Expression of proline-rich Akt-substrate PRAS40 in cell survival pathway and carcinogenesis

Comprehensive Transcriptomic and Proteomic Analyses Identify a Candidate Gene Set in Cross-Resistance for Endocrine Therapy in Breast Cancer

Endocrine therapy (ET) of selective estrogen receptor modulators (SERMs), selective estrogen receptor downregulators (SERDs), and aromatase inhibitors (AIs) has been used as the gold standard treatment for hormone-receptor-positive (HR+) breast cancer. Despite its clinical benefits, approximately 30% of patients develop ET resistance, which remains a major clinical challenge in patients with HR+ breast cancer. The mechanisms of ET resistance mainly focus on mutations in the ER and related pathways; however, other targets still exist from ligand-independent ER reactivation. Moreover, mutations in the ER that confer resistance to SERMs or AIs seldom appear in SERDs. To date, little research has been conducted to identify a critical target that appears in both SERMs/SERDs and AIs. In this study, we conducted comprehensive transcriptomic and proteomic analyses from two cohorts of The Cancer Genome Atlas Breast Invasive Carcinoma (TCGA-BRCA) to identify the critical targets for both SERMs/SERDs and AIs of ET resistance. From a treatment response cohort with treatment response for the initial ET regimen and an endocrine therapy cohort with survival outcomes, we identified candidate gene sets that appeared in both SERMs/SERDs and AIs of ET resistance. The candidate gene sets successfully differentiated progress/resistant groups (PD) from complete response groups (CR) and were significantly correlated with survival outcomes in both cohorts. In summary, this study provides valuable clinical implications for the critical roles played by candidate gene sets in the diagnosis, mechanism, and therapeutic strategy for both SERMs/SERDs and AIs of ET resistance for the future.

Hsp90-mediated regulation of DYRK3 couples stress granule disassembly and growth via mTORC1 signaling

Stress granules (SGs) are dynamic condensates associated with protein misfolding diseases. They sequester stalled mRNAs and signaling factors, such as the mTORC1 subunit raptor, suggesting that SGs coordinate cell growth during and after stress. However, the molecular mechanisms linking SG dynamics and signaling remain undefined. We report that the chaperone Hsp90 is required for SG dissolution. Hsp90 binds and stabilizes the dual‐specificity tyrosine‐phosphorylation‐regulated kinase 3 (DYRK3) in the cytosol. Upon Hsp90 inhibition, DYRK3 dissociates from Hsp90 and becomes inactive. Inactive DYRK3 is subjected to two different fates: it either partitions into SGs, where it is protected from irreversible aggregation, or it is degraded. In the presence of Hsp90, DYRK3 is active and promotes SG disassembly, restoring mTORC1 signaling and translation. Thus, Hsp90 links stress adaptation and cell growth by regulating the activity of a key kinase involved in condensate disassembly and translation restoration.

Identification of Novel Serological Autoantibodies in Takayasu Arteritis Patients Using HuProt Arrays

To identify novel autoantibodies of Takayasu arteritis (TAK) using HuProt array-based approach, a two-phase approach was adopted. In Phase I, serum samples collected from 40 TAK patients, 15 autoimmune disease patients, and 20 healthy subjects were screened to identify TAK-specific autoantibodies using human protein (HuProt) arrays. In phase II, the identified candidate autoantibodies were validated with TAK-focused arrays using an additional cohort comprised of 109 TAK patients, 110 autoimmune disease patients, and 96 healthy subjects. Subsequently, the TAK-specific autoantibodies validated in phase II were further confirmed using western blot analysis. We identified and validated eight autoantibodies as potential TAK-specific diagnostic biomarkers, including anti-SPATA7, -QDPR, -SLC25A2, -PRH2, -DIXDC1, -IL17RB, -ZFAND4, and -NOLC1 antibodies, with AUC of 0.803, 0.801, 0.780, 0.696, 0.695, 0.678, 0.635, and 0.613, respectively. SPATA7 could distinguish TAK from healthy and disease controls with 73.4% sensitivity at 85.4% specificity, while QDPR showed 71.6% sensitivity at 86.4% specificity. SLC25A22 showed the highest sensitivity of 80.7%, but at lower specificity of 67.0%. In addition, PRH2, IL17RB, and NOLC1 showed good specificities of 88.3%, 85.9%, and 86.9%, respectively, but at lower sensitivities (<50%). Finally, DIXDC1 and ZFAND4 showed moderate performance as compared with the other autoantibodies. Using a decision tree model, we could reach a specificity of 94.2% with AUC of 0.843, a significantly improved performance as compared with that by each individual biomarker. The performances of three autoantibodies, namely anti-SPATA7, -QDPR, and -PRH2, were successfully confirmed with western blot analysis. Using this two-phase strategy, we identified and validated eight novel autoantibodies as TAK-specific biomarker candidates, three of which could be readily adopted in a clinical setting.

Molecular dynamic study on PTEN frameshift mutations in breast cancer provide c2 domain as a potential biomarker

PTEN is a tumour suppressor gene known for regulating apoptosis, cell growth, and many other pathways. It is one of the most frequently mutated genes comprising the phosphatase domain (PD) and C terminal domain (C2). Direct therapeutic methods are not applicable for targeting PTEN because once gets mutated, it needs restoration. For mutant detection and restoration using PTEN mRNA there is a need to explore various mutations taking place in PTEN, identify their particular domains, and study their interactions within the cellular system. Here, we have tried to highlight a few such regions in the mutated PTEN of breast cancer patients. In this study, we have selected the top-most-occurring PTEN mutation in breast cancer and compared them to determine the specific properties of each mutation and its effect on functionality. Molecular dynamic simulation for 50 ns was performed on five structures to compare the structural behaviour of mutated PTEN in the system. Our finding suggests that frameshift mutations are more damaging and affect the c2 domain. Frameshift mutant fs_ACTT is the highest occurring as well as the most damaging mutation in all the compared structures. Docking study shows that substitution mutations D92H and R130Q causes loss of binding ability towards PIP2 in normal PTEN, interfering the dephosphorylation process. Overall, the C2 domain is more frequently mutated, and the amino acid residues in the C2 domain show more fluctuations compared to the other regions. Our study can provide the basis for selecting frequently mutated C2 domain as a potential therapeutic marker.Communicated by Ramaswamy H. Sarma.

M2-polarized tumor-associated macrophages promote epithelial-mesenchymal transition via activation of the AKT3/PRAS40 signaling pathway in intrahepatic cholangiocarcinoma

Tumor-associated macrophages (TAMs) have been considered as a major component of the tumor microenvironment. However, the crosstalk between M2-polarized tumor-associated macrophages (M2-TAMs) and intrahepatic cholangiocarcinoma (ICC) remains undetermined. In the present study, we aimed to clarify the role of M2-TAMs in ICC and the underlying mechanism. The in vitro assay demonstrated M2-TAMs promoted epithelial-mesenchymal transition (EMT) of ICC cells, resulting in enhanced cell invasion and metastasis ability. Moreover, M2-TAMs modulated the microenvironment of ICC by increasing the secretion of cytokines (GM-CSF, tumor necrosis factor-α [TNF-α], ICAM-1, interleukin-6 [IL-6], etc) and chemokines (CCL1, CCL3, etc). In addition, p-AKT (Ser473) and p-PRAS40 (Thr246) were upregulated in ICC cells when cocultured with M2-TAMs or treated with M2-TAMs secreted core cytokines (GM-CSF, TNF-α, ICAM-1, and IL-6). Consistently, AKT3 silencing (but not AKT1 silencing and AKT2 silencing) markedly inhibited phosphorylation of AKT and PRAS40 of ICC cells and inhibited the EMT process when cocultured with M2-TAMs. Taken together, the current data indicated that M2-TAMs promoted ICC cells EMT, partially through increasing secretion of cytokines and chemokines, thus modulating the microenvironment and activating the AKT3/PRAS40 signaling pathway.

Emerging Role of mTOR Signaling-Related miRNAs in Cardiovascular Diseases

Mechanistic/mammalian target of rapamycin (mTOR), an atypical serine/threonine kinase of the phosphoinositide 3-kinase- (PI3K-) related kinase family, elicits a vital role in diverse cellular processes, including cellular growth, proliferation, survival, protein synthesis, autophagy, and metabolism. In the cardiovascular system, the mTOR signaling pathway integrates both intracellular and extracellular signals and serves as a central regulator of both physiological and pathological processes. MicroRNAs (miRs), a class of short noncoding RNA, are an emerging intricate posttranscriptional modulator of critical gene expression for the development and maintenance of homeostasis across a wide array of tissues, including the cardiovascular system. Over the last decade, numerous studies have revealed an interplay between miRNAs and the mTOR signaling circuit in the different cardiovascular pathophysiology, like myocardial infarction, hypertrophy, fibrosis, heart failure, arrhythmia, inflammation, and atherosclerosis. In this review, we provide a comprehensive state of the current knowledge regarding the mechanisms of interactions between the mTOR signaling pathway and miRs. We have also highlighted the latest advances on mTOR-targeted therapy in clinical trials and the new perspective therapeutic strategies with mTOR-targeting miRs in cardiovascular diseases.

Overexpression of Na+/Mg2+ exchanger SLC41A1 attenuates pro-survival signaling

The Na⁺/Mg²⁺ exchanger SLC41A1 (A1), a key component of intracellular Mg homeostasis (IMH), is the major cellular Mg²⁺ efflux system, and its overexpression decreases [Mg²⁺]_{intracellular}. IMH plays an important role in the regulation of many cellular processes, including cellular signaling. However, whether the overexpression of A1 and the consequent drop of [Mg²⁺]_i impact on intracellular signaling is unknown. To examine the latter, we utilized dynamic mass redistribution (DMR) assay, PathScan^® RTK signaling antibody (PRSA) array, confirmatory Western blot (WB) analyses of phosphorylation of kinases selected by PRSA, and mag-fura 2-assisted fast filter spectrometry (FFS). We demonstrate here that the overexpression of A1 quantitatively and qualitatively changes the DMR signal evoked by the application of PAR-1-selective activating peptide and/or by changing [Mg²⁺]_{extracellular} in HEK293 cells. PRSA profiling of the phosphorylation of important signaling nodes followed by confirmatory WB has revealed that, in HEK293 cells, A1 overexpression significantly attenuates the phosphorylation of Akt/PKB on Thr³⁰⁸ and/or Ser⁴⁷³ and of Erk1/2 on Thr²⁰²/Tyr²⁰⁴ in the presence of 0 or 1 mM (physiological) Mg²⁺ in the bath solution. The latter is also true for SH-SY5Y and HeLa cells. Overexpression of A1 in HEK293 cells significantly lowers [Mg²⁺]_i in the presence of [Mg²⁺]_e = 0 or 1 mM. This correlates with the observed attenuation of prosurvival Akt/PKB - Erk1/2 signaling in these cells. Thus, A1 expression status and [Mg²⁺]_e (and consequently also [Mg²⁺]_i) modulate the complex physiological fingerprint of the cell and influence the activity of kinases involved in anti-apoptotic and, hence, pro-survival events in cells.

PRAS40 Connects Microenvironmental Stress Signaling to Exosome-Mediated Secretion

Secreted exosomes carrying lipids, proteins, and nucleic acids conduct cell-cell communications within the microenvironment of both physiological and pathological conditions. Exosome secretion is triggered by extracellular or intracellular stress signals. Little is known, however, about the signal transduction between stress cues and exosome secretion. To identify the linker protein, we took advantage of a unique finding in human keratinocytes. In these cells, although transforming growth factor alpha (TGF-α) and epidermal growth factor (EGF) share the same EGF receptor and previously indistinguishable intracellular signaling networks, only TGF-α stimulation causes exosome-mediated secretion. However, deduction of EGF-activated pathways from TGFα-activated pathways in the same cells allowed us to identify the proline-rich Akt substrate of 40 kDa (PRAS40) as the unique downstream effector of TGF-α but not EGF signaling via threonine 308-phosphorylated Akt. PRAS40 knockdown (KD) or PRAS40 dominant-negative (DN) mutant overexpression blocks not only TGF-α- but also hypoxia- and H₂O₂-induced exosome secretion in a variety of normal and tumor cells. Site-directed mutagenesis and gene rescue studies show that Akt-mediated activation of PRAS40 via threonine 246 phosphorylation is both necessary and sufficient to cause exosome secretion without affecting the endoplasmic reticulum/Golgi pathway. Identification of PRAS40 as a linker protein paves the way for understanding how stress regulates exosome secretion under pathophysiological conditions.

Proteomic profile of maternal-aged blastocoel fluid suggests a novel role for ubiquitin system in blastocyst quality

PURPOSE

The etiology of maternal aging, a common cause of female factor infertility and a rate-limiting step in vitro fertilization (IVF) success, remains still unclear. Proteomic changes responsible for the impaired successful pregnancy outcome after IVF with aged blastocysts have not been yet evaluated. The objective of this prospective study was to employ proteomic techniques and bioinformatic tools to enlight differences at the protein level in blastocoel fluid of aged and younger woman.

METHODS

Protein composition of human blastocoel fluid isolated by micromanipulation from 46 blastocysts of women aged <37 years (group A) and 29 of women aged ≥37 years (group B) have been identified by a shotgun proteomic approach based on high-resolution nano-liquid chromatography electrospray-ionization-tandem mass spectrometry (nLC-ESI-MS/MS) using label free for the relative quantification of their expression levels.

RESULTS

The proteomic analysis leads to the identification and quantification of 148 proteins; 132 and 116 proteins were identified in groups A and B, respectively. Interestingly, the identified proteins are mainly involved in processes aimed at fine tuning embryo implantation and development. Among the 100 proteins commonly expressed in both groups, 17 proteins are upregulated and 44 downregulated in group B compared to group A. Overall, the analysis identified 33 proteins, which were increased or present only in B while 76 were decreased in B or present only in A.

CONCLUSIONS

Data revealed that maternal aging mainly affects blastocyst survival and implantation through unbalancing the equilibrium of the ubiquitin system known to play a crucial role in fine-tuning several aspects required to ensure successful pregnancy outcome.

Tat-PRAS40 prevent hippocampal HT-22 cell death and oxidative stress induced animal brain ischemic insults

Proline rich Akt substrate (PRAS40) is a component of mammalian target of rapamycin complex 1 (mTORC1) and is known to play an important role against reactive oxygen species-induced cell death. However, the precise function of PRAS40 in ischemia remains unclear. Thus, we investigated whether Tat-PRAS40, a cell-permeable fusion protein, has a protective function against oxidative stress-induced hippocampal neuronal (HT-22) cell death in an animal model of ischemia. We showed that Tat-PRAS40 transduced into HT-22 cells, and significantly protected against cell death by reducing the levels of H2O2 and derived reactive species, and DNA fragmentation as well as via the regulation of Bcl-2, Bax, and caspase 3 expression levels in H2O2 treated cells. Also, we showed that transduced Tat-PARS40 protein markedly increased phosphorylated RRAS40 expression levels and 14-3-3σ complex via the Akt signaling pathway. In an animal ischemia model, Tat-PRAS40 effectively transduced into the hippocampus in animal brain and significantly protected against neuronal cell death in the CA1 region. We showed that Tat-PRAS40 protein effectively transduced into hippocampal neuronal cells and markedly protected against neuronal cell damage. Therefore, we suggest that Tat-PRAS40 protein may be used as a therapeutic protein for ischemia and oxidative stress-induced brain disorders.

MYO6 knockdown inhibits the growth and induces the apoptosis of prostate cancer cells by decreasing the phosphorylation of ERK1/2 and PRAS40

Prostate cancer is the second most frequently diagnosed cancer among males around the world. Myosin VI (MYO6), as a motor protein, has been reported to be implicated in cancer-related cell migration and cellular functions. To investigate the role of MYO6 in prostate cancer, immunohistochemical analysis was firstly applied to prostate cancer tissues and revealed that MYO6 was closely related with the Gleason score in prostate cancer. Then we used specific short hairpin RNA (shRNA) to downregulate MYO6 expression in DU145 and PC-3 cells and found that decreased MYO6 expression significantly suppressed cell proliferation, as determined by MTT and colony formation assays. Flow cytometry confirmed that the suppression of MYO6 promoted cell cycle arrest at the G2/M and sub-G1 phase in the DU145 cells. Furthermore, PathScan intracellular signaling array analysis demonstrated that the phosphorylation of ERK1/2 and PRAS40 was downregulated in the DU145 cells following MYO6 knockdown. Knockdown of MYO6 downregulated the expression of AKT3 and upregulated the expression of PARP, as confirmed by western blot analysis. These results suggest that MYO6 plays an essential role in the progression of prostate cancer and silencing of MYO6 may be a promising therapeutic approach for prostate cancer.

Inhibition of Phospho-S6 Kinase, a Protein Involved in the Compensatory Adaptive Response, Increases the Efficacy of Paclitaxel in Reducing the Viability of Matrix-Attached Ovarian Cancer Cells

OBJECTIVE

To identify the proteins involved the compensatory adaptive response to paclitaxel in ovarian cancer cells and to determine whether inhibition of the compensatory adaptive response increases the efficacy of paclitaxel in decreasing the viability of cancer cells.

METHODS

We used a reverse-phase protein array and western blot analysis to identify the proteins involved in the compensatory mechanism induced by paclitaxel in HeyA8 and SKOV3 ovarian cancer cells. We used a cell viability assay to examine whether inhibition of the proteins involved in the compensatory adaptive response influenced the effects of paclitaxel on cancer cell viability. All experiments were performed in three-dimensional cell cultures.

RESULTS

Paclitaxel induced the upregulation of pS6 (S240/S244) and pS6 (S235/S236) in HeyA8 and SKOV3 cells, and pPRAS40 (T246) in HeyA8 cells. BX795 and CCT128930 were chosen as inhibitors of pS6 (S240/S244), pS6 (S235/S236), and pPRAS40 (T246). BX795 and CCT128930 decreased pS6 (S240/S244) and pS6 (S235/S236) expression in HeyA8 and SKOV3 cells. However, pPRAS40 (T246) expression was inhibited only by BX795 and not by CCT128930 in HeyA8 cells. Compared with paclitaxel alone, addition of BX795 or CCT128930 to paclitaxel was more effective in decreasing the viability of HeyA8 and SKOV3 cells.

CONCLUSION

Addition of BX795 or CCT128930 to inhibit pS6 (S240/S244) or pS6 (S235/S236) restricted the compensatory adaptive response to paclitaxel in HeyA8 and SKOV3 cells. These inhibitors increased the efficacy of paclitaxel in reducing cancer cell viability.

Proline-rich AKT substrate of 40-kDa (PRAS40) in the pathophysiology of cancer

Dysregulation of PI3K-AKT-mTOR pathway has been reported in various pathologies, such as cancer and insulin resistance. The proline-rich AKT substrate of 40-kDa (PRAS40), also known as AKT substrate 1 (AKT1S1), lies at the crossroads of these cascades and inhibits the activity of the mTOR complex 1 (mTORC1) kinase. This review discusses the role of PRAS40 and possible feedback mechanisms, and alterations in AKT/PRAS40/mTOR signaling that have been implicated in the pathogenesis of tumor progression. Additionally, we probed new datasets extracted from Oncomine, a cancer microarray database containing datasets derived from patient samples, to further understand the role of PRAS40 (AKT1S1). These data strongly supports the hypothesis that PRAS40 may serve as a potential therapeutic target for various cancers.

Role of PRAS40 in Akt and mTOR signaling in health and disease

The proline-rich Akt substrate of 40 kDa (PRAS40) acts at the intersection of the Akt- and mammalian target of rapamycin (mTOR)-mediated signaling pathways. The protein kinase mTOR is the catalytic subunit of two distinct signaling complexes, mTOR complex 1 (mTORC1) and mTORC2, that link energy and nutrients to the regulation of cellular growth and energy metabolism. Activation of mTOR in response to nutrients and growth factors results in the phosphorylation of numerous substrates, including the phosphorylations of S6 kinase by mTORC1 and Akt by mTORC2. Alterations in Akt and mTOR activity have been linked to the progression of multiple diseases such as cancer and type 2 diabetes. Although PRAS40 was first reported as substrate for Akt, investigations toward mTOR-binding partners subsequently identified PRAS40 as both component and substrate of mTORC1. Phosphorylation of PRAS40 by Akt and by mTORC1 itself results in dissociation of PRAS40 from mTORC1 and may relieve an inhibitory constraint on mTORC1 activity. Adding to the complexity is that gene silencing studies indicate that PRAS40 is also necessary for the activity of the mTORC1 complex. This review summarizes the regulation and potential function(s) of PRAS40 in the complex Akt- and mTOR-signaling network in health and disease.

Mammalian target of rapamycin and the kidney. I. The signaling pathway

The mammalian target of rapamycin (mTOR) is a serine/threonine kinase that plays a fundamental role in regulating cellular homeostasis and metabolism. In a two-part review, we examine the complex molecular events involved in the regulation and downstream effects of mTOR, as well as the pivotal role played by this kinase in many renal diseases, particularly acute kidney injury, diabetic nephropathy, and polycystic kidney diseases. Here, in the first part of the review, we provide an overview of the complex signaling events and pathways governing mTOR activity and action. mTOR is a key component of two multiprotein complexes, known as mTOR complex 1 (mTORC1) and 2 (mTORC2). Some proteins are found in both mTORC1 and mTORC2, while others are unique to one or the other complex. Activation of mTORC1 promotes cell growth (increased cellular mass or size) and cell proliferation (increased cell number). mTORC1 acts as a metabolic "sensor," ensuring that conditions are optimal for both cell growth and proliferation. Its activity is tightly regulated by the availability of amino acids, growth factors, energy stores, and oxygen. The effects of mTORC2 activation are distinct from those of mTORC1. Cellular processes modulated by mTORC2 include cell survival, cell polarity, cytoskeletal organization, and activity of the aldosterone-sensitive sodium channel. Upstream events controlling mTORC2 activity are less well understood than those controlling mTORC1, although growth factors appear to stimulate both complexes. Rapamycin and its analogs inhibit the activity of mTORC1 only, and not that of mTORC2, while the newer "catalytic" mTOR inhibitors affect both complexes.

The functions of mTOR in ischemic diseases

Mammalian Target of Rapamycin (mTOR) is a serine/threonine kinase and that forms two multiprotein complexes known as the mTOR complex 1 (mTORC1) and mTOR complex 2 (mTORC2). mTOR regulates cell growth, proliferation and survival. mTORC1 is composed of the mTOR catalytic subunit and three associated proteins: raptor, mLST8/GβL and PRAS40. mTORC2 contains mTOR, rictor, mLST8/GβL, mSin1, and protor. Here, we discuss mTOR as a promising anti-ischemic agent. It is believed that mTORC2 lies down-stream of Akt and acts as a direct activator of Akt. The different functions of mTOR can be explained by the existence of two distinct mTOR complexes containing unique interacting proteins. The loss of TSC2, which is upstream of mTOR, activates S6K1, promotes cell growth and survival, activates mTOR kinase activities, inhibits mTORC1 and mTORC2 via mTOR inhibitors, and suppresses S6K1 and Akt. Although mTOR signaling pathways are often activated in human diseases, such as cancer, mTOR signaling pathways are deactivated in ischemic diseases. From Drosophila to humans, mTOR is necessary for Ser473 phosphorylation of Akt, and the regulation of Akt-mTOR signaling pathways may have a potential role in ischemic disease. This review evaluates the potential functions of mTOR in ischemic diseases. A novel mTOR-interacting protein deregulates over-expression in ischemic disease, representing a new mechanism for controlling mTOR signaling pathways and potential therapeutic strategies for ischemic diseases.

PIM1-activated PRAS40 regulates radioresistance in non-small cell lung cancer cells through interplay with FOXO3a, 14-3-3 and protein phosphatases

Resistance of cancer cells to ionizing radiation plays an important role in the clinical setting of lung cancer treatment. To date, however, the exact molecular mechanism of radiosensitivity has not been well explained. In this study, we compared radioresistance in two types of non-small cell lung cancer (NSCLC) cells, NCI-H460 and A549, and investigated the signaling pathways that confer radioresistance. In radioresistant cells, exposure to radiation led to overexpression of PIM1 and reduction of protein phosphatases (PP2A and PP5), which induced translocation of PIM1 into the nucleus. Increased nuclear PIM1 phosphorylated PRAS40. Consequently, pPRAS40 made a trimeric complex with 14-3-3 and AKT-activated pFOXO3a, which then moved rapidly to the cytoplasm. Cytoplasmic retention of FOXO3a was associated with downregulation of proapoptotic genes and possibly radioresistance. On the other hand, no suppressive effect of radiation on protein phosphatases was detected and, concomitantly, protein phosphatases downregulated PIM1 in radiosensitive cells. In this setting, PIM1-activated pPRAS40, AKT-activated pFOXO3a, and their complex formation with 14-3-3 could be key regulators of the radiation-induced radioresistance in NSCLC cells.

Proline-rich Akt substrate of 40kDa (PRAS40): a novel downstream target of PI3k/Akt signaling pathway

Modifications in signaling of the proline-rich Akt substrate of 40-kDa (PRAS40) pathway is implicated in type 2 diabetes and melanoma. PRAS40 is known for its ability to regulate the mammalian target of rapamycin complex 1 (mTORC1) kinase activity, possessing a key regulatory role at the cross point of signal transduction pathways activated by growth factor receptors. Recently it has been found that PRAS40 is regulated by its upstream phosphatidylinositol 3-kinase/Akt (PI3K/Akt) which is activated by many tyrosine kinase receptors growth factors including insulin-like growth factor 1. Also, PRAS40 functions downstream of mTORC1 and upstream from its effectors ribosomal protein S6 kinase 1 (S6K1) and eukaryotic initiation factor 4E binding protein 1 (4E-BP1). Phosphorylation of PRAS40 by Akt and mTORC1 disrupts the binding between mTORC1 and PRAS40, and relieves the inhibitory constraint of PRAS40 on mTORC1 activity. This review summarizes the signaling regulating PRAS40 phosphorylation, as well as the dual function of PRAS40 as substrate and inhibitor of mTORC1 upon growth factor stimulation and under pathophysiological conditions.

Dual inhibition of phosphatidylinositol 3-kinase/Akt and mammalian target of rapamycin signaling in human nonsmall cell lung cancer cells by a dietary flavonoid fisetin

Lung cancer is one of the most commonly occurring malignancies. It has been reported that mammalian target of rapamycin (mTOR) is phosphorylated in lung cancer and its activation was more frequent in tumors with overexpression of phosphatidylinositol 3-kinase (PI3K)/Akt. Therefore, dual inhibitors of PI3K/Akt and mTOR signaling could be valuable agents for treating lung cancer. In the present study, we show that fisetin, a dietary tetrahydroxyflavone inhibits cell growth with the concomitant suppression of PI3K/Akt and mTOR signaling in human nonsmall cell lung cancer (NSCLC) cells. Using autodock 4, we found that fisetin physically interacts with the mTOR complex at two sites. Fisetin treatment was also found to reduce the formation of A549 cell colonies in a dose-dependent manner. Treatment of cells with fisetin caused decrease in the protein expression of PI3K (p85 and p110), inhibition of phosphorylation of Akt, mTOR, p70S6K1, eIF-4E and 4E-BP1. Fisetin-treated cells also exhibited dose-dependent inhibition of the constituents of mTOR signaling complex such as Rictor, Raptor, GβL and PRAS40. There was an increase in the phosphorylation of AMPKα and a decrease in the phosphorylation of TSC2 on treatment of cells with fisetin. We also found that treatment of cells with mTOR inhibitor rapamycin and mTOR-siRNA caused decrease in phosphorylation of mTOR and its target proteins which were further downregulated on treatment with fisetin, suggesting that these effects are mediated in part, through mTOR signaling. Our results show that fisetin suppressed PI3K/Akt and mTOR signaling in NSCLC cells and thus, could be developed as a chemotherapeutic agent against human lung cancer.

Therapeutic Implications of Targeting AKT Signaling in Melanoma

Identification of key enzymes regulating melanoma progression and drug resistance has the potential to lead to the development of novel, more effective targeted agents for inhibiting this deadly form of skin cancer. The Akt3, also known as protein kinase B gamma, pathway enzymes regulate diverse cellular processes including proliferation, survival, and invasion thereby promoting the development of melanoma. Accumulating preclinical evidence demonstrates that therapeutic agents targeting these kinases alone or in combination with other pathway members could be effective for the long-term treatment of advanced-stage disease. However, currently, no selective and effective therapeutic agent targeting these kinases has been identified for clinical use. This paper provides an overview of the key enzymes of the PI3K pathway with emphasis placed on Akt3 and the negative regulator of this kinase called PTEN (phosphatase and tensin homolog deleted on chromosome 10). Mechanisms regulating these enzymes, their substrates and therapeutic implications of targeting these proteins to treat melanoma are also discussed. Finally, key issues that remain to be answered and future directions for interested researchers pertaining to this signaling cascade are highlighted.

PRAS40 acts as a nodal regulator of high glucose-induced TORC1 activation in glomerular mesangial cell hypertrophy

Diabetic nephropathy manifests aberrant activation of TORC1, which senses key signals to modulate protein synthesis and renal hypertrophy. PRAS40 has recently been identified as a raptor-interacting protein and is a component and a constitutive inhibitor of TORC1. The mechanism by which high glucose stimulates TORC1 activity is not known. PRAS40 was identified in the mesangial cells in renal glomeruli and in tubulointerstitium of rat kidney. Streptozotocin-induced diabetic renal hypertrophy was associated with phosphorylation of PRAS40 in the cortex and glomeruli. In vitro, high glucose concentration increased PRAS40 phosphorylation in a PI 3 kinase- and Akt-dependent manner, resulting in dissociation of raptor-PRAS40 complex in mesangial cells. High glucose augmented the inactivating and activating phosphorylation of 4EBP-1 and S6 kinase, respectively, with concomitant induction of protein synthesis and hypertrophy. Expression of TORC1-nonphosphorylatable mutant of 4EBP-1 and dominant-negative S6 kinase significantly inhibited high glucose-induced protein synthesis and hypertrophy. PRAS40 knockdown mimicked the effect of high glucose on phosphorylation of 4EBP-1 and S6 kinase, protein synthesis, and hypertrophy. To elucidate the role of PRAS40 phosphorylation, we used phosphorylation-deficient mutant of PRAS40, which in contrast to PRAS40 knockdown inhibited phosphorylation of 4EBP-1 and S6 kinase, leading to reduced mesangial cell hypertrophy. Thus, our data identify high glucose-induced phosphorylation and inactivation of PRAS40 as a central node for mesangial cell hypertrophy in diabetic nephropathy.

Alcohol and PRAS40 knockdown decrease mTOR activity and protein synthesis via AMPK signaling and changes in mTORC1 interaction

The mTORC1 protein kinase complex consists of mTOR, raptor, mLST8/GbetaL and PRAS40. Previously, we reported that mTOR plays an important role in regulating protein synthesis in response to alcohol (EtOH). However, the mechanisms by which EtOH regulates mTORC1 activity have not been established. Here, we investigated the effect of EtOH on the phosphorylation and interaction of components of mTORC1 in C2C12 myocytes. We also examined the specific role that PRAS40 plays in this process. Incubation of myocytes with EtOH (100 mM, 24 h) increased raptor and PRAS40 phosphorylation. Likewise, there were increased levels of the PRAS40 upstream regulators Akt and IRS-1. EtOH also caused changes in mTORC1 protein-protein interactions. EtOH enhanced the binding of raptor and PRAS40 with mTOR. These alterations occurred in concert with increased binding of 14-3-3 to raptor, while the PRAS40 and 14-3-3 interaction was not affected. The shRNA knockdown (KD) of PRAS40 decreased protein synthesis similarly to EtOH. PRAS40 KD increased raptor phosphorylation and its association with 14-3-3, whereas decreased GbetaL-mTOR binding. The effects of EtOH and PRAS40 KD were mediated by AMPK. Both factors increased in vitro AMPK activity towards the substrate raptor. In addition, KD enhanced the activity of AMPK towards TSC2. Collectively, our results indicate that EtOH stabilizes the association of raptor, PRAS40, and GbetaL with mTOR, while likewise increasing the interaction of raptor with 14-3-3. These data suggest a possible mechanism for the inhibitory effects of EtOH on mTOR kinase activity and protein synthesis in myocytes.

The regulation of protein synthesis in cancer

Translational control of cancer is a multifaceted process, involving alterations in translation factor levels and activities that are unique to the different types of cancers and the different stages of disease. Translational alterations in cancer include adaptations of the tumor itself, of the tumor microenvironment, an integral component in disease, and adaptations that occur as cancer progresses from development to local disease and ultimately to metastatic disease. Adaptations include the overexpression and increased activity of specific translation factors, the physical or functional loss of translation regulatory components, increased production of ribosomes, selective mRNA translation, and alteration of signal transduction pathways to permit unfettered activation of protein synthesis. There is intense clinical interest to capitalize on the emerging new understanding of translational control in cancer by targeting specific components of the translation apparatus that are altered in disease for the development of specific cancer therapeutics. Clinical trial data are nascent but encouraging, suggesting that translational control constitutes an important new area for drug development in human cancer.

PRAS40: target or modulator of mTORC1 signalling and insulin action?

Alterations in signalling via protein kinase B (PKB/Akt) and the mammalian target of rapamycin (mTOR) frequently occur in type 2 diabetes and various human malignancies. Proline-rich Akt substrate of 40-kDa (PRAS40) has a regulatory function at the intersection of these pathways. The interaction of PRAS40 with the mTOR complex 1 (mTORC1) inhibits the activity of mTORC1. Phosphorylation of PRAS40 by PKB/Akt and mTORC1 disrupts the binding between mTORC1 and PRAS40, and relieves the inhibitory constraint of PRAS40 on mTORC1 activity. This review summarizes the signalling pathways regulating PRAS40 phosphorylation, as well as the dual function of PRAS40 as substrate and inhibitor of mTORC1 in the physiological situation, and under pathological conditions, such as insulin resistance and cancer.

The PTEN-AKT3 signaling cascade as a therapeutic target in melanoma

Melanocytes undergo extensive genetic changes during transformation into aggressive melanomas. These changes deregulate genes whose aberrant activity promotes the development of this disease. The phosphoinositide-3-kinase (PI3K) and mitogen-activated protein (MAP) kinase pathways are two key signaling cascades that have been found to play prominent roles in melanoma development. These pathways relay extra-cellular signals via an ordered series of consecutive phosphorylation events from cell surface throughout the cytoplasm and nucleus regulating diverse cellular processes including proliferation, survival, invasion and angiogenesis. It is generally accepted that therapeutic agents would need to target these two pathways to be an effective therapy for the long-term treatment of advanced-stage melanoma patients. This review provides an overview of the PI3 kinase pathway focusing specifically on two members of the pathway, called PTEN and Akt3, which play important roles in melanoma development. Mechanisms leading to deregulation of these two proteins and therapeutic implications of targeting this signaling cascade to treat melanoma are detailed in this review.

mTOR complex 2 is required for the development of prostate cancer induced by Pten loss in mice

mTOR complex 2 (mTORC2) contains the mammalian target of rapamycin (mTOR) kinase and the Rictor regulatory protein and phosphorylates Akt. Whether this function of mTORC2 is critical for cancer progression is unknown. Here, we show that transformed human prostate epithelial cells lacking PTEN require mTORC2 to form tumors when injected into nude mice. Furthermore, we find that Rictor is a haploinsufficient gene and that deleting one copy protects Pten heterozygous mice from prostate cancer. Finally, we show that the development of prostate cancer caused by Pten deletion specifically in prostate epithelium requires mTORC2, but that for normal prostate epithelial cells, mTORC2 activity is nonessential. The selective requirement for mTORC2 in tumor development suggests that mTORC2 inhibitors may be of substantial clinical utility.

Mammalian target of rapamycin inhibition as a therapeutic strategy in the management of urologic malignancies

The mammalian target of rapamycin (mTOR) is a protein kinase that regulates protein translation, cell growth, and apoptosis. Recently, there has been an enormous increase in our understanding on molecular mechanisms underlying the therapeutics of rapamycin in cancer. Alterations in the pathway regulating mTOR occur in many solid malignancies including prostate, bladder, and kidney cancer; in vitro and in vivo models of prostate and bladder cancer have established the importance of the mTOR pathway in control of cancer progression and metastasis. Temsirolimus (Torisel) and everolimus (RAD-001), two ester analogues of rapamycin, as well as rapamycin itself have clear antitumor activity in in vitro and in vivo models and are under clinical trial investigations for prostate and bladder cancer. Phase II and III trials have already established the clinical efficacy of temsirolimus in renal cancer, and current renal trials are evaluating the combined effects of vascular endothelial growth factor and mTOR inhibition. Ongoing studies in prostate and bladder cancer will soon define the activity and safety profiles of everolimus and temsirolimus. Recent molecular advances have uncovered a startling complexity in the macromolecular function of mTOR complexes, with the identification of new mTOR partners (raptor, rictor, FKBP38, PRAS40, and mSIN1), putative cancer therapeutic/prognostic targets for future clinical trials.

Increased expression of a proline-rich Akt substrate (PRAS40) in human copper/zinc-superoxide dismutase transgenic rats protects motor neurons from death after spinal cord injury

The serine-threonine kinase, Akt, plays an important role in the cell survival signaling pathway. A proline-rich Akt substrate, PRAS40, has been characterized, and an increase in phospho-PRAS40 (pPRAS40) is neuroprotective after transient focal cerebral ischemia. However, the involvement of PRAS40 in the cell death/survival pathway after spinal cord injury (SCI) is unclear. Liposome-mediated PRAS40 transfection was performed to study whether overexpression of pPRAS40 is neuroprotective. We further examined the expression of pPRAS40 after SCI by immunohistochemistry and Western blot using copper/zinc-superoxide dismutase (SOD1) transgenic (Tg) rats and wild-type (Wt) littermates. We then examined the relationship between PRAS40 and Akt by injection of LY294002, a phosphatidylinositol 3-kinase (PI3K) pathway inhibitor, or Akt inhibitor IV, a compound that inhibits Akt activation after SCI. Our data demonstrated that increased pPRAS40 resulted in survival of more motor neurons compared with control complementary DNA transfection. Phosphorylated PRAS40 increased in the Wt rats after SCI, whereas there was a greater and prolonged increase in the SOD1 Tg rats. Coimmunoprecipitation showed that binding of pPRAS40 with 14-3-3 increased 1 day after SCI in the Wt rats, whereas there was a significant increase in the Tg rats. The inhibitor studies showed that phospho-Akt and pPRAS40 were decreased after injection of LY294002 or Akt inhibitor IV. We conclude that an increase in pPRAS40 by transfection after SCI results in survival of motor neurons, and overexpression of SOD1 in the Tg rats results in an increase in endogenous pPRAS40 and a decrease in motor neuron death through the PI3K/Akt pathway.

PRAS40 and PRR5-like protein are new mTOR interactors that regulate apoptosis

TOR (Target of Rapamycin) is a highly conserved protein kinase and a central controller of cell growth. TOR is found in two functionally and structurally distinct multiprotein complexes termed TOR complex 1 (TORC1) and TOR complex 2 (TORC2). In the present study, we developed a two-dimensional liquid chromatography tandem mass spectrometry (2D LC-MS/MS) based proteomic strategy to identify new mammalian TOR (mTOR) binding proteins. We report the identification of Proline-rich Akt substrate (PRAS40) and the hypothetical protein Q6MZQ0/FLJ14213/CAE45978 as new mTOR binding proteins. PRAS40 binds mTORC1 via Raptor, and is an mTOR phosphorylation substrate. PRAS40 inhibits mTORC1 autophosphorylation and mTORC1 kinase activity toward eIF-4E binding protein (4E-BP) and PRAS40 itself. HeLa cells in which PRAS40 was knocked down were protected against induction of apoptosis by TNFα and cycloheximide. Rapamycin failed to mimic the pro-apoptotic effect of PRAS40, suggesting that PRAS40 mediates apoptosis independently of its inhibitory effect on mTORC1. Q6MZQ0 is structurally similar to proline rich protein 5 (PRR5) and was therefore named PRR5-Like (PRR5L). PRR5L binds specifically to mTORC2, via Rictor and/or SIN1. Unlike other mTORC2 members, PRR5L is not required for mTORC2 integrity or kinase activity, but dissociates from mTORC2 upon knock down of tuberous sclerosis complex 1 (TSC1) and TSC2. Hyperactivation of mTOR by TSC1/2 knock down enhanced apoptosis whereas PRR5L knock down reduced apoptosis. PRR5L knock down reduced apoptosis also in mTORC2 deficient cells. The above suggests that mTORC2-dissociated PRR5L may promote apoptosis when mTOR is hyperactive. Thus, PRAS40 and PRR5L are novel mTOR-associated proteins that control the balance between cell growth and cell death.

Defining the role of mTOR in cancer

The mammalian target of rapamycin (mTOR) has emerged as a critical effector in cell-signaling pathways commonly deregulated in human cancers. This has led to the prediction that mTOR inhibitors may be useful in oncology, and derivatives of one such molecule, rapamycin (from which mTOR derives its name), are currently in clinical development. In this review, we discuss recent progress in understanding mTOR signaling, paying particular attention to its relevance in cancer. We further discuss the use of rapamycin in oncology and conclude with a discussion on the future of mTOR-targeted therapy.

The proline-rich Akt substrate of 40 kDa (PRAS40) is a physiological substrate of mammalian target of rapamycin complex 1

The proline-rich Akt substrate of 40 kilodaltons (PRAS40) was identified as a raptor-binding protein that is phosphorylated directly by mammalian target of rapamycin (mTOR) complex 1 (mTORC1) but not mTORC2 in vitro, predominantly at PRAS40 (Ser(183)). The binding of S6K1 and 4E-BP1 to raptor requires a TOR signaling (TOS) motif, which contains an essential Phe followed by four alternating acidic and small hydrophobic amino acids. PRAS40 binding to raptor was severely inhibited by mutation of PRAS40 (Phe(129) to Ala). Immediately carboxyl-terminal to Phe(129) are two small hydrophobic amino acid followed by two acidic residues. PRAS40 binding to raptor was also abolished by mutation of the major mTORC1 phosphorylation site, Ser(183), to Asp. PRAS40 (Ser(183)) was phosphorylated in intact cells; this phosphorylation was inhibited by rapamycin, by 2-deoxyglucose, and by overexpression of the tuberous sclerosis complex heterodimer. PRAS40 (Ser(183)) phosphorylation was also inhibited reversibly by withdrawal of all or of only the branched chain amino acids; this inhibition was reversed by overexpression of the Rheb GTPase. Overexpressed PRAS40 suppressed the phosphorylation of S6K1 and 4E-BP1 at their rapamycin-sensitive phosphorylation sites, and reciprocally, overexpression of S6K1 or 4E-BP1 suppressed phosphorylation of PRAS40 (Ser(183)) and its binding to raptor. RNA interference-induced depletion of PRAS40 enhanced the amino acid-stimulated phosphorylation of both S6K1 and 4E-BP1. These results establish PRAS40 as a physiological mTORC1 substrate that contains a variant TOS motif. Moreover, they indicate that the ability of raptor to bind endogenous substrates is limiting for the activity of mTORC1 in vivo and is therefore a potential locus of regulation.

PRAS40 is an insulin-regulated inhibitor of the mTORC1 protein kinase

The heterotrimeric mTORC1 protein kinase nucleates a signaling network that promotes cell growth in response to insulin and becomes constitutively active in cells missing the TSC1 or TSC2 tumor suppressors. Insulin stimulates the phosphorylation of S6K1, an mTORC1 substrate, but it is not known how mTORC1 kinase activity is regulated. We identify PRAS40 as a raptor-interacting protein that binds to mTORC1 in insulin-deprived cells and whose in vitro interaction with mTORC1 is disrupted by high salt concentrations. PRAS40 inhibits cell growth, S6K1 phosphorylation, and rheb-induced activation of the mTORC1 pathway, and in vitro it prevents the great increase in mTORC1 kinase activity induced by rheb1-GTP. Insulin stimulates Akt/PKB-mediated phosphorylation of PRAS40, which prevents its inhibition of mTORC1 in cells and in vitro. We propose that the relative strengths of the rheb- and PRAS40-mediated inputs to mTORC1 set overall pathway activity and that insulin activates mTORC1 through the coordinated regulation of both.

Expression changes in tolerant murine cardiac allografts after gene therapy with a lentiviral vector expressing alpha1,3 galactosyltransferase

Comparison of intragraft gene expression changes in tolerant cardiac allograft models may provide the basis for identifying pathways involved in graft survival. Our laboratory has previously demonstrated that tolerance to the gal alpha1,3 gal epitope, the major target of rejection of wild-type pig hearts in human cardiac transplantation, can be achieved after transplantation with bone marrow transduced with a lentiviral vector expressing alpha1,3 galactosyltransferase. We now present intracardiac gene expression changes associated with long-term tolerance in this model. Biotin-labeled cRNA was hybridized to Affymetrix GeneChip 430 2.0 Mouse Genome Arrays. Data were subjected to functional annotation analysis to identify genes of known function in which expression was increased or decreased by at least 2-fold (t-test, P < .05) in tolerant gal+/+ wild-type hearts as compared to transplanted syngeneic controls. Tolerant hearts demonstrated increased expression of genes associated with the stress response, modulation of immune function and cell survival (HSPa9a, CD56, and Akt1s1), and decreased expression of several immunoregulatory genes (CD209, CD26, and PDE4b). These data suggest that tolerance may be associated with activation of immunomodulatory and survival pathways.

PRAS40 Deregulates Apoptosis in Malignant Melanoma

Malignant melanoma is the most invasive and deadly form of skin cancer with no effective therapy to treat advanced disease, leading to poor survival rates. Akt3 signaling plays an important role in deregulating apoptosis in approximately 70% of melanomas. Thus, targeting Akt3 signaling in melanoma patients has significant therapeutic potential for inhibiting melanomas, but no Akt3-specific chemotherapeutic agent exists. Unfortunately, nonspecific Akt inhibitors can cause systemic toxicity or increase metastasis. Identifying and targeting the Akt3 substrate that deregulates apoptosis might circumvent these complications but would require demonstration of its functional importance in disrupting normal apoptosis. In this study, PRAS40 was identified as an Akt3 substrate that deregulated apoptosis to promote melanoma tumorigenesis. Levels of phosphorylated PRAS40 (pPRAS40) increased during melanoma tumor progression paralleling increasing Akt3 activity. Majority of melanomas from patients with elevated Akt activity also had correspondingly higher levels of pPRAS40. Targeting PRAS40 or upstream Akt3 similarly reduced anchorage-independent growth in culture and inhibited tumor development in mice. Mechanistically, decreased pPRAS40 increased tumor cell apoptosis as well as sensitivity of melanoma cells to apoptosis-inducing agents, thereby decreasing chemoresistance. Collectively, these studies provide a solid mechanistic basis for targeting PRAS40 to inhibit the Akt3 signaling cascade and thereby retard melanoma development.

Insulin signalling to mTOR mediated by the Akt/PKB substrate PRAS40

Insulin stimulates protein synthesis and cell growth by activation of the protein kinases Akt (also known as protein kinase B, PKB) and mammalian target of rapamycin (mTOR). It was reported that Akt activates mTOR by phosphorylation and inhibition of tuberous sclerosis complex 2 (TSC2). However, in recent studies the physiological requirement of Akt phosphorylation of TSC2 for mTOR activation has been questioned. Here, we identify PRAS40 (proline-rich Akt/PKB substrate 40 kDa) as a novel mTOR binding partner that mediates Akt signals to mTOR. PRAS40 binds the mTOR kinase domain and its interaction with mTOR is induced under conditions that inhibit mTOR signalling, such as nutrient or serum deprivation or mitochondrial metabolic inhibition. Binding of PRAS40 inhibits mTOR activity and suppresses constitutive activation of mTOR in cells lacking TSC2. PRAS40 silencing inactivates insulin-receptor substrate-1 (IRS-1) and Akt, and uncouples the response of mTOR to Akt signals. Furthermore, PRAS40 phosphorylation by Akt and association with 14-3-3, a cytosolic anchor protein, are crucial for insulin to stimulate mTOR. These findings identify PRAS40 as an important regulator of insulin sensitivity of the Akt-mTOR pathway and a potential target for the treatment of cancers, insulin resistance and hamartoma syndromes.

Alpha-synuclein activates stress signaling protein kinases in THP-1 cells and microglia

Here we show that alpha-synuclein, a major constituent of Lewy bodies, induces inflammation in human microglial and human THP-1 cells. Secretions from such stimulated THP-1 cells contain increased levels of IL-1beta and TNF-alpha. When stimulated by alpha-synuclein in combination with IFN-gamma, secretions from the cells also become toxic towards SH-SY5Y neuroblastoma cells. The A30P, E46K and A53T alpha-synuclein mutations, which induce Parkinson's disease, are more potent than normal alpha-synuclein in the induction of such cytotoxicity. To investigate the signaling mechanisms evoked, protein phosphorylation profiling was applied. At least 81 target phospho-sites were identified. Large increases were induced in the three major mitogen-activated protein (MAP) kinase pathways: p38 MAP kinase, extracellular regulated protein-serine kinase (ERK)1/2 and c-Jun-N-terminal kinase (JNK). Upregulation occurred within minutes following exposure to alpha-synuclein, which is consistent with a receptor-mediated effect. These findings demonstrate that alpha-synuclein acts as a potent inflammatory stimulator of microglial cells, and that inhibitors of such stimulation might be beneficial in the treatment of Parkinson's disease and other synucleinopathies.

Insulin-mediated phosphorylation of the proline-rich Akt substrate PRAS40 is impaired in insulin target tissues of high-fat diet-fed rats

Clinical insulin resistance is associated with decreased activation of phosphatidylinositol 3'-kinase (PI3K) and its downstream substrate protein kinase B (PKB)/Akt. However, its physiological protein substrates remain poorly characterized. In the present study, the effect of in vivo insulin action on phosphorylation of the PKB/Akt substrate 40 (PRAS40) was examined. In rat and mice, insulin stimulated PRAS40-Thr246 phosphorylation in skeletal and cardiac muscle, the liver, and adipose tissue in vivo. Physiological hyperinsulinemia increased PRAS40-Thr246 phosphorylation in human skeletal muscle biopsies. In cultured cell lines, insulin-mediated PRAS40 phosphorylation was prevented by the PI3K inhibitors wortmannin and LY294002. Immunohistochemical and immunofluorescence studies showed that phosphorylated PRAS40 is predominantly localized to the nucleus. Finally, in rats fed a high-fat diet (HFD), phosphorylation of PRAS40 was markedly reduced compared with low-fat diet-fed animals in all tissues examined. In conclusion, the current study identifies PRAS40 as a physiological target of in vivo insulin action. Phosphorylation of PRAS40 is increased by insulin in human, rat, and mouse insulin target tissues. In rats, this response is reduced under conditions of HFD-induced insulin resistance.