A collaborative Sino-Australian study led by scientists at the Institute of Ageing Research at Hangzhou Normal University in Chongqing has identified a new target for enhancing respiratory function and preventing stress-induced premature lung aging and fibrosis due to environmental stresses in mice.
Specifically, the F-box/WD repeat-containing protein 7 (FBW7) tumor suppressor protein was shown to mark the telomere protein TPP1 for degradation, prompting telomere uncapping, cancer or stem cell aging, and pulmonary fibrosis (PF).
"Our study has elucidated the telomere uncapping mechanism, the identification of which can lead to new approaches to unlocking the mechanisms of pulmonary fibrosis and guiding intervention," said corresponding author Jun-Ping Liu, a professor of immunology at Monash University in Melbourne.
This research also demonstrated that targeting FBW7 using a synthetic peptidomimetic could protect telomeres and offset the aging process, the study authors reported in the October 20, 2020, online edition of Cell Metabolism.
Idiopathic PF (IPF) is characterized by cell senescence in the lung epithelium and interstitial fibrogenesis leading to progressive irreparable and ultimately fatal lung damage.
IPF has a median survival of less that 3 years and, while some medications and treatments can alleviate symptoms, it remains essentially incurable, although some patients may be suitable for lung transplantation.
"Currently, there are no specific IPF therapeutics, apart from the anti-inflammatory tumor necrosis factor-beta inhibitor pirfenidone (Esbriet; Genentech/Roche) and the anti-vascular epithelial growth factor nintedanib (Ofev/Vargatef; Boehringer Ingelheim), which alleviate respiratory symptoms," said Liu.
While the exact mechanisms underlying IPF remain unclear, increasing evidence suggests it may be due to pulmonary alveolar stem cell DNA damage due to oxidative stress and impaired protective mechanisms.
Multiple cellular and genetic factors, including those required for telomere homeostasis, have also been implicated in sporadic PF.
Telomerase subunit genes mutations have been linked to familial PF and are associated with pulmonary alveolar epithelial type 2 (AEC2) stem cell senescence and differentiation to myofibroblasts.
A potential cause of aging-related diseases, telomere dysfunction results from mutations in shelterin proteins and telomerase, which protect cells from the telomere DNA damage response (DDR) and senescence.
Genetic mutagenesis of the shelterin component, telomere repeat binding factor 2 (TRF2) or telomerase genes, induces PF, whereas transgenic expression of the telomerase catalytic subunit TERT may alleviate the disease.
By interacting with genetic factors, environmental stressors including the DNA-damaging antibiotic bleomycin, which can also cause telomere shortening, ionizing radiation (IR) and reactive oxygen species have all been shown to cause PF.
FBW7 regulates oncogenic cell proliferation by directing a number of proto-oncoproteins for degradation, but FBW7 is one of the most frequently mutated genes underpinning tumorigenesis in human cancers.
Inactivation of FBW7 results in tissue stem cell expansion in the pancreatic, bone marrow hematopoietic, and testicular spermatogenic tissues via poorly understood mechanisms.
In their new Cell Metabolism study, the research team investigated how environmental stressors induced telomere dysfunction and accelerate aging and fibrosis.
"For more than 30 years now, exactly how stress triggers telomere shortening and pulmonary fibrosis has remained a mystery," Liu told BioWorld Science.
The team showed that in response to stress in the form of either radiation or the lung-damaging agent bleomycin, FBW7 bound to telomere protective protein 1 (TPP1) and marked it for degradation, leading to unprotected telomeres that triggered a senescence-inducing DNA damage response.
Using fluorescence labeling and microimaging, the researchers first examined nuclear protein kinases and ubiquitin E3 ligases responding to stress in cancer, mouse pulmonary epithelial stem/progenitor, and in human lung normal epithelial cells.
This revealed that FBW7 mediates cellular senescence by accelerating TPP1 turnover, triggering telomere uncapping and shortening.
In mouse AEC2 stem cells, Fbw7 knockout (KO) was shown to prevent bleomycin- or IR-induced telomere uncapping, pulmonary senescence, and fibrosis in a TPP1 stability-dependent manner.
Moreover, the researchers demonstrated the ability of a small peptidomimetic to specifically inhibit FBW7 binding to TPP1, thereby attenuating telomere uncapping.
"We have shown how oxidative stress triggers telomere capping protein TPP1 degradation, resulting in telomere protein cap loss, telomere DNA exposure, hence telomere DNA breakdown and shortening, cell senescence and transdifferentiation, and pulmonary fibrosis," noted Liu.
Screening synthetic peptides complementary to different regions of FBW7 identified a small telomere dysfunction inhibitor called telodin. This was shown to prevent TPP1 degradation, telomere shortening, alveolar stem cell senescence, and pulmonary fibrosis in mice exposed to bleomycin or IR.
Taken together, these study findings elucidate a key mechanism underlying stress-induced pulmonary epithelial stem cell senescence and fibrosis, paving the way for aging-related disorder interventions.
"We have shown that a small 8-amino acid residue peptide [Telodin] can prevent TPP1 degradation and telomere uncapping," Liu said.
"This was shown to lead to telomere lengthening and pulmonary stem cell mobilization, which resulted in enhanced normal respiratory function and resistance to stress-induced senescence and fibrogenesis in mice." (Wang, L. et al. Cell Metab 2020, 32: 1).