INTRODUCTION: Chemical inhibitors of p38 mitogen activated protein kinase (MAPK) block the induction of MMP expression by pro‐inflammatory cytokines in disc cells. Thus a proposed mechanism of inflammation‐induced MMP expression is through the p38 MAPK pathway. However given the existence of four mammalian p38 MAPK isoforms (α, β, γ, δ) and that most chemical inhibitors block the activity of multiple p38 isoforms, it is unclear whether different p38 isoform controls expression of specific MMP subsets. Here we focused on investigating how p38a and p38β individually controls the expression of MMP1 and MMP3 in TNF‐α stimulated disc cells.
METHODS: Human nucleus pulposus (hNP) cells were transduced with lentiviral particles expressing ShRNA to silence either p38a or p38β, and exposed to TNF‐a(5 ng/ml) for 3 days. MMP1 and MMP3 mRNA levels were determined by semi‐qRT‐PCR.
RESULTS: hNP cells transduced with ShRNA targeting p38a and p38β selectively reduced their mRNA and protein expression (˜40% of control). TNF‐α exposure stimulated MMP1 (114x) and MMP3 (245x) expression compared to untreated control. TNF‐aexposure with p38β silencing further increased MMP3 (804x) while reduced MMP1 (42x) expression. TNF‐aexposure with p38a silencing drastically increased MMP‐1 (860x) expression. These findings were confirmed using a dominant negative p38aprotein.
DISCUSSION: These results suggest that p38a and p38β perform the vital function to prevent excessive induction of two key MMPs implicated in disc matrix destruction, MMP1 and MMP3, respectively, in hNP cells under inflammatory stress. Thus the current therapeutic strategy of using inhibitors to block multiple p38 isoforms indiscriminately could potentially be detrimental to disc matrix homeostasis. Further studies are needed to dissect the roles of the different p38 isoforms in regulating disc MMP expression and matrix degradation.