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.

Neosis: a novel type of cell division in cancer.

Using computerized video time-lapse microscopy, we studied early cellular events during carcinogen-induced transformation of C3H10T1/2 cells. Multinucleate/polyploid giant cells (MN/PGs) formed due to DNA damage are thought to die via mitotic catastrophe. Before they die, some MN/PGs undergo a novel type of cell division, termed neosis, characterized by karyokinesis via nuclear budding followed by asymmetric, intracellular cytokinesis, producing several small mononuclear cells, termed the Raju cells, with extended mitotic life span (MLS). Mitotic derivatives of Raju cells give rise to transformed cell lines, inherit genomic instability, display a phenotype and transcriptome different from the neosis mother cell, and anchorage-independent growth. Neosis of MN/PGs also precedes spontaneous transformation of p53-/- mouse cells. Rodent neotic clones, and primary and metastatic human tumor cells undergo spontaneous or induced secondary/tertiary neosis. Neosis seems to extend the MLS of cells under conditions of genetic duress not favoring mitosis. It precedes tumorigenesis, occurs several times during tumor progression, yielding tumor-initiating Raju cells and introducing tumor cell heterogeneity subject to natural selection during tumor progression. Events during neosis, and its relevance to origin of established cell lines, multistep carcinogenesis, cancer stem cells, and therapeutic advantages of anti-neotic agents (neosicides) are discussed.

Neosis--a paradigm of self-renewal in cancer.

We recently described a novel form of cell division termed neosis, which appears to be the mode of escape of cells from senescence and is involved in the neoplastic transformation and progression of tumors (Cancer Biol & Therap 2004;3:207-18). Neosis is a parasexual somatic reduction division and is characterized by (1) DNA damage-induced senescence/mitotic crisis and polyploidization, (2) followed by production of aneuploid daughter cells via nuclear budding, (3) asymmetric cytokinesis and cellularization conferring extended, but, limited mitotic life span to the offspring, and (4) is repeated several times during tumor growth. The immediate neotic progeny are termed the Raju cells, which seem to transiently display stem cell properties. The Raju cells immediately undergo symmetric mitotic division and become mature tumor cells. Exposure of tumor cells to genotoxic agents yields neosis-derived Raju cell progenies that are resistant to genotoxins, thus contributing to the recurrence of drug-resistant tumor growth. Similar events have been described in the literature under different names through several decades, but have been neglected due to the lack of appreciation of the significance of this process in cancer biology. Here we review and interpret the literature in the light of our observations and the recent advances in self-renewal in cancer. Neosis paradigm of self-renewal of cancer growth is consistent with the telomere attrition, aging and origin of cancer cells after reactivation of telomerase, and constitutes an alternative to the cancer stem cell hypothesis. We summarize the arguments favoring Raju cells and not cancer stem cells, as the source of self-renewal in cancer and present a comprehensive hypothesis of carcinogenesis, encompassing various aspects of cancer biology including senescence, tumor suppressor genes, oncogenes, cell cycle checkpoints, genomic instability, polyploidy and aneuploidy, natural selection, apoptosis, endoapoptosis, development of resistance to radiotherapy and chemotherapy leading tumor progression into malignancy.