The regulation of class IA PI 3-kinases by inter-subunit interactions

MicroRNA-425: A Pivotal Regulator Participating in Tumorigenesis of Human Cancers

MicroRNAs (miRNAs) are small single-stranded regulatory RNAs that are shown to be dysregulated in a wide array of human cancers. MiRNAs play critical roles in cancer progression and function as either oncogenes or tumor suppressors through modulating various target genes. Therefore, they possess great potential as diagnostic and therapeutic targets for cancer detection and treatment. In particular, recent studies have illustrated that miR-425 is also dysregulated in various human malignancies and plays a fundamental role in cancer initiation and progression. miR-425 has been reported to function as a dual-role miRNA participating in the regulation of cellular processes, including metastasis, invasion, and cell proliferation by modulating multiple signaling pathways, such as TGF-β, Wnt, and P13K/AKT pathways. Therefore, regarding recent researches showing the high therapeutic potential of miR-425, in this review, we have noted the impact of its dysregulation on signaling pathways and various aspects of tumorigenesis in a variety of human cancers.

PI3K/AKT Signaling Pathway Mediated Autophagy in Oral Carcinoma - A Comprehensive Review

Oral cancer is the most heterogeneous cancer at clinical and histological levels. PI3K/AKT/mTOR pathway was identified as one of the most commonly modulated signals in oral cancer, which regulates major cellular and metabolic activity of the cell. Thus, various proteins of PI3K/AKT/mTOR pathway were used as therapeutic targets for oral cancer, to design more specific drugs with less off-target toxicity. This review sheds light on the regulation of PI3K/AKT/mTOR, and its role in controlling autophagy and associated apoptosis during the progression and metastasis of oral squamous type of malignancy (OSCC). In addition, we reviewed in detail the upstream activators and the downstream effectors of PI3K/AKT/mTOR signaling as potential therapeutic targets for oral cancer treatment.

The WW domain of IQGAP1 binds directly to the p110α catalytic subunit of PI 3-kinase

IQGAP1 is a multi-domain cancer-associated protein that serves as a scaffold protein for multiple signaling pathways. Numerous binding partners have been found for the calponin homology, IQ and GAP-related domains in IQGAP1. Identification of a binding partner for its WW domain has proven elusive, however, even though a cell-penetrating peptide derived from this domain has marked anti-tumor activity. Here, using in vitro binding assays with human proteins and co-precipitation from human cells, we show that the WW domain of human IQGAP1 binds directly to the p110α catalytic subunit of phosphoinositide 3-kinase (PI3K). In contrast, the WW domain does not bind to ERK1/2, MEK1/2, or the p85α regulatory subunit of PI3K when p85α is expressed alone. However, the WW domain is able to bind to the p110α/p85α heterodimer when both subunits are co-expressed, as well as to the mutationally activated p110α/p65α heterodimer. We present a model of the structure of the IQGAP1 WW domain, and experimentally identify key residues in the hydrophobic core and beta strands of the WW domain that are required for binding to p110α. These findings contribute to a more precise understanding of IQGAP1-mediated scaffolding, and of how IQGAP1-derived therapeutic peptides might inhibit tumorigenesis.

Macropinocytosis: mechanisms and regulation

Macropinocytosis is defined as an actin-dependent but coat- and dynamin-independent endocytic uptake process, which generates large intracellular vesicles (macropinosomes) containing a non-selective sampling of extracellular fluid. Macropinocytosis provides an important mechanism of immune surveillance by dendritic cells and macrophages, but also serves as an essential nutrient uptake pathway for unicellular organisms and tumor cells. This review examines the cell biological mechanisms that drive macropinocytosis, as well as the complex signaling pathways - GTPases, lipid and protein kinases and phosphatases, and actin regulatory proteins - that regulate macropinosome formation, internalization, and disposition.

Phosphoinositide 3-Kinase (PI3K) Inhibitors and Breast Cancer: An Overview of Current Achievements

The phosphatidylinositol 3-kinase (PI3K) pathway is one of the most altered pathways in human cancers, and it plays a central role in cellular growth, survival, metabolism, and cellular mobility, making it a particularly interesting therapeutic target. Recently, pan-inhibitors and then selective p110α subunit inhibitors of PI3K were developed. Breast cancer is the most frequent cancer in women and, despite therapeutic progress in recent years, advanced breast cancers remain incurable and early breast cancers are at risk of relapse. Breast cancer is divided in three molecular subtypes, each with its own molecular biology. However, PI3K mutations are found in all breast cancer subtypes in three main "hotspots". In this review, we report the results of the most recent and main ongoing studies evaluating pan-PI3K inhibitors and selective PI3K inhibitors in each breast cancer subtype. In addition, we discuss the future of their development, the various potential mechanisms of resistance to these inhibitors and the ways to circumvent them.

Dengue Virus Dysregulates Master Transcription Factors and PI3K/AKT/mTOR Signaling Pathway in Megakaryocytes

Dengue virus (DENV) infection can cause either self-limited dengue fever or hemorrhagic complications. Low platelet count is one of the manifestations of dengue fever. Megakaryocytes are the sole producers of platelets. However, the role of both host and viral factors in megakaryocyte development, maturation, and platelet production is largely unknown in DENV infection. PI3K/AKT/mTOR pathway plays a significant role in cell survival, maturation, and megakaryocyte development. We were interested to check whether pathogenic insult can impact this pathway. We observed decreased expression of most of the major key molecules associated with the PI3K/AKT/mTOR pathway in DENV infected MEG-01 cells. In this study, the involvement of PI3K/AKT/mTOR pathway in megakaryocyte development and maturation was confirmed with the use of specific inhibitors in infected MEG-01 cells. Our results showed that direct pharmacologic inhibition of this pathway greatly impacted megakaryopoiesis associated molecule CD61 and some essential transcription factors (GATA-1, GATA-2, and NF-E2). Additionally, we observed apoptosis in megakaryocytes due to DENV infection. Our results may suggest that DENV impairs PI3K/AKT/mTOR axis and molecules involved in the development and maturation of megakaryocytes. It is imperative to investigate the role of these molecules in the context of megakaryopoiesis during DENV infection to better understand the pathways and mechanisms, which in turn might provide insights into the development of antiviral strategies.

Molecular Determination of mirRNA-126 rs4636297, Phosphoinositide-3-Kinase Regulatory Subunit 1-Gene Variability rs7713645, rs706713 (Tyr73Tyr), rs3730089 (Met326Ile) and Their Association with Susceptibility to T2D

Type 2 diabetes is a metabolic disease characterized by elevated blood sugar. It has serious complications and socioeconomic impact. The MicroRNAs are short single-stranded and non-coding RNA molecules. They regulate gene expression at the post-transcriptional levels. They are important for many physiological processes including metabolism, growth, and others. The phosphoinositide 3-kinase (PI3K) is important for insulin signaling and glucose uptake. The genome wide association studies have identified the association of certain loci with diseases including T2D. In this study we have examined the association of miR126 rs4636297 and Phosphoinositide-3-kinase regulatory subunit 1 (PIK3R1) gene Variations rs7713645, rs706713 (Tyr73Tyr), and rs3730089 (Met326Ile) with T2D using the amplification refractory mutation system PCR. Results indicated that there was a significant different (p-value < 0.05) in the Mir126 rs4636297 genotypes distribution between cases and controls, and the minor allele of the rs4636297 was also associated with T2D with OR = 0.58, p-value < 0.05. In addition results showed that there were significant differences (p-value < 0.05) of rs4636297 genotype distribution of patients with normal and patient with abnormal lipid profile. Results also showed that the PIK3R1 rs7713645 and rs3730089 genotype distribution was significantly different between cases and controls with a p-values < 0.05. In addition, the minor allele of the rs7713645 and rs3730089 were associated with T2D with OR = 0.58, p-value < 0.05. We conclude that the Mir126 rs4636297 and PIK3R1 SNPs (rs7713645 and rs3730089) were associated with T2D. These results need verification in future studies with larger sample sizes and in different populations. Protein-protein interaction and enzyme assay studies are also required to uncover the effect of the SNPs on the PI3K regulatory subunit (PI3KR1) and PI3K catalytic activity.

p55PIK deficiency and its NH2-terminal derivative inhibit inflammation and emphysema in COPD mouse model

Chronic obstructive pulmonary disease (COPD) is composed of chronic airway inflammation and emphysema. Recent studies show that Class IA phosphatidylinositol 3-kinases (PI3Ks) play an important role in the regulation of inflammation and emphysema. However, there are few studies on their regulatory subunits. p55PIK is a regulatory subunit of Class IA PI3Ks, and its unique NH₂-terminal gives it special functions. p55PIK expression in the lungs of nonsmokers, smokers, and patients with COPD was examined. We established a fusion protein TAT-N15 from the NH₂-terminal effector sequence of p55PIK and TAT (the transduction domain of HIV transactivator protein) and investigated the effects of silencing p55PIK or adding TAT-N15 on cigarette smoke exposure at the cellular and animal level. p55PIK expression was increased in patients with COPD. p55PIK deficiency and TAT-N15 significantly inhibited the cigarette smoke extract-induced IL-6, IL-8, and activation of the Akt and the NF-κB pathway in BEAS-2B. p55PIK deficiency and TAT-N15 intranasal administration prevented emphysema and the lung function decline in mice exposed to smoke for 6 mo. p55PIK deficiency and TAT-N15 significantly inhibited lung inflammatory infiltration, reduced levels of IL-6 and KC in mice lung homogenate, and inhibited activation of the Akt and the NF-κB signaling in COPD mice lungs. Our studies indicate that p55PIK is involved in the pathogenesis of COPD, and its NH₂-terminal derivative TAT-N15 could be an effective drug in the treatment of COPD by inhibiting the activation of the Akt and the NF-κB pathway.

Biophysical and Structural Characterization of Novel RAS-Binding Domains (RBDs) of PI3Kα and PI3Kγ

Phosphatidylinositol-3-kinases (PI3Ks) are lipid kinases that phosphorylate phosphatidylinositol 4,5-bisphosphate to generate a key lipid second messenger, phosphatidylinositol 3,4,5-bisphosphate. PI3Kα and PI3Kγ require activation by RAS proteins to stimulate signaling pathways that control cellular growth, differentiation, motility and survival. Intriguingly, RAS binding to PI3K isoforms likely differ, as RAS mutations have been identified that discriminate between PI3Kα and PI3Kγ, consistent with low sequence homology (23%) between their RAS binding domains (RBDs). As disruption of the RAS/PI3Kα interaction reduces tumor growth in mice with RAS- and epidermal growth factor receptor driven skin and lung cancers, compounds that interfere with this key interaction may prove useful as anti-cancer agents. However, a structure of PI3Kα bound to RAS is lacking, limiting drug discovery efforts. Expression of full-length PI3K isoforms in insect cells has resulted in low yield and variable activity, limiting biophysical and structural studies of RAS/PI3K interactions. This led us to generate the first RBDs from PI3Kα and PI3Kγ that can be expressed at high yield in bacteria and bind to RAS with similar affinity to full-length PI3K. We also solved a 2.31 Å X-ray crystal structure of the PI3Kα-RBD, which aligns well to full-length PI3Kα. Structural differences between the PI3Kα and PI3Kγ RBDs are consistent with differences in thermal stability and may underly differential RAS recognition and RAS-mediated PI3K activation. These high expression, functional PI3K RBDs will aid in interrogating RAS interactions and could aid in identifying inhibitors of this key interaction.

Anti-Inflammatory Activity of Extracts and Pure Compounds Derived from Plants via Modulation of Signaling Pathways, Especially PI3K/AKT in Macrophages

The plant kingdom is a source of important therapeutic agents. Therefore, in this review, we focus on natural compounds that exhibit efficient anti-inflammatory activity via modulation signaling transduction pathways in macrophage cells. Both extracts and pure chemicals from different species and parts of plants such as leaves, roots, flowers, barks, rhizomes, and seeds rich in secondary metabolites from various groups such as terpenes or polyphenols were included. Selected extracts and phytochemicals control macrophages biology via modulation signaling molecules including NF-κB, MAPKs, AP-1, STAT1, STAT6, IRF-4, IRF-5, PPARγ, KLF4 and especially PI3K/AKT. Macrophages are important immune effector cells that take part in antigen presentation, phagocytosis, and immunomodulation. The M1 and M2 phenotypes are related to the production of pro- and anti-inflammatory agents, respectively. The successful resolution of inflammation mediated by M2, or failed resolution mediated by M1, may lead to tissue repair or chronic inflammation. Chronic inflammation is strictly related to several disorders. Thus, compounds of plant origin targeting inflammatory response may constitute promising therapeutic strategies.

CircRNA_0000392 promotes colorectal cancer progression through the miR-193a-5p/PIK3R3/AKT axis

Background

Circular RNAs (circRNAs), important members of the noncoding RNA family, have been recently revealed to play a role in the pathogenic progression of diseases, particularly in the malignant progression of cancer. With the application of high-throughput sequencing technology, a large number of circRNAs have been identified in tumor tissues, and some circRNAs have been demonstrated to act as oncogenes. In this study, we analyzed the circRNA expression profile in colorectal cancer (CRC) tissues and normal adjacent tissues by high-throughput sequencing. We focused on circRNA_0000392, a circRNA with significantly increased expression in CRCtissues, and further investigated its function in the progression of colorectal cancer.

Methods

The expression profile of circRNAs in 6 pairs of CRC tissues and normal adjacent tissues was analyzed by RNA sequencing. We verified the identified differentially expressed circRNAs in additional samples by qRT-PCR and selected circRNA_0000392 to evaluate its associations with clinicopathological features. Then, we knocked down circRNA_0000392 in CRC cells and investigated the in vitro and in vivo effects using functional experiments. Dual luciferase and RNA pull-down assays were performed to further explore the downstream potential molecular mechanisms.

Results

CircRNA_0000392 was significantly upregulated in CRC compared with normal adjacent tissues and cell lines. The expression level of circRNA_0000392 was positively correlated with the malignant progression of CRC. Functional studies revealed that reducing the expression of circRNA_0000392 could inhibit the proliferation and invasion of CRC both in vitro and in vivo. Mechanistically, circRNA_0000392 could act as a sponge of miR-193a-5p and regulate the expression of PIK3R3, affecting the activation of the AKT-mTOR pathway in CRC cells.

Conclusions

CircRNA_0000392 functions as an oncogene through the miR-193a-5p/PIK3R3/Akt axis in CRC cells, suggesting that circRNA_0000392 is a potential therapeutic target for the treatment of colorectal cancer and a predictive marker for CRC patients.

LRRC4 Suppresses E-Cadherin-Dependent Collective Cell Invasion and Metastasis in Epithelial Ovarian Cancer

Epithelial ovarian cancer (EOC) is the most malignant gynecological carcinoma and is of a high incidence of death due to detection at late stages when metastasis already occurs. However, the mechanism underlying metastasis of EOC remains unclear. Analysis of the open database and experiments with immunochemistry showed that LRRC4 is lowly expressed in high-grade serous ovarian cancer (HGSC) cells and during EOC metastasis. The 3D cell culture system and the orthotopic ovarian xenograft model infected with LRRC4-containing adeno-associated virus serotype 9 (AAV9) were used to confirm collective invasion and metastasis of cells in vitro and in vivo. Phos-tag SDS-PAGE was used to detect the phosphorylation of LRRC4 and PIK3R1. A number of experiments with methods such as co-immunoprecipitation and immunoblotting were performed to explore the mechanism for the actions of LRRC4 and PIK3R1 in EOC metastasis. An inverse correlation between LRRC4 and E-cadherin expression was detected in the regions of invasion in primary EOC tissues and metastatic ascites. LRRC4 binds to the cSH2 domain of PIK3R1 and inhibits the activity of PIK3R1, without disrupting the physical interactions between PIK3R1 and PIK3CA. LRRC4 inhibits EOC metastasis by targeting E-cadherin-dependent collective cell invasion and does so by inhibiting the PIK3R1-mediated AKT/GSK3β/β-catenin signaling pathway. LRRC4 functions as a tumor suppressor gene to inhibit EOC collective invasion and metastasis in vitro and in vivo and does so by directly binding to the cSH2 domain of PIK3R1 to exert its regulatory function. Our findings provide a potential novel approach for metastasis prognosis and a new strategy for the treatment of EOC.

The role of microRNAs in the regulation of insulin signaling pathway with respect to metabolic and mitogenic cascades: A review

Insulin resistance (IR) is a shared pathological condition among type 2 diabetes, obesity, cardiovascular disease, and other metabolic disorders. It is growing significantly all over the world and consequently, a substantial effort is needed for developing the potential novel diagnostics and therapeutics. An insulin signaling pathway is tightly modulated by different mechanisms including the epigenetic modifications. Today, a deal of great attention has been shifted towards the regulatory role of noncoding RNAs on target proteins of the insulin signaling pathway. Noncoding RNAs are a major area of the epigenetics which control gene expression at the posttranscriptional levels and include a large class of microRNAs (miRNAs). With this in view, many studies have implicated the mediatory effects of miRNAs on the downstream metabolic and mitogenic proteins of the insulin signaling pathway. Since providing new biomarkers for the early diagnosis of IR and related metabolic traits are very significant, we intended to review the possible role of miRNAs in the regulation of the insulin signaling pathway, with a primary focus on the downstream target proteins of the metabolic and mitogenic cascades.

Strategies to Overcome Resistance Mechanisms in T-Cell Acute Lymphoblastic Leukemia

Chemoresistance is a major cause of recurrence and death from T-cell acute lymphoblastic leukemia (T-ALL), both in adult and pediatric patients. In the majority of cases, drug-resistant disease is treated by selecting a combination of other drugs, without understanding the molecular mechanisms by which malignant cells escape chemotherapeutic treatments, even though a more detailed genomic characterization and the identification of actionable disease targets may enable informed decision of new agents to improve patient outcomes. In this work, we describe pathways of resistance to common chemotherapeutic agents including glucocorticoids and review the resistance mechanisms to targeted therapy such as IL7R, PI3K-AKT-mTOR, NOTCH1, BRD4/MYC, Cyclin D3: CDK4/CDK6, BCL2 inhibitors, and selective inhibitors of nuclear export (SINE). Finally, to overcome the limitations of the current trial-and-error method, we summarize the experiences of anti-cancer drug sensitivity resistance profiling (DSRP) approaches as a rapid and relevant strategy to infer drug activity and provide functional information to assist clinical decision one patient at a time.

The kinetics of folding of the NSH2 domain from p85

SH2 domains are protein domains that mediate protein-protein interaction through the recognition and binding of specific sequences containing phosphorylated tyrosines. The p85 protein is the regulatory subunit of the heterodimeric enzyme PI3K, an important enzyme involved in several molecular pathways. In this work we characterize the folding kinetics of the NSH2 domain of p85. Our data clearly reveal peculiar folding kinetics, characterized by an apparent mismatch between the observed folding and unfolding kinetics. Taking advantage of double mixing stopped flow experiments and site directed mutagenesis we demonstrate that such behavior is due to the cis/trans isomerization of the peptide bond between D73 and P74, being in a cis conformation in the native protein. Our data are discussed in comparison with previous works on the folding of other SH2 domains.

PI3Kβ-A Versatile Transducer for GPCR, RTK, and Small GTPase Signaling

The phosphoinositide 3-kinase (PI3K) family includes eight distinct catalytic subunits and seven regulatory subunits. Only two PI3Ks are directly regulated downstream from G protein-coupled receptors (GPCRs): the class I enzymes PI3Kβ and PI3Kγ. Both enzymes produce phosphatidylinositol 3,4,5-trisposphate in vivo and are regulated by both heterotrimeric G proteins and small GTPases from the Ras or Rho families. However, PI3Kβ is also regulated by direct interactions with receptor tyrosine kinases (RTKs) and their tyrosine phosphorylated substrates, and similar to the class II and III PI3Ks, it binds activated Rab5. The unusually complex regulation of PI3Kβ by small and trimeric G proteins and RTKs leads to a rich landscape of signaling responses at the cellular and organismic levels. This review focuses first on the regulation of PI3Kβ activity in vitro and in cells, and then summarizes the biology of PI3Kβ signaling in distinct tissues and in human disease.

Impact of p85α Alterations in Cancer

The phosphatidylinositol 3-kinase (PI3K) pathway plays a central role in the regulation of cell signaling, proliferation, survival, migration and vesicle trafficking in normal cells and is frequently deregulated in many cancers. The p85α protein is the most characterized regulatory subunit of the class IA PI3Ks, best known for its regulation of the p110-PI3K catalytic subunit. In this review, we will discuss the impact of p85α mutations or alterations in expression levels on the proteins p85α is known to bind and regulate. We will focus on alterations within the N-terminal half of p85α that primarily regulate Rab5 and some members of the Rho-family of GTPases, as well as those that regulate PTEN (phosphatase and tensin homologue deleted on chromosome 10), the enzyme that directly counteracts PI3K signaling. We highlight recent data, mapping the interaction surfaces of the PTEN⁻p85α breakpoint cluster region homology (BH) domain, which sheds new light on key residues in both proteins. As a multifunctional protein that binds and regulates many different proteins, p85α mutations at different sites have different impacts in cancer and would necessarily require distinct treatment strategies to be effective.

Insight into the PTEN - p85α interaction and lipid binding properties of the p85α BH domain

The phosphatidylinositol 3-kinase (PI3K) pathway plays a key role in regulating cell growth and cell survival and is frequently deregulated in cancer cells. p85α regulates the p110α lipid kinase, and also stabilizes and stimulates PTEN, the lipid phosphatase that downregulates this pathway. In this report, we determined that the p85α BH domain binds several phosphorylated phosphoinositide lipids, an interaction that could help localize p85α to membranes rich in these lipids. We also identified key residues responsible for mediating PTEN – p85α complex formation. Based on these experimental results, a docking model for the PTEN – p85α BH domain complex was developed that is consistent with the known binding interactions for both PTEN and p85α. This model involves extensive side-chain and peptide backbone contacts between both the PASE and C2 domains of PTEN with the p85α BH domains. The p85α BH domain residues shown to be important for PTEN binding were p85α residues E212, Q221, K225, R228 and H234. We also verified experimentally the importance of PTEN-E91 in mediating the interaction with the p85α BH domain. These results shed new light on the mechanism of PTEN regulation by p85α.

Class I Phosphoinositide 3-Kinase PIK3CA/p110α and PIK3CB/p110β Isoforms in Endometrial Cancer

The phosphoinositide 3-kinase (PI3K) signalling pathway is highly dysregulated in cancer, leading to elevated PI3K signalling and altered cellular processes that contribute to tumour development. The pathway is normally orchestrated by class I PI3K enzymes and negatively regulated by the phosphatase and tensin homologue, PTEN. Endometrial carcinomas harbour frequent alterations in components of the pathway, including changes in gene copy number and mutations, in particular in the oncogene PIK3CA, the gene encoding the PI3K catalytic subunit p110α, and the tumour suppressor PTEN. PIK3CB, encoding the other ubiquitously expressed class I isoform p110β, is less frequently altered but the few mutations identified to date are oncogenic. This isoform has received more research interest in recent years, particularly since PTEN-deficient tumours were found to be reliant on p110β activity to sustain transformation. In this review, we describe the current understanding of the common and distinct biochemical properties of the p110α and p110β isoforms, summarise their mutations and highlight how they are targeted in clinical trials in endometrial cancer.

Quantitation of class IA PI3Ks in mice reveals p110-free-p85s and isoform-selective subunit associations and recruitment to receptors

Class IA PI3Ks have many roles in health and disease. The rules that govern intersubunit and receptor associations, however, remain unclear. We engineered mouse lines in which individual endogenous class IA PI3K subunits were C-terminally tagged with 17aa that could be biotinylated in vivo. Using these tools we quantified PI3K subunits in streptavidin or PDGFR pull-downs and cell lysates. This revealed that p85α and β bound equivalently to p110α or p110β but p85α bound preferentially to p110δ. p85s were found in molar-excess over p110s in a number of contexts including MEFs (p85β, 20%) and liver (p85α, 30%). In serum-starved MEFs, p110-free-p85s were preferentially, compared with heterodimeric p85s, bound to PDGFRs, consistent with in vitro assays that demonstrated they bound PDGFR-based tyrosine-phosphorylated peptides with higher affinity and co-operativity; suggesting they may act to tune a PI3K activation threshold. p110α-heterodimers were recruited 5-6× more efficiently than p110β-heterodimers to activated PDGFRs in MEFs or to PDGFR-based tyrosine-phosphorylated peptides in MEF-lysates. This suggests that PI3Kα has a higher affinity for relevant tyrosine-phosphorylated motifs than PI3Kβ. Nevertheless, PI3Kβ contributes substantially to acute PDGF-stimulation of PIP3 and PKB in MEFs because it is synergistically, and possibly sequentially, activated by receptor-recruitment and small GTPases (Rac/CDC42) via its RBD, whereas parallel activation of PI3Kα is independent of its RBD. These results begin to provide molecular clarity to the rules of engagement between class IA PI3K subunits in vivo and past work describing "excess p85," p85α as a tumor suppressor, and differential receptor activation of PI3Kα and PI3Kβ.

Patient-derived mutations within the N-terminal domains of p85α impact PTEN or Rab5 binding and regulation

The p85α protein regulates flux through the PI3K/PTEN signaling pathway, and also controls receptor trafficking via regulation of Rab-family GTPases. In this report, we determined the impact of several cancer patient-derived p85α mutations located within the N-terminal domains of p85α previously shown to bind PTEN and Rab5, and regulate their respective functions. One p85α mutation, L30F, significantly reduced the steady state binding to PTEN, yet enhanced the stimulation of PTEN lipid phosphatase activity. Three other p85α mutations (E137K, K288Q, E297K) also altered the regulation of PTEN catalytic activity. In contrast, many p85α mutations reduced the binding to Rab5 (L30F, I69L, I82F, I177N, E217K), and several impacted the GAP activity of p85α towards Rab5 (E137K, I177N, E217K, E297K). We determined the crystal structure of several of these p85α BH domain mutants (E137K, E217K, R262T E297K) for bovine p85α BH and found that the mutations did not alter the overall domain structure. Thus, several p85α mutations found in human cancers may deregulate PTEN and/or Rab5 regulated pathways to contribute to oncogenesis. We also engineered several experimental mutations within the p85α BH domain and identified L191 and V263 as important for both binding and regulation of Rab5 activity.

Interaction of Calmodulin with the cSH2 Domain of the p85 Regulatory Subunit

Calmodulin (CaM) is a calcium sensor protein that directly interacts with the dual-specificity (lipid and protein) kinase PI3Kα through the SH2 domains of the p85 regulatory subunit. In adenocarcinomas, the CaM interaction removes the autoinhibition of the p110 catalytic subunit of PI3Kα, leading to activation of PI3Kα and promoting cell proliferation, survival, and migration. Here we demonstrate that the cSH2 domain of p85α engages its two CaM-binding motifs in the interaction with the N- and C-lobes of CaM as well as the flexible central linker, and our nuclear magnetic resonance experiments provide structural details. We show that in response to binding CaM, cSH2 exposes its tryptophan residue at the N-terminal region to the solvent. Because of the flexible nature of both CaM and cSH2, multiple binding modes of the interactions are possible. Binding of CaM to the cSH2 domain can help release the inhibition imposed on the p110 subunit, similar to the binding of the phosphorylated motif of RTK, or phosphorylated CaM (pCaM), to the SH2 domains. Amino acid sequence analysis shows that CaM-binding motifs are common in SH2 domains of non-RTKs. We speculate that CaM can also activate these kinases through similar mechanisms.

Differential expression of miRNAs are associated with the insulin signaling pathway in preeclampsia and gestational hypertension

OBJECTIVES

The objective of this study was to determine microRNAs (miRNAs) expression levels in placental tissue and serum samples from preeclampsia (PE) and gestational hypertensive (GH) patients.

STUDY DESIGN

Using a targeted qPCR method, the selected miRNAs putatively involved in the PE and GH were examined from normotensive (n = 32), PE (n = 32) and GH (n = 28) in South African women. Western blot analysis of protein expressions of AKT and PI3K was performed in the placental tissue of all three groups.

RESULTS

qPCR results of serum miR-222 expression levels showed a significant decrease in PE compared to GH and normotensive groups. miR-29a expression levels were significantly increased in PE and GH groups compared to normotensives. Serum expression levels of miR-181a in GH showed a significant increase compared to the PE and normotensive groups. Placental tissue expression levels of miR-181a were significantly increased in PE and GH groups compared to normotensives. Western blot results of placental tissue showed a decrease in the expression levels of AKT-serine and threonine in the PE groups compared to the normotensives and a significantly higher expression in the GH groups compared to normotensives. Phosphatidyl-inositol-3 kinase (PI3K) expression levels were significantly decreased in PE and GH groups compared to normotensives.

CONCLUSION

The present study, interestingly, demonstrates the differential expression of circulating miRNA in GH and a correlation between the expression levels of miRNAs with AKT/PI3K in the insulin signaling pathway, reinforcing the presence of metabolic dysregulation in PE and GH.

NEDL2 regulates enteric nervous system and kidney development in its Nedd8 ligase activity-dependent manner

The GDNF (Glial cell line-derived neurotrophic factor)/Ret/Akt signaling pathway is essential to the development of ENS (enteric nervous system) as well as kidney. We previously showed that the HECT-type E3 ligase NEDL2 (Nedd4-like ligase 2) is required for the ENS development by activating GDNF/Ret/Akt. However, the underlying mechanism remains unknown. Here we show that in addition to ENS, NEDL2 is also pivotal for kidney development since about 1/3 of Nedl2-deficient mice displayed postnatal unilateral or bilateral kidney hydronephrosis. Double knockout of Nedl1 and Nedl2 in mice leads to postnatal lethal within 2 weeks and the phenotypes resemble those of Nedl2 single knockout mice. Surprisingly, its close member NEDL1 is dispensable for ENS and kidney function and the reason is lack of NEDL1 expression in these systems during early development. Furthermore, biochemical analysis indicated that NEDL2 appears to act like a scaffold protein to recruit SHC, Grb2, PI3K (p110 and p85), PDK1 and Akt together to promote the signaling transduction. Intriguingly, we found that NEDL2 harbours intrinsic Nedd8 ligase activity with cysteine 1341 as the core site. NEDL2 upregulates GDNF-stimulated Akt activity dependent of its Nedd8 ligase activity but not its ubiquitin ligase activity. These findings demonstrate that NEDL2 but not NEDL1 is required for ENS and kidney development in a unique Nedd8 ligase-dependent manner.

Identification of the Binding Sites on Rab5 and p110beta Phosphatidylinositol 3-kinase

Rab5 is a small monomeric GTPase that mediates protein trafficking during endocytosis. Inactivation of Rab5 by GTP hydrolysis causes a conformational change that masks binding sites on its “switch regions” from downstream effectors. The p85 subunit of phosphatidylinositol 3-kinase (PI3K) is a GTPase activating protein (GAP) towards Rab5. Whereas p85 can bind with both Rab5-GTP and Rab5-GDP, the PI3K catalytic subunit p110β binds only Rab5-GTP, suggesting it interacts with the switch regions. Thus, the GAP functions of the catalytic arginine finger (from p85) and switch region stabilization (from p110β) may be provided by both proteins, acting together. To identify the Rab5 residues involved in binding p110β, residues in the Rab5 switch regions were mutated. A stabilized recombinant p110 protein, where the p85-iSH2 domain was fused to p110 (alpha or beta) was used in binding experiments. Eleven Rab5 mutants, including E80R and H83E, showed reduced p110β binding. The Rab5 binding site on p110β was also resolved through mutation of p110β in its Ras binding domain, and includes residues I234, E238 and Y244. This is a second region within p110β important for Rab5 binding. The Rab5-GTP:p110β interaction may be further elucidated through the characterization of these non-binding mutants in cells.

Rac1-stimulated macropinocytosis enhances Gβγ activation of PI3Kβ

Phosphoinositide 3-kinases (PI 3-kinases) are regulated by a diverse range of upstream activators, including receptor tyrosine kinases (RTKs), G-protein-coupled receptors (GPCRs), and small GTPases from the Ras, Rho and Rab families. For the Class IA PI 3-kinase PI3Kβ, two mechanisms for GPCR-mediated regulation have been described: direct binding of Gβγ subunits to the C2-helical domain linker of p110β, and Dock180/Elmo1-mediated activation of Rac1, which binds to the Ras-Binding Domain of p110β. We now show that the integration of these dual pathways is unexpectedly complex. In breast cancer cells, expression of constitutively activated Rac1 (CA-Rac1) along with either GPCR stimulation or expression of Gβγ led to an additive PI3Kβ-dependent activation of Akt. Whereas CA-Rac1-mediated activation of Akt was blocked in cells expressing a mutated PI3Kβ that cannot bind Gβγ, Gβγ and GPCR-mediated activation of Akt was preserved when Rac1 binding to PI3Kβ was blocked. Surprisingly, PI3Kβ-dependent CA-Rac1 signaling to Akt was still seen in cells expressing a mutant p110β that cannot bind Rac1. Instead of directly binding to PI3Kβ, CA-Rac1 acts by enhancing Gβγ coupling to PI3Kβ, as CA-Rac1-mediated Akt activation was blocked by inhibitors of Gβγ. Cells expressing CA-Rac1 exhibited a robust induction of macropinocytosis, and inhibitors of macropinocytosis blocked the activation of Akt by CA-Rac1 or lysophosphatidic acid. Our data suggest that Rac1 can potentiate the activation of PI3Kβ by GPCRs through an indirect mechanism, by driving the formation of macropinosomes that serve as signaling platforms for Gβγ coupling to PI3Kβ.

Genes with mutation significance were highly associated with the clinical pattern of patients with breast cancer

In the United States, breast cancer is the second leading cause of cancer death in women. Over the past 20 years, breast cancer incidence and mortality rates increased rapidly in developing regions. We aimed to identify the gene mutation patterns that associated with the clinical patterns, including survival status, histo-pathological classes and so forth, of breast cancer. We retrieved 1098 cases of the clinical information, and level-3 legacy data of mRNA expression level, protein expression data and mutation files from GDC data portal. The genes with mutation significance were obtained. We studied the impacts of mutation types on the expression levels of mRNA and protein. Different statistics methods were used to calculate the correlation between the mutation types and the expression data or histo-clinical measures. There were 24 genes with mutation significance identified. The most mutated genes were selected to study the role of specific mutations played on the patients with breast cancer. One interesting finding was the missense mutations on TP53 were related with high expression levels of mRNA and protein. The missense mutations on TP53 were highly related with the morphology, race, ER status, PR status and HER2 Status, while the truncated mutations were only related with the morphology, ER status and PR status. The missense mutation on PIK3CA was highly associated with the morphology, race, ER status and PR status. The mutants with different mutants and the wild type of the most mutated genes had different impacts on the histo-clinical measures that might help personalized therapy.

The PI3K Pathway in Human Disease

Phosphoinositide 3-kinase (PI3K) activity is stimulated by diverse oncogenes and growth factor receptors, and elevated PI3K signaling is considered a hallmark of cancer. Many PI3K pathway-targeted therapies have been tested in oncology trials, resulting in regulatory approval of one isoform-selective inhibitor (idelalisib) for treatment of certain blood cancers and a variety of other agents at different stages of development. In parallel to PI3K research by cancer biologists, investigations in other fields have uncovered exciting and often unpredicted roles for PI3K catalytic and regulatory subunits in normal cell function and in disease. Many of these functions impinge upon oncology by influencing the efficacy and toxicity of PI3K-targeted therapies. Here we provide a perspective on the roles of class I PI3Ks in the regulation of cellular metabolism and in immune system functions, two topics closely intertwined with cancer biology. We also discuss recent progress developing PI3K-targeted therapies for treatment of cancer and other diseases.

Domain analysis reveals striking functional differences between the regulatory subunits of phosphatidylinositol 3-kinase (PI3K), p85α and p85β

Our understanding of isoform-specific activities of phosphatidylinositol 3-kinase (PI3K) is still rudimentary, and yet, deep knowledge of these non-redundant functions in the PI3K family is essential for effective and safe control of PI3K in disease. The two major isoforms of the regulatory subunits of PI3K are p85α and p85β, encoded by the genes PIK3R1 and PIK3R2, respectively. These isoforms show distinct functional differences that affect and control cellular PI3K activity and signaling [1–4]. In this study, we have further explored the differences between p85α and p85β by genetic truncations and substitutions. We have discovered unexpected activities of the mutant proteins that reflect regulatory functions of distinct p85 domains. These results can be summarized as follows: Deletion of the SH3 domain increases oncogenic and PI3K signaling activity. Deletion of the combined SH3-RhoGAP domains abolishes these activities. In p85β, deletion of the cSH2 domain reduces oncogenic and signaling activities. In p85α, such a deletion has an activating effect. The deletions of the combined cSH2 and iSH2 domains and also the deletion of the cSH2, iSH2 and nSH2 domains yield results that go in the same direction, generally activating in p85α and reducing activity in p85β. The contrasting functions of the cSH2 domains are verified by domain exchanges with the cSH2 domain of p85β exerting an activating effect and the cSH2 domain of p85α an inactivating effect, even in the heterologous isoform. In the cell systems studied, protein stability was not correlated with oncogenic and signaling activity. These observations significantly expand our knowledge of the isoform-specific activities of p85α and p85β and of the functional significance of specific domains for regulating the catalytic subunits of class IA PI3K.

ATM mutations and E-cadherin expression define sensitivity to EGFR-targeted therapy in colorectal cancer

EGFR-targeted therapy is a key treatment approach in patients with RAS wildtype metastatic colorectal cancers (CRC). Still, also RAS wildtype CRC may be resistant to EGFR-targeted therapy, with few predictive markers available for improved stratification of patients. Here, we investigated response of 7 CRC cell lines (Caco-2, DLD1, HCT116, HT29, LS174T, RKO, SW480) to Cetuximab and correlated this to NGS-based mutation profiles, EGFR promoter methylation and EGFR expression status as well as to E-cadherin expression. Moreover, tissue specimens of primary and/or recurrent tumors as well as liver and/or lung metastases of 25 CRC patients having received Cetuximab and/or Panitumumab were examined for the same molecular markers. In vitro and in situ analyses showed that EGFR promoter methylation and EGFR expression as well as the MSI and or CIMP-type status did not guide treatment responses. In fact, EGFR-targeted treatment responses were also observed in RAS exon 2 p.G13 mutated CRC cell lines or CRC cases and were further linked to PIK3CA exon 9 mutations. In contrast, non-response to EGFR-targeted treatment was associated with ATM mutations and low E-cadherin expression. Moreover, down-regulation of E-cadherin by siRNA in otherwise Cetuximab responding E-cadherin positive cells abrogated their response. Hence, we here identify ATM and E-cadherin expression as potential novel supportive predictive markers for EGFR-targeted therapy.

Comparing sequencing assays and human-machine analyses in actionable genomics for glioblastoma

Objective:

To analyze a glioblastoma tumor specimen with 3 different platforms and compare potentially actionable calls from each.

Methods:

Tumor DNA was analyzed by a commercial targeted panel. In addition, tumor-normal DNA was analyzed by whole-genome sequencing (WGS) and tumor RNA was analyzed by RNA sequencing (RNA-seq). The WGS and RNA-seq data were analyzed by a team of bioinformaticians and cancer oncologists, and separately by IBM Watson Genomic Analytics (WGA), an automated system for prioritizing somatic variants and identifying drugs.

Results:

More variants were identified by WGS/RNA analysis than by targeted panels. WGA completed a comparable analysis in a fraction of the time required by the human analysts.

Conclusions:

The development of an effective human-machine interface in the analysis of deep cancer genomic datasets may provide potentially clinically actionable calls for individual patients in a more timely and efficient manner than currently possible.

ClinicalTrials.gov identifier:

NCT02725684.

Combining properties of different classes of PI3Kα inhibitors to understand the molecular features that confer selectivity

Phosphoinositide 3-kinases (PI3Ks) are major regulators of many cellular functions, and hyperactivation of PI3K cell signalling pathways is a major target for anticancer drug discovery. PI3Kα is the isoform most implicated in cancer, and our aim is to selectively inhibit this isoform, which may be more beneficial than concurrent inhibition of all Class I PI3Ks. We have used structure-guided design to merge high-selectivity and high-affinity characteristics found in existing compounds. Molecular docking, including the prediction of water-mediated interactions, was used to model interactions between the ligands and the PI3Kα affinity pocket. Inhibition was tested using lipid kinase assays, and active compounds were tested for effects on PI3K cell signalling. The first-generation compounds synthesized had IC₅₀ (half maximal inhibitory concentration) values >4 μM for PI3Kα yet were selective for PI3Kα over the other Class I isoforms (β, δ and γ). The second-generation compounds explored were predicted to better engage the affinity pocket through direct and water-mediated interactions with the enzyme, and the IC₅₀ values decreased by ∼30-fold. Cell signalling analysis showed that some of the new PI3Kα inhibitors were more active in the H1047R mutant bearing cell lines SK-OV-3 and T47D, compared with the E545K mutant harbouring MCF-7 cell line. In conclusion, we have used a structure-based design approach to combine features from two different compound classes to create new PI3Kα-selective inhibitors. This provides new insights into the contribution of different chemical units and interactions with different parts of the active site to the selectivity and potency of PI3Kα inhibitors.

KIT over-expression by p55PIK-PI3K leads to Imatinib-resistance in patients with gastrointestinal stromal tumors

Imatinib is the first-line drug for gastrointestinal stromal tumors (GISTs), as mutated KIT is closely associated with the occurrence of GIST. However, Imatinib resistance (IMA-resistance) occurs inevitably in most GIST patients. Although the over-expression of KIT in GIST is one of the major factors contributing to IMA-resistance, the underlying mechanism is still unclear. In this study, we demonstrate that p55PIK, an isoform of phosphoinositide 3-kinase (PI3K), increases KIT expression, leading to IMA-resistance in GISTs by activating NF-κB signaling pathway. Furthermore, down-regulation of p55PIK significantly decreases KIT expression and re-sensitizes IMA-resistance-GIST cells to Imatinib in vitro and in vivo. Interestingly, the expression of both p55PIK and KIT proteins is significantly increased in tumor samples from IMA-resistance-GIST patients, suggesting that p55PIK up-regulation may be important for IMA-resistance in the clinical setting. Altogether, our data provide evidence that p55PIK-PI3K signaling can contribute to IMA-resistance in GIST by increasing KIT expression. Moreover, p55PIK may be a novel potential drug target for treating tumors that develop IMA-resistance.

Coincident signals from GPCRs and receptor tyrosine kinases are uniquely transduced by PI3Kβ in myeloid cells

Class I phosphoinositide 3-kinases (PI3Ks) catalyze production of the lipid messenger phosphatidylinositol 3,4,5-trisphosphate (PIP3), which plays a central role in a complex signaling network regulating cell growth, survival, and movement. This network is overactivated in cancer and inflammation, and there is interest in determining the PI3K catalytic subunit (p110α, p110β, p110γ, or p110δ) that should be targeted in different therapeutic contexts. Previous studies have defined unique regulatory inputs for p110β, including direct interaction with Gβγ subunits, Rac, and Rab5. We generated mice with knock-in mutations of p110β that selectively blocked the interaction with Gβγ and investigated its contribution to the PI3K isoform dependency of receptor tyrosine kinase (RTK) and G protein (heterotrimeric guanine nucleotide-binding protein)-coupled receptor (GPCR) responses in primary macrophages and neutrophils. We discovered a unique role for p110β in supporting synergistic PIP3 formation in response to the coactivation of macrophages by macrophage colony-stimulating factor (M-CSF) and the complement protein C5a. In contrast, we found partially redundant roles for p110α, p110β, and p110δ downstream of M-CSF alone and a nonredundant role for p110γ downstream of C5a alone. This role for p110β completely depended on direct interaction with Gβγ, suggesting that p110β transduces GPCR signals in the context of coincident activation by an RTK. The p110β-Gβγ interaction was also required for neutrophils to generate reactive oxygen species in response to the Fcγ receptor-dependent recognition of immune complexes and for their β2 integrin-mediated adhesion to fibrinogen or poly-RGD+, directly implicating heterotrimeric G proteins in these two responses.

EPHA3 regulates the multidrug resistance of small cell lung cancer via the PI3K/BMX/STAT3 signaling pathway

Multidrug resistance (MDR) is a major obstacle to the treatment of small cell lung cancer (SCLC). EPHA3 has been revealed to be the most frequently mutated Eph receptor gene in lung cancer with abnormal expression. Growing evidence indicates that the signaling proteins of EPHA3 downstream, including PI3K, BMX and STAT3, play crucial roles in tumorigenesis and cancer progression. To explore the possible role of EPHA3 in MDR, we assessed the influence of EPHA3 on chemoresistance, cell cycle, apoptosis, and tumor growth, as well as the relationship between EPHA3 and the expression of PI3K, BMX, and STAT3 in SCLC. We observed that overexpression of EPHA3 in SCLC cells decreased chemoresistance by increasing apoptosis and inducing G0/G1 arrest, accompanied by reduced phosphorylation of PI3K/BMX/STAT3 signaling pathway. Knockdown of EPHA3 expression generated a resistant phenotype of SCLC, as a result of decreased apoptosis and induced G2/M phase arrest. And re-expression of EPHA3 in these cells reversed the resistant phenotype. Meanwhile, increased phosphorylation of PI3K/BMX/STAT3 signaling pathway was observed in these cells with EPHA3 deficiency. Notably, both PI3K inhibitor (LY294002) and BMX inhibitor (LFM-A13) impaired the chemoresistance enhanced by EPHA3 deficiency in SCLC cell lines. Furthermore, EPHA3 inhibited growth of SCLC cells in vivo and was correlated with longer overall survival of SCLC patients. Thus, we first provide the evidences that EPHA3 is involved in regulating the MDR of SCLC via PI3K/BMX/STAT3 signaling and may be a new therapeutic target in SCLC.

GPCR Signaling Mediates Tumor Metastasis via PI3Kβ

Inappropriate activation of PI3K signaling has been implicated strongly in human cancer. Although studies on the role of PI3K signaling in breast tumorigenesis and progression have focused most intensively on PI3Kα, a role for PI3Kβ has begun to emerge. The PI3Kβ isoform is unique among class IA PI3K enzymes in that it is activated by both receptor tyrosine kinases and G-protein-coupled receptors (GPCR). In previous work, we identified a mutation that specifically abolishes PI3Kβ binding to Gβγ (p110(526KK-DD)). Expression of this mutant in p110β-silenced breast cancer cells inhibits multiple steps of the metastatic cascade in vitro and in vivo and causes a cell autonomous defect in invadopodial matrix degradation. Our results identify a novel link between GPCRs and PI3Kβ in mediating metastasis, suggesting that disruption of this link might offer a novel therapeutic target to prevent the development of metastatic disease. Cancer Res; 76(10); 2944-53. ©2016 AACR.

Hypoxia enhances glucocorticoid-induced apoptosis and cell cycle arrest via the PI3K/Akt signaling pathway in osteoblastic cells

Although osteonecrosis of the femoral head is a known primary limitation of long-term or high-dose clinical administration of glucocorticoids, the mechanisms underlying this side effect remain unclear. Hypoxia is an important biological state under numerous pathological conditions. In this study, we investigated glucocorticoid-induced osteonecrosis under hypoxic conditions in the MC3T3-E1 osteoblast cell line using a cell cytotoxicity assay, flow cytometry, and western blotting. 6α-Methylprednisolone sodium succinate (MPSL) more effectively induced apoptosis and G0/G1 arrest of MC3T3-E1 osteoblasts under hypoxic conditions than under normoxic conditions. Correspondingly, MPSL more effectively upregulated cellular levels of cleaved caspase 3, p53, and its target p21, and downregulated cyclin D1 levels in hypoxia. Moreover, overexpression of Akt abrogated the MPSL activation of p53, p21, and cleaved caspase 3 and the attenuation of cyclin D1 expression and rescued osteoblasts from MPSL-induced cell cycle arrest and apoptosis, indicating that phosphatidylinositol 3-kinase (PI3K)/Akt signaling might play an essential role in MPSL-induced inhibition of osteoblasts. Furthermore, the suppression of PI3K/Akt signaling and upregualtion of cellular p85α monomer levels by MPSL were more pronounced under hypoxic conditions than under normoxic conditions. Finally, we found that the enhancement of the effects of MPSL under hypoxic conditions was attributed to hypoxia-upregulated glucocorticoid receptor activity. In conclusion, our results demonstrate that MPSL, a synthetic glucocorticoid receptor agonist, promotes the level of p85α and inhibits PI3K/Akt signaling to induce apoptosis and cell cycle arrest in osteoblasts, and that this effect is enhanced under hypoxic conditions.

Regulation of Glucose Homeostasis by Glucocorticoids

Glucocorticoids are steroid hormones that regulate multiple aspects of glucose homeostasis. Glucocorticoids promote gluconeogenesis in liver, whereas in skeletal muscle and white adipose tissue they decrease glucose uptake and utilization by antagonizing insulin response. Therefore, excess glucocorticoid exposure causes hyperglycemia and insulin resistance. Glucocorticoids also regulate glycogen metabolism. In liver, glucocorticoids increase glycogen storage, whereas in skeletal muscle they play a permissive role for catecholamine-induced glycogenolysis and/or inhibit insulin-stimulated glycogen synthesis. Moreover, glucocorticoids modulate the function of pancreatic α and β cells to regulate the secretion of glucagon and insulin, two hormones that play a pivotal role in the regulation of blood glucose levels. Overall, the major glucocorticoid effect on glucose homeostasis is to preserve plasma glucose for brain during stress, as transiently raising blood glucose is important to promote maximal brain function. In this chapter we will discuss the current understanding of the mechanisms underlying different aspects of glucocorticoid-regulated mammalian glucose homeostasis.

Assembly and Molecular Architecture of the Phosphoinositide 3-Kinase p85α Homodimer

Phosphoinositide 3-kinases (PI3Ks) are a family of lipid kinases that are activated by growth factor and G-protein-coupled receptors and propagate intracellular signals for growth, survival, proliferation, and metabolism. p85α, a modular protein consisting of five domains, binds and inhibits the enzymatic activity of class IA PI3K catalytic subunits. Here, we describe the structural states of the p85α dimer, based on data from in vivo and in vitro solution characterization. Our in vitro assembly and structural analyses have been enabled by the creation of cysteine-free p85α that is functionally equivalent to native p85α. Analytical ultracentrifugation studies showed that p85α undergoes rapidly reversible monomer-dimer assembly that is highly exothermic in nature. In addition to the documented SH3-PR1 dimerization interaction, we identified a second intermolecular interaction mediated by cSH2 domains at the C-terminal end of the polypeptide. We have demonstrated in vivo concentration-dependent dimerization of p85α using fluorescence fluctuation spectroscopy. Finally, we have defined solution conditions under which the protein is predominantly monomeric or dimeric, providing the basis for small angle x-ray scattering and chemical cross-linking structural analysis of the discrete dimer. These experimental data have been used for the integrative structure determination of the p85α dimer. Our study provides new insight into the structure and assembly of the p85α homodimer and suggests that this protein is a highly dynamic molecule whose conformational flexibility allows it to transiently associate with multiple binding proteins.

PI3King the right partner: unique interactions and signaling by p110β

Phosphoinositide 3-kinases (PI3Ks) are central regulators of cellular responses to extracellular stimuli, and are involved in growth, proliferation, migration, and metabolism. The Class I PI3Ks are activated by Receptor Tyrosine Kinases (RTKs) or G Protein-Coupled Receptors (GPCRs), and their signaling is commonly deregulated in disease conditions. Among the class I PI3Ks, the p110β isoform is unique in being activated by both RTKs and GPCRs, and its ability to bind Rho-GTPases and Rab5. Recent studies have characterized these p110β interacting partners, defining the binding mechanisms and regulation, and thus provide insight into the function of this kinase in physiology and disease. This review summarizes the developments in p110β research, focusing on the interacting partners and their role in p110β-mediated signaling.

Crossroads of PI3K and Rac pathways

Rac and PI3Ks are intracellular signal transducers able to regulate multiple signaling pathways fundamental for cell behavior. PI3Ks are lipid kinases that produce phosphorylated lipids which, in turn, transduce extracellular cues within the cell, while Rac is a small G protein that impacts on actin organization. Compelling evidence indicates that in multiple circumstances the 2 signaling pathways appear intermingled. For instance, phosphorylated lipids produced by PI3Ks recruit and activate GEF and GAP proteins, key modulators of Rac function. Conversely, PI3Ks interact with activated Rac, leading to Rac signaling amplification. This review summarizes the molecular mechanisms underlying the cross-talk between Rac and PI3K signaling in 2 different processes, cell migration and ROS production.

Oncogenic activity of the regulatory subunit p85β of phosphatidylinositol 3-kinase (PI3K)

Expression of the regulatory subunit p85β of PI3K induces oncogenic transformation of primary avian fibroblasts. The transformed cells proliferate at an increased rate compared with nontransformed controls and show elevated levels of PI3K signaling. The oncogenic activity of p85β requires an active PI3K-TOR signaling cascade and is mediated by the p110α and p110β isoforms of the PI3K catalytic subunit. The data suggest that p85β is a less effective inhibitor of the PI3K catalytic subunit than p85α and that this reduced level of p110 inhibition accounts for the oncogenic activity of p85β.

Investigating the structure and dynamics of the PIK3CA wild-type and H1047R oncogenic mutant

The PIK3CA gene is one of the most frequently mutated oncogenes in human cancers. It encodes p110α, the catalytic subunit of phosphatidylinositol 3-kinase alpha (PI3Kα), which activates signaling cascades leading to cell proliferation, survival, and cell growth. The most frequent mutation in PIK3CA is H1047R, which results in enzymatic overactivation. Understanding how the H1047R mutation causes the enhanced activity of the protein in atomic detail is central to developing mutant-specific therapeutics for cancer. To this end, Surface Plasmon Resonance (SPR) experiments and Molecular Dynamics (MD) simulations were carried out for both wild-type (WT) and H1047R mutant proteins. An expanded positive charge distribution on the membrane binding regions of the mutant with respect to the WT protein is observed through MD simulations, which justifies the increased ability of the mutated protein variant to bind to membranes rich in anionic lipids in our SPR experiments. Our results further support an auto-inhibitory role of the C-terminal tail in the WT protein, which is abolished in the mutant protein due to loss of crucial intermolecular interactions. Moreover, Functional Mode Analysis reveals that the H1047R mutation alters the twisting motion of the N-lobe of the kinase domain with respect to the C-lobe and shifts the position of the conserved P-loop residues in the vicinity of the active site. These findings demonstrate the dynamical and structural differences of the two proteins in atomic detail and propose a mechanism of overactivation for the mutant protein. The results may be further utilized for the design of mutant-specific PI3Kα inhibitors that exploit the altered mutant conformation.

The PI3Kα protein is involved in cellular processes such as cell growth, division, and formation of new blood vessels (angiogenesis) that aid cancer cell survival. In certain types of cancer cells, PI3Kα is found to be altered compared to healthy cells. These PI3Kα alterations, called mutations, are found in 27% of breast cancer patients, 24% of endometrial cancer patients, and 15% of colon cancer patients. PI3Kα mutations cause the protein to become overactivated and may contribute to tumor growth. The most common PI3Kα amino acid mutation is a histidine changed to an arginine: H1047R. Understanding how the H1047R mutation overactivates PI3Kα is central to developing therapeutics for cancer patients who bear PI3Kα mutations. To this end, we performed simulations and experiments of the mutated and physiological proteins to explain why the mutant protein becomes overactivated. Our results indicate structural and dynamical differences between the mutant and physiological proteins that may affect the PI3Kα function. Based on these differences, we propose a mechanism that highlights the series of events that lead to the mutant H1047R PI3Kα overactivation. This study provides insights into developing mutant-specific PI3Kα inhibitors that exploit the altered conformation of the mutant with respect to the physiological protein.

Too much of a good thing: immunodeficiency due to hyperactive PI3K signaling

Primary immune deficiency diseases arise due to heritable defects that often involve signaling molecules required for immune cell function. Typically, these genetic defects cause loss of gene function, resulting in primary immune deficiencies such as severe combined immune deficiency (SCID) and X-linked agammaglobulinemia (XLA); however, gain-of-function mutations may also promote immune deficiency. In this issue of the JCI, Deau et al. establish that gain-of-function mutations in PIK3R1, which encodes the p85α regulatory subunit of class IA PI3Ks, lead to immunodeficiency. These observations are consistent with previous reports that hyperactivating mutations in PIK3CD, which encodes the p110δ catalytic subunit, are capable of promoting immune deficiency. Mutations that reduce PI3K activity also result in defective lymphocyte development and function; therefore, these findings support the notion that too little or too much PI3K activity leads to immunodeficiency.

Phosphatidylinositol-3,4,5-trisphosphate: tool of choice for class I PI 3-kinases

Class I PI 3-kinases signal by producing the signaling lipid phosphatidylinositol(3,4,5) trisphosphate, which in turn acts by recruiting downstream effectors that contain specific lipid-binding domains. The class I PI 3-kinases comprise four distinct catalytic subunits linked to one of seven different regulatory subunits. All the class I PI 3-kinases produce the same signaling lipid, PIP3, and the different isoforms have overlapping expression patterns and are coupled to overlapping sets of upstream activators. Nonetheless, studies in cultured cells and in animals have demonstrated that the different isoforms are coupled to distinct ranges of downstream responses. This review focuses on the mechanisms by which the production of a common product, PIP3, can produce isoform-specific signaling by PI 3-kinases.

The structural basis of PI3K cancer mutations: from mechanism to therapy

While genetic alteration in the p85α-p110α (PI3K) complex represents one of the most frequent driver mutations in cancer, the wild-type complex is also required for driving cancer progression through mutations in related pathways. Understanding the mechanistic basis of the function of the phosphoinositide 3-kinase (PI3K) is essential for designing optimal therapeutic targeting strategies. Recent structural data of the p85α/p110α complex unraveled key insights into the molecular mechanisms of the activation of the complex and provided plausible explanations for the well-established biochemical data on p85/p110 dimer regulation. A wealth of biochemical and biologic information supported by recent genetic findings provides a strong basis for additional p110-independent function of p85α in the regulation of cell survival. In this article, we review the structural, biochemical, and biologic mechanisms through which p85α regulates the cancer cell life cycle with an emphasis on the recently discovered genetic alterations in cancer. As cancer progression is dependent on multiple biologic processes, targeting key drivers such as the PI3K may be required for efficacious therapy of heterogeneous tumors typically present in patients with late-stage disease.

Cell signaling associated with Na(+)/K(+)-ATPase: activation of phosphatidylinositide 3-kinase IA/Akt by ouabain is independent of Src

Exposure of intact cells to selective inhibitors of Na⁺/K⁺-ATPase such as ouabain activates several growth-related cell signaling pathways. It has been suggested that the initial event of these pathways is the binding of ouabain to a preexisting complex of Src with Na⁺/K⁺-ATPase of the plasma membrane. The aim of this work was to evaluate the role of Src in the ouabain-induced activation of phosphatidylinositide 3-kinase 1A (PI3K1A) and its downstream consequences. When fibroblasts devoid of Src (SYF cells) and controls (Src⁺⁺ cells) were exposed to ouabain, PI3K1A, Akt, and proliferative growth were similarly stimulated in both cell lines. Ouabain-induced activation of Akt was not prevented by the Src inhibitor PP2. In contrast, ERK1/2 were not activated by ouabain in SYF cells but were stimulated in Src⁺⁺ cells; this was prevented by PP2. In isolated adult mouse cardiac myocytes, where ouabain induces hypertrophic growth, PP2 also did not prevent ouabain-induced activation of Akt and the resulting hypertrophy. Ouabain-induced increases in the levels of co-immunoprecipitation of the α-subunit of Na⁺/K⁺-ATPase with the p85 subunit of PI3K1A were noted in SYF cells, Src⁺⁺ cells, and adult cardiac myocytes. In conjunction with previous findings, the results presented here indicate that (a) if there is a preformed complex of Src and Na⁺/K⁺-ATPase, it is irrelevant to ouabain-induced activation of the PI3K1A/Akt pathway through Na⁺/K⁺-ATPase and (b) a more likely, but not established, mechanism of linkage of Na⁺/K⁺-ATPase to PI3K1A is the ouabain-induced interaction of a proline-rich domain of the α-subunit of Na⁺/K⁺-ATPase with the SH3 domain of the p85 subunit of PI3K1A.

Novel approaches to inhibitor design for the p110β phosphoinositide 3-kinase

Phosphoinositide (PI) 3-kinases are essential regulators of cellular proliferation, survival, metabolism, and motility that are frequently dysregulated in human disease. The design of inhibitors to target the PI 3-kinase/mTOR pathway is a major area of investigation by both academic laboratories and the pharmaceutical industry. This review focuses on the Class IA PI 3-kinase p110β, which plays a unique role in thrombogenesis and in the growth of tumors with deletion or loss-of-function mutation of the Phosphatase and Tensin Homolog (PTEN) lipid phosphatase. Several p110β-selective inhibitors that target the ATP-binding site in the kinase domain have been identified. However, recent discoveries regarding the regulatory mechanisms that control p110β activity suggest alternative strategies by which to disrupt signaling by this PI 3-kinase isoform. This review summarizes the current status of p110β-specific inhibitors and discusses how these new insights into p110 regulation might be used to devise novel pharmacological inhibitors.