[Expression of p16INK4a in nucleus pulposus and its effect on degenerated intervertebral discs]

To investigate the expression of p16INK4a in nucleus pulposus (NP) and to clarify its relationship with intervertebral disc degeneration so as to provide evidence for biological repair of intervertebral disc.

The NP specimens were obtained from 17 patients with intervertebral disc degeneration undergoing discectomy, who aged 40-50 years (mean, 45.4 years). Based on the preoperative MRI, there were 10 cases of grade III degeneration, and 7 cases of grade IV degeneration. Cell senescence was evaluated by detecting senescence-associated β-galactosidase (SA-β-gal) activity. Senescence marker (p16INK4a) and disc degeneration markers [A disintegrin and metalloproteinase with thrombospondin motifs 5 (ADAMTS 5), Aggrecan, and Sry-related HMG box transcription factor 9 (Sox-9)] were determined in the NP specimens with immunohistochemistry and Western blot. The correlation between ADAMTS 5 and p16INK4a was analyzed.

Clustered distribution of green SA-β-gal-positive cells was seen in the NP with grade III and IV degeneration. A few single round SA-β-gal-positive NP cells (NPCs) wrapped by the layered extracellular matrix were also seen in the NP with grade III degeneration. It was difficult to see single distribution of NPCs in the NP with grade IV degeneration. The percentage of SA-β-gal-positive cells was 22.7% ± 5.4% and 37.1% ± 7.6% in the NP with grade III and IV degeneration respectively, showing significant difference (t=-9.666, P=0.000). The percentages of p16INK4a-positive and ADAMTS 5-positive NPCs in the NP with grade IV degeneration were significantly higher than those with grade III degeneration (P < 0.05). The percentages of Aggrecan-positive and Sox-9-positive NPCs in the NP with grade IV degeneration were significantly lower than those in the NP with grade III degeneration (P < 0.05). The protein expressions of Aggrecan and Sox-9 in the NP with grade IV degeneration were significantly lower than those in the NP with grade III degeneration (P < 0.05). The NP with grade IV degeneration showed significantly higher protein expressions of p16INK4a and ADAMTS 5 (P < 0.05). Importantly, there was a good correlation between p16INK4a and ADAMTS 5 protein expressions (r=0.908, P=0.000).

Premature senescent NPCs increase in the NP with the advancing disc degeneration. The expression of p16INK4a and its association with degeneration grades suggest that the p16INK4a may play a significant role in the pathogenesis of intervertebral disc degeneration.