Evidence for non-functional Dickkopf-1 (DKK-1) signaling in chronic lymphocytic leukemia (CLL)

Purpose: Wnt signaling was demonstrated to be activated in chronic lymphocytic leukemia (CLL). It is thought to be responsible for the extended survival of CLL cells in vivo. Dickkopf1 (DKK1) is known to antagonize Wnt signaling by direct high-affinity binding to the extracellular domain of WNT coreceptor lipoprotein receptor-related protein 6 (LRP6). The purpose of this study was to investigate the effect of DKK1 in B-CLL cells in vitro. Methods: Expression of DKK1 was estimated by Western blot and real-time PCR. B cells from patients with CLL and healthy donors were incubated with recombinant DKK1. Survival was measured by flow cytometry. Primers for real-time PCR were designed for extracellular domain of LRP6, responsible for DKK1 binding, and the intracellular region, essential for inhibiting GSK3 beta. Results: Healthy and CLL cells express equivalent mRNA levels of DKK1 and LRP6. After treatment of CLL cells with recombinant DKK1 (1 mu g/mL) for 3 h, there was no change in the levels of phosphorylated beta-catenin and total beta-catenin. Healthy B cells proved to have significantly higher levels of extracellular, DKK1 binding domain of LRP6. We estimated that in CLL cells every 6th LRP6 receptor is lacking the extracellular domain. Discussion: For the first time we show the expression of DKK1 in CLL cells. Unlike in similar tumors, the addition of DKK1 to culture of CLL cells does not inactivate WNT pathway. The reason for this could be the absence of the binding domain of LRP6. On the other hand, a truncated LRP6 without extracellular DKK1 binding domain could lead to an uncontrollable activation of WNT signaling.