Targeting PI3K signalling in cancer: opportunities, challenges and limitations

Bromelain inhibits nuclear factor kappa-B translocation, driving human epidermoid carcinoma A431 and melanoma A375 cells through G(2)/M arrest to apoptosis

Bromelain, obtained from pineapple, is already in use clinically as adjunct in chemotherapy. Our objective was to test its ability to act as a sole anti-cancer agent. Therefore, we describe its anti-proliferative, anti-inflammatory and subsequent anti-cancer effects in vitro, against human epidermoid carcinoma-A431 and melanoma-A375 cells. Bromelain exhibited reduction in proliferation of both these cell-lines and suppressed their potential for anchorage-independent growth. Further, suppression of inflammatory signaling by bromelain was evident by inhibition of Akt regulated-nuclear factor-kappaB activation via suppression of inhibitory-kappaBα phosphorylation and concomitant reduction in cyclooxygenase-2. Since, the inflammatory cascade is well-known to be closely allied to cancer; we studied the effect of bromelain on events/molecules central to it. Bromelain caused depletion of intracellular glutathione and generation of reactive oxygen-species followed by mitochondrial membrane depolarization. This led to bromelain-induced cell-cycle arrest at G(2)/M phase which was mediated by modulation of cyclin B1, phospho-cdc25C, Plk1, phospho-cdc2, and myt1. This was subsequently followed by induction of apoptosis, indicated by membrane-blebbing, modulation of Bax-Bcl-2 ratio, Apaf-1, caspase-9, and caspase-3; chromatin-condensation, increase in caspase-activity and DNA-fragmentation. Bromelain afforded substantial anti-cancer potential in these settings; hence we suggest it as a potential prospect for anti-cancer agent besides only an additive in chemotherapy.

Dual blockade of phosphatidylinositol 3'-kinase and mitogen-activated protein kinase pathways overcomes paclitaxel-resistance in colorectal cancer

Paclitaxel, one of key drugs to treat a wide range of malignancies, exhibits relative low sensitivity for colorectal cancer. The present study was to examine whether and how phosphatidylinositol 3'-kinase (PI3K) signals affect the sensitivity of colorectal cancer to paclitaxel. Four colorectal cancer cell lines were exposed to paclitaxel in the presence of PI3K signal inhibitors, such as LY294002, siRNA for Akt, or rapamycin, with or without MAPK inhibitor, PD98059. Cell viability and apoptosis were determined by MTT assay, cell cycle analysis in flow cytometer and Hoechst nuclear staining. To analyze the PI3K activity, the expression in phosphorylated Akt and downstream effectors of p70S6 kinase (S6K) were evaluated by Western blot analysis. Paclitaxel alone (5-10 nM) did not induce the apoptosis in all four cell lines. Although LY294002 alone did not affect the cell viability, it suppressed the Akt and S6K activities and induced the sub-G1 arrest/apoptosis when paclitaxel was co-administered, as well as the Akt siRNA and rapamycin did. Simultaneous blockade of PI3K and MAPK pathways more suppressed the S6K activity and further increased the apoptosis. In conclusion, PI3K is involved in low susceptibility of colorectal cancer to paclitaxel and dual PI3K/MAPK targeting agents may evolve a new paclitaxel-based chemotherapy for colorectal cancer.

Dependence on the MUC1-C oncoprotein in non-small cell lung cancer cells

Non-small cell lung cancer (NSCLC) cells are often associated with constitutive activation of the phosphoinositide 3-kinase (PI3K) → Akt → mTOR pathway. The mucin 1 (MUC1) heterodimeric glycoprotein is aberrantly overexpressed in NSCLC cells and induces gene signatures that are associated with poor survival of NSCLC patients. The present results show that the MUC1 C-terminal subunit (MUC1-C) cytoplasmic domain associates with PI3K p85 in NSCLC cells. We show that inhibition of MUC1-C with cell-penetrating peptides blocks this interaction with PI3K p85 and suppresses constitutive phosphorylation of Akt and its downstream effector, mTOR. In concert with these results, treatment of NSCLC cells with the MUC1-C peptide inhibitor GO-203 was associated with downregulation of PI3K → Akt signaling and inhibition of growth. GO-203 treatment was also associated with increases in reactive oxygen species (ROS) and induction of necrosis by a ROS-dependent mechanism. Moreover, GO-203 treatment of H1975 (EGFR L858R/T790M) and A549 (K-Ras G12S) xenografts growing in nude mice resulted in tumor regressions. These findings indicate that NSCLC cells are dependent on MUC1-C both for activation of the PI3K → Akt pathway and for survival.

New targeted therapies for gastric cancer

INTRODUCTION

Inhibitors targeting oncogenic kinases, especially receptor tyrosine kinases (RTKs), are being vigorously developed, and some have been demonstrated to be effective in clinical settings. The amplification of certain RTKs (ErbB2, c-Met and FGFR2) is associated with gastric cancer progression, but the only recently approved inhibitor is trastuzumab, ErbB2-targeting antibody. Other well-known oncogenic kinases (PI3K and RAF) are also activated in a small portion of gastric cancers. Drugs targeting these kinases are promising and should be approved in an appropriate and expeditious way.

AREAS COVERED

This article reviews novel inhibitors emerging in the field of advanced gastric cancer, based on basic research concerning altered oncogenes and the clinical trials of drugs targeting these oncogenes.

EXPERT OPINION

Promising inhibitors of gastric cancer may be found in not only new investigative agents but also agents currently being used against other malignancies. The appropriate design for clinical trials of molecularly targeted therapeutic agents is also important. Targeted therapies tailored to individual genomic profiles would provide a more personalized treatment for advanced gastric cancer.

Effect of PI3K- and mTOR-specific inhibitors on spontaneous B-cell follicular lymphomas in PTEN/LKB1-deficient mice

Background:

The PI3K–mTOR (phosphoinositide 3-kinase–mammalian target of rapamycin kinase) pathway is activated in the majority of tumours, and there is interest in assessing whether inhibitors of PI3K or mTOR kinase have efficacy in treating cancer. Here, we define the effectiveness of specific mTOR (AZD8055) and PI3K (GDC-0941) inhibitors, currently in clinical trials, in treating spontaneous B-cell follicular lymphoma that develops in PTEN^+/−LKB1^+/hypo mice.

Methods:

The PTEN^+/−LKB1^+/hypo mice were administered AZD8055 or GDC-0941, and the volumes of B-cell follicular lymphoma were measured by MRI. Tumour samples were analysed by immunohistochemistry, immunoblot and flow cytometry.

Results:

The AZD8055 or GDC-0941 induced ∼40% reduction in tumour volume within 2 weeks, accompanied by ablation of phosphorylation of AKT, S6K and SGK (serum and glucocorticoid protein kinase) protein kinases. The drugs reduced tumour cell proliferation, promoted apoptosis and suppressed centroblast population. The AZD8055 or GDC-0941 treatment beyond 3 weeks caused a moderate additional decrease in tumour volume, reaching ∼50% of the initial volume after 6 weeks of treatment. Tumours grew back at an increased rate and displayed similar high grade and diffuse morphology as the control untreated tumours upon cessation of drug treatment.

Conclusion:

These results define the effects that newly designed and specific mTOR and PI3K inhibitors have on a spontaneous tumour model, which may be more representative than xenograft models frequently employed to assess effectiveness of kinase inhibitors. Our data suggest that mTOR and PI3K inhibitors would benefit treatment of cancers in which the PI3K pathway is inappropriately activated; however, when administered alone, may not cause complete regression of such tumours.

MicroRNA-mediated downregulation of mTOR/FGFR3 controls tumor growth induced by Src-related oncogenic pathways

The tyrosine kinase c-Src is upregulated in various human cancers, but the molecular mechanisms underlying c-Src-mediated tumor growth remain unclear. Here we examined the involvement of microRNAs in the c-Src-mediated tumor growth. Microarray profiling revealed that c-Src activation downregulates a limited set of microRNAs, including miR-99a, which targets oncogenic mammalian target of rapamycin (mTOR) and fibroblast growth factor receptor 3 (FGFR3). Re-expression of miR-99a suppressed tumor growth of c-Src-transformed cells, and this effect was restored by the overexpression of mTOR. The downregulation of miR-99a was also observed in epidermal growth factor- and Ras-transformed cells, and it was suppressed by inhibiting the mitogen-activated protein kinase (MAPK) pathway. Furthermore, miR-99a downregulation is associated with mTOR/FGFR3 upregulation in various human lung cancer cells/tissues. The tumorigenicity of these cells was suppressed by the introduction of miR-99a. These findings suggest that the miR-99a-mTOR/FGFR3 pathway is crucial for controlling tumor growth in a wide range of human cancers that harbor upregulation of the Src-related oncogenic pathways.

The multi-targeted kinase inhibitor sorafenib inhibits human cytomegalovirus replication

Human cytomegalovirus (HCMV) is a major pathogen in immunocompromised individuals. Here, non-toxic concentrations of the anti-cancer kinase inhibitor sorafenib were shown to inhibit replication of different HCMV strains (including a ganciclovir-resistant strain) in different cell types. In contrast to established anti-HCMV drugs, sorafenib inhibited HCMV major immediate early promoter activity and HCMV immediate early antigen (IEA) expression. Sorafenib is known to inhibit Raf. Comparison of sorafenib with the MEK inhibitor U0126 suggested that sorafenib inhibits HCMV IEA expression through inhibition of Raf but independently of signaling through the Raf downstream kinase MEK 1/2. In concordance, siRNA-mediated depletion of Raf but not of MEK-reduced IEA expression. In conclusion, sorafenib diminished HCMV replication in clinically relevant concentrations and inhibited HCMV IEA expression, a pathophysiologically relevant event that is not affected by established anti-HCMV drugs. Moreover, we demonstrated for the first time that Raf activation is involved in HCMV IEA expression.

PIK3CA mutations in patients with advanced cancers treated with PI3K/AKT/mTOR axis inhibitors

Preclinical data suggest that PIK3CA mutations predict response to PI3K/AKT/mTOR inhibitors. Concomitant KRAS or BRAF mutations may mediate resistance. Therefore, tumors from patients referred to the phase I program for targeted therapy starting in October 2008 were analyzed for PIK3CA mutations using PCR-based DNA sequencing of exons 9 and 20. Consecutive patients with diverse tumor types and PIK3CA mutation were treated whenever possible with agents targeting the PI3K/AKT/mTOR pathway. Overall, PIK3CA mutations were detected in 25 of 217 patients (11.5%; exon 9, n = 11; exon 20, n = 14). In tumor types with more than 10 patients tested, PIK3CA mutations were most frequent in endometrial (3 of 14, 21%), ovarian (5 of 30, 17%), colorectal (9 of 54, 17%), breast (2 of 14, 14%), cervical (2 of 15, 13%), and squamous cell cancer of the head and neck (1 of 11, 9%). Of the 25 patients with PIK3CA mutations, 17 (68%) were treated on a protocol that included a PI3K/AKT/mTOR pathway inhibitor, and 6 (35%) achieved a partial response. In contrast, only 15 of 241 patients (6%) without documented PIK3CA mutations treated on the same protocols responded (P = 0.001). Of the 17 patients with PIK3CA mutations, 6 (35%) had simultaneous KRAS or BRAF mutations (colorectal, n = 4; ovarian, n = 2). Colorectal cancer patients with PIK3CA and KRAS mutations did not respond to therapy, whereas both ovarian cancer patients with PIK3CA and KRAS or BRAF mutations did. In conclusion, PIK3CA mutations were detected in 11.5% of patients with diverse solid tumors. The response rate was significantly higher for patients with PIK3CA mutations treated with PI3K/AKT/mTOR pathway inhibitors than for those without documented mutations.

Preclinical modeling of combined phosphatidylinositol-3-kinase inhibition with endocrine therapy for estrogen receptor-positive breast cancer

Introduction

Inhibition of phosphatidylinositol-3-kinase (PI3K) induces apoptosis when combined with estrogen deprivation in estrogen receptor (ER)-positive breast cancer. The aims of the present study were to identify effective PI3K pathway inhibitor and endocrine therapy combinations, to evaluate the effect of PI3K pathway mutations and estrogen dependency on tumor response, and to determine the relevance of PIK3CA mutation in recurrent disease.

Methods

The PI3K catalytic subunit inhibitor BKM120, the mammalian target of rapamycin (mTOR) inhibitor RAD001 and the dual PI3K/mTOR inhibitor BGT226 were tested against ER-positive breast cancer cell lines before and after long-term estrogen deprivation (LTED). The impact of estradiol deprivation and the ER downregulator fulvestrant on PI3K pathway inhibitor-induced apoptosis was assessed. PIK3CA hotspot mutation analysis was performed in 51 recurrent or metastatic breast cancers and correlated with ER status and survival.

Results

Drug-induced apoptosis was most marked in short-term estrogen-deprived cells with PIK3CA mutation and phosphatase and tensin homolog loss. Apoptosis was most highly induced by BGT226, followed by BKM120, and then RAD001. Estradiol antagonized PI3K inhibitor-induced apoptosis following short-term estrogen deprivation, emphasizing a role for estrogen-deprivation therapy in promoting PI3K inhibitor activity in the first-line setting. ER-positive MCF7 LTED cells exhibited relative resistance to PI3K pathway inhibition that was reversed by fulvestrant. In contrast, T47D LTED cells exhibited ER loss and ER-independent PI3K agent sensitivity. PIK3CA mutation was prevalent in relapsed ER-positive disease (48%) and was associated with persistent ER positivity and a late relapse pattern.

Conclusions

Estrogen deprivation increased the apoptotic effects of PI3K and dual PI3K/mTOR inhibitors in ER-positive disease, providing a rationale for PI3K/aromatase inhibitor combinations as first-line therapy. In LTED cells, differential effects on ER expression may be a relevant consideration. When ER was persistently expressed, fulvestrant strongly promoted PI3K drug activity. When ER was lost, PI3K inhibitor monotherapy was sufficient to induce high-level apoptosis. Although tumors with PIK3CA mutation had a late recurrence pattern, these mutations were common in metastatic disease and were most often associated with persistent ER expression. Targeting PIK3CA mutant tumors with a PI3K pathway inhibitor and fulvestrant is therefore a feasible strategy for aromatase-inhibitor-resistant ER-positive relapsed breast cancer.

Selected Topics in the Molecular Pathology of Endometrial Carcinoma

In the developed world, endometrial carcinoma is the most common malignant tumor of the female gynecologic tract. Numerous epidemiologic studies indicate that exposure to unopposed estrogen is a significant risk factor for developing endometrial cancer, particularly endometrioid-type endometrial carcinoma; however, a number of other molecular pathways and mechanisms are also important in endometrial cancer. In this review, the authors highlight some of the more interesting molecular pathways in endometrial cancer, such as the PTEN/PI3K/AKT pathway, microsatellite instability, and molecular mediators of epithelial-to-mesenchymal transition.

Polyphenols as small molecular inhibitors of signaling cascades in carcinogenesis

Multiple lines of evidences suggest that oxidative stress induced by reactive oxygen species are closely related to multi-stage carcinogenesis. Polyphenols, a group of chemicals with more than one phenol unit or building block per molecule, have been recognized for possessing many health benefits including cancer-preventive effects mainly due to their antioxidant activity. However, polyphenols can directly bind with signaling molecules involved in carcinogenesis and regulate its activity. Moreover, it is noteworthy that the binding between the polyphenol and the target protein is determined by their structural relationship, which implies that different polyphenols have different target proteins, leading to divergent chemopreventive effects. Extracellular stimuli transmit signals into a cell by activating their target signaling cascades involved in carcinogenesis. As an example, Src family kinase, a family of proto-oncogenic tyrosine kinases activated by a variety of oxidative stress and proinflammatory agents, is known to regulate cell proliferation, differentiation, survival and angiogenesis. Src family kinase subsequently activates downstream signal cascades including mitogen-activated protein kinase, phosphoinositol-3-kinase, and nuclear factor-kappaB, thereby inducing cell proliferation and causing cancer. Recent studies demonstrate that polyphenols can directly target signaling cascades involved in inflammation and the development of cancer. Inhibition of the kinases by polyphenols contributes to the attenuation of carcinogenesis. Therefore, the development of polyphenols as direct inhibitors against target proteins is regarded as a rational approach for chemoprevention. This review describes and discusses recent results about the direct interactions of polyphenols and protein kinases in cancer chemoprevention.

Targeting aberrant PI3K/Akt activation by PI103 restores sensitivity to TRAIL-induced apoptosis in neuroblastoma

PURPOSE

Because we recently identified Akt activation as a novel poor prognostic indicator in neuroblastoma, we investigated whether phosphoinositide 3'-kinase (PI3K) inhibition sensitizes neuroblastoma cells for TRAIL-induced apoptosis.

EXPERIMENTAL DESIGN

The effect of pharmacological or genetic inhibition of PI3K or mTOR was analyzed on apoptosis induction, clonogenic survival, and activation of apoptosis signaling pathways in vitro and in a neuroblastoma in vivo model. The functional relevance of individual Bcl-2 family proteins was examined by knockdown or overexpression experiments.

RESULTS

The PI3K inhibitor PI103 cooperates with TRAIL to synergistically induce apoptosis (combination index < 0.1), to suppress clonogenic survival, and to reduce tumor growth in a neuroblastoma in vivo model. Similarly, genetic silencing of PI3K significantly increases TRAIL-mediated apoptosis, whereas genetic or pharmacological blockage of mTOR fails to potentiate TRAIL-induced apoptosis. Combined treatment with PI103 and TRAIL enhances cleavage of Bid and the insertion of tBid into mitochondrial membranes, and reduces phosphorylation of Bim(EL). Additionally, PI103 decreases expression of Mcl-1, XIAP, and cFLIP, thereby promoting Bax/Bak activation, mitochondrial perturbations, and caspase-dependent apoptosis. Knockdown of Bid or Noxa or overexpression of Bcl-2 rescues cells from PI103- and TRAIL-induced apoptosis, whereas Mcl-1 silencing potentiates apoptosis. Bcl-2 overexpression also inhibits cleavage of caspase-3, caspase-8, and Bid pointing to a mitochondria-driven feedback amplification loop.

CONCLUSIONS

PI103 primes neuroblastoma cells for TRAIL-induced apoptosis by shifting the balance toward proapoptotic Bcl-2 family members and increased mitochondrial apoptosis. Thus, PI3K inhibitors represent a novel promising approach to enhance the efficacy of TRAIL-based treatment protocols in neuroblastoma.

Pathway-based identification of biomarkers for targeted therapeutics: personalized oncology with PI3K pathway inhibitors

Although we have made great progress in understanding the complex genetic alterations that underlie human cancer, it has proven difficult to identify which molecularly targeted therapeutics will benefit which patients. Drug-specific modulation of oncogenic signaling pathways in specific patient subpopulations can predict responsiveness to targeted therapy. Here, we report a pathway-based phosphoprofiling approach to identify and quantify clinically relevant, drug-specific biomarkers for phosphatidylinositol 3-kinase (PI3K) pathway inhibitors that target AKT, phosphoinositide-dependent kinase 1 (PDK1), and PI3K-mammalian target of rapamycin (mTOR). We quantified 375 nonredundant PI3K pathway-relevant phosphopeptides, all containing AKT, PDK1, or mitogen-activated protein kinase substrate recognition motifs. Of these phosphopeptides, 71 were drug-regulated, 11 of them by all three inhibitors. Drug-modulated phosphoproteins were enriched for involvement in cytoskeletal reorganization (filamin, stathmin, dynamin, PAK4, and PTPN14), vesicle transport (LARP1, VPS13D, and SLC20A1), and protein translation (S6RP and PRAS40). We then generated phosphospecific antibodies against selected, drug-regulated phosphorylation sites that would be suitable as biomarker tools for PI3K pathway inhibitors. As proof of concept, we show clinical translation feasibility for an antibody against phospho-PRAS40(Thr246). Evaluation of binding of this antibody in human cancer cell lines, a PTEN (phosphatase and tensin homolog deleted from chromosome 10)-deficient mouse prostate tumor model, and triple-negative breast tumor tissues showed that phospho-PRAS40(Thr246) positively correlates with PI3K pathway activation and predicts AKT inhibitor sensitivity. In contrast to phosphorylation of AKT(Thr308), the phospho-PRAS40(Thr246) epitope is highly stable in tissue samples and thus is ideal for immunohistochemistry. In summary, our study illustrates a rational approach for discovery of drug-specific biomarkers toward development of patient-tailored treatments.

Signal transduction inhibitors and antiangiogenic therapies for malignant glioma

Detailed characterization of the cancer genome in a large number of primary human glioblastomas has identified recurrent alterations that result in deregulation of signal transduction pathways and are "druggable" with a growing number of small molecule pharmaceuticals. While many of these compounds have shown clinical activity in other human cancers harboring similar genetic alterations, the clinical experience in glioblastoma has been disappointing thus far with only rare and transient radiographic responses. Our understanding of drug resistance is confounded by the uncertainty of drug delivery across the blood brain barrier and the limited knowledge to what extent the growth of these tumors depends on any particular signaling pathway. This uncertainty is, at least in part, due to shortcomings in the current approach to evaluate signal transduction inhibitors in glioma patients, including drug testing in molecularly unselected patient populations, limited documentation of drug penetration and target inhibition in tumor tissue, and use of radiographic response criteria that may not be optimal for the evaluation of these compounds. © 2011 Wiley-Liss, Inc.

Antitumor efficacy of PKI-587, a highly potent dual PI3K/mTOR kinase inhibitor

PURPOSE

The aim of this study was to show preclinical efficacy and clinical development potential of PKI-587, a dual phosphoinositide 3-kinase (PI3K)/mTOR inhibitor.

EXPERIMENTAL DESIGN

In vitro class 1 PI3K enzyme and human tumor cell growth inhibition assays and in vivo five tumor xenograft models were used to show efficacy.

RESULTS

In vitro, PKI-587 potently inhibited class I PI3Ks (IC(50) vs. PI3K-α = 0.4 nmol/L), PI3K-α mutants, and mTOR. PKI-587 inhibited growth of 50 diverse human tumor cell lines at IC(50) values of less than 100 nmol/L. PKI-587 suppressed phosphorylation of PI3K/mTOR effectors (e.g., Akt), and induced apoptosis in human tumor cell lines with elevated PI3K/mTOR signaling. MDA-MB-361 [breast; HER2(+), PIK3CA mutant (E545K)] was particularly sensitive to this effect, with cleaved PARP, an apoptosis marker, induced by 30 nmol/L PKI-587 at 4 hours. In vivo, PKI-587 inhibited tumor growth in breast (MDA-MB-361, BT474), colon (HCT116), lung (H1975), and glioma (U87MG) xenograft models. In MDA-MB-361 tumors, PKI-587 (25 mg/kg, single dose i.v.) suppressed Akt phosphorylation [at threonine(T)308 and serine(S)473] for up to 36 hours, with cleaved PARP (cPARP) evident up to 18 hours. PKI-587 at 25 mg/kg (once weekly) shrank large (∼1,000 mm(3)) MDA-MB-361 tumors and suppressed tumor regrowth. Tumor regression correlated with suppression of phosphorylated Akt in the MDA-MB-361 model. PKI-587 also caused regression in other tumor models, and efficacy was enhanced when given in combination with PD0325901 (MEK 1/2 inhibitor), irinotecan (topoisomerase I inhibitor), or HKI-272 (neratinib, HER2 inhibitor).

CONCLUSION

Significant antitumor efficacy and a favorable pharmacokinetic/safety profile justified phase 1 clinical evaluation of PKI-587.

Epigallocatechin gallate (EGCG), a major component of green tea, is a dual phosphoinositide-3-kinase/mTOR inhibitor

The PI3K signaling pathway is activated in a broad spectrum of human cancers, either directly by genetic mutation or indirectly via activation of receptor tyrosine kinases or inactivation of the PTEN tumor suppressor. The key nodes of this pathway have emerged as important therapeutic targets for the treatment of cancer. In this study, we show that (-)-epigallocatechin-3-gallate (EGCG), a major component of green tea, is an ATP-competitive inhibitor of both phosphoinositide-3-kinase (PI3K) and mammalian target of rapamycin (mTOR) with K(i) values of 380 and 320nM respectively. The potency of EGCG against PI3K and mTOR is within physiologically relevant concentrations. In addition, EGCG inhibits cell proliferation and AKT phosphorylation at Ser473 in MDA-MB-231 and A549 cells. Molecular docking studies show that EGCG binds well to the PI3K kinase domain active site, agreeing with the finding that EGCG competes for ATP binding. Our results suggest another important molecular mechanism for the anticancer activities of EGCG.

The extending network of FOXO transcriptional target genes

The evolutionarily conserved Forkhead box O (FOXO) family of transcription factors regulates multiple transcriptional targets involved in various cellular processes, including proliferation, stress resistance, apoptosis, and metabolism. Target gene regulation appears to be controlled in a cell-type-specific manner due to association of FOXO isoforms with specific cofactors. Many of the cellular processes modulated by FOXO are themselves deregulated in tumorigenesis, and deletion of Foxo genes has demonstrated that these transcription factors function as tumor suppressors. Our understanding of the regulation of FOXO activity, and defining specific transcriptional targets, may provide clues to the molecular mechanisms controlling cell fate decisions. In this review we describe the functional consequences of FOXO activation based on our current knowledge of transcriptional targets.

Sense and sensitivity: FOXO and ROS in cancer development and treatment

Forkhead box O (FOXO) transcription factors are at the center of an emerging paradigm that links longevity, cell fate, and tumor development. Key to these processes is the ability of FOXO to regulate, and be regulated by, oxidative stress. Perturbation of the mechanisms that tightly couple reactive oxygen species (ROS) production, oxidative stress signaling, and FOXO activity to the subsequent cellular response is a pivotal step in cancer development and progression. Consequently, the ROS-FOXO pathway is a major therapeutic target in cancer, not only as it mediates the cellular response to chemotherapy, but also because it underpins drug resistance. As the intimate and reciprocal relation between FOXO and ROS is being unravelled, new opportunities arise to develop more-effective cancer treatments that circumvent resistance to the conventional cytotoxic drugs.

Cancer genetics-guided discovery of serum biomarker signatures for diagnosis and prognosis of prostate cancer

A key barrier to the realization of personalized medicine for cancer is the identification of biomarkers. Here we describe a two-stage strategy for the discovery of serum biomarker signatures corresponding to specific cancer-causing mutations and its application to prostate cancer (PCa) in the context of the commonly occurring phosphatase and tensin homolog (PTEN) tumor-suppressor gene inactivation. In the first stage of our approach, we identified 775 N-linked glycoproteins from sera and prostate tissue of wild-type and Pten-null mice. Using label-free quantitative proteomics, we showed that Pten inactivation leads to measurable perturbations in the murine prostate and serum glycoproteome. Following bioinformatic prioritization, in a second stage we applied targeted proteomics to detect and quantify 39 human ortholog candidate biomarkers in the sera of PCa patients and control individuals. The resulting proteomic profiles were analyzed by machine learning to build predictive regression models for tissue PTEN status and diagnosis and grading of PCa. Our approach suggests a general path to rational cancer biomarker discovery and initial validation guided by cancer genetics and based on the integration of experimental mouse models, proteomics-based technologies, and computational modeling.

Beyond chemotherapy: new agents for targeted treatment of lymphoma

An improved understanding of the molecular biology of cancer cell growth and survival and the role of the microenvironment in supporting the survival of cancer cells, including lymphoma cells, has led to the identification of a number of potential therapeutic targets. Despite these advances, drug development for lymphoma remains slow, inefficient, and frequently unfocused. Future work should focus on identifying 'driver' molecular defects of oncogenic pathways that can be targeted therapeutically, discovering predictive biomarkers for treatment response, and prioritizing promising drugs to accelerate their approval. This Review summarizes the current development status of novel agents for lymphoma and discusses strategies to move the field forward.

Preclinical testing of the Akt inhibitor triciribine in T-cell acute lymphoblastic leukemia

Over the past 20 years, survival rates of T-cell acute lymphoblastic leukemia (T-ALL) patients have improved, mainly because of advances in polychemotherapy protocols. Despite these improvements, we still need novel and less toxic treatment strategies targeting aberrantly activated signaling networks which increase proliferation, survival, and drug resistance of T-ALL cells. One such network is represented by the phosphatidylinositol 3-kinase (PI3K)/Akt axis. PI3K inhibitors have displayed some promising effects in preclinical models of T-ALL. Here, we have analyzed the therapeutic potential of the Akt inhibitor, triciribine, in T-ALL cell lines. Triciribine caused cell cycle arrest and caspase-dependent apoptosis. Western blots demonstrated a dose-dependent dephosphorylation of Akt1/Akt2, and of mammalian target of rapamycin complex 1 downstream targets in response to triciribine. Triciribine induced autophagy, which could be interpreted as a defensive mechanism, because an autophagy inhibitor (chloroquine) increased triciribine-induced apoptosis. Triciribine synergized with vincristine, a chemotherapeutic drug employed for treating T-ALL patients, and targeted the side population of T-ALL cell lines, which might correspond to leukemia initiating cells. Our findings indicate that Akt inhibition, either alone or in combination with chemotherapeutic drugs, may serve as an efficient treatment towards T-ALL cells requiring upregulation of this signaling pathway for their proliferation and survival.

A unique spectrum of somatic PIK3CA (p110alpha) mutations within primary endometrial carcinomas

PURPOSE

The goal of this study was to comprehensively define the incidence of mutations in all exons of PIK3CA in both endometrioid endometrial cancer (EEC) and nonendometrioid endometrial cancer (NEEC).

EXPERIMENTAL DESIGN

We resequenced all coding exons of PIK3CA and PTEN, and exons 1 and 2 of KRAS, from 108 primary endometrial tumors. Somatic mutations were confirmed by sequencing matched normal DNAs. The biochemical properties of a subset of novel PIK3CA mutations were determined by exogenously expressing wild type and mutant constructs in U2OS cells and measuring levels of AKT(Ser473) phosphorylation.

RESULTS

Somatic PIK3CA mutations were detected in 52.4% of 42 EECs and 33.3% of 66 NEECs. Half (29 of 58) of all nonsynonymous PIK3CA mutations were in exons 1-7 and half were in exons 9 and 20. The exons 1-7 mutations localized to the ABD, ABD-RBD linker and C2 domains of p110α. Within these regions, Arg88, Arg93, Gly106, Lys111, Glu365, and Glu453, were recurrently mutated; Arg88, Arg93, and Lys111 formed mutation hotspots. The p110α-R93W, -G106R, -G106V, -K111E, -delP449-L455, and -E453K mutants led to increased levels of phospho-AKT(Ser473) compared to wild-type p110α. Overall, 62% of exons 1-7 PIK3CA mutants and 64% of exons 9-20 PIK3CA mutants were activating; 72% of exon 1-7 mutations have not previously been reported in endometrial cancer.

CONCLUSIONS

Our study identified a new subgroup of endometrial cancer patients with activating mutations in the amino-terminal domains of p110α; these patients might be appropriate for consideration in clinical trials of targeted therapies directed against the PI3K pathway.

Cell-to-cell variability in PI3K protein level regulates PI3K-AKT pathway activity in cell populations

BACKGROUND

Cell-to-cell variability in populations has been widely observed in mammalian cells. This heterogeneity can result from random stochastic events or can be deliberately maintained through regulatory processes. In the latter case, heterogeneity should confer a selective advantage that benefits the entire population.

RESULTS

Using multicolor flow cytometry, we have uncovered robust heterogeneity in phosphoinositide 3-kinase (PI3K) activity in MCF10A cell populations, which had been previously masked by techniques that only measure population averages. We show that AKT activity is bimodal in response to EGF stimulation and correlates with PI3K protein level, such that only cells with high PI3K protein can activate AKT. We further show that heterogeneity in PI3K protein levels is invariably maintained in cell populations through a degradation/resynthesis cycle that can be regulated by cell density.

CONCLUSIONS

Given that the PI3K pathway is one of the most frequently upregulated pathways in cancer, we propose that heterogeneity in PI3K activity is beneficial to normal tissues by restricting PI3K activation to only a subset of cells. This may serve to protect the population as a whole from overactivating the pathway, which can lead to cellular senescence or cancer. Consistent with this, we show that oncogenic mutations in p110α (H1047R and E545K) partially evade this negative regulation, resulting in increased AKT activity in the population.

New strategies in HER2-overexpressing breast cancer: many combinations of targeted drugs available

The anti-HER2 drugs trastuzumab and lapatinib are increasingly changing the natural history of early and metastatic HER2-overexpressing breast cancer. Many other agents targeted against the HER2 signaling network are in clinical development, and these are or will soon be combined with the currently approved anti-HER2 therapies. We review herein recent data in support of the early use of combinations of agents targeted to the HER2 network as the most rational approach against this subtype of breast cancer. We propose that the optimal combination or combinations of anti-HER2 agents delivered early in the natural history of HER2+ breast cancer should close to eliminate acquired drug resistance, shorten the duration of therapy, and potentially dispense with the need of concurrent chemotherapy.

Stathmin overexpression identifies high-risk patients and lymph node metastasis in endometrial cancer

PURPOSE

Overexpression of the oncogen Stathmin has been linked to aggressive endometrial carcinoma and a potential for PI3Kinase inhibitors in this disease. We wanted to validate the prognostic value of Stathmin expression in a large prospective multicenter setting. As lymph node sampling is part of current surgical staging, we also aimed to test if Stathmin expression in endometrial curettage specimens could predict lymph node metastasis.

EXPERIMENTAL DESIGN

A total of 1,076 endometrial cancer patients have been recruited from 10 centers to investigate the biological tumor marker Stathmin in relation to clinicopathologic variables, including lymph node status and survival. Stathmin immunohistochemical staining was carried out in 477 hysterectomy and 818 curettage specimens.

RESULTS

Seventy-one percent of the patients (n = 763) were subjected to lymph node sampling, of which 12% had metastatic nodes (n = 94). Overexpression of Stathmin was detected in 37% (302 of 818) of the curettage and in 18% (84 of 477) of the hysterectomy specimens investigated. Stathmin overexpression in curettage and hysterectomy specimens were highly correlated and significantly associated with nonendometrioid histology, high grade, and aneuploidy. Stathmin analysis in preoperative curettage samples significantly correlated with, and was an independent predictor of, lymph node metastases. High Stathmin expression was associated with poor disease-specific survival (P ≤ 0.002) both in curettage and hysterectomy specimens.

CONCLUSIONS

Stathmin immunohistochemical staining identifies endometrial carcinomas with lymph node metastases and poor survival. The value, as a predictive marker for response to PI3Kinase inhibition and as a tool to stratify patients for lymph node sampling in endometrial carcinomas, remains to be determined.

CAL-101, a p110delta selective phosphatidylinositol-3-kinase inhibitor for the treatment of B-cell malignancies, inhibits PI3K signaling and cellular viability

Phosphatidylinositol-3-kinase p110δ serves as a central integration point for signaling from cell surface receptors known to promote malignant B-cell proliferation and survival. This provides a rationale for the development of small molecule inhibitors that selectively target p110δ as a treatment approach for patients with B-cell malignancies. We thus identified 5-fluoro-3-phenyl-2-[(S)-1-(9H-purin-6-ylamino)-propyl]-3H-quinazolin-4-one (CAL-101), a highly selective and potent p110δ small molecule inhibitor (half-maximal effective concentration [EC(50)] = 8nM). Using tumor cell lines and primary patient samples representing multiple B-cell malignancies, we have demonstrated that constitutive phosphatidylinositol-3-kinase pathway activation is p110δ-dependent. CAL-101 blocked constitutive phosphatidylinositol-3-kinase signaling, resulting in decreased phosphorylation of Akt and other downstream effectors, an increase in poly(ADP-ribose) polymerase and caspase cleavage and an induction of apoptosis. These effects have been observed across a broad range of immature and mature B-cell malignancies, thereby providing a rationale for the ongoing clinical evaluation of CAL-101.

Transient PI3K inhibition induces apoptosis and overcomes HGF-mediated resistance to EGFR-TKIs in EGFR mutant lung cancer

PURPOSE

Epidermal growth factor receptor (EGFR) tyrosine kinase inhibitors (TKI), such as gefitinib and erlotinib, show favorable response to EGFR mutant lung cancer. However, the responders acquire resistance almost without exception. We recently reported that hepatocyte growth factor (HGF) induces EGFR-TKI resistance by activating MET that restores downstream mitogen activated protein kinase (MAPK)/extracellular signal regulated kinase (ERK)1/2 and phosphoinositide 3-kinase (PI3K)/Akt signaling. The purpose of this study was to determine whether inhibition of PI3K, a downstream molecule of both EGFR and MET, could overcome HGF-mediated EGFR-TKI resistance in EGFR mutant lung cancer cells PC-9 and HCC827.

EXPERIMENTAL DESIGN

We explored therapeutic effect of a class I PI3K inhibitor PI-103 on HGF-induced EGFR-TKI resistance in vitro and in vivo.

RESULTS

Unlike gefitinib or erlotinib, continuous exposure with PI-103 inhibited proliferation of PC-9 and HCC827 cells, even in the presence of HGF. On the other hand, in gefitinib-resistant xenograft model by using PC-9 cells mixed with HGF high producing fibroblasts, PI-103 monotherapy did not inhibit tumor growth. However, PI-103 combined with gefitinib successfully regressed gefitinib-resistant tumor. In vitro experiments by considering short half-life of PI-103 reveal that transient exposure of PI-103 combined with gefitinib caused sustained inhibition of Akt phosphorylation, but not ERK1/2 phosphorylation, resulting in induction of tumor cell apoptosis even in the presence of HGF.

CONCLUSIONS

These results indicate that transient blockade of PI3K/Akt pathway by PI-103 and gefitinib could overcome HGF-mediated resistance to EGFR-TKIs by inducing apoptosis in EGFR mutant lung cancer.

Epigenetic silencing mediated through activated PI3K/AKT signaling in breast cancer

Trimethylation of histone 3 lysine 27 (H3K27me3) is a critical epigenetic mark for the maintenance of gene silencing. Additional accumulation of DNA methylation in target loci is thought to cooperatively support this epigenetic silencing during tumorigenesis. However, molecular mechanisms underlying the complex interplay between the two marks remain to be explored. Here we show that activation of PI3K/AKT signaling can be a trigger of this epigenetic processing at many downstream target genes. We also find that DNA methylation can be acquired at the same loci in cancer cells, thereby reinforcing permanent repression in those losing the H3K27me3 mark. Because of a link between PI3K/AKT signaling and epigenetic alterations, we conducted epigenetic therapies in conjunction with the signaling-targeted treatment. These combined treatments synergistically relieve gene silencing and suppress cancer cell growth in vitro and in xenografts. The new finding has important implications for improving targeted cancer therapies in the future.

Phylogenomics of phosphoinositide lipid kinases: perspectives on the evolution of second messenger signaling and drug discovery

Background

Phosphoinositide lipid kinases (PIKs) generate specific phosphorylated variants of phosatidylinositols (PtdIns) that are critical for second messenger signaling and cellular membrane remodeling. Mammals have 19 PIK isoforms spread across three major families: the PtIns 3-kinases (PI3Ks), PtdIns 4-kinases (PI4Ks), and PtdIns-P (PIP) kinases (PIPKs). Other eukaryotes have fewer yet varying PIK complements. PIKs are also an important, emerging class of drug targets for many therapeutic areas including cancer, inflammatory and metabolic diseases and host-pathogen interactions. Here, we report the genomic occurrences and evolutionary relationships or phylogenomics of all three PIK families across major eukaryotic groups and suggest potential ramifications for drug discovery.

Results

Our analyses reveal four core eukaryotic PIKs which are type III PIK4A and PIK4B, and at least one homolog each from PI3K (possibly PIK3C3 as the ancestor) and PIP5K families. We also applied evolutionary analyses to PIK disease ontology and drug discovery. Mutated PIK3CA are known to be oncogenic and several inhibitors are in anti-cancer clinical trials. We found conservation of activating mutations of PIK3CA in paralogous isoforms suggesting specific functional constraints on these residues. By mapping published compound inhibition data (IC50s) onto a phylogeny of PI3Ks, type II PI4Ks and distantly related, MTOR, ATM, ATR and PRKDC kinases, we also show that compound polypharmacology corresponds to kinase evolutionary relationships. Finally, we extended the rationale for drugs targeting PIKs of malarial Plasmodium falciparum, and the parasites, Leishmania sp. and Trypanosoma sp. by identifying those PIKs highly divergent from human homologs.

Conclusion

Our phylogenomic analysis of PIKs provides new insights into the evolution of second messenger signaling. We postulate two waves of PIK diversification, the first in metazoans with a subsequent expansion in cold-blooded vertebrates that was post-emergence of Deutrostomia\Chordata but prior to the appearance of mammals. Reconstruction of the evolutionary relationships among these lipid kinases also adds to our understanding of their roles in various diseases and assists in their development as potential drug targets.

The molecular biology of head and neck cancer

Head and neck squamous cell carcinomas (HNSCCs) are caused by tobacco and alcohol consumption and by infection with high-risk types of human papillomavirus (HPV). Tumours often develop within preneoplastic fields of genetically altered cells. The persistence of these fields after treatment presents a major challenge, because it might lead to local recurrences and second primary tumours that are responsible for a large proportion of deaths. Aberrant signalling pathways have been identified in HNSCCs and inhibition of epidermal growth factor receptor (EGFR) has proved a successful therapeutic strategy. In this Review, we discuss the recent literature on tumour heterogeneity, field cancerization, molecular pathogenesis and the underlying causative cancer genes that can be exploited for novel and personalized treatments of patients with HNSCC.

Evolution in Endometrial Cancer: Evidence From an Immunohistochemical Study

Background:

It is poorly described how endometrial cancer biology changes during tumor evolution. We hypothesize that characterization of molecular targets in recurrent lesions is more relevant for targeting treatment.

Methods:

Paired biopsies from primary and recurrent endometrial cancer tumors (n = 85) were stained immunohistochemically for the following proteins: estrogen receptor (ER), progesterone receptor (PR), stathmin (correlating with phosphatidylinositol 3-kinase activity), HER-2/neu, WT1 (Wilms tumor gene 1), phospho-mammalian target of rapamycin (p-mTOR), and p53. Each tumor was scored, using a semiquantitative and subjective grading system. Discordance, a change in expression between primary and recurrent tumor, was defined as ≥2 step change; concordance was ≤1 step change. The frequency of change was correlated with established prognostic markers in endometrial cancer.

Results:

Fifty-six patients (67%) were diagnosed with endometrioid carcinoma, 23 (27%) with serous/clear cell carcinoma. A change in expression between primary and recurrent tumor was noted in 7% to 31% of patients for ER, PR, stathmin, HER-2/neu, WT1, p-mTOR, and p53. Concordant-positive cases for PR were significantly correlated with stage, tumor grade, and histological subtype. Expression of ER, p53, and p-mTOR in cytoplasm in the recurrent tumor correlated significantly with survival.

Conclusions:

Endometrial cancer biology changes over time. The decision on targeted treatment should preferably be based on recurrent tumor characteristics.

Preclinical pharmacology, antitumor activity, and development of pharmacodynamic markers for the novel, potent AKT inhibitor CCT128930

AKT is frequently deregulated in cancer, making it an attractive anticancer drug target. CCT128930 is a novel ATP-competitive AKT inhibitor discovered using fragment- and structure-based approaches. It is a potent, advanced lead pyrrolopyrimidine compound exhibiting selectivity for AKT over PKA, achieved by targeting a single amino acid difference. CCT128930 exhibited marked antiproliferative activity and inhibited the phosphorylation of a range of AKT substrates in multiple tumor cell lines in vitro, consistent with AKT inhibition. CCT128930 caused a G(1) arrest in PTEN-null U87MG human glioblastoma cells, consistent with AKT pathway blockade. Pharmacokinetic studies established that potentially active concentrations of CCT128930 could be achieved in human tumor xenografts. Furthermore, CCT128930 also blocked the phosphorylation of several downstream AKT biomarkers in U87MG tumor xenografts, indicating AKT inhibition in vivo. Antitumor activity was observed with CCT128930 in U87MG and HER2-positive, PIK3CA-mutant BT474 human breast cancer xenografts, consistent with its pharmacokinetic and pharmacodynamic properties. A quantitative immunofluorescence assay to measure the phosphorylation and total protein expression of the AKT substrate PRAS40 in hair follicles is presented. Significant decreases in pThr246 PRAS40 occurred in CCT128930-treated mouse whisker follicles in vivo and human hair follicles treated ex vivo, with minimal changes in total PRAS40. In conclusion, CCT128930 is a novel, selective, and potent AKT inhibitor that blocks AKT activity in vitro and in vivo and induces marked antitumor responses. We have also developed a novel biomarker assay for the inhibition of AKT in human hair follicles, which is currently being used in clinical trials.

The molecular biology of head and neck cancer

Head and neck squamous cell carcinomas (HNSCCs) are caused by tobacco and alcohol consumption and by infection with high-risk types of human papillomavirus (HPV). Tumours often develop within preneoplastic fields of genetically altered cells. The persistence of these fields after treatment presents a major challenge, because it might lead to local recurrences and second primary tumours that are responsible for a large proportion of deaths. Aberrant signalling pathways have been identified in HNSCCs and inhibition of epidermal growth factor receptor (EGFR) has proved a successful therapeutic strategy. In this Review, we discuss the recent literature on tumour heterogeneity, field cancerization, molecular pathogenesis and the underlying causative cancer genes that can be exploited for novel and personalized treatments of patients with HNSCC.

Targeting a common collaborator in cancer development

In this issue of Science Translational Medicine, Wallin et al. have identified a subset of breast and ovarian cancer cell lines that show synergistic response to the combination of doxorubicin and GDC-0941, a class IA phosphatidylinositol 3-kinase (PI3K) inhibitor. Here, we discuss the potential implications of these data on the clinical development of PI3K pathway inhibitors as cancer therapeutics.

Akt/protein kinase B in skeletal muscle physiology and pathology

The Akt/protein kinase B is critical regulator of cellular homeostasis with diminished Akt activity being associated with dysregulation of cellular metabolism and cell death while Akt over-activation has been linked to inappropriate cell growth and proliferation. Although the regulation of Akt function has been well characterized in vitro, much less is known regarding the function of Akt in vivo. Here we examine how skeletal muscle Akt expression and enzymatic activity are controlled, the role of Akt in the regulation of skeletal muscle contraction, stress response glucose utilization, and protein metabolism, and the potential participation of this important molecule in skeletal muscle atrophy, aging, and cancer.

Amino acids activate mammalian target of rapamycin complex 2 (mTORC2) via PI3K/Akt signaling

The activity of mammalian target of rapamycin (mTOR) complexes regulates essential cellular processes, such as growth, proliferation, or survival. Nutrients such as amino acids are important regulators of mTOR complex 1 (mTORC1) activation, thus affecting cell growth, protein synthesis, and autophagy. Here, we show that amino acids may also activate mTOR complex 2 (mTORC2). This activation is mediated by the activity of class I PI3K and of Akt. Amino acids induced a rapid phosphorylation of Akt at Thr-308 and Ser-473. Whereas both phosphorylations were dependent on the presence of mTOR, only Akt phosphorylation at Ser-473 was dependent on the presence of rictor, a specific component of mTORC2. Kinase assays confirmed mTORC2 activation by amino acids. This signaling was functional, as demonstrated by the phosphorylation of Akt substrate FOXO3a. Interestingly, using different starvation conditions, amino acids can selectively activate mTORC1 or mTORC2. These findings identify a new signaling pathway used by amino acids underscoring the crucial importance of these nutrients in cell metabolism and offering new mechanistic insights.

Novel strategies for treating relapsed/refractory urothelial carcinoma

Advanced urothelial cancer is associated with a poor prognosis and there has been no substantial progress over the past three decades since the development of platinum-based multiagent chemotherapy. Clinical trials evaluating novel agents and combinations including chemotherapeutic drugs, as well as targeted inhibitors, are desperately needed. With a better understanding of the complex molecular alterations that drive urothelial tumorigenesis, new targets for novel therapeutics are being defined. This article will describe the current state of advanced urothelial cancer treatment and provide a comprehensive discussion of novel agents in development.

CH05-10, a novel indinavir analog, is a broad-spectrum antitumor agent that induces cell cycle arrest, apoptosis, endoplasmic reticulum stress and autophagy

Indinavir, a human immunodeficiency virus (HIV) protease inhibitor, inhibits the growth of tumor cells in vivo but does not show any cytotoxicity against cancer cells in vitro. To optimize the anticancer activity of indinavir, two novel analogs, CH05-0 and CH05-10, were synthesized. CH05-10 was much more cytotoxic than indinavir and had similar cytotoxicity to nelfinavir, the one with the best anticancer activities among all HIV protease inhibitors examined. For 14 cell lines representing 10 different types of human malignancies, the 50% inhibitory concentration (IC(50)) values of CH05-10 are in the range of 4.64-38.87 μM. Further detailed studies using the lung cancer cell line A549 as the model system showed that the effect of CH05-10 on the A549 cell line is both time- and dose-dependent. The CH05-10 treatment not only induced cell cycle arrest at G(1) and caused caspase-dependent apoptosis, but also resulted in caspase-independent death via the induction of endoplasmic reticulum stress and unfolded protein response. These findings demonstrate that CH05-10, a novel indinavir analog, is a potent anticancer agent with pleiotropic effects.

A beta version of life: p110β takes center stage

The PI3K pathway is frequently activated in tumors, most commonly through p110α mutation or PTEN deletion. In contrast to p110α, p110β is oncogenic when over-expressed in the wild-type state, suggesting that its regulation by p85 is different than that of p110α. In this perspective, we summarize recent data concerning the regulation of p110β, which shows that wild-type p110β acts like an oncogenic mutant of p110α. We also discuss the significance of this altered regulation in tumor models of PTEN deletion, as well as the potential implications of the unique p110β regulation on GPCR-driven tumorigenesis.

Systematic quantification of negative feedback mechanisms in the extracellular signal-regulated kinase (ERK) signaling network

Cell responses are actuated by tightly controlled signal transduction pathways. Although the concept of an integrated signaling network replete with interpathway cross-talk and feedback regulation is broadly appreciated, kinetic data of the type needed to characterize such interactions in conjunction with mathematical models are lacking. In mammalian cells, the Ras/ERK pathway controls cell proliferation and other responses stimulated by growth factors, and several cross-talk and feedback mechanisms affecting its activation have been identified. In this work, we take a systematic approach to parse the magnitudes of multiple regulatory mechanisms that attenuate ERK activation through canonical (Ras-dependent) and non-canonical (PI3K-dependent) pathways. In addition to regulation of receptor and ligand levels, we consider three layers of ERK-dependent feedback: desensitization of Ras activation, negative regulation of MEK kinase (e.g. Raf) activities, and up-regulation of dual-specificity ERK phosphatases. Our results establish the second of these as the dominant mode of ERK self-regulation in mouse fibroblasts. We further demonstrate that kinetic models of signaling networks, trained on a sufficient diversity of quantitative data, can be reasonably comprehensive, accurate, and predictive in the dynamical sense.