Promiscuous affairs of PKB/AKT isoforms in metabolism

High glucose-induced downregulation of PTEN-Long is sufficient for proximal tubular cell injury in diabetic kidney disease

During the progression of diabetic kidney disease, proximal tubular epithelial cells respond to high glucose to induce hypertrophy and matrix expansion leading to renal fibrosis. Recently, a non-canonical PTEN has been shown to be translated from an upstream initiation codon CUG (leucine) to produce a longer protein called PTEN-Long (PTEN-L). Interestingly, the extended sequence present in PTEN-L contains cell secretion/penetration signal. Role of this non-canonical PTEN-L in diabetic renal tubular injury is not known. We show that high glucose decreases expression of PTEN-L. As a mechanism of its function, we find that reduced PTEN-L activates Akt-2, which phosphorylates and inactivate tuberin and PRAS40, resulting in activation of mTORC1 in tubular cells. Antibacterial agent acriflavine and antiviral agent ATA regulate translation from CUG codon. Acriflavine and ATA, respectively, decreased and increased expression of PTEN-L to altering Akt-2 and mTORC1 activation in the absence of change in expression of canonical PTEN. Consequently, acriflavine and ATA modulated high glucose-induced tubular cell hypertrophy and lamininγ1 expression. Importantly, expression of PTEN-L inhibited high glucose-stimulated Akt/mTORC1 activity to abrogate these processes. Since PTEN-L contains secretion/penetration signals, addition of conditioned medium containing PTEN-L blocked Akt-2/mTORC1 activity. Notably, in renal cortex of diabetic mice, we found reduced PTEN-L concomitant with Akt-2/mTORC1 activation, leading to renal hypertrophy and lamininγ1 expression. These results present first evidence for involvement of PTEN-L in diabetic kidney disease.

Peroxisome proliferator-activated receptor gamma coactivator-1 (PGC-1) family in physiological and pathophysiological process and diseases

Peroxisome proliferator-activated receptor gamma coactivator-1 (PGC-1) family (PGC-1s), consisting of three members encompassing PGC-1α, PGC-1β, and PGC-1-related coactivator (PRC), was discovered more than a quarter-century ago. PGC-1s are essential coordinators of many vital cellular events, including mitochondrial functions, oxidative stress, endoplasmic reticulum homeostasis, and inflammation. Accumulating evidence has shown that PGC-1s are implicated in many diseases, such as cancers, cardiac diseases and cardiovascular diseases, neurological disorders, kidney diseases, motor system diseases, and metabolic disorders. Examining the upstream modulators and co-activated partners of PGC-1s and identifying critical biological events modulated by downstream effectors of PGC-1s contribute to the presentation of the elaborate network of PGC-1s. Furthermore, discussing the correlation between PGC-1s and diseases as well as summarizing the therapy targeting PGC-1s helps make individualized and precise intervention methods. In this review, we summarize basic knowledge regarding the PGC-1s family as well as the molecular regulatory network, discuss the physio-pathological roles of PGC-1s in human diseases, review the application of PGC-1s, including the diagnostic and prognostic value of PGC-1s and several therapies in pre-clinical studies, and suggest several directions for future investigations. This review presents the immense potential of targeting PGC-1s in the treatment of diseases and hopefully facilitates the promotion of PGC-1s as new therapeutic targets.

Ube4A maintains metabolic homeostasis and facilitates insulin signaling in vivo

OBJECTIVE

Defining the regulators of cell metabolism and signaling is essential to design new therapeutic strategies in obesity and NAFLD/NASH. E3 ubiquitin ligases control diverse cellular functions by ubiquitination-mediated regulation of protein targets, and thus their functional aberration is associated with many diseases. The E3 ligase Ube4A has been implicated in human obesity, inflammation, and cancer. However, its in vivo function is unknown, and no animal models are available to study this novel protein.

METHODS

A whole-body Ube4A knockout (UKO) mouse model was generated, and various metabolic parameters were compared in chow- and high fat diet (HFD)-fed WT and UKO mice, and in their liver, adipose tissue, and serum. Lipidomics and RNA-Seq studies were performed in the liver samples of HFD-fed WT and UKO mice. Proteomic studies were conducted to identify Ube4A's targets in metabolism. Furthermore, a mechanism by which Ube4A regulates metabolism was identified.

RESULTS

Although the body weight and composition of young, chow-fed WT and UKO mice are similar, the knockouts exhibit mild hyperinsulinemia and insulin resistance. HFD feeding substantially augments obesity, hyperinsulinemia, and insulin resistance in both sexes of UKO mice. HFD-fed white and brown adipose tissue depots of UKO mice have increased insulin resistance and inflammation and reduced energy metabolism. Moreover, Ube4A deletion exacerbates hepatic steatosis, inflammation, and liver injury in HFD-fed mice with increased lipid uptake and lipogenesis in hepatocytes. Acute insulin treatment resulted in impaired activation of the insulin effector protein kinase Akt in liver and adipose tissue of chow-fed UKO mice. We identified the Akt activator protein APPL1 as a Ube4A interactor. The K63-linked ubiquitination (K63-Ub) of Akt and APPL1, known to facilitate insulin-induced Akt activation, is impaired in UKO mice. Furthermore, Ube4A K63-ubiquitinates Akt in vitro.

CONCLUSION

Ube4A is a novel regulator of obesity, insulin resistance, adipose tissue dysfunction and NAFLD, and preventing its downregulation may ameliorate these diseases.

AKT ISOFORMS-AS160-GLUT4: The defining axis of insulin resistance

The Akt isoforms-AS160-GLUT4 axis is the primary axis that governs glucose homeostasis in the body. The first step on the path to insulin resistance is deregulated Akt isoforms. This could be Akt isoform expression, its phosphorylation, or improper isoform-specific redistribution to the plasma membrane in a specific tissue system. The second step is deregulated AS160 expression, its phosphorylation, improper dissociation from glucose transporter storage vesicles (GSVs), or its inability to bind to 14-3-3 proteins, thus not allowing it to execute its function. The final step is improper GLUT4 translocation and aberrant glucose uptake. These processes lead to insulin resistance in a tissue-specific way affecting the whole-body glucose homeostasis, eventually progressing to an overt diabetic phenotype. Thus, the relationship between these three key proteins and their proper regulation comes out as the defining axis of insulin signaling and -resistance. This review summarizes the role of this central axis in insulin resistance and disease in a new light.

Molecular regulation of polycystic ovary syndrome: altered gene expression levels in mouse models pretreatment and post-treatment

Polycystic ovary syndrome (PCOS) is a complex disorder and genetic factors are believed to play a role. The main aim was to investigate expression levels of genes involved in PI3K/AKT signalling pathway pretreatment and post-treatment. Mouse models of PCOS were generated. Group one included control mice with no polycystic ovaries (n = 4), Group 2 included a PCOS mouse model (n = 8), Group 3 included PCOS mice treated with clomiphene citrate (n = 7) and Group 4 included PCOS mice treated with clomiphene citrate, metformin and pioglitazone (n = 8). Histochemical analyses were performed. Total RNA was extracted and cDNA was synthesized. Irs, Akt1 and Akt2, mTor and Pdpk1 gene expression levels were evaluated by RT-PCR amplification. In Group 1, cortex and medulla were evaluated as normal; in Group 2, ovarian cortex was composed of immature oocytes and cystic follicles with atretic follicles. In Groups 3 and 4, follicles were in the process of normal follicle differentiation. The expression levels of Akt1 and Pi3k were significantly different (P < 0.0001) between Groups 1 and 2. The significant differences in expression levels of Pi3k and Akt1 were also observed between the Group 1 and both Groups 3 and 4 (P < 0.0001). Furthermore, significant variations of the expression levels of mTor between Groups 1 and 4 were observed. The extrapolation of results of this study may imply that follicular development may be regulated by molecular pathways involving Pi3k, Akt1 and mTor expression. Therefore, genes in the PI3K/AKT pathway may have a direct regulatory role in the development of PCOS.

Culprits or consequences: Understanding the metabolic dysregulation of muscle in diabetes

The prevalence of type 2 diabetes (T2D) continues to rise despite the amount of research dedicated to finding the culprits of this debilitating disease. Skeletal muscle is arguably the most important contributor to glucose disposal making it a clear target in insulin resistance and T2D research. Within skeletal muscle there is a clear link to metabolic dysregulation during the progression of T2D but the determination of culprits vs consequences of the disease has been elusive. Emerging evidence in skeletal muscle implicates influential cross talk between a key anabolic regulatory protein, the mammalian target of rapamycin (mTOR) and its associated complexes (mTORC1 and mTORC2), and the well-described canonical signaling for insulin-stimulated glucose uptake. This new understanding of cellular signaling crosstalk has blurred the lines of what is a culprit and what is a consequence with regard to insulin resistance. Here, we briefly review the most recent understanding of insulin signaling in skeletal muscle, and how anabolic responses favoring anabolism directly impact cellular glucose disposal. This review highlights key cross-over interactions between protein and glucose regulatory pathways and the implications this may have for the design of new therapeutic targets for the control of glucoregulatory function in skeletal muscle.

Sevoflurane Modulates AKT Isoforms in Triple Negative Breast Cancer Cells. An Experimental Study

(1) Background: Triple negative breast cancer (TNBC) is a highly aggressive tumor, associated with high rates of early distant recurrence and short survival times, and treatment may require surgery, and thus anesthesia. The effects of anesthetic drugs on cancer progression are under scrutiny, but published data are controversial, and the involved mechanisms unclear. Anesthetic agents have been shown to modulate several molecular cascades, including PI3K/AKT/mTOR. AKT isoforms are frequently amplified in various malignant tumors and associated with malignant cell survival, proliferation and invasion. Their activation is often observed in human cancers and is associated with decreased survival rate. Certain anesthetics are known to affect hypoxia cell signaling mechanisms by upregulating hypoxia-inducible factors (HIFs). (2) Methods: MCF-10A and MDA-MB 231 cells were cultivated and CellTiter-Blue^® Cell Viability assay, 2D and 3D matrigel assay, immunofluorescence assays and gene expressions assay were performed after exposure to different sevoflurane concentrations. (3) Results: Sevoflurane exposure of TNBC cells results in morphological and behavioral changes. Sevoflurane differently influences the AKT isoforms expression in a time-dependent manner, with an important early AKT3 upregulation. The most significant effects occur at 72 h after 2 mM sevoflurane treatment and consist in increased viability, proliferation and aggressiveness and increased vimentin and HIF expression. (4) Conclusions: Sevoflurane exposure during surgery may contribute to cancer recurrence via AKT3 induced epithelial–mesenchymal transition (EMT) and by all three AKT isoforms enhanced cancer cell survival and proliferation.

Targeting the Inositol Pyrophosphate Biosynthetic Enzymes in Metabolic Diseases

In mammals, a family of three inositol hexakisphosphate kinases (IP6Ks) synthesizes the inositol pyrophosphate 5-IP7 from IP6. Genetic deletion of Ip6k1 protects mice from high fat diet induced obesity, insulin resistance and fatty liver. IP6K1 generated 5-IP7 promotes insulin secretion from pancreatic β-cells, whereas it reduces insulin signaling in metabolic tissues by inhibiting the protein kinase Akt. Thus, IP6K1 promotes high fat diet induced hyperinsulinemia and insulin resistance in mice while its deletion has the opposite effects. IP6K1 also promotes fat accumulation in the adipose tissue by inhibiting the protein kinase AMPK mediated energy expenditure. Genetic deletion of Ip6k3 protects mice from age induced fat accumulation and insulin resistance. Accordingly, the pan IP6K inhibitor TNP [N2-(m-trifluorobenzyl), N6-(p-nitrobenzyl)purine] ameliorates obesity, insulin resistance and fatty liver in diet induced obese mice by improving Akt and AMPK mediated insulin sensitivity and energy expenditure. TNP also protects mice from bone loss, myocardial infarction and ischemia reperfusion injury. Thus, the IP6K pathway is a potential target in obesity and other metabolic diseases. Here, we summarize the studies that established IP6Ks as a potential target in metabolic diseases. Further studies will reveal whether inhibition of this pathway has similar pleiotropic benefits on metabolic health of humans.

Exercise effects on γ3-AMPK activity, phosphorylation of Akt2 and AS160, and insulin-stimulated glucose uptake in insulin-resistant rat skeletal muscle

One exercise session can increase subsequent insulin-stimulated glucose uptake (ISGU) by skeletal muscle. Prior research on healthy muscle suggests that enhanced postexercise ISGU depends on elevated γ3-AMPK activity leading to greater phosphorylation of Akt substrate of 160 kDa (pAS160) on an AMPK-phosphomotif (Ser⁷⁰⁴). Phosphorylation of AS160^Ser704, in turn, may favor greater insulin-stimulated pAS160 on an Akt-phosphomotif (Thr⁶⁴²) that regulates ISGU. Accordingly, we tested if exercise-induced increases in γ3-AMPK activity and pAS160 on key regulatory sites accompany improved ISGU at 3 h postexercise (3hPEX) in insulin-resistant muscle. Rats fed a high-fat diet (HFD; 2-wk) that induces insulin resistance either performed acute swim-exercise (2 h) or were sedentary (SED). SED rats fed a low-fat diet (LFD; 2 wk) served as healthy controls. Isolated epitrochlearis muscles from 3hPEX and SED rats were analyzed for ISGU, pAS160, pAkt2 (Akt-isoform that phosphorylates pAS160^Thr642), and γ1-AMPK and γ3-AMPK activity. ISGU was lower in HFD-SED muscles versus LFD-SED, but this decrement was eliminated in the HFD-3hPEX group. γ3-AMPK activity, but not γ1-AMPK activity, was elevated in HFD-3hPEX muscles versus both SED controls. Furthermore, insulin-stimulated pAS160^Thr642, pAS160^Ser704, and pAkt2^Ser474 in HFD-3hPEX muscles were elevated above HFD-SED and equal to values in LFD-SED muscles, but insulin-independent pAS160^Ser704 was unaltered at 3hPEX. These results demonstrated, for the first time in an insulin-resistant model, that the postexercise increase in ISGU was accompanied by sustained enhancement of γ3-AMPK activation and greater pAkt2^Ser474. Our working hypothesis is that these changes along with enhanced insulin-stimulated pAS160 increase ISGU of insulin-resistant muscles to values equaling insulin-sensitive sedentary controls.NEW & NOTEWORTHY Earlier research focusing on signaling events linked to increased insulin sensitivity in muscle has rarely evaluated insulin resistant muscle after exercise. We assessed insulin resistant muscle after an exercise protocol that improved insulin-stimulated glucose uptake. Prior exercise also amplified several signaling steps expected to favor enhanced insulin-stimulated glucose uptake: increased γ3-AMP-activated protein kinase activity, greater insulin-stimulated Akt2 phosphorylation on Ser474, and elevated insulin-stimulated Akt substrate of 160 kDa phosphorylation on Ser588, Thr642, and Ser704.

Akt-ing Up Just About Everywhere: Compartment-Specific Akt Activation and Function in Receptor Tyrosine Kinase Signaling

The serine/threonine kinase Akt is a master regulator of many diverse cellular functions, including survival, growth, metabolism, migration, and differentiation. Receptor tyrosine kinases are critical regulators of Akt, as a result of activation of phosphatidylinositol-3-kinase (PI3K) signaling leading to Akt activation upon receptor stimulation. The signaling axis formed by receptor tyrosine kinases, PI3K and Akt, as well as the vast range of downstream substrates is thus central to control of cell physiology in many different contexts and tissues. This axis must be tightly regulated, as disruption of PI3K-Akt signaling underlies the pathology of many diseases such as cancer and diabetes. This sophisticated regulation of PI3K-Akt signaling is due in part to the spatial and temporal compartmentalization of Akt activation and function, including in specific nanoscale domains of the plasma membrane as well as in specific intracellular membrane compartments. Here, we review the evidence for localized activation of PI3K-Akt signaling by receptor tyrosine kinases in various specific cellular compartments, as well as that of compartment-specific functions of Akt leading to control of several fundamental cellular processes. This spatial and temporal control of Akt activation and function occurs by a large number of parallel molecular mechanisms that are central to regulation of cell physiology.

Maternal cinnamon intake during lactation led to visceral obesity and hepatic metabolic dysfunction in the adult male offspring

PURPOSE

Studies with foods, known to promote health benefits in addition to the nutritive value, show that their consumption by pregnant and/or lactating females could induce negative outcomes to the offspring. It is well characterized that cinnamon intake promotes benefits to energy homeostasis. The present study aimed to analyze the effects of the consumption of an aqueous extract of cinnamon by lactating female rats on the endocrine-metabolic outcomes in the adult offspring.

METHODS

Lactating dams (Wistar rats) were supplemented with cinnamon aqueous extract (400 mg/kg body weight/day) for the entire lactating period. The male adult offspring were evaluated at 180 days old (CinLac).

RESULTS

The offspring presented visceral obesity (P = 0.001), hyperleptinemia (P = 0.002), and hyperinsulinemia (P = 0.016). In the liver, CinLac exhibited reduced p-IRβ (P = 0.018) suggesting insulin resistance. However, phosphorylation of IRS1 (P = 0.041) and AKT (P = 0.050) were increased. JAK2 (P = 0.030) and p-STAT3 (P = 0.015) expressions were higher, suggesting that the activation of IRS1/AKT in the CinLac group could have resulted from the increased activation of leptin signaling. Although we observed no changes in the gluconeogenic pathway, the CinLac group exhibited lower hepatic glycogen content (P = 0.005) accompanied by increased p-GSK3β (P = 0.011). In addition, the CinLac group showed increased hepatic triacylglycerol content (P = 0.049) and a mild steatosis (P = 0.001), accompanied by reduced PPARα mRNA expression (P = 0.005).

CONCLUSION

We conclude that maternal intake of aqueous extract of cinnamon induces long-term molecular, metabolic, and hormonal changes in the adult progeny, including visceral obesity, higher lipid accumulation, and lower glycogen content in the liver.

Involvement of Hepatic SHIP2 and PI3K/Akt Signalling in the Regulation of Plasma Insulin by Xiaoyaosan in Chronic Immobilization-Stressed Rats

Background: Long-term exposure to chronic stress is thought to be a factor closely correlated with the development of metabolic disorders, such as diabetes mellitus and metabolic syndrome. Xiaoyaosan, a Chinese herbal formula, has been described in many previous studies to exert anxiolytic-like or antidepressant effects in chronically stressed rats. However, few studies have observed the effects of Xiaoyaosan on the metabolic disorders induced by chronic stress. Objective: We sought to investigate the effective regulation of Xiaoyaosan on 21-day chronic immobility stress (CIS, which is 3 h of restraint immobilization every day)-induced behavioural performance and metabolic responses and to further explore whether the effects of Xiaoyaosan were related to SHIP2 expression in the liver. Methods: Sixty male Sprague Dawley rats were randomly divided into a control group, a CIS group, a Xiaoyaosan group and a rosiglitazone group. The latter three groups were subjected to 21 days of CIS to generate the stress model. After 21 days of CIS, the effects of Xiaoyaosan on body weight, food intake, and behaviour in the open field test, the sucrose preference test and the forced swimming test were observed following chronic stress. Plasma insulin, cholesterol (CHOL), triglyceride (TG), low-density lipoprotein (LDL-C) and high-density lipoprotein (HDL-C) concentrations and blood glucose were examined, and the protein and mRNA expression levels of SHIP2, p85 and Akt in the liver were measured using RT-qPCR and immunohistochemical staining. Results: Rats exposed to CIS exhibited depression-like behaviours, decreased levels of plasma insulin, CHOL, LDL-C, TG and HDL-C, and increased blood glucose. Increased SHIP2 expression and reduced Akt, p-Akt and p85 expression were also observed in the liver. Xiaoyaosan exerted antidepressant effects and effectively reversed the changes caused by CIS. Conclusions: These results suggest that Xiaoyaosan attenuates depression-like behaviours and ameliorates stress-induced abnormal levels of insulin, blood glucose, CHOL, LDL-C and HDL-C in the plasma of stressed rats, which may be associated with the regulation of SHIP2 expression to enhance PI3K/Akt signalling activity in the liver.

Akt2 causes TGFβ-induced deptor downregulation facilitating mTOR to drive podocyte hypertrophy and matrix protein expression

TGFβ promotes podocyte hypertrophy and expression of matrix proteins in fibrotic kidney diseases such as diabetic nephropathy. Both mTORC1 and mTORC2 are hyperactive in response to TGFβ in various renal diseases. Deptor is a component of mTOR complexes and a constitutive inhibitor of their activities. We identified that deptor downregulation by TGFβ maintains hyperactive mTOR in podocytes. To unravel the mechanism, we found that TGFβ -initiated noncanonical signaling controls deptor inhibition. Pharmacological inhibitor of PI 3 kinase, Ly 294002 and pan Akt kinase inhibitor MK 2206 prevented the TGFβ induced downregulation of deptor, resulting in suppression of both mTORC1 and mTORC2 activities. However, specific isoform of Akt involved in this process is not known. We identified Akt2 as predominant isoform expressed in kidney cortex, glomeruli and podocytes. TGFβ time-dependently increased the activating phosphorylation of Akt2. Expression of dominant negative PI 3 kinase and its signaling inhibitor PTEN blocked Akt2 phosphorylation by TGFβ. Inhibition of Akt2 using a phospho-deficient mutant that inactivates its kinase activity, as well as siRNA against the kinase markedly diminished TGFβ -mediated deptor suppression, its association with mTOR and activation of mTORC1 and mTORC2. Importantly, inhibition of Akt2 blocked TGFβ -induced podocyte hypertrophy and expression of the matrix protein fibronectin. This inhibition was reversed by the downregulation of deptor. Interestingly, we detected increased phosphorylation of Akt2 concomitant with TGFβ expression in the kidneys of diabetic rats. Thus, our data identify previously unrecognized Akt2 kinase as a driver of TGFβ induced deptor downregulation and sustained mTORC1 and mTORC2 activation. Furthermore, we provide the first evidence that deptor downstream of Akt2 contributes to podocyte hypertrophy and matrix protein expression found in glomerulosclerosis in different renal diseases.

Hypoinsulinaemic, hypoketotic hypoglycaemia due to mosaic genetic activation of PI3-kinase

OBJECTIVE

Genetic activation of the insulin signal-transducing kinase AKT2 causes syndromic hypoketotic hypoglycaemia without elevated insulin. Mosaic activating mutations in class 1A phospatidylinositol-3-kinase (PI3K), upstream from AKT2 in insulin signalling, are known to cause segmental overgrowth, but the metabolic consequences have not been systematically reported. We assess the metabolic phenotype of 22 patients with mosaic activating mutations affecting PI3K, thereby providing new insight into the metabolic function of this complex node in insulin signal transduction.

METHODS

Three patients with megalencephaly, diffuse asymmetric overgrowth, hypoketotic, hypoinsulinaemic hypoglycaemia and no AKT2 mutation underwent further genetic, clinical and metabolic investigation. Signalling in dermal fibroblasts from one patient and efficacy of the mTOR inhibitor Sirolimus on pathway activation were examined. Finally, the metabolic profile of a cohort of 19 further patients with mosaic activating mutations in PI3K was assessed.

RESULTS

In the first three patients, mosaic mutations in PIK3CA (p.Gly118Asp or p.Glu726Lys) or PIK3R2 (p.Gly373Arg) were found. In different tissue samples available from one patient, the PIK3CA p.Glu726Lys mutation was present at burdens from 24% to 42%, with the highest level in the liver. Dermal fibroblasts showed increased basal AKT phosphorylation which was potently suppressed by Sirolimus. Nineteen further patients with mosaic mutations in PIK3CA had neither clinical nor biochemical evidence of hypoglycaemia.

CONCLUSIONS

Mosaic mutations activating class 1A PI3K cause severe non-ketotic hypoglycaemia in a subset of patients, with the metabolic phenotype presumably related to the extent of mosaicism within the liver. mTOR or PI3K inhibitors offer the prospect for future therapy.

Lysophosphatidic acid directly induces macrophage-derived foam cell formation by blocking the expression of SRBI

The leading cause of morbidity and mortality is the result of cardiovascular disease, mainly atherosclerosis. The formation of macrophage foam cells by ingesting ox-LDL and focal retention in the subendothelial space are the hallmarks of the early atherosclerotic lesion. Lysophosphatidic acid (LPA), which is a low-molecular weight lysophospholipid enriched in oxidized LDL, exerts a range of effects on the cardiovascular system. Previous reports show that LPA increases the uptake of ox-LDL to promote the formation of foam cells. However, as the most active component of ox-LDL, there is no report showing whether LPA directly affects foam cell formation. The aim of this study was to investigate the effects of LPA on foam cell formation, as well as to elucidate the underlying mechanism. Oil red O staining and a Cholesterol/cholesteryl ester quantitation assay were used to evaluate foam cell formation in Raw264.7 macrophage cells. We utilized a Western blot and RT-PCR to investigate the relationship between LPA receptors and lipid transport related proteins. We found that LPA promoted foam cell formation, using 200 μM for 24 h. Meanwhile, the expression of the Scavenger receptor BI (SRBI), which promotes the efflux of free cholesterol, was decreased. Furthermore, the LPA_1/3 receptor antagonist Ki16425 significantly abolished the LPA effects, indicating that LPA_1/3 was involved in the foam cell formation and SRBI expression induced by LPA. Additionally, the LPA-induced foam cell formation was blocked with an AKT inhibitor. Our results suggest that LPA-enhanced foam cell formation is mediated by LPA_1/3 -AKT activation and subsequent SRBI expression.

3,5-diiodothyronine (3,5-T2) reduces blood glucose independently of insulin sensitization in obese mice

AIM

Thyroid hormones regulate metabolic response. While triiodothyronine (T3) is usually considered to be the active form of thyroid hormone, one form of diiodothyronine (3,5-T2) exerts T3-like effects on energy consumption and lipid metabolism. 3,5-T2 also improves glucose tolerance in rats and 3,5-T2 levels correlate with fasting glucose in humans. Presently, however, little is known about mechanisms of 3,5-T2 effects on glucose metabolism. Here, we set out to compare effects of T3, 3,5-T2 and another form of T2 (3,3-T2) in a mouse model of diet-induced obesity and determined effects of T3 and 3,5-T2 on markers of classical insulin sensitization to understand how diiodothyronines influence blood glucose.

METHODS

Cell- and protein-based assays of thyroid hormone action. Assays of metabolic parameters in mice. Analysis of transcript and protein levels in different tissues by qRT-PCR and Western blot.

RESULTS

T3 and 3,5-T2 both reduce body weight, adiposity and body temperature despite increased food intake. 3,3'-T2 lacks these effects. T3 and 3,5-T2 reduce blood glucose levels, whereas 3,3'-T2 worsens glucose tolerance. Neither T3 nor 3,5-T2 affects markers of insulin sensitization in skeletal muscle or white adipose tissue (WAT), but both reduce hepatic GLUT2 glucose transporter levels and glucose output. T3 and 3,5-T2 also induce expression of mitochondrial uncoupling proteins (UCPs) 3 and 1 in skeletal muscle and WAT respectively.

CONCLUSIONS

3,5-T2 influences glucose metabolism in a manner that is distinct from insulin sensitization and involves reductions in hepatic glucose output and changes in energy utilization.

Insulin-stimulated glucose uptake in healthy and insulin-resistant skeletal muscle

Skeletal muscle is the largest tissues in the human body and is considered the primary target for insulin-stimulated glucose disposal. In skeletal muscle, binding of the insulin to insulin receptor (IR) initiates a signaling cascade that results in the translocation of the insulin-sensitive glucose transporter protein 4 (GLUT4) to the plasma membrane which leads to facilitated diffusion of glucose into the cell. Understanding the precise signaling events guiding insulin-stimulated glucose uptake is pivotal, because impairment in these signaling events leads to development of insulin resistance and type 2 diabetes. This review summarizes current understanding of insulin signaling pathways mediating glucose uptake in healthy and insulin-resistant skeletal muscle.

Astrocytic Expression of CTMP Following an Excitotoxic Lesion in the Mouse Hippocampus

Akt (also known as protein kinase B, PKB) has been seen to play a role in astrocyte activation of neuroprotection; however, the underlying mechanism on deregulation of Akt signaling in brain injuries is not fully understood. We investigated the role of carboxy-terminal modulator protein (CTMP), an endogenous Akt inhibitor, in brain injury following kainic acid (KA)-induced neurodegeneration of mouse hippocampus. In control mice, there was a weak signal for CTMP in the hippocampus, but CTMP was markedly increased in the astrocytes 3 days after KA treatment. To further investigate the effectiveness of Akt signaling, the phosphorylation of CTMP was examined. KA treatment induced an increased p-CTMP expression in the astrocytes of hippocampus at 1 day. LPS/IFN-γ-treatment on primary astrocytes promoted the p-CTMP was followed by phosphorylation of Akt and finally upregulation of CTMP and p-CREB. Time-dependent expression of p-CTMP, p-Akt, p-CREB, and CTMP indicate that LPS/IFN-γ-induced phosphorylation of CTMP can activate Akt/CREB signaling, whereas lately emerging enhancement of CTMP can inhibit it. These results suggest that elevation of CTMP in the astrocytes may suppress Akt activity and ultimately negatively affect the outcome of astrocyte activation (astroglisiois). Early time point enhancers of phosphorylation of CTMP and/or late time inhibitors specifically targeting CTMP may be beneficial in astrocyte activation for neuroprotection within treatment in neuroinflammatory conditions.

Insulin-induced Effects on the Subcellular Localization of AKT1, AKT2 and AS160 in Rat Skeletal Muscle

AKT1 and AKT2, the AKT isoforms that are highly expressed in skeletal muscle, have distinct and overlapping functions, with AKT2 more important for insulin-stimulated glucose metabolism. In adipocytes, AKT2 versus AKT1 has greater susceptibility for insulin-mediated redistribution from cytosolic to membrane localization, and insulin also causes subcellular redistribution of AKT Substrate of 160 kDa (AS160), an AKT2 substrate and crucial mediator of insulin-stimulated glucose transport. Although skeletal muscle is the major tissue for insulin-mediated glucose disposal, little is known about AKT1, AKT2 or AS160 subcellular localization in skeletal muscle. The major aim of this study was to determine insulin’s effects on the subcellular localization and phosphorylation of AKT1, AKT2 and AS160 in skeletal muscle. Rat skeletal muscles were incubated ex vivo ± insulin, and differential centrifugation was used to isolate cytosolic and membrane fractions. The results revealed that: 1) insulin increased muscle membrane localization of AKT2, but not AKT1; 2) insulin increased AKT2 phosphorylation in the cytosol and membrane fractions; 3) insulin increased AS160 localization to the cytosol and membranes; and 4) insulin increased AS160 phosphorylation in the cytosol, but not membranes. These results demonstrate distinctive insulin effects on the subcellular redistribution of AKT2 and its substrate AS160 in skeletal muscle.

Novel method demonstrates differential ligand activation and phosphatase-mediated deactivation of insulin receptor tyrosine-specific phosphorylation

Insulin receptor signaling is a complex cascade leading to a multitude of intracellular functional responses. Three natural ligands, insulin, IGF1 and IGF2, are each capable of binding with different affinities to the insulin receptor, and result in variable biological responses. However, it is likely these affinity differences alone cannot completely explain the myriad of diverse cellular outcomes. Ligand binding initiates activation of a signaling cascade resulting in phosphorylation of the IR itself and other intracellular proteins. The direct catalytic activity along with the temporally coordinated assembly of signaling proteins is critical for insulin receptor signaling. We hypothesized that determining differential phosphorylation among individual tyrosine sites activated by ligand binding or dephosphorylation by phosphatases could provide valuable insight into insulin receptor signaling. Here, we present a sensitive, novel immunoassay adapted from Meso Scale Discovery technology to quantitatively measure changes in site-specific phosphorylation levels on endogenous insulin receptors from HuH7 cells. We identified insulin receptor phosphorylation patterns generated upon differential ligand activation and phosphatase-mediated deactivation. The data demonstrate that insulin, IGF1 and IGF2 elicit different insulin receptor phosphorylation kinetics and potencies that translate to downstream signaling. Furthermore, we show that insulin receptor deactivation, regulated by tyrosine phosphatases, occurs distinctively across specific tyrosine residues. In summary, we present a novel, quantitative and high-throughput assay that has uncovered differential ligand activation and site-specific deactivation of the insulin receptor. These results may help elucidate some of the insulin signaling mechanisms, discriminate ligand activity and contribute to a better understanding of insulin receptor signaling. We propose this methodology as a powerful approach to characterize agonists and antagonists of the insulin receptor and can be adapted to serve as a platform to evaluate ligands of alternate receptor systems.

CX3CR1-Mediated Akt1 Activation Contributes to the Paclitaxel-Induced Painful Peripheral Neuropathy in Rats

Painful peripheral neuropathy is a serious dose-limiting side effect of paclitaxel therapy, which unfortunately often happens during the optimal clinical management of chemotherapy in cancer patients. Currently the underlying mechanisms of the painful peripheral neuropathy remain largely unknown. Here, we found that paclitaxel treatment (3 × 8 mg/kg, cumulative dose 24 mg/kg) upregulated the expression of CX3CR1 and phosphorylated Akt1 in DRG and spinal dorsal horn. Blocking of Akt1 pathway activation with different inhibitor (MK-2206 or LY294002) significantly attenuated mechanical allodynia and thermal hyperalgesia induced by paclitaxel. Furthermore, inhibition of CX3CR1 by using neutralizing antibody not only prevented Akt1 activation in DRG and spinal dorsal horn but also alleviated pain-related behavior induced by paclitaxel treatment. This study suggested that CX3CR1/Akt1 signaling pathway may be a potential target for prevention and reversion of the painful peripheral neuropathy induced by paclitaxel.

Regulation of PtdIns(3,4,5)P3/Akt signalling by inositol polyphosphate 5-phosphatases

The phosphoinositide 3-kinase (PI3K) generated lipid signals, PtdIns(3,4,5)P3 and PtdIns(3,4)P2, are both required for the maximal activation of the serine/threonine kinase proto-oncogene Akt. The inositol polyphosphate 5-phosphatases (5-phosphatases) hydrolyse the 5-position phosphate from the inositol head group of PtdIns(3,4,5)P3 to yield PtdIns(3,4)P2. Extensive work has revealed several 5-phosphatases inhibit PI3K-driven Akt signalling, by decreasing PtdIns(3,4,5)P3 despite increasing cellular levels of PtdIns(3,4)P2. The roles that 5-phosphatases play in suppressing cell proliferation and transformation are slow to emerge; however, the 5-phosphatase PIPP [proline-rich inositol polyphosphate 5-phosphatase; inositol polyphosphate 5-phosphatase (INPP5J)] has recently been identified as a putative tumour suppressor in melanoma and breast cancer and SHIP1 [SH2 (Src homology 2)-containing inositol phosphatase 1] inhibits haematopoietic cell proliferation. INPP5E regulates cilia stability and INPP5E mutations have been implicated ciliopathy syndromes. This review will examine 5-phosphatase regulation of PI3K/Akt signalling, focussing on the role PtdIns(3,4,5)P3 5-phosphatases play in developmental diseases and cancer.

G-Protein-Coupled Lysophosphatidic Acid Receptors and Their Regulation of AKT Signaling

A hallmark of G-protein-coupled receptors (GPCRs) is their ability to recognize and respond to chemically diverse ligands. Lysophospholipids constitute a relatively recent addition to these ligands and carry out their biological functions by activating G-proteins coupled to a large family of cell-surface receptors. This review aims to highlight salient features of cell signaling by one class of these receptors, known as lysophosphatidic acid (LPA) receptors, in the context of phosphatidylinositol 3-kinase (PI3K)-AKT pathway activation. LPA moieties efficiently activate AKT phosphorylation and activation in a multitude of cell types. The interplay between LPA, its receptors, the associated Gαi/o subunits, PI3K and AKT contributes to the regulation of cell survival, migration, proliferation and confers chemotherapy-resistance in certain cancers. However, detailed information on the regulation of PI3K-AKT signals induced by LPA receptors is missing from the literature. Here, some urgent issues for investigation are highlighted.

Akt2-Dependent Phosphorylation of Radixin in Regulation of Mrp-2 Trafficking in WIF-B Cells

BACKGROUND

The dominant ezrin/radixin/moesin protein in hepatocytes is radixin, which plays an important role in mediating the binding of F-actin to the plasma membrane after a conformational activation by phosphorylation at Thr564.

AIM

Here we have investigated the importance of Akt-mediated radixin Thr564 phosphorylation on Mrp-2 distribution and function in WIF-B cells. Mrp-2 is an adenosine triphosphate (ATP)-binding cassette transporter that plays an important role in detoxification and chemoprotection by transporting a wide range of compounds, especially conjugates of lipophilic substances with glutathione, organic anions, and drug metabolites such as glucuronides.

METHODS

Akt1 and Akt2 expression were manipulated using dominant active and negative constructs as well as Akt1 and Akt2 siRNA. Cellular distribution of radixin and Mrp-2 was visualized by fluorescence microscopy. A 5-chloromethylfluorescein diacetate, which is a substrate of the Mrp-2 and is actively transported in canalicular lumina, was used to measure Mrp-2 function.

RESULTS

Radixin phosphorylation was significantly increased in wild-type and dominant active Akt2 transfected cells. Furthermore, radixin and Mrp-2 were localized at the canalicular membrane, similar to control cells. In contrast, overexpression of dominant negative Akt2, siRNA knockdown of Akt2 and a specific Akt inhibitor prevented radixin phosphorylation and led to alteration of normal radixin and Mrp-2 localization; inhibition of Akt2, but not Akt1 function led to radixin localization to the cytoplasmic space. In addition, dominant negative and Akt2 knockdown led to a dramatically impaired hepatocyte secretory response, while wild-type and dominant active Akt2 transfected cells exhibited increased 5-chloromethylfluorescein diacetate excretion. In contrast to Akt2, Akt1 was not associated with radixin phosphorylation.

CONCLUSIONS

These studies, therefore, identify Akt2 as a critical kinase that regulates radixin phosphorylation and leads to Mrp-2 translocation and function.

Gene targets of mouse miR-709: regulation of distinct pools

MicroRNA (miRNA) are short non-coding RNA molecules that regulate multiple cellular processes, including development, cell differentiation, proliferation and death. Nevertheless, little is known on whether miRNA control the same gene networks in different tissues. miR-709 is an abundant miRNA expressed ubiquitously. Through transcriptome analysis, we have identified targets of miR-709 in hepatocytes. miR-709 represses genes implicated in cytoskeleton organization, extracellular matrix attachment, and fatty acid metabolism. Remarkably, none of the previously identified targets in non-hepatic tissues are silenced by miR-709 in hepatocytes, even though several of these genes are abundantly expressed in liver. In addition, miR-709 is upregulated in hepatocellular carcinoma, suggesting it participates in the genetic reprogramming that takes place during cell division, when cytoskeleton remodeling requires substantial changes in gene expression. In summary, the present study shows that miR-709 does not repress the same pool of genes in separate cell types. These results underscore the need for validating gene targets in every tissue a miRNA is expressed.

Retinoid X Receptor-selective Signaling in the Regulation of Akt/Protein Kinase B Isoform-specific Expression

The differentiation and fusion of myoblasts into mature myotubes are complex processes responding to multiple signaling pathways. The function of Akt/PKB is critical for myogenesis, but less is clear as to the regulation of its isoform-specific expression. Bexarotene is a drug already used clinically to treat cancer, and it has the ability to enhance the commitment of embryonic stem cells into skeletal muscle lineage. Whereas bexarotene regulates fundamental biological processes through retinoid X receptor (RXR)-mediated gene expression, molecular pathways underlying its positive effects on myogenesis remain unclear. In this study, we have examined the signaling pathways that transmit bexarotene action in the context of myoblast differentiation. We show that bexarotene promotes myoblast differentiation and fusion through the activation of RXR and the regulation of Akt/PKB isoform-specific expression. Interestingly, bexarotene signaling appears to correlate with residue-specific histone acetylation and is able to counteract the detrimental effects of cachectic factors on myogenic differentiation. We also signify an isoform-specific role for Akt/PKB in RXR-selective signaling to promote and to retain myoblast differentiation. Taken together, our findings establish the viability of applying bexarotene in the prevention and treatment of muscle-wasting disorders, particularly given the lack of drugs that promote myogenic differentiation available for potential clinical applications. Furthermore, the model of bexarotene-enhanced myogenic differentiation will provide an important avenue to identify additional genetic targets and specific molecular interactions that we can study and apply for the development of potential therapeutics in muscle regeneration and repair.

Molecular and biochemical trajectories from diabetes to Alzheimer’s disease: A critical appraisal

Diabetes mellitus (DM), a metabolic disorder is a major orchestra influencing brain and behavioral responses via direct or indirect mechanisms. Many lines of evidence suggest that diabetic patients apparently face severe brain complications, but the story is far from being fully understood. Type 2 diabetes, an ever increasing epidemic and its chronic brain complications are implicated in the development of Alzheimer’s disease (AD). Evidences from clinical and experimental studies suggest that insulin draws a clear trajectory from the peripheral system to the central nervous system. This review is a spot light on striking pathological, biochemical, molecular and behavioral commonalities of AD and DM. Incidence of cognitive decline in diabetic patients and diabetic symptoms in AD patients has brought the concept of brain diabetes to attention. Brain diabetes reflects insulin resistant brain state with oxidative stress, cognitive impairment, activation of various inflammatory cascade and mitochondrial vulnerability as a shared footprint of AD and DM. It has become extremely important for the investigators to understand the patho-physiology of brain complications in diabetes and put intensive pursuits for therapeutic interventions. Although, decades of research have yielded a range of molecules with potential beneficial effects, but they are yet to meet the expectations.

Impaired regeneration: A role for the muscle microenvironment in cancer cachexia

While changes in muscle protein synthesis and degradation have long been known to contribute to muscle wasting, a body of literature has arisen which suggests that regulation of the satellite cell and its ensuing regenerative program are impaired in atrophied muscle. Lessons learned from cancer cachexia suggest that this regulation is simply not a consequence, but a contributing factor to the wasting process. In addition to satellite cells, evidence from mouse models of cancer cachexia also suggests that non-satellite progenitor cells from the muscle microenvironment are also involved. This chapter in the series reviews the evidence of dysfunctional muscle repair in multiple wasting conditions. Potential mechanisms for this dysfunctional regeneration are discussed, particularly in the context of cancer cachexia.

Molecular Basis for the Regulation of Transcriptional Coactivator p300 in Myogenic Differentiation

Skeletal myogenesis is a highly ordered process which specifically depends on the function of transcriptional coactivator p300. Previous studies have established that Akt/protein kinase B (PKB), a positive regulator of p300 in proliferating cells, is also important for proper skeletal muscle development. Nevertheless, it is not clear as to how the p300 is regulated by myogenic signaling events given that both p300 and Akt are involved in many cellular processes. Our studies revealed that the levels of p300 protein are temporally maintained in ligand-enhanced skeletal myocyte development. Interestingly, this maintenance of p300 protein is observed at the stage of myoblast differentiation, which coincides with an increase in Akt phosphorylation. Moreover, regulation of p300 during myoblast differentiation appears to be mediated by Akt signaling. Blunting of p300 impairs myogenic expression and myoblast differentiation. Thus, our data suggests a particular role for Akt in myoblast differentiation through interaction with p300. Our studies also establish the potential of exploiting p300 regulation and Akt activation to decipher the complex signaling cascades involved in skeletal muscle development.

Specific and redundant roles of PKBα/AKT1 and PKBβ/AKT2 in human pancreatic islets

Protein kinase Bα (PKBα)/AKT1 and PKBβ/AKT2 are required for normal peripheral insulin action but their role in pancreatic β cells remains enigmatic as indicated by the relatively mild islet phenotype of mice with deficiency for either one of these two isoforms. In this study we have analysed proliferation, apoptosis, β cell size and glucose-stimulated insulin secretion in human islets overexpressing either PKBα or PKBβ. Our results reveal redundant and specific functions. Overexpression of either isoform resulted in increased β cell size, but insulin production and secretion remained unchanged. Proliferation and apoptosis of β cells were only significantly stimulated and inhibited, respectively, by PKBα/AKT1. Importantly, overexpression of PKBα/AKT1 in dissociated islets increased the ratio of β cells to non-β cells. These results confirm our previous findings obtained with rodent islets and strongly indicate that PKBα/AKT1 can regulate β cell mass also in human islets.

Fatty acid transport and transporters in muscle are critically regulated by Akt2

Muscle contains various fatty acid transporters (CD36, FABPpm, FATP1, FATP4). Physiological stimuli (insulin, contraction) induce the translocation of all four transporters to the sarcolemma to enhance fatty acid uptake similarly to glucose uptake stimulation via glucose transporter-4 (GLUT4) translocation. Akt2 mediates insulin-induced, but not contraction-induced, GLUT4 translocation, but its role in muscle fatty acid transporter translocation is unknown. In muscle from Akt2-knockout mice, we observed that Akt2 is critically involved in both insulin-induced and contraction-induced fatty acid transport and translocation of fatty acid translocase/CD36 (CD36) and FATP1, but not of translocation of fatty acid-binding protein (FABPpm) and FATP4. Instead, Akt2 mediates intracellular retention of both latter transporters. Collectively, our observations reveal novel complexities in signaling mechanisms regulating the translocation of fatty acid transporters in muscle.

miR-150 Deficiency Protects against FAS-Induced Acute Liver Injury in Mice through Regulation of AKT

Although miR-150 is implicated in the regulation of immune cell differentiation and activation, it remains unknown whether miR-150 is involved in liver biology and disease. This study was performed to explore the potential role of miR-150 in LPS/D-GalN and Fas-induced liver injuries by using wild type and miR-150 knockout (KO) mice. Whereas knockout of miR-150 did not significantly alter LPS/D-GalN-induced animal death and liver injury, it protected against Fas-induced liver injury and mortality. The Jo2-induced increase in serum transaminases, apoptotic hepatocytes, PARP cleavage, as well as caspase-3/7, caspase-8, and caspase-9 activities were significantly attenuated in miR-150 KO mice. The liver tissues from Jo2-treated miR-150 KO mice expressed higher levels of Akt1, Akt2, total Akt, as well as p-Akt(Ser473) compared to the wild type livers. Pretreatment with the Akt inhibitor V reversed Jo2-induced liver injury in miR-150 KO mice. The primary hepatocytes isolated from miR-150 KO mice also showed protection against Fas-induced apoptosis in vitro (characterized by less prominent PARP cleavage, less nuclear fragmentation and less caspase activation) in comparison to hepatocytes from wild type mice. Luciferase reporter assays in hepatocytes transfected with the Akt1 or Akt2 3’-UTR reporter constructs (with or without mutation of miR-150 binding site) established Akt1 and Akt2 as direct targets of miR-150. Tail vein injection of lentiviral particles containing pre-miR-150 enhanced Jo2-induced liver injury in miR-150 KO mice. These findings demonstrate that miR-150 deficiency prevents Fas-induced hepatocyte apoptosis and liver injury through regulation of the Akt pathway.

Signalling specificity in the Akt pathway in breast cancer

Aberrant activation of fundamental cellular processes, such as proliferation, migration and survival, underlies the development of numerous human pathophysiologies, including cancer. One of the most frequently hyperactivated pathways in cancer is the phosphoinositide 3-kinase (PI3K)/Akt signalling cascade. Three isoforms of the serine/threonine protein kinase Akt (Akt1, Akt2 and Akt3) function to regulate cell survival, growth, proliferation and metabolism. Strikingly, non-redundant and even opposing functions of Akt isoforms in the regulation of phenotypes associated with malignancy in humans have been described. However, the mechanisms by which Akt isoform-specificity is conferred are largely unknown. In the present review, we highlight recent findings that have contributed to our understanding of the complexity of Akt isoform-specific signalling and discussed potential mechanisms by which this isoform-specificity is conferred. An understanding of the mechanisms of Akt isoform-specificity has important implications for the development of isoform-specific Akt inhibitors and will be critical to finding novel targets to treat disease.

MicroRNA-124a is hyperexpressed in type 2 diabetic human pancreatic islets and negatively regulates insulin secretion

AIMS

MicroRNAs are a class of negative regulators of gene expression, which have been shown to be involved in the development of endocrine pancreas and in the regulation of insulin secretion. Since type 2 diabetes (T2D) is characterized by beta cell dysfunction, we aimed at evaluating expression levels of miR-124a and miR-375, both involved in the control of beta cell function, in human pancreatic islets obtained from T2D and from age-matched non-diabetic organ donors.

METHODS

We analyzed miR-124a and miR-375 expression by real-time qRT-PCR in human pancreatic islets and evaluated the potential role of miR-124a by overexpressing or silencing such miRNA in MIN6 pseudoislets.

RESULTS

We identified a major miR-124a hyperexpression in T2D human pancreatic islets with no differential expression of miR-375. Of note, miR-124a overexpression in MIN6 pseudoislets resulted in an impaired glucose-induced insulin secretion. In addition, miR-124a silencing in MIN6 pseudoislets resulted in increased expression of predicted target genes (Mtpn, Foxa2, Flot2, Akt3, Sirt1 and NeuroD1) involved in beta cell function. For Mtpn and Foxa2, we further demonstrated the actual binding of miR-124a to their 3UTR sequences by luciferase assay.

CONCLUSIONS

We uncovered a major hyperexpression of miR-124a in T2D islets, whose silencing resulted in increased expression of target genes of major importance for beta cell function and whose overexpression impaired glucose-stimulated insulin secretion, leading to the hypothesis that an altered miR-124a expression may contribute to beta cell dysfunction in type 2 diabetes.

The role of mouse Akt2 in insulin-dependent suppression of adipocyte lipolysis in vivo

AIM/HYPOTHESIS

The release of fatty acids from adipocytes, i.e. lipolysis, is maintained under tight control, primarily by the opposing actions of catecholamines and insulin. A widely accepted model is that insulin antagonises catecholamine-dependent lipolysis through phosphorylation and activation of cAMP phosphodiesterase 3B (PDE3B) by the serine-threonine protein kinase Akt (protein kinase B). Recently, this hypothesis has been challenged, as in cultured adipocytes insulin appears, under some conditions, to suppress lipolysis independently of Akt.

METHODS

To address the requirement for Akt2, the predominant isoform expressed in classic insulin target tissues, in the suppression of fatty acid release in vivo, we assessed lipolysis in mice lacking Akt2.

RESULTS

In the fed state and following an oral glucose challenge, Akt2 null mice were glucose intolerant and hyperinsulinaemic, but nonetheless exhibited normal serum NEFA and glycerol levels, suggestive of normal suppression of lipolysis. Furthermore, insulin partially inhibited lipolysis in Akt2 null mice during an insulin tolerance test (ITT) and hyperinsulinaemic-euglycaemic clamp, respectively. In support of these in vivo observations, insulin antagonised catecholamine-induced lipolysis in primary brown fat adipocytes from Akt2-deficient mice.

CONCLUSIONS/INTERPRETATION

These data suggest that suppression of lipolysis by insulin in hyperinsulinaemic states can take place in the absence of Akt2.

Reduced hepatic lipid content in Pten-haplodeficient mice because of enhanced AKT2/PKBβ activation in skeletal muscle

BACKGROUND & AIMS

Non-alcoholic fatty liver disease (NAFLD) is a major health problem and occurs frequently in the context of metabolic syndrome and type 2 diabetes mellitus. Hepatocyte-specific Pten-deficiency in mice was shown previously to result in hepatic steatosis due to hyperactivated AKT2. However, the role of peripheral insulin-sensitive tissues on PTEN- and AKT2-dependent accumulation of hepatic lipids has not been addressed.

METHODS

Effects of systemically perturbed PTEN/AKT2 signalling on hepatic lipid content were studied in Pten-haplodeficient (Pten(+/-) /Akt2(+/+) ) mice and Pten-haplodeficient mice lacking Akt2 (Pten(+/-) /Akt2(-/-) ). The liver and skeletal muscle were characterized by histology and/or analysis of insulin signalling. To assess the effects of AKT2 activity in skeletal muscle on hepatic lipid content, AKT2 mutants were expressed in skeletal muscle of Pten(+/+) /Akt2(+/+) and Pten(+/-) /Akt2(+/+) mice using adeno-associated virus 8.

RESULTS

Pten(+/-) /Akt2(+/+) mice were found to have a more than 2-fold reduction in hepatic lipid content, at a level similar to that observed in Pten(+/-) /Akt2(-/-) mice. Insulin signalling in the livers of Pten(+/-) /Akt2(+/+) mice was enhanced, indicating that extrahepatic factors prevent lipid accumulation. The skeletal muscle of Pten(+/-) /Akt2(+/+) mice also showed enhanced insulin signalling. Skeletal muscle-specific expression of constitutively active AKT2 reduced hepatic lipid content in Pten(+/+) /Akt2(+/+) mice, and dominant negative AKT2 led to an increase in accumulation of hepatic lipids in both Pten(+/+) /Akt2(+/+) and Pten(+/-) /Akt2(+/+) mice.

CONCLUSION

Our results demonstrate that AKT2 activity in skeletal muscle critically affects lipid accumulation in the livers of Pten(+/+) /Akt2(+/+) and Pten(+/-) /Akt2(+/+) mice, and emphasize the role of skeletal muscle in the pathology of NAFLD.

Novel individual metabolic profile characterizes the protein kinase B-alpha (pkbα-/-) in vivo model

CONTEXT

Type 2 diabetes and associated co-morbidities run epidemic waves worldwide. Since pathophysiological constellations are individual and display a wide spread of dysmetabolic profiles personalized health care assessments start to emerge. Therefore, we established a specific in silico assessment tool targeting metabolic characterizations individually.

MATERIALS AND METHODS

Values obtained from oral glucose and intraperitoneal insulin tolerance tests performed on pkbα(-/-) mice (KO) as well as age- and gender-matched controls (WT) were analysed using our established in silico model.

RESULTS

Generally, male pkbα(-/-) mice (KO) presented significantly increased insulin sensitivity at an age of 6 months compared with age-matched WTs (p = 0.036). Female KO and WT groups displayed improved glucose sensitivities compared with age-matched males (for WT p ≤ 0.011).

DISCUSSION AND CONCLUSION

Specific metabolic characterization should be assessed individually. Therefore, our in silico model enables novel insights inaugurating specific primary preventive strategies targeting individual metabolic profiling precisely.

Defect of insulin signal in peripheral tissues: Important role of ceramide

In healthy people, balance between glucose production and its utilization is precisely controlled. When circulating glucose reaches a critical threshold level, pancreatic β cells secrete insulin that has two major actions: to lower circulating glucose levels by facilitating its uptake mainly into skeletal muscle while inhibiting its production by the liver. Interestingly, dietary triglycerides are the main source of fatty acids to fulfill energy needs of oxidative tissues. Normally, the unconsumed fraction of excess of fatty acids is stored in lipid droplets that are localized in adipocytes to provide energy during fasting periods. Thus, adipose tissue acts as a trap for fatty acid excess liberated from plasma triglycerides. When the buffering action of adipose tissue to store fatty acids is impaired, fatty acids that build up in other tissues are metabolized as sphingolipid derivatives such as ceramides. Several studies suggest that ceramides are among the most active lipid second messengers to inhibit the insulin signaling pathway and this review describes the major role played by ceramide accumulation in the development of insulin resistance of peripherals tissues through the targeting of specific proteins of the insulin signaling pathway.

Role of AKT signaling in DNA repair and clinical response to cancer therapy

Effective cancer treatment has been limited by the emergence of resistant cancer cells. The results of many studies indicate that AKT activation plays an important role in the acquisition of resistance to anticancer therapy. AKT is a critical effector serine/threonine kinase in the receptor tyrosine kinase/phosphatase and tensin homolog/phospho-inositide 3-kinase pathway and controls a myriad of cellular functions. Activation of AKT not only supports tumor growth and progression but also contributes to tumor-cell evasion of the cytotoxic effects of cancer therapy through many avenues including the promotion of anti-apoptosis, proliferation, and migration and regulation of the cell cycle. Accumulating evidence has implicated AKT as a direct participant in the DNA damage response and repair induced by commonly used genotoxic agents. In this review, we discuss the molecular mechanisms by which genotoxic agents activate AKT and therefore contribute to resistance to cancer therapeutics, with particular emphasis on DNA repair.

Obese and lean porcine difference of FoxO1 and its regulation through C/EBPβ and PI3K/GSK3β signaling pathway

Forkhead box O 1 (FoxO1) is an important transcription factor implicated in adipogenesis. In this study, we detected the breed differences in FoxO1 between Bamei pigs (an obese breed) and Large White pigs (a lean breed). Compared with Large White pigs, the BW of Bamei pigs was lower (P < 0.01), but back fat thickness, fat percent, and intramuscular fat content were greater (P < 0.01). The levels of FoxO1 mRNA and protein were lower (P < 0.01) in subcutaneous adipose tissue (SAT) of Bamei pigs at 180 d, adipocytes and stromal-vascular fraction extracted from SAT of Bamei pigs at 1 d compared with Large White pigs. Knockdown of FoxO1 increased triglyceride content (P < 0.01) and upregulated the levels of adipocyte fatty-acid binding protein, PPARγ, and CCAAT enhancer-binding protein α (C/EBPα) at 6 d after porcine preadipocytes were induced. Furthermore, the transcriptional regulation of FoxO1 through C/EBPβ during early porcine preadipocyte differentiation and the effect of insulin on phosphoinositide 3 kinase (PI3K)/glycogen synthase kinase 3β (GSK3β) signal pathway by FoxO1 were examined. The results indicated that FoxO1 inhibited transcription activity of C/EBPβ, whereas C/EBPβ did not affect transcription activity of FoxO1. At 6 and 12 h of early differentiation, knockdown of FoxO1 triggered the transcription activity of C/EBPβ. In addition, FoxO1 protein interacted with C/EBPβ protein in porcine adipocytes at 12 h after induction. Under treatment with 100 nM insulin, knockdown or overexpression of FoxO1 mediated PI3K/GSK3β signaling via upregulating or downregulating the levels of GSK3β and its phosphorylation in adipocytes. Taken together, there is low, but detectable, expression of FoxO1 in SAT of obese pigs and FoxO1 inhibited adipogenesis through C/EBPβ and PI3K/GSK3β signaling pathway. These findings provide useful information to further the understanding of the function of FoxO1 in porcine adipogenesis.

Application of the [γ-32P] ATP kinase assay to study anabolic signaling in human skeletal muscle

AMPK (AMP-dependant protein kinase)-mTORC1 (mechanistic target of rapamycin in complex 1)-p70S6K1 (ribosomal protein S6 kinase 1 of 70 kDa) signaling plays a crucial role in muscle protein synthesis (MPS). Understanding this pathway has been advanced by the application of the Western blot (WB) technique. However, because many components of the mTORC1 pathway undergo numerous, multisite posttranslational modifications, solely studying the phosphorylation changes of mTORC1 and its substrates may not adequately represent the true metabolic signaling processes. The aim of this study was to develop and apply a quantitative in vitro [γ-(32)P] ATP kinase assay (KA) for p70S6K1 to assess kinase activity in human skeletal muscle to resistance exercise (RE) and protein feeding. In an initial series of experiments the assay was validated in tissue culture and in p70S6K1-knockout tissues. Following these experiments, the methodology was applied to assess p70S6K1 signaling responses to a physiologically relevant stimulus. Six men performed unilateral RE followed by the consumption of 20 g of protein. Muscle biopsies were obtained at pre-RE, and 1 and 3 h post-RE. In response to RE and protein consumption, p70S6K1 activity as assessed by the KA was significantly increased from pre-RE at 1 and 3 h post-RE. However, phosphorylated p70S6K1(thr389) was not significantly elevated. AMPK activity was suppressed from pre-RE at 3 h post-RE, whereas phosphorylated ACC(ser79) was unchanged. Total protein kinase B activity also was unchanged after RE from pre-RE levels. Of the other markers we assessed by WB, 4EBP1(thr37/46) phosphorylation was the only significant responder, being elevated at 3 h post-RE from pre-RE. These data highlight the utility of the KA to study skeletal muscle plasticity.

Insulin receptor signaling in normal and insulin-resistant states

In the wake of the worldwide increase in type-2 diabetes, a major focus of research is understanding the signaling pathways impacting this disease. Insulin signaling regulates glucose, lipid, and energy homeostasis, predominantly via action on liver, skeletal muscle, and adipose tissue. Precise modulation of this pathway is vital for adaption as the individual moves from the fed to the fasted state. The positive and negative modulators acting on different steps of the signaling pathway, as well as the diversity of protein isoform interaction, ensure a proper and coordinated biological response to insulin in different tissues. Whereas genetic mutations are causes of rare and severe insulin resistance, obesity can lead to insulin resistance through a variety of mechanisms. Understanding these pathways is essential for development of new drugs to treat diabetes, metabolic syndrome, and their complications.

The role of the dysfunctional akt-related pathway in cancer: establishment and maintenance of a malignant cell phenotype, resistance to therapy, and future strategies for drug development

Akt serine/threonine kinases, or PKB, are key players in the regulation of a wide variety of cellular activities, such as growth, proliferation, protection from apoptotic injuries, control of DNA damage responses and genome stability, metabolism, migration, and angiogenesis. The Akt-related pathway responds to the stimulation mediated by growth factors, cytokines, hormones, and several nutrients. Akt is present in three isoforms: Akt1, Akt2, and Akt3, which may be alternatively named PKB α , PKB β , and PKB γ , respectively. The Akt isoforms are encoded on three diverse chromosomes and their biological functions are predominantly distinct. Deregulations in the Akt-related pathway were observed in many human maladies, including cancer, cardiopathies, neurological diseases, and type-2 diabetes. This review discusses the significance of the abnormal activities of the Akt axis in promoting and sustaining malignancies, along with the development of tumor cell populations that exhibit enhanced resistance to chemo- and/or radiotherapy. This occurrence may be responsible for the relapse of the disease, which is unfortunately very often related to fatal consequences in patients.

Astrocytic phosphorylation of PDK1 on Tyr9 following an excitotoxic lesion in the mouse hippocampus

3-phosphoinositide-dependent kinase-1 (PDK1) is suggested to play important roles in the regulation of synaptic plasticity and neuronal cell survival in the mature CNS. Although few studies have investigated the roles of PDK1, little is known about PDK1 changes in glial cells under neuropathological conditions. In current report, phosphorylation of PDK1 was monitored specially on tyrosine residues, following the induction of an excitotoxic lesion in rat brain by using kainic acid administration. In injured hippocampal CA3 region, Tyr9 phosphorylation of PDK1 was increased from 4h until 3 day post-injection. Double immunohistochemistry further evaluated that these phosphorylated forms of PDK1 were localized in astrocytes not other cells. Overexpression of unphosphorylatable mutant, PDK1-Y9F leads to inhibit Protein kinase B (PKB/Akt) activation and cAMP responsive element binding protein (CREB) phosphorylation. In conclusion, our results suggested for the first time that tyrosine phosphorylation of PDK1 is required for PKB and CREB activation in KA-mediated excitotoxic lesion in mouse brain.

Sirt1 inhibits akt2-mediated porcine adipogenesis potentially by direct protein-protein interaction

Compared with the rodent, the domestic pig is a much better animal model for studying adipogenesis and obesity-related diseases. Currently, the role of Akt2 and Sirt1 in porcine adipogenesis remains elusive. In this study, we defined the effect of Akt2 and Sirt1 on porcine preadipocyte lipogenesis and the regulatory mechanism. First, we found that Akt2 was widely expressed in porcine various tissues and at high level in adipose tissue. Further analysis showed that the expression level of Akt2 was much higher in adipose tissue and adipocytes of the Bamei pig breed (a Chinese indigenous fatty pig) than in that of the Large White pig breed (a Lean type pig), whereas the level of Sirt1 expression was opposite. The expression levels of Sirt1 and Akt2 gradually increased during adipogenic differentiation. Adipogenesis was robustly inhibited in Akt2 deficient fat cells, whereas it was promoted in Sirt1 deficient cells using the lentiviral–mediated shRNA approach. Interestingly, adipogenesis returned to normal in Akt2 and Sirt1 dual–deficient cells, showing that the pro- and anti–adipogenic effects were balanced. Sirt1 inhibited transcriptional activity of Akt2 in a dose-dependent way. Interaction of endogenous Akt2 and Sirt1 was gradually enhanced before day 6 of differentiation, and then attenuated. Akt2 and Sirt1 also interacted with C/EBPα in adipocytes. Moreover, knockdown of Akt2 or/and Sirt1 affected pro–lipogenesis of insulin–stimulated by PI3K/Akt pathway. We further found that Sirt1 respectively interacted with PI3K and GSK3β which were key upstream and downstream components of PI3K/Akt pathway. Based on the above findings, we concluded that the crosstalk between C/EBPα and PI3K/Akt signaling pathways is implicated in Akt2 and Sirt1 regulation of adipogenesis.