In a recent study published in Metabolism of Naturethe researchers investigated the correlation between cellular damage, iron accumulation, aging and fibrosis.
Record
Fibrogenesis is a defense response to tissue damage that leads to life-threatening disorders. Senescent cells, also known as the senescence-associated secretory phenotype (SASP), are the main cause of fibrosis. Fibrosis causes organ damage and the development of cardiovascular, interstitial lung and kidney disorders by replacing healthy tissues with collagen-rich scar tissue. Pro-angiogenic factors such as tumor growth factor beta (TGF-β) and interleukin-11 (IL-11) can cause senescent cells to become fibrotic.
About the study
In the present study, researchers identified iron accumulation as a therapeutically exploitable cause of pathological aging and fibrosis.
The study investigated the function of iron accumulation in fibrous tissues, specifically in mice with pulmonary fibrosis and people with idiopathic pulmonary fibrosis (IPF). Perl’s Prussian blue staining was used to examine mouse kidneys and kidney samples from diabetic patients with kidney disease. They also investigated the gene signatures of iron transport and storage genes in the lung transcriptomes of healthy subjects and patients with IPF. The team examined the kinetics of iron during fibrogenesis in tissues taken from patients with acute respiratory distress syndrome (ARDS).
Mice were administered intratracheal bleomycin or intraperitoneal (ip) folic acid (FA) to assess the impact of iron accumulation in injured tissues. They also investigated whether iron accumulation was a transient effect or a causative factor in fibrogenesis.
The researchers further looked at whether intratracheal iron might cause damage, as vascular or hemolytic lesions can release iron from damaged red blood cells (RBCs), contributing to inflammation, aging, and fibrosis. They developed a mouse model (Tie2-Cre-ERT2; Rosa26-iDTR) in which injections of tamoxifen and diphtheria toxin can ablate a percentage of endothelial cells, resulting in microbleeds, mainly in the lungs, allowing the animals to survive.
To investigate the function of iron in FA-induced renal fibrosis, the researchers administered FA to mice in the presence or absence of the iron chelator deferiprone (DP). They investigated the effects of iron on in vitro-grown cells to improve mechanistic understanding of the relationship between iron and aging. In addition, the researchers assessed injury-induced iron release and accumulation in vitro.
The team used single-cell ribonucleic acid sequencing (scRNA-seq) of mouse lungs two and six days after iron exposure to improve understanding of the cell types that underpin these two stages of the iron response. They also reviewed a scRNA-seq meta-analysis of IPF patients for enrichment of interstitial fibrosis and tubular atrophy (IFTA score) and a magnetic resonance imaging (MRI) study of 13 renal allograft recipients for R2* signal in the renal cortex and level of fibrosis assessed in biopsies.
Results
Iron accumulation has been associated with cellular aging and different fibrotic disorders in experimental animals and humans. Hemolytic and vascular damage can cause iron deposition, causing aging and promoting fibrosis. Even after the extracellular increase in iron subsided, senescent cells continued to accumulate iron.
Cells subjected to various senescence-inducing insults accumulated excess forms of ferritin-bound iron, mainly within iron-labile lysosomes in large amounts, fueling the formation of SASP and reactive oxygen species (ROS). Magnetic resonance imaging of iron could allow noninvasive assessment of fibrotic burden in renal tissues of humans and mice with fibrosis.
Findings from snRNA sequencing revealed that iron stimulated the transient recruitment of neutrophils, the persistent growth of monocytes, macrophages and dendritic cells, and the gradual suppression of relatively abundant bronchial artery cells. Intratracheal injection of iron was sufficient to induce the hallmarks of fibrogenesis, including collagen deposition, senescence, vascular thinning, inflammation, and innate immune infiltration.
The team observed iron accumulation in fibrous cardiac tissues of mice with transgenic adrenergic receptor beta 1 (Adrb1) overexpression or ischemic heart disease resulting from coronary artery ligation. Similar findings were obtained in mouse renal fibrosis induced by intraperitoneal FA injection or unilateral ureteral occlusion. IPF patients revealed a significant enhancement of the iron transport signature in both datasets, demonstrating that altered gene expression of iron absorption and storage genes is a hallmark of IPF.
The researchers found a transient increase in the number of fibroblasts and goblet cells in the non-immune compartment, with the most notable changes being stimulation of the molecules plasminogen activator, tissue inhibitor of metalloproteinase 1 (TIMP1), and suppression of angiogenic molecules. vascular endothelial growth factor A (VEGF A) and angiopoietin 1 (Angpt1). IPF lung tissues were enriched for iron accumulation signature (IAS) genes in tumor lung tissue and multiple cell types.
The study findings indicated that iron accumulation contributes to aging and fibrosis, and iron metabolism may be a therapeutic target for aging-related disorders. Iron accumulation in fibrosis occurs early after tissue damage and develops as a distinctive feature of some fibrotic diseases. Excess iron can cause fibrogenesis and aging.
The researchers distinguished between “excess extracellular iron” as a putative trigger of senescence and “excess intracellular iron” as a factor of pathogenic consequences in senescent cells.
