Matrix Metalloproteinase (MMP)‐28 Activates Signal Transducer and Activator of Transcription 1 to Induce Macrophage M1 Polarization

Pro‐inflammatory M1 and anti‐inflammatory M2 macrophages are involved in cardiac repair after myocardial infarction (MI). Mice with matrix metalloproteinase (MMP)‐28 deletion display decreased inflammation at day 7 post‐MI. Accordingly, we hypothesized that MMP‐28 polarizes macrophages to a pro‐inflammatory M1 phenotype. In this study, we stimulated peritoneal macrophages isolated from 3 to 6 month old male C57/BL6J mice with MMP‐28 recombinant protein and evaluated cell phenotype. By quantitative RT‐PCR, the MMP‐28 catalytic domain only protein dose‐dependently upregulated pro‐inflammatory markers (Ccl3, Ccl5, Il1β, Il6, Il12a, and Tnfα, all p<0.05) 100–10,000 fold and downregulated anti‐inflammatory M2 markers (Arg1 and Tgfβ1, all p<0.05) 4–7 fold. This indicated that MMP‐28 is a potent activator of macrophage M1 polarization. Interestingly, the pro‐MMP‐28 full length recombinant protein demonstrated similar M1 polarization responses, demonstrating that MMP‐28 activity was not required. To evaluate if MMP‐28 can function as a signaling molecule to induce M1 polarization independent of its enzymatic activity, we measured the transcription factors nuclear factor (NF)‐κB and signal transducer and activator of transcription (STAT) 1, which are known M1 polarization signaling pathways. MMP‐28 catalytic domain‐stimulated macrophages demonstrated a significant activation of STAT1, evidenced by increased phospho‐STAT1 (400 fold, p<0.05). In contrast, NF‐κB activation was not significantly affected by MMP‐28 treatment. This indicates that the MMP‐28 effect is mediated through the STAT1 pathway. This study, for the first time, demonstrates that MMP‐28 functions as a signaling molecule and potently induces macrophage M1 polarization through activating the STAT1 pathway. Support or Funding Information This work was supported by the National Institutes of Health [HHSN268201000036C (N01‐HV‐00244), HL075360, and GM114833 to M.L.L., and P01HL051971 and P20GM104357]; the Biomedical Laboratory Research and Development Service of the Veterans Affairs Office of Research and Development Award [5I01BX000505 to M.L.L.]; and American Heart Association [14POST18770012 to R.P.I.] and [15SDG22930009 to Y.M.].