Low Density Lipoprotein Receptor-Related Protein 12 (LRP12) is a newly identified member of the LRP family. It is expressed in normal fibroblasts, but only at very low levels in human fibrosarcoma cells. A previous study in this laboratory suggested that the cytoplasmic tail of LRP12 interacts with several signaling proteins, including a truncated form of the SMAD Anchor for Receptor Activation (SARA). SARA is a protein in the TGF-β signaling pathway. To determine whether LRP12 formed a complex with SARA, we co-transfected LRP12 and full-length SARA DNA into 293 cells. These proteins co-immunoprecipitated, indicating complex formation. When we treated SHAC fibrosarcoma cells expressing LRP12 with TGFβ1 (2.5ng/ml), we found that Smad2 exhibited increased phosphorylation level than the vector controls. SHAC cells treated with TGFβ1 also expressed higher levels of Plasminogen Activator Inhibitor type 1 (PAI-1) protein in the cells and in the culture medium than the control cells when assayed by Western Blotting and ELISA. It is known that TGF-β signaling pathway can cross-talk with the MAPK pathway. Our results also showed that expression of LRP12 could enhance the TGFβ-induced activation of Erk/JNK/p38 kinases of the MAPK pathway. These data suggest that reduced expression of LRP12 in SHAC cells may lead to aberrant TGF-β signaling, which can be expected to play a role in the malignant transformation of these cells. (Supported by NIH CA98305 to JJM).
[Proc Amer Assoc Cancer Res, Volume 46, 2005]