Dysregulation of phosphatidylinositol 3-kinase and downstream effectors in human breast cancer

Phosphatidylinositol 3-kinase (PI3-K) is a growth factor-activated transforming lipid (and protein) kinase, involved in cell motility and invasion, that has multiple effectors. Relatively little is known about its expression and enzymatic activity in human breast cancer. Since growth factor receptors are amplified in breast cancer, and the tumor suppressor PTEN may be mutated in human breast cancer, it was hypothesized that PI3-K and its downstream effectors would be activated in this disease. In 11 resected tumors analyzed for expression of this kinase, a mean 3-fold increase in protein expression was observed over the corresponding adjacent control tissue. Using an in vitro lipid kinase assay of the immunoprecipitated PI3-K protein, a greater than 2-fold increase in activation was observed. These changes were observed in the absence of an activation of either protein kinase B (PKB, akt1) or p70 S6 kinase (p70 S6K). However, p21-activated kinase (Pak), p38 mitogen-activated protein kinase (p38 MAPK) and mitogen-activated protein kinase-activated protein kinase 2 (MAPKAPK 2) were all overexpressed and demonstrated increased enzyme activity. It may be concluded that aberrant mitogenic signaling in human breast cancer in vivo involves Pak, p38 MAPK and MAPKAPK2 downstream of PI3-K, but neither of PKB or p70 S6K. It is proposed that this pathway may serve as a useful targeting nexus for investigation of small molecule inhibitors in human breast cancer.     

Targeting the phosphatidylinositol 3-kinase signaling pathway in breast cancer

The phosphatidylinositol 3-kinase (PI3K)/AKT/mammalian target of rapamycin (mTOR) network plays a key regulatory function in cell survival, proliferation, migration, metabolism, angiogenesis, and apoptosis. Genetic aberrations found at different levels, either with activation of oncogenes or inactivation of tumor suppressors, make this pathway one of the most commonly disrupted in human breast cancer. The PI3K-dependent phosphorylation and activation of the serine/threonine kinase AKT is a key activator of cell survival mechanisms. The activation of the oncogene PIK3CA and the loss of regulators of AKT including the tumor suppressor gene PTEN are mutations commonly found in breast tumors. AKT relieves the negative regulation of mTOR to activate protein synthesis and cell proliferation through S6K and 4EBP1. The common activation of the PI3K pathway in breast cancer has led to the development of compounds targeting the effector mechanisms of the pathway including selective and pan-PI3K/pan-AKT inhibitors, rapamycin analogs for mTOR inhibition, and TOR-catalytic subunit inhibitors. The influences of other oncogenic pathways such as Ras-Raf-Mek on the PI3K pathway and the known feedback mechanisms of activation have prompted the use of compounds with broader effect at multiple levels and rational combination strategies to obtain a more potent antitumor activity and possibly a meaningful clinical effect. Here, we review the biology of the network, its role in the development and progression of breast cancer, and the evaluation of targeted therapies in clinical trials.     

TC21 mediates transformation and cell survival via activation of phosphatidylinositol 3-kinase/Akt and NF-kappaB signaling pathway

The signaling pathways of TC21-mediated transformation and cell survival are not well-established. In this study, we have investigated the role of PI3-K/Akt signaling pathway in oncogenic-TC21-mediated transformation and cell survival. We found that oncogenic-TC21 stimulated the PI3-K activity. This was associated with the activation of Akt, a key component of PI3-K signaling pathway. We also found that TC21 interacted and formed complex with PI3-K. Mutations in the GTP-binding region of TC21, which enhanced GTP-binding potential of this protein, also stimulated its association with PI3-K, suggesting that PI3-K may preferentially interact with the GTP-bound form. Suppression of PI3-K and Akt by specific inhibitors LY294002 and Wortmannin reversed TC21-induced transformation. Likewise, inhibition of PI3-K activity by the PI3-K phosphotase PTEN reduced TC21-mediated focus formation in NIH3T3 cells. Investigation of TC21's effect on cell survival revealed that mutant-TC21 expressing cells were more resistant to etoposide- and cisplatin-induced cell death, and this was associated with the activation of anti-apoptotic protein NF-kappaB, a downstream target of Akt. Treatment of PI3-K inhibitor LY294002 significantly suppressed TC21-mediated NF-kappaB activation. In conclusion, we have identified PI3-K as an effector of TC21 and demonstrated that the PI3-K/Akt signaling pathway plays important roles in TC21-mediated transformation and cell survival.     

Prevention of phosphatidylinositol 3'-kinase-Akt survival signaling pathway during topotecan-induced apoptosis

The serine/threonine kinase Akt (also known as protein kinase B) is a downstream effector of phosphatidylinositol-3'-kinase [PI(3)K] that is recognized as the major mediator of survival signals that protect cells from undergoing apoptosis. In the course of examining the target molecules of the topoisomerase I inhibitor topotecan, we found that topotecan treatment promoted Akt dephosphorylation that led to the inactivation of Akt in human lung cancer A549 cells. Transfection of the constitutively active akt cDNA into A549 cells resulted in the reduction of the cytotoxic effect of topotecan, indicating that inhibition of the Akt pathway played an important role in exhibition of topotecan-mediated cytotoxic effects. Further analysis of Akt dephosphorylation revealed that topotecan treatment suppressed upstream kinases of Akt, 3-phosphoinositide-dependent protein kinase 1, and PI(3)K. Overall, the results demonstrate that topotecan exhibited its cytotoxic effects by down-regulating the PI(3)K-Akt survival signaling pathway in addition to inhibiting topoisomerase I.     

肝癌细胞中PI3K信号通路对VEGF表达的影响

目的:研究磷脂酰肌醇3-激酶(PI3K)信号通路在肝癌细胞血管内皮生长因子(VEGF)表达调控中的作用.方法: 体外培养高转移性人肝癌细胞株HCCLM3,分为对照组、PI3K特异性抑制剂LY294002浓度5μmol/L组,LY294002浓度10μmol/L及20μmol/L组,处理6小时后,采用逆转录聚合酶链反应(RT-PCR)技术检测VEGF mRNA表达的变化.结果: LY294002可使肝癌细胞HCCLM3的VEGF mRNA的表达下降,且这种抑制作用随着LY294002浓度增大而增大(P<0.01),呈现剂量移赖性关系.结论: LY294002可以抑制肝癌细胞株HCCLM3中VEGF的表达,肝癌细胞VEGF的表达调控受PI3K信号通路调控.     

磷脂酰肌醇3-激酶在人乳腺癌中的活化

背景与目的：磷脂酰肌醇3-激酶（phosphatidylinsitol 3-kinase,P13-K)可被多种有丝分裂信号刺激活化,它参与有关细胞生长信号的调控,但其在人类肿瘤中的作用尚不清楚。本研究旨在探讨人乳腺癌中P13-K的蛋白表达、mRNA表达和激酶活性变化情况。方法：应用Western blot、免疫沉淀、激酶活性测定和RT-PCR方法检测了37例人乳腺癌及其周围非癌组织中P13-KP85亚单位的表达及P13-K活性变化。结果：与周围非癌组织相比,37例人乳腺癌中,32例（86.49%)P13-Kp85亚单位蛋白表达显著增加;35例（94.59%)P13-Kp85亚单位mRNA水平明显增高;25例（67.57%)p13-k激酶活性亦显著增高。结论：人乳腺癌中P13-K被活化,提示P13-K参与乳腺癌细胞的信号转导;它有可能成为人乳腺癌治疗的新靶点。     

K-Ras promotes growth transformation and invasion of immortalized human pancreatic cells by Raf and phosphatidylinositol 3-kinase signaling

Mutational activation of the K-Ras oncogene is well established as a key genetic step in the development and growth of pancreatic adenocarcinomas. However, the mechanism by which aberrant Ras signaling promotes uncontrolled pancreatic tumor cell growth remains to be fully elucidated. The recent use of primary human cells to study Ras-mediated oncogenesis provides important model cell systems to dissect this mechanism. We have used a model of telomerase-immortalized human pancreatic duct-derived cells (E6/E7/st) to study mechanisms of Ras growth transformation. First, we found that human papillomavirus E6 and E7 oncogenes, which block the function of the p53 and Rb tumor suppressors, respectively, and SV40 small t antigen were required to allow mutant K-Ras(12D) growth transformation. Second, K-Ras(12D) caused growth transformation in vitro, including enhanced growth rate and loss of density dependency for growth, anchorage independence, and invasion through reconstituted basement membrane proteins, and tumorigenic transformation in vivo. Third, we determined that the Raf, phosphatidylinositol 3-kinase (PI3K), and Ral guanine nucleotide exchange factor effector pathways were activated, although extracellular signal-regulated kinase (ERK) activity was not up-regulated persistently. Finally, pharmacologic inhibition of Raf/mitogen-activated protein kinase/ERK and PI3K signaling impaired K-Ras-induced anchorage-independent growth and invasion. In summary, our studies established, characterized, and validated E6/E7/st cells for the study of Ras-induced oncogenesis.     

Drug-resistant phosphatidylinositol 3-kinase: guidance for the preemptive strike

In this issue of Cancer Cell, Zunder et al. (2008) describe surprising findings from investigating inhibitor-resistant mutations in the affinity pocket of p110 alpha of phosphatidylinositol 3-kinase (PI3K). Information on these critical residues provides a road map for generating novel PI3K inhibitors that can overcome the anticipated resistance mutations.     

Targeting the phosphatidylinositol 3-kinase pathway in multiple myeloma.

Multiple myeloma is a plasma cell neoplasm with a median survival of 3 to 5 years. Recent advances have improved patient outlook, but the disease remains incurable. Therefore, continued efforts to develop new therapies that target aberrant signaling pathways are needed. The phosphatidylinositol 3-kinase pathway regulates apoptosis, cell cycle regulation, and tumor proliferation. This pathway is constitutively activated in multiple myeloma and its inhibition induces apoptosis. Advances in understanding the signaling cascades mediating proliferation and survival of multiple myeloma cells have markedly improved the treatment of this disease. In this article, we review the role of the phosphatidylinositol 3-kinase/Akt pathway in the pathogenesis of multiple myeloma and the potential therapeutic implications of targeting this pathway in the treatment of multiple myeloma.     

Retinol decreases phosphatidylinositol 3-kinase activity in colon cancer cells

Previously, we showed that retinol inhibited all-trans-retinoic acid (ATRA)-resistant human colon cancer cell invasion via a retinoic acid receptor-independent mechanism. Because phosphatidylinositol 3-kinase (PI3K) regulates cell invasion, the objective of the current study was to determine if retinol affected PI3K activity. Following 24 h of serum starvation, the ATRA resistant human colon cancer cell lines HCT-116 and SW620 were treated with 0, 1, or 10 microM retinol. Thirty minutes of retinol treatment resulted in a significant decrease in PI3K activity in both cell lines. To determine the mechanism by which retinol reduces PI3K activity, the levels and heterodimerization of the regulatory subunit, p85, and the catalytic subunit, p110, of PI3K were examined. Retinol treatment did not alter p85 or p110 protein levels or the heterodimerization of these subunits at any time point examined. To determine if retinol affected the ability of PI3K to phosphorylate the substrate, phosphatidylinositol (PI), PI3K was immunoprecipitated from control cells and incubated with 10 microg PI and increasing concentrations of retinol or 10 microg retinol and increasing concentrations of PI. Retinol decreased PI3K activity in a dose-responsive manner and increased PI suppressed the inhibitory effect of retinol on PI3K activity. Finally, the PI3K inhibitor, LY294002, mimicked the ability of retinol to decrease cell invasion. Computational modeling revealed that retinol may inhibit PI3K activity in a manner similar to that of wortmannin. Thus, a decrease in PI3K activity due to retinol treatment may confer the ability of retinol to inhibit ATRA-resistant colon cancer cell invasion.     

Cooperation between STAT5 and phosphatidylinositol 3-kinase in the IL-3-dependent survival of a bone marrow derived cell line

Cytokine-dependent activation of distinct signaling pathways is a common scheme thought to be required for the subsequent programmation into cell proliferation and survival. The PI 3-kinase/Akt, Ras/MAP kinase, Ras/NFIL3 and JAK/STAT pathways have been shown to participate in cytokine mediated suppression of apoptosis in various cell types. However the relative importance of these signaling pathways seems to depend on the cellular context. In several cases, individual inhibition of each pathway is not sufficient to completely abrogate cytokine mediated cell survival suggesting that cooperation between these pathways is required. Here we showed that individual inhibition of STAT5, PI 3-kinase or MEK activities did not or weakly affected the IL-3 dependent survival of the bone marrow derived Ba/F3 cell line. However, the simultaneous inhibition of STAT5 and PI 3-kinase activities but not that of STAT5 and MEK reduced the IL-3 dependent survival of Ba/F3. Analysis of the expression of the Bcl-2 members indicated that phosphorylation of Bad and Bcl-x expression which are respectively regulated by the PI 3-kinase/Akt pathway and STAT5 probably explain this cooperation. Furthermore, we showed by co-immunoprecipitation studies and pull down experiments with fusion proteins encoding the GST-SH2 domains of p85 that STAT5 in its phosphorylated form interacts with the p85 subunit of the PI 3-kinase. These results indicate that the activations of STAT5 and the PI 3-kinase by IL-3 in Ba/F3 cells are tightly connected and cooperate to mediate IL-3-dependent suppression of apoptosis by modulating Bad phosphorylation and Bcl-x expression.     

The phosphatidylinositol 3-kinase/Akt/mTOR signaling network as a therapeutic target in acute myelogenous leukemia patients

The phosphatidylinositol 3-kinase (PI3K)/Akt/mammalian target of rapamycin (mTOR) signaling axis plays a central role in cell proliferation, growth, and survival under physiological conditions. However, aberrant PI3K/Akt/mTOR signaling has been implicated in many human cancers, including acute myelogenous leukemia (AML). Therefore, the PI3K/Akt/mTOR network is considered as a validated target for innovative cancer therapy. The limit of acceptable toxicity for standard polychemotherapy has been reached in AML. Novel therapeutic strategies are therefore needed. This review highlights how the PI3K/Akt/mTOR signaling axis is constitutively active in AML patients, where it affects survival, proliferation, and drug-resistance of leukemic cells including leukemic stem cells. Effective targeting of this pathway with small molecule kinase inhibitors, employed alone or in combination with other drugs, could result in the suppression of leukemic cell growth. Furthermore, targeting the PI3K/Akt/mTOR signaling network with small pharmacological inhibitors, employed either alone or in combinations with other drugs, may result in less toxic and more efficacious treatment of AML patients. Efforts to exploit pharmacological inhibitors of the PI3K/Akt/mTOR cascade which show efficacy and safety in the clinical setting are now underway.     

Selective PDZ protein-dependent stimulation of phosphatidylinositol 3-kinase by the adenovirus E4-ORF1 oncoprotein

While PDZ domain-containing proteins represent cellular targets for several different viral oncoproteins, including human papillomavirus E6, human T-cell leukemia virus type 1 Tax, and human adenovirus E4-ORF1, the functional consequences for such interactions have not been elucidated. Here we report that, at the plasma membrane of cells, the adenovirus E4-ORF1 oncoprotein selectively and potently stimulates phosphatidylinositol 3-kinase (PI3K), triggering a downstream cascade of events that includes activation of both protein kinase B and p70S6-kinase. This activity of E4-ORF1 could be abrogated by overexpression of its PDZ-protein targets or by disruption of its PDZ domain-binding motif, which was shown to mediate complex formation between E4-ORF1 and PDZ proteins at the plasma membrane of cells. Furthermore, E4-ORF1 mutants unable to activate the PI3K pathway failed to transform cells in culture or to promote tumors in animals, and drugs that block either PI3K or p70S6-kinase inhibited E4-ORF1-induced transformation of cells. From these results, we propose that the transforming and tumorigenic potentials of the adenovirus E4-ORF1 oncoprotein depend on its capacity to activate PI3K through a novel PDZ protein-dependent mechanism of action.     

PWT-458, a novel pegylated-17-hydroxywortmannin, inhibits phosphatidylinositol 3-kinase signaling and suppresses growth of solid tumors

Deregulated phosphatidylinositol 3-kinase (PI3K) signaling pathway is widely implicated in tumor growth and resistance to chemotherapy. While a strong rationale exists for pharmacological targeting of PI3K, only a few proof-of-principle in vivo efficacy studies are currently available. PWT-458, pegylated-17-hydroxywortmannin, is a novel and highly potent inhibitor of PI3K in animal models. Upon in vivo cleavage of its poly(ethyleneglycol) (PEG), PWT-458 releases its active moiety 17-hydroxywortmannin (17-HWT), the most potent inhibitor in its class. Here we show that a single intravenous injection of PWT-458 rapidly inhibited PI3K signaling, as measured by a complete loss of AKT (Ser-473) phosphorylation in xenograft tumors grown in nude mice. Following a daily X5 dosing regimen, PWT-458 demonstrated single-agent antitumor activity in nude mouse xenograft models of U87MG glioma, nonsmall cell lung cancer (NSCLC) A549, and renal cell carcinoma (RCC) A498. Efficacious doses ranged from 0.5 mg/kg to 10 mg/kg, achieving a superior therapeutic index over 17-HWT. PWT-458 augmented anticancer efficacy of a suboptimal dose of paclitaxel against A549 and U87MG tumors. Combination treatment of PWT-458 and an mTOR inhibitor, Pegylated-Rapamycin (Peg-Rapa), resulted in an enhanced antitumor efficacy in U87MG. Finally, PWT-458 in combination with interferon-alpha (Intron-A) caused a dramatic regression of RCC A498, which was not achieved by either agent alone. These studies identify PWT-458 as an effective anticancer agent and provide strong proof-of-principle for targeting the PI3K pathway as novel anticancer therapy.     

Association of PTEN gene methylation with genetic alterations in the phosphatidylinositol 3-kinase/AKT signaling pathway in thyroid tumors

BACKGROUND: The phosphatidylinositol 3-kinase (PI3K)/AKT pathway plays an important role in thyroid tumorigenesis and progression. Genetic alterations, particularly PIK3CA amplification and mutations and ras mutations, are the major cause of aberrant activation of this pathway in thyroid tumors. Epigenetic silencing of the PTEN gene, a negative regulator of the PI3K/AKT pathway, also occurs in thyroid tumors, but its relationship with genetic alterations in this pathway is unclear. METHODS: By using quantitative methylation-specific polymerase chain reaction, the authors examined PTEN methylation and its relationship with genetic alterations in the PI3K/AKT pathway in various types of thyroid tumors. RESULTS: The authors found PTEN methylation to become progressively higher from benign thyroid adenoma to follicular thyroid cancer and to aggressive anaplastic thyroid cancer, which harbored activating genetic alterations in the PI3K/AKT pathway correspondingly with a progressively higher prevalence. The association of PTEN methylation was seen with both overall genetic alterations and individual genetic alterations, particularly PIK3CA alterations and ras mutations, in the PI3K/AKT pathway within each of the 3 types of thyroid tumors. In contrast, no such relationship was observed for the tumor suppressor gene RASSF1A. CONCLUSIONS: The authors found an interesting association of PTEN methylation with the activating genetic alterations in the PI3K/AKT pathway in thyroid tumors. This finding is consistent with a model in which aberrant methylation and hence silencing of the PTEN gene, which coexists with activating genetic alterations of the PI3K/AKT pathway, may enhance the signaling of this pathway aberrantly activated by genetic alterations and hence contribute to the progression of thyroid tumors. Cancer 2008.