Current Molecular Medicine - Volume 3, Issue 7, 2003

Cancer is a disease that has been in existence throughout the history of the human race. The term “carcinoma” was coined as early as in the fourth century B.C. Cancer is a unique and most feared malady that humankind has ever faced. At present, in technologically developed countries one out of every three individuals will be affected with cancer during his or her lifetime, and the prediction is that this may increase to one in two by the next decade. The last century, particularly the last three decades have witnessed remarkable progress in our understanding of the molecular basis of how cancer cells originate and spread to distant organs. It has now been well established that accumulated mutations of a cascade of specific genes in a single cell are required for a normal cell to become a cancer cell. The aim of this special issue of the Current Molecular Medicine on cancer is to provide comprehensive reviews of a number of key alterations in cell physiology, mentioned below, that collectively are considered to be involved in the genesis of cancer. First, genomic instability in cancer cells is considered to be the primary event by which a single cell may accumulate multiple genetic mutations that allow it to acquire properties that are crucial to become a cancer cell. Almost all cancer cells show abnormalities in chromosome structure and number. Even progenies of a cancer cell do not retain the structural and numerical changes in chromosomes present in the parental cell. Second, normal cells possess intrinsic genetic program that limits their replicative potential and leads to senescence. Cancer cells circumvent senescence and become immortal. Certain genes are up regulated in a tumor cell to sustain its limitless replicative potential. Third, there are a group of genes known as tumor suppressor genes, which normally prevent unscheduled cell division. Mutation or loss of tumor suppressor genes induces uncontrolled division of mutated cells in the body, which may lead to the development of cancer. Fourth, the balance between the generation of new cells and genetically programmed cell death, called apoptosis maintains tissue homeostasis (stability of the organ mass). This apoptotic pathway is disturbed in cancer cells due to mutations of specific genes, causing increased tumor mass. Fifth, as the tumor mass increases, it requires the formation of new blood vessels to supply cells with appropriate nutrients for further growth of the tumor. The sprouting of new vessles from the existing vessles is called angiogenesis. In cancer cells induction and maintenance of such a process occur due to gene mutations which stimulate synthesis of angiogenic factors. Sixth, a cancer cell must acquire the ability to invade surrounding tissues, and penetrate blood and Iymphatic vessels. Which allow the cell to migrate to distant organs. The cancer cell acquires this property by functional elimination of genes that suppress the cell's ability to invade tissues and metastasize. This compilation of reviews should help the reader conceptualize the unique nature of the disease called cancer, and to understand the reason why inspite of the enormous efforts that are being made by researchers worldwide, a cure for cancer has not yet been found. The real problem is when a single cell is converted to a cancer cell and starts dividing in an uncontrolled manner, and its progenies spread all over the body, each and every one of such cells cannot be killed without causing fatal side effects. Based on our present knowledge of cancer at the molecular and cellular levels, attempts are being made by researchers worldwide to try out various approaches to cure cancer. Using such approaches, significant progress has been made in the detection of some cancers at an early and treatable stage. Combination of presently available therapies, customized for specific cancer patients have proved successful in some cases. Improved drugs for chemotherapy, and radiation therapy have prolonged remission time for many cancer patients. However, the development of a magic bullet for curing most invading cancers appears remote at present. The major problem has to do with the fact that once the cancer metastasizes, it is virtually impossible to kill each and every cancer cell in the system, and a single surviving malignant cell may eventually cause the cancer to reappear. This has been the heart of the problem in curing metastasizing cancers, irrespective of the therapies that are presently being used. Thus, it may be more prudent to refocus our research priorities, and direct the bulk of the available research resources to further improve technologies for early detection and implementation of preventive measures to fight cancer. Let us hope that Dr. Robert A. Weinberg is right when he wrote - “One day, we imagine that cancer biology and treatment - at present, a patchwork quilt of cell biology, genetics, histopathology, biochemistry, immunology, and pharmacology - will become a science with a conceptual and logical coherence that rivals that of chemistry or physics.........those researching the cancer problem will be practicing a dramatically different type of science than we have experienced over the past 25 years”.

Tumorigenesis can be viewed as an imbalance between the mechanisms of cell-cycle control and mutation rates within the genes. Genomic instability is broadly classified into microsatellite instability (MIN) associated with mutator phenotype, and chromosome instability (CIN) recognized by gross chromosomal abnormalities. Three intracellular mechanisms are involved in DNA damage repair that leads to mutator phenotype. They include the nucleotide excision repair (NER), base excision repair (BER) and mismatch repair (MMR). The CIN pathway is typically associated with the accumulation of mutations in tumor suppressor genes and oncogenes. Defects in DNA MMR and CIN pathways are responsible for a variety of hereditary cancer predisposition syndromes including hereditary non-polyposis colorectal carcinoma (HNPCC), Bloom syndrome, ataxia-telangiectasia, and Fanconi anaemia. While there are many genetic contributors to CIN and MIN, there are also epigenetic factors that have emerged to be equally damaging to cell-cycle control. Hypermethylation of tumor suppressor and DNA MMR gene promoter regions, is an epigenetic mechanism of gene silencing that contributes to tumorigenesis. Telomere shortening has been shown to increase genetic instability and tumor formation in mice, underscoring the importance of telomere length and telomerase activity in maintaining genomic integrity. Mouse models have provided important insights for discovering critical pathways in the progression to cancer, as well as to elucidate cross talk among different pathways. This review examines various molecular mechanisms of genomic instability and their relevance to cancer.

Altering the rate of translation initiation of a specific gene can tightly regulate the synthesis of the corresponding polypeptide and is an important mechanism in the control of gene expression. For some time it has been known that many genes involved in cell proliferation, cell growth and apoptosis have atypical 5' untranslated regions (UTRs) containing a high degree of RNA secondary structure, upstream open reading frames and internal ribosome entry segments. These features play a key role in the regulation of protein synthesis. In this review we discuss how the rate of translation initiation of proto-oncogenes and tumour suppressor genes is affected by elements in their 5' and 3' UTRs and we focus on how changes in the control of gene expression at this level can contribute towards tumorigenesis.

Our current definitions of the tumor suppressor gene (TSG) have been guided by the identification of the prototypical gene, RB1, a TSG that is implicated in the development of both the inherited and sporadic forms of retinoblastoma. The hallmark feature of this TSG is loss of function in tumoral cells, which can be restored by reintroduction of a normally functioning protein with concomitant reversion of tumorigenicity. Key to this discovery was that loss of function is often achieved by deletion of a normal copy of the TSG and retention of a mutated allele, which was either inherited or acquired. Suppression of tumorigenicity and the loss-of-function concept of TSGs was also demonstrated in early studies where normal cellular growth was achieved when tumorigenic cells were fused with normal cells. Thus loss of genetic content and restoration of gene function has guided studies aimed at the discovery of novel TSGs. Here we review the successes of TSG discovery using three approaches that are based on the genetic analysis of inherited predisposition to cancer, tumors that display chromosome loss, and tumorigenic cells that display a suppression of tumorigenicity as a result of transfer of normal chromosomes. Based on a review of the literature we conclude that the discovery of TSGs has been highly successful in the genetic analysis of inherited predisposition to cancer with a dominant mode of inheritance. In contrast, the latter two approaches have yielded a paucity of TSGs that exhibit features similar to the prototypical RB1 in that they are rarely inactivated by somatic mutations in tumors displaying LOH, although decreased gene expression is observed. Nevertheless, some of these genes have been shown to suppress tumorigenicity when normal function is restored in tumorigenic cells consistent with the loss-of-function concept. These observations continue to challenge our current definition of TSG.

Metastasis, the process by which cancer spreads from a primary to a secondary site, is responsible for the majority of cancer related deaths. Yet despite the detrimental effects of metastasis, it is an extremely inefficient process by which very few of the cells that leave the primary tumor give rise to secondary tumors. Metastasis can be considered as a series of sequential steps that begins with a cell leaving a primary tumor, and concludes with the formation of a metastatic tumor in a distant site. During the process of metastasis cells are subjected to various apoptotic stimuli. Thus, in addition to genetic changes that promote unregulated proliferation, successful metastatic cells must have a decreased sensitivity to apoptotic stimuli. As many cancer cells exhibit aberrations in the level and function of key apoptotic regulators, exploiting these alterations to induce tumor cell apoptosis offers a promising therapeutic target. This review will examine the apoptotic regulators that are often aberrantly expressed in metastatic cells; the role that these regulators may play in metastasis; the steps of metastasis and their susceptibility to apoptosis; and finally, current and future cancer prognostics and treatment targets based on apoptotic regulators.

The process of angiogenesis encompasses the growth and regression of capillary blood vessels. Angiogenesis is finely regulated at the molecular and genetic levels, not unlike other physiologic processes such as coagulation, glucose metabolism, and blood pressure. During the development of the field of angiogenesis research over the past three decades, fundamental concepts have been introduced along the way in an attempt where possible, to unify new data from a variety of different laboratories. I have assembled here the major concepts which underlie the angiogenic process as we currently understand it. Many of these are now taken for granted, but this was not always the case, and I have tried to show how they were developed. My goal is to provide a conceptual framework for those basic scientists or clinicians who may enter this rapidly expanding field. Each concept discussed here is accompanied by a few key references as a guide to the pertinent literature.

Cancer is one of the leading causes of mortality in developed countries such as the USA. In 1998, there were more than 280,000 and 250,000 cancer related deaths in males and females, respectively. In males, lung and prostate cancers accounted for almost half of these deaths, whereas in females, lung and breast cancers were the leading causes of cancer mortalities. Therefore, the study of cancer has been of the utmost importance to patients, doctors, and researchers alike. A variety of cellular processes occur in the precancerous cells that contribute to the development and progression to cancer. Not surprisingly, all of these cellular processes have been targeted for anticancer therapy. A novel serpin, maspin, has demonstrated a robust effect on a variety of these cancer progression steps. A number of studies have shown that maspin inhibits angiogenesis and tumor cell growth and invasion both in vitro and in vivo. In addition, maspin promotes cell adhesion to the basement membrane and extracellular matrix components. Efforts underway to understand the molecular mechanisms involved in the diverse functions influenced by maspin have yielded promising results and shed light on the cancer pathways.

As tumors progress to increased malignancy, cells within them develop the ability to invade into surrounding normal tissues and through tissue boundaries to form new growths (metastases) at sites distinct from the primary tumor. The molecular mechanisms involved in this process are incompletely understood but those associated with cell-cell and cell-matrix adhesion, with the degradation of extracellular matrix, and with the initiation and maintenance of early growth at the new site are generally accepted to be critical. This article discusses current knowledge of molecular events involved in these various processes. The potential role of adhesion molecules (eg. integrins and cadherins) has undergone a major transition over the last ten years, as it has become apparent that such molecules play a major role in signaling from outside to inside a cell, thereby controlling how a cell is able (or not) to sense and interact with its local environment. Similarly the roles of proteolytic enzymes and their inhibitors (eg. matrix metalloproteinases and TIMPs) have also expanded as it has become apparent that they not only have the abilities to break down the components of the extracellular matrix but also are involved in the release of factors which can affect the growth of the tumor cells positively or negatively. Recent work has highlighted the importance of the later, postextravasational stages of metastasis, where adhesion and proteolysis are now known to play a role along with other processes such as apoptosis, dormancy, growth factor-receptor interactions and signal transduction. Recent work has also demonstrated that not only the immediate cellular microenvironment, in terms of specific cell-cell and cell-matrix interactions, but also the extended cellular microenvironment, in terms of vascular insufficiency and hypoxia in the primary tumor, can modify cellular gene expression and enhance metastasis. Mechanisms of metastasis appear to involve a complex array of genetic and epigenetic changes many of which appear to be specific both for different types of tumors and for different sites of metastasis. Our improved understanding of the expanded roles of the individual molecules involved has resulted in a mechanistic blurring of the previously described discrete stages of the metastatic process.