Isolation and characterization of human NBL4, a gene involved in the beta-catenin/Tcf signaling pathway

beta-catenin, a key regulator of cellular proliferation, is often mutated i n various types of human cancer. To investigate cellular responses related to the beta-catenin signaling pathway, we applied a differential display me thod using mouse cells transfected with an activated form of mutant beta-ca tenin, This analysis and subsequent northern-blot hybridization confirmed t hat expression of a murine gene encoding NBL4 (novel band 4.1-like protein 4) was up-regulated by activation of beta-catenin. To examine a possible ro le of NBL4 in cancer, we isolated the human homologue of the murine NBL4 ge ne by matching mNBL4 against the human EST (expressed sequence tag) databas e followed by 5' rapid amplification of cDNA ends (5'RACE), The cDNA of hNB L4 encoded a protein of 598 amino acids that shared 87% identity in amino a cid sequence with murine NBL4 and 71% with zebrafish NBL4. A 2.2-kb hNBL4 t ranscript was expressed in all human tissues examined with high levels of e xpression in brain, liver, thymus and peripheral blood leukocytes and low l evels of expression in heart, kidney testis and colon. We determined its ch romosomal localization at 5q22 by fluorescence in situ hybridization, Expre ssion of hNBL4 was significantly reduced when beta-catenin was depleted in SW480 cells, a human cancer cell line that constitutionally accumulates bet a-catenin. The results support the view that NBL4 is an important component of the beta-catenin/Tcf pathway and is probably related to determination o f cell polarity or proliferation.