Mitochondrial Uncoupling Protein‐2 Drives Fibroblast Senescence in Age‐Related Lung Fibrosis by Altering Bioenergetics and Reactive Oxygen Species
Authors: Rangarajan Sunad , Bagchi Rushita A. , Zmijewski Jaroslaw W. , Eickelberg Oliver , Thannickal Victor J. 
Published: April 2020

Background Mitochondrial dysfunction is a potential link between aging and age‐related pathology, including lung fibrosis (idiopathic pulmonary fibrosis, IPF). (Myo)Fibroblasts are the effector cells in fibrosis. Transcriptomic analyses revealed high expression of mitochondrial uncoupling protein‐2 (UCP2) in lung fibroblasts isolated from patients with idiopathic pulmonary fibrosis, compared to healthy controls. Our objective was to examine the role of UCP2 in mediating mitochondrial dysfunction, fibroblast senescence and myofibroblast differentiation in age‐related pathologic fibrosis.

Methods

We studied tissues and fibroblasts obtained from lung explants of IPF patients and utilized the murine model of lung fibrosis and delayed resolution following bleomycin‐induced injury in aged mice. We silenced UCP2 expression using siRNA in vitro and in vivo. Specific endpoints were analyzed by RT‐PCR, SDS‐PAGE western blotting, immunofluorescence, flow cytometry, mass spectrometry and extracellular flux analysis.

Results

UCP2 gene expression was increased in lungs (whole lung lysates as well as fibroblasts) of IPF patients compared to those without IPF; in particular, mesenchymal stromal cells derived from bronchoalveolar lavage of patients with rapidly progressive clinical course had higher UCP2 expression compared to those with a slower course. Lung fibroblasts from aged mice expressed higher UCP2 than younger counterparts, more pronounced after bleomycin‐induced lung injury. The lungs as well as fibroblasts isolated from IPF patients in addition, demonstrated lower steady state ATP levels than those from non‐IPF controls. UCP2 silencing in IPF fibroblasts restored ATP levels, decreased pro‐fibrotic markers such as α‐SMA and collagen; this was associated with enhanced mitochondrial biogenesis, decreased mitochondrial ROS (superoxide and hydrogen peroxide), increased ATP‐linked oxygen consumption and decreased aerobic glycolysis. In addition, UCP2 silencing induced a decrease in senescence markers and decreased expression of senescence‐associated secretory phenotype (SASP) genes. In aged mice after bleomycin injury, therapeutic oro‐tracheal administration of UCP2 siRNA resulted in decreased lung collagen and fibrotic remodeling, indicating enhanced resolution capacity.

Conclusion

Our results suggest that expression of mitochondrial UCP2 in lung fibroblasts increases with age. UCP2 is highly expressed in lung fibroblasts of patients with IPF, more so in those with rapidly progressive clinical course; UCP2 drives a senescent fibroblast phenotype involving lower ATP levels and impairs resolution of bleomycin injury‐induced fibrosis in aged mice. Further studies are warranted to study the link between cellular aging and UCP2, and the therapeutic potential of targeting UCP2 in age‐related fibrotic disorders such as IPF.

Support or Funding Information

NIH grants K08 HL135399 (SR); P01 HL114470 and R01 AG046210 (VJT), VA Merit Award I01BX003056 (VJT)

Schematic depicting the putative role played by UCP2 in aging fibroblast phenotypes.

Figure 1