Activation of p38 MAPK is a key signaling step in chronic inflammation. Inhibition of p38 MAPK is considered to be a promising future strategy to control inflammatory diseases, but studies of compounds to inhibit this kinase have so far been limited to investigation of their side effects. We undertook the present study to investigate which specific molecule, among 4 different isoforms of p38 MAPK (α, β, γ, and δ), is predominantly expressed and activated in inflammation. Such knowledge could allow more specific targeting of p38 MAPK in inflammatory disease.
Studies were performed on inflamed tissue from patients with rheumatoid arthritis, as a prototype of inflammatory disease. The expression and activation of the α, β, γ, and δ isoforms of p38 MAPK were examined by immunoblotting, immunoprecipitation, and immunohistochemistry.
Immunoblot analysis revealed that α and γ were the predominantly expressed p38 MAPK isoforms, whereas the other 2 isoforms were less frequently present. By immunohistochemistry, the expression of all p38 MAPK isoforms was localized to the synovial lining layer as well as to blood vessels. Colabeling with cell-specific markers revealed that macrophages expressed the α and γ isoforms, synovial fibroblasts the β and γ isoforms, and granulocytes the δ isoform, whereas T lymphocytes were rarely positive for any p38 MAPK isoform. Double-labeling with isoform-specific antibody and pan-p38 antibody against the phosphorylated form of p38 MAPK showed activation of the α and γ isoforms. Occasional activation of the β isoform was also noted in the synovial lining and the endothelium, whereas the δ isoform, although expressed in pericytes around blood vessels, was not phosphorylated. This phosphorylation pattern was confirmed in immunoprecipitation studies in which activated p38 MAPK from synovial tissue extracts was identified as p38 MAPKα and -γ but not p38 MAPKβ or -δ.
These data show that the α and γ isoforms of p38 MAPK dominate in chronic inflammation. Effective strategies to inhibit p38 MAPK should therefore aim to specifically target either or both of these isoforms.