Nemo-like kinase (NLK) is a serine/threonine kinase that suppresses the transcription activity of the β β β β-catenin-T-cell factor (TCF) complex through phosphorylation of TCF. Our previous study showed that NLK overexpression induces apoptosis in DLD-1 human colon cancer cells and that apoptosis induction presumably requires a mechanism other than the suppression of β β β β-catenin-TCF complex. Luciferase reporter gene assay with pNF-κ κ κ κB-Luc revealed that NLK could suppress transcription activity of NF-κ κ κ κB in a kinase-dependent manner. However, it appeared that transcription co-activators of NF-κ κ κ κB, such as CREB binding protein (CBP)/p300, were likely to be the direct targets of NLK, rather than NF-κ κ κ κB itself. Luciferase reporter gene analysis of GAL4-CBP fusion proteins revealed that the C-terminal region of CBP was critical for transcription suppression by NLK. In vitro kinase assay showed that NLK could phosphorylate the C-terminal domain of CBP. However, HAT activity was not suppressed by the induction of wild-type NLK in DLD-1 cells. Furthermore, we observed that NLK suppressed the transcription activity of AP-1, Smad, and p53, all of which also utilize CBP as a co-activator. The extent of suppression by NLK was similar among the transcription factors tested (50-60% reduction). Our results suggest that NLK may suppress a wide range of gene expression, possibly through CBP. he Nemo-like kinase (NLK) is an evolutionarily conserved serine/threonine kinase that plays an important role in the development of organisms. Genetic studies revealed that homologs of NLK function as suppressors of the Wnt signaling pathway in the development of C. elegans and Drosophila.
1-4)A recent study showed that Wnt5a-Ca 2+ signaling, which is a negative regulator of Wnt,5) can be an upstream activator of NLK for suppression of canonical Wnt-β-catenin signaling in the Xenopus system.6) The target molecules of NLK in the Wnt-β-catenin pathway are conserved between worm and mammals. NLK phosphorylates T-cell factor (TCF) and consequently suppresses the transcription activity of the β-catenin-TCF complex. 3,6,7) In C. elegans, LIT-1, a homolog of NLK, could phosphorylate a TCF related protein, POP-1, in vivo and in vitro and suppress its asymmetric distribution between two daughter cells mediated by the worm Wnt signaling pathway. 1,8) As Wnt-β-catenin signaling is known to be involved in human carcinogenesis, especially in colon and liver, 9, 10) one can speculate that NLK may play a suppressive role in carcinogenesis mediated by the activated Wnt-β-catenin-TCF signaling.We have recently showed that overexpression of NLK could induce growth suppression and apoptosis in a human colon cancer cell line, DLD-1. 11) Although the overall results of our study were consistent with the aforementioned studies, there were minor discrepancies. Two independent studies showed that overexpression of dominant-negative TCF could efficiently suppress the transcription activity of β-catenin-TCF complex, but did not induce apoptos...