Smad3 controls β-1,3-glucuronosyltransferase 1 expression in rat nucleus pulposus cells: implications of dysregulated...

To study the regulation of expression of β-1,3-glucuronosyltransferase 1 (GlcAT-1), an important regulator of glycosaminoglycan (GAG) synthesis, by Smad3 in nucleus pulposus (NP) cells.

GlcAT-1 expression was examined in rat NP and anulus fibrosus (AF) cells treated with transforming growth factor β (TGFβ). The effects of Smad signaling and Smad suppression on GlcAT-1 were examined in rat NP cells. GlcAT-1 expression was analyzed in the discs of Smad3-null mice and in degenerated human NP tissue.

TGFβ increased the expression of GlcAT-1 in rat NP but not rat AF cells. Suppression of GlcAT-1 promoter activity was evident with dominant-negative ALK-5 (DN-ALK-5). Cotransfection with Smad3 strongly induced promoter activity independent of TGFβ. Bioinformatics analysis indicated the presence of several Smad binding sites in the promoter; deletion analysis showed that the region between -274 and -123 bp was required for Smad3 response. DN-Smad3, Smad 3 small interfering RNA, and Smad7 strongly suppressed basal as well as TGFβ-induced promoter activity. Induction of promoter activity by Smad3 was significantly blocked by DN-Smad3; Smad7 had a very small effect. Lentiviral transduction of NP cells with short hairpin RNA Smad3 resulted in a decrease in GlcAT-1 expression and accumulation of GAG. Compared to wild-type mice, significantly lower expression of GlcAT-1 was seen in the discs of Smad3-null mice. Analysis of degenerated human NP tissue specimens showed no positive correlation between GlcAT-1 and TGFβ expression. Moreover, isolated cells from degenerated human tissue showed a lack of induction of GlcAT-1 expression following TGFβ treatment, suggesting an altered response.

Our findings demonstrate that in healthy NP cells, the TGFβ-Smad3 axis serves as a regulator of GlcAT-1 expression. However, an altered responsiveness to TGFβ during disc degeneration may compromise GAG synthesis.