BMP-2 and TGF-β Stimulate Expression of β1,3-Glucuronosyl Transferase 1 (GIcAT-1) in Nucleus Pulposus Cells Through AP1, TonEBP, and Sp1: Role of MAPKs

The goal of the study was to investigate bone morphogenetic protein 2 (BMP-2) and transforming growth factor beta (TGF-beta) control of the expression of beta 1,3-glucuronosyl transferase 1 (GIcAT-1), an important regulator of chondroitin sulfate synthesis in cells of the nucleus pulposus. Treatment with both growth factors resulted in induction of GIcAT-1 expression and promoter activity. Deletion analysis indicated that promoter constructs lacking AP1 and TonE sites were unresponsive to growth factor treatment. Experiments using dominant-negative proteins showed that these transcription factors along with Sp1 were required for induction of GIcAT-1 promoter activity. Moreover, when either AP1 or TonE binding sites were mutated, induction was suppressed. Both BMP-2 and TGF-beta increased c-Jun and TonEBP expression and phosphorylation of transactivation domains. We investigated the role of the mitogen-activated protein kinase (MAPK) signaling pathway following growth factor treatment; a robust and transient activation of ERK1/2, p38, and JNK was noted. Treatment with MAPK inhibitors blocked BMP-2- and TGF-beta-induced AP1 reporter function, GIcAT-1 expression, and GAG accumulation. We found that DN-ERK1 but not DN-ERK2 resulted in suppression of growth factor-mediated induction of GIcAT-1 promoter activity; we also showed that p38 delta was important in GIcAT-1 activation. Results of these studies demonstrate that BMP-2 and TGF-beta regulate GIcAT-1 expression in nucleus pulposus cells through a signaling network comprising MAPK, AP1, Sp1, and TonEBP. It is concluded that by controlling both GAG and aggrecan synthesis, these growth factors positively influence disk cell function. 2010 American Society for Bone and Mineral Research.