Human Nucleus Pulposus Cells React to IL-6: Independent Actions and Amplification of Response to IL-1 and TNF-α

STUDY DESIGN.: Human nucleus pulposus cells were activated with IL-6 plus IL-6 soluble receptor (sR) in the presence or absence of IL-1β or TNF-α. Cell production of factors modulating the anabolic/catabolic balance of the disc and proteoglycan synthesis were determined. OBJECTIVE.: To evaluate NP cell response to exogenous IL-6, and how IL-6 modulates IL-1 and TNF-α actions in these cells. SUMMARY OF BACKGROUND DATA.: Interleukin-6 (IL-6) is produced by cervical and lumbar herniated discs and is associated with neurological symptoms of intervertebral disc degeneration. It upregulates catabolic gene expression and downregulates matrix protein gene expression in chondrocytes. However, no studies have evaluated the effects of IL-6 on disc nucleus pulposus (NP) cells. METHODS.: NP cells from degenerated human discs were expanded in monolayer, maintained in alginate bead culture, and activated with IL-6 plus IL-6 soluble receptor (sR), in the presence or absence of IL-1β or TNF-α. Conditioned media was collected and analyzed for nitrite, PGE-2, TIMP-1, MMP-3, VEGF, and IL-8. Proteoglycan synthesis was assayed as S-sulfate incorporation normalized to DNA content and relative gene expression measured by rtPCR. RESULTS.: IL-6 + sR decreased collagen and aggrecan message, proteoglycan synthesis, and exacerbated the downregulation of proteoglycan synthesis effected by IL-1. PGE-2 synthesis was increased by IL-6 + sR, as was the induction of COX-2 mRNA. IL-6 + sR also enhanced IL-1 and TNF-α stimulated synthesis of PGE-2. IL-6 + sR induced MMP-3 approximately twofold and increased gene expression and synthesis in cells exposed to IL-1 and TNF-α. MMP-13 induction by TNF-α was also potentiated by IL-6 + sR. IL-6 + sR induced IL-6 gene expression and increased that stimulated by TNF-α fourfold. CONCLUSION.: The results suggest maneuvers to diminish IL-6 production in the disc could provide some protection against the adverse effects of IL-1 and TNF-α, thus, helping preserve disc composition, structure, and function.