A Reciprocal Role of the Smad4-Taz Axis in Osteogenesis and Adipogenesis of Mesenchymal Stem Cells

Abstract

Mesenchymal stem cells (MSCs) are multipotent cells that can differentiate into mature cells of various cell types. Although the differentiation process of MSCs requires lineage-specific transcription factors, the exact molecular mechanism that determines MSCs differentiation is not clearly addressed. Here, we demonstrate a Smad4-Taz axis as a new intrinsic regulator for adipoosteogenic differentiation of MSCs and show that this function of Smad4 is independent of the transforming growth factor-β signal. Smad4 directly bound to the Taz protein and facilitated nuclear localization of Taz through its nuclear localization signal. Nuclear retention of Taz by direct binding to Smad4 increased expression of osteogenic genes through enhancing Taz-runt-related transcription factor 2 (Runx2) interactions in the C3H10T1/2 MSC cell line and preosteoblastic MC3T3-E1 cells, whereas it suppressed expression of adipogenic genes through promoting Tazperoxisome proliferator-activated receptor-γ (PPARγ) interaction in C3H10T1/2 and preadipogenic 3T3-L1 cells. A reciprocal role of the Smad4 in osteogenic and adipogenic differentiation was also observed in human adipose tissue-derived stem cells (hASCs). Consequently, Smad4 depletion in C3H10T1/2 and hASCs reduced nuclear retention of Taz and thus caused the decreased interaction with Runx2 or PPARγ, resulting in delayed osteogenesis or enhanced adipogenesis of the MSC. Therefore, these findings provide insight into a novel function of Smad4 to regulate the balance of MSC lineage commitment through reciprocal targeting of the Taz protein in osteogenic and adipogenic differentiation pathways. STEM CELLS 2019;37:368-381 SIGNIFICANCE STATEMENTAlthough the Smad4 protein has been suggested to act as a common Smad in the transforming growth factor-β (TGF-ß) superfamily signaling pathway in human embryonic stem cells, it has been unclear whether Smad4 has a noncanonical role in adipo-osteogenic differentiation of mesenchymal stem cells (MSCs), independent of the TGF-ß and bone morphogenic protein pathways. The study demonstrated that Smad4 plays a crucial role in the regulation of lineage commitment of the MSCs, including human adipose tissue-derived stem cells, into osteoblasts and adipocytes through modulating the retention of Taz in the nucleus during MSC differentiation. The Smad4 is specific to Taz but not YAP. Therefore, the findings provide new insight into a novel mechanism of the Smad4-Taz axis in adipo-osteogenic differentiation of MSCs and demonstrate a reciprocal role of Smad4 as a positive and negative factor in osteogenesis and adipogenesis of MSCs, respectively.

<p>The Role of Tantalum Nanoparticles in Bone Regeneration Involves the BMP2/Smad4/Runx2 Signaling Pathway</p>

“…Smad4/Runx2 signaling is critical in osteogenesis. 31 Several research groups are investigating the role of Smad4/Runx2 signaling in mediating the cell response to biomaterials. 1,34 Increasing evidence indicates that Smad4/ Runx2 signaling is involved in the responses of cells to Ta NPs.…”

“…1,34 Increasing evidence indicates that Smad4/ Runx2 signaling is involved in the responses of cells to Ta NPs. As Smad4 nuclear translocation and accumulation constitute the markers of Smad4/Runx2 signaling activation, 31 we investigated Smad4 and Runx2 protein levels. Elevated nuclear Smad4 protein levels were found after Ta NP treatment, confirming the activation of Smad4/ Runx2 signaling by the Ta NPs.…”

“…In addition to playing a role in BMP2 signaling, Smad4 induces runt-related transcription factor 2 (Runx2) degradation in a ubiquitin proteasome-dependent manner, which directly affects osteoblast differentiation. 31 The Smad4 complex transcribes Runx2 and interacts with Runx2 to initiate the expression of other osteoblast genes. 32 Thus, Smad4 expression is mediated by Smad4/Runx2 signaling at the transcriptional level, and Smad4 regulates Runx2 activity and the expression of other osteoblast genes in a feedback loop.…”

The YAP/TAZ Pathway in Osteogenesis and Bone Sarcoma Pathogenesis

“…A recent study in which YAP and/or TAZ were inactivated at different times of mice embryonal development by knockout revealed opposing effects of YAP and TAZ at different stages of osteoblast development: in osteoblast progenitors, they block differentiation towards the osteoblast lineage by repressing RUNX2 and β-catenin activity, but, in mature osteoblasts and osteocytes, they promote bone formation and inhibit bone resorption [152]. In contrast, conditional knockout of YAP or TAZ in young adult mice demonstrated that YAP stabilizes nuclear β-catenin, while TAZ binds to SMAD4 co-activating RUNX2 to drive osteoblast differentiation of MSC and inhibit adipogenic peroxisome proliferator-activated receptor gamma (PPARγ)-induced gene transcription [153,154]. Interestingly, complexes between RUNX2, YAP/TAZ and SNAIL/SLUG were found associated with the promoter regions of multiple genes involved in osteoblastic differentiation and function in mice [137].…”

Role of Heparin Sodium Salt in the Modulation of Human Umbilical Cord-Derived Mesenchymal Stem Cell Differentiation

“…The WWTR1 nuclear retention leading to its active transcription regulating hippo pathway eventually leads to hard matrix formation that yields formation of osteocytes. 24 Initial findings of WWTR1expression was in relation to RUNX2 where this protein was identified to promote osteoblast differentiation and its regulation along with repressing PPARG and adipogenesis. The findings of this study are in line with Hong et al's study.…”