NPM/ALK downregulates p27 ^Kip1 in a PI-3K-dependent manner

Anaplastic large-cell lymphomas (ALCL) are frequently associated with the chromosomal translocation t(2;5) (p23;q35) resulting in the NPM/ALK fusion gene that encodes a constitutively activated tyrosine kinase. We showed that NPM/ALK stimulated cell proliferation and that PI-3K/AKT pathway played an important role in this effect. p27 ^Kip1 is a member of the CDK family inhibitory proteins regulating the entry into S phase. It was reported that p27 ^Kip1 function is impaired in many tumors. In this study we investigated the role of PI-3K/AKT in NPM/ALK-dependent downregulation of p27 ^Kip1 protein. To investigate this phenomenon the pro-B cell line BaF3, BaF3 cell line stably expressing NPM/ALK, and ALCL SUP-M2 cell line were used. The p27 ^Kip1 protein expression before and after LY294002, wortmannin, or epoxomicin treatment and phosphorylation status of AKT were measured in parental and NPM/ALK ⁺ cells by Western analysis. Also, the localization of p27 ^Kip1 protein was analyzed by fractionation and immunoblotting. p27 ^Kip1 was found to be downregulated in NPM/ALK-transformed hematopoietic cells, but inhibition of proteasome-dependent degradation pathway by epoxomicin reversed this effect. In addition, treatment of NPM/ALK ⁺ cells with LY294002, the PI-3K inhibitor, caused elevation of p27 ^Kip1 protein expression and its nuclear localization. Taken together, we postulate that NPM/ALK-PI-3K pathway stimulates cell proliferation by regulation of the expression and nuclear localization of p27 ^Kip1.