Up-regulated expression of matrilysin and neutrophil collagenase in human herniated discs

Spontaneous resorption of herniated nucleus pulposus (HNP) is commonly observed when there is substantial contact of the disc with the spinal canal. We already demonstrated the expression of matrix metalloproteinase (MMP)-3 (stromelysin-1) in the granulation tissues of HNP, suggesting its role in the resorption process of HNP. Recent studies of osteoarthritic cartilages reported an up-regulated expression of metalloproteinases including MMP-7 (matrilysin) and MMP-8 (neutrophil collagenase), suggesting their roles in the matrix degradation. To clarify the expression of MMP-7 and MMP-8 in HNP, immunohistological analysis of various types of HNP was performed. We found MMP-7 was expressed in infiltrated mononuclear cells and chondrocytes, whereas MMP-8 was specifically expressed in chondrocytes. The positive rate for both MMP-7 and MMP-8 significantly increased when HNP was exposed to the epidural space (p < 0.01). Our data suggest that not only MMP-3 but also MMP-7 and MMP-8 may play a role in the resorption process of HNP.