LRP6 mediates Wnt/β-catenin signaling and regulates adipogenic differentiation in human mesenchymal stem cells.

Human mesenchymal stem cells (hMSC) are subjected to the control of several signal transduction pathways during regeneration processes, whereby Wnt/β-catenin signaling is of pivotal importance. Since there exists only fragmentary knowledge concerning the molecular function of the Wnt-coreceptors LRP5 and LRP6 (low-density lipoprotein receptor-related protein) in hMSC, we studied their impact on Wnt/β-catenin signal transduction by RNA interference. For monitoring changes in β-catenin-dependent transcription in a highly sensitive and specific manner, hMSC were stably transfected with a TCF/LEF reporter gene plasmid. In the presence of the activator Wnt3a, knockdown of LRP6 led to a strong decreased Wnt/β-catenin signaling, while RNAi against LRP5 exhibited no effect in this setting. In a reverse approach, ectopic expression of LRP6 resulted in a strong enhancement of Wnt/β-catenin signaling, whereas overexpression of LRP5 exhibited no increased signaling capacity. Furthermore, only the ectopic expression of LRP6--but not that of LRP5--was able to restore Wnt3a-mediated β-catenin signaling after knockdown of endogenously expressed LRP6. These results demonstrate LRP6 as the predominant Wnt3a LRP-receptor in hMSC, which cannot be substituted by LRP5. In addition, we observed enhanced differentiation toward the adipogenic lineage after RNAi against LRP6 which was associated with the induction of PPAR-γ and fat vacuole formation. Thus, LRP6 is not only indispensable for Wnt3a/β-catenin signaling, but also for the suppression of differentiation of hMSC into the adipogenic lineage. Based on these observations, LRP6 may represent an attractive drug target for manipulating hMSC in cell and tissue regeneration approaches.