Thiazolidinediones Inhibit Mouse Osteoblastic MC3T3-E1 Cell Proliferation in Part Through the Wnt Signaling Pathway

Background: Peroxisome proliferator-activated receptor gamma (PPAR gamma) and Wnt play different roles in bone homeostasis. Thiazolidinediones are PPAR gamma agonists that cause bone mineral density loss. This study investigated the relationship between PPAR gamma and Wnt/beta-catenin signaling in mouse osteoblastic MC3T3-E1 cells. Methods: MC3T3-E1 cells were treated with either pioglitazone (Pio) or rosiglitazone (Rosi), thiazolidinediones, for 24 hours at 10 to 40-mu M concentrations. Recombinant mouse Wnt3a protein (50 ng/mL) for 6 hours was also used to treat the 20-mu M Pio and Rosi pretreated cells. Cell proliferation was measured by MTT, and apoptosis with flow cytometry using annexin V/propidium iodide staining; reverse transcriptase-polymerase chain reaction measured mRNA expression levels of LRP5/6 (low-density lipoprotein-related protein 5/6), glycogen synthase kinase 3 beta (GSK3 beta), TCF7L2 (transcription factor 7-like 2), PPAR gamma, and cyclin D1, and Western blots detected beta-catenin and p-GSK3 beta proteins. Results: Pioglitazone and Rosi decreased MC3T3-E1 cell viability by 28.07% and 18.14% at 20 mu M, respectively (P < 0.05). Apoptosis increased compared with controls (7.21%), after 20-mu M treatment with Pio or Rosi, to 10.45% and 12.10%, respectively (P < 0.05). Both Pio and Rosi decreased beta-catenin protein levels and increased p-GSK3 beta, but the LRP5/6, GSK3 beta, and TCF7L2 mRNA levels were constant. Upon activation of the Wnt pathway by mouse Wnt3a protein, beta-catenin and p-GSK3 beta protein levels were reversed, accompanied with increased proliferation, but apoptosis remained high. Conclusions: Activation of PPAR gamma in osteoblasts accompanied Wnt signaling suppression. Activation of Wnt signaling alleviated the PPAR gamma proliferation decreases but not the apoptosis increases. The thiazolidinedione PPAR gamma agonists act in part through inhibition of the Wnt signaling pathway, showing there is a relationship between PPAR gamma and Wnt signaling.