Ezrin function is required for ROCK-mediated fibroblast transformation by the Net and Dbl oncogenes.

The small G protein RhoA and its GDP/GTP exchange factors (GEFs) Net and Dbl can transform NIH 3T3 fibroblasts, dependent on the activity of the RhoA effector kinase ROCK. We investigated the role of the cytoskeletal linker protein ezrin in this process. RhoA effector loop mutants which can bind ROCK induce relocalization of ezrin to dorsal actin-containing cell surface protrusions, as do Net and Dbl. Both processes are inhibited by the ROCK inhibitor Y27632, which also inhibits association of ezrin with the cytoskeleton, and phosphorylation of T567, conserved between ezrin and its relatives radixin and moesin. ROCK can phosphorylate the ezrin C-terminus in vitro. The ezrin mutant T567A cannot be relocalized by activated RhoA, Net or Dbl or by ROCK itself, and also inhibits RhoA-mediated contractility and focal adhesion formation. Moreover, ezrin T567A, but not wild-type ezrin, restores contact inhibition to Net- and Dbl-transformed cells, and inhibits the activity of Net and Ras in focus formation assays. These results implicate ROCK-mediated ezrin C-terminal phosphorylation in transformation by RhoGEFs.

Erythroid cell development and leukemic transformation: interplay between signal transduction, cell cycle control and oncogenes.

Studies using genetically modified mice and ex vivo tissue culture of erythroid progenitors converge to show that generation of mature erythroid cells depends on the interplay between specific transcriptional regulators and intracellular signals controlled by cytokines and growth factors. These studies also show that terminal differentiation in the erythroid lineage is unusual since the acquisition of the phenotypic traits of mature cells occurs while the cells are still actively dividing. Furthermore, under specific stress conditions, a massive and sustained self-renewal of committed erythroid progenitors can take place to replenish the pool of terminally differentiated cells. We review here how the erythroid genetic program and its interplay with specific cytokines, growth factors and hormones controls survival, proliferation and differentiation of erythroid progenitors both in normal and stress conditions. Special emphasis is laid on our present understanding of the differences in cell cycle control, which result either in self-renewal of erythroid progenitors or in the particular cell divisions which accompany terminal differentiation. Finally, we discuss how deregulation of the various aspects of the physiological control of erythroid progenitor survival, proliferation and differentiation can lead to erythroblast transformation and erythroleukemia.