Cell size reduction induced by inhibition of the mTOR/S6K-signaling pathway protects Jurkat cells from apoptosis

PIKfyve-dependent regulation of the Cl- channel ClC-2

The widely expressed chloride channel ClC-2 is stimulated by the serum and glucocorticoid inducible kinase SGK1. The SGK1-dependent regulation of several carriers involves the mammalian phosphatidylinositol-3-phosphate-5-kinase PIKfyve (PIP5K3). The present experiments explored whether SGK1-dependent regulation of ClC-2 similarly involves PIKfyve. The conductance of Xenopus oocytes is increased more than eightfold by ClC-2 expression. In ClC-2-expressing oocytes, but not in water-injected oocytes, the current was further enhanced by coexpression of either, PIKfyve or constitutively active (S422D)SGK1. Coexpression of the inactive SGK1 mutant (K127N)SGK1 did not significantly alter the current in ClC-2-expressing oocytes and abrogated the stimulation of the current by PIKfyve-coexpression. The stimulating effect of PIKfyve was abolished by replacement of the serine with alanine in the SGK1 consensus sequence ((S318A)PIKfyve). Coexpression of (S318A)PIKfyve significantly blunted the stimulating effect of (S422D)SGK1 on ClC-2-activity. In conclusion, PIKfyve is a potent stimulator of ClC-2-activity and contributes to SGK1-dependent regulation of ClC-2.

Rapamycin induces transactivation of the EGFR and increases cell survival

The mammalian target of rapamycin (mTOR) signaling network regulates cell growth, proliferation and cell survival. Deregulated activation of this pathway is a common event in diverse human diseases such as cancers, cardiac hypertrophy, vascular restenosis and nephrotic hypertrophy. Although mTOR inhibitor, rapamycin, has been widely used to inhibit the aberrant signaling due to mTOR activation that plays a major role in hyperproliferative diseases, in some cases rapamycin does not attenuate the cell proliferation and survival. Thus, we studied the mechanism(s) by which cells may confer resistance to rapamycin. Our data show that in a variety of cell types the mTOR inhibitor rapamycin activates extracellularly regulated kinases (Erk1/2) signaling. Rapamycin-mediated activation of the Erk1/2 signaling requires (a) the epidermal growth factor receptor (EGFR), (b) its tyrosine kinase activity and (c) intact autophosphorylation sites on the receptor. Rapamycin treatment increases tyrosine phosphorylation of EGFR without the addition of growth factor and this transactivation of receptor involves activation of c-Src. We also show that rapamycin treatment triggers activation of cell survival signaling pathway by activating the prosurvival kinases Erk1/2 and p90RSK. These studies provide a novel paradigm by which cells escape the apoptotic actions of rapamycin and its derivatives that inhibit the mTOR pathway.

PIKfyve in the SGK1 mediated regulation of the creatine transporter SLC6A8

The Na(+),Cl(-),creatine transporter CreaT (SLC6A8) mediates concentrative cellular uptake of creatine into a wide variety of cells. Previous observations disclosed that SLC6A8 transport activity is enhanced by mammalian target of rapamycin (mTOR) at least partially through the serum and glucocorticoid inducible kinase isoforms SGK1 and SGK3. As SLC6A8 does not contain a putative SGK consensus motif, the mechanism linking SGK1 with SLC6A8 activity remained elusive. A candidate kinase is the mammalian phosphatidylinositol-3-phosphate-5-kinase PIKfyve (PIP5K3), which has previously been shown to regulate the glucose transporter GLUT4. The present experiments explored the possibility that SLC6A8 is regulated by PIKfyve. In Xenopus oocytes expressing SLC6A8 but not in water injected oocytes creatine induced a current which was significantly enhanced by coexpression of PIKfyve. The effect of PIKfyve on SLC6A8 was blunted by additional coexpression of the inactive mutant of the serum and glucocorticoid inducible kinase (K127N)SGK1. The stimulating effect of PIKfyve was abrogated by replacement of the serine in the SGK consensus sequence by alanine ((S318A)PIKfyve). Moreover, coexpression of ( S318A)PIKfyve blunted the effect of SGK1 on SLC6A8 activity. The observations suggest that SGK1 regulates the creatine transporter SLC6A8 at least partially through phosphorylation and activation of PIKfyve and subsequent formation of PI(3,5)P(2).

Regulation of the Na(+), glucose cotransporter by PIKfyve and the serum and glucocorticoid inducible kinase SGK1

The Na(+), glucose cotransporter SGLT1 (SLC5A1) accomplishes Na(+)-dependent concentrative cellular glucose uptake. SGLT1 activity is enhanced by the serum and glucocorticoid inducible kinase SGK1. As shown recently, the stimulating effect of protein kinase B on the glucose carrier GLUT4 involves the mammalian phosphatidylinositol-3-phosphate-5-kinase PIKfyve (PIP5K3). The present experiments thus explored whether PIKfyve is similarly involved in the SGK1-dependent regulation of SLC5A1. In Xenopus oocytes expressing SLC5A1 but not in water injected oocytes glucose induced a current which was significantly enhanced by coexpression of PIKfyve. The effect of PIKfyve on SLC5A1 was blunted by additional coexpression of the inactive mutant of the serum and glucocorticoid inducible kinase (K119N)SGK1 and mimicked by coexpression of constitutively active (S422D)SGK1. The stimulating effect of PIKfyve was abrogated by replacement of the serine in the SGK consensus sequence by alanine ((S138A)PIKfyve). Moreover, coexpression of (S138A)PIKfyve significantly blunted the effect of SGK1 on SLC5A1 activity. The observations disclose that PIKfyve participates in the SGK1-dependent regulation of SLC5A1.

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.

Rapamycin confers preconditioning-like protection against ischemia-reperfusion injury in isolated mouse heart and cardiomyocytes

Rapamycin (sirolimus) is an antibiotic that inhibits protein synthesis through mammalian target of rapamycin (mTOR) signaling and is used as an immunosuppressant in the treatment of organ rejection in transplant recipients. Recently, the antigrowth properties of rapamycin have been utilized for cardiovascular benefit as stents impregnated with rapamycin effectively reduce coronary restenosis. We report here a novel role of this drug in protection against ischemia/reperfusion (I/R) injury. Adult male ICR mice were treated with rapamycin (0.25 mg/kg, IP) or volume-matched DMSO (solvent for rapamycin). The hearts were subjected to 20 min of global ischemia and 30 min of reperfusion in Langendorff mode. The blocker of mitochondrial KATP channel, 5-hydroxydecanoate (5-HD, 100 microM) was given 10 min before ischemia. Infarct size in the DMSO treated group was 28.2 +/- 1.3% and was reduced to 10.1 +/- 2.8% in the rapamycin-treated mice (64% decrease, P < 0.001). 5-HD blocked the protective effect (infarct area 32.2 +/- 1.8%, P < 0.001 vs. rapamycin). The infarct limiting effect of rapamycin was not associated with improved recovery of ventricular function. We further examined the effect of rapamycin in protection against necrosis and apoptosis in adult cardiomyocytes subjected to simulated ischemia and reoxygenation. Myocytes treated with rapamycin in doses from 25-100 nM demonstrated significantly lower trypan blue-positive necrotic cells and TUNEL-positive apoptotic nuclei, supporting the protective role of drug in the intact heart. These data suggest that rapamycin induces potent preconditioning-like effect against myocardial infarction through opening of mitochondrial KATP channels. We propose that rapamycin may be a novel therapeutic strategy to limit infarction, apoptosis, and remodeling following I/R injury in the heart.

Investigating mammalian target of rapamycin inhibitors for their anticancer properties

The mammalian target of rapamycin (mTOR) is a key element of the PI3KAkt (protein kinase B) signalling pathway, responsible for the regulation of cell growth and proliferation. There are two main downstream messengers of the mTOR kinase, eukaryotic initiation factor 4E-binding protein-1 and the 40S ribosomal protein S6 kinase 1, that control translation and cell-cycle progression. Abnormal activation of the mTOR pathway occurs frequently in numerous human malignancies; therefore, mTOR represents an attractive target for anticancer drug development. Rapamycin and its analogues CCI-779, RAD-001 and AP-23573 are known specific inhibitors of the mTOR kinase. Several clinical Phase I/II trials showed their activity in solid tumours and haematological malignancies. Moreover, inhibitors of mTOR were found to synergise with some cytostatics or other biological agents, which seems to be a promising direction for future strategies of antitumour treatment.

mTOR and cancer: insights into a complex relationship

mTOR (mammalian target of rapamycin) has come a long way since its humble beginnings as a kinase of unknown function. As part of the mTORC1 and mTORC2 complexes mTOR has key roles in several pathways that are involved in human cancer, stimulating interest in mTOR inhibitors and placing it on the radar of the pharmaceutical industry. Here, I discuss the rationale for the use of drugs that target mTOR, the unexpectedly complex mechanism of action of existing mTOR inhibitors and the potential benefits of developing drugs that function through different mechanisms. The purpose is not to cover all aspects of mTOR history and signalling, but rather to foster discussion by presenting some occasionally provocative ideas.

Creatine as a compatible osmolyte in muscle cells exposed to hypertonic stress

Exposure of C2C12 muscle cells to hypertonic stress induced an increase in cell content of creatine transporter mRNA and of creatine transport activity, which peaked after about 24 h incubation at 0.45 osmol (kg H(2)O)(-1). This induction of transport activity was prevented by addition of either cycloheximide, to inhibit protein synthesis, or of actinomycin D, to inhibit RNA synthesis. Creatine uptake by these cells is largely Na(+) dependent and kinetic analysis revealed that its increase under hypertonic conditions resulted from an increase in V(max) of the Na(+)-dependent component, with no significant change in the K(m) value of about 75 mumol l(-1). Quantitative real-time PCR revealed a more than threefold increase in the expression of creatine transporter mRNA in cells exposed to hypertonicity. Creatine supplementation significantly enhanced survival of C2C12 cells incubated under hypertonic conditions and its effect was similar to that obtained with the well known compatible osmolytes, betaine, taurine and myo-inositol. This effect seemed not to be linked to the energy status of the C2C12 cells because hypertonic incubation caused a decrease in their ATP content, with or without the addition of creatine at 20 mmol l(-1) to the medium. This induction of creatine transport activity by hypertonicity is not confined to muscle cells: a similar induction was shown in porcine endothelial cells.

Stimulation of the intestinal phosphate transporter SLC34A2 by the protein kinase mTOR

Adequate phosphate homeostasis is of critical importance for a wide variety of functions including bone mineralization and energy metabolism. Phosphate balance is a function of intestinal absorption and renal elimination, which are both under tight hormonal control. Intestinal phosphate absorption is accomplished by the Na(+), phosphate cotransporter NaPi IIb (SLC34A2). Signaling mechanisms mediating hormonal regulation of SLC34A2 are incompletely understood. The mammalian target of rapamycin (mTOR) is a kinase regulating a variety of nutrient transporters. The present experiments explored whether mTOR regulates the activity of SLC34A2. In Xenopus oocytes expressing SLC34A2 but not in water injected oocytes phosphate (1 mM) induced a current (Ip) which was significantly enhanced by coexpression of mTOR. Preincubation of the oocytes for 24 h with rapamycin (50 nM) did not significantly affect Ip in the absence of mTOR but virtually abolished the increase of Ip following coexpression of mTOR. The wild type serum and glucocorticoid inducible kinase SGK1 and the constitutively active (S422D)SGK1 similarly stimulated Ip, an effect again reversed by rapamycin. Coexpression of the inactive mutant of the serum and glucocorticoid inducible kinase (K119N)SGK1 significantly decreased Ip and abrogated the stimulating effect of mTOR on Ip. In conclusion, mTOR and SGK1 cooperate in the stimulation of the intestinal phosphate transporter SLC34A2.

Stimulation of the creatine transporter SLC6A8 by the protein kinase mTOR

Cellular accumulation of creatine is accomplished by the Na(+), Cl(-), and creatine transporter CreaT (SLC6A8). The mammalian target of rapamycin (mTOR) is a kinase stimulating cellular nutrient uptake. The present experiments explored whether SLC6A8 is regulated by mTOR. In Xenopus oocytes expressing SLC6A8 but not in water injected oocytes, creatine-induced a current which was significantly enhanced by coexpression of mTOR. Kinetic analysis revealed that mTOR enhanced maximal current without significantly altering affinity. Preincubation of the oocytes for 32 h with rapamycin (50 nM) decreased the creatine-induced current and abrogated its stimulation by mTOR. The effect of mTOR on CreaT was blunted by additional coexpression of the inactive mutant of the serum and glucocorticoid-inducible kinase (K119N)SGK1 and mimicked by coexpression of wild type SGK1. In conclusion, mTOR stimulates the creatine transporter SLC6A8 through mechanisms at least partially shared by the serum and glucocorticoid-inducible kinase SGK1.

Rapamycin inhibits liver growth during refeeding in rats via control of ribosomal protein translation but not cap-dependent translation initiation

We examined the role of the mammalian target of rapamycin (mTOR) in hepatic cell growth. To dissociate cell growth from cell proliferation, we employed an in vivo model of nonproliferative liver growth in rats, refeeding after 48 h of food deprivation. Starvation resulted in a decrease in liver mass, liver protein, and cell size, all of which were largely restored after 24 h of refeeding. Administration of the mTOR inhibitor, rapamycin, before the refeeding period partially inhibited the restoration of liver protein content. Refeeding was also associated with an increase in ribosomal protein S6 phosphorylation and phosphorylation of the eukaryotic initiation factor (eIF) 4E binding protein 1 (4E-BP1). 4E-BP1 phosphorylation was accompanied by a decrease in the abundance of the complex containing 4E-BP1 with eIF4E. These changes were prevented by rapamycin administration. However, association of eIF4E and eIF4G and eIF2alpha phosphorylation, both of which are stimulated by refeeding, were insensitive to rapamycin. The functional importance of these observations was confirmed by polysome fractionation, which showed that translation initiation of 5' oligopyrimidine tract-containing mRNAs, which encode ribosomal proteins, was inhibited by rapamycin, whereas translation of signal transducer and activator of transcription 1 (STAT1), a cap-dependent mRNA, was unaffected. The abundance of ribosomal proteins paralleled total protein content during refeeding in both control and rapamycin-injected rats. We conclude that accretion of liver protein during refeeding is dependent on mTOR-mediated activation of the translation of ribosomal proteins but not dependent on mTOR-mediated activation of cap-dependent translation initiation.

Sublethal mitochondrial stress with an attendant stoichiometric augmentation of reactive oxygen species may precipitate many of the beneficial alterations in cellular physiology produced by caloric restriction, intermittent fasting, exercise and dietary phytonutrients: "Mitohormesis" for health and vitality

The precise mechanistic sequence producing the beneficial effects on health and lifespan seen with interventions as diverse as caloric restriction, intermittent fasting, exercise, and consumption of dietary phytonutrients is still under active characterization, with large swaths of the research community kept in relative isolation from one another. Among the explanatory models capable of assisting in the identification of precipitating elements responsible for beneficial influences on physiology seen in these states, the hormesis perspective on biological systems under stress has yielded considerable insight into likely evolutionarily consistent organizing principles functioning in all four conditions. Recent experimental findings provide the tantalizing initial lodestones for an entirely new research front examining molecular substrates of stress resistance. In this novel body of research, a surprising new twist has emerged: Reactive oxygen species, derived from the mitochondrial electron transport system, may be necessary triggering elements for a sequence of events that result in benefits ranging from the transiently cytoprotective to organismal-level longevity. With the recent appreciation that reactive oxygen species and reactive nitrogen species function as signaling elements in a interconnected matrix of signal transduction, the entire basis of many widely accepted theories of aging that predominated in the past may need to be reconsidered to facilitate the formulation of an new perspective more correctly informed by the most contemporaneous experimental findings. This perspective, the mitohormesis theory, can be used in many disparate domains of inquiry to potentially explain previous findings, as well as point to new targets of research. The utility of this perspective for research on aging is significant, but beyond that this perspective emphasizes the pressing need to rigorously characterize the specific contribution of the stoichiometry of reactive oxygen species and reactive nitrogen species in the various compartments of the cell to cytoprotection and vitality. Previous findings regarding the influences of free radical chemistry on cellular physiology may have represented assessments examining the consequences of isolated elevation of signaling elements within a larger signal transductive apparatus, rather than definitive characterizations of the only modality of reactive oxygen species (and reactive nitrogen species) influence. In applying this perspective, it may be necessary for the research community, as well as the practicing clinician, to engender a more sanguine perspective on organelle level physiology, as it is now plausible that such entities have an evolutionarily orchestrated capacity to self-regulate that may be pathologically disturbed by overzealous use of antioxidants, particularly in the healthy.