Akt1 governs breast cancer progression in vivo

Polypeptide N-acetylgalactosaminyltransferase 6 disrupts mammary acinar morphogenesis through O-glycosylation of fibronectin

A high expression of short and immature O-glycans is one of the prominent features of breast cancer cells, which would be attributed to the upregulated expression of glycosyltransferases. Therefore, a detailed elucidation of glycosyltransferases and their substrate(s) may improve our understandings for their roles in mammary carcinogenesis. Here we report that overexpression of polypeptide N-acetylgalactosaminyltransferase 6 (GALNT6), a glycosyltransferase involved in the initial step of O-glycosylation, has transformational potentials through disruptive acinar morphogenesis and cellular changes similar to epithelial-to-mesenchymal transition in normal mammary epithelial cell, MCF10A. As one of the critical O-glycan substrates, we identified fibronectin that was O-glycosylated in vivo and thereby stabilized by GALNT6. Because knockdown of fibronectin abrogated the disruptive proliferation caused by introduction of GALNT6 into epithelial cells, our findings suggest that GALNT6-fibronectin pathway should be a critical component for breast cancer development and progression.

Drug discovery using chemical systems biology: weak inhibition of multiple kinases may contribute to the anti-cancer effect of nelfinavir

Nelfinavir is a potent HIV-protease inhibitor with pleiotropic effects in cancer cells. Experimental studies connect its anti-cancer effects to the suppression of the Akt signaling pathway, but the actual molecular targets remain unknown. Using a structural proteome-wide off-target pipeline, which integrates molecular dynamics simulation and MM/GBSA free energy calculations with ligand binding site comparison and biological network analysis, we identified putative human off-targets of Nelfinavir and analyzed the impact on the associated biological processes. Our results suggest that Nelfinavir is able to inhibit multiple members of the protein kinase-like superfamily, which are involved in the regulation of cellular processes vital for carcinogenesis and metastasis. The computational predictions are supported by kinase activity assays and are consistent with existing experimental and clinical evidence. This finding provides a molecular basis to explain the broad-spectrum anti-cancer effect of Nelfinavir and presents opportunities to optimize the drug as a targeted polypharmacology agent.

Selective activation of Akt1 by mammalian target of rapamycin complex 2 regulates cancer cell migration, invasion, and metastasis

Mammalian target of rapamycin complex (mTORC) regulates a variety of cellular responses including proliferation, growth, differentiation and cell migration. In this study, we show that mammalian target of rapamycin complex 2 (mTORC2) regulates invasive cancer cell migration through selective activation of Akt1. Insulin-like growth factor-1 (IGF-1)-induced SKOV-3 cell migration was completely abolished by phosphatidylinositol 3-kinase (PI3K) (LY294002, 10 μM) or Akt inhibitors (SH-5, 50 μM), whereas inhibition of extracellular-regulated kinase by an ERK inhibitor (PD98059, 10 μM) or inhibition of mammalian target of rapamycin complex 1 (mTORC1) by an mTORC1 inhibitor (Rapamycin, 100 nM) did not affect IGF-1-induced SKOV-3 cell migration. Inactivation of mTORC2 by silencing Rapamycin-insensitive companion of mTOR (Rictor), abolished IGF-1-induced SKOV-3 cell migration as well as activation of Akt. However, inactivation of mTORC1 by silencing of Raptor had no effect. Silencing of Akt1 but not Akt2 attenuated IGF-1-induced SKOV-3 cell migration. Rictor was preferentially associated with Akt1 rather than Akt2, and over-expression of Rictor facilitated IGF-1-induced Akt1 activation. Expression of PIP3-dependent Rac exchanger1 (P-Rex1), a Rac guanosine exchange factor and a component of the mTOR complex, strongly stimulated activation of Akt1. Furthermore, knockdown of P-Rex1 attenuated Akt activation as well as IGF-1-induced SKOV-3 cell migration. Silencing of Akt1 or P-Rex1 abolished IGF-1-induced SKOV-3 cell invasion. Finally, silencing of Akt1 blocked in vivo metastasis, whereas silencing of Akt2 did not. Given these results, we suggest that selective activation of Akt1 through mTORC2 and P-Rex1 regulates cancer cell migration, invasion and metastasis.

Tip30 deletion in MMTV-Neu mice leads to enhanced EGFR signaling and development of estrogen receptor-positive and progesterone receptor-negative mammary tumors

Estrogen receptor-positive and progesterone receptor-negative (ER+/PR-) breast cancers account for 15% to 25% of all human breast cancers and display more aggressive malignant characteristics than ER+/PR+ cancers. However, the molecular mechanism underlying development of ER+/PR- breast cancers still remains elusive. We show here that Tip30 deletion dramatically accelerated the onset of mammary tumors in the MMTV-Neu mouse model of breast cancer. The mammary tumors arising in Tip30(-/-)/MMTV-Neu mice were exclusively ER+/PR-. The growth of these ER+/PR- tumors depends not only on estrogen but also on progesterone despite the absence of detectable PR. Tip30 is predominantly expressed in ER+ mammary epithelial cells, and its deletion leads to an increase in the number of phospho-ERα-positive cells in mammary glands and accelerated activation of Akt in MMTV-Neu mice. Moreover, we found that Tip30 regulates the EGFR pathway through controlling endocytic downregulation of EGFR protein level and signaling. Together, these findings suggest a novel mechanism in which loss of Tip30 cooperates with Neu activation to enhance the activation of Akt signaling, leading to the development of ER+/PR- mammary tumors.

The canonical NF-kappaB pathway governs mammary tumorigenesis in transgenic mice and tumor stem cell expansion

The role of mammary epithelial cell (MEC) NF-κB in tumor progression in vivo is unknown, as murine NF-κB components and kinases either are required for murine survival or interfere with normal mammary gland development. As NF-κB inhibitors block both tumor-associated macrophages (TAM) and MEC NF-κB, the importance of MEC NF-κB to tumor progression in vivo remained to be determined. Herein, an MEC-targeted inducible transgenic inhibitor of NF-κB (IκBαSR) was developed in ErbB2 mammary oncomice. Inducible suppression of NF-κB in the adult mammary epithelium delayed the onset and number of new tumors. Within similar sized breast tumors, TAM and tumor neoangiogenesis was reduced. Coculture experiments demonstrated MEC NF-κB enhanced TAM recruitment. Genome-wide expression and proteomic analysis showed that IκBαSR inhibited tumor stem cell pathways. IκBαSR inhibited breast tumor stem cell markers in transgenic tumors, reduced stem cell expansion in vitro, and repressed expression of Nanog and Sox2 in vivo and in vitro. MEC NF-κB contributes to mammary tumorigenesis. As we show that NF-κB contributes to expansion of breast tumor stem cells and heterotypic signals that enhance TAM and vasculogenesis, these processes may contribute to NF-κB-dependent mammary tumorigenesis.

Akt1 deletion prevents lung tumorigenesis by mutant K-ras

K-ras mutations are associated with smoking-induced lung cancer and poor clinical outcomes. In mice, K-ras mutations are sufficient to induce lung tumors, which require phosphoinoside-3-kinase (PI3K) and further downstream, mammalian target of rapamycin (mTOR) activation. However, the roles of individual Akt isoforms that link PI3K and mTOR are unknown. Here, we show that deletion of Akt1 but not Akt2 or Akt3 prevents lung tumorigenesis in a tobacco carcinogen-induced model and a genetic model. Akt1 deletion prevented tumor initiation as well as tumor progression, coincident with decreased Akt signaling in tumor tissues. In contrast, deletion of Akt3 increased tumor multiplicity in the carcinogen model and increased tumor size in the genetic model. Fibroblasts lacking Akt1 are resistant to transformation by mutant K-ras and stimulation by epidermal growth factor. Human lung cancer cells with mutant K-ras and diminished Akt1 levels fail to grow in vivo. These data suggest that Akt1 is the primary Akt isoform activated by mutant K-ras in lung tumors, and that Akt3 may oppose Akt1 in lung tumorigenesis and lung tumor progression. Given that Akt inhibitors in clinical development as cancer therapeutics are not isoform selective, these studies support specific targeting of Akt1 to mitigate the effects of mutant K-ras in lung cancer.

Inference of patient-specific pathway activities from multi-dimensional cancer genomics data using PARADIGM

Motivation: High-throughput data is providing a comprehensive view of the molecular changes in cancer tissues. New technologies allow for the simultaneous genome-wide assay of the state of genome copy number variation, gene expression, DNA methylation and epigenetics of tumor samples and cancer cell lines.

Analyses of current data sets find that genetic alterations between patients can differ but often involve common pathways. It is therefore critical to identify relevant pathways involved in cancer progression and detect how they are altered in different patients.

Results: We present a novel method for inferring patient-specific genetic activities incorporating curated pathway interactions among genes. A gene is modeled by a factor graph as a set of interconnected variables encoding the expression and known activity of a gene and its products, allowing the incorporation of many types of omic data as evidence. The method predicts the degree to which a pathway's activities (e.g. internal gene states, interactions or high-level ‘outputs’) are altered in the patient using probabilistic inference.

Compared with a competing pathway activity inference approach called SPIA, our method identifies altered activities in cancer-related pathways with fewer false-positives in both a glioblastoma multiform (GBM) and a breast cancer dataset. PARADIGM identified consistent pathway-level activities for subsets of the GBM patients that are overlooked when genes are considered in isolation. Further, grouping GBM patients based on their significant pathway perturbations divides them into clinically-relevant subgroups having significantly different survival outcomes. These findings suggest that therapeutics might be chosen that target genes at critical points in the commonly perturbed pathway(s) of a group of patients.

Availability:Source code available at http://sbenz.github.com/Paradigm

Contact:jstuart@soe.ucsc.edu

Supplementary information:Supplementary data are available at Bioinformatics online.

From man to mouse and back again: advances in defining tumor AKTivities in vivo

AKT hyperactivation is a common event in human cancers, and inhibition of oncogenic AKT activation is a major goal of drug discovery programs. Mouse tumor models that replicate AKT activation typical of human cancers provide a powerful means by which to investigate mechanisms of oncogenic signaling, identify potential therapeutic targets and determine treatment regimes with maximal therapeutic efficacy. This Perspective highlights recent advances using in vivo studies that reveal how AKT signaling supports tumor formation, cooperates with other mutations to promote tumor progression and facilitates tumor-cell dissemination, focusing on well-characterized prostate carcinoma mouse models that are highly sensitive to AKT activation. The implications of these findings on the therapeutic targeting of AKT and potential new drug targets are also explored.

Expression of metabolic, tissue remodeling, oxidative stress, and inflammatory pathways in mammary tissue during involution in lactating dairy cows

Histological and functional changes associated with involution in the mammary gland are partly regulated by changes in gene expression. At 42 d postpartum, Holstein cows underwent a period of 5 d during which they were milked 1X daily until complete cessation of milking. Percutaneous mammary biopsies (n = 5/time point) were obtained on d 1, 5, 14, and 21 relative to the start of 1X milking for transcript profiling via qPCR of 57 genes associated with metabolism, apoptosis/proliferation, immune response/inflammation, oxidative stress, and tissue remodeling. Not surprisingly, there was clear downregulation of genes associated with milk fat synthesis (FASN, ACACA, CD36, FABP3, SCD) and lipid-related transcription regulation (SREBF1, SREBF2). Similar to milk fat synthesis-related genes, those encoding proteins required for glucose uptake (SLC2A1), casein synthesis (CSN2, CSN3), and lactose synthesis (LALBA) decreased during involution. Unlike metabolic genes, those associated with immune response and inflammation (C3, LTF, SAA3), oxidative stress (GPX1, SOD2), and pro-inflammatory cytokine signaling (SPP1, TNF) increased to peak levels by d 14 from the start of 1X milking. These adaptations appeared to be related with tissue remodeling as indicated by upregulation of proteins encoding matrix proteinases (MMP2), IGFBP3, and transcriptional regulation of apoptosis/cell proliferation (MYC). In contrast, the concerted upregulation of STAT3, TGFB1, and TGFB1R during the first 14 d was suggestive of an activation of these signaling pathways probably as an acute response to regulate differentiation and/or mammary cell survival upon the onset of a marked pro-inflammatory and oxidative stress response induced by the gradual reduction in milk removal. Results suggest a central role of STAT3, MYC, PPARG, SREBF1, and SREBF2 in regulating concerted alterations in metabolic and cell survival mechanisms, which were induced partly via oxidative stressed-triggered inflammation and the decline in metabolic activity.

Vimentin is a novel AKT1 target mediating motility and invasion

The PI3K/AKT signaling pathway is aberrant in a wide variety of cancers. Downstream effectors of AKT are involved in survival, growth and metabolic-related pathways. In contrast, contradictory data relating to AKT effects on cell motility and invasion, crucial prometastatic processes, have been reported pointing to a potential cell type and isoform type-specific AKT-driven function. By implication, study of AKT signaling should optimally be conducted in an appropriate intracellular environment. Prognosis in soft-tissue sarcoma (STS), the aggressive malignancies of mesenchymal origin, is poor, reflecting our modest ability to control metastasis, an effort hampered by lack of insight into molecular mechanisms driving STS progression and dissemination. We examined the impact of the cancer progression-relevant AKT pathway on the mesenchymal tumor cell internal milieu. We demonstrate that AKT1 activation induces STS cell motility and invasiveness at least partially through a novel interaction with the intermediate filament vimentin (Vim). The binding of AKT (tail region) to Vim (head region) results in Vim Ser39 phosphorylation enhancing the ability of Vim to induce motility and invasion while protecting Vim from caspase-induced proteolysis. Moreover, vimentin phosphorylation was shown to enhance tumor and metastasis growth in vivo. Insights into this mesenchymal-related molecular mechanism may facilitate the development of critically lacking therapeutic options for these devastating malignancies.

Oncogenes and tumor suppressor genes

Breast cancer progression involves multiple genetic events, which can activate dominant-acting oncogenes and disrupt the function of specific tumor suppressor genes. This article describes several key oncogene and tumor suppressor signaling networks that have been implicated in breast cancer progression. Among the tumor suppressors, the article emphasizes BRCA1/2 and p53 tumor suppressors. In addition to these well characterized tumor suppressors, the article highlights the importance of PTEN tumor suppressor in counteracting PI3K signaling from activated oncogenes such as ErbB2. This article discusses the use of mouse models of human breast that recapitulate the key genetic events involved in the initiation and progression of breast cancer. Finally, the therapeutic potential of targeting these key tumor suppressor and oncogene signaling networks is discussed.

Synthesis and evaluation of indazole based analog sensitive Akt inhibitors

The kinase Akt is a key signaling node in regulating cellular growth and survival. It is implicated in cancer by mutation and its role in the downstream transmission of aberrant PI3K signaling. For these reasons, Akt has become an increasingly important target of drug development efforts and several inhibitors are now reaching clinical trials. Paradoxically it has been observed that active site kinase inhibitors of Akt lead to hyperphosphorylation of Akt itself. To investigate this phenomenon we here describe the application of a chemical genetics strategy that replaces native Akt with a mutant version containing an active site substitution that allows for the binding of an engineered inhibitor. This analog sensitive strategy allows for the selective inhibition of a single kinase. In order to create the inhibitor selective for the analog sensitive kinase, a diversity of synthetic approaches was required, finally resulting in the compound PrINZ, a 7-substituted version of the Abbott Labs Akt inhibitor A-443654.

Responsiveness to PI3K and MEK inhibitors in breast cancer. Use of a 3D culture system to study pathways related to hormone independence in mice

BACKGROUND

A significant proportion of breast cancer patients face failure of endocrine therapy due to the acquisition of endocrine resistance. We have explored mechanisms involved in such disease progression by using a mouse breast cancer model that is induced by medroxyprogesterone acetate (MPA). These tumors transit through different stages of hormone sensitivity. However, when cells from tumor variants were seeded on plastic, all were stimulated by progestins and inhibited by antiprogestins such as RU486. Furthermore, cells from a RU486-resistant tumor variant recovered antiprogestin sensitivity.

HYPOTHESIS

A three-dimensional (3D) culture system, by maintaining differential cellular organization that is typical of each tumor variant, may allow for the maintenance of particular hormone responses and thus be appropriate for the study of the effects of specific inhibitors of signaling pathways associated with disease progression.

METHOD

We compared the behavior of tumors growing in vivo and cancer cells ex vivo (in 3D Matrigel). In this system, we evaluated the effects of kinase inhibitors and hormone antagonists on tumor growth.

PRINCIPAL FINDINGS

LY294002, a PI3K/AKT pathway inhibitor, decreased both tumor growth in vivo and cell survival in Matrigel in MPA-independent tumors with higher AKT activity. Induction of cell death by anti-hormones such as ICI182780 and ZK230211 was more effective in MPA-dependent tumors with lower AKT activity. Inhibition of MEK with PD98059 did not affect tumor growth in any tested variant. Finally, while Matrigel reproduced differential responsiveness of MPA-dependent and -independent breast cancer cells, it was not sufficient to preserve antiprogestin resistance of RU486-resistant tumors.

CONCLUSION

We demonstrated that the PI3K/AKT pathway is relevant for MPA-independent tumor growth. Three-dimensional cultures were useful to test the effects of kinase inhibitors on breast cancer growth and highlight the need for in vivo models to validate experimental tools used for selective therapeutic targeting.

Blockade of CCN6 (WISP3) activates growth factor-independent survival and resistance to anoikis in human mammary epithelial cells

CCN6 is a secreted cysteine-rich matricellular protein (36.9 kDa) that exerts growth-inhibitory functions in breast cancer. Reduction or loss of CCN6 protein has been reported in invasive carcinomas of the breast with lymph node metastasis and in inflammatory breast cancer. However, the mechanism by which CCN6 loss promotes breast cancer growth remains to be defined. In the present study, we developed lentiviral-mediated short hairpin RNA CCN6 knockdown (KD) in nontumorigenic mammary epithelial cells MCF10A and HME. We discovered that CCN6 KD protects mammary epithelial cells from apoptosis and activates growth factor-independent survival. In the absence of exogenous growth factors, CCN6 KD was able to promote growth under anchorage-independent conditions and triggered resistance to detachment-induced cell death (anoikis). On serum starvation, CCN6 KD was sufficient for activation of the phosphatidylinositol 3-kinase (PI3K)/Akt pathway. Growth factor-independent cell survival was stunted in CCN6 KD cells when treated with either human recombinant CCN6 protein or the PI3K inhibitor LY294002. Targeted inhibition of Akt isoforms revealed that the survival advantage rendered by CCN6 KD requires specific activation of Akt-1. The relevance of our studies to human breast cancer is highlighted by the finding that low CCN6 protein levels are associated with upregulated expression of phospho-Akt-1 (Ser(473)) in 21% of invasive breast carcinomas. These results enable us to pinpoint one mechanism by which CCN6 controls survival of breast cells mediated by the PI3K/Akt-1 pathway.

Distinct biological roles for the akt family in mammary tumor progression

The phosphatidylinositol 3' kinase/Akt pathway is frequently dysregulated in cancer, which can have unfavorable consequences in terms of cell proliferation, survival, metabolism, and migration. Increasing evidence suggests that Akt1, Akt2, and Akt3 play unique roles in breast cancer initiation and progression. We have recently shown that in contrast to Akt1, which accelerates mammary tumor induction in transgenic mice, Akt2 promotes metastasis of tumor cells without affecting the latency of tumor development. Despite the distinct phenotypic outputs resulting from Akt1 or Akt2 activation, very little is known about the mode by which such unique functions originate from these highly related kinases. Here we discuss potential mechanisms contributing to the differing functional specificity of Akt1 and Akt2 with respect to migration, invasion, and metastasis.

Subcellular localization of activated AKT in estrogen receptor- and progesterone receptor-expressing breast cancers: potential clinical implications

Activated v-AKT murine thymoma viral oncogene homolog 1 (AKT)/protein kinase B (PKB) kinase (pAKT) is localized to the plasma membrane, cytoplasm, and/or nucleus in 50% of cancers. The clinical importance of pAKT localization and the mechanism(s) controlling this compartmentalization are unknown. In this study, we examined nuclear and cytoplasmic phospho-AKT (pAKT) expression by immunohistochemistry in a breast cancer tissue microarray (n = 377) with approximately 15 years follow-up and integrated these data with the expression of estrogen receptor (ER)alpha, progesterone receptor (PR), and FOXA1. Nuclear localization of pAKT (nuclear-pAKT) was associated with long-term survival (P = 0.004). Within the ERalpha+/PR+ subgroup, patients with nuclear-pAKT positivity had better survival than nuclear-pAKT-negative patients (P < or = 0.05). The association of nuclear-pAKT with the ERalpha+/PR+ subgroup was validated in an independent cohort (n = 145). TCL1 family proteins regulate nuclear transport and/or activation of AKT. TCL1B is overexpressed in ERalpha-positive compared with ERalpha-negative breast cancers and in lung metastasis-free breast cancers. Therefore, we examined the possible control of TCL1 family member(s) expression by the estrogen:ERalpha pathway. Estradiol increased TCL1B expression and increased nuclear-pAKT levels in breast cancer cells; short- interfering RNA against TCL1B reduced nuclear-pAKT. Overexpression of nuclear-targeted AKT1 in MCF-7 cells increased cell proliferation without compromising sensitivity to the anti-estrogen, tamoxifen. These results suggest that subcellular localization of activated AKT plays a significant role in determining its function in breast cancer, which in part is dependent on TCL1B expression.

Exploring the gain of function contribution of AKT to mammary tumorigenesis in mouse models

Elevated expression of AKT has been noted in a significant percentage of primary human breast cancers, mainly as a consequence of the PTEN/PI3K pathway deregulation. To investigate the mechanistic basis of the AKT gain of function-dependent mechanisms of breast tumorigenesis, we explored the phenotype induced by activated AKT transgenes in a quantitative manner. We generated several transgenic mice lines expressing different levels of constitutively active AKT in the mammary gland. We thoroughly analyzed the preneoplastic and neoplastic mammary lesions of these mice and correlated the process of tumorigenesis to AKT levels. Finally, we analyzed the impact that a possible senescent checkpoint might have in the tumor promotion inhibition observed, crossing these lines to mammary specific p53(R172H) mutant expression, and to p27 knock-out mice. We analyzed the benign, premalignant and malignant lesions extensively by pathology and at molecular level analysing the expression of proteins involved in the PI3K/AKT pathway and in cellular senescence. Our findings revealed an increased preneoplastic phenotype depending upon AKT signaling which was not altered by p27 or p53 loss. However, p53 inactivation by R172H point mutation combined with myrAKT transgenic expression significantly increased the percentage and size of mammary carcinoma observed, but was not sufficient to promote full penetrance of the tumorigenic phenotype. Molecular analysis suggest that tumors from double myrAKT;p53(R172H) mice result from acceleration of initiated p53(R172H) tumors and not from bypass of AKT-induced oncogenic senescence. Our work suggests that tumors are not the consequence of the bypass of senescence in MIN. We also show that AKT-induced oncogenic senescence is dependent of pRb but not of p53. Finally, our work also suggests that the cooperation observed between mutant p53 and activated AKT is due to AKT-induced acceleration of mutant p53-induced tumors. Finally, our work shows that levels of activated AKT are not essential in the induction of benign or premalignant tumors, or in the cooperation of AKT with other tumorigenic signal such as mutant p53, once AKT pathway is activated, the relative level of activity seems not to determine the phenotype.

A structural determinant that renders G alpha(i) sensitive to activation by GIV/girdin is required to promote cell migration

Although several non-receptor activators of heterotrimeric G proteins have been identified, the structural features of G proteins that determine their interaction with such activators and the subsequent biological effects are poorly understood. Here we investigated the structural determinants in G alpha(i3) necessary for its regulation by GIV/girdin, a guanine-nucleotide exchange factor (GEF) that activates G alpha(i) subunits. Using G protein activity and in vitro pulldown assays we demonstrate that G alpha(i3) is a better substrate for GIV than the highly homologous G alpha(o). We identified Trp-258 in the G alpha(i) subunit as a novel structural determinant for GIV binding by comparing GIV binding to G alpha(i3)/G alpha(o) chimeras. Mutation of Trp-258 to the corresponding Phe in G alpha(o) decreased GIV binding in vitro and in cultured cells but did not perturb interaction with other G alpha-binding partners, i.e. G betagamma, AGS3 (a guanine nucleotide dissociation inhibitor), GAIP/RGS19 (a GTPase-activating protein), and LPAR1 (a G protein-coupled receptor). Activation of G alpha(i3) by GIV was also dramatically reduced when Trp-258 was replaced with Tyr, Leu, Ser, His, Asp, or Ala, highlighting that Trp is required for maximal activation. Moreover, when mutant G alpha(i3) W258F was expressed in HeLa cells they failed to undergo cell migration and to enhance Akt signaling after growth factor or G protein-coupled receptor stimulation. Thus activation of G alpha(i3) by GIV is essential for biological functions associated with G alpha(i3) activation. In conclusion, we have discovered a novel structural determinant on G alpha(i) that plays a key role in defining the selectivity and efficiency of the GEF activity of GIV on G alpha(i) and that represents an attractive target site for designing small molecules to disrupt the G alpha(i)-GIV interface for therapeutic purposes.

Insulin-mediated acceleration of breast cancer development and progression in a nonobese model of type 2 diabetes

Epidemiologic studies suggest that type 2 diabetes (T2D) increases breast cancer risk and mortality, but there is limited experimental evidence supporting this association. Moreover, there has not been any definition of a pathophysiological pathway that diabetes may use to promote tumorigenesis. In the present study, we used the MKR mouse model of T2D to investigate molecular mechanisms that link T2D to breast cancer development and progression. MKR mice harbor a transgene encoding a dominant-negative, kinase-dead human insulin-like growth factor-I receptor (IGF-IR) that is expressed exclusively in skeletal muscle, where it acts to inactivate endogenous insulin receptor (IR) and IGF-IR. Although lean female MKR mice are insulin resistant and glucose intolerant, displaying accelerated mammary gland development and enhanced phosphorylation of IR/IGF-IR and Akt in mammary tissue, in the context of three different mouse models of breast cancer, these metabolic abnormalities were found to accelerate the development of hyperplastic precancerous lesions. Normal or malignant mammary tissue isolated from these mice exhibited increased phosphorylation of IR/IGF-IR and Akt, whereas extracellular signal-regulated kinase 1/2 phosphorylation was largely unaffected. Tumor-promoting effects of T2D in the models were reversed by pharmacological blockade of IR/IGF-IR signaling by the small-molecule tyrosine kinase inhibitor BMS-536924. Our findings offer compelling experimental evidence that T2D accelerates mammary gland development and carcinogenesis,and that the IR and/or the IGF-IR are major mediators of these effects.

c-Jun induces mammary epithelial cellular invasion and breast cancer stem cell expansion

The molecular mechanisms governing breast tumor cellular self-renewal contribute to breast cancer progression and therapeutic resistance. The ErbB2 oncogene is overexpressed in approximately 30% of human breast cancers. c-Jun, the first cellular proto-oncogene, is overexpressed in human breast cancer. However, the role of endogenous c-Jun in mammary tumor progression is unknown. Herein, transgenic mice expressing the mammary gland-targeted ErbB2 oncogene were crossed with c-jun(f/f) transgenic mice to determine the role of endogenous c-Jun in mammary tumor invasion and stem cell function. The excision of c-jun by Cre recombinase reduced cellular migration, invasion, and mammosphere formation of ErbB2-induced mammary tumors. Proteomic analysis identified a subset of secreted proteins (stem cell factor (SCF) and CCL5) induced by ErbB2 expression that were dependent upon endogenous c-Jun expression. SCF and CCL5 were identified as transcriptionally induced by c-Jun. CCL5 rescued the c-Jun-deficient breast tumor cellular invasion phenotype. SCF rescued the c-Jun-deficient mammosphere production. Endogenous c-Jun thus contributes to ErbB2-induced mammary tumor cell invasion and self-renewal.

Activating peroxisome proliferator-activated receptor gamma mutant promotes tumor growth in vivo by enhancing angiogenesis

Peroxisome proliferator-activated receptor gamma (PPARgamma) is expressed in a variety of cancer cells. The addition of ligand activates the receptor by inducing a conformational change in the receptor, which can be recapitulated by mutation. To investigate the role of activated PPARgamma signaling in breast cancer, we compared the function of a constitutively active PPARgamma (PgammaCA) mutant with the wild-type PPARgamma in ErbB2-induced mammary tumorigenesis in vivo. Tumor cells transduced with either PPARgamma or PgammaCA were implanted into immunocompetent FVB mice. Enhanced tumor growth was observed in PgammaCA-transduced cells, which was associated with increased angiogenesis and endothelial stem cells as evidenced by increased number of cells stained with von Willebrand factor, c-Kit, CD133, and CD31. Genome-wide expression profiling identified a group of genes within the angiogenesis pathway, including Angptl4, as targets of activated PPARgamma; PgammaCA also induced Angptl4 protein secretion in ErbB2-transformed mammary epithelial cells. Angptl4 promoted vascular endothelial cell migration; conversely, immunodepletion of Angptl4 reduced PgammaCA-mediated cellular migration. Collectively, these studies suggest that activated PPARgamma induces Angptl4 to promote tumor growth through enhanced angiogenesis in vivo.

The Dachshund gene in development and hormone-responsive tumorigenesis

The dachshund (dac) gene was initially described as a mutant phenotype in flies featuring extremely short legs relative to their body length. Functioning as a dominant suppressor of the ellipse mutation, a hypermorphic allele of the Epidermal Growth Factor Receptor (EGFR), the dac gene plays a key role in metazoan development, regulating ocular, limb, brain, and gonadal development. In the Drosophila eye, dac is a key component of the Retinal Determination Gene Network (RDGN) governing the normal initiation of the morphogenetic furrow and thereby eye development. Recent studies have demonstrated an important role for human Dachshund homologue (DACH1) in tumorigenesis, in particular, breast, prostate and ovarian cancer. The molecular mechanisms by which DACH1 regulates differentiation and tumorigenesis are discussed herein.

AKT1 pleckstrin homology domain E17K activating mutation in endometrial carcinoma

OBJECTIVES

The PI3K/AKT pathway is frequently activated in endometrial carcinoma (EC) mainly due to mutations in the PIK3CA and PTEN genes. These events are common and believed to be the key to endometrial carcinogenesis. Recently, a somatic activating mutation in the AKT1 gene (E17K) was identified in several cancer types. In this study we explored the frequency of this AKT1 mutation in endometrial carcinoma.

METHODS

Tumor DNA, extracted from 73 EC was analyzed for AKT1 E17K mutation (G49A) using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS). In addition, the tumors were screened for coexisting common mutations in PTEN, PIK3CA and KRAS.

RESULTS

The AKT1 E17K mutation was detected in 4% of EC. One of the AKT1-mutated tumors showed coexisting PTEN loss-of-function mutation.

CONCLUSION

We identified the AKT1 E17K mutation in 4% of endometrial carcinomas. The presence of double AKT1/ PTEN mutants is in accord with the hypothesis that in EC more than one hit is required to completely activate the PI3K pathway. Furthermore, AKT1 mutations were limited to high grade, advanced stage tumors suggesting that this mutation confers a more aggressive tumor behavior.

MicroRNAs differentially regulated by Akt isoforms control EMT and stem cell renewal in cancer cells

Although Akt is known to play a role in human cancer, the relative contribution of its three isoforms to oncogenesis remains to be determined. We expressed each isoform individually in an Akt1(-/-)/Akt2(-/-)/Akt3(-/-) cell line. MicroRNA profiling of growth factor-stimulated cells revealed unique microRNA signatures for cells with each isoform. Among the differentially regulated microRNAs, the abundance of the miR-200 family was decreased in cells bearing Akt2. Knockdown of Akt1 in transforming growth factor-beta (TGFbeta)-treated MCF10A cells also decreased the abundance of miR-200; however, knockdown of Akt2, or of both Akt1 and Akt2, did not. Furthermore, Akt1 knockdown in MCF10A cells promoted TGFbeta-induced epithelial-mesenchymal transition (EMT) and a stem cell-like phenotype. Carcinomas developing in MMTV-cErbB2/Akt1(-/-) mice showed increased invasiveness because of miR-200 down-regulation. Finally, the ratio of Akt1 to Akt2 and the abundance of miR-200 and of the messenger RNA encoding E-cadherin in a set of primary and metastatic human breast cancers were consistent with the hypothesis that in many cases breast cancer metastasis may be under the control of the Akt-miR-200-E-cadherin axis. We conclude that induction of EMT is controlled by microRNAs whose abundance depends on the balance between Akt1 and Akt2 rather than on the overall activity of Akt.

PI3K/PTEN signaling in angiogenesis and tumorigenesis

Phosphatidylinositol 3-kinase (PI3K) and phosphatase and tensin homolog deleted on chromosome 10 (PTEN) signaling pathway play an important role in multiple cellular functions such as cell metabolism, proliferation, cell-cycle progression, and survival. PI3K is activated by growth factors and angiogenesis inducers such as vascular endothelial growth factor (VEGF) and angiopoietins. The amplification and mutations of PI3K and the loss of the tumor suppressor PTEN are common in various kinds of human solid tumors. The genetic alterations of upstream and downstream of PI3K signaling molecules such as receptor tyrosine kinases and AKT, respectively, are also frequently altered in human cancer. PI3K signaling regulates tumor growth and angiogenesis by activating AKT and other targets, and by inducing HIF-1 and VEGF expression. Angiogenesis is required for tumor growth and metastasis. In this review, we highlight the recent studies on the roles and mechanisms of PI3K and PTEN in regulating tumorigenesis and angiogenesis, and the roles of the downstream targets of PI3K for transmitting the signals. We also discuss the crosstalk of these signaling molecules and cellular events during tumor growth, metastasis, and tumor angiogenesis. Finally, we summarize the potential applications of PI3K, AKT, and mTOR inhibitors and their outcome in clinical trials for cancer treatment.

EphA2 mediates ligand-dependent inhibition and ligand-independent promotion of cell migration and invasion via a reciprocal regulatory loop with Akt

Both pro- and antioncogenic properties have been attributed to EphA2 kinase. We report that a possible cause for this apparent paradox is diametrically opposite roles of EphA2 in regulating cell migration and invasion. While activation of EphA2 with its ligand ephrin-A1 inhibited chemotactic migration of glioma and prostate cancer cells, EphA2 overexpression promoted migration in a ligand-independent manner. Surprisingly, the latter effects required phosphorylation of EphA2 on serine 897 by Akt, and S897A mutation abolished ligand-independent promotion of cell motility. Ephrin-A1 stimulation of EphA2 negated Akt activation by growth factors and caused EphA2 dephosphorylation on S897. In human astrocytoma, S897 phosphorylation was correlated with tumor grades and Akt activation, suggesting that the Akt-EphA2 crosstalk may contribute to brain tumor progression.

Akt1 and akt2 play distinct roles in the initiation and metastatic phases of mammary tumor progression

The phosphatidylinositol 3-kinase (PI3K)/Akt survival pathway is often dysregulated in cancer. Our previous studies have shown that coexpression of activated Akt1 with activated ErbB2 or polyoma virus middle T antigen uncoupled from the PI3K pathway (PyVmT Y315/322F) accelerates mammary tumor development but cannot rescue the metastatic phenotype associated with these models. Here, we report the generation of transgenic mice expressing activated Akt2 in the mammary epithelium. Like the mouse mammary tumor virus-Akt1 strain, mammary-specific expression of Akt2 delayed mammary gland involution. However, in contrast to Akt1, coexpression of Akt2 with activated ErbB2 or PyVmT Y315/322F in the mammary glands of transgenic mice did not affect the latency of tumor development. Strikingly, Akt2 coexpresssion markedly increased the incidence of pulmonary metastases in both tumor models, demonstrating a unique role in tumor progression. Together, these observations argue that these highly conserved kinases have distinct biological and biochemical outputs that play opposing roles in mammary tumor induction and metastasis.

Akt is a direct target for myricetin to inhibit cell transformation

Akt, a serine/threonine kinase, is a critical regulator in many cellular processes including cell growth, proliferation, and apoptosis. In this study, we found that myricetin, a typical flavonol existing in many fruits and vegetables, could directly target Akt to inhibit cell transformation. Binding assay revealed that myricetin bound to Akt directly by competing with ATP. In vitro and ex vivo data confirmed that myricetin inhibited the phosphorylation and kinase activity of Akt. Molecular modeling suggested that myricetin easily docks to the ATP-binding site of Akt with hydrogen bonds. Signaling analysis data further demonstrated that myricetin inhibited Akt-mediated activator protein-1 (AP-1) transactivation, cyclin D1 expression and cell transformation. Overall, our results indicate that Akt is a direct target for myricetin to inhibit cell transformation.

Galpha(i1) and Galpha(i3) are required for epidermal growth factor-mediated activation of the Akt-mTORC1 pathway

The precise mechanism whereby epidermal growth factor (EGF) activates the serine-threonine kinase Akt and the mammalian target of rapamycin (mTOR) complex 1 (mTORC1) remains elusive. Here, we report that the alpha subunits of the heterotrimeric guanine nucleotide-binding proteins (G proteins) Galpha(i1) and Galpha(i3) are critical for this activation process. Both Galpha(i1) and Galpha(i3) formed complexes with growth factor receptor binding 2 (Grb2)-associated binding protein 1 (Gab1) and the EGF receptor (EGFR) and were required for the phosphorylation of Gab1 and its subsequent interaction with the p85 subunit of phosphatidylinositol 3-kinase in response to EGF. Loss of Galpha(i1) and Galpha(i3) severely impaired the activation of Akt and of p70 S6 kinase and 4E-BP1, downstream targets of mTORC1, in response to EGF, heparin-binding EGF-like growth factor, and transforming growth factor alpha, but not insulin, insulin-like growth factor, or platelet-derived growth factor. In addition, ablation of Galpha(i1) and Galpha(i3) largely inhibited EGF-induced cell growth, migration, and survival and the accumulation of cyclin D1. Overall, this study suggests that Galpha(i1) and Galpha(i3) lie downstream of EGFR, but upstream of Gab1-mediated activation of Akt and mTORC1, thus revealing a role for Galpha(i) proteins in mediating EGFR signaling.

Nuclear factor-kappaB enhances ErbB2-induced mammary tumorigenesis and neoangiogenesis in vivo

The (HER2/Neu) ErbB2 oncogene is commonly overexpressed in human breast cancer and is sufficient for mammary tumorigenesis in transgenic mice. Nuclear factor (NF)-kappaB activity is increased in both human and murine breast tumors. The immune response to mammary tumorigenesis may regulate tumor progression. The role of endogenous mammary epithelial cell NF-kappaB had not previously been determined in immune-competent animals. Furthermore, the role of the NF-kappaB components, p50 and p65, in tumor growth was not known. Herein, the expression of a stabilized form of the NF-kappaB-inhibiting IkappaBalpha protein (IkappaBalphaSR) in breast tumor cell lines that express oncogenic ErbB2 inhibited DNA synthesis and growth in both two- and three-dimensional cultures. Either NF-kappaB inhibition or selective silencing of p50 or p65 led to a loss of contact-independent tumor growth in vitro. IkappaBalphaSR reversed the features of the oncogene-induced phenotype under three-dimensional growth conditions. The NF-kappaB blockade inhibited ErbB2-induced mammary tumor growth in both immune-competent and immune-deficient mice. These findings were associated with both reduced tumor microvascular density and a reduction in the amount of vascular endothelial growth factor. The expression of IkappaBalphaSR in breast cancer tumors inhibited angiogenesis. Thus, mammary epithelial cell NF-kappaB activity enhances ErbB2-mediated mammary tumorigenesis in vivo by promoting both growth and survival signaling via the promotion of tumor vasculogenesis.

Staged stromal extracellular 3D matrices differentially regulate breast cancer cell responses through PI3K and beta1-integrins

Background

Interactions between cancer cells and stroma are critical for growth and invasiveness of epithelial tumors. The biochemical mechanisms behind tumor-stromal interactions leading to increased invasiveness and metastasis are mostly unknown. The goal of this study was to analyze the direct effects of staged stroma-derived extracellular matrices on breast cancer cell behavior.

Methods

Early and late three-dimensional matrices were produced by NIH-3T3 and tumor-associated murine fibroblasts, respectively. After removing fibroblasts, extracted matrices were re-cultured with breast epithelial cells of assorted characteristics: MCF-10A (non-tumorigenic), MCF-7 (tumorigenic, non-invasive), and MDA-MB-231 (tumorigenic, invasive). Effects prompted by staged matrices on epithelial cell's growth, morphology and invasion were determined. Also, matrix-induced velocity, directionality and relative track orientation of invasive cells were assessed in the presence or absence of inhibitors of phosphoinositide-3 kinase (PI3K) and/or beta-1 integrin.

Results

We observed that assorted breast epithelial cells reacted differently to two-dimensional vs. staged, control (early) and tumor-associated (late), three-dimensional matrices. MCF-10A had a proliferative advantage on two-dimensional substrates while MCF-7 and MDA-MB-231 showed no difference. MCF-10A and MCF-7 formed morphologically distinguishable aggregates within three-dimensional matrices, while MDA-MB-231 exhibited increased spindle-shape morphologies and directional movements within three-dimensional matrices. Furthermore, MDA-MB-231 acquired a pattern of parallel oriented organization within tumor-associated, but not control matrices. Moreover, tumor-associated matrices induced PI3K and beta1-integrin dependent Akt/PKB activity in MDA-MB-231 cells. Interestingly, beta1-integrin (but not PI3K) regulated tumor-associated matrix-induced mesenchymal invasion which, when inhibited, resulted in a change of invasive strategy rather than impeding invasion altogether.

Conclusion

We propose that both cells and matrices are important to promote effective breast cancer cell invasion through three-dimensional matrices and that beta1-integrin inhibition is not necessarily sufficient to block tumor-matrix induced breast cancer cell invasion. Additionally, we believe that characterizing stroma staging (e.g., early vs. late or tumor-associated) might be beneficial for predicting matrix-induced cancer cell responses in order to facilitate the selection of therapies.

Cell biology of the movement of breast cancer cells: intracellular signalling and the actin cytoskeleton

Cell motility is a critical step in cancer invasion and metastasis that must be unravelled to gain an appropriate understanding of the behaviour of cancer cells. A broad spectrum of motility mechanisms that facilitate invasion of extramammary tissues and metastasis exists in breast cancer cells (e.g. reorganization of the actin cytoskeleton, regulation of focal adhesion, changes in response to a different microenvironment, epithelial mesenchymal transition, and control of membrane proteins through endocytosis). These cellular responses are tightly regulated by intracellular signalling pathways evoked by humoral factors that include growth factors, chemokines, and cytokines. Learning more about the cellular and molecular basis of these different motility programmes will aid in the development of treatments for breast cancer invasion and metastasis. This review of recent literature focuses on aspects of cell biology related to motility and metastasis, and suggests some directions for future breast cancer research.

GIV is a nonreceptor GEF for G alpha i with a unique motif that regulates Akt signaling

Heterotrimeric G proteins are molecular switches that control signal transduction. Ligand-occupied, G protein-coupled receptors serve as the canonical guanine nucleotide exchange factors (GEFs) that activate heterotrimeric G proteins. A few unrelated nonreceptor GEFs have also been described, but little or nothing is known about their structure, mechanism of action, or cellular functions in mammals. We have discovered that GIV/Girdin serves as a nonreceptor GEF for G alpha i through an evolutionarily conserved motif that shares sequence homology with the synthetic GEF peptide KB-752. Using the available structure of the KB-752 x G alpha i1 complex as a template, we modeled the G alpha i-GIV interface and identified the key residues that are required to form it. Mutation of these key residues disrupts the interaction and impairs Akt enhancement, actin remodeling, and cell migration in cancer cells. Mechanistically, we demonstrate that the GEF motif is capable of activating as well as sequestering the G alpha-subunit, thereby enhancing Akt signaling via the G betagamma-PI3K pathway. Recently, GIV has been implicated in cancer metastasis by virtue of its ability to enhance Akt activity and remodel the actin cytoskeleton during cancer invasion. Thus, the novel regulatory motif described here provides the structural and biochemical basis for the prometastatic features of GIV, making the functional disruption of this unique G alpha i-GIV interface a promising target for therapy against cancer metastasis.

Genetic modelling of the PTEN/AKT pathway in cancer research

The focus on targeted therapies has been fuelled by extensive research on molecular pathways and their role in tumorigenesis. Novel models of human cancer have been created to evaluate the role of specific genes in the different stages of cancer. Currently, mouse modelling of human cancer is possible through the expression of oncogenes, specific genetic mutations or the inactivation of tumour suppressor genes, and these models have begun to provide us with an understanding of the molecular pathways involved in tumour initiation and progression at the physiological level. Additionally, these mouse models serve as an excellent system to evaluate the efficacy of currently developed molecular targeted therapies and identify new potential targets for future therapies. The PTEN/AKT pathway is implicated in signal transduction through tyrosine kinase receptors and heterotrimeric G protein-linked receptors. Deregulation of the PTEN/AKT pathway is a common event in human cancer. Despite the abundant literature, the physiological role of each element of the pathway has begun to be uncovered thanks to genetically engineered mice. This review will summarise some of the key animal models which have helped us to understand this signalling network and its contribution to tumorigenesis.

Alterations in cell growth and signaling in ErbB3 binding protein-1 (Ebp1) deficient mice

Background

The ErbB3 binding protein-1 (Ebp1) belongs to a family of DNA/RNA binding proteins implicated in cell growth, apoptosis and differentiation. However, the physiological role of Ebp1 in the whole organism is not known. Therefore, we generated Ebp1-deficient mice carrying a gene trap insertion in intron 2 of the Ebp1 (pa2g4) gene.

Results

Ebp1^-/- mice were on average 30% smaller than wild type and heterozygous sex matched littermates. Growth retardation was apparent from Day 10 until Day 30. IGF-1 production and IGBP-3 and 4 protein levels were reduced in both embryo fibroblasts and adult knock-out mice. The proliferation of fibroblasts derived from Day 12.5 knock out embryos was also decreased as compared to that of wild type cells. Microarray expression analysis revealed changes in genes important in cell growth including members of the MAPK signal transduction pathway. In addition, the expression or activation of proliferation related genes such as AKT and the androgen receptor, previously demonstrated to be affected by Ebp1 expression in vitro, was altered in adult tissues.

Conclusion

These results indicate that Ebp1 can affect growth in an animal model, but that the expression of proliferation related genes is cell and context specific. The Ebp1^-/- mouse line represents a new in vivo model to investigate Ebp1 function in the whole organism.

Akt determines replicative senescence and oxidative or oncogenic premature senescence and sensitizes cells to oxidative apoptosis

Akt deficiency causes resistance to replicative senescence, to oxidative stress- and oncogenic Ras-induced premature senescence, and to reactive oxygen species (ROS)-mediated apoptosis. Akt activation induces premature senescence and sensitizes cells to ROS-mediated apoptosis by increasing intracellular ROS through increased oxygen consumption and by inhibiting the expression of ROS scavengers downstream of FoxO, particularly sestrin 3. This uncovers an Achilles' heel of Akt, since in contrast to its ability to inhibit apoptosis induced by multiple apoptotic stimuli, Akt could not inhibit ROS-mediated apoptosis. Furthermore, treatment with rapamycin that led to further Akt activation and resistance to etoposide hypersensitized cancer cells to ROS-mediated apoptosis. Given that rapamycin alone is mainly cytostatic, this constitutes a strategy for cancer therapy that selectively eradicates cancer cells via Akt activation.

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.

Ablation of PI3K blocks BCR-ABL leukemogenesis in mice, and a dual PI3K/mTOR inhibitor prevents expansion of human BCR-ABL+ leukemia cells

Some cases of pre-B cell acute lymphoblastic leukemia (pre-B-ALL) are caused by the Philadelphia (Ph) chromosome-encoded BCR-ABL oncogene, and these tend to have a poor prognosis. Inhibitors of the PI3K/AKT pathway reduce BCR-ABL-mediated transformation in vitro; however, the specific PI3K isoforms involved are poorly defined. Using a murine model of Ph+ pre-B-ALL, we found that deletion of both Pik3r1 and Pik3r2, genes encoding class IA PI3K regulatory isoforms, severely impaired transformation. BCR-ABL-dependent pre/pro-B cell lines could be established at low frequency from progenitors that lacked these genes, but the cells were smaller, proliferated more slowly, and failed to cause leukemia in vivo. These cell lines displayed nearly undetectable PI3K signaling function and were resistant to the PI3K inhibitor wortmannin. However, they maintained activation of mammalian target of rapamycin (mTOR) and were more sensitive to rapamycin. Treatment with rapamycin caused feedback activation of AKT in WT cell lines but not PI3K-deficient lines. A dual inhibitor of PI3K and mTOR, PI-103, was more effective than rapamycin at suppressing proliferation of mouse pre-B-ALL and human CD19+CD34+)Ph+ ALL leukemia cells treated with the ABL kinase inhibitor imatinib. Our findings provide mechanistic insights into PI3K dependency in oncogenic networks and provide a rationale for targeting class IA PI3K, alone or together with mTOR, in the treatment of Ph+ ALL.

Akt inhibition promotes autophagy and sensitizes PTEN-null tumors to lysosomotropic agents

Although Akt is known as a survival kinase, inhibitors of the phosphatidylinositol 3-kinase (PI3K)-Akt pathway do not always induce substantial apoptosis. We show that silencing Akt1 alone, or any combination of Akt isoforms, can suppress the growth of tumors established from phosphatase and tensin homologue-null human cancer cells. Although these findings indicate that Akt is essential for tumor maintenance, most tumors eventually rebound. Akt knockdown or inactivation with small molecule inhibitors did not induce significant apoptosis but rather markedly increased autophagy. Further treatment with the lysosomotropic agent chloroquine caused accumulation of abnormal autophagolysosomes and reactive oxygen species, leading to accelerated cell death in vitro and complete tumor remission in vivo. Cell death was also promoted when Akt inhibition was combined with the vacuolar H(+)-adenosine triphosphatase inhibitor bafilomycin A1 or with cathepsin inhibition. These results suggest that blocking lysosomal degradation can be detrimental to cancer cell survival when autophagy is activated, providing rationale for a new therapeutic approach to enhancing the anticancer efficacy of PI3K-Akt pathway inhibition.

Radiation-induced CXCL16 release by breast cancer cells attracts effector T cells

Recruitment of effector T cells to inflamed peripheral tissues is regulated by chemokines and their receptors, but the factors regulating recruitment to tumors remain largely undefined. Ionizing radiation (IR) therapy is a common treatment modality for breast and other cancers. Used as a cytocidal agent for proliferating cancer cells, IR in combination with immunotherapy has been shown to promote immune-mediated tumor destruction in preclinical studies. In this study we demonstrate that IR markedly enhanced the secretion by mouse and human breast cancer cells of CXCL16, a chemokine that binds to CXCR6 on Th1 and activated CD8 effector T cells, and plays an important role in their recruitment to sites of inflammation. Using a poorly immunogenic mouse model of breast cancer, we found that irradiation increased the migration of CD8(+)CXCR6(+) activated T cells to tumors in vitro and in vivo. CXCR6-deficient mice showed reduced infiltration of tumors by activated CD8 T cells and impaired tumor regression following treatment with local IR to the tumor and Abs blocking the negative regulator of T cell activation, CTLA-4. These results provide the first evidence that IR can induce the secretion by cancer cells of proinflammatory chemotactic factors that recruit antitumor effector T cells. The ability of IR to convert tumors into "inflamed" peripheral tissues could be exploited to overcome obstacles at the effector phase of the antitumor immune response and improve the therapeutic efficacy of immunotherapy.

Signal transduction in transgenic mouse models of human breast cancer--implications for human breast cancer

The advent of genetically engineered mouse models (GEMs) of human breast cancer, have provided important insight into molecular basis or human breast cancer. This review will focus on two of the most extensively studied mouse models for human breast cancer involving mammary gland specific expression of the polyoma middle T (PyV MT) antigen and of the ErbB2. In addition, this review will discuss past and recent advances in understanding relative contribution of the signaling pathways in tumor induction and metastasis by these potent mammary oncogenes.

Activated Akt1 accelerates MMTV-c-ErbB2 mammary tumourigenesis in mice without activation of ErbB3

Introduction

ErbB2, a member of the epidermal growth factor receptor (EGFR) family, is overexpressed in 20% to 30% of human breast cancer cases and forms oncogenic signalling complexes when dimerised to ErbB3 or other EGFR family members.

Methods

We crossed mouse mammary tumour virus (MMTV)-myr-Akt1 transgenic mice (which express constitutively active Akt1 in the mammary gland) with MMTV-c-ErbB2 transgenic mice to evaluate the role of Akt1 activation in ErbB2-induced mammary carcinoma using immunoblot analysis, magnetic resonance spectroscopy and histological analyses.

Results

Bitransgenic MMTV-c-ErbB2, MMTV-myr-Akt1 mice develop mammary tumours twice as fast as MMTV-c-ErbB2 mice. The bitransgenic tumours were less organised, had more mitotic figures and fewer apoptotic cells. However, many bitransgenic tumours displayed areas of extensive necrosis compared with tumours from MMTV-c-ErbB2 mice. The two tumour types demonstrate dramatically different expression and activation of EGFR family members, as well as different metabolic profiles. c-ErbB2 tumours demonstrate overexpression of EGFR, ErbB2, ErbB3 and ErbB4, and activation/phosphorylation of both ErbB2 and ErbB3, underscoring the importance of the entire EGFR family in ErbB2-induced tumourigenesis. Tumours from bitransgenic mice overexpress the myr-Akt1 and ErbB2 transgenes, but there was dramatically less overexpression and phosphorylation of ErbB3, diminished phosphorylation of ErbB2, decreased level of EGFR protein and undetectable ErbB4 protein. There was also an observable attenuation in a subset of tyrosine-phosphorylated secondary signalling molecules in the bitransgenic tumours compared with c-ErbB2 tumours, but Erk was activated/phosphorylated in both tumour types. Finally, the bitransgenic tumours were metabolically more active as indicated by increased glucose transporter 1 (GLUT1) expression, elevated lactate production and decreased intracellular glucose (suggesting increased glycolysis).

Conclusion

Expression of activated Akt1 in MMTV-c-ErbB2 mice accelerates tumourigenesis with a reduced requirement for signalling through the EGFR family, as well as a reduced requirement for a subset of downstream signaling molecules with a metabolic shift in the tumours from bitransgenic mice. The reduction in signalling downstream of ErbB2 when Akt is activated suggest a possible mechanism by which tumour cells can become resistant to ErbB2-targeted therapies, necessitating therapies that target oncogenic signalling events downstream of ErbB2.

PHLiPPing the switch on Akt and protein kinase C signaling

The Ser/Thr-specific phosphatase PHLPP [pleckstrin homology (PH) domain leucine-rich repeat protein phosphatase] provides 'the brakes' for Akt and protein kinase C (PKC) signaling. The two isoforms of this recently discovered family, PHLPP1 and PHLPP2, control the amplitude and duration of signaling of Akt and PKC by catalyzing the dephosphorylation of the hydrophobic phosphorylation motif, a C-terminal phosphorylation switch that controls these kinases. Aberrant regulation of either kinase accompanies many diseases, notably diabetes and cancer. By specifically dephosphorylating the hydrophobic motif, PHLPP controls the degree of agonist-evoked signaling by Akt and the cellular levels of PKC. This review focuses on the function of PHLPP1 and PHLPP2 in modulating signaling by Akt and PKC.

The nuclear receptor coactivator amplified in breast cancer-1 is required for Neu (ErbB2/HER2) activation, signaling, and mammary tumorigenesis in mice

Overexpression of the oncogene amplified in breast cancer 1 (AIB1)/steroid receptor coactivator-3 (SRC-3) induces mammary tumorigenesis in mice. In breast cancer, high levels of AIB1/SRC-3 and the growth factor receptor HER2/neu predict resistance to endocrine therapy and poor outcome. However, a mechanistic relationship between AIB1/SRC-3 and HER2/neu in the development of breast cancer has not been shown. Here, we show that deletion of one allele of SRC-3 significantly delays Neu-induced mammary tumor development in mice. Homozygous deletion of SRC-3 in mice completely prevents Neu-induced tumor formation. By ages 3 to 4 months, Neu/SRC-3(+/-) mice exhibit a noticeable reduction in lateral side-bud formation, accompanied by reduced cellular levels of phosphorylated Neu compared with Neu/SRC-3(wt) mice. In Neu-induced tumors, high levels of SRC-3, phosphorylated Neu, cyclin D1, cyclin E, and proliferating cell nuclear antigen expression are observed, accompanied by activation of the AKT and c-Jun NH(2) kinase (JNK) signaling pathways. In comparison, phosphorylated Neu, cyclin D1, and cyclin E are significantly decreased in Neu/SRC-3(+/-) tumors, proliferation is reduced, and AKT and JNK activation is barely detectable. Our data indicate that AIB1/SRC-3 is required for HER2/neu oncogenic activity and for the phosphorylation and activation of the HER2/neu receptor. We predict that reducing AIB1/SRC-3 levels or activity in the mammary epithelium could potentiate therapies aimed at inhibiting HER2/neu signaling in breast cancer.

Molecular targets of breast cancer: AKTing in concert

Despite significant advancement in the diagnosis and treatment of breast cancer, many patients succumb to this disease. The elucidation of aberrant signaling pathways that lead to breast cancer should help develop more effective therapeutic strategies. The Akt signaling pathway plays an important role in the development and progression of breast cancer. Overexpression/activation of Akt has been associated with poor prognosis and resistance to hormonal and chemotherapy. Although mutations in Akt are rare in breast cancer, the activity of Akt is regulated by hormones, growth factors, growth factor receptors, oncogenes and tumor suppressor genes that are often deregulated in breast cancer. The objective of this commentary is to discuss recent literature on how activation of Akt by various signaling pathways contributes to breast cancer and confers resistance to current therapy.

Dachshund inhibits oncogene-induced breast cancer cellular migration and invasion through suppression of interleukin-8

Oncogene-mediated signaling to the host environment induces a subset of cytokines and chemokines. The Drosophila Dac gene promotes migration of the morphogenetic furrow during eye development. Expression of the cell-fate determination factor Dachshund (DACH1) was lost in poor prognosis invasive breast cancer. Mouse embryo fibroblasts derived from Dach1(-/-) mice demonstrated endogenous Dach1 constitutively represses cellular migration. DACH1 inhibited cellular migration and invasion of oncogene (Ras, Myc, ErbB2, c-Raf)-transformed human breast epithelial cells. An unbiased proteomic analysis identified and immunoneutralizing antibody and reconstitution experiments demonstrated IL-8 is a critical target of DACH1 mediating breast cancer cellular migration and metastasis in vivo. DACH1 bound the endogenous IL-8 promoter in ChIP assays and repressed the IL-8 promoter through the AP-1 and NF-kappaB binding sites. Collectively, our data identify a pathway by which an endogenous cell-fate determination factor blocks oncogene-dependent tumor metastasis via a key heterotypic mediator.

Characterization of an Akt kinase inhibitor with potent pharmacodynamic and antitumor activity

Akt kinases 1, 2, and 3 are important regulators of cell survival and have been shown to be constitutively active in a variety of human tumors. GSK690693 is a novel ATP-competitive, low-nanomolar pan-Akt kinase inhibitor. It is selective for the Akt isoforms versus the majority of kinases in other families; however, it does inhibit additional members of the AGC kinase family. It causes dose-dependent reductions in the phosphorylation state of multiple proteins downstream of Akt, including GSK3 beta, PRAS40, and Forkhead. GSK690693 inhibited proliferation and induced apoptosis in a subset of tumor cells with potency consistent with intracellular inhibition of Akt kinase activity. In immune-compromised mice implanted with human BT474 breast carcinoma xenografts, a single i.p. administration of GSK690693 inhibited GSK3 beta phosphorylation in a dose- and time-dependent manner. After a single dose of GSK690693, >3 micromol/L drug concentration in BT474 tumor xenografts correlated with a sustained decrease in GSK3 beta phosphorylation. Consistent with the role of Akt in insulin signaling, treatment with GSK690693 resulted in acute and transient increases in blood glucose level. Daily administration of GSK690693 produced significant antitumor activity in mice bearing established human SKOV-3 ovarian, LNCaP prostate, and BT474 and HCC-1954 breast carcinoma xenografts. Immunohistochemical analysis of tumor xenografts after repeat dosing with GSK690693 showed reductions in phosphorylated Akt substrates in vivo. These results support further evaluation of GSK690693 as an anticancer agent.

An actin-binding protein Girdin regulates the motility of breast cancer cells

Girdin (girders of actin filaments) is a novel actin-binding Akt substrate that plays an important role in actin organization and Akt-dependent cell motility in fibroblasts. Here, we find that Girdin is expressed in a variety of cancer cell lines, including the breast cancer cell line MDA-MB-231, and is phosphorylated by the stimulation of insulin-like growth factor (IGF-I). In vitro migration and invasion assays showed that Girdin is required for the IGF-I-dependent cell movement of MDA-MB-231 cells. Short hairpin interfering RNA directed against Girdin markedly inhibited the metastasis of s.c. transplanted MDA-MB-231 cells in nude mice. In addition, Girdin is highly expressed in a variety of human malignant tissues, including breast, colon, lung, and uterine cervical carcinomas. These findings highlight the important role of Girdin in tumor progression in which the Akt signaling pathway is aberrantly activated.