DNA damaging agents and p53 do not cause senescence in quiescent cells, while consecutive re-activation of mTOR is associated with conversion to senescence

Abstract

When the cell cycle is arrested, growth-promoting pathways such as mTOR (Target of Rapamycin) drive cellular senescence, characterized by cellular hyper-activation, hypertrophy and permanent loss of the proliferative potential. While arresting cell cycle, p53 (under certain conditions) can inhibit the mTOR pathway. Senescence occurs when p53 fails to inhibit mTOR. Low concentrations of DNA-damaging drugs induce p53 at levels that do not inhibit mTOR, thus causing senescence. In quiescence caused by serum starvation, mTOR is deactivated. This predicts that induction of p53 will not cause senescence in such quiescent cells. Here we tested this prediction. In proliferating normal cells, etoposide caused senescence (cells could not resume proliferation after removal of etoposide). Serum starvation prevented induction of senescence, but not of p53, by etoposide. When etoposide was removed, such cells resumed proliferation upon addition of serum. Also, doxorubicin did not cause senescent morphology in the absence of serum. Re-addition of serum caused mTOR-dependent senescence in the presence of etoposide or doxorubicin. Also, serum-starvation prevented senescent morphology caused by nutlin-3a in MCF-7 and Mel-10 cells. We conclude that induction of p53 does not activate the senescence program in quiescent cells. In cells with induced p53, re-activation of mTOR by serum stimulation causes senescence, as an equivalent of cellular growth.

A set of miRNAs participates in the cellular senescence program in human diploid fibroblasts

“…24 Finally, we showed that the mTOR pathway was an essential driver of senescence induced by senescence-associated miRNAs as SA-b-gal staining was significantly reduced by treatment with rapamycin. [20][21][22] Of note, our preliminary observations, by studying the expression levels of the senescence-inducing miRNAs in human skin broblasts from young (age [17][18][19][20][21][22][23][24][25] or old (age 89-94) donors, show that the highest expression levels of miR-376a* and miR-494 were found in one old donor cell line, and expression of miR-486-5p was on average two-fold higher in old with respect to young cells.…”

“…The mammalian target of rapamycin (mTOR) pathway was demonstrated to drive senescence in cells treated with several factors, including oxidative agents. [20][21][22] Thus, initially, we asked whether miRNA transfection caused any change in mTOR pathway activation, by measuring the phosphorylation of the ribosomal S6 protein (S6); S6 is phosphorylated by p70-S6 kinase (p70S6K), which, in turn, is activated by mTOR. 23 IMR90 cells featured high levels of S6 phosphorylation, and adoptive overexpression of any of the five miRNAs (miR-210, miR-376a*, miR-486-5p, miR-494, and miR-542-5p) did not cause detectable change in the levels of phospho-S6 (Supplementary Figure 3).…”

p53 expression controls prostate cancer sensitivity to chemotherapy and the MDM2 inhibitor Nutlin-3

“…To determine the sensitivities of each cell line with different TP53 statuses to common anticancer drugs used in the treatment of prostate cancer, MTT assays were an increase/decrease in cell cycle regulating and apoptotic proteins needed for the correction of cell damage, cell death or the induction of cellular senescence. 6,[17][18][19][20][21][22] Inhibition of one or more of these controlled processes could result in systematic deregulation within the cell, causing abnormal cell cycle progression, chromosome instability or anti-apoptotic effects. 23,24 The function of p53 as a master regulator in the cell cycle and its role in cancer development is well-documented.…”

“…To do this, we turned to the use of a MDM2-specific inhibitor, Nutlin-3. 3,7,[20][21][22][50][51][52] LNCaP cells express two wild-type TP53 alleles and demonstrated the most significant cellular response to alterations in p53 signaling when mutant DN p53DD was introduced. Therefore, we next performed MTT analysis with LNCaP cells treated with a constant concentration of Nutlin-3 performed.…”

Δ2-Troglitazone promotes cytostatic rather than pro-apoptotic effects in breast cancer cells cultured in high serum conditions

“…6,44 mTOR pathway could be involved in this process since it is inhibited in low serum conditions and mTOR inhibitors like rapamycin sensitize cancer cells to anticancer drugs. 45,46 Then, it could be on purpose to test the potentiating effect of the co-treatment rapamycin/D2-TGZ on breast cancer cells. Moreover, the starvation-based method was named the differential stress resistance since it potentiated the action of the chemotherapeutic agent toward cancer cells while being less deleterious for normal cells.…”