IL-1beta sensitizes intervertebral disc annulus cells to fluid-induced shear stress

Chronic inflammation and altered mechanical loading are implicated as contributors to intervertebral disc degeneration. Biomechanical and biochemical factors play a role in disc degeneration but have received limited study. Mechanically, intervertebral discs are sheared during bending or twisting of the trunk. Biochemically, IL-1beta, detected in degenerative discs, promotes metalloproteinase expression. We hypothesized that disc cells might respond to shear stress and IL-1beta in a calcium signaling response. We measured the effect of single and combined stimuli on intracellular calcium concentration ([Ca2+]ic) and signaling. Cells were isolated from annulus tissue, cultured to quiescence, plated on collagen-bonded Culture Slips and incubated with Fura-2AM. Cells then were incubated in IL-1beta. Cell response to the effects of fluid flow was tested using FlexFlo, a laminar flow device. Human annulus (hAN) cells responded to laminar fluid flow with a one to three-fold increase in [Ca2+]ic. IL-1beta alone produced a small, transient stimulation. hAN cells pretreated with IL-1beta responded to shear with a more dramatic and sustained increase in [Ca2+]ic, six to ten-fold over basal level, when compared to shear then IL-1beta or shear and IL-1beta alone (P<0.001 for all comparisons). This is the first study documenting synergism of a signaling response to biomechanical and biochemical stimuli in human disc cells. IL-1beta treatment appeared to "sensitize" annulus cells to mechanical load. This increased responsiveness to mechanical load in the face of inflammatory cytokines may imply that the sensitivity of annulus cells to shear increases during inflammation and may affect initiation and progression of disc degeneration.