Study design: In vitro studies of the effects of proinflammatory cytokines on the production of nerve growth factor (NGF) by human intervertebral disc (IVD) cells.
Objective: To determine the constitutive expression and production of NGF and the effect of cytokines on the expression of NGF by human IVD cells.
Summary of the background data: NGF may play a role in the collateral sprouting of sensory axons, neural survival, and regulation of nociceptive sensory neurons. NGF is known to be up-regulated by proinflammatory cytokines.
Methods: The presence of NGF protein was analyzed by immunohistochemistry using human IVD cells obtained from cadaveric human spines with no known disc disease (MRI Thompson grades 2-4). The effects of interleukin-1beta (IL-1beta) and tumor necrosis factor-alpha (TNF-alpha) on NGF production and mRNA expression of NGF by IVD cells were examined. The expression of NGF receptors, trkA and p75, was also assessed immunohistochemically.
Results: Cadaveric anulus fibrosus (AF) and nucleus pulposus (NP) cells cultured in vitro in monolayer and in alginate beads positively stained with an anti-NGF antibody. The constitutive production of NGF protein in IVD cells was low (NP) or not detectable (AF). The expression of NGF mRNA was detectable in both cell types. IL-1beta and TNF-alpha up-regulated the NGF mRNA expression and the secretion of NGF protein into the media. TrkA was immunolocalized in AF and NP cells.
Conclusion: Our results demonstrate, for the first time, that human AF and NP cells constitutively express NGF protein and mRNA, and that the proinflammatory cytokines IL-1beta and TNF-alpha stimulate the production of NGF. The precise role of NGF produced by IVD cells in the generation of discogenic pain or on the metabolism of IVD cells, especially under certain physiologic conditions in which cytokines are up-regulated, needs to be clarified in future experimentation.