Idiopathic pulmonary fibrosis (IPF) is a disease of aging. IPF carries a high morbidity and mortality, with a median survival rate of less than three years. The incidence and prevalence of IPF increase drastically with age; however, despite this strong association, cellular/molecular mechanisms that account for the aging predilection to fibrotic disease have not been elucidated. Recent studies from our laboratory have identified a reactive oxygen species (ROS)-generating enzyme, NADPH oxidase-4 (Nox4), in mediating differentiation of fibroblasts (Fbs) to myofibroblasts (MFbs), key effectors of fibrogenesis, and in in-vivo lung fibrosis in murine models of lung injury. Our preliminary studies indicate that the biological actions of Nox4 may be modulated by the expression of SIRT3, a mitochondrial sirtuin. Both cellular senescence and TGF-?1 mediate suppression of SIRT3; our preliminary studies support a role for epigenetic silencing of SIRT3 involving both DNA methylation and histone modification. Decreased expression of SIRT3 promotes a senescent and pro-fibrotic Fb phenotype. TGF-?1-induced down-regulation of SIRT3 is associated with hyper-acetylation of mitochondrial proteins, supporting a role for altered mitochondrial bioenergetics in MFbs. Human subjects with IPF express low levels of SIRT3 in myofibroblastic foci (by immunohistochemistry), as well as in ex-vivo Fbs isolated from IPF lungs. We have developed a novel aging model of non-resolving fibrosis in mice; fibrosis in young mice (2 months) resolves by >50% by 4 months post-bleomycin, whereas aged mice (18 months) show persistent fibrotic response. While SIRT3 levels decrease during the fibrogenic phase in both groups, young mice demonstrate a capacity to recover SIRT3 levels during resolution; in contrast, aged mice manifest sustained down-regulation of SIRT3. The central hypothesis to be tested in this grant proposal is that, in the context of aging, lung injury results in sustained, epigenetically-regulated SIRT3 silencing that leads to mitochondrial dysfunction, MFb senescence and apoptosis resistance, leading to persistent fibrosis with aging.
Our specific aims are to: (1) determine epigenetic mechanisms for SIRT3 down-regulation with cellular senescence and with TGF-?1 signaling in lung Fbs; (2) determine the role of SIRT3 in regulating mitochondrial bioenergetics, Fb senescence and apoptosis resistance; (3) determine whether whole-animal SIRT3 knockout and/or conditional genetic deletion of SIRT3 in collagen-producing/mesenchymal cells induce(s) persistent fibrosis in young mice. The completion of the Aims in this proposal will: (a) elucidate epigenetic mechanisms that control SIRT3 expression with cellular senescence/aging; (b) provide mechanistic insights into the role of SIRT3 in maintenance of mitochondrial bioenergetics and cellular plasticity/fate; (c) provide proof-of-concept that SIRT3 induction in the context of age-associated fibrosis facilitates fibrosis resolution, uncovering a novel therapeutic approach to non-resolving fibrotic disorders such as IPF.
Idiopathic pulmonary fibrosis (IPF) is a disease of aging, with limited available therapies. This project studies the link between aging and pathogenesis of IPF, and evaluates the role of sirtuins (in particular, SIRT3) in the resolution of fibrosis in murine models of lung injury.