Collagen-Derived N-Acetylated Proline-Glycine-Proline in Intervertebral Discs Modulates CXCR1/2 Expression and Activation in Cartilage Endplate Stem Cells to Induce Migration and Differentiation Toward a Pro-Inflammatory Phenotype

The factors that regulate the migration and differentiation of cartilage endplate stem cells (CESCs) remain unknown. N-Acetylated proline-glycine-proline (N-Ac-PGP) is a chemokine that is involved in inflammatory diseases. The purpose of this study was to detect N-Ac-PGP in degenerative intervertebral discs (IVDs) and to determine its roles in the migration and differentiation of CESCs. Enzyme-linked immunosorbent assay (ELISA) and liquid chromatography-mass spectrometry results indicated that the levels of the proteases that generate N-Ac-PGP as well as N-Ac-PGP levels themselves increase with the progression of IVD degeneration. Immunohistochemistry and an N-Ac-PGP generation assay demonstrated that nucleus pulposus (NP) cells generate N-Ac-PGP from collagen. The effects of N-Ac-PGP on the migration and differentiation of CESCs were determined using migration assays, RT-PCR, immunoblot analysis, and ELISA. The results showed that the expression of N-Ac-PGP receptors (CXCR1 and CXCR2) in CESCs was upregulated by N-Ac-PGP. Additionally, N-Ac-PGP induced F-actin cytoskeletal rearrangement in CESCs and increased CESC chemotaxis. Furthermore, N-Ac-PGP recruited chondrocytes and spindle-shaped cells from the cartilage endplate (CEP) into the NP in vivo. These spindle-shaped cells expressed CD105 and Stro-1 (mesenchymal stem cell markers). N-Ac-PGP induced the differentiation of CESCs toward a pro-inflammatory phenotype with increased production of inflammatory cytokines rather than toward an NP-like phenotype. Our study indicated that, in the complex microenvironment of a degenerative disc, N-Ac-PGP is generated by NP cells and induces the migration of CESCs from the CEP into the NP. N-Ac-PGP induces a pro-inflammatory phenotype in CESCs, and these cells promote the inflammatory response in degenerative discs.