Finally the potential contribution of CD b cells to the
Finally, the potential contribution of CD11b+ S63845 to the regenerative microenvironment highlights the experimental challenges in defining the mechanism of lung growth. The distinct transcriptional profiles of the CD11b+ cells, and their migration into different anatomic compartments, suggests that the CD11b+ cells participate in a complex network of signaling interactions. Our results suggest that traditional experimental manipulations—such as ablation and reconstitution experiments—are unlikely to clarify the relative contribution of blood-borne leukocytes to lung growth. In addition, the design-based morphometry used to assess growth (e.g. mean linear intercept) are too insensitive to detect “accessory” contributions to the regenerative microenvironment. Rather, we anticipate that an understanding of leukocyte contributions to lung growth will require an understanding of the network of intercellular signaling interactions relevant to neoalveolarization (Chamoto et al., 2011; Lin et al., 2011)—the data presented here represents a contribution toward that goal.
Acknowledgments This work was supported by NIH grants HL75426, HL94567 and HL007734 as well as by the Uehara Memorial Foundation and the JSPS Postdoctoral Fellowships for Research Abroad.
Introduction Obesity is linked with many metabolic disorders including type 2 diabetes, cardiovascular diseases, Alzheimer\'s disease, and even certain cancers (Cornier et al., 2008; Craft, 2007; Hsu et al., 2007). Increases in both adipocyte number and size are the fundamental processes of obesity. The increase in adipocyte number is the result of commitment of mesenchymal stem cells (MSCs) to preadipocytes (Yu et al., 1997) and the mitotic clonal expansion of committed preadipocytes (Tang et al., 2003) during the adipocyte differentiation program. Since MSCs have the capacity to commit to various lineages, e.g., adipocytes, osteocytes, myocytes, and chondrocytes, it is critical for the study of obesity to understand how they commit to the adipocytic lineage. The commitment of stem cells to these different lineages is triggered by many cues in the local tissue microenvironment. Recently, it has been demonstrated that treatment with BMP2/4 induces the commitment of C3H10T1/2 mesenchymal stem cells to the adipocyte lineage (Huang et al., 2009, 2011; Tang et al., 2004), and an independent line of investigation also supports the role of BMPs in the commitment of MSCs to the adipocyte lineage (Bowers et al., 2006). The epithelial–mesenchymal transition (EMT) is a biological process that causes epithelial cells to lose their epithelial phenotype and acquire properties of mesenchymal cells. EMT is characterized by loss of epithelial markers such as Cdh1 and Ocln; the decrease in epithelial genes expression is accompanied by increased expression of mesenchymal genes (Vim, Cdh2, etc.) (Furuse et al., 1993; Hay, 1995; Ikenouchi et al., 2003; Kalluri and Neilson, 2003; Rastaldi, 2006). These alterations are usually accompanied by an increase of cell motility. The epithelial–mesenchymal transition plays crucial roles in the differentiation of multiple tissues and organs (Dedhar et al., 2006). Recent studies have demonstrated that BMP4 induces an EMT (Huber et al., 2005; Satoh et al., 2007; Theriault et al., 2007). We therefore speculated that an EMT also occurs during adipocyte lineage commitment. Lox, a copper-containing amine oxidase, has been shown to be induced by BMP2/4 during adipocyte lineage commitment. Moreover, knockdown of Lox expression prevents adipocyte commitment and differentiation of MEFs (mouse embryonic fibroblasts) as well as C3H10T1/2 stem cells (Huang et al., 2009). Lox is also known to be selectively expressed and to be implicated in hypoxia-induced EMT (Erler et al., 2006; Higgins et al., 2007; Schietke et al., 2010). These findings raised the question of whether BMP4 could regulate Lox expression and thus induce the EMT during adipocyte lineage commitment. In this study, we found that BMP4 induced an EMT-like response during adipocyte lineage commitment and the EMT-like response is required for adipocyte lineage commitment. Our further studies showed that Lox contributes to the BMP4-induced EMT-like response during this commitment process. EMT can be transcriptionally regulated by developmentally important transcription factors including Snai1, Snai2, Twist, and Zeb2 (Bolos et al., 2003; Comijn et al., 2001; Nawshad et al., 2004; Yang et al., 2004). Our studies also demonstrated that only Twist expression was correlated with LOX expression and contributed to adipocyte lineage commitment.