Toward a Biochemical Understanding of Human Intervertebral Disc Degeneration and Herniation

Study Design. Normal and herniated human intervertebral disc specimens were cultured to study the effects of interleukin-1β on the production of nitric oxide, interleukin-6, prostaglandin E2, and matrix metalloproteinases. The effects of endogenously produced nitric oxide on the synthesis of other mediators also were studied. Objectives. To test the hypothesis that the cells of the intervertebral disc are metabolically active and are capable of responding to biochemical stimuli such as interleukin-1β in a manner that could engender degenerative changes. As part of this study, the authors also investigated some of the possible autocrine regulatory mechanisms that may operate during the biochemical responses of disc cells. Summary of Background Data. The authors previously showed, for the first time, that herniated cervical and lumbar disc specimens spontaneously produce increased amounts of nitric oxide, interleukin-6, prostaglandin E2, and certain matrix metalloproteinases. These results suggest that these biochemical agents are in some manner involved with degenerative processes in the intervertebral disc. This novel hypothesis merits further evaluation; the current communication reports the results of experiments designed to do so. Methods. Fourteen normal, nondegenerated discs (control group) were obtained from seven patients undergoing anterior spinal surgery for trauma or lumbar scoliosis. Thirty-six herniated discs (18 lumbar and 18 cervical) were obtained from 30 patients undergoing surgery for persistent radiculopathy. The specimens were placed into tissue culture and incubated for 72 hours in the presence or absence of interleukin-1β and NG-monomethyl-L-arginine, an inhibitor of nitric oxide synthases, and the media were subsequently collected for biochemical analysis. Biochemical assays for matrix metalloproteinases, nitric oxide, interleukin-6, and prostaglandin E2 were performed. Results. Normal, control disc specimens significantly increased their production of matrix metalloproteinases, nitric oxide, interleukin-6, and prostaglandin E2 in response to interleukin-1β. Herniated lumbar and cervical discs, which were spontaneously releasing increased levels of these biochemical agents, further increased their production of nitric oxide, interleukin-6, and prostaglandin E2 in response to interleukin-1β. Blocking the biosynthesis of nitric oxide in interleukin-1β-stimulated disc cells provoked a large increase in the production of interleukin-6. Conclusions. Cells of the intervertebral discs are biologically responsive and increase their production of matrix metalloproteinases, nitric oxide, interleukin-6, and prostaglandin E2 when stimulated by interleukin-1β. The effect is more dramatic in normal, nondegenerated discs where spontaneous synthesis of these mediators is low. Nevertheless, cells of the herniated degenerated discs where spontaneous production was high were still capable of further increasing their synthesis of several of these biochemical agents in response to interleukin-1β. Endogenously produced nitric oxide appears to have a strong inhibitory effect on the production of interleukin-6, which suggests that autocrine mechanisms play an important role in the regulation of disc cell metabolism.