 1 diabetic BB/Wor rats. To differentiate the contribution by insulin deficiency vs. that by hyperglycemia on neuronal apoptosis, we examined the activities of various apoptotic pathways in hippocampi from type 1 diabetic BB/Wor rats (hyperglycemic and insulinopenic) and type 2 diabetic BBZDR/Wor rats (hyperglycemic and hyperinsulinemic). DNA fragmentation was demonstrated by LM-PCR in type 1 diabetic BB/Wor rats, but was not detectable in duration- and hyperglycemia-matched type 2 BBZDR/Wor rats. Of various apoptotic pathways, Fas activations, 8-OHdG expression, and caspase-12 were demonstrated in type 1 diabetic BB/Wor rats only. In contrast, perturbations of the IGF and NGF systems and PARP activation were demonstrated in type 1 and to a lesser extent in type 2 diabetes. Expressions of Bax and active caspase-3 were significantly increased in type 1, but not in type 2, diabetic rats. These data suggest a lesser apoptogenic stress in type 2 vs. type 1 diabetes. These differences translated into a more profound neuronal loss in the hippocampus of type 1 rats. The results demonstrate that caspase-dependent apoptotic activities dominate in type 1 diabetes, whereas PARP-mediated caspase-independent apoptotic stress is present in both type 1 and type 2 diabetes. The findings suggest that insulin deficiency plays a compounding role to that of hyperglycemia in neuronal apoptosis underpinning primary diabetic encephalopathy.

Insulin reduces apoptosis and increases DNA synthesis and cell size via distinct signalling pathways in Drosophila Kc cells

During development of Drosophila, cell proliferation and size are known to be regulated by insulin. Here we use Drosophila Kc cells to examine the molecular basis for the control of cell growth by insulin. Growing cells in the presence of insulin increased cell number above control levels at 16, 24, 48 and 72 h. We have demonstrated a novel anti-apoptotic effect of insulin (approximately 50%) in these cells, measured by caspase 3-like activity, which contributed to the increase in cell number. The anti-apoptotic effect was observed both in control cells and those in which apoptosis was induced by ultraviolet irradiation. An approximately 2-fold stimulation of bromodeoxyuridine incorporation demonstrated that insulin also increased Kc cell proliferation by stimulating new DNA synthesis. The ability of insulin to increase cell number, stimulate bromodeoxyuridine incorporation and reduce caspase 3-like activity was prevented by PD98059, which inhibits activation of the Drosophila extracellular signal regulated kinase (DERK) pathway, and was unaffected by wortmannin, an inhibitor of Drosophila phosphatidylinositol 3-kinase (DPI3K). Insulin also increased cell size approximately 2-fold and this was prevented by wortmannin and rapamycin, an inhibitor of Drosphilia target of rapamycin (DTOR). In summary, we show that DERK plays an important role in mediating the effect of insulin to reduce apoptosis and increase DNA synthesis whereas the DPI3K/DTOR/Dp70S6 kinase pathway mediates effects of insulin on cell size in Drosophila Kc cells.

Insulin, C-peptide, hyperglycemia, and central nervous system complications in diabetes

Diabetes is an increasingly common disorder which causes and contributes to a variety of central nervous system (CNS) complications which are often associated with cognitive deficits. There appear to be two types of diabetic encephalopathy. Primary diabetic encephalopathy is caused by hyperglycemia and impaired insulin action, which evolves in a diabetes duration-related fashion and is associated with apoptotic neuronal loss and cognitive decline. This appears to be particularly associated with insulin-deficient diabetes. Secondary diabetic encephalopathy appears to arise from hypoxic-ischemic insults due to underlying microvascular disease or as a consequence of hypoglycemia. This type of cerebral diabetic complication is more common in the type 2 diabetic population. Here, we will review the clinical and experimental data supporting this conceptual division of diabetic CNS complications and discuss the underlying metabolic, molecular, and functional aberrations.

C-peptide enhances insulin-mediated cell growth and protection against high glucose-induced apoptosis in SH-SY5Y cells

BACKGROUND

We have previously reported that C-peptide exerts preventive and therapeutic effects on diabetic neuropathy in type 1 diabetic BB/Wor-rats and that it prevents duration-dependent hippocampal apoptosis in the same animal model. In the present study, we examined human neuroblastoma SH-SY5Y cells to examine whether C-peptide stimulates cell proliferation/neurite outgrowth and whether it has antiapoptotic effects.

METHODS

For neurite outgrowth, serum-starved cultures were treated with C-peptide and/or insulin or IGF-1. Neurites were visualized with NF-L antibody and measured morphometrically. Cell numbers were determined using an electronic cell counter. Scrambled C-peptide was used as a negative control. For assessment of apoptosis, SH-SY5Y cells were incubated with 100 mM glucose for 24 h, and the effects of C-peptide and/or insulin or IGF-1 were examined. Apoptosis was demonstrated by transferase-mediated dUTP nick-end labeling (TUNEL)/4,6-diamidino-2-phenylindole (DAPI) stainings, flow cytometry and changes in the expression of Bcl2. Activation of insulin signaling intermediaries was determined by Western blots. Translocation of NF-kappaB was demonstrated immunocytochemically.

RESULTS

C-peptide but not scrambled C-peptide stimulated cell proliferation and neurite outgrowth. In the presence of 4 nM insulin, 3 nM C-peptide significantly increased autophosphorylation of the insulin receptor (IR) but not that of the insulin-like growth factor 1 receptor (IGF-1R). It stimulated phosphoinositide 3-kinase (PI-3 kinase) and p38 mitogen-activated protein (MAP) kinase activation, enhanced the expression and translocation of nuclear factor-kappaB (NF-kappaB), promoted the expression of Bcl2 and reduced c-jun N-terminal kinase (JNK) phosphorylation in excess of that of insulin alone.

CONCLUSIONS

C-peptide in the presence of insulin exerts synergistic effects on cell proliferation, neurite outgrowth and has in the presence of insulin an antiapoptotic effect on high glucose-induced apoptosis but less so on hyperosmolar-induced apoptosis. These effects are likely to be mediated via interactions with the insulin signaling pathway.

A dominant-negative p38 MAPK mutant and novel selective inhibitors of p38 MAPK reduce insulin-stimulated glucose uptake in 3T3-L1 adipocytes without affecting GLUT4 translocation

Participation of p38 mitogen-activated protein kinase (p38) in insulin-induced glucose uptake was suggested using pyridinylimidazole p38 inhibitors (e.g. SB203580). However, the role of p38 in insulin action remains controversial. We further test p38 participation in glucose uptake using a dominant-negative p38 mutant and two novel pharmacological p38 inhibitors related to but different from SB203580. We present the structures and activities of the azaazulene pharmacophores A291077 and A304000. p38 kinase activity was inhibited in vitro by A291077 and A304000 (IC(50) = 0.6 and 4.7 microm). At higher concentrations A291077 but not A304000 inhibited JNK2alpha (IC(50) = 3.5 microm). Pretreatment of 3T3-L1 adipocytes and L6 myotubes expressing GLUT4myc (L6-GLUT4myc myotubes) with A291077, A304000, SB202190, or SB203580 reduced insulin-stimulated glucose uptake by 50-60%, whereas chemical analogues inert toward p38 were ineffective. Expression of an inducible, dominant-negative p38 mutant in 3T3-L1 adipocytes reduced insulin-stimulated glucose uptake. GLUT4 translocation to the cell surface, immunodetected on plasma membrane lawns of 3T3-L1 adipocytes or on intact L6-GLUT4myc myotubes, was not altered by chemical or molecular inhibition of p38. We propose that p38 contributes to enhancing GLUT4 activity, thereby increasing glucose uptake. In addition, the azaazulene class of inhibitors described will be useful to decipher cellular actions of p38 and JNK.

Hippocampal neuronal apoptosis in type 1 diabetes

Duration-related cognitive impairment is an increasingly recognized complication of type 1 diabetes. To explore potential underlying mechanisms, we examined hippocampal abnormalities in the spontaneously type 1 diabetic BB/W rat. As a functional assay of cognition, the Morris water maze test showed significantly prolonged latencies in 8-month diabetic rats not present at 2 months of diabetes. These abnormalities were associated with DNA fragmentation, positive TUNEL staining, elevated Bax/Bcl-x(L) ratio, increased caspase 3 activities and decreased neuronal densities in diabetic hippocampi. These changes were not caused by hypoglycemic episodes or reduced weight in diabetic animals. To explore potential mechanisms responsible for the apoptosis, we examined the expression of the IGF system. Western blotting and in situ hybridization revealed significant reductions in the expression of IGF-I, IGF-II, IGF-IR and IR preceding (2 months) and accompanying (8 months) the functional cognitive impairments and the apoptotic neuronal loss in hippocampus. These data suggest that a duration-related apoptosis-induced neuronal loss occurs in type 1 diabetes associated with cognitive impairment. The data also suggest that this is at least in part related to impaired insulin and/or IGF activities.

Antiapoptotic effects induced by different wavelengths of ultraviolet light

Cells receive signals for survival as well as for death, and the balance between the two ultimately determines the fate of the cells. UV-triggered apoptotic signaling has been well documented, whereas UV-induced survival effects have received little attention. We have reported previously that UVB irradiation prevented apoptosis, which is partly dependent on activation of the phosphatidylinositol 3-kinase (PI3-kinase)-Akt pathway (Ibuki Y. and Goto, R. [2000] Biochem. Biophys. Res. Commun. 279, 872-878). In this study, antiapoptotic effects and survival signals of UV with different wavelength ranges, UVA, UVB and UVC, were examined. NIH3T3 cells showed apoptotic cell death by detachment from the extracellular matrix under serum-free conditions, which was prevented by all wavelengths. However, the effect of UVA was less than those of UVB and UVC, as determined by metabolism of fluoresceine diacetate and the appearance of chromatin-condensed cells. Furthermore, the effects of three wavelengths of UV on the apoptotic pathway upstream of the nuclear signals were examined. Reduction of mitochondrial transmembrane potential (delta psi) and activation of caspase-9 and -3 were suppressed by all three wavelengths of UV, showing wavelength-dependent effects as mentioned previously. Shorter wavelengths showed stronger inhibitory effects on caspase-8 activity. The P13-kinase inhibitor wortmannin partially inhibited the UVB- and UVC-induced suppression of apoptosis but not the inhibitory effect of UVA. Furthermore, normal delta psi maintained by UVA was not changed in the presence of wortmannin, but those by UVB and UVC were reduced. Akt was clearly phosphorylated by all three wavelengths. The phosphorylation by UVB and UVC was completely inhibited by addition of wortmannin, but that by UVA was not, in agreement with the results of survival and of delta psi. These results suggested the existence of two different survival pathways leading to suppression of apoptosis, one for UVA that is independent of the PI3-kinase-Akt pathway and the other for UVB and UVC that is dependent on this pathway.

Selection of highly productive mammalian cells based on an inducible growth advantage using an antibody/receptor chimera

In mammalian cell culture, the selection of high producers is a critical step in efficient recombinant protein production. Drug-resistance selection has been commonly used, but does not always give a pure population of high producers. In this study, we propose a novel selection method in which the growth of high producers is specifically promoted. Two plasmids encoding (i) a hybrid receptor composed of the V(H) portion of anti-hen egg lysozyme antibody HyHEL-10 and an N-terminally truncated erythropoietin receptor (V(H)-EpoR), and (ii) a V(L)-EpoR fusion derived from the same construct as in (i), were employed. The second plasmid contained enhanced green fluorescent protein (EGFP) as a model recombinant protein that was flanked by the internal ribosomal entry sequence. Both plasmids were used simultaneously to transfect an IL-3-dependent murine myeloid cell line, 32D. The transfectants, after antigen selection in the absence of IL-3, showed a clear antigen-induced dose-dependent proliferation. In addition, a high EGFP expression level was observed by flow cytometry in comparison with the cells before antigen selection. The results clearly demonstrate the advantage of our method over conventional drug-resistance selection. We propose the term AMEGA (Antigen MEdiated Genetically-modified cell Amplification) for such an approach.

Wortmannin-sensitive pathway is required for insulin-stimulated phosphorylation of inhibitor kappaBalpha

The aim of this study was to examine the signaling pathways by which insulin promotes activation of nuclear factor kappaB (NFkappaB) through the regulation of inhibitor kappaBalpha (IkappaBalpha). We show here that although insulin increased kappaB-dependent reporter gene expression and augmented nuclear translocation of the p65/RelA subunit of NFkappaB and its DNA binding, it was able to induce a time-dependent accumulation of phosphorylated and ubiquitinated IkappaBalpha without its proteolytic degradation. In contrast, cell stimulation with the cytokine TNFalpha allowed activation of NFkappaB through phosphorylation, ubiquitination, and subsequent degradation of IkappaBalpha. Immunofluorescence studies revealed the presence of a large pool of phosphorylated IkappaBalpha in the nucleus of unstimulated and insulin-treated cells. IkappaB kinase alpha and beta, central players in the phosphorylation of IkappaBalpha, were rapidly induced following exposure to TNFalpha but not insulin. Furthermore, insulin-stimulated IkappaBalpha phosphorylation did not depend on activation of the Ras/ERK cascade. Expression of a dominant-negative mutant of Akt1 or class I PI3K inhibited the insulin stimulation of PI3K/Akt1 signaling without affecting phosphorylation of IkappaBalpha. Interestingly, the PI3K inhibitors wortmannin and LY294002 blocked insulin-stimulated class I PI3K-dependent events at much lower doses than that required to inhibit phosphorylation of IkappaBalpha. These data demonstrate that insulin regulates IkappaBalpha function through a distinct low-affinity wortmannin-sensitive pathway.

Differential effects of phosphatidylinositol 3-kinase inhibition on intracellular signals regulating GLUT4 translocation and glucose transport

Phosphatidylinositol (PI) 3-kinase is required for insulin-stimulated translocation of GLUT4 to the surface of muscle and fat cells. Recent evidence suggests that the full stimulation of glucose uptake by insulin also requires activation of GLUT4, possibly via a p38 mitogen-activated protein kinase (p38 MAPK)-dependent pathway. Here we used L6 myotubes expressing Myc-tagged GLUT4 to examine at what level the signals regulating GLUT4 translocation and activation bifurcate. We compared the sensitivity of each process, as well as of signals leading to GLUT4 translocation (Akt and atypical protein kinase C) to PI 3-kinase inhibition. Wortmannin inhibited insulin-stimulated glucose uptake with an IC(50) of 3 nm. In contrast, GLUT4myc appearance at the cell surface was less sensitive to inhibition (IC(50) = 43 nm). This dissociation between insulin-stimulated glucose uptake and GLUT4myc translocation was not observed with LY294002 (IC(50) = 8 and 10 microm, respectively). The sensitivity of insulin-stimulated activation of PKC zeta/lambda, Akt1, Akt2, and Akt3 to wortmannin (IC(50) = 24, 30, 35, and 60 nm, respectively) correlated closely with inhibition of GLUT4 translocation. In contrast, insulin-dependent p38 MAPK phosphorylation was efficiently reduced in cells pretreated with wortmannin, with an IC(50) of 7 nm. Insulin-dependent p38 alpha and p38 beta MAPK activities were also markedly reduced by wortmannin (IC(50) = 6 and 2 nm, respectively). LY294002 or transient expression of a dominant inhibitory PI 3-kinase construct (Delta p85), however, did not affect p38 MAPK phosphorylation. These results uncover a striking correlation between PI 3-kinase, Akt, PKC zeta/lambda, and GLUT4 translocation on one hand and their segregation from glucose uptake and p38 MAPK activation on the other, based on their wortmannin sensitivity. We propose that a distinct, high affinity target of wortmannin, other than PI 3-kinase, may be necessary for activation of p38 MAPK and GLUT4 in response to insulin.

GLUT4 translocation precedes the stimulation of glucose uptake by insulin in muscle cells: potential activation of GLUT4 via p38 mitogen-activated protein kinase

We previously reported that SB203580, an inhibitor of p38 mitogen-activated protein kinase (p38 MAPK), attenuates insulin-stimulated glucose uptake without altering GLUT4 translocation. These results suggested that insulin might activate GLUT4 via a p38 MAPK-dependent pathway. Here we explore this hypothesis by temporal and kinetic analyses of the stimulation of GLUT4 translocation, glucose uptake and activation of p38 MAPK isoforms by insulin. In L6 myotubes stably expressing GLUT4 with an exofacial Myc epitope, we found that GLUT4 translocation (t(1/2)=2.5 min) preceded the stimulation of 2-deoxyglucose uptake (t(1/2)=6 min). This segregation of glucose uptake from GLUT4 translocation became more apparent when the two parameters were measured at 22 degrees C. Preincubation with the p38 MAPK inhibitors SB202190 and SB203580 reduced insulin-stimulated transport of either 2-deoxyglucose or 3-O-methylglucose by 40-60%. Pretreatment with SB203580 lowered the apparent transport V(max) of insulin-mediated 2-deoxyglucose and 3-O-methylglucose without any significant change in the apparent K(m) for either hexose. The IC(50) values for the partial inhibition of 2-deoxyglucose uptake by SB202190 and SB203580 were 1 and 2 microM respectively, and correlated with the IC(50) for full inhibition of p38 MAPK by the two inhibitors in myotubes (2 and 1.4 microM, respectively). Insulin caused a dose- (EC(50)=15 nM) and time- (t(1/2)=3 min) dependent increase in p38 MAPK phosphorylation, which peaked at 10 min (2.3+/-0.3-fold). In vitro kinase assay of immunoprecipitates from insulin-stimulated myotubes showed activation of p38 alpha (2.6+/-0.3-fold) and p38 beta (2.3+/-0.2-fold) MAPK. These results suggest that activation of GLUT4 follows GLUT4 translocation and that both mechanisms contribute to the full stimulation of glucose uptake by insulin. Furthermore, activation of GLUT4 may occur via an SB203580-sensitive pathway, possibly involving p38 MAPK.

Constitutively active phosphatidylinositol 3-kinase and AKT are sufficient to stimulate the epithelial Na+/H+ exchanger 3

Phosphatidylinositol 3-kinase (PI 3-kinase) is a cytoplasmic signaling molecule that is recruited to activated growth factor receptors and has been shown to be involved in regulation of stimulated exocytosis and endocytosis. One of the downstream signaling molecules activated by PI 3-kinase is the protein kinase Akt. Previous studies have indicated that PI 3-kinase is necessary for basal Na(+)/H(+) exchanger 3 (NHE3) transport and for fibroblast growth factor-stimulated NHE3 activity in PS120 fibroblasts. However, it is not known whether activation of PI 3-kinase is sufficient to stimulate NHE3 activity or whether Akt is involved in this PI 3-kinase effect. We used an adenoviral infection system to test the possibility that activation of PI 3-kinase or Akt alone is sufficient to stimulate NHE3 activity. This hypothesis was investigated in PS120 fibroblasts stably expressing NHE3 after somatic gene transfer using a replication-deficient recombinant adenovirus containing constitutively active catalytic subunit of PI 3-kinase or constitutively active Akt. The adenovirus construct used was engineered with an upstream ecdysone promoter to allow time-regulated expression. Adenoviral infection was nearly 100% at 48 h after infection. Forty-eight hours after infection (24 h after activation of the ecdysone promoter), PI 3-kinase and Akt amount and activity were increased. Increases in both PI 3-kinase activity and Akt activity stimulated NHE3 transport. In addition, a membrane-permeant synthetic 10-mer peptide that binds polyphosphoinositides and increases PI 3-kinase activity similarly enhanced NHE3 transport activity and also increased the percentage of NHE3 on the plasma membrane. The magnitudes of stimulation of NHE3 by constitutively active PI 3-kinase, PI 3-kinase peptide, and constitutively active Akt were similar to each other. These results demonstrate that activation of PI 3-kinase or Akt is sufficient to stimulate NHE3 transport activity in PS120/NHE3 cells.

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.

The common Arg972 polymorphism in insulin receptor substrate-1 causes apoptosis of human pancreatic islets

Molecular scanning of human IRS-1 gene revealed a common polymorphism causing Gly-->Arg972 change. Diabetic and pre-diabetic carriers of Arg972 IRS-1 are characterized by low fasting levels of insulin and C-peptide. To investigate directly whether the Arg 972 IRS-1 affects human islet cells survival, we took advantage of the unique opportunity to analyze pancreatic islets isolated from three donors heterozygous for the Arg972 and six donors carrying wild-type IRS-1. Islets from carriers of Arg972 IRS-1 showed a two-fold increase in the number of apoptotic cells as compared with wild-type. IRS-1-associated PI3-kinase activity was decreased in islets from carriers of Arg972 IRS-1. Same results were reproduced in RIN rat b-cell lines stably expressing wild-type IRS-1 or Arg972 IRS-1. Using these cells, we characterized the downstream pathway by which Arg972 IRS-1 impairs b-cell survival. RIN-Arg972 cells exhibited a marked impairment in the sequential activation of PI3-kinase, Akt, and BAD as compared with RI N-WT. Impaired BAD phosphorylation resulted in increased binding to Bcl-XL instead of 14-3-3 protein, thus sequestering the Bcl-XL antiapoptotic protein to promote survival. Both caspase-9 and caspase-3 activities were increased in RIN-Arg972 cells. The results show that the common Arg972 polymorphism in IRS-1 impairs human b-cell survival and causes resistance to antiapoptotic effects of insulin by affecting the PI3-kinase/Akt survival pathway. These findings establish an important role for the insulin signaling in human b-cell survival and suggest that genetic defects in early steps of insulin signaling may contribute to b-cell failure.

Suppression of apoptosis by UVB irradiation: survival signaling via PI3-kinase/Akt pathway

UVB irradiation induces apoptosis in several cell types. However, we report here that UVB irradiation prevents induction of apoptosis in cells detached from the extracellular matrix under serum-free conditions. NIH3T3 cells cultured in bovine serum albumin-coated dishes (detached from the extracellular matrix) underwent apoptosis under serum-free conditions, which was inhibited by UVB (<0.1 J/cm(2)) irradiation, keeping suspension conditions, as determined by chromatin condensation and the appearance of a subG1 DNA fraction. Furthermore, UVB irradiation decreased caspase-3/7, -8/6, and -9 activation and eliminated loss of mitochondrial inner transmembrane potential, suggesting suppression upstream of the caspase cascade. Treatment with PI3-kinase inhibitors, wortmannin, and LY294002 partly eliminated the UV-mediated inhibition of cell death and recovered the inhibited caspase-3/7 activity. Phosphorylation of Akt was observed from 15 min after UVB irradiation. These results suggested that UVB irradiation transduced a survival signal via PI3 kinase activation and phosphorylation of Akt, and induced some apoptosis inhibition factors upstream of the caspase cascade.

Brain ischemia and reperfusion: molecular mechanisms of neuronal injury

Brain ischemia and reperfusion engage multiple independently-fatal terminal pathways involving loss of membrane integrity in partitioning ions, progressive proteolysis, and inability to check these processes because of loss of general translation competence and reduced survival signal-transduction. Ischemia results in rapid loss of high-energy phosphate compounds and generalized depolarization, which induces release of glutamate and, in selectively vulnerable neurons (SVNs), opening of both voltage-dependent and glutamate-regulated calcium channels. This allows a large increase in cytosolic Ca(2+) associated with activation of mu-calpain, calcineurin, and phospholipases with consequent proteolysis of calpain substrates (including spectrin and eIF4G), activation of NOS and potentially of Bad, and accumulation of free arachidonic acid, which can induce depletion of Ca(2+) from the ER lumen. A kinase that shuts off translation initiation by phosphorylating the alpha-subunit of eukaryotic initiation factor-2 (eIF2alpha) is activated either by adenosine degradation products or depletion of ER lumenal Ca(2+). Early during reperfusion, oxidative metabolism of arachidonate causes a burst of excess oxygen radicals, iron is released from storage proteins by superoxide-mediated reduction, and NO is generated. These events result in peroxynitrite generation, inappropriate protein nitrosylation, and lipid peroxidation, which ultrastructurally appears to principally damage the plasmalemma of SVNs. The initial recovery of ATP supports very rapid eIF2alpha phosphorylation that in SVNs is prolonged and associated with a major reduction in protein synthesis. High catecholamine levels induced by the ischemic episode itself and/or drug administration down-regulate insulin secretion and induce inhibition of growth-factor receptor tyrosine kinase activity, effects associated with down-regulation of survival signal-transduction through the Ras pathway. Caspase activation occurs during the early hours of reperfusion following mitochondrial release of caspase 9 and cytochrome c. The SVNs find themselves with substantial membrane damage, calpain-mediated proteolytic degradation of eIF4G and cytoskeletal proteins, altered translation initiation mechanisms that substantially reduce total protein synthesis and impose major alterations in message selection, down-regulated survival signal-transduction, and caspase activation. This picture argues powerfully that, for therapy of brain ischemia and reperfusion, the concept of single drug intervention (which has characterized the approaches of basic research, the pharmaceutical industry, and clinical trials) cannot be effective. Although rigorous study of multi-drug protocols is very demanding, effective therapy is likely to require (1) peptide growth factors for early activation of survival-signaling pathways and recovery of translation competence, (2) inhibition of lipid peroxidation, (3) inhibition of calpain, and (4) caspase inhibition. Examination of such protocols will require not only characterization of functional and histopathologic outcome, but also study of biochemical markers of the injury processes to establish the role of each drug.

Insulin administered at reoxygenation exerts a cardioprotective effect in myocytes by a possible anti-apoptotic mechanism

The metabolic cocktail of glucose-insulin-potassium (GIK) has been shown to reduce mortality in humans and reduce infarct size in the rat when administered from the onset of reperfusion following an ischemic insult. The mechanisms underlying GIK mediated cardioprotection are, however, still unclear. Recent data implicates insulin "alone" as the major protagonist of cardioprotection when administered at the time of reperfusion. We have therefore begun to investigate an insulin activated signalling pathway and the putative role of apoptosis in this insulin-induced cardioprotection. Simulated ischemia and reoxygenation were induced in rat neonatal cardiocyte experiments. The administration of insulin [0.3 mU/ml] at the moment of reoxygenation (Ins(R)) enhanced myocardial cell viablility as assessed by trypan blue exclusion compared to vehicle alone treated control myocytes (Ins(R)50+/-2%v controls 70+/-1%, P<0.001). This insulin-mediated cardioprotection was due, in part to a reduction in myocyte apoptosis as measured by TUNEL (Ins(R)29+/-2%v controls 49+/-3%, P<0.001) and Annexin V staining (Ins(R)34+/-2%v controls 65+/-3%, P<0.001). These cardioprotective and anti-apoptotic effects of insulin were completely abolished by the tyrosine kinase inhibitor lavendustin A and by the phosphatidylinositol 3-kinase (PI3-kinase) inhibitor wortmannin. Thus, we conclude that the early administration of insulin appears to be an effective modality to reduce reoxgygenation injury in cardiocytes, in part, via the attenuation of ischemia/reoxygenation-induced apoptosis. Moreover, the cardioprotective and anti-apoptotic effects of insulin are mediated via tyrosine kinase and PI3-kinase signalling pathways.

Insulin antiapoptotic signaling involves insulin activation of the nuclear factor kappaB-dependent survival genes encoding tumor necrosis factor receptor-associated factor 2 and manganese-superoxide dismutase

We recently showed that the antiapoptotic function of insulin requires nuclear factor kappaB (NF-kappaB) activation (Bertrand, F., Atfi, A., Cadoret, A., L'Allemain, G., Robin, H., Lascols, O., Capeau, J., and Cherqui, G. (1998) J. Biol. Chem. 273, 2931-2938). Here we sought to identify the NF-kappaB-dependent survival genes that are activated by insulin to mediate this function. Insulin increased the expression of tumor necrosis factor receptor-associated factor 2 (TRAF2) mRNA and protein in Chinese hamster ovary cells overexpressing insulin receptors (IRs). This effect required (i) IR activation since it was abrogated by IR mutation at tyrosines 1162 and 1163 and (ii) NF-kappaB activation since it was abolished by overexpression of dominant-negative IkappaB-alpha(A32/36) and mimicked by overexpression of the NF-kappaB c-Rel subunit. TRAF2 contributed to insulin protection against serum withdrawal-induced apoptosis since TRAF2 overexpression mimicked insulin protection, whereas overexpression of dominant-negative TRAF2-(87-501) reduced this process. Along with its protective effect, overexpressed TRAF2 increased basal and insulin-stimulated NF-kappaB activities. All effects were inhibited by IkappaB-alpha(A32/36), suggesting that an amplification loop involving TRAF2 activation of NF-kappaB is implicated in insulin antiapoptotic signaling. We also show that insulin increased manganese-superoxide dismutase (Mn-SOD) mRNA expression through NF-kappaB activation and that Mn-SOD contributed to insulin antiapoptotic signaling since expression of antisense Mn-SOD RNA decreased this process. This study provides the first evidence that insulin activates the NF-kappaB-dependent survival genes encoding TRAF2 and Mn-SOD and thereby clarifies the role of NF-kappaB in the antiapoptotic function of insulin.

Induction of apoptosis in luteinized granulosa cells by the MAP kinase kinase (MEK) inhibitor PD98059

Our objective is to test the hypothesis that inhibition of mitogen-activated protein (MAP) kinase kinase (MEK) with PD98059 in human luteinized granulosa cells will block epidermal growth (EGF)-stimulated MAP kinase activity and induce apoptosis. Luteinized granulosa cells from human in vitro fertilization aspirates were cultured and treated with the following: (1) vehicle; (2) PD98059; (3) EGF; (4) PD98059 + EGF. Treatment with PD98059 suppressed MAP kinase activity, inhibited MAP kinase phosphorylation by Western blot analysis, blocked nuclear translocation of phosphorylated MAP kinase by confocal microscopy, and increased percentages of subdiploid apoptotic nuclei by flow cytometry. Our data are the first evidence that a relationship may exist between the MAP kinase pathway and control of apoptosis in human luteinized granulosa cells. These results support the hypothesis that suppression of the MAP kinase pathway may lead to apoptosis in these cells.

An inhibitor of p38 mitogen-activated protein kinase prevents insulin-stimulated glucose transport but not glucose transporter translocation in 3T3-L1 adipocytes and L6 myotubes

The precise mechanisms underlying insulin-stimulated glucose transport still require investigation. Here we assessed the effect of SB203580, an inhibitor of the p38 MAP kinase family, on insulin-stimulated glucose transport in 3T3-L1 adipocytes and L6 myotubes. We found that SB203580, but not its inactive analogue (SB202474), prevented insulin-stimulated glucose transport in both cell types with an IC50 similar to that for inhibition of p38 MAP kinase (0.6 microM). Basal glucose uptake was not affected. Moreover, SB203580 added only during the transport assay did not inhibit basal or insulin-stimulated transport. SB203580 did not inhibit insulin-stimulated translocation of the glucose transporters GLUT1 or GLUT4 in 3T3-L1 adipocytes as assessed by immunoblotting of subcellular fractions or by immunofluorescence of membrane lawns. L6 muscle cells expressing GLUT4 tagged on an extracellular domain with a Myc epitope (GLUT4myc) were used to assess the functional insertion of GLUT4 into the plasma membrane. SB203580 did not affect the insulin-induced gain in GLUT4myc exposure at the cell surface but largely reduced the stimulation of glucose uptake. SB203580 had no effect on insulin-dependent insulin receptor substrate-1 phosphorylation, association of the p85 subunit of phosphatidylinositol 3-kinase with insulin receptor substrate-1, nor on phosphatidylinositol 3-kinase, Akt1, Akt2, or Akt3 activities in 3T3-L1 adipocytes. In conclusion, in the presence of SB203580, insulin caused normal translocation and cell surface membrane insertion of glucose transporters without stimulating glucose transport. We propose that insulin stimulates two independent signals contributing to stimulation of glucose transport: phosphatidylinositol 3-kinase leads to glucose transporter translocation and a pathway involving p38 MAP kinase leads to activation of the recruited glucose transporter at the membrane.