TI - LRP5Delta is required for breast tumor cell growth in vitro and in vivo in SCID mice . AB - Control siRNA , siLRP5wt , siLRP5Delta and siLRP5tot were transfected to MCF7 cells and the cell viability was determined 24 hrs and 48 hrs later . Growth inhibition and induction of cell death were seen only with siLRP5Delta and siLRP5tot at both time points ( Figure 5A ) . Reduced expression of LRP5Delta also lead to growth inhibition and cell death in sHPT-1 parathyroid tumor cells , but not in HeLa cells that do express LRP5wt and not LRP5Delta [26] . Tumor growth was then evaluated in a xenograft SCID mouse model . Tumor growth was significantly reduced in transplants of MCF7 cells pretransfected with siLRP5Delta and siLRP5tot , but not with control siRNA when compared to non-transfected cells (Figure 5B) . Thus , LRP5Delta appeared to be necessary for breast tumor cell growth both in cell culture and in SCID mice . If breast tumor cell growth is dependent on LRP5Delta , as strongly suggested by the above experiments , an appropriate anti-LRP5 antibody may reduce cell viability as well as beta-catenin activity . The anti-LRP5 goat polyclonal antibody , that immunoprecipitated both LRP5wt and LRP5Delta ( Figure 1B , Figure 2B and Figure 5C ) , significantly attenuated the non-PHOSphorylated active beta-catenin level and the beta-catenin activity in MCF7 cells , and caused reduced cell viability (Figure 5D) . This was not observed in HeLa cells that only express LRP5wt (Figure 5D) . We also determined effects of the LRP5 antibody on T-47D breast cancer cells , since these cells expressed both LRP5wt and LRP5Delta , showed nuclear accumulation of beta-catenin [25] , and displayed endogenous beta-catenin activity as determined by the TOPFLASH assay (Figure 5C) [14] . The LRP5 antibody significantly inhibited cell growth and attenuated beta-catenin activity also in these cells (Figure 5D) . Treatment with the anti-LRP5 antibody induced significant apoptosis in both breast cancer cell lines , and not in HeLa cells (Figure 5E) . Thus , breast tumor cell growth was dependent on maintained expression of LRP5Delta and continued beta-catenin signaling by the receptor . The latter result is in line with the observation that WNT antagonist SFRPs were shown to suppress MCF7 and T-47D cell colony formation [25] . Compared to LRP5wt , LRP5Delta activated beta-catenin driven transcription more strongly in the presence of WNT3 ligand and in a DKK1-insensitive way , both likely contributing to the oncogenic potential of LRP5Delta . In addition to dephosphorylaTION/PHOSphorylation , specific ubiquitination by Rad6B has been suggested to control beta-catenin stabilization in MDA-MB-231 breast cancer cells [35] . LRP5Delta was found not to be expressed in this cell line ( not shown ) . Although the mechanism by which the LRP5Delta receptor is made and how beta-catenin signaling is activated remain to be understood , the results presented here strongly support an important role of the truncated LRP5 receptor in mammary gland tumorigenesis , and its expression may present an early event during disease progression . Our results furthermore suggest that antibody therapy directed against LRP5Delta , possibly in combination with chemotherapy [39] -[41] , present future treatment options of breast cancer .