INDUCED EXPRESSION OF A UBIQUITIN COOH-TERMINAL HYDROLASE IN ACUTE LYMPHOBLASTIC-LEUKEMIA

To identify potential effector molecules of human B-cell differentiati on, the acute lymphoblastic leukemia cells (Reh) were induced to termi nal differentiation in vitro using the phorbol ester, 12-O-tetradecano ylphorbol-13-acetate. Proteins of parent and differentiated Reh cells were mapped using powerful two-dimensional gel electrophoresis coupled with ultrasensitive silver stain. New protein (MW-pl; p34-5.3) spot w as induced in the differentiated Reh cells. The NH2-terminal sequencin g of this protein revealed 100% homology with the ubiquitin COOH-termi nal hydrolase isozyme L1 (UCH-L1). The presence of UCH-L1 protein in t he differentiated Reh cells but not in parent cells was confirmed by i mmunocytochemistry and Western blot. The cDNA for this enzyme was clon ed from the differentiated Reh cells, and the UCH-L1 mRNA was detected in both parent and 12-O-tetradecanoylphorbol-13-acetate-induced cells . However, the message was more abundant in the differentiated cells t han parent cells, indicating a posttranscriptional regulation. Until n ow, UCH-L1 was thought to be neuron-specific. The induced expression o f UCH-L1 in differentiated Reh cells argues for a role of this enzyme, and the ubiquitin system, in B-cell differentiation.