Effect of High Fat Diet on the Severity and Repair of Lung Fibrosis in Mice

Lung fibrosis is a progressive fatal disease, and the underlying mechanisms remain unclear. These involve a combination of altered fibroblasts, excessive accumulation of extracellular matrix, inflammation, and aberrant activation of epithelial cells. Previously, we showed that high-fat diet (HFD) induces lung inflammation, aberrant activation of stem cells, and lung mitochondria impairment. Therefore, we hypothesized that HFD-induced changes would influence lung fibrosis. Mice were fed standard diet (SD) or HFD, administered bleomycin, then examined for fibrosis severity and the start of repair 3 weeks after injury, and for fibrosis repair/resolution 6-9 weeks after injury. At 3 weeks, no significant differences in inflammation and fibrosis severity were observed between SD- and HFD-fed mice. However, infiltration of alveolar type (AT)-2 cells and bronchioalveolar stem cells (BASCs) into the fibrotic areas (the start of repair) was impaired in HFD-fed mice. At 6 weeks, SD-fed mice showed near-complete resolution/repair of fibrosis and inflammation, while HFD-fed mice still showed residual fibrosis and inflammation. Infiltration of the fibrotic areas with AT2 cells was observed, but very few BASCs were detectable. At 9 weeks, mice from both groups showed complete resolution/repair of fibrosis and inflammation, indicating that HFD induced delayed, rather than failed, resolution of fibrosis and alveolar repair. To further confirm the direct role of enhanced fatty-acid oxidation (FAO) in delayed resolution/repair, we administered etomoxir, a FAO inhibitor, to HFD-fed mice for 3-6 weeks after bleomycin injury. Inhibition of FAO abolished the HFD-induced delay in alveolar repair and fibrosis resolution at both time points. In conclusion, after a fibrosis-inducing injury, HFD slows resolution of fibrosis/inflammation and delays alveolar repair by slowing the contribution of AT2 stem cells and abolishing the contribution of BASCs in the repair process. FAO activation appears to be involved in this delay mechanism; thus, inhibiting FAO may be useful in the treatment of lung injury and fibrosis.