Mesenchymal Stem Cells Self-Regulate Their Differentiation To Maintain The Bone Marrow Stromal System.

Abstract Mesenchymal stem cells (MSCs) account for a very small population in bone marrow stroma as a non-hematopoietic component with multipotency of differentiation into adipocytes, osteocytes and chondrocytes. MSC-derived cells are known to have hematopoiesis-supporting and immunomodulatory abilities. Although clinical applications of MSCs have already been conducted for the suppression of graft versus host disease in allogeneic stem cell transplantation and for tissue regeneration, underlying mechanisms of the biological events are still obscure. Previously, we established a differentiation model of MSCs using a mouse embryo fibroblast cell line, C3H10T1/2 (10T1/2) (Nishikawa M et al: Blood81:1184–1192, 1993). Preadipocyte (A54) and myoblast (M1601) cell lines were cloned by treatment with 5-azacytidine. A54 cells and M1601 cells can terminally differentiate into adipocytes and myotubes, respectively, under appropriate conditions, while parent 10T1/2 cells remain undifferentiated. Moreover, A54 cells show a higher ability to support hematopoiesis compared with the other cell lines. In this study, we analyzed gene expression profiles of the three cell lines by using DNA microarray and real-time PCR to investigate molecular mechanisms for maintaining immaturity of parent 10T1/2 cells. In A54 cells, 202 genes were up-regulated, including those encoding critical factors for hematopoiesis such as SCF, Angiopoietin-1, and SDF-1 as well as genes known to be involved in adipocyte differentiation such as C/EBPα, C/EBPδ and PPAR-γ genes. These data are consistent with the hematopoiesis-supporting ability of A54 cells. During adipocyte differentiation, SCF and SDF-1 expression levels decreased in A54 cells while C/EBPα expression showed a steady level. Recently, osteoblasts have been reported to play crucial roles in “niche” for self-renewal of hematopoietic stem cells. Our results also implicate that precursor cells of non-hematopoietic components may have important roles for hematopoiesis in bone marrow. Meanwhile, in parent 10T1/2 cells, 105 genes were up-regulated, including CD90, Dlk, Wnt5α and many functionally unknown genes. Although C/EBPα expression was induced in 10T1/2 cells without differentiation under the adipocyte differentiation conditions, CD90 expression decreased, Dlk showed a steady level and Wnt5α was up-regulated. Assuming that some regulatory mechanisms are needed to keep an immature state of parent 10T1/2 cells even under the differentiation-inducible conditions, we performed following experiments. First, enforced Dlk expression in A54 cells did not inhibit terminal differentiation to adipocytes under the differentiation conditions. Second, when we cultured A54 cells in the conditioned media of parent 10T1/2 cells under the differentiation-inducible conditions, adipocyte differentiation was inhibited, suggesting that 10T1/2 cells produce some soluble molecules that can inhibit adipocyte differentiation. Since Wnt family is known to be involved in the regulation of self-renewal of several stem cells, Wnt5α may be one candidate for maintenance of “stemness” of MSCs. Taken together, the data of 10T1/2 cells suggest that MSCs can self-regulate their differentiation in the bone marrow stromal system. This concept may be important to investigate the fatty change of bone marrow in aging and in aplastic anemia.