Transcriptional activation of the tyrosine phosphatase gene, OST-PTP, during osteoblast differentiation

Protein tyrosine phosphatases (PTPs) are critical regulators of cellular phosphorylation functioning in processes such as cell growth, differentiation, and adhesion. Osteotesticular PTP (OST) is the only characterized member of this superfamily whose expression is regulated in osteoblasts and critical for their in vitro differentiation. Such evidence would suggest that this molecule is a key modulator of signaling events during osteogenesis, yet little is known about its genetic regulation. In an effort to examine the molecular mechanisms involved in the cellular regulation of OST, we have characterized its expression in MC3T3 osteoblasts during differentiation. Northern analysis revealed that murine OST mRNA is dramatically regulated during the preosteoblast to osteoblast progression, with predominant expression in differentiated and early mineralizing osteoblasts. This expression pattern is unique to this phosphatase since, in comparison, the structurally similar receptor PTP, LAR, and the intracellular PTP1B show little change during differentiation. Cell density contributes to this upregulated expression as confluent cultures display an increase in OST transcripts within 4 h post-plating. Transient transfection of the OST promoter in differentiating MC3T3 results in a significant increase in transcriptional activation from day 0 to day 5 of differentiation, similar in timing and intensity to the observed upregulation of the endogenous gene. This activation appears to be specific to osteoblasts, since progression to a myoblast phenotype results in no change in reporter gene activity. Culturing these preosteoblast cells in the absence of critical co-factors results in an inhibition of differentiation and leads to a delayed induction of OST transcripts as well as the attenuation of transcriptional activation. These results show that the murine OST gene is regulated at the transcriptional level in an osteoblast-specific, differentiation-dependent manner during the differentiation of MC3T3 osteoblasts. Future studies will help determine the essential regulatory elements within the OST-PTP promoter and the critical signaling pathways important in this regulation.