Running m(o)TOR with the brakes on leads to catastrophe at mitosis.

Two parallel and concordant activities characterize cell cycle progression. One is associated with preparation for and execution of DNA replication followed by mitosis (“cell cycle”). Cyclins are the motors of the cell cycle, as they activate the respective CDKs and thereby drive the cell through the sequential phases of the cycle. Their antagonists are CKIs, which inhibit the CDKs stalling the cycle progression. The second type of activities involves anabolic processes that contribute to growth in cell size and mass (“cellular growth”). Constitutive signaling along the mTOR/S6K pathways is the key factor mediating these anabolic processes. During unperturbed and balanced growth these two activities are flawlessly coordinated. This synchronization ensures that the cell size, as well the ratio of protein or RNA content to DNA, remains invariable for cells in particular phases of the cycle or for particular cell type. However, during arrest in cell cycle progression, for example, when induced by inhibitors of DNA replication, these activities become uncoupled. The cell growth continues, resulting in an “unbalanced growth” phenotype when the ratio of cell protein/mass to DNA content is greatly augmented. While this phenomenon was initially observed nearly five decades ago, recent evidence underscores its importance and links it mechanistically with senescence and aging.- Specifically, it has been postulated that cell cycle arrest when is concurrent with the ongoing or intensified mTOR/S6K signaling (growth cycle) results in induction of the unbalanced growth phenotype (cell hypertrophy), which is a characteristic feature of cell senescence as well as considered to be the primary cause of organismal aging.-

In the currently published article, Leontieva et al. describe that constitutive mTOR signaling during the cell cycle arrest, induced by upregulation of p21, contributed to cell senescence (geroconversion); these cells were characterized by greatly increased levels of cyclin D1 and cyclin E as well as being under replication stress, manifested by markers of DNA damage signaling. When the cycle progression was restored by downregulation of p21, the cells were able to pass through S and G2 and decrease the level of cyclins D1 and E, but then they underwent either mitotic catastrophe or entered higher DNA ploidy by endoreduplication. Suppression of mTOR signaling, either by rapamycin or by nutlin 3a in the cells arrested by p21, prevented geroconversion, lowered the level of cyclin D1 expression and, after removal of p21, restored ability to proliferate. In another model cell system, in which nutlin 3a was unable to suppress mTOR signaling but was inducing arrest in G1 and the senescent phenotype, removal of nutlin 3a led to initiation of DNA replication but could not restore capability to proliferate.

The findings presented by Leontieva et al. underscore the role of mTOR signaling during cell cycle arrest in the induction of either cell senescence or quiescence and restoration of replicative potential. Of particular interest is the observation of massive upregulation of cyclins D1 and E, which, upon restoration of the cycle progression, appeared to initially enhance DNA replication rate (providing the “hyper-mitogenic drive”), but then likely to contribute to the mitotic catastrophe. Clearly, progression of hypertrophic cells additionally accelerated during S appears to be catastrophic when passing later through mitosis.

Highly unbalanced (“unscheduled”) expression of not only cyclins D and E but also cyclins A and B1 was previously observed in cells arrested and synchronized at the G1/S boundary by the inhibitors of DNA replication aphidicolin, mimosine or excess of thymidine. These cells, in which the chromosome cycle and growth cycle were also dissociated, resulting in their hypertrophy, when released from the arrest and progressing through S phase, had several-fold higher levels of all the cyclins (D, E, A and B1) compared with S-phase cells in untreated cultures. The elevated level of cyclin A was likely reporting the replication stress, while the elevated level of cyclin B1 was considered to be due to the increased half-life of this protein stabilized by overexpression of cyclin E. Interestingly, following successful mitosis and cytokinesis, the immediate G1 progeny of these synchronized cells, while they had normal levels of the respective cyclins, still showed some degree of imbalance, characterized by > 30% higher protein to DNA ratio compared with G1 cells from exponentially growing cultures, and had proliferative capacity. These findings collectively with observations of Leontieva et al. suggest that there is a threshold level of the growth imbalance (cell hypertrophy) defining the “point of no return.” The cells that pass the threshold are losing the reproductive potential either through mitotic catastrophe or endoreduplication. The cells below this threshold attempt to return to balanced growth either through accelerated rate of cell cycle progression vis-à-vis growth cycle, by autophagy or by both mechanisms and preserve their replicative potential. The intensity of mTOR signaling combined with duration of the cell cycle arrest plays a critical role as to whether the cells do pass this point or not.

The possibility of decoupling “cell cycle” from the “growth cycle” to induce cell hypertrophy and irreversible senescence suggests an interesting anticancer strategy. The activation of many oncogenes and/or dysfunction of tumor suppressor genes, either one leading downstream to an increased mTOR signaling, is a hallmark of most cancers. Cells of such tumors, therefore, would be more predisposed to undergo senescence under conditions of persistent chromosome cycle arrest that induces DNA replication stress compared with normal cells. We have recently reported that prolonged treatment of the non-small cell lung carcinoma cells, characterized by strong mTOR signaling, with very low doses of DNA alkylating drug mitomycin C, induced cell cycle arrest, replication stress and, similarly, as in the case of p21-induced arrest, endoreduplication. This led to their senescence and irreversible loss of reproductive capability. The induction of cell senescence by the low doses of DNA damaging drugs or other means of arresting the cell cycle rather than induction of apoptosis by high drug doses, typical of standard chemotherapy, may be therefore more effective in treatment of cancers characterized by high level of mTOR signaling, with less side effects for the patient. Screening tumors for the presence of activation of such oncogenes may select patients sensitive for this type of chemotherapy, providing a personalized cancer treatment approach.