O-Linked beta-N-acetylglucosamine (O-G1cNAc) modification, a reversible post-translational modification, has been implicated in the regulation of protein stability, subcellular localization of proteins and protein-protein interaction. Here, we demonstrate that O-G1cNAc modification regulates the expression of osteocalcin, an osteoblast-specific marker, via Runx2 transcriptional activity in osteoblastic differentiation. Protein-associated O-G1cNAc was increased during osteoblastic differentiation in MC3T3-E1 preosteoblasts. In addition, PUGNAc, an inhibitor of O-G1cNAcase, potentiated the expression of osteocalcin caused by ascorbic acid, parathyroid hormone (PTH) and forskolin. By conducting activity assays of the osteocalcin promoter and transcription factor, we found that the OSE2 site in the osteocalcin promoter and Runx2 were important for increased osteocalcin promoter activity by PUGNAc. Furthermore, PUGNAc led to increased O-G1cNAe modification of Runx2, which regulated the transcription of its target gene osteocalcin. Thus, these data provide evidence that O-G1cNAc modification may be a new mode of osteoblastic differentiation regulation. (C) 2007 Elsevier Inc. All rights reserved.