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


RESULTS
Healthy and CLL cells express equivalent mRNA levels of DKK1 and LRP6. After treatment of CLL cells with recombinant DKK1 (1 μg/mL) for 3 h, there was no change in the levels of phosphorylated β-catenin and total β-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.


CONCLUSIONS
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.
Filipovich
Alexandra
A
Department I of Internal Medicine I, University at Cologne, Cologne, Germany.
Gandhirajan
Rajesh K
RK
Gehrke
Iris
I
Poll-Wolbeck
Simon J
SJ
Kreuzer
Karl-Anton
KA
D002470:Cell Survival; D038362:Glycogen Synthase Kinase 3 / Q000378:metabolism; D000071679:Glycogen Synthase Kinase 3 beta; D006801:Humans; D036341:Intercellular Signaling Peptides and Proteins / Q000235:genetics / Q000378:metabolism* / Q000494:pharmacology; D015451:Leukemia, Lymphocytic, Chronic, B-Cell / Q000235:genetics / Q000378:metabolism* / Q000473:pathology; D060488:Low Density Lipoprotein Receptor-Related Protein-6; D016133:Polymerase Chain Reaction; D011485:Protein Binding; D011973:Receptors, LDL / Q000378:metabolism; D011994:Recombinant Proteins / Q000235:genetics / Q000378:metabolism / Q000494:pharmacology; D015398:Signal Transduction / Q000187:drug effects; D014407:Tumor Cells, Cultured; D051153:Wnt Proteins / Q000378:metabolism
C501126:DKK1 protein, human; D036341:Intercellular Signaling Peptides and Proteins; C555475:LRP6 protein, human; D060488:Low Density Lipoprotein Receptor-Related Protein-6; D011973:Receptors, LDL; D011994:Recombinant Proteins; D051153:Wnt Proteins; D000071679:Glycogen Synthase Kinase 3 beta; D038362:Glycogen Synthase Kinase 3