Mesenchymal stem cells (MSC) that can differentiate to various connective tissue cells may be useful for autologous cell transplantation to defects of bone, cartilage, and tendon, if MSC can be expanded in vitro. However, a short life span of MSC and a reduction in their differentiation potential in culture have limited their clinical application. The purpose of this study is to identify a growth factor(s) involved in self-renewal of MSC and the maintenance of their multilineage differentiation potential. Fibroblast growth factor-2 (FGF-2) markedly increased the growth rate and the life span of rabbit, canine, and human bone marrow MSC in monolayer cultures. This effect of FGF-2 was more prominent in low-density cultures than in high-density cultures. In addition, all MSC expanded in vitro with FGF-2, but not without FGF-2, differentiated to chondrocytes in pellet cultures. The FGF+ MSC also retained the osteogenic and adipogenic potential throughout many mitotic divisions. These findings suggest that FGFs play a crucial role in self-renewal of MSC.

How cell death and cell proliferation are coordinated during development and after stress is a fundamental question that is critical for an understanding of growth regulation. Our data suggest that dp53 may carry out an ancestral function that promotes animal survival through the coordination of responses leading to compensatory growth after tissue damage.

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tob is a member of antiproliferative family genes. Mice lacking tob are prone to spontaneous formation of tumors. The occurrence rate of diethylnitrosamine-induced liver tumors is higher in tob −/− mice than in wildtype mice. tob −/− p53 −/− mice show accelerated tumor formation in comparison with single null mice. Expression of cyclin D1 mRNA is increased in the absence of Tob and is reduced by Tob. Tob acts as a transcriptional corepressor and suppresses the cyclin D1 promoter activity through an interaction with histone deacetylase. Levels of tob mRNA are often decreased in human cancers, implicating tob in cancer development.Supplemental material is available at http://www.genesdev.org. There is accumulating evidence that genes involved in the negative control of cell growth can function as tumor suppressors. In humans, tob, tob2, ana, pc3b, btg1, and btg2 comprise a family (tob family) of antiproliferative genes (Bradbury et al. 1991;Fletcher et al. 1991;Rouault et al. 1992;Matsuda et al. 1996;Guehenneux et al. 1997;Yoshida et al. 1998;Ikematsu et al. 1999;Buanne et al. 2000). Exogenous expression of Tob family proteins suppresses growth of NIH-3T3 cells by inhibiting G1 progression of the cell cycle (Yoshida et al. 1998;Ikematsu et al. 1999;Guardavaccaro et al. 2000;Maekawa et al. 2002;Suzuki et al. 2002). We showed previously that Tob is a substrate of Erk MAPK, and unphosphorylated Tob suppresses cell-cycle entry of quiescent cells. Erk phosphorylation of Tob blocks the antiproliferative activity (Maekawa et al. 2002;Suzuki et al. 2002), which, at least in part, describes the importance of Erk activation in the cells stimulated by growth factors. When Tob is depleted, Cyclin D1 continues to be expressed and readily progress into S phase during serum starvation (Suzuki et al. 2002). In addition, the antiproliferative activity of Tob is impaired in the presence of exogenously coexpressed Cyclin D1 (Suzuki et al. 2002). These data suggest that tob functions as a tumor suppressor. However, possible involvement of Tob in tumorigenesis and roles of Tob in the control of cyclin D1 expression are unclear.Tob family proteins associate with transcription factors. Virtually all of the Tob family members interact with Caf1 (Rouault et al. 1998;Ikematsu et al. 1999;Yoshida et al. 2001), whose yeast homolog is a component of the CCR4-NOT transcriptional complex (Albert et al. 2000). The CCR4-NOT complex participates in the control of specific sets of genes such as those involved in the late mitotic phase of the cell cycle (Liu et al. 1997). Both BTG1 and BTG2 associate with HoxB9 and estrogen receptor ␣, and modulate their transcription activity (Prevot et al. 2000(Prevot et al. , 2001. Tob associates with Smads transcription complex and affects Smad-mediated gene expression (Yoshida et al. 2000;Tzachanis et al. 2001). This suggests that Tob family proteins are regulators of gene transcription, functioning as either coactivators or corepressors.Here, we report that mice lacking tob are prone to spontaneous formation o...

The mRNA level of basic helix-loop-helix transcription factor DEC1 (BHLHB2)/Stra13/Sharp2 was up-regulated during chondrocyte differentiation in cultures of ATDC5 cells and growth plate chondrocytes, and in growth plate cartilage in vivo. Forced expression of DEC1 in ATDC5 cells induced chondrogenic differentiation, and insulin increased this effect of DEC1 overexpression. Parathyroid hormone (PTH) and PTH-related peptide (PTHrP) suppressed DEC1 expression and the differentiation of ATDC5 cells, but DEC1 overexpression antagonized this inhibitory action of PTH/PTHrP. Transforming growth factor-␤ or bone morphogenetic protein-2, as well as insulin, induced DEC1 expression in ATDC5 cultures where it induced chondrogenic differentiation. In pellet cultures of bone marrow mesenchymal stem cells exposed to transforming growth factor-␤ and insulin, DEC1 was induced at the earliest stage of chondrocyte differentiation and also at the hypertrophic stage. Overexpression of DEC1 in the mesenchymal cells induced the mRNA expressions of type II collagen, Indian hedgehog, and Runx2, as well as cartilage matrix accumulation; overexpression of DEC1 in growth plate chondrocytes at the prehypertrophic stage increased the mRNA levels of Indian hedgehog, Runx2, and type X collagen, and also increased alkaline phosphatase activity and mineralization. To our knowledge, DEC1 is the first transcription factor that can promote both chondrogenic differentiation and terminal differentiation.The development of the vertebrate long bones occurs through the process of endochondral ossification, which is initiated in the embryo with the condensation of mesenchymal cells and then progresses with their commitment and differentiation into chondrogenic cells. By the late embryonic stage, the epiphyseal growth plate has developed with distinguishable, well organized and spatially distinct zones of resting, proliferating, and post-proliferative hypertrophic chondrocytes. The hypertrophic cartilage calcifies and is invaded by capillaries, and is subsequently replaced by new bone (1). Recent studies have identified several transcription factors involved in endochondral ossification. Among these, Sox9 is required for the condensation of prechondrogenic mesenchymal cells, and Sox5 and Sox6, as well as Sox9, are required for the activation of type II collagen expression during chondrogenesis (2). In addition, different sets of Smads are involved in stimulation or inhibition of chondrocyte hypertrophy by transforming growth factor-␤ (TGF-␤) superfamily members (3, 4). Runx2/Cbfa1/AML3/PEBP2␣-A is essential for intramembranous ossification, and mutations in this gene are responsible for cleidocranial dysplasia, a syndrome characterized by open fontanelles and hypoplastic clavicles (5-8). Furthermore, Runx2 plays a crucial role in endochondral ossification. In Runx2-deficient mice, chondrocyte hypertrophy, mineralization, and vascular invasion are suppressed in most parts of the skeleton (9 -13). On the other hand, little is known about the role of the bas...