Runx3 plays a critical role in restriction-point and defense against cellular transformation

The restriction (R)-point decision is fundamental to normal differentiation and the G(1)–S transition, and the decision-making machinery is perturbed in nearly all cancer cells. The mechanisms underlying the cellular context–dependent R-point decision remain poorly understood. We found that the R-point was dysregulated in Runx3(−/−)mouse embryonic fibroblasts (MEFs), which formed tumors in nude mice. Ectopic expression of Runx3 restored the R-point and abolished the tumorigenicity of Runx3(−/−)MEFs and K-Ras–activated Runx3(−/−)MEFs (Runx3(−/−);K-Ras(G12D/+)). During the R-point, Runx3 transiently formed a complex with pRb and Brd2 and induced Cdkn1a (p21(Waf1/Cip1/Sdi1); p21), a key regulator of the R-point transition. Cyclin D–CDK4/6 promoted dissociation of the pRb–Runx3–Brd2 complex, thus turning off p21 expression. However, cells harboring oncogenic K-Ras maintained the pRb–Runx3–Brd2 complex and p21 expression even after introduction of Cyclin D1. Thus, Runx3 plays a critical role in R-point regulation and defense against cellular transformation.