, a member of the PDZRN (or LNX) family of proteins, is essential for the differentiation of mesenchymal stem cells into myotubes, but it plays an inhibitory role in the differentiation of these cells into osteoblasts. Given that mesenchymal stem cells also differentiate into adipocytes, we examined the possible role of PDZRN3 in adipogenesis in mouse 3T3-L1 preadipocytes. The expression of PDZRN3 decreased at both the mRNA and protein levels during adipogenic differentiation. RNAi-mediated depletion of PDZRN3 enhanced the differentiation of 3T3-L1 cells into adipocytes as assessed on the basis of lipid accumulation. The upregulation of aP2 and CCAAT/enhancerbinding protein (C/EBP)- during adipocyte differentiation was also enhanced in the PDZRN3-depleted cells, as was the induction of peroxisome proliferator-activated receptor-␥ (PPAR␥), an upstream regulator of aP2 and C/EBP␣, at both the mRNA and protein levels. Among transcription factors that control the expression of PPAR␥, we found that STAT5b, but not STAT5a, was upregulated in PDZRN3-depleted cells at both mRNA and protein levels. Tyrosine phosphorylation of STAT5b, but not that of STAT5a, was also enhanced at an early stage of differentiation by PDZRN3 depletion. In addition, the expression of C/EBP during the induction of differentiation was enhanced at the mRNA and protein levels in PDZRN3-depleted cells. Our results thus suggest that PDZRN3 negatively regulates adipogenesis in 3T3-L1 cells through downregulation of STAT5b and C/EBP and consequent suppression of PPAR␥ expression. adipogenesis; C/EBP; differentiation; PDZRN3; PPAR␥; STAT5 THE PDZ (PSD-95/DISCS-LARGE ZO-1) domain mediates protein-protein interactions and thereby contributes to intracellular signaling. Members of the PDZ domain-containing RING finger (PDZRN or LNX) family of proteins share a common domain organization that includes an NH 2 -terminal RING domain, two or four PDZ domains in the central region, and a COOHterminal consensus motif for binding to the PDZ domain (9). PDZRN3 and PDZRN4 constitute a subfamily of PDZRN proteins with two PDZ domains. PDZRN3 is expressed in a variety of human tissues including heart, skeletal muscle, liver, and brain (11). We previously showed that PDZRN3 is essential for myogenic differentiation of myoblasts into myotubes with the use of C2C12 mesenchymal progenitor cells (11). PDZRN3 also regulates surface expression of the musclespecific receptor tyrosine kinase (MuSK) at the neuromuscular junction by functioning as a synapse-associated E3 ubiquitin ligase (12). On the other hand, PDZRN3 plays an inhibitory role in the differentiation of C2C12 cells into osteoblasts by suppressing Wnt signaling (8). In addition to mesenchymal stem cells, the expression of PDZRN3 was recently detected in the central nervous system of zebrafish including rhombomere 1, ventral retina, thalamus, and motor neurons, suggesting that the protein may also play a role during neural development (3).Mesenchymal stem cells differentiate into adipocytes in addition to myo...