BACKGROUND

It has been demonstrated that vitamin D deficiency is associated with an increased risk of patients developing lumbar disc herniation. However, intervertebral disc degeneration caused by active vitamin D deficiency has not been reported. Thus, the purpose of this study was to e investigate the role and mechanism of 1,25-dihydroxyvitamin D (1,25(OH)D) insufficiency in promoting intervertebral disc degeneration.

METHODS

The phenotypes of intervertebral discs were compared in wild-type mice and mice with heterozygous deletion of 1α-hydroxylase [1α(OH)ase] at 8 mouths of age using iconography, histology and molecular biology. A mouse model that overexpressed Sirt1 in mesenchymal stem cells on a 1α(OH)ase background (Sirt1/1α(OH)ase) was generated by crossing Prx1-Sirt1 transgenic mice with 1α(OH)ase mice and comparing their intervertebral disc phenotypes with those of Sirt1, 1α(OH)ase and wild-type littermates at 8 months of age. A vitamin D receptor (VDR)-deficient cellular model was generated by knock-down of endogenous VDR using Ad-siVDR transfection into nucleus pulposus cells; VDR-deficient nucleus pulposus cells were then treated with or without resveratrol. The interactions between Sirt1 and acetylated p65, and p65 nuclear localization, were examined using co-immunoprecipitation, Western blots and immunofluorescence staining. VDR-deficient nucleus pulposus cells were also treated with 1,25(OH)D, or resveratrol or 1,25(OH)D plus Ex527 (an inhibitor of Sirt1). Effects on Sirt1 expression, cell proliferation, cell senescence, extracellular matrix protein synthesis and degradation, nuclear factor-κB (NF-κB), and expression of inflammatory molecules, were examined, using immunofluorescence staining, Western blots and real-time RT-PCR.

RESULTS

1,25(OH)D insufficiency accelerated intervertebral disc degeneration by reducing extracellular matrix protein synthesis and enhancing extracellular matrix protein degradation with reduced Sirt1 expression in nucleus pulposus tissues. Overexpression of Sirt1 in MSCs protected against 1,25(OH)D deficiency-induced intervertebral disc degeneration by decreasing acetylation and phosphorylation of p65 and inhibiting the NF-κB inflammatory pathway. VDR or resveratrol activated Sirt1 to deacetylate p65 and inhibit its nuclear translocation into nucleus pulposus cells. Knockdown of VDR decreased VDR expression and significantly reduced the proliferation and extracellular matrix protein synthesis of nucleus pulposus cells, significantly increased the senescence of nucleus pulposus cells and significantly downregulated Sirt1 expression, and upregulated matrix metallopeptidase 13 (MMP13), tumor necrosis factor-α (TNF-α) and interleukin 1β (IL-1β) expression; the ratios of acetylated and phosphorylated p65/p65 in nucleus pulposus cells were also increased. Treatment of nucleus pulposus cells with VDR reduction using 1,25(OH)D or resveratrol partially rescued the degeneration phenotypes, by up-regulating Sirt1 expression and inhibiting NF-κB inflammatory pathway; these effects in nucleus pulposus cells were blocked by inhibition of Sirt1.

CONCLUSION

Results from this study indicate that the 1,25(OH)D/VDR pathway can prevent the degeneration of nucleus pulposus cells by inhibiting the NF-κB inflammatory pathway mediated by Sirt1.: This study provides new insights into the use of 1,25(OH)D to prevent and treat intervertebral disc degeneration caused by vitamin D deficiency.