Intervertebral disc cell apoptosis by nitric oxide: biological understanding of intervertebral disc degeneration

While the unphysiological mechanical load is a central etiologic factor of disc degeneration, biologic factors including nitric oxide (NO) seems to play an important role in this condition. It is, therefore, investigated whether NO is related to the degeneration of intervertebral disc by way of inducing the disc cell apoptosis. Twelve herniated lumbar disc and eight control specimens were obtained from the patients underwent surgery. Apoptotic cells were identified by TUNEL procedure and the percent apoptotic cell index (ACI%) of each sample was calculated. Detection of iNOS expression was performed by immunohistochemical analysis. Disc cell monolayer culture was prepared from the surgical specimen of the patients with lumbar disc herniation. NO generation of the disc cells was measured by Griess reaction. Cell proliferation was detected by measuring the incorporation of 3H-Thymidine. The extent of fragmented DNA induced by NO donor, NOC-18, was also measured by an enzyme-linked immunosorbent assay. The incidence of apoptotic cell death (ACI%) was greater in the herniated group (61.3 +/- 24.5%) than that of control group (5.6 +/- 6.8%; P < 0.001). iNOS positive disc cells were detected in all the samples. NO production of disc cells was enhanced by the stimulation of IL-1 alpha. Suppression of 3H-Thymidine incorporation and DNA fragmentation in the disc cells were promoted by treatment of 100 microM NOC-18. These results suggest that disc cells are able to release NO and NO may play an important role in the pathogenesis of disc degeneration through the induction of apoptosis of disc cells in situ.