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Discussion
hMSCs are one of the human tissue stem cells, and they maintain the homeostasis of bone and cartilage. hMSCs are found in adult human bone marrow, and those obtained from patients until late adulthood still exhibit osteogenic potency [29]. Thus, it is thought that hMSCs maintain self-renewal and differentiation capacity in vivo throughout life. However, our previous studies have shown that the self-renewal potency of hMSCs is decreased by long-term culture in vitro [27]. The results of this study suggest that cellular senescence was induced in hMSCs ( Fig. 2) following G1 cell growth arrest through increases of p16INK4a, p21Cip1, and p53 mRNA Dabrafenib (GSK2118436) levels ( Fig. 2 and Fig. 3) due to long-term culture in vitro. Since it was reported that cellular senescence was induced by the stress of culture [30], hMSCs would also be subject to finite proliferation due to many unknown stresses in our studies.
It was reported that TGF-β1 induced changes in hMSC morphology [28]. In the present study, after 5 days’ treatment with TGF-β1 and TGF-β2 (TGF-βs), hMSCs were spread out, some of the cells were stained blue by SA-β-Gal staining (Fig. 1A–F), and they had decreased DNA replicative potential (Fig. 1G). In human prostate stromal cells, TGF-β1 induced similar morphological changes, but had no effect on cellular senescence [31]. However, we hypothesize that the changes in hMSC morphology induced by TGF-βs were due to cellular senescence because the conditions of our study differed from those of previous studies: hMSCs were treated with 5 ng/ml TGF-βs for 5 days in this study (Fig. 1), whereas hMSCs were treated with 1 ng/ml TGF-β1 for 3 days in the previous study [31]. Moreover, the responses to TGF-β stimulation may depend on the kind of cell.