Cytokine response gene 8 (CR8) regulates the cell cycle G1-S phase transition and promotes cellular survival

Cellular proliferation and survival are modulated by the expression of spec ific genes. Cytokine response gene 8 (CR8), which was originally cloned as an IL-2-induced gene in human T lymphocytes, encodes a basic helix-loop-hel ix (bHLH) transcription factor, The CR8 gene product is highly conserved am ong human, mouse and rat, and contains sequence motifs that distinguish it from other bHLH families, The CR8 gene is ubiquitously expressed, and CR8 g ene expression is induced by both growth-promoting as well as growth-inhibi tory stimuli. As bHLH proteins have been found to regulate both the G1-S ph ase cell cycle transition, as well as cellular survival, the effects of CR8 on these processes were investigated. Ectopic CR8 expression in asynchrono us U2OS cell cultures reduces the percentage of cells in the cell cycle S p hase, and also slows the entry of G1-synchronized cells into S phase. The p rolonged G1 interval correlates with impaired elevation of cyclin E protein and prolonged p21 protein expression in G1, CR8 expression also protects U 2OS cells from serum-withdrawal induced apoptosis, These results indicate t hat CR8 is an important modulator of both the G1-S phase cell cycle transit ion, and cellular survival.