Small-molecule-inhibitor-based bone differentiation has been recently exploited as a novel approach to regulating osteogenesis-related signaling pathways. In this study, we identified 1-Azakenpaullone, a highly selective inhibitor of glycogen synthase kinase-3β (GSK-3β), as a powerful inducer of osteoblastic differentiation and mineralization of human mesenchymal stem cells (MSCs). GSK-3β is a serine-threonine protein kinase that plays a major role in different disease development. GSK-3β is a key regulator of Runx2 activity in osteoblastic formation. We evaluated alkaline phosphatase activity and staining assays to assess osteoblast differentiation and Alizarin Red staining to assess the mineralization of cultured human MSCs. Gene expression profiling was assessed using an Agilent microarray platform, and bioinformatics were performed using Ingenuity Pathway Analysis software. Human MSCs treated with 1-Azakenpaullone showed higher ALP activity, increased in vitro mineralized matrix formation, and the upregulation of osteoblast-specific marker gene expression. Global gene expression profiling of 1-Azakenpaullone-treated human MSCs identified 1750 upregulated and 2171 downregulated mRNA transcripts compared to control cells. It also suggested possible changes in various signaling pathways, including Wnt, TGFβ, and Hedgehog. Further bioinformatics analysis employing Ingenuity Pathway Analysis recognized significant enrichment in the 1-Azakenpaullone-treated cells of genetic networks involved in CAMP, PI3K (Complex), P38 MAPK, and HIF1A signaling and functional categories associated with connective tissue development. Our results suggest that 1-Azakenpaullone significantly induced the osteoblastic differentiation and mineralization of human MSCs mediated by the activation of Wnt signaling and the nuclear accumulation of β-catenin, leading to the upregulation of Runx2, a key transcription factor that ultimately promotes the expression of osteoblast-specific genes. Thus, 1-Azakenpaullone could be used as an osteo-promotor factor in bone tissue engineering.

TGF-β is a multifunctional pathway that controls significant cellular and physiological processes and several pathological activities. TGF-β-induced signaling can be triggered upon binding to specific receptors to initiate the transcriptional activation of several genes and cellular processes. However, the detailed role of TGF-β signaling in osteoblast differentiation remains to be explicated. SB525334, a selective TGF-βRI inhibitor, was investigated for its effect on the osteoblastic differentiation of human bone marrow MSCs at different concentrations. Alkaline phosphatase (ALP) activity was used to assess osteoblast differentiation marker, while Alizarin red staining was used as a marker for mineralization. Expressions of osteoblast-specific genes were evaluated using real-time PCR. A migration assay was performed to assess the effect of TGF-β on wound healing. Moreover, immunofluorescent staining for SMAD2/3 and SMAD4 was employed to confirm the activation of the TGF-β pathway. The inhibition of TGF-β1 signaling using a high concentration of SB525334 (3 µM) significantly reduced ALP activity and mineralization and downregulated osteoblast-specific genes. However, the opposite effect was reported using a lower concentration (0.03 µM), where osteoblast-associated genes were significantly upregulated, and ALP activity and mineralization were higher. Significant scratch/wound healing was achieved at a lower concentration of SB525334, while a higher concentration of SB525334 resulted in lower healing. Moreover, a low concentration of SB525334 demonstrated nuclear translocation of SMAD 2/3 and 4. Our study confirms that the effect of TGF-β signaling in bone formation and wound healing is dose-dependent, and the use of TGF-β is recommended as a valuable therapeutic approach.

Glycogen synthase kinase-3 (GSK3) inhibition is a commonly used approach to promote osteogenic differentiation through activation of Wnt signaling. However, 6-bromoindirubin-3′-oxime (BIO), which is commonly used for GSK3 inhibition, also targets JAK/STAT, raising the possibility of dual pathway interference during osteoblast differentiation, as both GSK3 and JAK/STAT pathways are critical regulators of osteoblastogenesis. In this study, we investigated the effect of BIO on the osteoblast differentiation of hMSCs-TERT4. While BIO had no significant effect on cell viability or apoptosis, it markedly inhibited osteoblast differentiation, as evidenced by reduced ALP activity, decreased matrix mineralization, and downregulation of osteoblast-associated markers. Microarray analysis followed by qRT-PCR validation revealed downregulation of Wnt and TGF-β pathway genes. These findings show that BIO suppresses osteoblast commitment and osteogenic differentiation, accompanied by altered Wnt- and TGF-β-related gene expression. This study provides mechanistic insight into the off-target consequences of widely used small molecules and highlights the importance of dissecting pathway-specific roles in stem cell differentiation. Understanding the interplay between GSK3 and JAK signaling is essential for optimizing pharmacological strategies in skeletal regenerative medicine. This study highlights the importance of pathway selectivity when using small molecules in stem cell-based therapies for bone regeneration.