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c-Maf and you won’t see fat
Laurie K. McCauley
Department of Periodontics and Oral Medicine, School of Dentistry, and Department of Pathology, Medical School, University of Michigan, Ann Arbor, Michigan, USA.
Address correspondence to: Laurie K. McCauley, University of Michigan, 1011 N. University Ave., Room 3397, Dept. POM, Ann Arbor, Michigan 48109, USA. Phone: 734.647.3206; Fax: 734.763.5503; E-mail: mccauley@umich.edu.
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Published September 27, 2010 - More info
J Clin Invest. 2010;120(10):3440–3442. https://doi.org/10.1172/JCI44786.
© 2010 The American Society for Clinical Investigation
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Abstract
Aging leads to the disruption of the homeostatic balance of multiple biological systems. In bone marrow multipotent mesenchymal cells undergo differentiation into various anchorage-dependent cell types, including osteoblasts and adipocytes. With age as well as with treatment of antidiabetic drugs such as thiazolidinediones, mesenchymal cells favor differentiation into adipocytes, resulting in an increased number of adipocytes and a decreased number of osteoblasts, causing osteoporosis. The mechanism behind this differentiation switch is unknown. Here we show an age-related decrease in the expression of Maf in mouse mesenchymal cells, which regulated mesenchymal cell bifurcation into osteoblasts and adipocytes by cooperating with the osteogenic transcription factor Runx2 and inhibiting the expression of the adipogenic transcription factor Pparg. The crucial role of Maf in both osteogenesis and adipogenesis was underscored by in vivo observations of delayed bone formation in perinatal Maf–/– mice and an accelerated formation of fatty marrow associated with bone loss in aged Maf+/– mice. This study identifies a transcriptional mechanism for an age-related switch in cell fate determination and may provide a molecular basis for novel therapeutic strategies against age-related bone diseases.
Authors
Keizo Nishikawa, Tomoki Nakashima, Shu Takeda, Masashi Isogai, Michito Hamada, Ayako Kimura, Tatsuhiko Kodama, Akira Yamaguchi, Michael J. Owen, Satoru Takahashi, Hiroshi Takayanagi
Osteoporosis is a metabolic bone disease that results from an imbalance between the processes of bone formation and bone resorption, leading to reduced bone mass and increased susceptibility to fracture. Osteoporosis is commonly age associated, and one out of two women and one out of four men over the age of 50 will experience an osteoporotic fracture during their lifetime, with patient care costs estimated at $14 billion per year (1, 2). Recent research has identified a correlative relationship of fat deposition in bone marrow with bone density. Many but not all studies link the two in an inverse relationship that suggests marrow fat may be driven at the expense of new bone formation. While the role of fat in bone remains unclear, current thinking supports a reciprocal relationship between adipogenesis and osteogenesis governed by mesenchymal stem cell (MSC) lineage allocation.
The bone marrow houses a wide variety of cell types at various stages of differentiation. Cells of the mesenchymal lineage reside in the marrow and give rise to osteoblasts and adipocytes, among other cell types (3). Over the past decade, a considerable effort has been devoted toward understanding how MSC fates can be directed toward adipogenic and osteogenic lineages. The critical determinants of this switch are unclear, but a complete understanding of the process may direct the development of therapies for age-associated bone loss. The mutual exclusivity of the osteoblast and adipocyte cell fates suggests that signals that direct cells down one lineage may prevent them from traversing down the other. Prime examples of these switch signals are CCAAT/enhancer binding protein (C/EBP), a trigger for adipogenesis; PPARγ, which promotes adipocyte maturation; and Runx2, an osteoblastic transcriptional mediator (4). Impaired PPARγ signaling shifts the fate of MSCs in the marrow toward the osteoblast lineage (5). The wnt pathway can suppress PPARγ, favoring MSC differentiation to osteoblasts (6). Other transcriptional mediators associated with osteoblast/adipocyte specification include ΔFosB, TAZ, Esr1, Msx2, C/EBPβ, and Id4, though their specificity as determinants of the age-related switch in bone is unclear (7, 8).
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Identification of Maf as a cell fate switch factor
In this issue, Nishikawa et al. demonstrate, in an elegant series of studies, that the basic leucine-zipper (bZIP) transcription factor Maf (musculoaponeurotic fibrosarcoma, also known as c-Maf) is central to osteoblast lineage commitment with age (9) (Figure 1). Via a genome-wide screening of transcription factors expressed in cells derived from calvaria, Maf was identified as a factor that increased more than 4-fold during osteoblastogenesis. It was further found to be highly expressed in bone marrow stromal cells (BMSCs) and reduced in expression in BMSCs during aging (32- vs. 8-week-old mice).
Figure 1Maf expression determines whether MSCs differentiate toward the adipocyte or osteoblast lineage. The transcription factor Maf partners with Runx2 to regulate osteoblastic genes, such as osteocalcin (OCN). Maf is downregulated during aging and under the influence of reactive oxygen species (ROS) and thiazolidinediones (TZDs), pharmaceutical drugs for the treatment of diabetes. Downregulation of Maf leads to increased PPARG through the activation of C/EBPδ/α and leads to increased adipogenesis. PPARG itself also downregulates Maf, promulgating a cycle of adipogenesis. Antioxidants promote Maf expression and reverse the adipogenic phenotype, resulting in greater bone formation. Other factors that increase Maf may provide promising therapeutic strategies for the treatment of age-associated bone loss.
The Maf transcription factor was originally identified as a retina-specific factor and a T helper cell factor responsible for IL-4 production (10, 11). The first description of its deletion in mice indicated runting that was attributed to a cartilage phenotype (11). Maf is a member of the group of large Maf family members characterized by the presence of the bZIP domain (12). The bZIP domain consists of a region rich in basic residues, followed by heptad repeats of hydrophobic residues, forming a leucine zipper. A conserved amino-terminal domain in the large Maf proteins is rich in Asp, Glu, Ser, Thr, and Pro residues, which are targets for phosphorylation that likely regulate Maf function (13). Several groups of bZIP proteins have been well described and have prominent roles in bone biology, including the AP-1 and CREB/ATF families (14). These other bZIP proteins bind relatively small consensus sequences of 7–8 bp in length, whereas the Maf proteins bind longer consensus sequences of 13–14 bp, termed Maf-recognition elements (MAREs) (12). This study by Nishikawa et al. found 5 MARE sequences in the osteocalcin promoter and demonstrated a physical interaction between Maf and Runx2 on the osteocalcin gene.
A series of experiments with Maf–/– and heterozygous mice unraveled the downstream mechanisms of Maf in the skeleton. Because Maf–/– mice suffer perinatal lethality, studies focused on early osteogenesis. Bone formation was severely impaired with low alkaline phosphatase and osteocalcin gene expression. Interestingly, Runx2 expression and rates of osteoblast proliferation and apoptosis were not altered. Since Maf deletion was previously found to result in a severe cartilage phenotype, the impact of the cartilaginous development on osteogenesis was questioned. Defective bone formation in intramembraneous bones along with cell-autonomous osteoblast differentiation defects in vitro suggest that the role of Maf in skeletal development could not be explained entirely by the cartilage phenotype. Instead, the in vitro cell studies pointed to Maf as a critical determinant of adipogenic (e.g., PPARγ, C/EBPα, and FABP4) versus osteoblastogenic (alkaline phosphatase, osteocalcin) gene expression. Furthermore, studies with Maf+/– mice revealed reduced bone formation and increased numbers of bone marrow adipocytes with age in the absence of a cartilage phenotype. This bone phenotype was rescued with the addition of osteoblastic cells overexpressing Maf.
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