Highdose TGFβ1 degrades human nucleus pulposus cells via ALK1Smad1/5/8 activation

Transforming growth factor β1 (TGFβ1) can promote the proliferation and differentiation of intervertebral disc cells and participates in its repair process. However, whether TGFβ1 engages in the process of disc degeneration has not yet been fully elucidated. The present study aimed to investigate the function of highdose TGFβ1 on the metabolism of nucleus pulposus cells (NPCs). TGFβ1 levels in human degenerative intervertebral disc tissues and tumor necrosis factor (TNF)αinduced degenerative NPCs were analyzed. Furthermore, NPCs were treated with TGFβ1 and inhibitors of TGFβ1 receptors [ALK tyrosine kinase receptor (ALK) 1 and ALK5] to determine the effect of the receptors in the mediation of NPC degeneration. The NPC state was determined by the components of secretory collagen I/II, tissue inhibitor of metalloproteinase3 (TIMP3) and matrix metalloproteinase (MMP)13. The mRNA expression of Smad1/2/3/5/8, the downstream gene of TGFβ1 mediated by ALK, was also measured. Results showed that TGFβ1 and ALK1 were positively associated with the degree of degeneration of NP or NPCs in vitro, but negatively associated with ALK5. Furthermore, highdoses of TGFβ1 suppressed collagen II, but enhanced collagen I, TIMP3, MMP13, ALK1/5 and Smad1/2/3/5/8 expression. ALK5 inhibition induced the suppression of Smad2/3 and aggravated highdose TGFβ1induced NPC degeneration, as shown by the reduction in collagen II and increase in collagen I, TIMP3 and MMP13. By contrast, ALK1 inhibition resulted in Smad1/5/8 suppression and alleviated highdose TGFβ1induced NPC degeneration. Taken together, it was concluded that highdoses of TGFβ1 contributed to the degeneration of NPCs via the upregulation of ALK1 and Smad1/5/8.