Time-lapse microscopic observation of non-dividing cells in cultured human osteosarcoma MG-63 cell line

Analogous to normal tissue-specific stem cells, cancer stem cells have been suggested to possess quiescence. Although quiescence is believed to be maintained only in vivo by the stem cell niche (for normal stem cells) and by the tumor stromal microenvironment (for cancer stem cells), it has been reported that cultured cancer cell lines, in which all cells are provided with the same environment (oxygen and nutrients), contain subpopulations of rarely dividing and non-dividing cells, and a fraction of them displays some cancer stem cells markers.

These data appear to be contradictory and inconclusive. Moreover, the existence of slow-proliferating and non-dividing cells in stable cancer cell lines that are cultured under the normal conditions, cannot not be explained by hypoxia and/or exposure to chemotherapeutic agents. The fate of these cells and their progeny, and whether and how such slow-proliferating and non-dividing cells are maintained at a constant ratio from passage to passage, remains poorly understood. Importantly, it is generally recognized that during the tumor cell evolution, the malignant cells acquire a significant growth advantage, over the non-malignant cells, and later over the neighboring cells once the tumor is formed.

Alternatively, it has been suggested that cancer stem cells can enter prolonged but reversible quiescence in response to chemotherapy (as well as the lack of oxygen), and remain dormant until activated through some poorly understood processes.

Researchers led by Dr. Alexander Kofman at Dakota Wesleyan University (currently an assistant professor at Troy University) in the United States used time-lapse microscopy and the novel imaging algorithm to monitor actively dividing and non-dividing MG-63 (human osteosarcoma) cells under the normal culture conditions.  Their research work is now published in Journal, Cell Cycle. They found that whereas MG-63 cell line harbors non-dividing cells under the normal culture conditions and without being exposed to chemotherapeutic agents, such cells do not have a growth advantage over the other cultured cells and, therefore, their belonging to the cancer stem cells pool is questionable. Of note, neither their observations, nor the observations of earlier publications present any proof that such non-diving cells are quiescent, that is, they are able to re-enter the proliferation cycle. Authors also observed failed cytokinesis and multi-lobed nucleus in one of non-dividing cells, which indicated the possible aneuploidy and/or mitotic catastrophe. They suggested that the inability of cultured cancer cells to enter a proliferative cell cycle may be due to high genomic instability. The latter is known to be characteristic for most tumor cells and may result in significant structural changes, including inactivation of cell cycle genes.

In contrast to the previous reports about slowly proliferating cells in stable cancer cell lines the presented data demonstrate that, at least, some rarely dividing and non-dividing cells in cancer cell lines do not provide a contribution to the cancer stem cells population. The study also highlights the growing potential of time-lapse microscopy as a powerful tool to further elucidate these cells.