Atypical Cell Populations Associated with Acquired Resistance to Cytostatics and Cancer Stem Cell Features: The Role of Mitochondria in Nuclear Encapsulation

Abstract

Until recently, acquired resistance to cytostatics had mostly been attributed to biochemical mechanisms such as decreased intake and/or increased efflux of therapeutics, enhanced DNA repair, and altered activity or deregulation of target proteins. Although these mechanisms have been widely investigated, little is known about membrane barriers responsible for the chemical imperviousness of cell compartments and cellular segregation in cytostatic-treated tumors. In highly heterogeneous cross-resistant and radiorefractory cell populations selected by exposure to anticancer agents, we found a number of atypical recurrent cell types in (1) tumor cell cultures of different embryonic origins, (2) mouse xenografts, and (3) paraffin sections from patient tumors. Alongside morphologic peculiarities, these populations presented cancer stem cell markers, aberrant signaling pathways, and a set of deregulated miRNAs known to confer both stem-cell phenotypes and highly aggressive tumor behavior. The first type, named spiral cells, is marked by a spiral arrangement of nuclei. The second type, monastery cells, is characterized by prominent walls inside which daughter cells can be seen maturing amid a rich mitochondrial environment. The third type, called pregnant cells, is a giant cell with a syncytium-like morphology, a main nucleus, and many endoreplicative functional progeny cells. A rare fourth cell type identified in leukemia was christened shepherd cells, as it was always associated with clusters of smaller cells. Furthermore, a portion of resistant tumor cells displayed nuclear encapsulation via mitochondrial aggregation in the nuclear perimeter in response to cytostatic insults, probably conferring imperviousness to drugs and long periods of dormancy until nuclear eclosion takes place. This phenomenon was correlated with an increase in both intracellular and intercellular mitochondrial traffic as well as with the uptake of free extracellular mitochondria. All these cellular disorders could, in fact, be found in untreated tumor cells but were more pronounced in resistant entities, suggesting a natural mechanism of cell survival triggered by chemical injury, or a primitive strategy to ensure stemming, self-renewal, and differentiation under adverse conditions, a fact that may play a significant role in chemotherapy outcomes.

The “virgin birth”, polyploidy, and the origin of cancer

“…Endopolyploidy, multi-nucleation with multiplication of pronuclei and/or polar bodies are seen in pathological human eggs (numerous examples are seen in the pictures from the Advanced Fertility Center of Chicago http://www.advancedfertility.com/abnormal-ivf-egg-pictures.htm). These pictures resemble those of multi-nucleated (“pregnant”) giant tumour cells presented by Diaz-Carballo, et al [51] and Zhang, et al [50]. Interesting changes are observed in giant tumour cells after reaching “the totipotency checkpoint”.…”

“…These experiments were performed on tumour cell lines representing almost all cancer types (breast, ovarian, bladder, colon, glioblastoma, fibrosarcoma, osteosarcoma, retinoblastoma, lymphoma). Moreover, it was shown that these giant polyploid tumour cells possessing large subnuclei ultimately bud smaller cells [51] of fibroblastic shape and with markers of epithelial-mesenchymal transition (EMT) [50]. The occurrence of asymmetric mitotic divisions in the late ETC which precede cellularisation and the release of rejuvenated sub-cells was also suggested by us previously [31].…”

“…Various proposals have been made including; meiosis-like reduction divisions [15, 22]; reduction through diplochromosomes and haploidy [28, 29]; multi-polar mitoses [16, 46, 47]; and “a-mitotic budding” of descendent sub-cells [48-51]. Recently, we attempted to unite and order most of these various events as a necessary sequence of steps in a prolonged process of specific rearrangements [31, 52].…”

“…This link is also made more apparent with a series of recent studies reporting the spherogenicity and malignancy of endopolyploid tumour cells (ETC). In these experiments, polyploid giant cancer cells were sorted either manually [62], or chemically – using the hypoxiamimic CoCl 2 [50, 63] or by serial selections in etoposide [51]. These ETC displayed increased resistance to chemo-radiotherapy, expressed key ESC and germline factors (Oct4/Nanog, Sox2, SCF, c-kit), and surface markers (CD44, CD133) as well as an ESC-like microRNA profiles.…”

“…Here, we should explain why tumour ‘pseudo-blastomeres’ are polyploid. During cleavage of the tumour pseudo-embryo the pseudo-blastomeres do not undergo full cytotomy until the blastula-equivalent stage, after which the giant cell sub-nuclei undergo cellularisation (sequestration of individual cytoplasm territories) preceding the ‘budding’ of daughter cells (described in [26, 31, 48, 50, 51]. This type of cleavage where a syncytial blastoderm is formed and shortly afterwards a cluster of germ cells is separated along with cellularisation is well-characterised in Drosophila, perhaps the best understood of all developmental systems [69].…”