Contrasting effects of myeloid specific ablation of statin‐responsive KLF2 in atherosclerosis and skeletal muscle injury

Abstract

Krüppel‐like factor 2 (KLF2) is a statin‐responsive transcription factor. The anti‐inflammatory and vascular protective actions of statins are due in part to the ability of statins to induce KLF2 expression and activity in various myeloid‐derived cell types. Our lab has shown that myeloid specific knockout of KLF2 (myeKLF2−/−) produces robust atherosclerosis in hypercholesterolemic mice. The increased atherosclerosis correspond to enhanced cytokine production, elevated adhesion of KLF2‐deficient macrophages to endothelial cells and their infiltration into atherosclerotic lesion areas. On par, skeletal muscle injury also induces massive infiltration of immune cells; however, this robust inflammatory response has a beneficial role in muscle repair. Whether or not myeKLF2−/− mice display an elevated inflammatory response to muscle injury, and if such elevation alleviates or worsens repair is not known. We hypothesized that reduction or suppression of myeKLF2 may enhance the inflammatory phase of muscle repair and improve regeneration. To test this hypothesis, we compared the inflammatory response in injured skeletal muscles of control and myeKLF2−/− mice. The gastrocnemius muscle of control and myeKLF2−/− mice was treated with 100μl of 10μM cardiotoxin (Naja pallida) or PBS (sham) and regeneration was followed for 21 days post injury. RTQPCR data reveal that on day 3 post injury, expression of a specific set of pro‐inflammatory mediators (IL6, CCL2, CCL4, CCL5, and TNFα) is significantly increased in myeKLF2−/− compared to control mice. Histological studies show that by day 8, myeKLF2−/− mice have effectively repaired the damaged muscle, while WT muscle still contains necrotic and fibrotic tissue, inflammatory cells, and few regenerating cells. In addition, markers of muscle growth and regeneration are significantly increased in injured muscle of myeKLF2−/− mice compared to WT. Flow cytometry analysis using a single‐cell suspension of CTX‐treated muscle showed that injured muscles of myeKLF2−/− mice contain a high percentage of Ly6Cpos CD11B+ F4/80+ macrophages during the early inflammatory phase of repair. This population of macrophages subsequently transitions to anti‐inflammatory, Ly6Cneg CD11B+ F4/80+ macrophages which presumably support myogenesis. Injured muscles of myeKLF2−/− mice showed a significantly greater percentage of Ly6Cpos macrophages during the inflammation phase of repair, and a greater percentage of Ly6Cneg macrophages during the regeneration phase. These data suggest that myeKLF2−/− triggers robust inflammatory macrophage infiltration in skeletal muscle injury, similar to atherosclerosis. However, in injured muscle, the infiltrating macrophages enhance inflammation and transition earlier to an anti‐inflammatory phenotype which improves regeneration; whereas, in atherosclerosis they transition to foam cells and the inflammation fails to resolve.Support or Funding InformationNIH ‐ NATIONAL INSTITUTE OF ARTHRITIS AND MUSCULOSKELETAL AND SKIN DISEASES (NIAMS) ‐ RO1 grantThis abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.