THE PROTEIN-KINASE-C INHIBITOR H7 BLOCKS PHOSPHORYLATION OF STATHMIN DURING TPA-INDUCED GROWTH-INHIBITION OF HUMAN PRE-B LEUKEMIA REH6 CELLS

The human pre-B acute lymphoblastic leukemia cell line REH6 was used t o analyze the regulation of a ubiquitous intracellular phosphoprotein stathmin (Mr 19,000, pl=5.6-6.2). We demonstrated by P-32-labeling tha t the short (1 h) treatment of the REH6 cells with the tumor promoting phorbol ester, 12-O-tetradecanoylphorbol-13-acetate (TPA), resulted i n a rapid phosphorylation of at least three (P1, P2 and P3) stathmin i soforms without an alteration of stathmin isoform expression. Furtherm ore, Western blot analysis with specific antiserum showed that the pro longed period (48 h) of IPA treatment partially reduced protein levels particularly of two (N2 and P2) stathmin isoforms. The potent and rel atively specific protein kinase C (PKC) inhibitor, 1,(5-isoquinolinesu lphonyl)2methylpiperasine dihydrochloride (H7), partially inhibited th ese TPA effects, whereas the specific calmodulin inhibitor R24571 (cal midazolium) had no effect upon these events. Our findings suggest that stathmin phosphorylation in REH6 cells could be in part mediated by P KC activation.