Cell growth inhibition by the multifunctional multivalent zinc-finger factor CTCF

The 11-zinc finger protein CCTC-binding factor (CTCF) employs different set s of zinc fingers to form distinct complexes with varying CTCF-target seque nces (CTSs) that mediate the repression or activation of gene expression an d the creation of hormone-responsive gene silencer, and of diverse vertebra te enhancer-blocking elements (chromatin insulators). To determine how thes e varying effects would integrate in vivo, we engineered a variety of expre ssion systems to study effects of CTCF on cell growth. Here we show that ec topic expression of CTCF in many cell types inhibits cell clonogenicity by causing profound growth retardation without apoptosis. In asynchronous cult ures, the cell-cycle profile of CTCF-expressing cells remained unaltered, w hich suggested that progression through the cycle was slowed at multiple po ints. Although conditionally induced CTCF caused the S-phase block, CTCF ca n also arrest cell division. Viable CTCF-expressing cells could be maintain ed without dividing for several days. While MYC is the well-characterized C TCF target, the inhibitory effects of CTCF on cell growth could not be ascr ibed solely to repression of MYC, suggesting that additional CTS-driven gen es involved in growth-regulatory circuits, such as p19ARF, are likely to co ntribute to CTCF-induced growth arrest. These findings indicate that CTCF m ay regulate cell-cycle progression at multiple steps within the cycle, and ade. to the growing evidence for the function of CTCF as a tumor suppresser gene.