Nutlin-3 downregulates the expression of the oncogene TCL1 in primary B chronic lymphocytic leukemic cells

Purpose: The oncogene TCL1 plays a key role in the development of B chronic lymphocytic leukemia (B-CLL), but it is not known whether TCL1 could be modulated by therapeutic approaches. Experimental Design: B-CLL patient samples (n = 35) and B leukemic cell lines (EHEB, JVM2, JVM3, MEC1, MEC2, and BJAB) with different p53 status were exposed to Nutlin-3, a small-molecule inhibitor of the p53-MDM2 interaction. Modulations of the steady-state mRNA levels of TCL1 were analyzed by quantitative real-time PCR and Western blotting in both primary B-CLL samples and leukemic cell lines. In addition, transfection experiments with either p53 siRNA or with a TCL1 expression plasmid were carried out in the EHEB B-CLL cell line. Results: Upon ex vivo treatment with Nutlin-3, TCL1 was significantly (P <0.05) decreased in 23 of 28 B-CLL p53 ^wild-type. The functionality of the p53 pathway in the same leukemic cell samples was underscored by the concomitant ability of Nutlin-3 to significantly (P <0.05) upregulate the p53 target gene MDM2 in the p53 ^wild-type leukemic cells. The dependence of TCL1 downregulation by a functional p53 pathway was confirmed in a panel of B lymphoblastoid cell lines and by p53 knockdown experiments with p53 siRNA. The importance of TCL1 in promoting leukemic cell survival was underscored in transfection experiments, in which TCL1 overexpression significantly counteracted the Nutlin-3-mediated induction of apoptosis in EHEB. Conclusions: Our data indicate that the Nutlin-3 downregulates TCL1 mRNA and protein, which likely represents an important molecular determinant in the proapoptotic activity of Nutlin-3.