PDZRN3 Negatively Regulates BMP-2�Cinduced Osteoblast Differentiation through Inhibition of Wnt Signaling

PDZRN3 is a member of the PDZ domain–containing RING finger family of proteins. We previously showed that PDZRN3 is essential for the differentiation of C2C12 mouse mesenchymal progenitor cells into myotubes. Mesenchymal progenitor cells differentiate into osteoblasts, chondrocytes, and adipocytes in addition to myotubes, and we have now examined the potential role of PDZRN3 in the differentiation of C2C12 cells into osteoblasts. The abundance of PDZRN3 in C2C12 cells was increased after the induction of osteoblast differentiation by exposure to bone morphogenetic protein (BMP)-2 in low-serum medium. Depletion of PDZRN3 in C2C12 cells by RNA interference resulted in marked enhancement of the BMP-2–induced up-regulation of alkaline phosphatase (ALP) activity. Dkk1, an inhibitor of Wnt signaling, markedly attenuated the enhancement of the BMP-2–induced increase in ALP activity by PDZRN3 depletion. The up-regulation of ALP activity by Wnta3a was also promoted by depletion of PDZRN3. Furthermore, the expression and Wnt3a-induced phosphorylation of LRP6 as well as the increase in the cytosolic abundance of β-catenin induced by Wnt3a were potentiated in PDZRN3-depleted cells. These results indicate that PDZRN3 plays an important role in negative feedback control of BMP-2–induced osteoblast differentiation in C2C12 cells through inhibition of Wnt–β-catenin signaling.     

Ubiquitin-proteasome-mediated degradation, intracellular localization, and protein synthesis of MyoD and Id1 during muscle differentiation

Mammalian skeletal myogenesis results in the differentiation of myoblasts to mature syncytial myotubes, a process regulated by an intricate genetic network of at least three protein families: muscle regulatory factors, E proteins, and Id proteins. MyoD, a key muscle regulatory factor, and its negative regulator Id1 have both been shown to be degraded by the ubiquitin-proteasome system. Using C2C12 cells and confocal fluorescence microscopy, we showed that MyoD and Id1 co-localize within the nucleus in proliferating myoblasts. In mature myotubes, in contrast, they reside in distinctive subcellular compartments, with MyoD within the nucleus and Id1 exclusively in the cytoplasm. Cellular abundance of Id1 was markedly diminished from the very onset of muscle differentiation, whereas MyoD abundance was reduced to a much lesser extent and only at the later stages of differentiation. These reductions in MyoD and Id1 protein levels seem to result from a change in the rate of protein synthesis rather than the rate of degradation. In vivo protein stability studies revealed that the rates of ubiquitin-proteasome-mediated MyoD and Id1 degradation are independent of myogenic differentiation state. Id1 and MyoD were both rapidly degraded, each with a t 1/2 approximately = 1 h in myoblasts and in myotubes. Furthermore, relative protein synthesis rates for MyoD and Id1 were significantly diminished during myoblast to myotube differentiation. These results provide insight as to the interaction between MyoD and Id1 in the process of muscle differentiation and have implications for the involvement of the ubiquitin-proteasome-mediated protein degradation and protein synthesis in muscle differentiation and metabolism under abnormal and pathological conditions.     

Inhibitory effect of p53 on mitochondrial content and function during adipogenesis

The p53 protein is known as a guardian of the genome and is involved in energy metabolism. Since the metabolic system is uniquely regulated in each tissue, we have anticipated that p53 also would play differential roles in each tissue. In this study, we focused on the functions of p53 in white adipose tissue (adipocytes) and skeletal muscle (myotubes), which are important peripheral tissues involved in energy metabolism. We found that in 3T3-L1 preadipocytes, but not in C2C12 myoblasts, p53 stabilization or overexpression downregulates the expression of Ppargc1a, a master regulator of mitochondrial biogenesis. Next, by using p53-knockdown C2C12 myotubes or 3T3-L1 preadipocytes, we further examined the relationship between p53 and mitochondrial regulation. In C2C12 myoblasts, p53 knockdown did not significantly affect Ppargc1a expression and mtDNA, but did suppress differentiation to myotubes, as previously reported. However, in 3T3-L1 preadipocytes and mouse embryonic fibroblasts, p53 downregulation enhanced both differentiation into adipocytes and mitochondrial DNA content. Furthermore, p53-depleted 3T3-L1 cells showed increase in mitochondrial proteins and enhancement of both Citrate Synthase and Complex IV activities during adipogenesis. These results show that p53 differentially regulates cell differentiation and mitochondrial biogenesis between adipocytes and myotubes, and provide evidence that p53 is an inhibitory factor of mitochondrial regulation in adipocyte lineage.