Current Molecular Medicine - Volume 6, Issue 7, 2006

OVERVIEW The concept that the retinoblastoma arises as a result of two discrete genetic hits in the same tumor suppressor gene has been in existence for greater than 20 years [1-4]. Cloning and analyses of the retinoblastoma tumor suppressor gene (Rb) revealed that this same tumor suppressor is mutated not only in retinoblastoma, but in a litany of other tumor types (e.g. bladder cancer, osteosarcoma, lung cancer and breast cancer) [5-9]. Subsequent studies demonstrated that in many additional tumors, the RB protein can be inactivated by a multitude of mechanisms [10-12]. For example, the E7 protein of human papilloma virus directly binds and inactivates the RB protein in cervical cancer [13, 14]. Additionally, RB is functionally inactivated by deregulated phosphorylation in those tumors which lack the p16ink4a tumor suppressor or harbor excessive CDK4 and cyclin D1 oncogenes [10, 12, 15]. The frequency of functional disruption of the retinoblastoma tumor suppressor in human cancers has precipitated significant efforts to define its modes of action. In general, these studies have been focused on two areas: 1. Defining Physiological/Biochemical Function Analyses of RB function in vitro or in cell culture models have defined the mechanisms through which RB is regulated and those biological processes which are governed by RB. RB action encompasses control of cell cycle, regulation of apoptosis, control of genomic stability, and modulation of differentiation [10, 16-19]. Each process is known to be altered in human cancer, and the influence of RB on each pathway is reviewed in this issue. 2. Delineating Tissue Specific Actions in Tumor Suppression RB and critical interacting factors are conserved in metazoans. As a result, it has been possible to study the functionality of RB in multiple organisms [20-23]. These models have been used to uncover the consequence of RB loss related to organismal development and tumorigenesis, and have revealed that RB likely utilizes distinct mechanisms to suppress tumorigenesis in specific tissues. Corresponding reviews discuss the disparate functions of RB in discrete model organisms, context dependent RB action and implications for tumor suppression. Taken together the body of work described in this issue and additional research, which regrettably due to space constraints could not be included herein, have provided significant insights into the action of RB in cancer. CHALLENGE FOR THE FUTURE Cloning of the Rb tumor suppressor, provided great promise that knowledge of such inhibitors of oncogenic proliferation would represent ideal nodes for the treatment of cancer. The concept that molecules like RB could be used to treat cancer was originally supported by studies demonstrating that reintroduction of functional RB protein into retinoblastoma cell lines could inhibit tumorigenic proliferation [24]. Indeed, today we can readily inhibit the proliferation of virtually any tumor cell type via manipulation of RB function [25-27]. While it is well appreciated that restoring the activity of a factor, such as RB, lost in cancer can be quite difficult, there are now clear instances where restoration of tumor suppressive signaling pathways can be achieved through pharmacological means. This is perhaps best demonstrated in the context of the p53 tumor suppressor [28-30], wherein multiple therapeutic agents have been shown to unleash the nascent p53 activity present in many tumor cells. Such approaches can be similarly applied to the RB pathway, wherein in many instances RB inactivation is achieved not through mutation but through other mechanisms. Unfortunately, RB has apparently failed to receive the level of exploration directed against p53 and other tumor suppressive pathways. Since RB loss occurs in many cancers and modifies the response to a variety of therapeutic agents [31-33], RB status could provide a critical diagnostic/prognostic basis upon with to direct treatment. While this concept is not new and a plethora of studies have evaluated RB status in disease outcome and therapeutic response, testing for RB status is largely confined to retinoblastoma susceptibility and is not yet used as the basis to direct therapy [34-37]. In summary, identification and study of the RB tumor suppressor has provided an essential knowledge basis for delineating endogenous mechanisms that protect against tumor formation. Significant advances in our understanding of RB funtion have unexpectedly revealed that its tumor suppressive activity extends to many disparate cellular pathways and that RB function is often tissue specific. Current challenges remain on how to harness this information and apply this detailed understanding of RB action to the improvement of cancer diagnosis and therapy....

All forms of life on Earth share a common ancestry. As a consequence, Homo sapiens shares a large number of genes essential for the development and maintenance of multicellular life with "simple" animals, such as the fruit fly Drosophila melanogaster and the nematode worm Caenorhabdites elegans. Indeed, Drosophila and C. elegans have successfully been used to unravel fundamental mechanisms underlying animal development. The sequencing of their genomes has revealed that a surprisingly large proportion of genes relevant for human disease have counterparts in the worm and in the fly. This includes many oncogenes and tumour suppressor genes and provides us with a unique opportunity to exploit the advantages of simple model organisms to further our understanding of the molecular basis of cancer. Recent work on the fly and worm homologs of the Retinoblastoma tumour suppressor (pRb) has uncovered some unexpected pRb functions: Evolutionary conserved pRb complexes participate in cell fate determination, repress germline-specific gene expression and interact with RNA interference pathways. Similar complexes appear to operate in human cells.

The retinoblastoma (RB) gene was the first tumor suppressor to be identified, and it continues to be the subject of intense scientific interest. Not only is the RB gene mutated in the rare eye tumor and some other cancers, the Rb protein is functionally inactivated in virtually all human cancers, suggesting that it plays a general role in cellular homeostasis. Rb initially was envisaged as a simple ‘on-off’ regulator of the cell cycle, and this function was thought to account for its role as a tumor suppressor. Subsequently, however, closer scrutiny revealed unexpected and complex properties of Rb that together contribute to the unique role of Rb in cell biology. For example, Rb appears to be dispensable for normal cell cycling, but it has a special role in triggering permanent cell cycle exit associated with differentiation and senescence. Further, although the role of Rb as tumor suppressor is firmly established, it also has the ability to block apoptosis, which is generally thought to be a property of oncogenes. Our lab has been interested in understanding the dual and seemingly incongruous roles of Rb in cell cycle control and apoptosis. For many of these studies, we have chosen the melanocyte lineage as a model cell system because of the established role for Rb in melanocyte differentiation and survival, and the frequent deregulation of the Rb pathway in melanoma.

Regulators of the cell cycle machinery play a major role in modulating a variety of cellular phenomena including proliferation, quiescence, differentiation, senescence and apoptosis. Studies in the past decade have clearly established a role for the retinoblastoma tumor suppressor protein, Rb, and its primary downstream target E2F1, in the above processes. While the role of the Rb protein in the regulation of cell cycle progression has been analyzed in great detail, its potential roles in apoptosis as well as senescence are relatively less studied. It has become increasingly clear that the anti-apoptotic functions of Rb contribute significantly to the genesis and progression of tumors. This is especially relevant in neuronal systems, since terminally differentiated neurons do not proliferate; therefore the normal anti-proliferative functions of Rb in neurons are not very dominant. This chapter describes the current thoughts on the role of Rb function in the apoptosis and senescence of cells, both of neuronal and non-neuronal origin. Recent studies have also addressed how Rb function is differentially modulated by proliferative and apoptotic signals received at the cell surface, though both lead to Rb inactivation. The contribution of Rb to inducing cellular senescence has been long recognized, but the underlying molecular mechanisms are being elucidated only recently; the contribution of this function of Rb to tumor suppression remains to be understood in detail. It can be expected that an understanding of Rb function in cellular apoptosis and senescence will enhance our ability to develop novel agents and strategies to combat cancer.

Deregulation of E2F transcriptional activity as a result of alterations in the p16INK4a-cyclin D1-Rb pathway is a hallmark of human cancer. E2F is a family of related factors that controls the expression of genes important for cell cycle progression as well as other processes such as apoptosis, DNA repair, and differentiation. Some E2F family members are associated with the activation of transcription and the promotion of proliferation while others are implicated in repressing transcription and inhibiting cell growth. It is now becoming clear however, that this view of the E2F family is overly simplistic and that the role of a given E2F in regulating transcription and cell growth is highly dependent on context. This complexity is also evident when analyzing how perturbations in E2F modulate tumor development. As expected, some E2F family members are found to be critical for mediating the oncogenic effects of Rb loss. On the other hand, several E2Fs have tumor suppressive properties in mouse models and this appears to be reflected in some human cancers with decreased E2F expression. Surprisingly, tumor suppressive activity is not associated with the repressor E2Fs but instead is associated with the same E2Fs shown to have oncogenic activities. For example, deregulated E2F1 expression can either promote or inhibit tumorigenesis depending on the nature of the other oncogenic mutations that are present. Thus, the ability of some E2F family members to behave as both oncogene and tumor suppressor gene can be reconciled by putting E2F into context.

Since the discovery almost fifteen years ago that E2F transcription factors are key targets of the retinoblastoma protein (RB), studies of the E2F family have uncovered critical roles in the control of transcription, cell cycle and apoptosis. E2F proteins are encoded by at least eight genes, E2F1 through E2F8. While specific roles for individual E2Fs in mediating the effects of RB loss are emerging, it is also becoming clear that there are no simple divisions of labor among the E2F family. Instead, an individual E2F can function to activate or repress transcription, promote or impede cell cycle progression and enhance or inhibit cell death, dependent on the cellular context. While functional redundancy among E2Fs and the striking influences of cellular context on the effects of E2F loss or gain of function have prevented a simple delineation of unique functions within the E2F family, these complexities undoubtedly reflect the extensive regulation and importance of this transcription factor family.

The retinoblastoma tumor suppressor (RB) is functionally inactivated at high frequency in human cancers. Based on the role of RB as a negative regulator of cell cycle this event would be expected to contribute to deregulated proliferation. However, evidence suggests that loss of RB not only mediates aberrant proliferation, but compromises the fidelity of cell cycle transitions leading to a breakdown in genome integrity. This review is focused on the mechanisms underlying this facet of RB function and the contibution of this process to tumorigenesis.

The RB gene was discovered 20 years ago because of its role in the childhood eye cancer retinoblastoma. However, surprisingly little progress was made in defining the role of RB protein in the retina. In the last two years, new models exploiting conditional deletion of the mouse Rb gene have altered this picture radically. These models provide insight into the first Rb function, the cell of origin of retinoblastoma, the window during which Rb acts, distinct cell-specific defenses against Rb loss, the number and type of post-Rb lesions required for transformation, why pediatric tumors exist, the controversial role of the p53 pathway in retinoblastoma, and the reason why the disease is virtually unique to humans. Two years have dramatically improved our understanding of Rb function in the tissue that gave us this important tumor suppressor.

Lung cancer is the leading cause of cancer related deaths accounting for more deaths than breast, colon and prostate cancers combined. The Rb-p16 regulatory pathway plays an essential role in tumor suppression in the lung epithelium. This is evidenced by the nearly universal alterations in Rbp16 pathway components in lung cancer, and the increased incidence of pulmonary carcinomas in persons with germline Rb mutations. Interestingly, p16 loss and Rb mutations preferentially occur in phenotypically distinct lung cancer subtypes. Analysis of human tumors has identified progressive preneoplastic lesions that accumulate molecular alterations in an orderly sequence. Epigenetic p16 gene modifications represent an early event in lung cancer progression. This review summarizes the human studies that demonstrate a critical role for the Rb-p16 tumor suppressor pathway in lung carcinogenesis, and discusses how these findings in combination with genetically engineered mouse models have significantly contributed to our understanding of lung cancer pathogenesis.

Infection with human papillomaviruses (HPVs) is a major public health burden worldwide and is associated with benign and malignant lesions of the skin and genital tract. HPV causes cervical cancer, which represents the second most prevalent cancer in women worldwide. Functions of the viral oncogenes E6 and E7 are essential for carcinogenesis and for support of the viral life cycle. We will begin by discussing the relationship between HPV infection and disease, followed by a review of E6 and E7 activities and their respective cellular targets. Particular emphasis will be placed on established and newly discovered mechanisms by which E7 inhibits members of the cellular retinoblastoma protein family. We will then describe how current research links the above molecular interactions to malignant transformation as well as to aspects of the viral life cycle in vitro and in vivo. As a result of decades of intense HPV research, promising therapies to prevent infection and to treat HPV associated cancers are now on the horizon. We will conclude our review by a description of potential gene therapeutic and hormonal approaches and of new developments in the design of effective vaccines.

Osteogenic sarcoma (osteosarcoma) is the most common primary tumor of bone. It accounts for approximately 19% of all malignant tumors of the bone. Of all the molecular targets altered during the genesis of osteosarcoma, the retinoblastoma gene (RB1) shows the highest frequency of inactivation. Published data from human osteosarcoma tumors and in vivo and in vitro model systems support a role for the retinoblastoma gene family in bone development and tumorigenesis. Although a variety of bone tumors, depending on the cell of origin, including osteoclasts or osteoclast-like cells, chondroblasts, and fibroblasts, are described, for the purpose of this review we will focus primarily on the tumors arising from the osteoblast lineage.