Fibroblast-Specific Genetic Manipulation of p38 MAPK in vivo Reveals its Central Regulatory Role in Fibrosis

BACKGROUND—: In the heart acute injury induces a fibrotic healing response that generates collagen rich scarring that is at first protective but if inappropriately sustained can worsen heart disease. The fibrotic process is initiated by cytokines, neuroendocrine effectors and mechanical strain that promote resident fibroblast differentiation into contractile and extracellular matrix producing myofibroblasts. The mitogen-activated protein kinase (MAPK) p38α (Mapk14 gene) is known to influence the cardiac injury response, but its direct role in orchestrating programmed fibroblast differentiation and fibrosis in vivo is unknown. 

METHODS—: A conditional Mapk14 allele was used to delete the p38α encoding gene specifically in cardiac fibroblasts or myofibroblasts using 2 different tamoxifen-inducible Cre recombinase expressing gene-targeted mouse lines. Mice were subjected to ischemic injury or chronic neurohumoral stimulation and monitored for survival, cardiac function and fibrotic remodeling. Antithetically, mice with fibroblast-specific transgenic overexpression of activated MAPK kinase 6 (MKK6), a direct inducer of p38, were generated to investigate if this pathway can directly drive myofibroblast formation and the cardiac fibrotic response. 

RESULTS—: In mice loss of Mapk14 blocked cardiac fibroblast differentiation into myofibroblasts and ensuing fibrosis in response to ischemic injury or chronic neurohumoral stimulation. A similar inhibition of myofibroblast formation and healing was also observed in a dermal wounding model with deletion of Mapk14. Transgenic mice with fibroblast-specific activation of MKK6-p38 developed interstitial and perivascular fibrosis in the heart, lung and kidney due to enhanced myofibroblast numbers. Mechanistic experiments show that p38 transduces cytokine and mechanical signals into myofibroblast differentiation through the transcription factor serum-response factor (SRF) and the signaling effector calcineurin. 

CONCLUSIONS—: These findings suggest that signals from diverse modes of injury converge on p38α MAPK within the fibroblast to program the fibrotic response and myofibroblast formation in vivo, suggesting a novel therapeutic approach with p38 inhibitors for future clinical application.