Mesenchymal stem cells (MSCs) are multipotent cells, mainly from bone marrow, and an ideal source of cells in bone and cartilage tissue engineering. A study of the chondrogenic differentiation of MSCs is of particular interest for MSCs-based cartilage regeneration. In this study, we aimed to optimize the conditions for the chrondogenic differentiation of MSCs by regulating WNT signaling using the small molecule WNT inhibitor PKF118-310 and activator BIO. Human mesenchymal stem cells (hMSCs) were isolated from bone marrow aspirates and cultured in hMSCs proliferation medium. Pellet culture was subsequently established for three-dimensional chondrogenic differentiation of 5 weeks. WNT signaling was increased by the small molecule glycogen synthase kinase-3 inhibitor 6-bromoindirubin-3-oxim (BIO) and decreased by the WNT inhibitor PKF118-310 (PKF). The effects of BIO and PKF on the chondrogenesis of hMSCs was examined by real-time PCR, histological methods, and ELISA. We found that activation of canonical WNT-signaling by BIO significantly downregulated the expression of cartilage-specific genes SOX9, COL2A1, and ACAN, and matrix metalloproteinase genes MMP1/3/9/13, but increased ADAMTS 4/5. Inhibition of WNT signaling by PKF increased the expression of SOX9, COL2A1, ACAN, and MMP9, but decreased MMP13 and ADAMTS4/5. In addition, a high level of WNT signaling induced the expression of hypertrophic markers COL10A1, ALPL, and RUNX2, the dedifferentiation marker COL1A1, and glycolysis genes GULT1 and PGK1. Deposition of glycosaminoglycan (GAG) and collagen type II in the pellet matrix was significantly lost in the BIO-treated group and increased in the PKF-treated group. The protein level of COL10A1 was also highly induced in the BIO group. Interestingly, BIO decreased the number of apoptotic cells while PKF significantly induced apoptosis during chondrogenesis. The natural WNT antagonist DKK1 and the protein level of MMP1 in the pellet culture medium were decreased after PKF treatment. All of these chondrogenic effects appeared to be mediated through the canonical WNT signaling pathway, since the target gene Axin2 and other WNT members, such as TCF4 and β-catenin, were upregulated by BIO and downregulated by PKF, respectively, and BIO induced nuclear translocation of β-catenin while PKF inhibited β-catenin translocation into the nucleus. We concluded that addition of BIO to a chondrogenic medium of hMSCs resulted in a loss of cartilage formation, while PKF induced chondrogenic differentiation and cartilage matrix deposition and inhibited hypertrophic differentiation. However, BIO promoted cell survival by inhibiting apoptosis while PKF induced cell apoptosis. This result indicates that either an overexpression or overinhibition of WNT signaling to some extent causes harmful effects on chondrogenic differentiation. Cartilage tissue engineering could benefit from the adjustment of the critical level of WNT signaling during chondrogenesis of hMSC.
Keywords: WNT; cartilage; chondrogenesis; hMSC; tissue engineering.