Carcinogenesis in relation to the stem-cell-mutation hypothesis

Review Paper: Cancer Stem Cells and Cancer Nonstem Cells: From Adult Stem Cells or from Reprogramming of Differentiated Somatic Cells

Two opposing hypotheses of the origin of cancer have existed for many decades. One hypothesis postulates that the adult stem cell is needed to initiate the carcinogenic process, whereas the other claims a somatic differentiated cell can dedifferentiate or be reprogrammed to regain properties associated with cancer cells. Recent reemergence of the cancer stem cell hypothesis and the isolation of presumptive cancer stem cells from many types of cancer have forced a reexamination of these 2 hypotheses of the origin of cancer. In addition, normal embryonic and adult stem cells have now been isolated and partially characterized. Furthermore, the demonstration of embryonic-like stem cells, being isolated from adult-differentiated skin fibroblast cells of mice, monkey, and human beings, provides a newer opportunity to determine which of these 2 hypotheses might explain the cell type for initiating the carcinogenic process. The goal of this review is to integrate these recent findings, concerning the isolation of normal and cancer stem cells, with several of the classical concepts of carcinogenesis (initiation/promotion/progression; mutation/epigenetic; stem cell theory/dedifferentiation hypotheses; oncogenetumor suppressor theory). Although the weight of the evidence in this review seems to support the stem cell hypothesis, only future studies, probably using comparative animal and human oncologic studies, will determine if targeting the cancer stem cell, with individualized medical approaches, will improve cancer prevention and therapy.

Potential of neural stem cells for the treatment of brain tumors

Neural stem cells (NSCs) are self-renewing multipotent cells that generate the main phenotypes of the nervous system, neurons, astrocytes and oligodendrocytes. As such they hold the promise to treat a broad range of neurological diseases and injuries. Neural progenitor and stem cells have been isolated and characterized in vitro, from adult, fetal and post-mortem tissues, providing sources of material for cellular therapy. However, NSCs are still elusive cells and remain to be unequivocally identified and characterized, limiting their potential use for therapy. Neural progenitor and stem cells, isolated and cultured in vitro, can be genetically modified and when transplanted migrate to tumor sites in the brain. These intrinsic properties of neural progenitor and stem cells provide tremendous potential to bolster the translation of NSC research to therapy. It is proposed to combine gene therapy and cellular therapy to treat brain cancers. Hence, neural progenitor and stem cells provide new opportunities for the treatment of brain cancers.

Stem cells, senescence, neosis and self-renewal in cancer

We describe the basic tenets of the current concepts of cancer biology, and review the recent advances on the suppressor role of senescence in tumor growth and the breakdown of this barrier during the origin of tumor growth. Senescence phenotype can be induced by (1) telomere attrition-induced senescence at the end of the cellular mitotic life span (MLS*) and (2) also by replication history-independent, accelerated senescence due to inadvertent activation of oncogenes or by exposure of cells to genotoxins. Tumor suppressor genes p53/pRB/p16INK4A and related senescence checkpoints are involved in effecting the onset of senescence. However, senescence as a tumor suppressor mechanism is a leaky process and senescent cells with mutations or epimutations in these genes escape mitotic catastrophe-induced cell death by becoming polyploid cells. These polyploid giant cells, before they die, give rise to several cells with viable genomes via nuclear budding and asymmetric cytokinesis. This mode of cell division has been termed neosis and the immediate neotic offspring the Raju cells. The latter inherit genomic instability and transiently display stem cell properties in that they differentiate into tumor cells and display extended, but, limited MLS, at the end of which they enter senescent phase and can undergo secondary/tertiary neosis to produce the next generation of Raju cells. Neosis is repeated several times during tumor growth in a non-synchronized fashion, is the mode of origin of resistant tumor growth and contributes to tumor cell heterogeneity and continuity. The main event during neosis appears to be the production of mitotically viable daughter genome after epigenetic modulation from the non-viable polyploid genome of neosis mother cell (NMC). This leads to the growth of resistant tumor cells. Since during neosis, spindle checkpoint is not activated, this may give rise to aneuploidy. Thus, tumor cells also are destined to die due to senescence, but may escape senescence due to mutations or epimutations in the senescent checkpoint pathway. A historical review of neosis-like events is presented and implications of neosis in relation to the current dogmas of cancer biology are discussed. Genesis and repetitive re-genesis of Raju cells with transient "stemness" via neosis are of vital importance to the origin and continuous growth of tumors, a process that appears to be common to all types of tumors. We suggest that unlike current anti-mitotic therapy of cancers, anti-neotic therapy would not cause undesirable side effects. We propose a rational hypothesis for the origin and progression of tumors in which neosis plays a major role in the multistep carcinogenesis in different types of cancers. We define cancers as a single disease of uncontrolled neosis due to failure of senescent checkpoint controls.

Intercellular communication and human prostate carcinogenesis

Gap-junction-mediated intercellular communication (GJIC) is required for completion of embryonic development, tissue homeostasis, and regulation of cell proliferation and death. Although, as emphasized in several reports, defects or disruption of GJIC may be important in carcinogenesis, the potential role of GJIC in the onset and progression of human prostate cancer remains ill-defined. The gap junction channel-forming connexins (Cx) comprise a multigene family of highly conserved proteins that are differentially expressed in a tissue- and development-specific manner; changes in connexin expression are also commonly seen during cellular differentiation. However, when multiple connexins are concurrently expressed, gap junction channels may consist of more than one connexin species. This is important, because only certain pairings give rise to functional channels. In our studies, we investigated GJIC in a panel of both nontumorigenic (RWPE-1) and malignant (RWPE-2, LNCaP, DU-145) human prostate epithelial cells, compared to a normal rat liver epithelial F344 (WB-1) cell line, as it was found to be junctionally proficient. In addition, expression and regulation of Cx43 and Cx32 were also inspected using western blot analysis. The ability of hormones, antihormones, and the antihypertensive drug forskolin to restore GJIC in nontumorigenic and malignant human prostate epithelial cells was examined by the scrape-loading/dye transfer (SL/DT) or fluorescence recovery after photobleaching (FRAP) methods using an Ultima laser cytometer. Results from both assays showed that neither nontumorigenic nor malignant prostate cells have functional GJIC. However, both estrone (E1) and forskolin (FK) induced a significant increase (4.4- and 2.8-fold, respectively) in cell-cell communication only in the RWPE-1 cells. Interestingly, the use of Matrigel, a solubilized basement membrane, as substrate for cell attachment and growth resulted in the rescue of GJIC activity in RWPE-1 cells, as revealed by the SL/DT method. Furthermore, E1 induced a twofold increase in connexin 43 (Cx43), whereas forskolin caused a 50% reduction in Cx32 expression in RWPE-1 cells. These data suggest that agents that increase Cx43:Cx32 ratio may restore GJIC in junctionally deficient cells, providing a basis for the development of new strategies for the prevention and treatment of human prostate cancer.

Stem cell characteristics of transplanted rat mammary clonogens

Rat mammary glands contain a subpopulation of clonogenic epithelial cells with large proliferation and differentiation potentials. When transplanted, the clonogens in monodispersed rat mammary epithelial cell suspensions give rise to either alveolar units (AUs) or ductal units (DUs) depending on the nature of the hormonal milieu in the graft recipient. Clonogens are also the primary cells of origin of mammary cancer following exposure to ionizing radiation or chemical carcinogens. Given the other stem cell characteristics of mammary clonogens, it would be expected that the primary AUs and DUs to which they give rise when grafted and hormonally stimulated (a) would be derived from the same clonogenic cell subpopulation, (b) would contain all of the functionally differentiated cell types of homologous parts of comparably stimulated mammary glands in situ, and (c) would also contain clonogen subpopulations capable when subtransplanted of giving rise to secondary AUs and DUs of similar cell composition. The current experiments were designed to test these expectations. The data are discussed in the context of results of previous studies with this and other experimental models. The results further support the conclusion that rat mammary clonogens are multipotent mammary stem cells.

Oxidative stress, signal transduction, and intercellular communication in radiation carcinogenesis

During the evolution of multicellular organisms, survival in an aerobic environment came about by adaptive responses, both to the endogenous oxidative metabolism within the cells of the organism as well as the chemicals and low-level radiation to which they are exposed. In addition to defense mechanisms shared with single-cell organisms, multicellular organisms are equipped with gap junctions which allow electrotonic and/or metabolic synchronization of processes between coupled cells. The connexin genes, which code for the proteins comprising the gap junctions, provide homeostatic regulation of cell proliferation, differentiation, and adaptive responses of individual cells through a mechanism of "gap junctional intercellular communication." The biological consequences of the response of a multicellular organism to low-level radiation exceeding the background level of oxidative damage to a cell in a tissue could be apoptosis, cell proliferation, or cell differentiation.

The stem cells of the liver ? a selective review

The current status of the much-debated question of the still-hypothetical stem cells of the liver is reviewed, with an emphasis on their role in hepatocarcinogenesis. The widely held view of the primacy of the hepatocyte, notably of the mononuclear diploid type, in this process--the "hepatocytic theory"--has been compared with variants of the "stem cell hypothesis" based on the "non-parenchymal epithelial cells" of the liver--the "oval" or biliary ductular cells, the "nondescript periductular" cells and the "primitive" bipotential epithelial cells. An attempt has been made to concentrate mainly on the more recent publications, in an effort to balance the conflicting opinions expressed by comparing results obtained by the newer procedures currently in use. Despite some interesting and relevant findings it appears that the evidence in favour of the stem-cell hypothesis is still circumstantial and that the hepatocytic theory has not been invalidated. Presumably the question of the hepatic stem cells will be answered when the riddle of hepatocarcinogenesis has been solved.

Altruistic cell suicide in relation to radiation hormesis

The high radiosensitivity to killing of undifferentiated primordial cells (Bergonié and Tribondeau 1906) can be described as a manifestation of the suicide of injured cells for the benefit of an organism as a whole if their suicide stimulates proliferation of healthy cells to replace them, resulting in complete elimination of injury. This process is called cell-replacement repair, to distinguish it from DNA repair which is rarely complete. 'Cell suicide', 'programmed death' and 'apoptosis' are terms used for the same type of active cell death. Cell suicide is not always altruistic. Altruistic suicide in Drosophila, mice, humans, plants, and E. coli is reviewed in this paper to illustrate its widely different facets. The hypothesis that in animals, radiation hormesis results from altruistic cell suicide is proposed. This hypothesis can explain the hormetic effect of low doses of radiation on the immune system in mice. In contrast, in plants, radiation hormesis seems to be mainly due to non-altruistic cell death. HORMESIS--'the stimulating effect of small doses of substances which in larger doses are inhibitory' (British Medical Dictionary, Caxton Publ. Co., 1961).

The inheritance of epigenetic defects

Evidence from many sources shows that the control of gene expression in higher organisms is related to the methylation of cytosine in DNA, and that the pattern of methylation is inherited. Loss of methylation, which can result from DNA damage, will lead to heritable abnormalities in gene expression, and these may be important in oncogenesis and aging. Transformed permanent lines often lose gene activity through de novo methylation. It is proposed that epigenetic defects in germline cells due to loss of methylation can be repaired by recombination at meiosis but that some are transmitted to offspring.

Evidence for cell-replacement repair of X-ray-induced teratogenic damage in male genital imaginal discs of Drosophila melanogaster

Male genital imaginal discs from old (late-third-instar) larvae of Drosophila that had been X-irradiated with appropriate doses developed into severely damaged adult genitalia when implanted into old larvae, but they developed into completely normal adult genitalia when transplanted into 2-day-younger larvae. The major cause of X-ray-initiated teratogenesis in the genital disc was DNA damage because development of the genital discs of strain mei-9a with defective DNA repair was twice as sensitive as that of the wild-type strain to impairment by X-irradiation, just as larvae of this strain were twice as sensitive to killing by X-irradiation as those of the wild-type strain. Complete repair of X-ray-induced teratogenic damage in the genital discs on transplantation into young host larvae was similar in the wild-type and mei-9a strains. These results are discussed in relation to the hypothesis that repair of X-ray-induced teratogenic damage depends not on DNA repair but on replacement of damage-bearing primordial cells by healthy ones after suicidal elimination of the former.

Polyoma virus-induced hemangiomas in grafts of visceral yolk sac and of embryos

The development of hemangiomas in grafts of visceral yolk sac and embryos after in vitro infection with polyoma virus is described. These hemangiomas were shown to be positive for the polyoma tumor-specific transplantation antigen(s) and to develop as well in recipient virgin as in pregnant rats. In grafts of fetal skin or gut infected with polyoma virus neoplastic transformation was never observed.