Low-Frequency Cyclic Stretch Upregulates the Expression of Nuclear Factor Erythroid 2-Related Factor 2 in Human Nucleus Pulposus Cells to Inhibit the Resistin-Induced Interleukin-20 Expression

Background: Resistin may connect obesity and intervertebral disc (IVD) degeneration (IDD) and is linked with chronic inflammation. Furthermore, human IDD is characterized by high expression of interleukin-20 (IL-20). The response of human nucleus pulposus (NP) cells to tensile forces depends on both the duration and magnitude of the force applied. Nonetheless, the interactions among resistin, IL-20, and tensile forces in regulating the function of IVD NP cells remain yet to be fully understood. Nuclear factor erythroid 2-related factor 2 (NRF2) has been identified as a pleiotropic protein that enhances cellular resistance to stress stimuli and inflammatory challenges. The role of NRF2 in NP cells is not well defined, and whether tensile force influences NRF2 levels in NP cells is not known.

Aims: This study investigated the role of NRF2 in human NP cells subjected to low-frequency cyclic stretch stimulation, and the underlying mechanisms involved.

Materials and methods: Human NP cells were cultured in chambers, serum-starved for 12 h, then subjected to 5% cyclic strain at 0.1 Hz in a bioreactor. Control chambers had no stretching. Cells were lysed for analysis post-loading.

Results: Resistin stimulation induced the expression of IL-20 in human NP cells in a dose- and time-dependent manner. The activation of the p38 mitogen-activated protein kinase, and Akt signaling pathways, as well as the production of toll-like receptor 4, are necessary to render resistin to cause the release of IL-20. In NP cells, transcription factor enzyme-linked immunosorbent assays revealed that resistin led to an increase in nuclear factor (NF)-κB-DNA binding activities. Exposure of NP cells to 5% cyclic stretch at 0.1 Hz inhibited this resistin-induced NF-κB activation and IL-20 expression.

Discussion: These findings elucidate the molecular mechanisms through which resistin induces IL-20 expression in NP cells and also demonstrate that low-frequency cyclic stretch can protect against this induction.