gpr109a inhibitor br Conflicts of interest br Acknowledgemen

Conflicts of interest
Acknowledgement This research was supported by a grant (2017001360001) from the Korea Ministry of Environment (MOE).
Introduction Mesenchymal stem gpr109a inhibitor (MSCs) have been widely adopted in tissue engineering and regenerative medicine for their multilineage differentiation potential. Bone marrow derived mesenchymal stem cells (BMSCs) were reported to have achieved the most predictable results in bone tissue engineering, but the clinical application of autogenous BMSCs was restricted for limited harvest of the cells and additional donor site morbidity (Mezey, 2011). Adipose stem cells (ASCs) have common origin and similar differentiation potential with BMSCs and thus were proposed a promising alternative (Cai et al., 2011; Konno et al., 2013), for they are more abundant in source and more accessible (Dai et al., 2016). The effects of mechanical stimuli on ASCs have been well documented (Delaine-Smith and Reilly, 2012). The effects of several types of mechanical loading, such as vibrations, stretch and extracorporeal shockwaves, on the osteogenic differentiation of ASCs have been widely investigated. (Maredziak et al., 2017; Virjula et al., 2017; Catalano et al., 2017). As plenty of literature and our previous study reported, similar to BMSCs, under tensile stress, ASCs up-regulate the expression of osteogenic-related genes while adipogenesis and chondrogenesis are inhibited (Charoenpanich et al., 2011; Yang et al., 2012; Li et al., 2015). In most cells, integrin coupled with cytoskeleton, and stretch-activated ion channels coupled with g-protein function as membrane receptors of mechanical stimuli and activate downstream mitogen-activated protein kinases (MAPK) pathways (Delaine-Smith and Reilly, 2012), including ERK1/2, p38 and c-Jun amino (N)-terminal kinases (JNK), MAPKs are activated by a distinct kinase cascade in which a mitogen-activated protein kinase kinase kinase (MEKK) phosphorylates and stimulates a downstream mitogen-activated protein kinase kinase (MEK), which results in a conformational change that evokes MAPKs activity, subsequently regulates the proliferation and differentiation of mechano-responsive cells (Chang and Karin, 2001; Suzuki et al., 2002; Simmons et al., 2003; Liu et al., 2014). In recent years, endeavors to investigate mechanisms that participated in the osteogenic differentiation of ASCs enhanced by mechanical stimuli achieved substantial advances. Under tensile stress, miR-154-5p negatively regulates ASCs osteogenic differentiation through the Wnt/PCP, the upstream pathway of MAPK pathways (Li et al., 2015). It has also been reported that extracorporeal shockwaves enhance the osteogenic differentiation of ASCs via ERK1/2 pathways (Catalano et al., 2017). However, the molecular mechanisms through which tensile stress improves osteogenesis of ASCs are rarely reported. Our previous study showed that cyclically stretching ASCs for either two or six consecutive hours was shown to significantly promote the expression of BMP-2 and Runx2 while loading for 17 min each day for three, seven, or ten consecutive days made no difference, indicating that the mechanisms in stretch-induced osteogenic differentiation may have time-dependent features (Yang et al., 2010; Yang et al., 2012).
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Discussion The mechanisms through which mechanical signals regulate stem cell differentiation have been investigated under varied contexts. The immediate membrane and intracellular receptors including integrin, focal adhesion kinases (FAKs), and components of cytoskeleton have been well identified (Huang et al., 2004). Activated by these proteins, downstream signaling pathways subsequently regulate cell behaviors. However, in ASCs, pathways involved in mechanical signal transduction and consequent change in direction or progress of differentiation are under-investigated. It has been reported that long term mechanical stimulation elicits epigenetic modifications controlling osteogenic differentiation of ASCs and contributes to accelerated osteogenesis in vitro (Vlaikou et al., 2017). Uniaxial mechanical tension is reported to induce the osteogenic differentiation of tendon stem cell via the Wnt5a/Wnt5b/JNK signaling pathway (Liu et al., 2015). Moreover, negative pressure wound therapy, which acts as tensile stress exerted on cells, promotes muscle-derived stem cell osteogenic differentiation through MAPK pathways (Liu et al., 2018).