Background & aims: The Wnt signaling pathway is activated constitutively in the majority of colorectal cancers as a result of mutation in either the adenomatous polyposis coli or the CTNNB1 gene, and blockage of the pathway has been considered feasible as molecular therapy against colorectal cancer. DNA topoisomerase IIalpha (Topo IIalpha) is a component of the beta-catenin/T-cell factor-4 (TCF-4) nuclear complex. We examined the functional significance of Topo IIalpha in Wnt signaling.
Methods: The physical and functional interaction between Topo IIalpha and the beta-catenin/TCF-4 nuclear complex was evaluated by immunoprecipitation, immunofluorescence microscopy, 2-hybrid assay, and luciferase reporter assay.
Results: Amino acids 951-1301 of Topo IIalpha were necessary for binding to beta-catenin. Over expression of Topo IIalpha enhanced the TCF/lymphoid enhancer factor transcriptional activity in a dose-dependent manner, and knockdown of Topo IIalpha by RNA interference conversely attenuated the transcriptional activity. The Topo II inhibitors, merbarone and etoposide, suppressed the beta-catenin-mediated TCF/lymphoid enhancer factor transcriptional activity. The catalytic activity of Topo II was augmented by overexpression of beta-catenin as measured by the decatenation of kinetoplast DNA. Topo IIalpha was highly expressed and colocalized with beta-catenin in tumor cells of patients with familial adenomatous polyposis syndrome and patients with sporadic colorectal cancer.
Conclusions: Topo IIalpha interacts with beta-catenin as a novel transcriptional co-activator. A new drug targeting the interaction of Topo IIalpha with beta-catenin as well as its catalytic activity might be more effective for suppressing aberrant Wnt signaling and proliferation of colorectal cancer cells than the current Topo II inhibitors.