Function of Akt/PKB signaling to cell motility,invasion and the tumor stroma in cancer

The serine/threonine protein kinase Akt is a major signal transducer of the phosphoinositide 3-kinase (PI 3-K) pathway in all cells and tissues and plays a pivotal role in the maintenance of cellular processes including cell growth, proliferation, survival and metabolism. The frequent aberrant activation of the PI 3-K/Akt pathway in human cancer has made it an attractive therapeutic target. Numerous studies have provided a comprehensive understanding of the specific functions of Akt signaling in cancer cells as well as the surrounding tumor microenvironment and this has informed and enabled the development of therapeutic drugs to target both PI 3-K and Akt. However, recent studies have provided evidence for distinct functions of the three mammalian Akt isoforms, particularly with respect to the regulation of cell motility and metastasis of breast cancer. Here we discuss the mechanisms by which Akt signaling contributes to invasive migration and tumor metastasis, and highlight recent advances in our understanding of the contribution of the Akt pathway in the tumor-associated stroma.     

Regulation of MDR1 expression and drug resistance by a positive feedback loop involving hyaluronan, phosphoinositide 3-kinase, and ErbB2

Multidrug resistance is a potent barrier to effective, long term therapy in cancer patients. It is frequently attributed to enhanced expression of multidrug transporters or to the action of receptor kinases, such as ErbB2, and downstream anti-apoptotic signaling pathways, such as the phosphoinositide 3-kinase/Akt pathway. However, very few connections have been made between receptor kinases or anti-apoptotic pathways and multidrug transporter expression or function. Data presented herein show that constitutive interaction of the pericellular polysaccharide, hyaluronan, with its receptor, CD44, regulates assembly and activation of an ErbB2-containing signaling complex, which in turn stimulates phosphoinositide 3-kinase activity in multidrug-resistant MCF-7/Adr human breast carcinoma cells. Phosphoinositide 3-kinase activates Akt and downstream anti-apoptotic events, which contribute to drug resistance. However, hyaluronan and phosphoinositide 3-kinase stimulate expression of the multidrug transporter, MDR1 (P-glycoprotein), in an interdependent, but Akt-independent, manner. Furthermore, constitutively active phosphoinositide 3-kinase, but not Akt, stimulates hyaluronan production. These Akt-independent effects are dominant over the effects of Akt on doxorubicin resistance in MCF-7/Adr cells. Thus hyaluronan, phosphoinositide 3-kinase, and ErbB2 form a positive feedback loop that strongly amplifies MDR1 expression and regulates drug resistance in these cells. This pathway may also be important in progression of other malignant characteristics. These results illustrate the potential importance of hyaluronan as a therapeutic target in multidrug-resistant carcinomas.     

Regulation of multidrug resistance in cancer cells by hyaluronan

Multidrug resistance in cancer cells is often due to ATP-dependent efflux pumps, but is also linked to alterations in cell survival and apoptotic signaling pathways. We have found previously that perturbation of hyaluronan-tumor cell interaction by treatment with hyaluronan oligosaccharides suppresses the phosphoinositide 3-kinase/Akt cell survival signaling pathway in cancer cells and reduces tumor growth in vivo. Here we find that these oligomers suppress both the MAP kinase and phosphoinositide 3-kinase pathways in multidrug resistant tumor cells and sensitize these cells to a variety of chemotherapeutic drugs. On the other hand, increased hyaluronan production induces resistance in drug-sensitive tumor cells. Likewise, increased expression of emmprin, which is a glycoprotein that is present on the surface of most malignant cancer cells and that stimulates hyaluronan production, also induces increased resistance. Thus, perturbation of hyaluronan signaling may provide a dual therapeutic role, since it has intrinsic suppressive effects on tumor growth as well as sensitizing cancer cells to chemotherapeutic agents.     

Akt protein kinase inhibits Rac1-GTP binding through phosphorylation at serine 71 of Rac1

A putative Akt kinase phosphorylation site ((64)ydRIRplSYp(73)) was found in Rac1/CDC42 and Rho family proteins (RhoA, RhoB, RhoC, and RhoG). Phosphorylation of Rac1 by Akt kinase was assayed with recombinant Rac1 protein and the fluorescein-labeled Rac1 peptide. It was shown that the Rac1 peptide and the recombinant protein were phosphorylated by the activated recombinant Akt kinase and the lysate of SK-MEL28 cells, a human melanoma cell line. The phosphorylation of Rac1 inhibited its GTP-binding activity without any significant change in GTPase activity. Both the GTP-binding and GTPase activities of Rac1 S71A protein (with the serine residue to be phosphorylated replaced with alanine) were abolished regardless of the treatment of Akt kinase. Akt kinase activity and Rac1 peptide phosphorylation were down-regulated by the treatment of SK-MEL28 cells with wortmannin or LY294002 (a phosphoinositide 3-kinase inhibitor), but JNK/SAPK kinase activity was up-regulated. Thus, the results suggest that Akt kinase of the phosphoinositide 3-kinase signal transduction pathway phosphorylates serine 71 of Rac1 as one of its authentic substrates and modulates the Rac1 signal transduction pathway through phosphorylation.     

Akt and RhoA inhibition promotes anoikis of aggregated B16F10 melanoma cells

In the highly metastatic B16F10 melanoma cell line, activation of the signalling molecules that promote cell proliferation and survival on conventional adhesive culture dishes may also be responsible for the growth and resistance to anoikis of aggregates on a non-adhesive substratum. We have examined the influence of bacterial ADP-ribosyltransferases C3-like exoenzymes, which selectively modify RhoA, B and C proteins and inhibit signal pathways controlled by them. RNA interference [siRNA (small interfering RNA) Akt (also known as protein kinase B)] and a PI3K (phosphoinositide 3-kinase) inhibitor were used to analyse the changes caused by inhibiting the PI3K/Akt pathway. Inhibiting the activation of RhoA, B, C and Akt expression resulted in a decrease of the number of cells cultured in aggregates, and caspase 3 activation. RhoA activation and RhoB and RhoC expression were controlled by Akt, but not RhoA expression. Inhibiting Akt and RhoA reduced the expression of α5 integrin, and inactivated FAK (focal adhesion kinase) in B16F10 cells cultured as aggregates. Thus, inhibiting Rho subfamily proteins and Akt expression inactivates the FAK pathway and induces anoikis in anoikis-resistant cells. The activation of RhoA in melanoma cells can depend on PI3K/Akt activation, suggesting that PI3K/Akt is a suitable target for new therapeutic approaches.     

Down-regulation of the tumor suppressor gene retinoic acid receptor beta2 through the phosphoinositide 3-kinase/Akt signaling pathway

The retinoic acid receptor beta2 (RARbeta2) is a potent, retinoid-inducible tumor suppressor gene, which is a critical molecular relay for retinoid actions in cells. Its down-regulation, or loss of expression, leads to resistance of cancer cells to retinoid treatment. Up to now, no primary mechanism underlying the repression of the RARbeta2 gene expression, hence affecting cellular retinoid sensitivity, has been identified. Here, we demonstrate that the phosphoinositide 3-kinase/Akt signaling pathway affects cellular retinoid sensitivity, by regulating corepressor recruitment to the RARbeta2 promoter. Through direct phosphorylation of the corepressor silencing mediator for retinoic and thyroid hormone receptors (SMRT), Akt stabilized RAR/SMRT interaction, leading to an increased tethering of SMRT to the RARbeta2 promoter, decreased histone acetylation, down-regulation of the RARbeta2 expression, and impaired cellular differentiation in response to retinoid. The phosphoinositide 3-kinase/Akt signaling pathway, an important modulator of cellular survival, has thus a direct impact on cellular retinoid sensitivity, and its deregulation may be the triggering event in retinoid resistance of cancer cells.     

The phosphoinositide 3-kinase/Akt pathway is activated by daunorubicin in human acute myeloid leukemia cell lines.

Daunorubicin induces apoptosis in myeloid leukemia cells by activation of neutral sphingomyelinase and ceramide generation occurring 4-10 min after daunorubicin addition. We show here that daunorubicin is able to increase the phosphoinositide 3-kinase activity and enhance intracellular phosphoinositide 3-kinase lipid products prior to ceramide generation. Daunorubicin activates Akt, a downstream phosphoinositide 3-kinase effector. Interestingly, the phosphoinositide 3-kinase inhibitors wortmannin and LY294002 accelerate daunorubicin-induced apoptosis in U937 cells. The phosphoinositide 3-kinase/Akt pathway has been involved in cell survival following serum deprivation, tumor necrosis factor alpha, anti-Fas and UV radiations. Our results suggest that anti-tumor agents such as daunorubicin may also activate anti-apoptotic signals that could contribute to drug resistance.     

The tumor suppressor PTEN positively regulates macroautophagy by inhibiting the phosphatidylinositol 3-kinase/protein kinase B pathway

The tumor suppressor PTEN is a dual protein and phosphoinositide phosphatase that negatively controls the phosphatidylinositol (PI) 3-kinase/protein kinase B (Akt/PKB) signaling pathway. Interleukin-13 via the activation of the class I PI 3-kinase has been shown to inhibit the macroautophagic pathway in the human colon cancer HT-29 cells. Here we demonstrate that the wild-type PTEN is expressed in this cell line. Its overexpression directed by an inducible promoter counteracts the interleukin-13 down-regulation of macroautophagy. This effect was dependent upon the phosphoinositide phosphatase activity of PTEN as determined by using the mutant G129E, which has only protein phosphatase activity. The role of Akt/PKB in the signaling control of interleukin-13-dependent macroautophagy was investigated by expressing a constitutively active form of the kinase ((Myr)PKB). Under these conditions a dramatic inhibition of macroautophagy was observed. By contrast a high rate of autophagy was observed in cells expressing a dominant negative form of PKB. These data demonstrate that the signaling control of macroautophagy overlaps with the well known PI 3-kinase/PKB survival pathway and that the loss of PTEN function in cancer cells inhibits a major catabolic pathway.     

Akt and PI 3-kinase signaling in cardiomyocyte hypertrophy and survival

In many systems, activation of the "protein and lipid kinase" phosphoinositide 3-kinase (PI 3-kinase) and its downstream serine-threonine kinase effector, Akt (or Protein Kinase B), provide a potent stimulus for cell proliferation, growth, and survival. In the heart, constrained by the limited proliferative capacity of cardiomyocytes, this pathway plays a key role in regulating cardiomyocyte growth and survival, with little effect on proliferation. Simultaneously, PI 3-kinase and Akt are important modulators of metabolic substrate utilization and cardiomyocyte function. Thus, the convergent signaling pathways controlling so many clinically important phenotypes of the cardiomyocyte suggest it holds promise as a therapeutic target in a variety of cardiac diseases. However, the similar role of PI 3-kinase/Akt signaling in neoplasia suggests the difficulty of activating this pathway in the heart without invoking adverse consequences elsewhere. Here we review evidence regarding the role of PI 3-kinase/Akt in controlling cardiomyocyte growth and survival, and discuss the implications for therapeutic strategies.     

Akt and PI 3-Kinase Signaling in Cardiomyocyte Hypertrophy and Survival

In many systems, activation of the “protein and lipid kinase” phosphoinositide 3-kinase (PI 3-kinase) and its downstream serine-threonine kinase effector, Akt (or Protein Kinase B), provide a potent stimulus for cell proliferation, growth, and survival. In the heart, constrained by the limited proliferative capacity of cardiomyocytes, this pathway plays a key role in regulating cardiomyocyte growth and survival, with little effect on proliferation. Simultaneously, PI 3-kinase and Akt are important modulators of metabolic substrate utilization and cardiomyocyte function. Thus, the convergent signaling pathways controlling so many clinical important phenotypes of the cardiomyocyte suggest it holds promise as a therapeutic target in a variety of cardiac diseases. However, the similar role of PI 3-kinase/Akt signaling in neoplasia suggests the difficulty of activating this pathway in the heart without invoking adverse consequences elsewhere. Here we review evidence regarding the role of PI 3-kinase/Akt in controlling cardiomyocyte growth and survival, and discuss the implications for therapeutic strategies.     

Mammalian target of rapamycin promotes vincristine resistance through multiple mechanisms independent of maintained glycolytic rate

Deregulation of the phosphoinositide 3-kinase-Akt pathway is a major contributor to oncogenesis and resistance to cancer therapy. Recent work has shown mammalian target of rapamycin (mTOR) to be a major target downstream of Akt that contributes to both transformation and therapeutic resistance. Although inhibitors of Akt are not yet clinically available, rapamycin, a mTOR-specific inhibitor, has long been used as an immunosuppressant, and several rapamycin analogues are now in clinical trials in oncology. Recent data indicate that a mTOR complex phosphorylates Akt, and this complex is insensitive to rapamycin. We show that dominant-negative mTOR diminishes phosphorylation of endogenous Akt and exogenous myristoylated Akt (mAkt), that prolonged exposure to rapamycin also inhibits Akt activation, and that this inhibition is dependent on new protein synthesis. These data suggest that mTOR facilitates Akt activation through mechanisms other than direct phosphorylation. A constitutively active mTOR mutant that fails to enhance Akt phosphorylation nevertheless promotes resistance to multiple antimicrotubule agents, indicating that mTOR also mediates survival independent of Akt. Although Akt- and mTOR-mediated survival has been linked to regulation of cellular metabolism, we also show that survival and metabolic control are separable. The hexokinase inhibitor 5-thioglucose markedly inhibits glycolytic rate but does not diminish vincristine resistance mediated by mAkt or mTOR, and it has only a minor effect on mTOR- or mAkt-mediated resistance to growth factor withdrawal, suggesting that Akt-mTOR-mediated resistance is largely independent of maintenance of glycolytic rate. We conclude that mTOR activity can promote resistance through multiple mechanisms independent of maintained glycolytic rate.     

PI3K/Akt/mTOR信号通路与自噬及肿瘤的关系

自噬是一种以胞质内出现双层膜结构包裹长寿命蛋白和细胞器的自噬体为特征的细胞“自我消化”过程,在维持细胞内稳态、发育、肿瘤发生和感染中发挥重要作用。近来,诸多研究表明,自噬作为一把“双刃剑”,对肿瘤的发生发展既有促进作用,也有抑制作用。PI3K/Akt/mTOR通路由PI3激酶(PI3K)、蛋白激酶B（PKB/Akt）和哺乳动物类雷帕霉素靶蛋白（mTOR）3个作用分子组成,是一个中心的调节机构,对肿瘤细胞的生长与增殖有促进作用,同时对自噬进行抑制。本文就PI3K/Akt/mTOR通路与自噬及肿瘤发生发展的关系作一综述。     

Deregulated PTEN/PI3K/AKT/mTOR signaling in prostate cancer: Still a potential druggable target?

Although the prognosis of patients with localized prostate cancer is good after surgery, with a favorable response to androgen deprivation therapy, about one third of them invariably relapse, and progress to castration-resistant prostate cancer. Overall, prostate cancer therapies remain scarcely effective, thus it is mandatory to devise alternative treatments enhancing the efficacy of surgical castration and hormone administration.Dysregulation of the phosphoinositide 3-kinase pathway has attracted growing attention in prostate cancer due to the highly frequent association of epigenetic and post-translational modifications as well as to genetic alterations of both phosphoinositide 3-kinase and PTEN to onset and/or progression of this malignancy, and to resistance to canonical androgen-deprivation therapy.Here we provide a summary of the biological functions of the major players of this cascade and their deregulation in prostate cancer, summarizing the results of preclinical and clinical studies with PI3K signaling inhibitors and the reasons of failure independent from genomic changes.     

Ramentaceone,a Naphthoquinone Derived from Drosera sp., Induces Apoptosis by Suppressing PI3K/Akt Signaling in Breast Cancer Cells

The phosphoinositide 3-kinase (PI3K) signaling pathway plays an important role in processes critical for breast cancer progression and its upregulation confers increased resistance of cancer cells to chemotherapy and radiation. The present study aimed at determining the activity of ramentaceone, a constituent of species in the plant genera Drosera, toward breast cancer cells and defining the involvement of PI3K/Akt inhibition in ramentaceone-mediated cell death induction. The results showed that ramentaceone exhibited high antiproliferative activity toward breast cancer cells, in particular HER2-overexpressing breast cancer cells. The mode of cell death induced by ramentaceone was through apoptosis as determined by cytometric analysis of caspase activity and Annexin V staining. Apoptosis induction was found to be mediated by inhibition of PI3K/Akt signaling and through targeting its downstream anti-apoptotic effectors. Ramentaceone inhibited PI3-kinase activity, reduced the expression of the PI3K protein and inhibited the phosphorylation of the Akt protein in breast cancer cells. The expression of the anti-apoptotic Bcl-2 protein was decreased and the levels of the pro-apoptotic proteins, Bax and Bak, were elevated. Moreover, inhibition of PI3K and silencing of Akt expression increased the sensitivity of cells to ramentaceone-induced apoptosis. In conclusion, our results indicate that ramentaceone induces apoptosis in breast cancer cells through PI3K/Akt signaling inhibition. These findings suggest further investigation of ramentaceone as a potential therapeutic agent in breast cancer therapy, in particular HER2-positive breast cancer.     

AIB1 promotes DNA replication by JNK repression and AKT activation during cellular stress

Amplified in breast cancer 1 (AIB1) is a member of the p160 family of nuclear receptor coactivator protein. Recent studies have reported that high-level AIB1 production is involved in the phosphoinositide 3-kinase (PI3K)/Akt signaling pathway for progression to malignant carcinoma in a steroid-independent manner. Here we demonstrate that, in AIB1-knockout DT40 chicken B-lymphocytes, loss of AIB1 results in induction of phosphorylation of c-Jun N-terminal kinase (JNK) and c-Jun, in addition to the inhibition of DNA replication. In contrast, high-level AIB1 production prevents proapoptotic activation of the JNK/c-Jun signal transduction pathway and induces DNA replication through phosphorylation of the Akt/p65 NF-kappaB subunit RelA under cellular stresses such as UV irradiation or serum deprivation. Moreover, we have found that AIB1 is essential for the phosphorylation of histone H3 at serine 10, which is associated with the signal transduction to chromatin, leading to the transient expression of immediate-early genes in response to UV stimulation. Our results therefore suggest that AIB1 directly links to cell cycle control mechanisms in concern with the balance between apoptosis and proliferation.     

国家“重大新药创制”科技重大专项重点关注靶点分析解读——PI3K-Akt-mTOR 通路

由磷脂酰肌醇3- 激酶（PI3K）、丝氨酸/ 苏氨酸蛋白激酶（Akt）和哺乳动物雷帕霉素靶蛋白（mTOR）组成的PI3K-Akt-mTOR通路是细胞内非常重要的信号转导途径,该通路的紊乱会引起一系列的疾病,包括癌症、神经病变、自身免疫性疾病和血液淋巴系统疾病。近年来,PI3K-Akt-mTOR 通路作为药物靶点备受关注。结合汤森路透数据库资源——Thomson Reuters Integrity 和Cortellis for Competitive Intelligence,对PI3K-Akt-mTOR 通路的机制、相关药物研究进展、适应证、研发公司、交易、专利、文献等情报进行了数据层面的分析。     

Dual regulation of MMP-2 expression by the type 1 insulin-like growth factor receptor: the phosphatidylinositol 3-kinase/Akt and Raf/ERK pathways transmit opposing signals

The matrix metalloproteinase (MMP)-2 has been recognized as a major mediator of basement membrane degradation, angiogenesis, tumor invasion, and metastasis. The factors that regulate its expression have not, however, been fully elucidated. We previously identified the type I insulin-like growth factor (IGF-I) receptor as a regulator of MMP-2 synthesis. The objective of the present study was to investigate the signal transduction pathway(s) mediating this regulation. We show here that in Lewis lung carcinoma subline H-59 cells treated with IGF-I (10 ng/ml), the PI 3-kinase (phosphatidylinositol 3'-kinase) /protein kinase B (Akt) and C-Raf/ERK pathways were activated, and MMP-2 promoter activity, mRNA, and protein synthesis were induced. MMP-2 induction was blocked by the PI 3-kinase inhibitors LY294002 and wortmannin, by overexpression of a dominant-negative Akt or wild-type PTEN (phosphatase and tensin homologue deleted on chromosome 10), and by rapamycin. In contrast, a MEK inhibitor PD98059 failed to reduce MMP-2 promoter activation and actually increased MMP-2 mRNA and protein synthesis by up to 30%. Interestingly, suppression of PI 3-kinase signaling by a dominant-negative Akt enhanced ERK activity in cells stimulated with 10 ng/ml but not with 100 ng/ml IGF-I. Furthermore, at the higher (100 ng/ml) IGF-I concentration, C-Raf and ERK, but not PI 3-kinase activation, was enhanced, and this resulted in down-regulation of MMP-2 synthesis. This effect was reversed in cells expressing a dominant-negative ERK mutant. The results suggest that IGF-I can up-regulate MMP-2 synthesis via PI 3-kinase/Akt/mTOR (the mammalian target of rapamycin) signaling while concomitantly transmitting a negative regulatory signal via the Raf/ERK pathway. The outcome of IGF-IR (the receptor for IGF-I) activation may ultimately depend on factors, such as ligand bioavailability, that can shift the balance preferentially toward one pathway or the other.     

The phosphoinositide 3-kinase pathway and cancer

Human tumours emerge as the result of multiple genetic and epigenetic aberrations that allow the proto-cancer cell to escape normal social control. Many signal transduction pathways become constitutively active during this process, and one whose importance is increasingly being appreciated involves phosphoinositide 3-kinase (PI3-kinase). This pathway normally regulates important cell decisions such as growth, division, survival and migration, and when deregulated it can confer malignant potential to the ensuing tumour. However, constitutive activation of the PI3-kinase pathway might provide attractive therapeutic targets for the design of small-molecule inhibitors. This review discusses events upstream and downstream of PI3-kinase activity in the PI3-kinase signalling pathway, how PI3-kinase is deregulated in human tumourigenesis, and how this is being targeted clinically.     

蛋白激酶B/Akt抑制剂

磷脂酰肌醇-3-激酶(phosphatidylinositol 3-kinase,PI3K)/蛋白激酶B(protein kinase B,PKB/Akt)信号通路在细胞生长与存活中起着关键作用,PI3K/Akt通路的过度激活在多种肿瘤中常见。Akt激酶本身以及Akt激酶上游调节分子,例如PTEN和PI3K,在超过50%的人类肿瘤中均有异常变化。因此Akt成为肿瘤预防和肿瘤靶向治疗的热点之一。许多小分子化合物通过不同机制抑制Akt活性,根据小分子抑制剂与激酶的结合部位和化学结构不同,主要分为ATP竞争性抑制剂、Akt变构抑制剂和磷脂酰肌醇类似物抑制剂。本文综述了PI3K/Akt通路与肿瘤的关系和Akt抑制剂的研究现状,为新型抗癌药物的设计研究提供参考。