Cancer initiation and progression: involvement of stem cells and the microenvironment

The molecular events that lead to the cancer-initiating cell involve critical mutations in genes regulating normal cell growth and differentiation. Cancer stem cells, or cancer initiating cells have been described in the context of acute myeloid leukemia, breast, brain, bone, lung, melanoma and prostate. These cells have been shown to be critical in tumor development and should harbor the mutations needed to initiate a tumor. The origin of the cancer stem cells is not clear. They may be derived from stem cell pools, progenitor cells or differentiated cells that undergo trans-differentiation processes. It has been suggested that cell fusion and/or horizontal gene transfer events, which may occur in tissue repair processes, also might play an important role in tumor initiation and progression. Fusion between somatic cells that have undergone a set of specific mutations and normal stem cells might explain the extensive chromosomal derangements seen in early tumors. Centrosome deregulation can be an integrating factor in many of the mechanisms involved in tumor development. The regulation of the balance between cell renewal and cell death is critical in cancer. Increased knowledge of developmental aspects in relation to self-renewal and differentiation, both under normal and deregulated conditions, will probably shed more light on the mechanisms that lead to tumor initiation and progression.     

Surviving Drosophila eye development: integrating cell death with differentiation during formation of a neural structure

Normal differentiation requires an appropriately orchestrated sequence of developmental events. Regulation of cell survival and cell death is integrated with these events to achieve proper cell number, cell type, and tissue structure. Here we review regulation of cell survival in the context of a precisely patterned neural structure: the Drosophila compound eye. Numerous mutations lead to altered differentiation and are frequently accompanied by altered patterns of cell death. We discuss various critical times of normal eye development, highlighting how inappropriate regulation of cell death contributes to different mutant phenotypes associated with genes that specify the entire eye primordia, others that pattern the retina, and those that eliminate extraneous cells to refine the precise pigment cell lattice. Finally, we address how the Drosophila eye may allow identification of additional mechanisms that contribute to the normal integration of cell survival with appropriate events of cellular differentiation.     

Mitochondrial membrane potential in cardiac myocytes

Mitochondria are involved in cellular functions that transcend the traditional role of these organelles as the energy factory of the cell. Their relative inaccessibility and the difficulties involved in attempts to study them in their natural environment -- the cytosol -- has delayed much of this understanding and they still have many secrets to yield. One of the relatively new fields in this respect is undoubtedly the analysis of mitochondrial membrane potential. The realization that its alteration may have important pathophysiological consequences has led to an increased interest in measuring this variable in a variety of biological settings, including cardiovascular diseases. Measurements of mitochondrial membrane potential tell us much about the role of mitochondria in normal cell function and in processes leading to cell death. However, we must be aware of the limitations of using isolated mitochondria, single cells and different fluorescent indicators.     

Many genomic regions are required for normal embryonic programmed cell death in Caenorhabditis elegans

To identify genes involved in programmed cell death (PCD) in Caenorhabditis elegans, we screened a comprehensive set of chromosomal deficiencies for alterations in the pattern of PCD throughout embryonic development. From a set of 58 deficiencies, which collectively remove approximately 74% of the genome, four distinct classes were identified. In class I (20 deficiencies), no significant deviation from wild type in the temporal pattern of cell corpses was observed, indicating that much of the genome does not contain zygotic genes that perform conspicuous roles in embryonic PCD. The class II deficiencies (16 deficiencies defining at least 11 distinct genomic regions) led to no or fewer-than-normal cell corpses. Some of these cause premature cell division arrest, probably explaining the diminution in cell corpse number; however, others have little effect on cell proliferation, indicating that the reduced cell corpse number is not a direct result of premature embryonic arrest. In class III (18 deficiencies defining at least 16 unique regions), an excess of cell corpses was observed. The developmental stage at which the extra corpses were observed varied among the class III deficiencies, suggesting the existence of genes that perform temporal-specific functions in PCD. The four deficiencies in class IV (defining at least three unique regions), showed unusually large corpses that were, in some cases, attributable to extremely premature arrest in cell division without a concomitant block in PCD. Deficiencies in this last class suggest that the cell death program does not require normal embryonic cell proliferation to be activated and suggest that while some genes required for cell division might also be required for cell death, others are not. Most of the regions identified by these deficiencies do not contain previously identified zygotic cell death genes. There are, therefore, a substantial number of as yet unidentified genes required for normal PCD in C. elegans.     

Characterizing rice lesion mimic mutants and identifying a mutant with broad-spectrum resistance to rice blast and bacterial blight

Many plant mutants develop spontaneous lesions that resemble disease symptoms in the absence of pathogen attack. In several pathosystems, lesion mimic mutations have been shown to be involved in programmed cell death, which in some instances leads to enhanced disease resistance to multiple pathogens. We investigated the relationship between spontaneous cell death and disease resistance in rice with nine mutants with a range of lesion mimic phenotypes. All nine mutations are controlled by recessive genes and some of these mutants have stunted growth and other abnormal characteristics. The lesion mimics that appeared on the leaves of these mutants were caused by cell death as measured by trypan blue staining. Activation of six defense-related genes was observed in most of the mutants when the mimic lesions developed. Four mutants exhibited significant enhanced resistance to rice blast. One of the mutants, spl11, confers non-race-specific resistance not only to blast but also to bacterial blight. The level of resistance in the spl11 mutant to the two pathogens correlates with the defense-related gene expression and lesion development on the leaves. The results suggest that some lesion mimic mutations in rice may be involved in disease resistance, and cloning of these genes may provide a clue to developing broad-spectrum resistance to diverse pathogens.     

The molecular biology of cancer

The process by which normal cells become progressively transformed to malignancy is now known to require the sequential acquisition of mutations which arise as a consequence of damage to the genome. This damage can be the result of endogenous processes such as errors in replication of DNA, the intrinsic chemical instability of certain DNA bases or from attack by free radicals generated during metabolism. DNA damage can also result from interactions with exogenous agents such as ionizing radiation, UV radiation and chemical carcinogens. Cells have evolved means to repair such damage, but for various reasons errors occur and permanent changes in the genome, mutations, are introduced. Some inactivating mutations occur in genes responsible for maintaining genomic integrity facilitating the acquisition of additional mutations. This review seeks first to identify sources of mutational damage so as to identify the basic causes of human cancer. Through an understanding of cause, prevention may be possible. The evolution of the normal cell to a malignant one involves processes by which genes involved in normal homeostatic mechanisms that control proliferation and cell death suffer mutational damage which results in the activation of genes stimulating proliferation or protection against cell death, the oncogenes, and the inactivation of genes which would normally inhibit proliferation, the tumor suppressor genes. Finally, having overcome normal controls on cell birth and cell death, an aspiring cancer cell faces two new challenges: it must overcome replicative senescence and become immortal and it must obtain adequate supplies of nutrients and oxygen to maintain this high rate of proliferation. This review examines the process of the sequential acquisition of mutations from the prospective of Darwinian evolution. Here, the fittest cell is one that survives to form a new population of genetically distinct cells, the tumor. This review does not attempt to be comprehensive but identifies key genes directly involved in carcinogenesis and demonstrates how mutations in these genes allow cells to circumvent cellular controls. This detailed understanding of the process of carcinogenesis at the molecular level has only been possible because of the advent of modern molecular biology. This new discipline, by precisely identifying the molecular basis of the differences between normal and malignant cells, has created novel opportunities and provided the means to specifically target these modified genes. Whenever possible this review highlights these opportunities and the attempts being made to generate novel, molecular based therapies against cancer. Successful use of these new therapies will rely upon a detailed knowledge of the genetic defects in individual tumors. The review concludes with a discussion of how the use of high throughput molecular arrays will allow the molecular pathologist/therapist to identify these defects and direct specific therapies to specific mutations.     

Altered miRNA expression in canine retinas during normal development and in models of retinal degeneration

Background

Although more than 246 loci/genes are associated with inherited retinal diseases, the mechanistic events that link genetic mutations to photoreceptor cell death are poorly understood. miRNAs play a relevant role during retinal development and disease. Thus, as a first step in characterizing miRNA involvement during disease expression and progression, we examined miRNAs expression changes in normal retinal development and in four canine models of retinal degenerative disease.

Results

The initial microarray analysis showed that 50 miRNAs were differentially expressed (DE) early (3 vs. 7 wks) in normal retina development, while only 2 were DE between 7 and 16 wks, when the dog retina is fully mature. miRNA expression profiles were similar between dogs affected with xlpra2, an early-onset retinal disease caused by a microdeletion in RPGRORF15, and normal dogs early in development (3 wks) and at the peak of photoreceptor death (7 wks), when only 2 miRNAs were DE. However, the expression varied much more markedly during the chronic cell death stage at 16 wks (118 up-/55 down-regulated miRNAs). Functional analyses indicated that these DE miRNAs are associated with an increased inflammatory response, as well as cell death/survival. qRT-PCR of selected apoptosis-related miRNAs (“apoptomirs”) confirmed the microarray results in xlpra2, and extended the analysis to the early-onset retinal diseases rcd1 (PDE6B-mutation) and erd (STK38L-mutation), as well as the slowly progressing prcd (PRCD-mutation). The results showed up-regulation of anti-apoptotic (miR-9, -19a, -20, -21, -29b, -146a, -155, -221) and down-regulation of pro-apoptotic (miR-122, -129) apoptomirs in the early-onset diseases and, with few exceptions, also in the prcd-mutants.

Conclusions

Our results suggest that apoptomirs might be expressed by diseased retinas in an attempt to counteract the degenerative process. The pattern of expression in diseased retinas mirrored the morphology and cell death kinetics previously described for these diseases. This study suggests that common miRNA regulatory mechanisms may be involved in retinal degeneration processes and provides attractive opportunities for the development of novel miRNA-based therapies to delay the progression of the degenerative process.

Electronic supplementary material

The online version of this article (doi:10.1186/1471-2164-15-172) contains supplementary material, which is available to authorized users.     

Genetically Induced Abnormalities in Drosophila: Two or Three Patterns?

SYNOPSIS. The removal of a portion of a Drosophila imaginaldisc stimulates pattern regulation whereby the missing portionis regenerated or extra copies of the surviving portion (duplicationsand triplications) are produced. The results of recent experimentsin which temperature-sensitive cell-lethal mutations were usedto induce pattern regulation are considered here in an attemptto answer a major question: What is the nature of the pattern-formingsystem in the newly formed tissues? One current hypothesis isthat the observed pattern regulation phenotypes are the resultof interactions between portions of a single pattern. The evidencefrom studies in which Drosophila genetic investigative techniqueswere applied to developing leg duplications indicate that initiationand growth of the duplicate are similar or identical to normaldevelopment in terms of cell number, growth rate and patternof cell lineage (compartments). This suggests that portionsof the genetic mechanism underlying normal development may bereactivated and used to control abnormal development. Leg triplicationsconsist of one complete set of leg structures and two partialsets arranged in a mirror symmetrical cuticular process. Theseprocesses either become more or less complete distally (theydiverge or converge, respectively) by the addition or deletionof structures at the lines of symmetry. Whether a particularprocess diverges or converges is related to its circumferentiallocation. These leg duplication and triplication phenotypescan be explained as the result of cell death in situ followedby interaction between the surviving portions of the originalleg pattern following the rules of the polar coordinate modelof positional information.     

Bringing Down Cancer Aircraft: Searching for Essential Hypomutated Proteins in Skin Melanoma

We propose an approach to detection of essential genes/proteins required for cancer cell survival. A gene is considered essential if a mutation with high impact upon the function of encoded protein causes death of the cancer cell. We draw an analogy between essential cancer proteins and well-known Abraham Wald’s work on estimating the plane critical areas using data on survivability of aircraft encountering enemy fire. Wald reasoned that parts with no bullet holes on the airplanes returned to the airbase from a combat flight are the most crucial ones for the airplane functioning: a hit in one of these parts downs an airplane, so it does not return back for the survey. We have envisaged that the airplane surface is a cancer genome and the bullets are somatic mutations with high impact upon protein function. Similarly we propose that genes specifically essential for tumor cell survival should carry less high-impact mutations in cancer cells compared to polymorphisms found in normal cells. We used data on mutations from the Cancer Genome Atlas and polymorphisms found in healthy humans (from 1000 Genomes Project) to predict 91 protein-coding genes essential for melanoma. These genes were selected according to several criteria, including negative selection, expression in melanocytes and decrease in the proportion of high-impact mutations in cancer compared with normal cells. The Gene Ontology analysis revealed enrichment of essential proteins related to membrane and cell periphery. We speculate that this could be a sign of immune system-driven negative selection of cancer neo-antigens. Another finding is the overrepresentation of semaphorin receptors, which can mediate distinctive signaling cascades and are involved in various aspects of tumor development. Cytokine receptors CCR5 and CXCR1 were also identified as cancer essential proteins and this is confirmed by other studies. Overall, our goal was to illustrate the idea of detecting proteins whose sequence integrity and functioning is important for cancer cell survival. Hopefully, this prediction of essential cancer proteins may point to new targets for anti-tumor therapies.     

TRAIL and vitamins: opting for keys to castle of cancer proteome instead of open sesame

ABSTRACT: Cancer is a multifaceted molecular disorder that is modulated by a combination of genetic, metabolic and signal transduction aberrations, which severely impair the normal homeostasis of cell growth and death. Accumulating findings highlight the fact that different genetic alterations, such as mutations in tumor suppressor genes, might be related to distinct and differential sensitivity to targeted therapies. It is becoming increasingly apparent that a multipronged approach that addresses genetic milieu (alterations in upstream and/or parallel pathways) eventually determines the response of individual tumors to therapy. Cancerous cells often acquire the ability to evade death by attenuating cell death pathways that normally function to eliminate damaged and harmful cells. Therefore impaired cell death nanomachinery and withdrawal of death receptors from cell surface are some of major determinants for the development of chemotherapeutic resistance encountered during treatment. It is therefore essential to emphasize underlying factors which predispose cells to refractoriness against TRAIL mediated cell death pathway and the relevant regulatory components involved. We bring to limelight the strategies to re-sensitize TRAIL resistant cells via vitamins to induce apoptosis.     

Neurogenesis and cell cycle-reactivated neuronal death during pathogenic tau aggregation

The aim of the present study was to investigate the relation between neurogenesis, cell cycle reactivation and neuronal death during tau pathology in a novel tau transgenic mouse line THY-Tau22 with two frontotemporal dementia with parkinsonism linked to chromosome-17 mutations in a human tau isoform. This mouse displays all Alzheimer disease features of neurodegeneration and a broad timely resolution of tau pathology with hyperphosphorylation of tau at younger age (up to 6 months) and abnormal tau phosphorylation and tau aggregation in aged mice (by 10 months). Here, we present a follow-up of cell cycle markers with aging in control and transgenic mice from different ages. We show that there is an increased neurogenesis during tau hyperphosphorylation and cell cycle events during abnormal tau phosphorylation and tau aggregation preceding neuronal death and neurodegeneration. However, besides phosphorylation, other mechanisms including tau mutations and changes in tau expression and/or splicing may be also involved in these mechanisms of cell cycle reactivation. Altogether, these data suggest that cell cycle events in THY-Tau22 are resulting from neurogenesis in young animals and cell death in older ones. It suggests that neuronal cell death in such models is much more complex than believed.     

Chronic lymphocytic leukemia: A disease of dysregulated programmed cell death

Chronic lymphocytic leukemia (CLL) represents a quintessential example of a human malignancy which is caused principally by defects that prevent cell turnover due to programmed cell death rather than by alterations in cell cycle regulation. In the vast majority of patients, CLL cells are predominantly G₀ quiescent lymphocytes that gradually accumulate in the patient's body not because they are dividing more rapidly than normal, but because they are surviving too long. Investigations of the genetics of CLL therefore seem likely to teach us much about the molecular mechanisms that regulate programmed cell death (PCD). Moreover, since defects in the pathway for PCD can render neoplastic cells resistant to the cytotoxic effects of chemotherapeutic drugs and radiation, investigations of the aberrant regulation of cell death in CLL may also prove informative for gaining a better understanding of drug- and radiation-resistance mechanisms. In this review, I summarize current knowledge about the cytogenetic abnormalities associated with CLL and the role of deregulated cell death in the pathogenesis of this most common of the adult leukemias.     

Mycotoxins reveal connections between plants and animals in apoptosis and ceramide signaling

Plants undergo programmed cell death during development and disease in contexts that are functionally analogous to apoptosis in animals. Recent studies involving plant cell death induced by mycotoxins, pathogens and lethal mutations along with the cell-autonomous death during development now point to several conserved connections to apoptosis in animals. Morphological markers indicative of apoptosis recently reported in plants include TUNEL positive cells, DNA ladders, Ca2+-activated nucleosomal DNA cleavage, and formation of apoptotic-like bodies that occur in some but not all situations involving ordered cell death. In parallel studies with animal and plant cells treated with sphinganine analog mycotoxins our results indicate that the induction and inhibition of death may be mediated by ceramide-linked signaling systems. The presence and significance of ceramide-linked second messenger systems is well documented in animals but is virtually unknown in plants. Further research will discern the manner in which the important function of programmed cell death is conserved as well as diverged between the two kingdoms.     

Baculovirus p35 prevents developmentally programmed cell death and rescues a ced-9 mutant in the nematode Caenorhabditis elegans.

Programmed cell death, or apoptosis, occurs throughout the course of normal development in most animals and can also be elicited by a number of stimuli such as growth factor deprivation and viral infection. Certain morphological and biochemical characteristics of programmed cell death are similar among different tissues and species. During development of the nematode Caenorhabditis elegans, a single genetic pathway promotes the death of selected cells in a lineally fixed pattern. This pathway appears to be conserved among animal species. The baculovirus p35-encoding gene (p35) is an inhibitor of virus-induced apoptosis in insect cells. Here we demonstrate that expression of p35 in C. elegans prevents death of cells normally programmed to die. This suppression of developmentally programmed cell death results in appearance of extra surviving cells. Expression of p35 can rescue the embryonic lethality of a mutation in ced-9, an endogenous gene homologous to the mammalian apoptotic suppressor bcl-2, whose absence leads to ectopic cell deaths. These results support the hypothesis that viral infection can activate the same cell death pathway as is used during normal development and suggest that baculovirus p35 may act downstream or independently of ced-9 in this pathway.     

Does neuronal loss in Parkinson's disease involve programmed cell death?

Recently it has been hypothesized that apoptotic cell death is involved in several neuropathological conditions including Parkinson's disease (PD). Initial morphological studies assessing the presence of apoptosis in Parkinsonian brain tissues yielded mixed results. Based on more recent studies in human PD brains as well in animal and cell culture models of the disease, a picture is emerging, however, that strongly suggests that many of the molecular players thought to participate in this type of neuronal cell death are active in the disease. The task of researchers in the field is now to deduce how these players may be interacting with one another to bring about cell death in PD and to design effective therapies to interfere with these processes.