Enhancement by adenosine of insulin-induced activation of phosphoinositide 3-kinase and protein kinase B in rat adipocytes

Loss of brain energy metabolism control as a driver for memory impairment upon insulin resistance

The pathophysiological mechanisms intersecting metabolic and neurodegenerative disorders include insulin resistance, which has a strong involvement of environmental factors. Besides central regulation of whole-body homeostasis, insulin in the central nervous system controls molecular signalling that is critical for cognitive performance, namely signalling through pathways that modulate synaptic transmission and plasticity, and metabolism in neurons and astrocytes. This review provides an overview on how insulin signalling in the brain might regulate brain energy metabolism, and further identified molecular mechanisms by which brain insulin resistance might impair synaptic fuelling, and lead to cognitive deterioration.

Adenosine deaminase gene variant in diabetes and obesity

Purpose

Personal medicine is a new notion for individualizing treatment in the future. Studying pathogenic markers including genetic variants would be beneficial in better diagnosis and management of complex diseases such as diabetes and obesity. Adenosine deaminase (ADA) is a purine metabolic enzyme and modulates insulin activity in various tissues through several different mechanisms. Increased ADA activity is associated with decreased glucose uptake. A significant increase in serum deaminase activity has been reported in patients with T2DM and obesity. ADA gene polymorphisms seem to affect ADA enzymatic activity and a polymorphism at the position 4223 in the first intron of ADA gene (ADA 4223 A/C) has been previously associated with obesity. The aim of this study was to explore ADA gene 4223 A/C polymorphism and its association with obesity in patients with Type 2 diabetes.

Methods

Obese patients (N = 133: 64 diabetic +69 non-diabetic) with BMI ≥ 30 and subjects with BMI < 30 (N = 152: 83 diabetics +69 non-diabetic) were recruited into a case-control association study. Blood samples were collected and after DNA extraction, the allele and genotype frequency for ADA gene polymorphism was determined using PCR-RFLP technique.

Results

We observed a significant increase for the frequency of AA+CA genotype in non-obese patients with diabetes compared to obese patients with diabetes (P = 0.04, OR = 2.1, 95%CI; 0.93-4.9).

Conclusion

The higher frequency of AA+CA genotype in none obese diabetes individuals and lower frequency of this genotype in obese diabetes subjects indicates an important role for ADA gene polymorphism in diabetes subjects without obesity.

Adenosine A2A receptor (A2AR) activation triggers Akt signaling and enhances nuclear localization of β-catenin in osteoblasts

Osteoblast differentiation and proliferation are regulated by several modulators, among which are adenosine A_2A receptors (A2ARs) and Wingless/Integrated-β-catenin pathways. Cytosolic β-catenin stabilization promotes its nuclear translocation and transcriptional activity. In the present study, we seek to determine whether there is a connection between A2AR stimulation and cellular β-catenin levels in osteoblasts. Osteoblast precursor cell line (MC3T3-E1) and primary murine osteoblasts were treated with CGS21680, a highly selective A2AR agonist. We analyzed cellular content and nuclear translocation of phosphorylated (p)-serine 552 (S552) β-catenin in response to A2AR stimulation in MC3T3-E1 cells, in both wild-type and A2AR knockout (A2AKO) mice. Moreover, we measured cellular β-catenin levels in MC3T3-E1 cells transfected with scrambled or protein kinase B (Akt) small interfering RNA following A2AR activation. CGS21680 (1 μM) stimulated an increase in both the cellular content and nuclear translocation of p-S552 β-catenin after 15 min of incubation. A2AR activation had no tangible effect on the cellular β-catenin level either in A2AKO mice or in osteoblasts with diminished Akt content. Our findings demonstrate an interaction between A2AR, β-catenin, and Akt signaling in osteoblasts. The existence of such a crosstalk has significant repercussions in the development of novel therapeutic approaches targeting medical conditions associated with reduced bone density.-Borhani, S., Corciulo, C., Larranaga-Vera, A., Cronstein, B. N. Adenosine A_2A receptor (A2AR) activation triggers Akt signaling and enhances nuclear localization of β-catenin in osteoblasts.

Lipopolysaccharide Potentiates Insulin-Driven Hypoglycemic Shock

Critically ill patients typically present with hyperglycemia. Treatment with conventional insulin therapy (targeting 144-180 mg/dl) improves patient survival; however, intensive insulin therapy (IIT) targeting normal blood glucose levels (81-108 mg/dl) increases the incidence of moderate and severe hypoglycemia, and increases mortality. Septic patients are especially prone to IIT-induced hypoglycemia, but the mechanism remains unknown. Here, we show that codelivery of insulin with otherwise sublethal doses of LPS induced hypoglycemic shock in mice within 1-2 h. LPS impaired clearance of insulin, which amplified insulin receptor signaling. These effects were mediated by caspase-11, TLR4, and complement, each of which trigger eicosanoid production that potentiates insulin signaling. Finally, in an animal model of sepsis, we observed that Salmonella typhimurium-infected mice exhibited simultaneous impaired insulin clearance coexisting with insulin resistance. Our results raise the possibility that septic patients have impaired insulin clearance, which could increase their susceptibility to hypoglycemia during IIT, contraindicating its use.

Molecular implications of adenosine in obesity

Adenosine has broad activities in organisms due to the existence of multiple receptors, the differential adenosine concentrations necessary to activate these receptors and the presence of proteins able to synthetize, degrade or transport this nucleoside. All adenosine receptors have been reported to be involved in glucose homeostasis, inflammation, adipogenesis, insulin resistance, and thermogenesis, indicating that adenosine could participate in the process of obesity. Since adenosine seems to be associated with several effects, it is plausible that adenosine participates in the initiation and development of obesity or may function to prevent it. Thus, the purpose of this review was to explore the involvement of adenosine in adipogenesis, insulin resistance and thermogenesis, with the aim of understanding how adenosine could be used to avoid, treat or improve the metabolic state of obesity. Treatment with specific agonists and/or antagonists of adenosine receptors could reverse the obesity state, since adenosine receptors normalizes several mechanisms involved in obesity, such as lipolysis, insulin sensitivity and thermogenesis. Furthermore, obesity is a preventable state, and the specific activation of adenosine receptors could aid in the prevention of obesity. Nevertheless, for the treatment of obesity and its consequences, more studies and therapeutic strategies in addition to adenosine are necessary.

Diagnostic Value of Adenosine Deaminase and Its Isoforms in Type II Diabetes Mellitus

Background and Aims. In the present study, we have investigated the activity of adenosine deaminase (ADA) as a diagnostic marker in type 2 (or II) diabetes mellitus (T2DM). Design and Methods. The deaminase activity of ADA1 and ADA2 was determined in serum from 33 patients with type 2 (or II) diabetes mellitus and 35 healthy controls. We also determined the proportion of glycated hemoglobin (HbA1c). Results. Our results showed significant differences between total serum ADA (tADA) and ADA2 activities in the diabetic groups with HbA1c < 8 (%) and HbA1c ≥ 8 (%) with respect to the values in healthy individuals (p < 0.001). ADA2 activity in patients with high HbA1c was found to be much higher than that in patients with low HbA1c (p = 0.0001). In addition, total ADA activity showed a significant correlation with HbA1c (r = 0.6, p < 0.0001). Conclusions. Total serum ADA activity, specially that due to ADA2, could be useful test for the diagnosis of type 2 (or II) diabetes mellitus.

CL316243 induces phosphatidylinositol 3,4,5-triphosphate production in rat adipocytes in an adenosine deaminase-, pertussis toxin-, or wortmannin-sensitive manner

The effect of beta(3)-adrenoceptor (beta(3)-AR) agonists on adipocytes treated or not treated with signaling modulators has not been sufficiently elucidated. Using rat epididymal adipocytes (adipocytes) labeled with [(32)P]orthophosphate, we found that treatment with the selective beta(3)-AR agonist CL316243 (CL; 1 microM) induces phosphatidylinositol (PI) 3,4,5-triphosphate (PI[3,4,5]P(3)) production and that this response is inhibited by adenosine deaminase (ADA, an adenosine-degrading enzyme; 2 U/ml), pertussis toxin (PTX, an inactivator of inhibitory guanine-nucleotide-binding protein; 1 microg/ml), or wortmannin (WT, a PI-kinase inhibitor; 3 microM). The results showed that CL induced PI(3,4,5)P(3) production in intact adipocytes and that this production was affected by signaling modulators. Taken together, our findings indicate that CL produces PI(3,4,5)P(3) in an ADA-sensitive, PTX-sensitive, or WT-sensitive manner and will advance understanding of the effect of beta(3)-AR agonists on adipocytes.

Deletion of CD73 promotes dyslipidemia and intramyocellular lipid accumulation in muscle of mice

CONTEXT

CD73 converts extracellular AMP to adenosine which is well known to inhibit lipolysis. It is unknown, however, whether adenosine formed directly by CD73 is functionally relevant in this process.

OBJECTIVE

We therefore explored the effect of CD73-derived adenosine on body fat of aged mice.

RESULTS

In lean mice, extracellular adenosine formation by adipocytes is dependent on CD73. High fat diet down-regulates the expression of CD73 in wildtype mice similar to ob/ob mice. Transgenic mice chronically lacking CD73 (CD73(-/-)) gain significantly less body weight and show decreased superficial white fat content as well as increased serum free fatty acids and triglycerides. In addition, intramyocellular lipid levels are significantly increased. This phenotype is accompanied by an increase in blood glucose and serum insulin levels although insulin secretion and the level of insulin degrading enzyme are unaltered. Additionally, insulin-induced Akt phosphorylation is reduced in skeletal muscle of CD73(-/-) mice.

CONCLUSION

CD73-derived adenosine is functionally involved in body fat homeostasis.

Several agents and pathways regulate lipolysis in adipocytes

Adipose tissue is the only tissue capable of hydrolyzing its stores of triacylglycerol (TAG) and of mobilizing fatty acids and glycerol in the bloodstream so that they can be used by other tissues. The full hydrolysis of TAG depends on the activity of three enzymes, adipose triglyceride lipase (ATGL), hormone-sensitive lipase (HSL) and monoacylglycerol lipase, each of which possesses a distinct regulatory mechanism. Although more is known about HSL than about the other two enzymes, it has recently been shown that HLS and ATGL can be activated simultaneously, such that the mechanism that enables HSL to access the surface of lipid droplets also permits the stimulation of ATGL. The classical pathway of lipolysis activation in adipocytes is cAMP-dependent. The production of cAMP is modulated by G-protein-coupled receptors of the Gs/Gi family and cAMP degradation is regulated by phosphodiesterase. However, other pathways that activate TAG hydrolysis are currently under investigation. Lipolysis can also be started by G-protein-coupled receptors of the Gq family, through molecular mechanisms that involve phospholipase C, calmodulin and protein kinase C. There is also evidence that increased lipolytic activity in adipocytes occurs after stimulation of the mitogen-activated protein kinase pathway or after cGMP accumulation and activation of protein kinase G. Several agents contribute to the control of lipolysis in adipocytes by modulating the activity of HSL and ATGL. In this review, we have summarized the signalling pathways activated by several agents involved in the regulation of TAG hydrolysis in adipocytes.

Acadesine inhibits tissue factor induction and thrombus formation by activating the phosphoinositide 3-kinase/Akt signaling pathway

OBJECTIVE

Acadesine, an adenosine-regulating agent and activator of AMP-activated protein kinase, has been shown to possess antiinflammatory activity. This study investigated whether and how acadesine inhibits tissue factor (TF) expression and thrombus formation.

METHODS AND RESULTS

Human umbilical vein endothelial cells and human peripheral blood monocytes were stimulated with lipopolysaccharide to induce TF expression. Pretreatment with acadesine dramatically suppressed the clotting activity and expression of TF (protein and mRNA). These inhibitory effects of acadesine were unchanged for endothelial cells treated with ZM241385 (a specific adenosine A(2A) receptor antagonist) or AMP-activated protein kinase inhibitor compound C, and in macrophages lacking adenosine A(2A) receptor or alpha1-AMP-activated protein kinase. In endothelial cells and macrophages, acadesine activated the phosphoinositide 3-kinase/Akt signaling pathway, reduced the activity of mitogen-activated protein kinases, and consequently suppressed TF expression by inhibiting the activator protein-1 and NF-kappaB pathways. In mice, acadesine suppressed lipopolysaccharide-mediated increases in blood coagulation, decreased TF expression in atherosclerotic lesions, and reduced deep vein thrombus formation.

CONCLUSION

Acadesine inhibits TF expression and thrombus formation by activating the phosphoinositide 3-kinase/Akt pathway. This novel finding implicates acadesine as a potentially useful treatment for many disorders associated with thrombotic pathology, such as angina pain, deep vein thrombosis, and sepsis.

A1 adenosine receptor: role in diabetes and obesity

Adenosine mediates its diverse effects via four subtypes (A(1), A(2A), A(2B) and A(3)) of G-protein-coupled receptors. The A(1) adenosine receptor (A(1)AR) subtype is the most extensively studied and is well characterized in various organ systems. The A(1)ARs are highly expressed in adipose tissue, and endogenous adenosine has been shown to tonically activate adipose tissue A(1)ARs. Activation of the A(1)ARs in adipocytes reduces adenylate cyclase and cAMP content and causes inhibition of lipolysis. The role of A(1)ARs in lipolysis has been well characterized by using several selective A(1)AR agonists as well as A(1)AR knockout mice. However, the contribution of A(1)ARs to the regulation of lipolysis in pathological conditions like insulin resistance, diabetes and dyslipidemia, where free fatty acids (FFA) play an important role, has not been well characterized. Pharmacological agents that reduce the release of FFA from adipose tissue and thus the availability of circulating FFA have the potential to be useful for insulin resistance and hyperlipidemia. Toward this goal, several selective and efficacious agonists of the A(1)ARs are now available, and some have entered early-phase clinical trials; however, none have received regulatory approval yet. Here we review the existing knowledge on the role of A(1)ARs in insulin resistance, diabetes and obesity, and the progress made in the development of A(1)AR agonists as antilipolytic agents, including the challenges associated with this approach.

Adenosine A1 receptors regulate lipolysis and lipogenesis in mouse adipose tissue-interactions with insulin

Adenosine acting at adenosine A1 receptors is considered to be one major regulator of adipose tissue physiology. We have examined the role of adenosine and its interactions with insulin in adipose tissue by using A1R knock out (-/-) mice. Removal of endogenous adenosine with adenosine deaminase caused lipolysis in A1R (+/+), but not A1R (-/-) adipocytes. The adenosine analogue, 2-chloroadenosine, inhibited noradrenaline-stimulated lipolysis and cAMP accumulation in A1R (+/+), but not in A1R (-/-) adipocytes. Insulin reduces lipolysis and cAMP via another mechanism than adenosine and acted additively, but not synergistically, with adenosine. Plasma levels of free fatty acids, glycerol and triglycerides were significantly lower in A1R (+/+) than in A1R (-/-) mice after administration of an adenosine analogue. 2-chloroadenosine induced lipogenesis in presence of insulin in A1R (+/+), but not in A1R (-/-) adipocytes. There were no changes in mRNA levels for several genes involved in fat synthesis in adipose tissue between genotypes. Body weight was similar in young A1R (+/+) and A1R (-/-) mice, but old male A1R (-/-) mice were heavier than wild type controls. In conclusion, adenosine inhibits lipolysis via the adenosine A1 receptor and other adenosine receptors play no significant role. Adenosine and insulin mediate additive but not synergistic antilipolytic effects and 2-chloroadenosine stimulates lipogenesis via adenosine A1 receptors. Thus deletion of adenosine A1 receptors should increase lipolysis and decrease lipogenesis, but in fact an increased fat mass was observed, indicating that other actions of adenosine A1 receptors, possibly outside adipose tissue, are also important.

Activation of the A1adenosine receptor increases insulin-stimulated glucose transport in isolated rat soleus muscle

The A1adenosine receptor (A1AR) has been suggested to participate in insulin- and contraction-stimulated glucose transport in skeletal muscle, but the qualitative and quantitative nature of the effect are controversial. We sought to determine if A1AR is expressed in rat soleus muscle and then characterize its role in glucose transport in this muscle. A1AR mRNA and protein expression were determined by RT-PCR and Western blotting, respectively. To examine the role of adenosine in 3-O-methylglucose transport, isolated muscles were exposed to adenosine deaminase and α,β-methylene adenosine diphosphate to remove endogenous adenosine and were left unstimulated (basal) or stimulated with insulin. To assess the functional participation of A1AR in 3-O-methylglucose transport, muscles were incubated with A1-selective agonist and (or) antagonist in the absence of endogenous adenosine and with or without insulin. A1AR mRNA was expressed in soleus muscle and A1AR was present at the plasma membrane. Removal of endogenous adenosine reduced glucose transport in response to 100 μU/mL insulin (~50%). The A1-selective agonist, N6-cyclopentyladenosine, increased submaximal (100 μU/mL) insulin-stimulated glucose transport in a dose-dependent manner (0.001–1.0 μmol/L). This stimulatory effect was inhibited by the A1-selective receptor antagonist 1,3-dipropyl-8-cyclopentylxanthine in a concentration-dependent manner (0.001–1.0 μmol/L). However, neither activation nor inhibition of A1AR altered basal or maximal (10 mU/mL) insulin-stimulated glucose transport. Our results suggest that adenosine contributes ~50% to insulin-stimulated muscle glucose transport by activating the A1AR. This effect is limited to increasing insulin sensitivity, but not to either basal or maximal insulin-stimulated glucose uptake in rat soleus muscle.

Antilipolytic Activity of a Novel Partial A1Adenosine Receptor Agonist Devoid of Cardiovascular Effects: Comparison with Nicotinic Acid

Elevated lipolysis and circulating free fatty acid (FFA) levels have been linked to the pathogenesis of insulin resistance. A1 adenosine receptor agonists are potent inhibitors of lipolysis. Several A1 agonists have been tested as potential antilipolytic agents; however, their effect on the cardiovascular system remains a potential problem for development of these agents as drugs. In the present study, we report that CVT-3619 [(2-{6-[((1R,2R)-2-hydroxycyclopentyl) amino] purin9-yl} (4S,5 S,2R,3R)5-[(2fluorophenylthio) methyl] oxolane-3,4-diol)], a novel partial A1 receptor agonist, significantly reduces circulating FFA levels without any effect on heart rate and blood pressure in awake rats. Rats were implanted with indwelling arterial and venous cannulas to obtain serial blood samples, record arterial pressure, and administer drug. CVT-3619 decreased FFA levels in a dose-dependent manner at doses from 1 up to 10 mg/kg. The FFA-lowering effect was blocked by the A1 receptor antagonist, 1,3-dipropyl-8-cyclopentylxanthine. Triglyceride (TG) levels were also significantly reduced by CVT-3619 treatment in the absence and presence of Triton. Tachyphylaxis of the antilipolytic effect of CVT-3619 (1 mg/kg i.v. bolus) was not observed with three consecutive treatments. An acute reduction of FFA by CVT-3619 was not followed by a rebound increase of FFA as seen with nicotinic acid. The potency of insulin to decrease lipolysis was increased 4-fold (p < 0.01) in the presence of CVT-3619 (0.5 mg/kg). In summary, CVT-3619 is an orally bioavailable A1 agonist that lowers circulating FFA and TG levels by inhibiting lipolysis. CVT-3619 has antilipolytic effects at doses that do not elicit cardiovascular effects.

Generation of survival signal by differential interaction of p38MAPKalpha and p38MAPKbeta with caveolin-1 and caveolin-3 in the adapted heart

Sphingomyelin breakdown product ceramide has recently been found to induce an adaptive response and reduce myocardial ischemia/reperfusion injury. Since activation of MAP kinases plays an essential role in myocardial adaptation to ischemic stress and since ceramide is involved in lipid raft formation where MAP kinases can be translocated in response to stress, we reasoned that preconditioning may potentiate the translocation of MAP kinases into the lipid raft. To test the hypothesis, rats were divided into five groups: (i) control, (ii) ischemia/reperfusion (I/R), (iii) I/R+C-2 ceramide, (iv) adapted and (v) adapted+desipramine, an inhibitor of ceramide formation. Isolated hearts were preperfused for 15 min with Krebs Henseleit bicarbonate (KHB) buffer in the absence or presence of 10 microM desipramine followed by adaptation induced by four cyclic episodes of 5 min ischemia and 10 min reperfusion. For myocardial adaptation to ischemia with ceramide, the hearts were perfused with 1 microM C-2 ceramide. All hearts were then subjected to 30 min ischemia and 2 h of reperfusion. As expected, both ischemic adaptation and ceramide adaptation made the heart resistant to I/R injury as evidenced by improved ventricular performance and reduced myocardial infarct size and cardiomyocyte apoptosis, which were significantly blocked with desipramine indicating the involvement of ceramide in ischemic adaptation. Ceramide also participated in the formation of lipid raft, and desipramine disrupted the raft formation. In the adapted hearts, there was an increased association of the proapoptotic p38MAPKalpha with caveolin-1 while there was a reduced association of anti-apoptotic p38MAPKbeta with caveolin-3 indicating reduced amount of p38MAPKalpha and increased amount of p38MAPKbeta were available to the adapted hearts thereby generating a survival signal. Desipramine decreased the association of P38MAPKalpha and C-2 ceramide increased the association of P38MAPKalpha with the lipid raft. The survival signal was further confirmed by increased phosphorylation of AKT and enhanced induction of expression of Bcl-2 during adaptation and its reversal with desipramine. The results indicated a unique ceramide signaling the ischemic and PC hearts involving lipid rafts, which generated a survival signal by differentially associating the p38MAPKalpha and p38MAPKbeta with the caveolin-1 and caveoli-3, respectively.

Melatonin stimulates glucose transport via insulin receptor substrate-1/phosphatidylinositol 3-kinase pathway in C2C12 murine skeletal muscle cells

The prevalence of diabetes has exponentially increased in recent decades due to environmental factors such as nocturnal lifestyle and aging, both of which influence the amount of melatonin produced in the pineal gland. The present study investigated the effect of melatonin on signaling pathways of glucose transport in C2C12 mouse skeletal muscle cells. Intriguingly, treatment of C2C12 cells with melatonin (1 nm) stimulated glucose uptake twofold increase. Melatonin-stimulated glucose transport was inhibited with co-treatment with the melatonin receptor antagonist luzindole. Furthermore, treatment of stably over-expressed melatonin receptor type 2B containing C2C12 myotubes with melatonin amplified glucose transport c. 13-fold. Melatonin also increased the phosphorylation level of insulin receptor substrate-1 (IRS-1) and the activity of phosphoinositide 3-kinase (PI-3-kinase). However, 3',5'-cyclic adenosine monophosphate-activated protein kinase (AMPK), another important glucose transport stimulatory mediator via an insulin-independent pathway, was not influenced by melatonin treatment. Activity of p38 mitogen-activated protein kinase (MAPK), a downstream mediator of AMPK, was also not changed by melatonin. In addition, melatonin increased the expression level of forkhead box A2, which was recently discovered to regulate fatty acid oxidation and to be inhibited by insulin. In summary, melatonin stimulates glucose transport to skeletal muscle cells via IRS-1/PI-3-kinase pathway, which implies, at the molecular level, its role in glucose homeostasis and possibly in diabetes. Additionally, exposure to light at night and aging, both of which lower endogenous melatonin levels may contribute to the incidence and/or development of diabetes.

A naphthoquinone derivative, shikonin, has insulin-like actions by inhibiting both phosphatase and tensin homolog deleted on chromosome 10 and tyrosine phosphatases

The 1,4-naphthoquinone derivative, shikonin, has been shown to increase glucose uptake by adipocytes and myocytes with minor effects on protein tyrosine phosphorylation in the cells (Biochem Biophys Res Commun 292:642-651, 2002). The present study was performed to examine the mechanism of this action of shikonin. Shikonin inhibited the phosphatidylinositol 3,4,5-triphosphate (PtdIns-3,4,5-P3) phosphatase activity of recombinant phosphatase and tensin homolog deleted on chromosome 10 (PTEN) with an IC50 value of 2.7 microM. Shikonin induced marked accumulation of PtdIns-3,4,5-P3 and activation of protein kinase B (PKB) in Chinese hamster ovary cells expressing insulin receptors. In addition to its effect on PTEN, shikonin was found to inhibit several protein phosphatases in cell-free systems. Its effect on tyrosine phosphorylation in intact cells was far weaker than that of pervanadate, a widely used tyrosine phosphatase inhibitor, despite the observation that the effect of shikonin on PKB was more potent than that of pervanadate. These results suggested that the inhibition of PTEN provides a clue to its potent insulin-like actions. We also found that naphthoquinones, including 1,2-naphthoquinone, inhibit PTEN in the cell-free system, which suggested that the effect on PTEN (and thus the effect on phosphatidylinositol 3-kinase signaling) should be taken into account when examining the pharmacological actions of naphthoquinone derivatives.

Adenosine downregulates DPPIV on HT-29 colon cancer cells by stimulating protein tyrosine phosphatase(s) and reducing ERK1/2 activity via a novel pathway

The multifunctional cell-surface protein dipeptidyl peptidase IV (DPPIV/CD26) is aberrantly expressed in many cancers and plays a key role in tumorigenesis and metastasis. Its diverse cellular roles include modulation of chemokine activity by cleaving dipeptides from the chemokine NH(2)-terminus, perturbation of extracellular nucleoside metabolism by binding the ecto-enzyme adenosine deaminase, and interaction with the extracellular matrix by binding proteins such as collagen and fibronectin. We have recently shown that DPPIV can be downregulated from the cell surface of HT-29 colorectal carcinoma cells by adenosine, which is a metabolite that becomes concentrated in the extracellular fluid of hypoxic solid tumors. Most of the known responses to adenosine are mediated through four different subtypes of G protein-coupled adenosine receptors: A(1), A(2A), A(2B), and A(3). We report here that adenosine downregulation of DPPIV from the surface of HT-29 cells occurs independently of these classic receptor subtypes, and is mediated by a novel cell-surface mechanism that induces an increase in protein tyrosine phosphatase activity. The increase in protein tyrosine phosphatase activity leads to a decrease in the tyrosine phosphorylation of ERK1/2 MAP kinase that in turn links to the decline in DPPIV mRNA and protein. The downregulation of DPPIV occurs independently of changes in the activities of protein kinases A or C, phosphatidylinositol 3-kinase, other serine/threonine phosphatases, or the p38 or JNK MAP kinases. This novel action of adenosine has implications for our ability to manipulate adenosine-dependent events within the solid tumor microenvironment.

Specific role for p85/p110beta in GTP-binding-protein-mediated activation of Akt

We prepared CHO (Chinese hamster ovary) cells expressing both IR (insulin receptor) and A1R (A1 adenosine receptor). Treatment of the cells with insulin or PIA [N6-(2-phenylisopropyl)adenosine], a specific A(1)R agonist increased Akt activity in the cells in a PI3K- (phosphoinositide 3-kinase) dependent manner. Transfection of p110beta into the cells augmented the action of PIA with little effect on insulin. Introduction of a pH1 vector producing shRNA (short hairpin RNA) that targets p110beta abolished PIA-induced Akt activation. By contrast, an shRNA probe targeting p110alpha did not impair the effects of PIA. The effect of PIA in p110alpha-deficient cells was attenuated effectively by both Deltap85 and betaARK-CT (beta-adrenergic receptor kinase-C-terminal peptide). A Deltap85-derived protein possessing point mutations in its two SH2 domains did not impair PIA action. These results suggest that tyrosine-phosphorylated proteins and Gbetagamma (betagamma subunits of GTP-binding protein) are necessary for the specific function of p110beta in intact cells. The p110beta-middle (middle part of p110beta) may play an important role in signal reception from GPCRs (GTP-binding-protein-coupled receptor), because transfection of the middle part impaired PIA sensitivity.

Caffeine Use in Sports, Pharmacokinetics in Man, and Cellular Mechanisms of Action

Caffeine is the most widely consumed psychoactive 'drug' in the world and probably one of the most commonly used stimulants in sports. This is not surprising, since it is one of the few ergogenic aids with documented efficiency and minimal side effects. Caffeine is rapidly and completely absorbed by the gastrointestinal tract and is readily distributed throughout all tissues of the body. Peak plasma concentrations after normal consumption are usually around 50 microM, and half-lives for elimination range between 2.5-10 h. The parent compound is extensively metabolized in the liver microsomes to more than 25 derivatives, while considerably less than 5% of the ingested dose is excreted unchanged in the urine. There is, however, considerable inter-individual variability in the handling of caffeine by the body, due to both environmental and genetic factors. Evidence from in vitro studies provides a wealth of different cellular actions that could potentially contribute to the observed effects of caffeine in humans in vivo. These include potentiation of muscle contractility via induction of sarcoplasmic reticulum calcium release, inhibition of phosphodiesterase isoenzymes and concomitant cyclic monophosphate accumulation, inhibition of glycogen phosphorylase enzymes in liver and muscle, non-selective adenosine receptor antagonism, stimulation of the cellular membrane sodium/potassium pump, impairment of phosphoinositide metabolism, as well as other, less thoroughly characterized actions. Not all, however, seem to account for the observed effects in vivo, although a variable degree of contribution cannot be readily discounted on the basis of experimental data. The most physiologically relevant mechanism of action is probably the blockade of adenosine receptors, but evidence suggests that, at least under certain conditions, other biochemical mechanisms may also be operational.

ATP stimulates Na+-glucose cotransporter activity via cAMP and p38 MAPK in renal proximal tubule cells

Extracellular ATP plays an important role in the regulation of renal function. However, the effect of ATP on the Na(+)-glucose cotransporters (SGLTs) has not been elucidated in proximal tubule cells (PTCs). Therefore, this study was performed to examine the action of ATP on SGLTs and their related signal pathways in primary cultured rabbit renal PTCs. ATP increased [(14)C]-alpha-methyl-d-glucopyranoside (alpha-MG) uptake in a time-dependent (>1 h) and dose-dependent (>10(-6) M) manner. ATP stimulated alpha-MG uptake by increasing in V(max) without affecting K(m). ATP-induced increase of alpha-MG uptake was correlated with the increase in both SGLT1 and SGLT2 protein expression levels. ATP-induced stimulation of alpha-MG uptake was blocked by suramin (nonspecific P2 receptor antagonist), RB-2 (P2Y receptor antagonist), and MRS-2179 (P2Y(1) receptor antagonist), suggesting a role for the P2Y receptor. ATP-induced stimulation of alpha-MG uptake was blocked by pertussis toxin (PTX, a G(i) protein inhibitor), SQ-22536 (an adenylate cyclase inhibitor), and PKA inhibitor amide 14-22 (PKI). ATP also increased cAMP formation, which was blocked by PTX and RB-2. However, pretreatment of adenosine deaminase did not block ATP-induced cAMP formation. In addition, ATP-induced stimulation of alpha-MG uptake was blocked by SB-203580 (p38 MAPK inhibitor), but not by PD-98059 (p44/42 MAPK inhibitor) or SP-600125 (JNK inhibitor). Indeed, ATP induced phosphorylation of p38 MAPK. In conclusion, ATP increases alpha-MG uptake via cAMP and p38 MAPK in renal PTCs.

Resveratrol-Mediated Activation of cAMP Response Element-Binding Protein through Adenosine A3 Receptor by Akt-Dependent and -Independent Pathways

A recent study documented a role of adenosine A(3)-Akt-cAMP response element-binding protein (CREB) survival signaling in resveratrol preconditioning of the heart. In this study, we demonstrate that resveratrol-mediated CREB activation can also occur through an Akt-independent pathway. Isolated rat hearts were perfused for 15 min with Krebs-Henseleit bicarbonate (KHB) buffer containing resveratrol in the absence or presence of adenosine A(3) receptor blocker MRS-1191 [3-ethyl-5-benzyl-2-methyl-4-phenylethynyl-6-phenyl-1,4-(+/-)-dihydropyridine-3,5-dicar-boxylate], phosphatidylinositol 3 (PI3)-kinase inhibitor LY294002 [2-(4-morpholinyl)-8-phenyl-1(4H)-benzopyran-4-one hydrochloride], mitogen-activated extracellular signal-regulated protein kinase inhibitor PD098059 [2-(2-amino-3-methoxyphenyl)-4H-1-benzopyran-4-one], or a combination of LY294002 and PD098059. All hearts were subsequently subjected to 30-min ischemia followed by 2-h reperfusion. Cardioprotection was examined by determining infarct size, cardiomyocyte apoptosis, and ventricular recovery. Resveratrol phosphorylated both Akt and CREB that was blocked by MRS-1191, which also abolished cardioprotective abilities of resveratrol. LY294002 completely inhibited Akt phosphorylation but partially blocked the phosphorylation of CREB. Inhibition of PI3-kinase also partially blocked resveratrol's ability to precondition the heart. PD098059 partially blocked the phosphorylation of CREB and resveratrol-mediated cardioprotection. Preperfusing the hearts with LY294002 and PD098059 together completely abolished the phosphorylation of CREB, simultaneously inhibiting resveratrol-mediated cardioprotection. The results indicate that resveratrol preconditions the hearts through adenosine A(3) receptor signaling that triggers the phosphorylation of CREB through both Akt-dependent and -independent pathways, leading to cardioprotection.

Pharmacological preconditioning with resveratrol: role of CREB-dependent Bcl-2 signaling via adenosine A3 receptor activation

Recent studies demonstrated that resveratrol, a grape-derived polyphenolic phytoalexin, provides pharmacological preconditioning (PC) of the heart through a NO-dependent mechanism. Because adenosine receptors play a role in PC, we examined whether they play any role in resveratrol PC. Rats were randomly assigned to groups perfused for 15 min with 1) Krebs-Henseleit bicarbonate buffer (KHB) only; 2) KHB containing 10 microM resveratrol; 3) 10 microM resveratrol + 1 microM 8-cyclopentyl-1,3-dimethylxanthine (CPT; adenosine A(1) receptor blocker); 4) 10 microM resveratrol + 1 microM 8-(3-chlorostyryl)caffeine (CSC; adenosine A(2a) receptor blocker); 5) 10 microM resveratrol + 1 microM 3-ethyl-5-benzyl-2-methyl-4-phenylethynyl-6-phenyl-1,4-(+/-)-dihydropyridine-3,5-dicarboxylate (MRS-1191; adenosine A(3) receptor blocker); or 6) 10 microM resveratrol + 3 microM 2-(4-morpholinyl)-8-phenyl-1(4H)-benzopyran-4-one hydrochloride [LY-294002, phosphatidylinositol (PI)3-kinase inhibitor], and groups perfused with adenosine receptor blockers alone. Hearts were then subjected to 30-min ischemia followed by 2-h reperfusion. The results demonstrated significant cardioprotection with resveratrol evidenced by improved ventricular recovery and reduced infarct size and cardiomyocyte apoptosis. CPT and MRS 1191, but not CSC, abrogated the cardioprotective abilities of resveratrol, suggesting a role of adenosine A(1) and A(3) receptors in resveratrol PC. Resveratrol induced expression of Bcl-2 and caused its phosphorylation along with phosphorylation of cAMP response element-binding protein (CREB), Akt, and Bad. CPT blocked phosphorylation of Akt and Bad without affecting CREB, whereas MRS 1191 blocked phosphorylation of all compounds, including CREB. LY-294002 partially blocked the cardioprotective abilities of resveratrol. The results indicate that resveratrol preconditions the heart through activation of adenosine A(1) and A(3) receptors, the former transmitting a survival signal through PI3-kinase-Akt-Bcl-2 signaling pathway and the latter protecting the heart through a CREB-dependent Bcl-2 pathway in addition to an Akt-Bcl-2 pathway.

Activation of insulin receptors by lagerstroemin

Lagerstroemin, an ellagitannin isolated from the leaves of Lagerstroemia speciosa (L.) Pers. (Lythraceae), was examined for its biological activities. In rat adipocytes, the compound increased the rate of glucose uptake and decreased the isoproterenol-induced glycerol release. In Chinese hamster ovary cells expressing human insulin receptors, it increased the Erk activity. These insulin-like actions were accompanied by the increased tyrosine-phosphorylation of the beta-subunit of the insulin receptors. Tryptic digestion of the extracellular sites of the insulin receptors markedly increased the effective concentrations of insulin without changing those of lagerstroemin. Thus lagerstroemin was considered to cause its insulin-like actions by a mechanism different from that employed by insulin.

Inhibition of glucose uptake in murine cardiomyocyte cell line HL-1 by cardioprotective drugs dilazep and dipyridamole

Inhibition of adenosine reuptake by nucleoside transport inhibitors, such as dipyridamole and dilazep, is proposed to increase extracellular levels of adenosine and thereby potentiate adenosine receptor-dependent pathways that promote cardiovascular health. Thus adenosine can act as a paracrine and/or autocrine hormone, which has been shown to regulate glucose uptake in some cell types. However, the role of adenosine in modulating glucose transport in cardiomyocytes is not clear. Therefore, we investigated whether exogenously applied adenosine or inhibition of adenosine transport by S-(4-nitrobenzyl)-6-thioinosine (NBTI), dipyridamole, or dilazep modulated basal and insulin-stimulated glucose uptake in the murine cardiomyocyte cell line HL-1. HL-1 cell lysates were subjected to SDS-PAGE and immunoblotting to determine which GLUT isoforms are present. Glucose uptake was measured in the presence of dipyridamole (3-300 microM), dilazep (1-100 microM), NBTI (10-500 nM), and adenosine (50-250 microM) or the nonmetabolizable adenosine analog 2-chloro-adenosine (250 microM). Our results demonstrated that HL-1 cells possess GLUT1 and GLUT4, the isoforms typically present in cardiomyocytes. We found no evidence for adenosine-dependent regulation of basal or insulin-stimulated glucose transport in HL-1 cardiomyocytes. However, we did observe a dose-dependent inhibition of glucose transport by dipyridamole (basal, IC(50) = 12.2 microM, insulin stimulated, IC(50) = 13.09 microM) and dilazep (basal, IC(50) = 5.7 microM, insulin stimulated, IC(50) = 19 microM) but not NBTI. Thus our data suggest that dipyridamole and dilazep, which are widely used to specifically inhibit nucleoside transport, have a broader spectrum of transport inhibition than previously described. Moreover, these data may explain previous observations, in which dipyridamole was noted to be proischemic at high doses.

Dual Phosphorylation of Phosphoinositide 3-Kinase Adaptor Grb2-Associated Binder 2 Is Responsible for Superoxide Formation Synergistically Stimulated by Fcγ and Formyl-Methionyl-Leucyl-Phenylalanine Receptors in Differentiated THP-1 Cells

The class Ia phosphoinositide (PI) 3-kinase consisting of p110 catalytic and p85 regulatory subunits is activated by Tyr kinase-linked membrane receptors such as FcgammaRII through the association of p85 with the phosphorylated receptors or adaptors. The heterodimeric PI 3-kinase is also activated by G protein-coupled chemotactic fMLP receptors, and activation of the lipid kinase plays an important role in various immune responses, including superoxide formation in neutrophils. Although fMLP-induced superoxide formation is markedly enhanced in FcgammaRII-primed neutrophils, the molecular mechanisms remain poorly characterized. In this study, we identified two Tyr-phosphorylated proteins, c-Cbl (Casitas B-lineage lymphoma) and Grb2-associated binder 2 (Gab2), as PI 3-kinase adaptors that are Tyr phosphorylated upon the stimulation of FcgammaRII in differentiated neutrophil-like THP-1 cells. Interestingly, Gab2 was, but c-Cbl was not, further Ser/Thr phosphorylated by fMLP. Thus, the adaptor Gab2 appeared to be dually phosphorylated at the Ser/Thr and Tyr residues through the two different types of membrane receptors. The Ser/Thr phosphorylation of Gab2 required the activation of extracellular signal-regulated kinase, and fMLP receptor stimulation indeed activated extracellular signal-regulated kinase in the cells. Enhanced superoxide formation in response to Fcgamma and fMLP was markedly attenuated when the Gab2 Ser/Thr phosphorylation was inhibited. These results show the importance of the dual phosphorylation of PI 3-kinase adaptor Gab2 for the enhanced superoxide formation in neutrophil-type cells.

P2Y6 nucleotide receptor activates PKC to protect 1321N1 astrocytoma cells against tumor necrosis factor-induced apoptosis

1. We recently reported that the activation by UDP of rat P2Y6 nucleotide receptors expressed in 1321N1 astrocytoma cells protected them from TNFalpha-induced apoptosis by suppressing activation of caspase 3 and 8. This study aims to characterize the involvement of intracellular signaling pathways, including kinases involved in the antiapoptotic effect of UDP. 2. Cell death was induced in 1321N1 astrocytoma cells permanently expressing the rat P2Y6 receptor by exposure to TNFalpha in the presence of cycloheximide. The apoptotic fraction was analyzed using flow cytometry. 3. The activation of P2Y6 receptors by UDP both protected the astrocytes from TNF-alpha induced apoptosis and activated protein kinase C (PKC) isotypes. The phorbol ester PMA also activated PKC and protected the cells from TNFalpha-induced cell death. The alpha- and epsilon-isotypes of PKC were both activated in a persistent fashion upon 5-min exposure to either UDP (10 microM) or the phorbol ester PMA (100 nM). The PKCzeta isotype was markedly activated upon UDP treatment. 4. The addition of PKC inhibitors, GF109203X or Gö6976, partially antagonized the protective effect of UDP and reduced the UDP-induced phosphorylation of extracellular signal-regulated protein kinases (Erk). The inhibitors of Erk, PD98,059 or U0126, antagonized UDP-induced protection. 5. The antiapoptotic protein, Akt, was not affected by P2Y6 receptor activation. Incubation of the astrocytes with calcium modifiers BAPTA-AM or dantrolene, did not affect the UDP-induced protection from apoptosis. 6. The addition of phospholipase C (PLC) inhibitors, D609 or U73122, partially antagonized both UDP-induced protection and PKC activation.

Calcium receptor-induced serotonin secretion by parafollicular cells: role of phosphatidylinositol 3-kinase-dependent signal transduction pathways

Elevation of extracellular Ca2+ (increase[Ca2+]e) stimulates the Ca2+ receptor (CaR) to induce secretion of 5-hydroxytryptamine (5-HT) from the calcium-sensing parafollicular (PF) cells. The CaR has been reported to couple to Galpha(q) with subsequent activation of protein kinase C-gamma (PKCgamma). We have identified a parallel transduction pathway in primary cultures of sheep PF cells by using a combinatorial approach in which we expressed adenoviral-encoded dominant-negative signaling proteins and performed in vitro kinase assays. The role of the CaR was established by expression of a dominant-negative CaR that eliminated calcium-induced 5-HT secretion but not secretion in response to KCl or phorbol esters. The calcium-induced secretion was inhibited by a dominant-negative p85 regulatory subunit of phosphatidylinositol 3-kinase (PI3-K). PI3-K activity was also assayed using isoform-specific antibodies. The activity of p85/p110beta (PI3-Kbeta) immunocomplexes was elevated by increase[Ca2+]e and activated by Gbetagamma subunits. In addition, secretion of 5-HT was antagonized by the expression of a minigene encoding a peptide scavenger of Gbetagamma subunits (C-terminal fragment peptide of bovine beta-adrenergic receptor kinase). One target of PI3-K activity is phosphoinositide-dependent kinase-1 (PDK1), which in turn activated PKCzeta. Expression of a dominant-negative PKCzeta in PF cells reduced 5-HT secretion. Together, these observations establish that increase[Ca2+]e evokes 5-HT secretion from PF cells by stimulating both Galpha(q)- and Gbetagamma-signaling pathways downstream of the CaR. The betagamma cascade subsequently activates PI3-Kbeta-dependent signaling that is coupled to PDK1 and the downstream effector PKCzeta, and results in an increase in 5-HT release.

Direct effects of caffeine and theophylline on p110 delta and other phosphoinositide 3-kinases. Differential effects on lipid kinase and protein kinase activities

We investigated the effects of methylxanthines on enzymatic activity of phosphoinositide 3-kinases (PI3Ks). We found that caffeine inhibits the in vitro lipid kinase of class I PI3Ks (IC(50) = 75 microm for p110 delta, 400 microm for p110 alpha and p110 beta, and 1 mm for p110 gamma), and theophylline has similar effects (IC(50) = 75 microm for p110 delta, 300 microm for p110 alpha, and 800 microm for p110 beta and p110 gamma) and also inhibits the alpha isoform of class II PI3K (PI3K-C2 alpha) (IC(50) approximately 400 microm). However, four other xanthine derivatives tested (3-isobutyl-1-methylxanthine, 3-propylxanthine, alloxazine, and PD116948 (8-cyclopentyl-1,3-dipropylxanthine)) were an order of magnitude less effective. Surprisingly the triazoloquinazoline CGS15943 (9-chloro-2-(2-furyl)(1,2,d)triazolo(1,5-c)quinazolin-5-amine) also selectively inhibits p110 delta (IC(50) < 10 microm). Caffeine and theophylline also inhibit the intrinsic protein kinase activity of the class IA PI3Ks and DNA-dependent protein kinase, although with a much lower potency than that for the lipid kinase (IC(50) approximately 10 mm for p110 alpha, 3 mm for p110 beta, and 10 mm for DNA-dependent protein kinase). In CHO-IR cells and rat soleus muscle, theophylline and caffeine block the ability of insulin to stimulate protein kinase B with IC(50) values similar to those for inhibition of PI3K activity, whereas insulin stimulation of ERK1 or ERK2 was not inhibited at concentrations up to 10 mm. Theophylline and caffeine also blocked insulin stimulation of glucose transport in CHO-IR cells. These results demonstrate that these methylxanthines are direct inhibitors of PI3K lipid kinase activity but are distinctly less effective against serine kinase activity and thus could be of potential use in dissecting these two distinct kinase activities. Theophylline, caffeine, and CGS15943 may be of particular use in dissecting the specific role of the p110 delta lipid kinase. Finally, we conclude that inhibition of PI3K (p110 delta in particular) is likely explain some of the physiological and pharmacological properties of caffeine and theophylline.

Adenosine-mediated activation of Akt/protein kinase B in the rat hippocampus in vitro and in vivo

Adenosine is considered an endogenous neuroprotective metabolite that through activation of the A(1) receptor results in reduction of neuronal damage following cerebral ischemia. Protein kinase B, also known as Akt/PKB, is part of an endogenous pathway that exerts effective neuroprotection from both necrotic and apoptotic cell death. Using a rat model of unilateral common carotid artery occlusion coupled with hypoxia, and using in vitro rat hippocampal slices, we examined the ability of adenosine to directly activate Akt/PKB. Western blot analysis revealed that levels of phosphorylated Akt/PKB were elevated in vivo under ischemic conditions in an adenosine A(1)-dependent manner and elevated in hippocampal slices treated with an adenosine A(1) agonist. We conclude from these studies that the activation of an adenosine A(1) receptor-mediated signal transduction pathway, either by endogenous adenosine (in vivo) or by an adenosine A(1) agonist (in vitro), results in the activation of the neurotrophic kinase Akt/PKB.

Dose-dependent activation of antiapoptotic and proapoptotic pathways by ethanol treatment in human vascular endothelial cells: differential involvement of adenosine

Moderate but not heavy drinking has been found to have a protective effect against cardiovascular morbidity. We investigated the effects of ethanol (EtOH) treatment on the cell survival-promoting phosphatidylinositol 3-kinase (PI3K)/Akt pathway in cultured human umbilical vein endothelial cells (HUVEC). Exposure of cells to 2-20 mm EtOH resulted in rapid (<10 min) induction of Akt phosphorylation that could be prevented by pertussis toxin or the PI3K inhibitors wortmannin and LY294002. Among the downstream effectors of PI3K/Akt, p70S6 kinase, glycogen synthase kinase 3alpha/beta, and IkappaB-alpha were phosphorylated, the latter resulting in 3-fold activation of NF-kappaB. EtOH also activated p44/42 mitogen-activated protein kinase in a PI3K-dependent manner. Low concentrations of EtOH increased endothelial nitric-oxide synthase activity, which could be blocked by transfection of HUVEC with dominant-negative Akt, implicating the PI3K/Akt pathway in this effect. The adenosine A1 receptor antagonist 1,3-dipopylcyclopentylxanthine prevented the phosphorylation of Akt observed in the presence of EtOH, adenosine, or the A1 agonist N(6)-cyclopentyladenosine. Incubation of HUVEC with 50-100 mm EtOH resulted in mitochondrial permeability transition and caspase-3 activation followed by apoptosis, as documented by DNA fragmentation and TUNEL assays. EtOH-induced apoptosis was unaffected by DPCPX and was potentiated by wortmannin or LY294002. We conclude that treatment with low concentrations of EtOH activates the cell survival promoting PI3K/Akt pathway in endothelial cells by an adenosine receptor-dependent mechanism and activation of the proapoptotic caspase pathway by higher concentrations of EtOH via an adenosine-independent mechanism can mask or counteract such effects.

ATP stimulates glucose transport through activation of P2 purinergic receptors in C(2)C(12) skeletal muscle cells

Extracellular ATP acts as a signal that regulates a variety of cellular processes via binding to P2 purinergic receptors (P2 receptors). We herein investigated the effects and signaling pathways of ATP on glucose uptake in C(2)C(12) skeletal muscle cells. ATP as well as P2 receptor agonists (ATP-gamma S) stimulated the rate of glucose uptake, while P2 receptor antagonists (suramin) inhibited the stimulatory effect of ATP, indicating that P2 receptors are involved. This ATP-stimulated glucose transport was blocked by specific inhibitors of Gi protein (pertusiss toxin), phospholipase C (U73122), protein kinase C (GF109203X), and phosphatidylinositol (PI) 3-kinase (LY294002). ATP stimulated PI 3-kinase activity and P2 receptor antagonists blocked this activation. In C(2)C(12) myotubes expressing glucose transporter GLUT4, ATP increased basal and insulin-stimulated glucose transport. Finally, ATP facilitated translocation of GLUT1 and GLUT4 into plasma membrane. These results together suggest that cells respond to extracellular ATP to increase glucose transport through P2 receptors.

Shikonin stimulates glucose uptake in 3T3-L1 adipocytes via an insulin-independent tyrosine kinase pathway

Type 2 diabetes is due to defects in both insulin action and secretion. In an attempt to discover small molecules that stimulate glucose uptake, similar to insulin, a cell-based glucose uptake screening assay was performed using 3T3-L1 adipocytes. Shikonin, a substance originally isolated from the root of the Chinese plant that has been used as an ointment for wound healing, was thus identified. Shikonin stimulated glucose uptake and potentiated insulin-stimulated glucose uptake in a concentration-dependent manner in 3T3-L1 adipocytes. Stimulation of glucose uptake was also observed in rat primary adipocytes and cardiomyocytes. Like insulin, shikonin-stimulated glucose uptake was inhibited by genistein, a tyrosine kinase inhibitor, and enhanced by vanadate, a tyrosine phosphatase inhibitor. However, in contrast to insulin, shikonin-stimulated glucose uptake was not strongly inhibited by wortmannin, a specific inhibitor of phosphatidylinositol 3-kinase (PI3K). In vitro phosphorylation analyses revealed that shikonin did not induce tyrosine phosphorylation of the insulin receptor, but significantly induced both Thr-308 and Ser-473 phosphorylation of Akt. Our results suggest that in 3T3-L1 adipocytes, shikonin action is not mediated primarily via the insulin receptor/PI3K pathway, but rather via another distinct tyrosine kinase-dependent pathway leading to glucose uptake involving Akt phosphorylation.

Sphingosine 1-phosphate and isoform-specific activation of phosphoinositide 3-kinase beta. Evidence for divergence and convergence of receptor-regulated endothelial nitric-oxide synthase signaling pathways

Sphingosine 1-phosphate (S1P) is a platelet-derived sphingolipid that elicits diverse biological responses, including angiogenesis, via the activation of G protein-coupled EDG receptors. S1P activates the endothelial isoform of nitric-oxide synthase (eNOS), associated with eNOS phosphorylation at Ser-1179, a site phosphorylated by protein kinase Akt. We explored the proximal signaling pathways that mediate Akt activation and eNOS regulation by S1P/EDG receptors. Akt is regulated by the lipid kinase phosphoinositide 3-kinase (PI3-K). We found that bovine aortic endothelial cells (BAEC) express both alpha and beta isoforms of PI3-K, while lacking the gamma isoform. S1P treatment led to the rapid and isoform-specific activation of PI3-Kbeta in BAEC. PI3-Kbeta can be regulated by G protein betagamma subunits (Gbetagamma). The overexpression of a peptide inhibitor of Gbetagamma attenuated S1P-induced eNOS enzyme activation, as well as S1P-induced phosphorylation of eNOS and Akt. In contrast, bradykinin, a classical eNOS agonist, neither activated any PI3-K isoform nor induced eNOS phosphorylation at Ser-1179, despite activating eNOS in BAEC. Vascular endothelial growth factor activated both PI3-Kalpha and PI3-Kbeta via tyrosine kinase pathways and promoted eNOS phosphorylation that was unaffected by Gbetagamma inhibition. These findings indicate that PI3-Kbeta (regulated by Gbetagamma) may represent a novel molecular locus for eNOS activation by EDG receptors in vascular endothelial cells. These studies also indicate that different eNOS agonists activate distinct signaling pathways that diverge proximally following receptor activation but converge distally to activate eNOS.

Further evidence for the involvement of insulin receptor substrates in epidermal growth factor-induced activation of phosphatidylinositol 3-kinase

In accordance with our recent results obtained with cultured rat hepatocytes [Fujioka, T. & Ui, M. (2001) Eur. J. Biochem. 268, 25-34], epidermal growth factor (EGF) gave rise to transient tyrosine phosphorylation of insulin receptor substrates (IRS-1 and IRS-2), thereby activating the bound phosphatidylinositol 3-kinase in human epidermoid carcinoma A431 cells normally abundant in EGF receptors (EGFR) and Chinese hamster ovary (CHO) cells transfected with full-length EGFR. These actions of EGF, although much smaller in magnitude than those of insulin or IGF-I in the same cells, were accompanied by tyrosine phosphorylation of EGFR rather than insulin or IGF-I receptors, never observed in wild-type CHO cells expressing no EGFR, and totally inhibited by an inhibitor of EGFR kinase, AG1478, that was without effect on insulin or IGF-I actions. Recombinant IRS-1 was phosphorylated on tyrosines upon incubation with purified EGFR from A431 cells and 32P-labeled ATP. When CHO cells were transfected with C-terminal truncated EGFR lacking three NPXY motifs responsible for direct binding to phosphotyrosine-binding domains of IRSs, no effect of EGF could be observed. We suggest that tyrosine phosphorylation of IRS-1 or IRS-2 could mediate EGFR-induced activation of phosphatidylinositol 3-kinase in mammalian cells.

Placental alkaline phosphatase, insulin, and adenine nucleotides or adenosine synergistically promote long-term survival of serum-starved mouse embryo and human fetus fibroblasts

Earlier we showed that in serum-starved fibroblasts placental alkaline phosphatase (PALP) can exert growth factor-like effects. Here we report that in mouse embryo (NIH 3T3) and human fetus (HTB-157) fibroblasts, PALP (200 nM) alone provided full protection against serum starvation-induced cell death for 5 days. After 12 days, substantial effects of PALP on cell survival required the copresence of insulin (500 nM) and ATP or adenosine (100 microM). In serum-starved NIH 3T3 cells, PALP induced activating phosphorylation of p42/p44 mitogen-activated protein (MAP) kinases; insulin, but not ATP, had small additional effects. PALP also stimulated the expression of various cyclins; ATP both prolonged and enhanced PALP-induced expression of cyclins A and E. Finally, ATP/adenosine enhanced activation of Akt kinase by insulin. The results suggest that PALP may be a regulator of growth and remodeling of fetal tissues during the second and third trimester of pregnancy when it is expressed.

Activation of protein kinase B by the A(1)-adenosine receptor in DDT(1)MF-2 cells

In this study the effect of insulin and A(1)-adenosine receptor stimulation on protein kinase B (PKB) activation has been investigated in the hamster vas deferens smooth muscle cell line DDT(1)MF-2. Increases in PKB phosphorylation were determined by Western blotting using an antibody that detects PKB phosphorylation at Ser(473). Insulin, a recognized activator of PKB, stimulated a concentration-dependent increase in PKB phosphorylation in DDT(1)MF-2 cells (EC(50) 5+/-1 pM). The selective A(1)-adenosine receptor agonist N(6)-cyclopentyladenosine (CPA) stimulated time and concentration-dependent increases in PKB phosphorylation in DDT(1)MF-2 cells (EC(50) 1.3+/-0.5 nM). CPA-mediated increases in PKB phosphorylation were antagonized by the A(1)-adenosine receptor selective antagonist 1,3-dipropylcyclopentylxanthine (DPCPX) yielding an apparent K(D) value of 2.3 nM. Pre-treatment of DDT(1)MF-2 cells with pertussis toxin (PTX, 100 ng ml(-1) for 16 h), to block G(i)/G(o)-dependent pathways, abolished CPA (1 microM) induced phosphorylation of PKB. In contrast, responses to insulin (100 nM) were resistant to PTX pre-treatment. The phosphatidylinositol 3-kinase (PI-3K) inhibitors wortmannin (IC(50) 10.3+/-0.6 nM) and LY 294002 (IC(50) 10.3+/-1.2 microM) attenuated the phosphorylation of PKB elicited by CPA (1 microM) in a concentration-dependent manner. Wortmannin (30 nM) and LY 294002 (30 microM) also blocked responses to insulin (100 nM). Removal of extracellular Ca(2+) and chelation of intracellular Ca(2+) with BAPTA had no significant effect on CPA-induced PKB phosphorylation. Similarly, pretreatment (30 min) with inhibitors of protein kinase C (Ro 31-8220; 10 microM), tyrosine kinase (genistein; 100 microM), mitogen-activated protein (MAP) kinase kinase (PD 98059; 50 microM) and p38 MAPK (SB 203580; 20 microM) had no significant effect on CPA-induced PKB phosphorylation. In conclusion, these data demonstrate that A(1)-adenosine receptor stimulation in DDT(1)MF-2 cells increases PKB phosphorylation through a PTX and PI-3K-sensitive pathway.

Differential regulation of the phosphatidylinositol 3-kinase/Akt and p70 S6 kinase pathways by the alpha(1A)-adrenergic receptor in rat-1 fibroblasts

Phosphatidylinositol (PI) 3-kinase and its downstream effector Akt are thought to be signaling intermediates that link cell surface receptors to p70 S6 kinase. We examined the effect of a G(q)-coupled receptor on PI 3-kinase/Akt signaling and p70 S6 kinase activation using Rat-1 fibroblasts stably expressing the human alpha(1A)-adrenergic receptor. Treatment of the cells with phenylephrine, a specific alpha(1)-adrenergic receptor agonist, activated p70 S6 kinase but did not activate PI 3-kinase or any of the three known isoforms of Akt. Furthermore, phenylephrine blocked the insulin-like growth factor-I (IGF-I)-induced activation of PI 3-kinase and the phosphorylation and activation of Akt-1. The effect of phenylephrine was not confined to signaling pathways that include insulin receptor substrate-1, as the alpha(1)-adrenergic receptor agonist also inhibited the platelet-derived growth factor-induced activation of PI 3-kinase and Akt-1. Although increasing the intracellular Ca(2+) concentration with the ionophore A23187 inhibited the activation of Akt-1 by IGF-I, Ca(2+) does not appear to play a role in the phenylephrine-mediated inhibition of the PI 3-kinase/Akt pathway. The differential ability of phenylephrine and IGF-I to activate Akt-1 resulted in a differential ability to protect cells from UV-induced apoptosis. These results demonstrate that activation of p70 S6 kinase by the alpha(1A)-adrenergic receptor in Rat-1 fibroblasts occurs in the absence of PI 3-kinase/Akt signaling. Furthermore, this receptor negatively regulates the PI 3-kinase/Akt pathway, resulting in enhanced cell death following apoptotic insult.