PMID- 24889617 OWN - NLM STAT- MEDLINE DCOM- 20140912 LR - 20211203 IS - 1091-6490 (Electronic) IS - 0027-8424 (Print) IS - 0027-8424 (Linking) VI - 111 IP - 24 DP - 2014 Jun 17 TI - Contact inhibition and high cell density deactivate the mammalian target of rapamycin pathway, thus suppressing the senescence program. PG - 8832-7 LID - 10.1073/pnas.1405723111 [doi] AB - During cell cycle arrest caused by contact inhibition (CI), cells do not undergo senescence, thus resuming proliferation after replating. The mechanism of senescence avoidance during CI is unknown. Recently, it was demonstrated that the senescence program, namely conversion from cell cycle arrest to senescence (i.e., geroconversion), requires mammalian target of rapamycin (mTOR). Geroconversion can be suppressed by serum starvation, rapamycin, and hypoxia, which all inhibit mTOR. Here we demonstrate that CI, as evidenced by p27 induction in normal cells, was associated with inhibition of the mTOR pathway. Furthermore, CI antagonized senescence caused by CDK inhibitors. Stimulation of mTOR in contact-inhibited cells favored senescence. In cancer cells lacking p27 induction and CI, mTOR was still inhibited in confluent culture as a result of conditioning of the medium. This inhibition of mTOR suppressed p21-induced senescence. Also, trapping of malignant cells among contact-inhibited normal cells antagonized p21-induced senescence. Thus, we identified two nonmutually exclusive mechanisms of mTOR inhibition in high cell density: (i) CI associated with p27 induction in normal cells and (ii) conditioning of the medium, especially in cancer cells. Both mechanisms can coincide in various proportions in various cells. Our work explains why CI is reversible and, most importantly, why cells avoid senescence in vivo, given that cells are contact-inhibited in the organism. FAU - Leontieva, Olga V AU - Leontieva OV AD - Department of Cell Stress Biology, Roswell Park Cancer Institute, Buffalo, NY 14263. FAU - Demidenko, Zoya N AU - Demidenko ZN AD - Department of Cell Stress Biology, Roswell Park Cancer Institute, Buffalo, NY 14263. FAU - Blagosklonny, Mikhail V AU - Blagosklonny MV AD - Department of Cell Stress Biology, Roswell Park Cancer Institute, Buffalo, NY 14263 blagosklonny@oncotarget.com. LA - eng PT - Journal Article PT - Research Support, Non-U.S. Gov't DEP - 20140602 PL - United States TA - Proc Natl Acad Sci U S A JT - Proceedings of the National Academy of Sciences of the United States of America JID - 7505876 RN - 0 (CDKN1A protein, human) RN - 0 (CDKN1B protein, human) RN - 0 (Culture Media, Conditioned) RN - 0 (Cyclin-Dependent Kinase Inhibitor p21) RN - 147604-94-2 (Cyclin-Dependent Kinase Inhibitor p27) RN - EC 2.7.1.1 (MTOR protein, human) RN - EC 2.7.11.1 (TOR Serine-Threonine Kinases) RN - EC 3.2.1.23 (beta-Galactosidase) SB - IM MH - Cell Cycle MH - Cell Cycle Checkpoints MH - Cell Line, Tumor MH - Cell Proliferation MH - *Cellular Senescence MH - *Contact Inhibition MH - Culture Media, Conditioned MH - Cyclin-Dependent Kinase Inhibitor p21/*metabolism MH - Cyclin-Dependent Kinase Inhibitor p27/*metabolism MH - Fibrosarcoma/metabolism MH - Flow Cytometry MH - Humans MH - Neoplasms/*metabolism MH - Retinal Pigment Epithelium/cytology MH - Signal Transduction MH - TOR Serine-Threonine Kinases/*metabolism MH - beta-Galactosidase/metabolism PMC - PMC4066505 COIS- The authors declare no conflict of interest. EDAT- 2014/06/04 06:00 MHDA- 2014/09/13 06:00 PMCR- 2014/12/17 CRDT- 2014/06/04 06:00 PHST- 2014/06/04 06:00 [entrez] PHST- 2014/06/04 06:00 [pubmed] PHST- 2014/09/13 06:00 [medline] PHST- 2014/12/17 00:00 [pmc-release] AID - 1405723111 [pii] AID - 201405723 [pii] AID - 10.1073/pnas.1405723111 [doi] PST - ppublish SO - Proc Natl Acad Sci U S A. 2014 Jun 17;111(24):8832-7. doi: 10.1073/pnas.1405723111. Epub 2014 Jun 2.