How Cancers Escape Their Oncogene Habit

Many recent reports demonstrate that at least initially, the inactivation of an oncogene can induce sustained regression of even a highly invasive and genetically complex cancer. However, upon prolonged oncogene inactivation, some cancers ultimately relapse, becoming independent of the very oncogene that initiated the process of tumorigenesis. Understanding the specific mechanisms by which cancers can escape dependence upon a particular oncogene will be critical to anticipate mechanisms by which human cancers will evade therapies that target individual oncogenes. Thereby, more effective strategies will be developed to clinically treat cancer.     

How cancers escape their oncogene habit

Many recent reports demonstrate that at least initially, the inactivation of an oncogene can induce sustained regression of even a highly invasive and genetically complex cancer. However, upon prolonged oncogene inactivation, some cancers ultimately relapse, becoming independent of the very oncogene that initiated the process of tumorigenesis. Understanding the specific mechanisms by which cancers can escape dependence upon a particular oncogene will be critical to anticipate mechanisms by which human cancers will evade therapies that target individual oncogenes. Thereby, more effective strategies will be developed to clinically treat cancer.     

Rehabilitation of cancer through oncogene inactivation

The inactivation of the MYC oncogene alone can reverse tumorigenesis. Upon MYC inactivation, tumors stereotypically reverse, undergoing proliferative arrest, cellular differentiation and/or apoptosis. The precise consequences of MYC inactivation appear to depend upon both genetic and epigenetic parameters. In some types of cancer following MYC inactivation, tumor cells become well differentiated and biologically and histologically normal, inducing sustained tumor regression. However, in some cases, these normal-appearing cells are actually dormant tumor cells and upon MYC reactivation they rapidly recover their tumorigenic properties. Future therapies to treat cancer will need to address the possibility that tumor cells can camouflage a normal phenotype following treatment, resting in a dormant, latently cancerous state.     

Transformation effector and suppressor genes.

Much has been learned about the molecular basis of cancer from the study of the dominantly acting viral and cellular oncogenes and their normal progenitors, the proto-oncogenes. More recent studies have resulted in the isolation and characterization of several genes prototypic of a second class of cancer genes. Whereas oncogenes act to promote the growth of cells, members of this latter class of genes act to inhibit cellular growth and are believed to contribute to the tumorigenic phenotype only when their activities are absent. This new class of cancer genes is referred to by a number of different names including; anti-oncogenes, recessive oncogenes, growth suppressor genes, tumor suppressor genes and emerogenes. Although only a few of these cancer genes have been identified, to date, it is likely that many additional genes of this class await identification. A third class of genes, necessary for the development of the cancer phenotype, is comprised of the transformation effector genes. These are normal cellular genes that encode proteins that cooperate with or activate oncogene functions and thereby induce the development of the neoplastic phenotype. The inactivation of transformation effector functions would therefore inhibit the ability of certain dominantly acting oncogenes to transform cells. The approaches outlined here describe functional assays for the isolation and molecular characterization of transformation effector and suppressor genes.     

Reversibility of oncogene-induced cancer

Cancer can largely be conceived as a consequence of genomic catastrophes resulting in genetic events that usurp physiologic function of a normal cell. These genetic events mediate their pathologic effects by either activating oncogenes or inactivating tumor-suppressor genes. The targeted repair or inactivation of these damaged gene products may counteract the effects of these genetic events, reversing tumorigenesis and thereby serve as an effective therapy for cancer. However, because they are the result of many genetic events, the inactivation of no single mutant gene product may be sufficient to reverse cancer. Despite this caveat, compelling recent evidence suggests that there are circumstances when even the brief interruption of activation of a single oncogene can be sufficient to reverse tumorigenesis. Understanding how and when oncogene inactivation reverses cancer will be important in both defining the molecular pathogenesis of cancer as well as developing new molecularly based treatments.     

Combined Inactivation of MYC and K-Ras oncogenes reverses tumorigenesis in lung adenocarcinomas and lymphomas

Background

Conditional transgenic models have established that tumors require sustained oncogene activation for tumor maintenance, exhibiting the phenomenon known as “oncogene-addiction.” However, most cancers are caused by multiple genetic events making it difficult to determine which oncogenes or combination of oncogenes will be the most effective targets for their treatment.

Methodology/Principal Findings

To examine how the MYC and K-ras^G12D oncogenes cooperate for the initiation and maintenance of tumorigenesis, we generated double conditional transgenic tumor models of lung adenocarcinoma and lymphoma. The ability of MYC and K-ras^G12D to cooperate for tumorigenesis and the ability of the inactivation of these oncogenes to result in tumor regression depended upon the specific tissue context. MYC-, K-ras^G12D- or MYC/K-ras^G12D-induced lymphomas exhibited sustained regression upon the inactivation of either or both oncogenes. However, in marked contrast, MYC-induced lung tumors failed to regress completely upon oncogene inactivation; whereas K-ras^G12D-induced lung tumors regressed completely. Importantly, the combined inactivation of both MYC and K-ras^G12D resulted more frequently in complete lung tumor regression. To account for the different roles of MYC and K-ras^G12D in maintenance of lung tumors, we found that the down-stream mediators of K-ras^G12D signaling, Stat3 and Stat5, are dephosphorylated following conditional K-ras^G12D but not MYC inactivation. In contrast, Stat3 becomes dephosphorylated in lymphoma cells upon inactivation of MYC and/or K-ras^G12D. Interestingly, MYC-induced lung tumors that failed to regress upon MYC inactivation were found to have persistent Stat3 and Stat5 phosphorylation.

Conclusions/Significance

Taken together, our findings point to the importance of the K-Ras and associated down-stream Stat effector pathways in the initiation and maintenance of lymphomas and lung tumors. We suggest that combined targeting of oncogenic pathways is more likely to be effective in the treatment of lung cancers and lymphomas.     

Stem-cell driven cancer: "Hands-off" regulation of cancer development

A cancer dogma states that inactivation of oncogene(s) can cause cancer remission, implying that oncogenes are the Achilles' heel of cancers. This current “hands on” model of cancer has kept oncogenes firmly in focus as therapeutic targets and is in agreement with the fact that in human cancers all cancerous cells, with independence of the cellular heterogeneity existing within the tumour, carry the same oncogenic genetic lesions. This rule has now been broken in a study of the effect of the BCR-ABL oncogene in cancer development in a mouse model in which oncogene expression is restricted to the stem cell compartment. BCR-ABL is linked to chronic myeloid leukemia (CML) disease in humans, and this study shows that by limiting the oncogene expression to Sca1+ cells CML arises, indicating that maintenance of oncogene expression is not critical for the generation of differentiated tumor cells and showing a “hands off” role for BCR-ABL in regulating cancer formation. Here we provide an update on the use of this system for modeling human cancer and its potential application for therapeutic targeting of cancer stem cells (CSCs) and the hands-off function of oncogenes.     

A unified theory for the development of cancer

It is postulated that cancer is the result of genetic and epigenetic changes that occur mainly in stem (precursor) cells of various cell types. I propose that there are three classes of genes which are involved in the development of cancer. These are: Class I, II and III oncogenes. The classification is based on the way the oncogene acts at the cellular level to further the development of cancer. Genetic changes, that is point mutations, deletions, inversions, amplifications and chromosome translocations, gains or losses in the genes themselves or epigenetic changes in the genes (e.g. DNA hypomethylation) or in the gene products (RNA or protein) are responsible for the development of cancer. Changes of oncogene activity have a genetic or epigenetic origin or both and result in quantitative or qualitative differences in the oncogene products. These are involved in changing normal cells into the cells demonstrating a cancer phenotype (usually a form of dedifferentiated cell) in a multistep process. There are several pathways to cancer and the intermediate steps are not necessarily defined in an orderly fashion. Activation of a particular Class I or II oncogene and inactivation of a Class III oncogene could occur at any step during the development of cancer. Most benign or malignant tumors consist of a heterogeneous mixture of dedifferentiated cells arising from a single cell.     

3D view to tumor suppression: lkb1, polarity and the arrest of oncogenic c-myc

Machiavelli wrote, in his famous political treatise Il Principe, about disrupting organization by planting seeds of dissension or by eliminating necessary support elements. Tumor cells do exactly that by disrupting the organized architecture of epithelial cell layers during progression from contained benign tumor to full-blown invasive cancer. However, it is still unclear whether tumor cells primarily break free by activating oncogenes powerful enough to cause chaos or by eliminating tumor suppressor genes guarding the order of the epithelial organization. Studies in Drosophila have exposed genes that encode key regulators of the epithelial apicobasal polarity and which, upon inactivation, cause disorganization of the epithelial layers and promote unscheduled cell proliferation. These polarity regulator/tumor suppressor proteins, which include products of neoplastic tumor suppressor genes (nTSGs), are carefully positioned in polarized epithelial cells to maintain the order of epithelial structures and to impose a restraint on cell proliferation. In this review, we have explored the presence and prevalence of somatic mutations in the human counterparts of Drosophila polarity regulator/tumor suppressor genes across the human cancers. The screen points out LKB1, which is a causal genetic lesion in Peutz-Jeghers cancer syndrome, a gene mutated in certain sporadic cancers and a human homologue of the fly polarity gene par-4. We review the evidence linking Lkb1 to polarity regulation in the scope of our recent results suggesting a coupled role for Lkb1 as an architect of organized acinar structures and a suppressor of oncogenic c-Myc. We finally present models to explain how Lkb1-dependent formation of epithelial architecture is coupled to suppression of normal and oncogene-induced proliferation.     

Tumorigenesis in transgenic mice: identification and characterization of synergizing oncogenes.

Transgenic mice carrying oncogenes present a useful model with which to assess the tissue-specific action of oncogenes. These mice are usually predisposed to a specific type of neoplastic growth. The tumors that arise are usually monoclonal in origin and become only apparent after a variable latency period, suggesting that additional events are required for tumor formation. Identification of these additional events is highly relevant: it might give access to the genes that can synergize with a preselected oncogene in tumorigenesis and could facilitate the identification of the biochemical pathways in which these genes act. Retroviruses can be instrumental in identifying cooperating oncogenes. Proto-oncogene activation or tumor suppressor gene inactivation by insertional mutagenesis is an important mechanism by which the non-acute transforming retroviruses can induce tumors in several species. Owing to the sequence tag provided by the provirus, the relevant proto-oncogene can be directly identified by cloning of the DNA flanking the proviral insertion site. We have exploited this potential of retroviruses by infecting E mu-pim-1 and E mu-myc transgenic mice, which are predisposed to lymphomagenesis, with Moloney murine leukemia virus (MuLV). A strong acceleration of tumor induction ensued upon infection of these mice with MuLV. More importantly, it allowed us to identify a number of additional common insertion sites marking both previously known as well as new (putative) oncogenes. In a significant portion of the tumors more than one oncogene was found to be activated, indicating that within this system the synergistic effect of at least three genes can be established.(ABSTRACT TRUNCATED AT 250 WORDS)     

反义核酸在肿瘤研究中的应用

反义核酸研究已活跃于肿瘤研究及基因治疗领域,反义核酸通过碱基配对待异性地抑制基因表达,因此为研究肿瘤中癌基因和生长因子的功能及癌基因突变检测提供了更为有效的手段,并为肿瘤的基因治疗提供了可能途径.文章综述了反义核酸在基因治疗中所面临的问题及部分解决办法.     

Oncogene cooperation in cellular transformation

Oncogenic cell transformation induces major changes in the structure and physiology of the cells: modifications of morphology, differentiation block, disorganisation of cytoskeleton and extracellular matrix, alterations in growth control. The identification of oncogenes relies upon transfer into host normal cells of DNA isolated from cancer cells. The recent development of DNA transfer into germinal cells has provided new insights into the genetic control of tumorogenesis in vivo. In most cases, full transformation into leukemic or tumor cell requires the cooperation of several oncogenes. These observations support the hypothesis of cancer as a multistep process. However, many of the cooperative oncogenes have not yet been identified, especially in human cancers. The recent discovery of genes acting as repressors of cell growth in normal cells has brought to light a new class of potential recessive oncogenes that might have a contributory function in cancer development.     

Cellular oncogenes and cancer

Human oncogenes have been identified either by the ability of normal or tumor DNAs to induce transformation of cells in culture or as the targets of chromosome translocations or DNA amplification in neoplasms. By the combination of these approaches, approximately 40 different genes have been implicated as potential contributors to the development of human neoplasms. The proteins which are encoded by these potential human oncogenes include plasma membrane proteins with tyrosine kinase activity, plasma membrane guanine nucleotide binding proteins, cytoplasmic proteins with serine/threonine kinase activity and nuclear proteins. In many tumors, more than 1 potential oncogene has been activated, suggesting that multiple genes may contribute to neoplasm pathogenesis. I will discuss the identification of these genes, their modes of activation, the diversity of their protein products and their potential roles in both neoplastic and normal cells.     

Mutations and epimutations in the origin of cancer

Cancer is traditionally viewed as a disease of abnormal cell proliferation controlled by a series of mutations. Mutations typically affect oncogenes or tumor suppressor genes thereby conferring growth advantage. Genomic instability facilitates mutation accumulation. Recent findings demonstrate that activation of oncogenes and inactivation of tumor suppressor genes, as well as genomic instability, can be achieved by epigenetic mechanisms as well. Unlike genetic mutations, epimutations do not change the base sequence of DNA and are potentially reversible. Similar to genetic mutations, epimutations are associated with specific patterns of gene expression that are heritable through cell divisions. Knudson's hypothesis postulates that inactivation of tumor suppressor genes requires two hits, with the first hit occurring either in somatic cells (sporadic cancer) or in the germline (hereditary cancer) and the second one always being somatic. Studies on hereditary and sporadic forms of colorectal carcinoma have made it evident that, apart from genetic mutations, epimutations may serve as either hit or both. Furthermore, recent next-generation sequencing studies show that epigenetic genes, such as those encoding histone modifying enzymes and subunits for chromatin remodeling systems, are themselves frequent targets of somatic mutations in cancer and can act like tumor suppressor genes or oncogenes. This review discusses genetic vs. epigenetic origin of cancer, including cancer susceptibility, in light of recent discoveries. Situations in which mutations and epimutations occur to serve analogous purposes are highlighted.     

Oncogene-associated tumor antigens as targets for immunotherapy

Cellular antigens encoded by tumor viruses and some antigens encoded by cellular oncogenes offer advantages as targets for immunotherapy by being inextricably associated with the neoplastic phenotype. For example, monoclonal antibodies (MAb) specific for an antigen encoded by the neu oncogene have a direct inhibitory effect on proliferation of antigen-positive tumor cells. Many of the oncogene-encoded cell surface molecules are growth factor receptors, as are some tumor-associated differentiation antigens (TADAs). Therefore, it is not surprising that their level of cancer specificity is similar. There have been some promising findings from using TADAs as targets for various forms of immunotherapy, and one would expect the results to further improve by targeting to molecules that are more directly involved in cell proliferation and/or in maintaining the malignant state.     

Function of oncogenes in cancer development: a changing paradigm

Tumour‐associated oncogenes induce unscheduled proliferation as well as genomic and chromosomal instability. According to current models, therapeutic strategies that block oncogene activity are likely to selectively target tumour cells. However, recent evidences have revealed that oncogenes are only essential for the proliferation of some specific tumour cell types, but not all. Indeed, the latest studies of the interactions between the oncogene and its target cell have shown that oncogenes contribute to cancer development not only by inducing proliferation but also by developmental reprogramming of the epigenome. This provides the first evidence that tumorigenesis can be initiated by stem cell reprogramming, and uncovers a new role for oncogenes in the origin of cancer. Here we analyse these evidences and propose an updated model of oncogene function that can explain the full range of genotype–phenotype associations found in human cancer. Finally, we discuss how this vision opens new avenues for developing novel anti‐cancer interventions.     

Oncogene activation induces metabolic transformation resulting in insulin-independence in human breast cancer cells

Normal breast epithelial cells require insulin and EGF for growth in serum-free media. We previously demonstrated that over expression of breast cancer oncogenes transforms MCF10A cells to an insulin-independent phenotype. Additionally, most breast cancer cell lines are insulin-independent for growth. In this study, we investigated the mechanism by which oncogene over expression transforms MCF10A cells to an insulin-independent phenotype. Analysis of the effects of various concentrations of insulin and/or IGF-I on proliferation of MCF10A cells demonstrated that some of the effects of insulin were independent from those of IGF-I, suggesting that oncogene over expression drives a true insulin-independent proliferative phenotype. To test this hypothesis, we examined metabolic functions of insulin signaling in insulin-dependent and insulin-independent cells. HER2 over expression in MCF10A cells resulted in glucose uptake in the absence of insulin at a rate equal to insulin-induced glucose uptake in non-transduced cells. We found that a diverse set of oncogenes induced the same result. To gain insight into how HER2 oncogene signaling affected increased insulin-independent glucose uptake we compared HER2-regulated gene expression signatures in MCF10A and HER2 over expressing MCF10A cells by differential analysis of time series gene expression data from cells treated with a HER2 inhibitor. This analysis identified genes specifically regulated by the HER2 oncogene, including VAMP8 and PHGDH, which have known functions in glucose uptake and processing of glycolytic intermediates, respectively. Moreover, these genes specifically implicated in HER2 oncogene-driven transformation are commonly altered in human breast cancer cells. These results highlight the diversity of oncogene effects on cell regulatory pathways and the importance of oncogene-driven metabolic transformation in breast cancer.     

Signal transduction events in mammalian cells in response to ionizing radiation

Ionizing radiations elicit a variety of biological effects in mammalian cells. In recent years altered signal transduction has been recognized as a key cellular response to ionizing radiation. Several oncogenes, the products of which are components of signal transduction pathways and which are over-expressed in many tumors, are specifically induced in cells exposed to radiation. It has also become evident that the oncogene ras and the serine/threonine protein kinase oncogenes raf and PKC confer radio-resistance to tumor cells. Modulation of these genes or their activity by natural compounds may offer a strategy to treat cancer by enhancing radiation-induced apoptosis of tumor cells.     

Amplification of cellular oncogenes in cancer cells

Regulatory or structural alterations of cellular oncogenes have been implicated in the causation of various cancers. Oncogene alteration by point mutations can result in a protein product with strongly enhanced oncogenic potential. Aberrant expression of cellular oncogenes may be due to tumor-specific chromosomal translocations that dysregulate the normal functions of a proto-oncogene. Amplification of cellular oncogenes can also augment their expression by increasing the amount of DNA template available for the production of mRNA. It appears that amplification of certain oncogenes is a common correlate of the progression of some tumours and also occurs as a rare sporadic event affecting various oncogenes in different types of cancer. Amplified copies of oncogenes may or may not be associated with chromosomal abnormalities signifying DNA amplification: double minute chromosomes and homogeneously staining chromosomal regions. Amplified oncogenes, whether sporadic or tumour type-specific, are expressed at elevated levels, in some cases in cells where their diploid forms are normally silent. Increased dosage of an amplified oncogene may contribute to the multistep progression of at least some cancers.