Chronic resistance training activates autophagy and reduces apoptosis of muscle cells by modulating IGF-1 and its receptors,Akt/mTOR and Akt/FOXO3a signaling in aged rats

Resistance exercise training (RET) remains the most effective treatment for the loss of muscle mass and strength in elderly people. However, the underlying cellular and molecular mechanisms are not well understood. Recent evidence suggests that autophagic signaling is altered in aged skeletal muscles. This study aimed to investigate if RET affects IGF-1 and its receptors, the Akt/mTOR, and Akt/FOXO3a signaling pathways and regulates autophagy and apoptosis in the gastrocnemius muscles of 18–20 month old rats. The results showed that 9 weeks of RET prevented the loss of muscle mass and improved muscle strength, accompanied by reduced LC3-II/LC3-I ratio, reduced p62 protein levels, and increased levels of autophagy regulatory proteins, including Beclin 1, Atg5/12, Atg7, and the lysosomal enzyme cathepsin L. RET also reduced cytochrome c level in the cytosol but increased its level in mitochondrial fraction, and inhibited cleaved caspase 3 production and apoptosis. Furthermore, RET upregulated the expression of IGF-1 and its receptors but downregulated the phosphorylation of Akt and mTOR. In addition, RET upregulated the expression of total AMPK, phosphorylated AMPK, and FOXO3a. Taken together, these results suggest that the benefits of RET are associated with increased autophagy activity and reduced apoptosis of muscle cells by modulating IGF-1 and its receptors, the Akt/mTOR and Akt/FOXO3a signaling pathways in aged skeletal muscles.     

Exercise training eliminates age-related differences in skeletal muscle insulin receptor and IRS-1 abundance in rats

Insulin resistance is common in old age, and exercise training can improve insulin sensitivity. The purpose of this study was to determine the influence of age (6 vs 26 months) and exercise training (10 weeks of treadmill running) on insulin signaling protein abundance in skeletal muscle from male Fisher 344 rats. Muscle levels of insulin receptor (IR), insulin receptor substrate-1 (IRS-1), phosphatidylinositol 3-kinase (PI3K), and Akt1, a serine-threonine kinase, were determined. IRS-1 was reduced with aging, IR and PI3K abundance was greater in old rats, and Akt1 was unchanged. IRS-1 was increased by training in old but not young rats, and IR was increased by training in young but not old rats. PI3K tended to increase and Akt1 did not change with training, regardless of age. Aging does not uniformly affect insulin signaling protein abundance, and exercise differentially alters IR and IRS-1 in young and old rats, thereby eliminating age-related differences in these proteins.     

Cardioprotective stimuli mediate phosphoinositide 3-kinase and phosphoinositide dependent kinase 1 nuclear accumulation in cardiomyocytes

The phosphoinositide 3-kinase (PI3K)/phosphoinositide dependent kinase 1 (PDK1) signaling pathway exerts cardioprotective effects in the myocardium through activation of key proteins including Akt. Activated Akt accumulates in nuclei of cardiomyocytes suggesting that biologically relevant targets are located in that subcellular compartment. Nuclear Akt activity could be potentiated in both intensity and duration by the presence of a nuclear-associated PI3K/PDK1 signaling cascade as has been described in other non-myocyte cell types. PI3K/PDK1 distribution was determined in vitro and in vivo by immunostaining and nuclear extraction of cultured rat neonatal cardiomyocytes or transgenic mouse hearts. Results show that PI3K and PDK1 are present at a basal level in cardiomyocytes nuclei and that cardioprotective stimulation with atrial natriuretic peptide (ANP) increases their nuclear localization. In comparison, overexpression of nuclear-targeted Akt does not mediate increased translocation of either PI3K or PDK1 indicating that accumulation of Akt does not drive PI3K or PDK1 into the nuclear compartment. Furthermore, PI3K and phospho-Akt⁴⁷³ show parallel temporal accumulation in the nucleus following (MI) infarction challenge. These findings demonstrate the presence of a dynamically regulated nuclear-associated signaling cascade involving PI3K and PDK that presumably influences nuclear Akt activation.     

Glimepiride induces proliferation and differentiation of rat osteoblasts via the PI3-kinase/Akt pathway

Glimepiride is a third-generation sulfonylurea agent and is widely used in the treatment of type 2 diabetes mellitus. In addition to the stimulatory effects on pancreatic insulin secretion, glimepiride has also been reported to have extrapancreatic functions including activation of PI3 kinase (PI3K) and Akt in rat adipocytes and skeletal muscle. PI3-kinase and Akt are important signaling molecules in the regulation of proliferation and differentiation in various cells. This study investigated the actions of glimepiride in rat osteoblasts and the role of PI3K/Akt pathway. Cell proliferation was determined by measuring absorbance at 550 nm. Supernatant assay was used for measuring alkaline phosphatase activity. Western blot analysis was used for determining collagen I, insulin receptor substrate-1/2, PI3K/Akt, and endothelial nitric oxide synthase expression. We found that glimepiride significantly enhanced proliferation and differentiation of osteoblasts and led to activation of several key signaling molecules including insulin receptor substrate-1/2, PI3K/Akt, and endothelial nitric oxide synthase. Furthermore, a specific inhibitor of PI3K abolished the stimulatory effects of glimepiride on proliferation and differentiation. Taken together, these observations provide concrete evidence that glimepiride activates the PI3K/Akt pathway; and this activation is likely required for glimepiride to stimulate proliferation and differentiation of rat osteoblasts.     

Exercise training promotes SIRT1 activity in aged rats

The objective of this study was to determine the effects of aging and exercise training on SIRT1 activity and to identify a pathway linking SIRT1 to antioxidant response and cell cycle regulation in rats. SIRT1 is a NAD(+)-dependent deacetylase involved in the oxidative stress response and aging. The effects of aging and of moderate and prolonged exercise training in rats are unknown. We measured SIRT1 activity in heart and adipose tissue of young (6 months old), sedentary old (24 months), and trained old (24 months) rats using an assay kit. Peroxidative damage was determined by measuring levels of thiobarbituric reactive substances (TBARS) and the protein-aldehyde adduct 4-hydroxynonenal (4-HNE). MnSOD, catalase, and FOXO3a levels were evaluated by Western blot, and GADD45a, cyclin D(2), and FOXO3a mRNA by RT-PCR. Aging significantly reduced SIRT1 activity in heart, but not in adipose tissue, increased TBARS and 4-HNE and decreased Mn-SOD and catalase expression in both heart and adipose tissue. Aging did not affect FOXO3a protein expression in the heart or FOXO3a mRNA in adipose tissue. Exercise training significantly increased FOXO3a protein in the heart and FOXO3a mRNA in adipose tissue of aged rats. It also significantly increased Mn-SOD and catalase levels in both heart and adipose tissue. The exercise-induced increase in SIRT1 activity in the heart caused a decrease in cyclin D(2) and an increase in GADD45a mRNA expression. There was a similar decrease in cyclin D(2), and no changes in GADD45a mRNA expression in adipose tissue. We concluded that exercise training, which significantly increases SIRT1 activity, could counteract age-related systems impairment.     

The role of PTEN/Akt/PI3K signaling in the maintenance and viability of prostate cancer stem-like cell populations

Characterization of the molecular pathways that are required for the viability and maintenance of self-renewing tumor-initiating cells may ultimately lead to improved therapies for cancer. In this study, we show that a CD133⁺/CD44⁺ population of cells enriched in prostate cancer progenitors (PCaPs) has tumor-initiating potential and that these progenitors can be expanded under nonadherent, serum-free, sphere-forming conditions. Cells grown under these conditions have increased in vitro clonogenic and in vivo tumorigenic potential. mRNA expression analysis of cells grown under sphere-forming conditions, compared with long-term monolayer cultures, revealed preferential activation of the PI3K/AKT signaling pathway. PI3K p110α and β-protein levels were higher in cells grown under sphere-forming conditions, and phosphatase and tensin homolog (PTEN) knockdown by shRNA led to an increase in sphere formation as well as increased clonogenic and tumorigenic potential. Similarly, shRNA knockdown of FoxO3a led to an increase in tumorigenic potential. Consistent with these results, inhibition of PI3K activity by the dual PI3K/mTOR inhibitor NVP-BEZ235 led to growth inhibition of PCaPs. Taken together, our data strongly suggest that the PTEN/PI3K/Akt pathways are critical for prostate cancer stem-like cell maintenance and that targeting PI3K signaling may be beneficial in prostate cancer treatment by eliminating prostate cancer stem-like cells.     

Role of phosphoinositide 3-kinase IA (PI3K-IA) activation in cardioprotection induced by ouabain preconditioning

Acute myocardial infarction, the clinical manifestation of ischemia–reperfusion (IR) injury, is a leading cause of death worldwide. Like ischemic preconditioning (IPC) induced by brief episodes of ischemia and reperfusion, ouabain preconditioning (OPC) mediated by Na/K-ATPase signaling protects the heart against IR injury. Class I PI3K activation is required for IPC, but its role in OPC has not been investigated. While PI3K-IB is critical to IPC, studies have suggested that ouabain signaling is PI3K-IA-specific. Hence, a pharmacological approach was used to test the hypothesis that OPC and IPC rely on distinct PI3K-I isoforms. In Langendorff-perfused mouse hearts, OPC was initiated by 4 min of ouabain 10 μM and IPC was triggered by 4 cycles of 5 min ischemia and reperfusion prior to 40 min of global ischemia and 30 min of reperfusion. Without affecting PI3K-IB, ouabain doubled PI3K-IA activity and Akt phosphorylation at Ser⁴⁷³. IPC and OPC significantly preserved cardiac contractile function and tissue viability as evidenced by left ventricular developed pressure and end-diastolic pressure recovery, reduced lactate dehydrogenase release, and decreased infarct size. OPC protection was blunted by the PI3K-IA inhibitor PI-103, but not by the PI3K-IB inhibitor AS-604850. In contrast, IPC-mediated protection was not affected by PI-103 but was blocked by AS-604850, suggesting that PI3K-IA activation is required for OPC while PI3K-IB activation is needed for IPC. Mechanistically, PI3K-IA activity is required for ouabain-induced Akt activation but not PKCε translocation. However, in contrast to PKCε translocation which is critical to protection, Akt activity was not required for OPC. Further studies shall reveal the identity of the downstream targets of this new PI3K IA-dependent branch of OPC. These findings may be of clinical relevance in patients at risk for myocardial infarction with underlying diseases and/or medication that could differentially affect the integrity of cardiac PI3K-IA and IB pathways.     

Stimulation of mTORC2 by integrin αIIbβ3 is required for PI3Kβ-dependent activation of Akt but is dispensable for platelet spreading on fibrinogen

Abstract

Phosphatidylinositol 3 kinase (PI3K) is a major player in platelet activation and regulates thrombus formation and stabilization. The β isoform of PI3K is implicated in integrin αIIbβ3 outside-in signaling, is required for the phosphorylation of Akt, and controls efficient platelet spreading upon adhesion to fibrinogen. In this study we found that during integrin αIIbβ3 outside-in signaling PI3Kβ-dependent phosphorylation of Akt on Serine473 is mediated by the mammalian target of rapamycin complex 2 (mTORC2). The activity of mTORC2 is stimulated upon platelet adhesion to fibrinogen, as documented by increased autophosphorylation. However, mTORC2 activation downstream of integrin αIIbβ3 is PI3Kβ-independent. Inhibition of mTORC2, but not mTORC1, also prevents Akt phosphorylation of Threonine308 and affects Akt activity, resulting in the inhibition of GSK3α/β phosphorylation. Nevertheless, mTORC2 or Akt inhibition does not alter PI3Kβ-dependent platelet spreading on fibrinogen. The activation of the small GTPase Rap1b downstream of integrin αIIbβ3 is regulated by PI3Kβ but is not affected upon inhibition of either mTORC2 or Akt. Altogether, these results demonstrate for the first time the activation of mTORC2 and its involvement in Akt phosphorylation and stimulation during integrin αIIbβ3 outside-in signaling. Moreover, the results demonstrate that the mTORC2/Akt pathway is dispensable for PI3Kβ-regulated platelet spreading on fibrinogen.     

Aging is associated with decreased pancreatic acinar cell regeneration and phosphatidylinositol 3-kinase/Akt activation

BACKGROUND & AIMS: The effects of aging on pancreatic acinar cell proliferation have not been clearly defined. Phosphatidylinositol 3-kinase (PI3K)-mediated phosphorylation of Akt is a critical step for proliferation of various cell types and insulin secretion from pancreatic endocrine cells; however, its role in acinar cell proliferation is not known. The purpose of this study was to (1) delineate the effects of aging on pancreatic regeneration after partial pancreatectomy (Px) and (2) define the involvement of the PI3K/Akt pathway in pancreatic regeneration. METHODS: Following partial Px, pancreatic regeneration and activation of the PI3K pathway were compared in young and aged mice. Activation of the PI3K/Akt pathway was evaluated by Akt phosphorylation (pAkt). The role of the PI3K pathway in pancreatic regeneration after partial Px was assessed by effects of a pharmacologic PI3K inhibitor wortmannin or small interfering RNA (siRNA) to the p85alpha regulatory subunit. To confirm further the critical role of the PI3K/Akt pathway in pancreatic acinar cell proliferation, IGF-1-mediated cell proliferation was determined in cultured acinar cells pretreated with wortmannin or p85alpha siRNA. RESULTS: Pancreatic regeneration and pAkt expression after partial Px were significantly decreased with aging. Treatment with wortmannin or p85alpha siRNA reduced pancreatic regeneration after partial Px. The IGF-1-mediated cell proliferation in vitro was completely blocked by wortmannin or p85alpha siRNA but not by the MEK/ERK inhibitor PD98059. CONCLUSIONS: PI3K/Akt activation plays a critical role in the regeneration of pancreatic acini after resection. Furthermore, pancreatic regeneration is markedly attenuated in the aged pancreas most likely because of decreased PI3K/Akt activation.     

Combined targeting of MEK/MAPK and PI3K/Akt signalling in multiple myeloma

So-called RAS-dependent pathways, such as those signalling via mitogen-activated protein kinase kinase (MEK)/mitogen-activated protein kinase (MAPK) and phosphoinositide-3 kinase (PI3K)/Akt, are implicated in proliferation and survival of multiple myeloma (MM) cells. However, the effects of their combined blockade and its potential therapeutic utility for the treatment of RAS-mutated MM have not systematically been analysed. Here, we tested the functional consequences of single versus combined inhibition of the MEK/MAPK and PI3K/Akt pathways in a large series of primary MM samples (n = 55) and MM cell lines (n = 11). Additionally, the anti-myeloma activity of different treatments was analysed with respect to the RAS mutation status. PI3K/Akt blockade was generally more pro-apoptotic than blockade of MEK/MAPK both in cell lines and in primary MM samples. Simultaneous blockade of both pathways led to significantly enhanced anti-myeloma activity in 75% of primary MM samples, whereas the remainder was largely resistant. Resistance to combination blockade was exclusively observed in RAS wildtype cases, whereas sensitivity was noted in RAS wildtype and in RAS mutated MM. These results suggest that oncogenic RAS is a predictor of sensitivity to combination treatment with PI3K/Akt and MEK/MAPK inhibitors and that such an approach might therefore be beneficial for this genetically well-defined subgroup of MM patients.     

Defective phosphatidylinositol 3-kinase signaling in central control of cardiovascular effects in the nucleus tractus solitarii of spontaneously hypertensive rats

Recently we have shown functional involvement of the phosphatidylinositol 3-kinase (PI3K)-Akt-nitric oxide synthase (NOS) signaling pathway in central control of cardiovascular effects in the nucleus tractus solitarii (NTS) of normotensive Wistar-Kyoto (WKY) rats. In this study we determined whether PI3K/Akt signaling was defective in spontaneously hypertensive rats (SHR). WKY rats and SHR were anesthetized with urethane. Mean blood pressure (MBP) and heart rate (HR) were monitored intra-arterially. Unilateral microinjection (60 nL) of insulin (100 IU/mL) into the NTS produced prominent depressor and bradycardic effects in 8- and 16-week-old normotensive WKY and 8-week-old SHR. However, no significant cardiovascular effects were found in 16-week-old SHR after insulin injection. Furthermore, pretreatment with PI3K inhibitor LY294002 and NOS inhibitor L-NAME into the NTS attenuated the cardiovascular response evoked by insulin in WKY and 8-week-old SHR but not in 16-week-old SHR. Unilateral microinjection of 1 mmol/L of PI(3,4,5)P(3) (phosphatidylinositol 3,4,5-triphosphate), a phospholipids second messenger produced by PI3K, into the NTS produced prominent depressor and bradycardic effects in 8- or 16-week-old WKY rats as well as 8-week-old SHR but not in 16-week-old SHR. Western blot analysis showed no significant increase in Akt phosphorylation in 8-week-old pre-hypertensive SHR after insulin injection. Similar results were also found in hypertensive 16-week-old SHR. Our results indicate that the Akt-independent signaling pathway is involved in NOS activation to regulate cardiovascular effects in the NTS of 8-week-old pre-hypertensive SHR. Both Akt-dependent and Akt-independent signaling pathways are defective in hypertensive 16-week-old SHR.     

DIDS对I/RI心肌组织中信号分子PI3K/Akt及ROS的调控

目的:探讨氯离子通道阻断剂4,4-二异硫氰基芪-2,2′-二磺酸（4,4′-Diisothiocyanostilbene-2,2′-disulfonic acid,DIDS）对缺血/再灌注损伤(Ischemia/reperfusion injury,I/RI)心肌组织中信号分子磷脂酰肌醇-3激酶（Phosphoinositide-3 kinase,PI3K）/蛋白激酶B（Protein kinase B,Akt）及活性氧（ROS）的调控。方法: 常规建立SD大鼠I/RI心脏模型,32只,随机分成4组（n=8）,分为假手术组、I/R组、过氧化氢酶(CAT)组及DIDS组。伊文思蓝和红四氮唑（TTC）双染色法检测大鼠心肌梗死面积、荧光分光光度计法检测心肌组织中ROS的含量、用western blot 蛋白印迹法检测Akt和p-Akt的水平,TUNNEL法检测心肌细胞凋亡,以及检测血清肌酸激酶（CK）、乳酸脱氢酶（LDH）、超氧化物歧化酶(SOD)的活性及丙二醛（MDA）的含量。结果: ①与I/R组对比,DIDS组和CAT两组的心肌梗死面积、细胞凋亡数、血清CK、LDH的活性均明显减少（P<0.05）;但DIDS和CAT二组间无差异。②与I/R组对比,DIDS组和CAT组血清中MDA的含量明显减小;而SOD的活性明显升高（P<0.05）;与DIDS组比较,CAT组SOD的活性明显升高（P<0.05）、MDA的含量明显减少（P<0.05）。③与I/R组对比,CAT组和DIDS组心肌组织中ROS的含量显著降低（P<0.05）;CAT组较DIDS组降低的更明显（P<0.05）。④各组心肌组织中总Akt的表达无显著差异。与假手术组相比,I/R、DIDS 和CAT 3组p-Akt的水平均有明显升高（P<0.05）;DIDS组p-Akt的水平显著高于I/R和CAT组（P<0.05）;但I/R和CAT两组间无差异。结论: DIDS同时具有激活PI3K/Akt信号通路及降低ROS水平,减轻I/RI,达到保护心肌组织的作用。     

Differential effect of long-term leucine supplementation on skeletal muscle and adipose tissue in old rats: an insulin signaling pathway approach

Leucine acts as a signal nutrient in promoting protein synthesis in skeletal muscle and adipose tissue via mTOR pathway activation, and may be of interest in age-related sarcopenia. However, hyper-activation of mTOR/S6K1 has been suggested to inhibit the first steps of insulin signaling and finally promote insulin resistance. The impact of long-term dietary leucine supplementation on insulin signaling and sensitivity was investigated in old rats (18 months old) fed a 15% protein diet supplemented (LEU group) or not (C group) with 4.5% leucine for 6 months. The resulting effects on muscle and fat were examined. mTOR/S6K1 signaling pathway was not significantly altered in muscle from old rats subjected to long-term dietary leucine excess, whereas it was increased in adipose tissue. Overall glucose tolerance was not changed but insulin-stimulated glucose transport was improved in muscles from leucine-supplemented rats related to improvement in Akt expression and phosphorylation in response to food intake. No change in skeletal muscle mass was observed, whereas perirenal adipose tissue mass accumulated (+45%) in leucine-supplemented rats. A prolonged leucine supplementation in old rats differently modulates mTOR/S6K pathways in muscle and adipose tissue. It does not increase muscle mass but seems to promote hypertrophy and hyperplasia of adipose tissue that did not result in insulin resistance.     

Caloric restriction attenuates aging-induced cardiac insulin resistance in male Wistar rats through activation of PI3K/Akt pathway

Background and Aim

Caloric restriction (CR) improves insulin sensitivity and is one of the dietetic strategies most commonly used to enlarge life and to prevent aging-induced cardiovascular alterations. The aim of this study was to analyze the possible beneficial effects of caloric restriction (CR) preventing the aging-induced insulin resistance in the heart of male Wistar rats.