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Intrinsic and Pharmacological Mechanisms of Tumor Suppression
Summary: Scott Lowe is interested in characterizing tumor-suppressor networks and how mutations in network components influence tumorigenesis and resistance to chemotherapy. His goal is to identify new therapeutic targets and develop better strategies for using existing cancer drugs.
Cancer arises through an evolutionary process whereby normal cells acquire mutations that erode growth controls, leading to the inappropriate expansion of aberrantly proliferating cells. Such mutations can involve activation of oncogenes or inactivation of tumor-suppressor genes, each contributing one or more new capabilities to the developing cancer cell. However, cancer is not an inevitable consequence of oncogenic mutations; instead, cells acquiring such mutations can be eliminated or kept in check by innate tumor-suppressor programs that can be activated in these damaged cells.
Apoptosis and cellular senescence are two antiproliferative processes that limit the development and progression of malignant tumors. Apoptosis is a cell death program that controls normal tissue homeostasis but can also eliminate cells in response to cellular damage or stress. Senescence is a particularly stable form of proliferative arrest, also triggered by cellular damage, that produces "genetic death" in that the senescent cell is incapable of further propagation. Both programs are controlled by overlapping signaling networks whose components are frequent targets of mutation in cancer cells.
Our laboratory studies tumor-suppressor networks controlling apoptosis and senescence and how their disruption influences malignant behavior. We previously showed that apoptosis and cellular senescence are potent barriers to oncogene-driven tumorigenesis and that each contributes to the antitumor action of many chemotherapeutic drugs. Thus, not only do mutations that disrupt apoptosis and senescence promote tumor progression but, depending on the particular lesion, they can also reduce the efficacy of cancer therapy.
By understanding the mechanisms whereby proliferation is coupled to cell death or senescence, we hope to exploit intrinsic tumor-suppressor mechanisms to eliminate incipient cancer cells. We have shown that mitogenic oncogenes such as adenovirus E1A, myc, or ras can engage the p53 tumor-suppressor network to promote apoptosis or senescence and that loss of p53 function—or other components of the p53 network—can disable these antiproliferative responses, thereby facilitating malignant transformation. We continue to identify new network components and, using animal models or human tumor specimens, explore their action in vivo.
As one example, we recently showed that Cbx7, a chromobox family protein and a component of the Polycomb repressive complex 1 (PRC1) that suppresses the Ink4a/Arf tumor-suppressor locus, is overexpressed in germinal center-derived follicular lymphomas. By targeting Cbx7 expression to the lymphoid compartment in mice, we showed that Cbx7 can initiate T cell lymphomagenesis and cooperate with c-Myc to produce highly aggressive B cell lymphomas, in part by disabling p53 and its ability to promote apoptosis. In addition to these overexpression studies, we are also using RNA interference (RNAi) technology to identify new network components.
Control of cellular senescence involves the collaborative action of the p16-retinoblastoma (Rb) and p53 tumor-suppressor networks. We previously showed that certain senescent cells accumulate a new type of heterochromatin structure whose formation requires the action of Rb and the high-mobility group A (HMGA) proteins and is associated with the stable repression of certain proliferation-associated genes. More recently, we conducted a series of experiments where we showed that p53-deficient liver carcinoma cells could undergo senescence following p53 reactivation in vivo. Remarkably, although senescence is a cytostatic program, the tumors underwent massive regressions as a result of an attack of the immune system on the senescent cells. Thus, despite the cytostatic nature of the senescence program, senescent cells can turn over in vivo. Although it is established that chronic inflammation triggered by senescent stromal cells or other factors can promote tumorigenesis, our study illustrates how innate immune cells—when targeted against senescent tumor cells—can have antitumor effects as well.
To understand how tumor-suppressor networks influence the action of anticancer agents in real tumors, we focus on genetic studies in mouse models of human cancer. Initially, we focused on the Eμ-myc model, a transgenic mouse that expresses the myc oncogene from the immunoglobulin enhancer and develops B cell lymphoma. This model is particularly amenable to performing treatment studies and, in addition, we have developed methods for rapidly producing lymphomas of different genotypes, allowing us to study the impact of many genes and gene combinations on treatment behavior. These studies illustrate how evolutionary trajectory of a tumor dictates its subsequent response to therapy. To extend these efforts, we are also developing powerful mouse models of acute myelogenous leukemia to determine how heterogeneity in cancer genotypes influences the action of new “molecularly targeted” therapeutics.
We continue to develop new technologies for studying tumor-suppressor networks in cultured cells and in animal models. Working with Gregory Hannon (HHMI, Cold Spring Harbor Laboratory), we recently showed that polymerase II (Pol II) promoters expressing rationally designed primary microRNA-based short hairpin RNAs (shRNA) produce potent, stable, and regulatable gene knockdown in cultured cells and in animals, even when present at a single copy in the genome. We are taking advantage of these powerful technologies to explore various aspects of tumor-suppressor gene networks in vitro and in vivo.
One advantage of using microRNA-based shRNAs to silence specific genes is that they are amenable to the same types of manipulations that have been previously used to control the expression of protein-coding cDNAs. In fact, by adapting the tetracycline (tet)-responsive system previously used for conditional gene overexpression, we developed a simple transgenic system for reversible RNAi in mice. We showed that transgenic mice harboring a tet-responsive promoter driving a microRNA-based shRNA targeting the tumor-suppressor p53 express shRNA in a doxycyclin (a tetracycline analog)-dependent manner, which can be both reversible and tissue-specific when transgenic animals are crossed to existing mouse strains expressing tet transactivators. We showed that p53 knockdown in the hematopoietic compartment significantly accelerated lymphomagenesis in the Eμ-myc mouse model and that tumors presenting in these mice rapidly regressed following p53 reactivation in vivo. In addition to creating a powerful tool for studying p53 action, these studies illustrate how similar systems might be used to spatially, temporally, and reversibly control the expression of any endogenous gene in mice.
Finally, to accelerate the pace at which we can gain information about cancer genes and cancer biology, we have initiated a broad program to integrate our animal-modeling approaches with genomic technologies. As a first step, we took an oncogenomic approach to identify and characterize molecular changes that contribute to human cancer. Genome-wide analyses of liver tumors derived from our mouse models and from human patients revealed a recurrent amplification event in tumors arising in both species. Gene-expression analyses delineated cIAP1, a known inhibitor of apoptosis, and Yap, a transcription factor, as candidate oncogenes in the amplicon. In the genetic context of their amplification, both cIAP1 and Yap accelerated tumorigenesis and both were required to sustain rapid growth of amplicon-containing tumors. Furthermore, cIAP1 and Yap cooperated to promote tumorigenesis. Our results identify two oncogenes that cooperate by virtue of their coamplification in the same genomic locus and suggest a strategy for the annotation of human cancer genes. As human cancers are highly heterogeneous and genomically unstable, we believe such integrative approaches will be crucial for identifying those cancer genes that will serve as the best targets for new therapies. Current studies are aimed at applying similar approaches to the identification and characterization of tumor-suppressor genes.
Grants from the National Cancer Institute, the National Institute on Aging, and the Leukemia and Lymphoma Society of America provide partial support for this research.
Last updated August 29, 2007
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