Over-expression of the p110beta but not p110alpha isoform of PI 3-kinase inhibits motility in breast cancer cells

Isoform-specific activities of the regulatory subunits of phosphatidylinositol 3-kinases - potentially novel therapeutic targets

INTRODUCTION

The main regulatory subunits of Class IA phosphatidylinositol 3-kinase (PI3K), p85α and p85β, initiate diverse cellular activities independent of binding to the catalytic subunit p110. Several of these signaling processes directly or indirectly contribute to a regulation of PI3K and could become targets for therapeutic efforts. Areas covered: This review will highlight two general areas of p85 activity: (1) direct interaction with regulatory proteins and with determinants of the cytoskeleton, and (2) a genetic analysis by deletion and domain switches identifying new functions for p85 domains. Expert Opinion: Isoform-specific activities of regulatory subunits have long been at the periphery of the PI3K field. Our understanding of these unique functions of the regulatory subunits is fragmentary and raises many important questions. At this time, there is insufficient information to translate this knowledge into the clinic, but some tempting targets have emerged that could move the field forward with the help of novel technologies in drug design and identification.

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.

PI3K p110β isoform synergizes with JNK in the regulation of glioblastoma cell proliferation and migration through Akt and FAK inhibition

BACKGROUND

Glioblastoma multiforme is the most aggressive malignant primary brain tumor, characterized by rapid growth and extensive infiltration to neighboring normal brain parenchyma. Both PI3K/Akt and JNK pathways are essential to glioblastoma cell survival, migration and invasion. Due to their hyperactivation in glioblastoma cells, PI3K and JNK are promising targets for glioblastoma treatment.

METHODS

To investigate the combination effects of class IA PI3K catalytic isoforms (p110α, p110β and p110δ) and JNK inhibition on tumor cell growth and motility, glioblastoma cells and xenografts in nude mice were treated with isoform-selective PI3K inhibitors in combination with JNK inhibitor.

RESULTS

We showed that combined inhibition of these PI3K isoforms and JNK exerted divergent effects on the proliferation, migration and invasion of glioblastoma cells in vitro. Pharmacological inhibition of p110β or p110δ, but not p110α, displayed synergistic inhibitory effect with JNK inhibition on glioblastoma cell proliferation and migration through decreasing phosphorylation of Akt, FAK and zyxin, leading to blockade of lamellipodia and membrane ruffles formation. No synergistic effect on invasion was observed in all the combination treatment. In vivo, combination of p110β and JNK inhibitors significantly reduced xenograft tumor growth compared with single inhibitor alone.

CONCLUSION

Concurrent inhibition of p110β and JNK exhibited synergistic effects on suppressing glioblastoma cell proliferation and migration in vitro and xenograft tumor growth in vivo. Our data suggest that combined inhibition of PI3K p110β isoform and JNK may serve as a potent and promising therapeutic approach for glioblastoma multiforme.

Status of PI3K/Akt/mTOR pathway inhibitors in lymphoma

The phosphatidylinositol-3-kinase (PI3K) pathway is well known to regulate a wide variety of essential cellular functions, including glucose metabolism, translational regulation of protein synthesis, cell proliferation, apoptosis, and survival. Aberrations in the PI3K pathway are among the most frequently observed in cancer, and include amplifications, rearrangements, mutations, and loss of regulators. As a net result of these anomalies, the PI3K pathway is activated in many malignancies, including in Hodgkin and non-Hodgkin lymphomas, and yields a competitive growth and survival advantage, increased metastatic ability, and resistance to conventional therapy. Numerous inhibitors targeting various nodes in the PI3K pathway are undergoing clinical development, and their current status in lymphoma will be the focus of this review.

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.

Expression of PI3Kp110α and PI3Kp110β in colorectal canceration

AIM: To investigate the expression of PI3Kp110α and PI3Kp110β in colorectal canceration and to analyze their correlation.

METHODS: PI3Kp110α and PI3Kp110β expression was detected by immunohistochemistry in 30 cases of normal colorectal mucosa, 46 cases of low-grade intraepithelial neoplasia (LGIN), 32 cases of high-grade intraepithelial neoplasia (HGIN) and 60 cases of colorectal carcinoma.

RESULTS: The expression of PI3Kp110α in colorectal carcinoma was significantly higher than that in normal colorectal mucosa and LGIN (both P < 0.05), but there was no significant difference between colorectal carcinoma and HGIN (P > 0.05). The expression of PI3Kp110β was highest in colorectal carcinoma, followed by HGIN, LGIN and normal colorectal mucosa (P < 0.05). PI3Kp110α expression was correlated with patient age, tumor size and tumor differentiation (all P < 0.05). PI3Kp110β expression was correlated with TNM stage and degree of tumor differentiation (both P < 0.05). The expression of PI3Kp110α was positively correlated with that of PI3Kp110β in LGIN (P < 0.05). There was no significant correlation between the expression of PI3Kp110α and that of PI3Kp110β in colorectal carcinoma, HGIN or normal colorectal mucosa (all P > 0.05).

CONCLUSION: PI3Kp110α and p110β may play different roles in colorectal canceration.

Enhanced Akt phosphorylation and myogenic differentiation in PI3K p110β-deficient myoblasts is mediated by PI3K p110α and mTORC2

Phosphoinositide 3-kinase (PI3K) is a principal regulator of Akt activation and myogenesis; however, the function of PI3K p110β in these processes is not well defined. To address this, we investigated the role of p110β in Akt activation and skeletal muscle cell differentiation. We found that Akt phosphorylation was enhanced in p110β-deficient myoblasts in response to Insulin-like Growth Factor-I (IGF-I), epidermal growth factor, or p110α overexpression, as compared to p110β-sufficient cells. This effect was associated with increased mammalian target of rapamycin complex 2 activation, even in myoblasts deficient in mSin1 and rictor. Conversely, in response to the G-protein-coupled receptor agonist lysophosphatidic acid, Akt phosphorylation was attenuated in p110β-deficient myoblasts. Loss of p110β also enhanced the expression of myogenic markers at the myoblast stage and during the first 48 h of differentiation. These data demonstrate that reductions in p110β are associated with agonist-specific Akt hyperactivation and accelerated myogenesis, thus revealing a negative role for p110β in Akt activation and during myoblast differentiation.

Nuclear but not cytosolic phosphoinositide 3-kinase beta has an essential function in cell survival

Class I(A) phosphoinositide 3-kinases (PI3Ks) are heterodimeric enzymes composed of a p85 regulatory and a p110 catalytic subunit that induce the formation of 3-polyphosphoinositides, which mediate cell survival, division, and migration. There are two ubiquitous PI3K isoforms p110α and p110β that have nonredundant functions in embryonic development and cell division. However, whereas p110α concentrates in the cytoplasm, p110β localizes to the nucleus and modulates nuclear processes such as DNA replication and repair. At present, the structural features that determine p110β nuclear localization remain unknown. We describe here that association with the p85β regulatory subunit controls p110β nuclear localization. We identified a nuclear localization signal (NLS) in p110β C2 domain that mediates its nuclear entry, as well as a nuclear export sequence (NES) in p85β. Deletion of p110β induced apoptosis, and complementation with the cytoplasmic C2-NLS p110β mutant was unable to restore cell survival. These studies show that p110β NLS and p85β NES regulate p85β/p110β nuclear localization, supporting the idea that nuclear, but not cytoplasmic, p110β controls cell survival.

Adaptor protein XB130 is a Rac-controlled component of lamellipodia that regulates cell motility and invasion

XB130 is a newly described cytosolic adaptor protein and tyrosine kinase substrate, involved in Src- and RET/PTC-dependent signaling. Although XB130 has been cloned as a homologue of actin-filament-associated protein (AFAP-110), its potential regulation by the actin skeleton and its putative roles in cytoskeleton regulation have not been addressed. Here, we show that XB130 (in contrast to AFAP-110) exhibited robust translocation to the cell periphery in response to various stimuli (including epidermal growth factor, wounding and expression of constitutively active Rac) that elicit lamellipodium formation. In stimulated cells, XB130 localized to the lamellipodial F-actin meshwork. Genetic and pharmacological data suggest that the key trigger for XB130 recruitment is the formation of the branched F-actin itself. Structure-function analysis revealed that both the XB130 N-terminus (167 amino acids) and C-terminus (63 amino acids) harbor crucial regions for its translocation to lamellipodia, whereas the PH domains and Src-targeted tyrosines are dispensable. Importantly, in TPC1 thyroid papillary carcinoma cells, silencing endogenous XB130 decreased the rate of wound closure, inhibited matrigel invasion, reduced lamellipodial persistence and slowed down spreading. Thus, XB130 is a novel Rac- and cytoskeleton-regulated and cytoskeleton-regulating adaptor protein that exhibits high affinity to lamellipodial (branched) F-actin and impacts motility and invasiveness of tumor cells.

A biochemical mechanism for the oncogenic potential of the p110beta catalytic subunit of phosphoinositide 3-kinase

Class I PI3-kinases signal downstream of receptor tyrosine kinases and G protein-coupled receptors and have been implicated in tumorigenesis. Although the oncogenic potential of the PI3-kinase subunit p110α requires its mutational activation, other p110 isoforms can induce transformation when overexpressed in the wild-type state. In wild-type p110α, N345 in the C2 domain forms hydrogen bonds with D560 and N564 in the inter-SH2 (iSH2) domain of p85, and mutations of p110α or p85 that disrupt this interface lead to increased basal activity and transformation. Sequence analysis reveals that N345 in p110α aligns with K342 in p110β. This difference makes wild-type p110β analogous to a previously described oncogenic mutant, p110α-N345K. We now show that p110β is inhibited by p85 to a lesser extent than p110α and is not differentially inhibited by wild-type p85 versus p85 mutants that disrupt the C2-iSH2 domain interface. Similar results were seen in soft agar and focus-formation assays, where p110β was similar to p110α-N345K in transforming potential. Inhibition of p110β by p85 was enhanced by a K342N mutation in p110β, which led to decreased activity in vitro, decreased basal Akt and ribosomal protein S6 kinase (S6K1) activation, and decreased transformation in NIH 3T3 cells. Moreover, unlike wild-type p110β, p110β-K342N was differentially regulated by wild-type and mutant p85, suggesting that the inhibitory C2-iSH2 interface is functional in this mutant. This study shows that the enhanced transforming potential of p110β is the result of its decreased inhibition by p85, due to the disruption of an inhibitory C2-iSH2 domain interface.

Correlative fluorescence and transmission electron microscopy: an elegant tool to study the actin cytoskeleton of whole-mount (breast) cancer cells

Elucidating the structure and dynamics of lamellipodia and filopodia in response to different stimuli is a topic of continuing interest in cancer cells as these structures may be attractive targets for therapeutic purposes. Interestingly, a close functional relationship between these actin-rich protrusions and specialized membrane domains has been recently demonstrated. The aim of this study was therefore to investigate the fine organization of these actin-rich structures and examine how they structurally may relate to detergent-resistant membrane (DRM) domains in the MTLn3 EGF/serum starvation model. For this reason, we designed a straightforward and alternative method to study cytoskeleton arrays and their associated structures by means of correlative fluorescence (/laser)- and electron microscopy (CFEM). CFEM on whole mounted breast cancer cells revealed that a lamellipodium is composed of an intricate filamentous actin web organized in various patterns after different treatments. Both actin dots and DRM's were resolved, and were closely interconnected with the surrounding cytoskeleton. Long actin filaments were repeatedly observed extending beyond the leading edge and their density and length varied after different treatments. Furthermore, CFEM also allowed us to demonstrate the close structural association of DRMs with the cytoskeleton in general and the filamentous/dot-like structural complexes in particular, suggesting that they are all functionally linked and consequently may regulate the cell's fingertip dynamics. Finally, electron tomographic modelling on the same CFEM samples confirmed that these extensions are clearly embedded within the cytoskeletal matrix of the lamellipodium.

Distinct roles of class IA PI3K isoforms in primary and immortalised macrophages

The class IA isoforms of phosphoinositide 3-kinase (p110alpha, p110beta and p110delta) often have non-redundant functions in a given cell type. However, for reasons that are unclear, the role of a specific PI3K isoform can vary between cell types. Here, we compare the relative contributions of PI3K isoforms in primary and immortalised macrophages. In primary macrophages stimulated with the tyrosine kinase ligand colony-stimulating factor 1 (CSF1), all class IA PI3K isoforms participate in the regulation of Rac1, whereas p110delta selectively controls the activities of Akt, RhoA and PTEN, in addition to controlling proliferation and chemotaxis. The prominent role of p110delta in these cells correlates with it being the main PI3K isoform that is recruited to the activated CSF1 receptor (CSF1R). In immortalised BAC1.2F5 macrophages, however, the CSF1R also engages p110alpha, which takes up a more prominent role in CSF1R signalling, in processes including Akt phosphorylation and regulation of DNA synthesis. Cell migration, however, remains dependent mainly on p110delta. In other immortalised macrophage cell lines, such as IC-21 and J774.2, p110alpha also becomes more prominently involved in CSF1-induced Akt phosphorylation, at the expense of p110delta.These data show that PI3K isoforms can be differentially regulated in distinct cellular contexts, with the dominant role of the p110delta isoform in Akt phosphorylation and proliferation being lost upon cell immortalisation. These findings suggest that p110delta-selective PI3K inhibitors may be more effective in inflammation than in cancer.

Class I PI3K in oncogenic cellular transformation

Class I phosphoinositide 3-kinase (PI3K) is a dimeric enzyme, consisting of a catalytic and a regulatory subunit. The catalytic subunit occurs in four isoforms designated as p110 alpha, p110 beta, p110 gamma and p110 delta. These isoforms combine with several regulatory subunits; for p110 alpha, beta and delta, the standard regulatory subunit is p85, for p110 gamma, it is p101. PI3Ks play important roles in human cancer. PIK3CA, the gene encoding p110 alpha, is mutated frequently in common cancers, including carcinoma of the breast, prostate, colon and endometrium. Eighty percent of these mutations are represented by one of the three amino-acid substitutions in the helical or kinase domains of the enzyme. The mutant p110 alpha shows a gain of function in enzymatic and signaling activity and is oncogenic in cell culture and in animal model systems. Structural and genetic data suggest that the mutations affect regulatory inter- and intramolecular interactions and support the conclusion that there are at least two molecular mechanisms for the gain of function in p110 alpha. One of these mechanisms operates largely independently of binding to p85, the other abolishes the requirement for an interaction with Ras. The non-alpha isoforms of p110 do not show cancer-specific mutations. However, they are often differentially expressed in cancer and, in contrast to p110 alpha, wild-type non-alpha isoforms of p110 are oncogenic when overexpressed in cell culture. The isoforms of p110 have become promising drug targets. Isoform-selective inhibitors have been identified. Inhibitors that target exclusively the cancer-specific mutants of p110 alpha constitute an important goal and challenge for current drug development.

Differential regulation of class IA phosphoinositide 3-kinase catalytic subunits p110 alpha and beta by protease-activated receptor 2 and beta-arrestins

PAR-2 (protease-activated receptor 2) is a GPCR (G-protein-coupled receptor) that can elicit both G-protein-dependent and -independent signals. We have shown previously that PAR-2 simultaneously promotes Galphaq/Ca2+-dependent activation and beta-arrestin-1-dependent inhibition of class IA PI3K (phosphoinositide 3-kinase), and we sought to characterize further the role of beta-arrestins in the regulation of PI3K activity. Whereas the ability of beta-arrestin-1 to inhibit p110alpha (PI3K catalytic subunit alpha) has been demonstrated, the role of beta-arrestin-2 in PI3K regulation and possible differences in the regulation of the two catalytic subunits (p110alpha and p110beta) associated with p85alpha (PI3K regulatory subunit) have not been examined. In the present study we have demonstrated that: (i) PAR-2 increases p110alpha- and p110beta-associated lipid kinase activities, and both p110alpha and p110beta are inhibited by over-expression of either beta-arrestin-1 or -2; (ii) both beta-arrestin-1 and -2 directly inhibit the p110alpha catalytic subunit in vitro, whereas only beta-arrestin-2 directly inhibited p110beta; (iii) examination of upstream pathways revealed that PAR-2-induced PI3K activity required the small GTPase Cdc (cell-division cycle)42, but not tyrosine phosphorylation of p85; and (iv) beta-arrestins inhibit PAR-2-induced Cdc42 activation. Taken together, these results indicated that beta-arrestins could inhibit PAR-2-stimulated PI3K activity, both directly and through interference with upstream pathways, and that the two beta-arrestins differ in their ability to inhibit the p110alpha and p110beta catalytic subunits. These results are particularly important in light of the growing interest in PAR-2 as a pharmacological target, as commonly used biochemical assays that monitor G-protein coupling would not screen for beta-arrestin-dependent signalling events.

Oncogenic signaling of class I PI3K isoforms

The catalytic subunits of class I PI3Ks comprise four isoforms: p110alpha, p110beta, p110delta and p110gamma. Cancer-specific gain-of-function mutations in p110alpha have been identified in various malignancies. Cancer-specific mutations in the non-alpha isoforms of class I PI3K have not yet been identified, however overexpression of either wild-type p110beta, p110gamma or p110delta is sufficient to induce cellular transformation in chicken embryo fibroblasts. The mechanism whereby these non-alpha isoforms of class I mediate oncogenic signals is unknown. Here we show that potently transforming class I isoforms signal via Akt/mTOR, inhibit GSK3beta and cause degradation of FoxO1. A functional Erk pathway is required for p110gamma and p110beta transformation but not for transformation by p110delta or the H1047R mutant of p110alpha. Transformation and signaling by p110gamma and p110beta are sensitive to loss of interaction with Ras, which acts as a membrane anchor. Mutations in the C2 domain of p110delta reduce transformation, most likely by interfering with membrane association. Several small molecule inhibitors potently and specifically inhibit the oncogenic signaling and transformation of each of the class I PI3K, and, when used in combination with MEK inhibitors, can additively reduce the transformation induced by p110beta and p110gamma.

Phosphoinositide 3-kinase signalling in lung disease: leucocytes and beyond

The family of lipid kinases termed phosphoinositide-3-kinase (PI3K) is known to contribute at multiple levels to innate and adaptive immune responses, and is hence an attractive target for drug discovery in inflammatory and autoimmune disease, including respiratory diseases. The development of isoform-selective pharmacological inhibitors, targeted gene manipulation and short interfering RNA (siRNA) target validation have facilitated a better understanding of the role that each member of this family of kinases plays in the physiology and pathology of the respiratory system. In this review, we will evaluate the evidence for the roles of specific PI3K isoforms in the lung and airways, and discuss their potential as targets for novel drug therapies.

Label-free and real-time cell-based kinase assay for screening selective and potent receptor tyrosine kinase inhibitors using microelectronic sensor array

Kinases are the 2nd largest group of therapeutic targets in the human genome. In this article, a label-free and real-time cell-based receptor tyrosine kinase (RTK) assay that addresses limitation of existing kinase assays and can be used for high-throughput screening and lead optimization studies was validated and characterized. Using impedance, growth factor-induced morphological changes were quantitatively assessed in real time and used as a measure of RTK activity. COS7 cells treated with epidermal growth factor (EGF) and insulin results in a rapid increase in cell impedance. Assessment of these growth factor-induced morphological changes and levels of receptor autophosphorylation using fluorescent microscopy and enzyme-linked immunosorbent assay, respectively, demonstrates that these changes correlate with changes in impedance. This assay was used to screen, identify, and characterize a potent EGF receptor inhibitor from a compound library. This report describes an assay that is simple in that it does not require intensive optimization or special reagents such as peptides, antibodies, or probes. More important, because the assay is cell based, the studies are done in a physiologically relevant environment, allowing for concurrent assessment of a compound's solubility, stability, membrane permeability, cytotoxicity, and off-target interaction effects.

Oncogenic transformation induced by the p110beta, -gamma, and -delta isoforms of class I phosphoinositide 3-kinase

Class I phosphoinositide 3-kinase contains four isoforms of the catalytic subunit, p110alpha, -beta, -gamma, and -delta. At physiological levels of expression, the wild-type p110alpha isoform lacks oncogenic potential, but gain-of-function mutations and overexpression of p110alpha are correlated with oncogenicity. The p110beta, -gamma, and -delta isoforms induce transformation of cultured cells as wild-type proteins. This oncogenic potential requires kinase activity and can be suppressed by the target of rapamycin inhibitor rapamycin. The p110delta isoform constitutively activates the Akt signaling pathway; p110gamma activates Akt only in the presence of serum. The isoforms differ in their requirements for upstream signaling. The transforming activity of the p110gamma isoform depends on rat sarcoma viral oncogene homolog (Ras) binding; preliminary data suggest the same for p110beta and indicate Ras-independent oncogenic potential of p110delta. The surprising oncogenic potential of the wild-type non-alpha isoforms of class I phosphoinositide 3-kinase may explain the dearth of cancer-specific mutations in these proteins, because these non-alpha isoforms could contribute to the oncogenic phenotype of the cell by differential expression.

Exploiting the PI3K/AKT pathway for cancer drug discovery

Evolving studies with several different targeted therapeutic agents are demonstrating that patients with genomic alterations of the target, including amplification, translocation and mutation, are more likely to respond to the therapy. Recent studies indicate that numerous components of the phosphatidylinositol-3-kinase (PI3K)/AKT pathway are targeted by amplification, mutation and translocation more frequently than any other pathway in cancer patients, with resultant activation of the pathway. This warrants exploiting the PI3K/AKT pathway for cancer drug discovery.