Using structure to define the function of phosphoinositide 3-kinase family members

Combinatorial genetic manipulation of Cx50, PI3K and PTEN alters postnatal mouse lens growth and homeostasis

Introduction

Phosphoinositide 3-kinase (PI3K), Phosphatase and tensin homolog (PTEN) and connexin50 (Cx50) have individually been shown to play critical roles in the growth, development and maintenance of the lens and to functionally interact in vitro. To elucidate how gap junctional coupling mediated by Cx50 and intracellular signaling mediated by PI3K and PTEN synergistically interact to regulate lens homeostasis in vivo, we generated and characterized double knockout animal models lacking the p110α subunit of PI3K and Cx50, or PTEN and Cx50.

Methods

We interbred lens specific p110α and PTEN conditional knockout animals with Cx50 deficient mice to generate double knockouts. Animals and eyes were weighed, lenses were dissected, photographed, measured, fixed and sectioned for histological analysis. Lens epithelial cell proliferation was determined using 5-ethynyl-2'-deoxyuridine (EdU) labeling.

Results

Double knockout of p110α and Cx50 led to a significant reduction in lens and eye size, and a high rate of lens rupture. The individual cell proliferation defects of the Cx50 and p110α single knockout lenses both persisted in the double KO. Double deletion of Cx50 and PTEN produced severe lens defects, including cataract, aberrant cell migration, altered cell proliferation, vacuole formation and lens rupture.

Conclusion

The severe phenotypes in p110α/Cx50 and PTEN/Cx50 double deficient lenses suggest that PI3K, PTEN and Cx50 participate in both distinct and common regulatory pathways that are necessary to maintain normal lens growth and homeostasis.

Molecular Basis of Oncogenic PI3K Proteins

The dysregulation of phosphatidylinositol 3-kinase (PI3K) signaling plays a pivotal role in driving neoplastic transformation by promoting uncontrolled cell survival and proliferation. This oncogenic activity is primarily caused by mutations that are frequently found in PI3K genes and constitutively activate the PI3K signaling pathway. However, tumorigenesis can also arise from nonmutated PI3K proteins adopting unique active conformations, further complicating the understanding of PI3K-driven cancers. Recent structural studies have illuminated the functional divergence among highly homologous PI3K proteins, revealing how subtle structural alterations significantly impact their activity and contribute to tumorigenesis. In this review, we summarize current knowledge of Class I PI3K proteins and aim to unravel the complex mechanism underlying their oncogenic traits. These insights will not only enhance our understanding of PI3K-mediated oncogenesis but also pave the way for the design of novel PI3K-based therapies to combat cancers driven by this signaling pathway.

Structural modeling of phosphatidylinositol 3-kinase-γ with novel derivatives of stilbenoids

Phosphatidylinositol 3-kinases (PI3K) form a family of lipid kinases that catalyze the phosphorylation of 3-hydroxyl group of the inositol ring of phosphatidylinositol and its derivatives. It is implicated in inflammatory disorders and cancer thus making it an attractive drug target. Crystal structure of human PI3Kγ was taken and structure was completed using MODELLER and validated using PROCHECK. Stilbenoid molecules, piceatannol and resveratrol, were docked to kinase domain of PI3Kγ using AutoDock Vina and docked complexes were subjected to molecular dynamic simulations using Desmond suite of programmes. Based on the structural analysis of these complexes, modified derivatives of the native molecules were designed, docked and molecular dynamic simulations were performed. Kinase domain has a bi-lobar structure with ATP binding site lying in the cleft connecting the two lobes that are primarily composed of 12 α-helices and 8 β-strands. Piceatannol and resveratrol bind at the ATP binding site, with one its rings in a position primarily occupied by adenine of ATP making a hydrogen bond with backbone of Val882. Molecules also make interactions with Lys833 and several isoleucine residues. Interactions with Ser806 appear to be crucial for the loop conformation and compactness. Derivative molecules of stilbenoids also occupy the ATP binding cleft and the chemical modifications result in hydrogen bonded interactions to Glu880, and ionic interactions to Lys833 and Lys808 thereby enhancing their potencies in comparison to native molecules. Biophysical parameters and quality of interactions of stilbenoid derivatives augurs well for development of potent and specific inhibitory molecules against PI3Kγ enzyme.

New insights into the regulation and roles of phosphatidylinositol 3,4-bisphosphate

Phosphoinositides (PIPs) are phospholipids and components of the cellular membrane. In mammals, seven phosphorylated derivatives of PIPs have been identified. Among them, phosphatidylinositol 3,4-bisphosphate [PI(3,4)P2] is produced by lipid phosphatases (e.g., SHIP2) or by lipid kinases PI3KC2α and PI3KC2β. Although PI(3,4)P2 is undetectable in normal mouse or human tissues and common cell lines, it appears in a mouse prostate cancer model and in cells exposed to oxidative stress, indicating that PI(3,4)P2 is involved in the pathogenesis of some diseases. Here, I summarize recent findings on the cellular roles and pathophysiological significance of PI(3,4)P2.

Bioinformatics and network pharmacology identify promotional effects and potential mechanisms of ethanol on esophageal squamous cell carcinoma and experimental validation

Ethanol is an important risk factor for esophageal squamous cell carcinoma (ESCC); however, the molecular mechanisms behind how ethanol promotes ESCC development remain poorly understood. In this study, ethanol-ESCC-associated target genes were constructed and screened using network pharmacology and subjected to Kyoto Encyclopedia of Genes and Genomes (KEGG) and bioinformatics analysis. A mouse ethanol-exposed esophageal cancer model was constructed with 4-nitroquinoline-1-oxide (4-NQO) to assess its survival and tumor lesion status, and the mechanism of ethanol-promoted ESCC lesions was verified by qRT-PCR and Western blotting. The results showed that 126 ethanol-ESCC crossover genes were obtained, which were significantly enriched in the PI3K/AKT signaling pathway. Bioinformatics results showed that the target genes TNF, IL6, IL1β and JUN were highly expressed in esophageal tumor samples and positively correlated with tumor proliferation and apoptosis genes, and the genetic information of these genes was mutated to different degrees. Animal model experiments showed that ethanol decreased the survival rate and aggravated the occurrence of esophageal cancer in mice. qRT-PCR showed that ethanol promoted the expression of TNF, IL6, IL1β and JUN mRNA in mouse esophageal tumor tissues, and Western blotting showed that ethanol promoted p-PI3K and p-AKT protein expression in mouse esophageal tumor tissues. In conclusion, ethanol promotes esophageal carcinogenesis by increasing the expression of TNF, IL6, IL1β and JUN and activating the PI3K/AKT signaling pathway.

Platelet p110β mediates platelet-leukocyte interaction and curtails bacterial dissemination in pneumococcal pneumonia

Phosphatidylinositol 3-kinase catalytic subunit p110β is involved in tumorigenesis and hemostasis. However, it remains unclear if p110β also regulates platelet-mediated immune responses, which could have important consequences for immune modulation during anti-cancer treatment with p110β inhibitors. Thus, we investigate how platelet p110β affects inflammation and infection. Using a mouse model of Streptococcus pneumoniae-induced pneumonia, we find that both platelet-specific p110β deficiency and pharmacologic inhibition of p110β with TGX-221 exacerbate disease pathogenesis by preventing platelet-monocyte and neutrophil interactions, diminishing their infiltration and enhancing bacterial dissemination. Platelet p110β mediates neutrophil phagocytosis of S. pneumoniae in vitro and curtails bacteremia in vivo. Genetic deficiency or inhibition of platelet p110β also impairs macrophage recruitment in an independent model of sterile peritonitis. Our results demonstrate that platelet p110β dysfunction exacerbates pulmonary infection by impeding leukocyte functions. Thereby, our findings provide important insights into the immunomodulatory potential of PI3K inhibitors in bacterial infection.

Striking a balance: PIP2 and PIP3 signaling in neuronal health and disease

Phosphoinositides are membrane phospholipids involved in a variety of cellular processes like growth, development, metabolism, and transport. This review focuses on the maintenance of cellular homeostasis of phosphatidylinositol 4,5-bisphosphate (PIP₂), and phosphatidylinositol 3,4,5-trisphosphate (PIP₃). The critical balance of these PIPs is crucial for regulation of neuronal form and function. The activity of PIP₂ and PIP₃ can be regulated through kinases, phosphatases, phospholipases and cholesterol microdomains. PIP₂ and PIP₃ carry out their functions either indirectly through their effectors activating integral signaling pathways, or through direct regulation of membrane channels, transporters, and cytoskeletal proteins. Any perturbations to the balance between PIP₂ and PIP₃ signaling result in neurodevelopmental and neurodegenerative disorders. This review will discuss the upstream modulators and downstream effectors of the PIP₂ and PIP₃ signaling, in the context of neuronal health and disease.

The Upstream Pathway of mTOR-Mediated Autophagy in Liver Diseases

Autophagy, originally found in liver experiments, is a cellular process that degrades damaged organelle or protein aggregation. This process frees cells from various stress states is a cell survival mechanism under stress stimulation. It is now known that dysregulation of autophagy can cause many liver diseases. Therefore, how to properly regulate autophagy is the key to the treatment of liver injury. mechanistic target of rapamycin (mTOR)is the core hub regulating autophagy, which is subject to different upstream signaling pathways to regulate autophagy. This review summarizes three upstream pathways of mTOR: the phosphoinositide 3-kinase (PI3K)/protein kinase (AKT) signaling pathway, the adenosine monophosphate-activated protein kinase (AMPK) signaling pathway, and the rat sarcoma (Ras)/rapidly accelerated fibrosarcoma (Raf)/mitogen-extracellular activated protein kinase kinase (MEK)/ extracellular-signal-regulated kinase (ERK) signaling pathway, specifically explored their role in liver fibrosis, hepatitis B, non-alcoholic fatty liver, liver cancer, hepatic ischemia reperfusion and other liver diseases through the regulation of mTOR-mediated autophagy. Moreover, we also analyzed the crosstalk between these three pathways, aiming to find new targets for the treatment of human liver disease based on autophagy.

Distinct functions of AKT isoforms in breast cancer: a comprehensive review

BACKGROUND

AKT, also known as protein kinase B, is a key element of the PI3K/AKT signaling pathway. Moreover, AKT regulates the hallmarks of cancer, e.g. tumor growth, survival and invasiveness of tumor cells. After AKT was discovered in the early 1990s, further studies revealed that there are three different AKT isoforms, namely AKT1, AKT2 and AKT3. Despite their high similarity of 80%, the distinct AKT isoforms exert non-redundant, partly even opposing effects under physiological and pathological conditions. Breast cancer as the most common cancer entity in women, frequently shows alterations of the PI3K/AKT signaling.

MAIN CONTENT

A plethora of studies addressed the impact of AKT isoforms on tumor growth, metastasis and angiogenesis of breast cancer as well as on therapy response and overall survival in patients. Therefore, this review aimed to give a comprehensive overview about the isoform-specific effects of AKT in breast cancer and to summarize known downstream and upstream mechanisms. Taking account of conflicting findings among the studies, the majority of the studies reported a tumor initiating role of AKT1, whereas AKT2 is mainly responsible for tumor progression and metastasis. In detail, AKT1 increases cell proliferation through cell cycle proteins like p21, p27 and cyclin D1 and impairs apoptosis e.g. via p53. On the downside AKT1 decreases migration of breast cancer cells, for instance by regulating TSC2, palladin and EMT-proteins. However, AKT2 promotes migration and invasion most notably through regulation of β-integrins, EMT-proteins and F-actin. Whilst AKT3 is associated with a negative ER-status, findings about the role of AKT3 in regulation of the key properties of breast cancer are sparse. Accordingly, AKT1 is mutated and AKT2 is amplified in some cases of breast cancer and AKT isoforms are associated with overall survival and therapy response in an isoform-specific manner.

CONCLUSIONS

Although there are several discussed hypotheses how isoform specificity is achieved, the mechanisms behind the isoform-specific effects remain mostly unrevealed. As a consequence, further effort is necessary to achieve deeper insights into an isoform-specific AKT signaling in breast cancer and the mechanism behind it.

Testis developmental related gene 1 regulates the chemosensitivity of seminoma TCam-2 cells to cisplatin via autophagy

We previously identified testis developmental related gene 1 (TDRG1), a gene implicated in proliferation of TCam-2 seminoma cells. Recent evidence has revealed that autophagy influences the chemosensitivity of cancer cells to chemotherapy. However, whether TDRG1 protein regulates autophagy in seminoma cells and influences their sensitivity to cis-dichlorodiammine platinum (CDDP) remains unknown. In this study, we used TCam-2 cells and male athymic BALB/c nude mice with xenografts of TCam-2 cells to investigate autophagy, cell viability, apoptosis and the p110β/Rab5/Vps34 (PI3-kinase Class III) pathway under the conditions of TDRG1 overexpression or knockdown and with or without CDDP treatment. We found that TDRG1 upregulation promoted autophagy in both TCam-2 cells and seminoma xenografts via p110β/Rab5/Vps34 activation. Inhibition of autophagy reduced cell viability and promoted apoptosis during CDDP treatment of TCam-2 cells. Similarly, TDRG1 knockdown inhibited autophagy, reduced cell viability and promoted apoptosis during CDDP treatment of TCam-2 cells. TDRG1 knockdown inhibited tumour growth and promoted apoptosis in TCam-2 cell xenografts, whereas TDRG1 overexpression had the opposite effect. According to these results, we propose that high expression of TDRG1 promotes autophagy through the p110β/Rab5/Vps34 pathway in TCam-2 cells. TDRG1 overexpression promotes autophagy and leads to CDDP resistance, whereas TDRG1 knockdown inhibits autophagy and promotes chemosensitivity to CDDP both in vivo and in vitro. This study has uncovered a novel role of TDRG1 in reducing chemoresistance during CDDP treatment and provides potential therapeutic strategies for the treatment of human seminoma.

PI3K isoforms in cell signalling and vesicle trafficking

PI3Ks are a family of lipid kinases that phosphorylate intracellular inositol lipids to regulate signalling and intracellular vesicular traffic. Mammals have eight isoforms of PI3K, divided into three classes. The class I PI3Ks generate 3-phosphoinositide lipids, which directly activate signal transduction pathways. In addition to being frequently genetically activated in cancer, similar mutations in class I PI3Ks have now also been found in a human non-malignant overgrowth syndrome and a primary immune disorder that predisposes to lymphoma. The class II and class III PI3Ks are regulators of membrane traffic along the endocytic route, in endosomal recycling and autophagy, with an often indirect effect on cell signalling. Here, we summarize current knowledge of the different PI3K classes and isoforms, focusing on recently uncovered biological functions and the mechanisms by which these kinases are activated. Deeper insight into the PI3K isoforms will undoubtedly continue to contribute to a better understanding of fundamental cell biological processes and, ultimately, of human disease.

Disrupted apolipoprotein L1-miR193a axis dedifferentiates podocytes through autophagy blockade in an APOL1 risk milieu

We hypothesized that a functional apolipoprotein LI (APOL1)-miR193a axis (inverse relationship) preserves, but disruption alters, the podocyte molecular phenotype through the modulation of autophagy flux. Podocyte-expressing APOL1G0 (G0-podocytes) showed downregulation but podocyte-expressing APOL1G1 (G1-podocytes) and APOL1G2 (G2-podocytes) displayed enhanced miR193a expression. G0-, G1-, and G2-podocytes showed enhanced expression of light chain (LC) 3-II and beclin-1, but a disparate expression of p62 (low in wild-type but high in risk alleles). G0-podocytes showed enhanced, whereas G1- and G2-podocytes displayed decreased, phosphorylation of Unc-51-like autophagy-activating kinase (ULK)1 and class III phosphatidylinositol 3-kinase (PI3KC3). Podocytes overexpressing miR193a (miR193a-podocytes), G1, and G2 showed decreased transcription of PIK3R3 (PI3KC3's regulatory unit). Since 3-methyladenine (3-MA) enhanced miR193a expression but inhibited PIK3R3 transcription, it appears that 3-MA inhibits autophagy and induces podocyte dedifferentiation via miR193a generation. miR193a-, G1-, and G2-podocytes also showed decreased phosphorylation of mammalian target of rapamycin (mTOR) that could repress lysosome reformation. G1- and G2-podocytes showed enhanced expression of run domain beclin-1-interacting and cysteine-rich domain-containing protein (Rubicon); however, its silencing prevented their dedifferentiation. Docking, protein-protein interaction, and immunoprecipitation studies with antiautophagy-related gene (ATG)14L, anti-UV radiation resistance-associated gene (UVRAG), or Rubicon antibodies suggested the formation of ATG14L complex I and UVRAG complex II in G0-podocytes and the formation of Rubicon complex III in G1- and G2-podocytes. These findings suggest that the APOL1 risk alleles favor podocyte dedifferentiation through blockade of multiple autophagy pathways.

p110α and p110β isoforms of PI3K are involved in protection against H2O2 induced oxidative stress in cancer cells

PURPOSE

Phosphatidylinositol-3 kinases (PI3Ks) are involved in regulating cell growth, proliferation, differentiation, apoptosis and survival. p110α and p110β, two ubiquitously expressed isoforms of PI3K signalling, are involved in growth factor mediated signaling and survival by generating second messengers. Earlier, we have generated GFP-fusion proteins of p110α and p110β and expressed them in normal and cancer cell-lines to investigate their subcellular localization and their role in various activities. Here, we sought to examine the role of p110α and p110β isoforms in protecting MCF-7 breast cancer cells against oxidative stress.

MATERIAL METHODS

We performed cytotoxicity assays, DNA transfection, Plasmid DNA preparation, western blotting, flourscence microscopy and statistical analysis.

RESULTS

To know whether p110α and p110β are involved in protecting MCF-7 breast cancer cells against oxidative stress, we subjected MCF-7 cells to H2O2 treatment and observed a dose dependent decrease in cell viability and a marked increase in the levels of pro-apoptotic markers which include PARP, Bcl-2, Bax and procaspase-9. We then over-expressed recombinant GFP-fusion p110α and p110β proteins in MCF-7 cells and observed a significant decrease in apoptosis and a concomitant increase in pAkt levels.

CONCLUSION

We report the involvement of p110α and p110β isoforms of Class 1A PI3K signalling in rescue from oxidative stress-induced apoptosis in MCF-7 cells in Akt dependent manner.

PI3Kγ promotes vascular smooth muscle cell phenotypic modulation and transplant arteriosclerosis via a SOX9-dependent mechanism

Background

Transplant arteriosclerosis (TA) remains the major cause of chronic graft failure in solid organ transplantation. The phenotypic modulation of vascular smooth muscle cells (VSMCs) is a key event for the initiation and progression of neointimal formation and TA. This study aims to explore the role and underlying mechanism of phosphoinositide 3-kinases γ (PI3Kγ) in VSMC phenotypic modulation and TA.

Methods

The rat model of aortic transplantation was established to detect PI3Kγ expression and its role in neointimal formation and vascular remodeling in vivo. PI3Kγ shRNA transfection was employed to knockdown PI3Kγ gene. Aortic VSMCs was cultured and treated with TNF-α to explore the role and molecular mechanism of PI3Kγ in VSMC phenotypic modulation.

Findings

Activated PI3Kγ/p-Akt signaling was observed in aortic allografts and in TNF-α-treated VSMCs. Lentivirus-mediated shRNA transfection effectively inhibited PI3Kγ expression in medial VSMCs while restoring the expression of VSMC contractile genes, associated with impaired neointimal formation in aortic allografts. In cultured VSMCs, PI3Kγ blockade with pharmacological inhibitor or genetic knockdown markedly abrogated TNF-α-induced downregulation of VSMC contractile genes and increase in cellular proliferation and migration. Moreover, SOX9 located in nucleus competitively inhibited the interaction of Myocardin and SRF, while PI3Kγ inhibition robustly reduced SOX9 expression and its nuclear translocation and repaired the Myocardin/SRF association.

Interpretation

These results suggest that PI3Kγ plays a critical role in VSMC phenotypic modulation via a SOX9-dependent mechanism. Therefore, PI3Kγ in VSMCs may represent a promising therapeutic target for the treatment of TA.

Fund

National Natural Science Foundation of China.

Kinetic and structural analyses reveal residues in phosphoinositide 3-kinase α that are critical for catalysis and substrate recognition

Phosphoinositide 3-kinases (PI3Ks) are ubiquitous lipid kinases that activate signaling cascades controlling cell survival, proliferation, protein synthesis, and vesicle trafficking. PI3Ks have dual kinase specificity: a lipid kinase activity that phosphorylates the 3'-hydroxyl of phosphoinositides and a protein-kinase activity that includes autophosphorylation. Despite the wealth of biochemical and structural information on PI3Kα, little is known about the identity and roles of individual active-site residues in catalysis. To close this gap, we explored the roles of residues of the catalytic domain and the regulatory subunit of human PI3Kα in lipid and protein phosphorylation. Using site-directed mutagenesis, kinetic assays, and quantitative mass spectrometry, we precisely mapped key residues involved in substrate recognition and catalysis by PI3Kα. Our results revealed that Lys-776, located in the P-loop of PI3Kα, is essential for the recognition of lipid and ATP substrates and also plays an important role in PI3Kα autophosphorylation. Replacement of the residues His-936 and His-917 in the activation and catalytic loops, respectively, with alanine dramatically changed PI3Kα kinetics. Although H936A inactivated the lipid kinase activity without affecting autophosphorylation, H917A abolished both the lipid and protein kinase activities of PI3Kα. On the basis of these kinetic and structural analyses, we propose possible mechanistic roles of these critical residues in PI3Kα catalysis.

The Phosphoinosotide 3-Kinase Catalytic Subunit p110α is Required for Normal Lens Growth

Purpose

Signal transduction pathways influence lens growth, but little is known about the role(s) of the class 1A phosphoinositide 3-kinases (PI3Ks). To further investigate how signaling regulates lens growth, we generated and characterized mice in which the p110α and p110β catalytic subunits of PI3K were conditionally deleted in the mouse lens.

Methods

Floxed alleles of the catalytic subunits of PI3K were conditionally deleted in the lens by using MLR10-cre transgenic mice. Lenses of age-matched animals were dissected and photographed. Postnatal lenses were fixed, paraffin embedded, sectioned, and stained with hematoxylin-eosin. Cell proliferation was quantified by labeling S-phase cells in intact lenses with 5-ethynyl-2′-deoxyuridine. Protein kinase B (AKT) activation was examined by Western blotting.

Results

Lens-specific deletion of p110α resulted in a significant reduction of eye and lens size, without compromising lens clarity. Conditional knockout of p110β had no effect on lens size or clarity, and deletion of both the p110α and p110β subunits resulted in a phenotype that resembled the p110α single-knockout phenotype. Levels of activated AKT were decreased more in p110α- than in p110β-deficient lenses. A significant reduction in proliferating cells in the germinative zone was observed on postnatal day 0 in p110α knockout mice, which was temporally correlated with decreased lens volume.

Conclusions

These data suggest that the class 1A PI3K signaling pathway plays an important role in the regulation of lens size by influencing the extent and spatial location of cell proliferation in the perinatal period.

Interplay of the iron-regulated metastasis suppressor NDRG1 with epidermal growth factor receptor (EGFR) and oncogenic signaling

The iron-regulated metastasis suppressor N-myc downstream-regulated gene 1 (NDRG1) has been shown to inhibit numerous oncogenic signaling pathways in cancer cells. Recent findings have demonstrated that NDRG1 inhibits the ErbB family of receptors, which function as key inducers of carcinogenesis. NDRG1 attenuates ErbB signaling by inhibiting formation of epidermal growth factor receptor (EGFR)/human epidermal growth factor receptor 2 (HER2) and HER2/HER3 heterodimers and by down-regulating EGFR via a mechanism involving its degradation. Understanding the complex interplay between NDRG1, iron, and ErbB signaling is vital for identifying novel, more effective targets for cancer therapy.

Podocyte Autophagy: A Potential Therapeutic Target to Prevent the Progression of Diabetic Nephropathy

Diabetic nephropathy (DN), a leading cause of end-stage renal disease (ESRD), becomes a worldwide problem. Ultrastructural changes of the glomerular filtration barrier, especially the pathological changes of podocytes, lead to proteinuria in patients with diabetes. Podocytes are major components of glomerular filtration barrier, lining outside of the glomerular basement membrane (GBM) to maintain the permeability of the GBM. Autophagy is a high conserved cellular process in lysosomes including impaired protein, cell organelles, and other contents in the cytoplasm. Recent studies suggest that activation of autophagy in podocytes may be a potential therapy to prevent the progression of DN. Here, we review the mechanisms of autophagy in podocytes and discuss the current studies about alleviating proteinuria via activating podocyte autophagy.

p110α and p110β isoforms of PI3K signaling: are they two sides of the same coin?

Class-1 phosphatidylinositol-3-kinases (PI3Ks) are activated by a variety of extracellular stimuli and have been implicated in a wide range of cellular processes. p110α and p110β are the two most studied isoforms of the class-1A PI3K signaling pathway. Although these two isoforms are ubiquitously expressed and play multiple redundant roles, they also have distinct functions within the cell. More recently, p110α and p110β isoforms have been shown to translocate into the nucleus and play a role in DNA replication and repair, and in cell cycle progression. In the following Review article, we discuss the overlapping and unique roles of p110α and p110β isoforms with a particular focus on their structure, expression analysis, subcellular localization, and signaling contributions in various cell types and model organisms.

Induction of Central Host Signaling Kinases during Pneumococcal Infection of Human THP-1 Cells

Streptococcus pneumoniae is a widespread colonizer of the mucosal epithelia of the upper respiratory tract of human. However, pneumococci are also responsible for numerous local as well as severe systemic infections, especially in children under the age of five and the elderly. Under certain conditions, pneumococci are able to conquer the epithelial barrier, which can lead to a dissemination of the bacteria into underlying tissues and the bloodstream. Here, specialized macrophages represent an essential part of the innate immune system against bacterial intruders. Recognition of the bacteria through different receptors on the surface of macrophages leads thereby to an uptake and elimination of bacteria. Accompanied cytokine release triggers the migration of leukocytes from peripheral blood to the site of infection, where monocytes differentiate into mature macrophages. The rearrangement of the actin cytoskeleton during phagocytosis, resulting in the engulfment of bacteria, is thereby tightly regulated by receptor-mediated phosphorylation cascades of different protein kinases. The molecular cellular processes including the modulation of central protein kinases are only partially solved. In this study, the human monocytic THP-1 cell line was used as a model system to examine the activation of Fcγ and complement receptor-independent signal cascades during infection with S. pneumoniae. Pneumococci cultured either in chemically defined or complex medium showed no significant differences in pneumococcal phagocytosis by phorbol 12-myristate 13-acetate (PMA) differentiated THP-1 cells. Double immuno-fluorescence microscopy and antibiotic protection assays demonstrated a time-dependent uptake and killing of S. pneumoniae 35A inside of macrophages. Infections of THP-1 cells in the presence of specific pharmacological inhibitors revealed a crucial role of actin polymerization and importance of the phosphoinositide 3-kinase (PI3K) and Protein kinase B (Akt) as well during bacterial uptake. The participation of essential host cell signaling kinases in pneumococcal phagocytosis was deciphered for the kinase Akt, ERK1/2, and p38 and phosphoimmunoblots showed an increased phosphorylation and thus activation upon infection with pneumococci. Taken together, this study deciphers host cell kinases in innate immune cells that are induced upon infection with pneumococci and interfere with bacterial clearance after phagocytosis.

Myristoylated p110α Causes Embryonic Death Due to Developmental and Vascular Defects

The phosphatidylinositol 3-kinase (PI3K) signaling pathway regulates many important cellular functions. The functional impact of deregulating the PIK3CA gene, encoding the p110α catalytic subunit of PI3K, is validated by frequent gain of function mutations in a range of human cancers. We generated a mouse model with an inducible constitutively active form of PI3K. In this model Cre recombinase activates expression of a myristoylated form of p110α (myr-p110α). The myristoylated version of p110α brings the protein to the cytoplasmic side of the cell membrane, which mimics the normal activation mechanism for the p110α catalytic subunit and activates the PI3K enzyme. Constitutively activated PI3K signaling induced by myr-p110α in all cells of the developing mouse caused lethality during embryonic development. Transgenic Cre;myr-p110α heterozygous embryos displayed morphological malformation and poor vascular development with extremely dilated blood vessels and hemorrhage in the embryo and the extraembryonic yolk sac. Previous studies demonstrated that loss of p110α during embryonic development causes angiogenic disruption and here we show that constitutive activation of p110α by gain of function mutation during development also disrupts vasculogenesis/angiogenesis in what appears to be a similar manner. These finding demonstrate the importance of tight regulation of PI3K signaling during embryonic vasculogenesis/angiogenesis..

Targeting Protein Kinase C Downstream of Growth Factor and Adhesion Signalling

The signaling outputs of Receptor Tyrosine Kinases, G-protein coupled receptors and integrins converge to mediate key cell process such as cell adhesion, cell migration, cell invasion and cell proliferation. Once activated by their ligands, these cell surface proteins recruit and direct a diverse range of proteins to disseminate the appropriate response downstream of the specific environmental cues. One of the key groups of proteins required to regulate these activities is the family of serine/threonine intracellular kinases called Protein Kinase Cs. The activity and subcellular location of PKCs are mediated by a series of tightly regulated events and is dependent on several posttranslational modifications and the availability of second messengers. Protein Kinase Cs exhibit both pro- and anti-tumorigenic effects making them an interesting target for anti-cancer treatment.

Phosphatidylinositol-3,4,5-triphosphate and cellular signaling: implications for obesity and diabetes

Phosphatidylinositol-3,4,5-triphosphate (PtdIns(3,4,5)P₃) is one of the most important phosphoinositides and is capable of activating a wide range of proteins through its interaction with their specific binding domains. Localization and activation of these effector proteins regulate a number of cellular functions, including cell survival, proliferation, cytoskeletal rearrangement, intracellular vesicle trafficking, and cell metabolism. Phosphoinositides have been investigated as an important agonist-dependent second messenger in the regulation of diverse physiological events depending upon the phosphorylation status of their inositol group. Dysregulation in formation as well as metabolism of phosphoinositides is associated with various pathophysiological disorders such as inflammation, allergy, cardiovascular diseases, cancer, and metabolic diseases. Recent studies have demonstrated that the impaired metabolism of PtdIns(3,4,5)P₃ is a prime mediator of insulin resistance associated with various metabolic diseases including obesity and diabetes. This review examines the current status of the role of PtdIns(3,4,5)P₃ signaling in the regulation of various cellular functions and the implications of dysregulated PtdIns(3,4,5)P₃ signaling in obesity, diabetes, and their associated complications.

p85 protein expression is associated with poor survival in HER2-positive patients with advanced breast cancer treated with trastuzumab

To investigate the immunohistochemical expression of p85 in a cohort of trastuzumab-treated HER2-positive and HER2-negative metastatic breast cancer patients. The medical records of all patients with metastatic breast cancer treated with trastuzumab-based regimens between 1998 and 2010 were reviewed and clinical information was obtained. Formalin-fixed paraffin-embedded tumor tissue samples with adequate material were retrospectively collected from 183 patients. Samples were evaluated by immunohistochemistry for p85, estrogen receptors (ER), progesterone receptors (PgR), HER2, Ki67, PTEN and phosphorylated Akt (S473 and T308). HER2 status was studied by fluorescence in situ hybridization, as well. PIK3CA mutational status was also evaluated. Median follow-up for all patients was 72 months. Central re-evaluation for HER2 revealed only 111 HER2-positive cases, with the remaining 72 patients being HER2-negative. Median survival was longer in HER2-positive patients (50.7 months) compared to HER2-negative patients (36.6 months) both treated with trastuzumab, but this difference has not reached significance (p = 0.068). In total, 62% of the patients were found positive for p85, however the p85 protein was not found to be differentially expressed in HER2-positive versus HER2-negative cases. There were no significant associations between protein expression of p85 and any of the markers under study, or with time to progression. Positive p85 protein expression was however associated with poor survival in trastuzumab-treated HER2-positive patients. In our cohort of trastuzumab-treated HER2-positive breast cancer patients, positive p85 protein expression appears to be a prognostic factor of poor survival and, if validated, might have important implications in the treatment of such patients.

Expression pattern of class I phosphoinositide 3-kinase and distribution of its product, phosphatidylinositol-3,4,5-trisphosphate, during Drosophila embryogenesis

The class I phosphoinositide 3-kinase (PI3K) can be activated by a large variety of extracellular stimuli and is responsible for generating phosphatidylinositol-3,4,5-trisphosphate (PI(3,4,5)P(3)) from phosphatidylinositol-4,5-bisphosphate at the plasma membrane. The expression pattern of the class I PI3K and distribution of PI(3,4,5)P(3), visualized by its specific binding protein, GRP1-PH, were examined during Drosophila embryogenesis. We found that the RNA of Pi3K21B, encoding the Drosophila p60 regulatory subunit of the class I PI3Ks, was expressed maternally and expressed primarily in pole cells after cellularization until completion of germ band elongation. The RNA of Pi3K92E, encoding the Drosophila p110 catalytic subunit of the class I PI3Ks, was also expressed maternally. During gastrulation, its transcript level became lower and was slightly enriched in invaginating cells. Both Pi3K21B and Pi3K92E were expressed ubiquitously after germ band elongation and persisted during germ band shortening. PI(3,4,5)P(3) was distributed at the apical region of the invaginating cells during gastrulation. These findings suggest a possible involvement of class I PI3K and PI(3,4,5)P(3) in the regulation of invagination during gastrulation.

Neuron-specific regulation of class I PI3K catalytic subunits and their dysfunction in brain disorders

The phosphoinositide 3-kinase (PI3K) complex plays important roles in virtually all cells of the body. The enzymatic activity of PI3K to phosphorylate phosphoinositides in the membrane is mediated by a group of catalytic and regulatory subunits. Among those, the class I catalytic subunits, p110α, p110β, p110γ, and p110δ, have recently drawn attention in the neuroscience field due to their specific dysregulation in diverse brain disorders. While in non-neuronal cells these catalytic subunits may have partially redundant functions, there is increasing evidence that in neurons their roles are more specialized, and confined to distinct receptor-dependent pathways. This review will summarize the emerging role of class I PI3K catalytic subunits in neurotransmitter-regulated neuronal signaling, and their dysfunction in a variety of neurological diseases, including fragile X syndrome, schizophrenia, and epilepsy. We will discuss recent literature describing the use of PI3K subunit-selective inhibitors to rescue brain disease-associated phenotypes in in vitro and animal models. These studies give rise to the exciting prospect that these drugs, originally designed for cancer treatment, may be repurposed as therapeutic drugs for brain disorders in the future.

Theoretical studies on beta and delta isoform-specific binding mechanisms of phosphoinositide 3-kinase inhibitors

Phosphoinositide 3-kinase (PI3K) is known to be closely related to tumorigenesis and cell proliferation, and controls a variety of cellular processes, including proliferation, growth, apoptosis, migration, metabolism, etc. The PI3K family comprises eight catalytic isoforms, which are subdivided into three classes. Recently, the discovery of inhibitors that block a single isoform of PI3K has continued to attract special attention because they may have higher selectivity for certain tumors and less toxicity for healthy cells. The PI3Kβ and PI3Kδ share fewer studies than α/γ, and therefore, in this work, the combination of molecular dynamics simulations and free energy calculations was employed to explore the binding of three isoform-specific PI3K inhibitors (COM8, IC87114, and GDC-0941) to PI3Kβ or PI3Kδ. The isoform specificities of the studied inhibitors derived from the predicted binding free energies are in good agreement with the experimental data. In addition, the key residues critical for PI3Kβ or PI3Kδ selectivity were highlighted by decomposing the binding free energies into the contributions from individual residues. It was observed that although PI3Kβ and PI3Kδ share the conserved ATP-binding pockets, individual residues do behave differently, particularly the residues critical for PI3Kβ or PI3Kδ selectivity. It can be concluded that the inhibitor specificity between PI3Kβ and PI3Kδ is determined by the additive contributions from multiple residues, not just a single one. This study provides valuable information for understanding the isoform-specific binding mechanisms of PI3K inhibitors, and should be useful for the rational design of novel and selective PI3K inhibitors.

Class IA PI3K p110β subunit promotes autophagy through Rab5 small GTPase in response to growth factor limitation

Autophagy is an evolutionarily conserved membrane trafficking process. Induction of autophagy in response to nutrient limitation or cellular stress occurs by similar mechanisms in organisms from yeast to mammals. Unlike yeast, metazoan cells rely more on growth factor signaling for a wide variety of cellular activities including nutrient uptake. How growth factor availability regulates autophagy is poorly understood. Here we show that, upon growth factor limitation, the p110β catalytic subunit of the class IA phosphoinositide 3-kinases (PI3Ks) dissociates from growth factor receptor complexes and increases its interaction with the small GTPase Rab5. This p110β-Rab5 association maintains Rab5 in its guanosine triphosphate (GTP)-bound state and enhances the Rab5-Vps34 interaction that promotes autophagy. p110β mutants that fail to interact with Rab5 are defective in autophagy promotion. Hence, in mammalian cells, p110β acts as a molecular sensor for growth factor availability and induces autophagy by activating a Rab5-mediated signaling cascade.

Hepatitis C virus upregulates Beclin1 for induction of autophagy and activates mTOR signaling

Hepatitis C virus (HCV) induces autophagosome formation in infected human hepatocytes. We have previously reported that HCV exploits autophagic machinery in favor of virus growth and survival in host cells (S. Shrivastava et al., Hepatology 53:406-414, 2011); however, the mechanisms for autophagy induction is poorly understood. In the present study, we observed that HCV infection transcriptionally upregulates Beclin1, which forms complex with Vps34, the class III phosphatidylinositol 3-kinase, as a first step for autophagy initiation. Although Bcl-2 has an anti-autophagy effect by its association with Beclin1 in nutrient-deprived cells, our studies revealed that HCV-mediated autophagy occurs independent of Beclin1-Bcl-2 dissociation. Mammalian target of rapamycin (mTOR) is a positive regulator of cell growth and is recognized as an inhibitor of autophagy induction. Our results demonstrated that HCV infection enhances phospho-mTOR expression and its downstream target 4EBP1 activation, suggesting that mTOR is not a negative regulator of HCV-induced autophagy. On the other hand, HCV infection in autophagy-impaired cells reduced phospho-mTOR, mTOR, and phospho-4EBP1 expression. Together, these results suggested that HCV induces autophagy by upregulating Beclin1 and activates mTOR signaling pathway, which in turn may promote hepatocyte growth.

Modulation of Neuronal Survival Factor MEF2 by Kinases in Parkinson's Disease

Parkinson’s disease (PD) is the second most common neurodegenerative disorder due to selective death of neurons in the substantia nigra pars compacta. The cause of cell death remains largely unknown. Myocyte enhancer factor 2 (MEF2) is a group of transcriptional factors required to regulate neuronal development, synaptic plasticity, as well as survival. Recent studies show that MEF2 functions are regulated in multiple subcellular organelles and suggest that dysregulation of MEF2 plays essential roles in the pathogenesis of PD. Many kinases associated with transcription, translation, protein misfolding, autophagy, and cellular energy homeostasis are involved in the neurodegenerative process. Following the first demonstration that mitogen-activated protein kinase p38 (p38 MAPK) directly phosphorylates and activates MEF2 to promote neuronal survival, several other kinase regulators of MEF2s have been identified. These include protein kinase A and extracellular signal regulated kinase 5 as positive MEF2 regulators, and cyclin-dependent kinase 5 (Cdk5) and glycogen synthase kinase 3β as negative regulators in response to diverse toxic signals relevant to PD. It is clear that MEF2 has emerged as a key point where survival and death signals converge to exert their regulatory effects, and dysregulation of MEF2 function in multiple subcellular organelles may underlie PD pathogenesis. Moreover, several other kinases such as leucine-rich repeat kinase 2 and PTEN-induced putative kinase 1 (PINK1) are of particular interest due to their potential interaction with MEF2.

Identification of ETP-46321, a potent and orally bioavailable PI3K α, δ inhibitor

Phosphoinositide-3-kinase (PI3K) is an important target for cancer therapeutics due to the deregulation of this signaling pathway in a wide variety of human cancers. Herein, we describe the optimization of imidazo [1,2-a] pyrazines, which allow us to identify compound 14 (ETP-46321), with potent biochemical and cellular activity and good pharmacokinetic properties (PK) after oral dosing. ETP-46321 PK/PD studies showed time dependent downregulation of AKT(Ser473) phosphorylation, which correlates with compound levels in tumor tissue and demonstrating to be efficacious in a GEMM mouse tumor model driven by a K-Ras(G12V) oncogenic mutation. Treatment with ETP-46321 resulted in significant tumor growth inhibition.

Imidazo[1,2-a]pyrazines as novel PI3K inhibitors

Phosphoinositide-3-kinase (PI3K) is an important target for cancer therapeutics due to the deregulation of its signaling pathway in a wide variety of human cancers. We describe herein a novel series of imidazo[1,2-a]pyrazines as PI3K inhibitors.

ARNO regulates VEGF-dependent tissue responses by stabilizing endothelial VEGFR-2 surface expression

AIMS

The vascular endothelial growth factor (VEGF) stimulates angiogenesis by induction of vessel permeability, proliferation, and migration of endothelial cells, an important process in ischaemic diseases. ADP-ribosylation factor (ARF) nucleotide-binding site opener (ARNO) (cytohesin-2) is a guanine exchange factor important for cellular signalling through ARF GTPases. However, a role for ARNO in VEGF-dependent endothelial processes has so far not been documented. Therefore, we investigated whether ARNO has a role in VEGF-dependent activation of endothelial cells and thus vessel permeability.

METHODS AND RESULTS

ARNO expression was observed in endothelial cells in vitro by RT-PCR, western blotting, and immunofluorescence as well as ex vivo by immunohistochemical staining of mouse aorta. Treatment with the cytohesin inhibitor SecinH3 or with an ARNO siRNA prevented VEGF-dependent Akt activation, assessed by detection of phosphorylated Akt, and proliferation of endothelial cells in vitro, measured by methylthiazoletetrazolium (MTT) reduction. In addition, ARNO suppression reduced VEGF-induced permeability in vessels of the mouse (C57BL/6) cremaster muscle in vivo, as measured by extravasation of fluorescein isothiocyanate (FITC)-dextran. Moreover, ARNO knock-down accelerated ligand-induced reduction in vascular endothelial growth factor receptor-2 (VEGFR-2) surface expression, internalization, and degradation, as assessed by flow cytometry and western blotting, respectively.

CONCLUSION

Our findings indicate an important and novel role for endothelial ARNO in VEGF-dependent initiation of angiogenesis by regulation of VEGFR-2 internalization in endothelial cells, resulting in the activation of the Akt pathway, vessel permeability, and ultimately endothelial proliferation. Thus, ARNO may be a new essential player in endothelial signalling and angiogenesis.

Characterization of molecular recognition of phosphoinositide-3-kinase α inhibitor through molecular dynamics simulation

Phosphatidylinositol 3-kinase α (PI3Kα) is a promising target for anticancer drug discovery due to its overactivation in tumor cells. To systematically investigate the interactions between PI3Kα and PIK75 which is the most selective PI3Kα inhibitor reported to date, molecular docking, molecular dynamics simulation, and ensuing energetic analysis were utilized. The binding free energy between PI3Kα and PIK75 is -10.04 kcal•mol(-1) using MMPBSA method, while -13.88 kcal•mol(-1) using MMGBSA method, which is beneficial for the binding. The van der Waals/hydrophobic and electrostatic interactions play critical roles for the binding. The binding mode of PIK75 for PI3Kα is predicted. The conserved hydrophobic adenine region of PI3Kα made up of Ile800, Ile848, Val850, Val851, Met922, Phe930, and Ile932 accommodates the flat 6-bromine imidazo[1,2-a]pyridine ring of PIK75. The 2-methyl-5-nitrophenyl group of PIK75 extends to the P-loop region, and has four hydrogen-bond arms with the backbone and side chain of Ser773 and Ser774. And the distinct conformation of the P-loop induced by PIK75 is speculated to be responsible for the selectivity profile of PIK75. The predicted binding mode of PIK75 for PI3Kα presented in this study may help design high affinity and selective compounds to target PI3Kα.

Identification of mutations that delay somatic or reproductive aging of Caenorhabditis elegans

Aging is an important feature of animal biology characterized by progressive, degenerative changes in somatic and reproductive tissues. The rate of age-related degeneration is genetically controlled, since genes that influence lifespan have been identified. However, little is known about genes that affect reproductive aging or aging of specific somatic tissues. To identify genes that are important for controlling these degenerative changes, we used chemical mutagenesis to perform forward genetic screens in Caenorhabditis elegans. By conducting a screen focused on somatic aging, we identified mutant hermaphrodites that displayed extended periods of pharyngeal pumping, body movement, or survival. One of these mutations is a novel allele of the age-1 gene. age-1 encodes a phosphatidylinositol-3-kinase (PI3K) that functions in the insulin/insulin-like growth factor-1 (IGF-1) signaling pathway. age-1(am88) creates a missense change in the conserved PIK domain and causes dramatic extensions of the pharyngeal pumping and body movement spans, as well as a twofold extension of the lifespan. By conducting screens focused on reproductive aging in mated hermaphrodites, we identified mutants that displayed increased progeny production late in life. To characterize these mutations, we developed quantitative measurements of age-related morphological changes in the gonad. The am117 mutation delayed age-related declines in progeny production and morphological changes in the gonad. These studies provide new insights into the genetic regulation of age-related degenerative changes in somatic and reproductive tissues.

EGFR-PI3K-AKT-mTOR signaling in head and neck squamous cell carcinomas: attractive targets for molecular-oriented therapy

IMPORTANCE OF THE FIELD

Recent advances in the understanding of the oncogenesis of head and neck squamous cell carcinomas (HNSCC) have revealed multiple dysregulated signaling pathways. One frequently altered axis is the EGFR-PI3K-Akt-mTOR pathway. This pathway plays a central role in numerous cellular processes including metabolism, cell growth, apoptosis, survival and differentiation, which ultimately contributes to HNSCC progression.

AREAS COVERED IN THIS REVIEW

Books, journals, databases and websites have been searched to provide a current review on the subject.

WHAT THE READER WILL GAIN

This article reviews the current understanding of EGFR-PI3K-Akt-mTOR signaling in HNSCC, including the impact of both genetic and epigenetic alterations. This review further highlights the potential of targeting this signaling cascade as a promising therapeutic approach in the treatment of HNSCC.

TAKE HOME MESSAGE

Genetic alterations of several nodes within this pathway, including both genetic and epigenetic changes, leading to either oncogene activation or inactivation of tumor suppressors have frequently been implicated in HNSCC. Consequently, drugs that target the central nodes of this pathway have become attractive for molecular oriented cancer therapies. Numerous preclinical and clinical studies are being performed in HNSCC; however, more studies are still needed to better understand the biology of this pathway.

The class IA phosphatidylinositol 3-kinase p110-beta subunit is a positive regulator of autophagy

Autophagy is an evolutionarily conserved cell renewal process that depends on phosphatidylinositol 3-phosphate (PtdIns(3)P). In metazoans, autophagy is inhibited by PtdIns(3,4,5)P(3), the product of class IA PI3Ks, which mediates the activation of the Akt-TOR kinase cascade. However, the precise function of class IA PI3Ks in autophagy remains undetermined. Class IA PI3Ks are heterodimeric proteins consisting of an 85-kD regulatory subunit and a 110-kD catalytic subunit. Here we show that the class IA p110-β catalytic subunit is a positive regulator of autophagy. Genetic deletion of p110-β results in impaired autophagy in mouse embryonic fibroblasts, liver, and heart. p110-β does not promote autophagy by affecting the Akt-TOR pathway. Rather, it associates with the autophagy-promoting Vps34-Vps15-Beclin 1-Atg14L complex and facilitates the generation of cellular PtdIns(3)P. Our results unveil a previously unknown function for p110-β as a positive regulator of autophagy in multicellular organisms.

The α-isoform of class II phosphoinositide 3-kinase is necessary for the activation of ERK but not Akt/PKB

Phosphoinositide 3-kinases (PI3Ks) are key enzymes that activate intracellular signaling molecules when a number of different growth factors bind to cell surface receptors. PI3Ks are divided into three classes (I, II, III), and enzymes of each class have different tissue specificities and physiological functions. The α-isoform (PI3K-C2α) of class II PI3Ks is considered ubiquitous and preferentially activated by insulin. Our previous study showed that suppression of PI3K-C2α leads to apoptotic cell death. The aim of this study is to determine whether depletion of PI3K-C2α affects ERK or PKB/Akt activity following stimulation with serum and insulin growth factors in Chinese hamster ovary cells expressing human insulin receptors (CHO-IR) and human HepG2 liver cells. Different antisense oligonucleotides (ODNs), which were designed based on the sequence of the C2 domain of the human PI3K-C2α gene, were transfected into cells to inhibit PI3K-C2α expression. Insulin- or serum-induced stimulation of ERK was significantly suppressed by depletion of PI3K-C2α, whereas phosphorylation of IRS-1 and the stimulation of PKB/Akt by insulin were not affected. The number of apoptotic cells was also increased by depletion of PI3K-C2α protein levels. Taken together, our data indicate that PI3K-C2α may be a crucial factor in the stimulation of ERK activity in response to serum or insulin, whereas it is less important for the stimulation of PKB/Akt activity in response to insulin.

Phosphatidylinositol 3-kinase signaling in thymocytes: the need for stringent control

The thymus serves as the primary site for the lifelong formation of new T lymphocytes; hence, it is essential for the maintenance of an effective immune system. Although thymocyte development has been widely studied, the mechanisms involved are incompletely defined. A comprehensive understanding of the molecular events that control regular thymocyte development will not only shed light on the physiological control of T cell differentiation but also probably provide insight into the pathophysiology of T cell immunodeficiencies, the molecular basis that underpins autoimmunity, and the mechanisms that instigate the formation of T cell lymphomas. Phosphatidylinositol 3-kinases (PI3Ks) play a critical role in thymocyte development, although not all of their downstream mediators have yet been identified. Here, we discuss experimental evidence that argues for a critical role of the PI3K-phosphoinositide-dependent protein kinase (PDK1)-protein kinase B (PKB) signaling pathway in the development of both normal and malignant thymocytes, and we highlight molecules that can potentially be targeted therapeutically.