[Show abstract][Hide abstract] ABSTRACT: Unlabelled:
Previous work has shown that prostate cancer in a Pten-null murine model is dependent on the p110β isoform of phosphatidylinositol 3-kinase (PI3K), while breast cancer driven by either polyoma middle T antigen (MT) or HER2 is p110α dependent. Whether these differences in isoform dependence arise from tissue specificity or from the nature of the oncogenic signal activating the PI3K pathway is important, given increasing interest in using isoform-specific PI3K inhibitors in cancer therapy. To approach this question, we studied the PI3K isoform dependence of our recently constructed prostate cancer model driven by MT. Since MT activates a number of signaling pathways, we first confirmed that the MT-driven prostate cancer model was actually dependent on PI3K. A newly generated transgenic prostate line expressing an MT allele (Y315F) known to be defective for PI3K binding displayed a markedly reduced ability to drive tumor formation. We next selectively ablated expression of either p110α or p110β in mice in which wild-type MT was expressed in the prostate. We found that tumor formation driven by MT was significantly delayed by the loss of p110α expression, while ablation of p110β had no effect. Since the tumor formation driven by MT is p110α dependent in the prostate as well as in the mammary gland, our data suggest that PI3K isoform dependence is driven by the mode of PI3K pathway activation rather than by tissue type.
Middle T antigen (MT), the oncogene of polyomavirus, can drive tumor formation in a variety of cell types and tissues. Interestingly, MT has no intrinsic enzymatic activity but instead functions by binding and activating cellular signaling proteins. One of the most important of these is the lipid kinase PI3K, which was first studied in MT immunoprecipitates. Ubiquitously expressed PI3K comes in two major isoforms: p110α and p110β. Previous work in animal models showed that p110α was the key isoform in breast tumors driven by oncogenes, including MT and HER2, while p110β was key in prostate tumors driven by Pten loss. We asked the simple question of whether a prostate tumor driven by MT depends on p110α, which would suggest that the mode of activation determines p110 isoform dependence, or p110β, which would suggest that tissue type determines isoform dependence. The clear answer is that MT depends on p110α in both the prostate and breast.
Preview · Article · Jul 2014 · Journal of Virology
[Show abstract][Hide abstract] ABSTRACT: There has been increasing interest in the use of isoform-selective inhibitors of phosphatidylinositide-3-kinase (PI3K) in cancer therapy. Using conditional deletion of the p110 catalytic isoforms of PI3K to predict sensitivity of cancer types to such inhibitors, we and others have demonstrated that tumors deficient of the phosphatase and tensin homolog (PTEN) are often dependent on the p110β isoform of PI3K. Because human cancers usually arise due to multiple genetic events, determining whether other genetic alterations might alter the p110 isoform requirements of PTEN-null tumors becomes a critical question. To investigate further the roles of p110 isoforms in PTEN-deficient tumors, we used a mouse model of ovarian endometrioid adenocarcinoma driven by concomitant activation of the rat sarcoma protein Kras, which is known to activate p110α, and loss of PTEN. In this model, ablation of p110β had no effect on tumor growth, whereas p110α ablation blocked tumor formation. Because ablation of PTEN alone is often p110β dependent, we wondered if the same held true in the ovary. Because PTEN loss alone in the ovary did not result in tumor formation, we tested PI3K isoform dependence in ovarian surface epithelium (OSE) cells deficient in both PTEN and p53. These cells were indeed p110β dependent, whereas OSEs expressing activated Kras with or without PTEN loss were p110α dependent. Furthermore, isoform-selective inhibitors showed a similar pattern of the isoform dependence in established Kras(G12D)/PTEN-deficient tumors. Taken together, our data suggest that, whereas in some tissues PTEN-null tumors appear to inherently depend on p110β, the p110 isoform reliance of PTEN-deficient tumors may be altered by concurrent mutations that activate p110α.
Preview · Article · Apr 2014 · Proceedings of the National Academy of Sciences
[Show abstract][Hide abstract] ABSTRACT: Class Ia phosphatidylinositol 3 kinase (PI3K) is required for oncogenic receptor-mediated transformation; however, the individual roles of the two commonly expressed class Ia PI3K isoforms in oncogenic receptor signaling have not been elucidated in vivo. Here, we show that genetic ablation of p110α blocks tumor formation in both polyoma middle T antigen (MT) and HER2/Neu transgenic models of breast cancer. Surprisingly, p110β ablation results in both increased ductal branching and tumorigenesis. Biochemical analyses suggest a competition model in which the less active p110β competes with the more active p110α for receptor binding sites, thereby modulating the level of PI3K activity associated with activated receptors. Our findings demonstrate a novel p110β-based regulatory role in receptor-mediated PI3K activity and identify p110α as an important target for treatment of HER2-positive disease.
Preview · Article · Jul 2012 · Genes & development
[Show abstract][Hide abstract] ABSTRACT: The PI3K pathway is frequently activated in cancer; therefore, considerable effort is focused on identifying compounds that can inhibit specific pathway components, particularly the hallmark oncogene PIK3CA. Although targeted inhibition of a cancer survival gene holds significant promise, there are concerns that drug resistance may emerge within the cancerous cells, thus limiting clinical efficacy. Using genetically defined human mammary epithelial cells, we evolved resistance to the PI3K/mammalian target of rapamycin (mTOR) inhibitor BEZ235, and by genome-wide copy number analyses, we identified MYC and eIF4E amplification within the resistant cells. Importantly, either MYC or eukaryotic translation initiation factor 4E (eIF4E) was required to bypass pharmacological PI3K/mTOR inhibition in resistant cells. Furthermore, these cells displayed elevated 5' cap-dependent protein translation. Collectively, these findings suggest that analysis of drivers of protein translation could facilitate the identification of cancer lesions that confer resistance to PI3K pathway-targeted drugs.
Full-text · Article · Aug 2011 · Proceedings of the National Academy of Sciences
[Show abstract][Hide abstract] ABSTRACT: The middle T (MT) antigen of polyomavirus has provided fundamental insights into the regulation of mammalian cell growth in vitro and important animal models for the analysis of tumor induction. The mouse mammary tumor virus (MMTV)-MT model of breast cancer has been important for probing the cellular signaling pathways in mammary tumorigenesis. MT itself has no intrinsic enzymatic activity but, rather, transforms by binding to and activating key intracellular signaling molecules, phosphatidylinositol 3-kinase (PI3-kinase) being the best studied of these. Thus, MT mimics a constitutively activated receptor tyrosine kinase (RTK). Our recent work suggests that MT signaling, like that of RTKs, is often quite dependent on cellular context in vitro. Here, we examine contextual effects on signaling in animal models as well. In this study, we generated transgenic mice in which MT is expressed in the mouse prostate under the control of an (ARR)2-Probasin promoter. All male transgenic mice displayed mouse prostatic intraepithelial neoplasia (mPIN) in the ventral and dorsal/lateral prostate as early as 8 weeks of age. Notably, during the course of tumor development over time, invasive cancer, reactive stroma, and infiltration of inflammatory cells were seen. Transcriptional profiling analyses show regulation of multiple pathways, with marked upregulation of both the NF-κB and inflammatory pathways. Comparison of expression profiles of our MT prostate model with those from an MMTV-MT breast model (23) shows both tissue-specific and tissue-independent MT effects. The signature of genes regulated by MT in a tissue-independent manner may have prognostic value.
Full-text · Article · Mar 2011 · Journal of Virology
[Show abstract][Hide abstract] ABSTRACT: Middle T antigen (MT) of polyomavirus is known to play an important role in virus-mediated cellular transformation. While MT has been extensively examined in spontaneously immortalized rodent fibroblasts, its interactions with cells of other types and species are less well understood. We have undertaken a cross-species and cross-cell-type comparison of MT-induced transformation in cells with genetically defined backgrounds. We tested the transforming abilities of a panel of MT mutants, Y250F, Y315F, and Y322F, that are selectively mutated in the binding sites for the principal effectors of MT--Src homology 2 domain-containing transforming protein, phosphatidylinositol 3-kinase (PI3K), and phospholipase C-gamma, respectively--in fibroblasts and epithelial cells of murine or human origin. We found that the Y315F mutation disabled the ability of MT to induce transformation in all cell types and species tested. While Y315F also failed to activate the PI3K pathway in these cells, genetic evidence has indicated Y315 may make other contributions to transformation. To confirm the role of PI3K, the PIK3CA gene, encoding p110alpha, the prime effector of PI3K signaling downstream from activated growth factor receptors, was genetically ablated. This abolished the transforming activity of MT, demonstrating the essential role for this PI3K isoform in MT-mediated transformation. The Y250F mutant was able to transform the human, but not the murine, cells that were examined. Interestingly, this mutant fully activates the PI3K pathway in human cells but activated PI3K signaling poorly in the murine cells used in the study. This again points to the importance of PI3K activation for transformation and suggests that the mechanism by which MT activates the PI3K pathway differs in different species.
Preview · Article · Aug 2007 · Journal of Virology