RASSF1A Mediates p21-Dependent Cell Cycle Arrest and Senescence through Modulation of the Raf-MEK-ERK Pathway and Inhibition of Akt

Promoter hypermethylation preventing expression of the RAS association domain family 1 isoform A (RASSF1A) gene product is among the most abundant epigenetic deregulations in human cancer. Restoration of RASSF1A inhibits tumor cell growth in vitro and in murine xenograft models. Rassf1adeficient mice feature increased spontaneous and carcinogeninduced tumor formation. Mechanistically, RASSF1A affects several cellular functions, such as microtubule dynamics, migration, proliferation, and apoptosis; however, its tumorsuppressive mechanism is incompletely understood. To study the functional consequences of RASSF1A expression in human cancer cells, we made use of a doxycycline-inducible expression system and a RASSF1A-deficient lung cancer cell line. We observed that RASSF1A induces cell cycle arrest in G1 phase and senescence in vitro and in tumors established in immunodeficient mice. RASSF1A-mediated growth inhibition was accompanied by the up-regulation of the cyclin-dependent kinase inhibitor p21 and proceeded independently of p53, p14, and p16. Loss of p21 or coexpression of the human papilloma virus 16 oncoprotein E7 was found to override RASSF1A-induced cell cycle arrest and senescence. Conditional RASSF1A affected mitogen-activated protein kinase and protein kinase B/Akt signaling to upregulate p21 and to facilitate its nuclear localization. In summary, RASSF1A can mediate cell cycle arrest and senescence in human cancer cells by p53-independent regulation of p21. [Cancer Res 2009;69(5):1748–57]