Senescence-associated exosomes transfer miRNA-induced fibrosis to neighboring cells

Radiation-induced fibrosis is a common side effect of radiotherapy, which is the most common treatment for cancer. However, radiation also causes p53-mediated cell cycle arrest, prolonged expression of p21, and the development of senescence in normal cells that reside in irradiated tissues. Bone marrow-derived mesenchymal stem cells (MSCs) accumulate in primary tumor sites because of their natural tropism for inflammatory and fibrotic tissues.

MSCs are extremely sensitive to low doses of ionizing radiation and acquire senescence as a result of bystander radiation effects. Senescent cells remain metabolically active but develop a potent senescence-associated secretory phenotype (SASP) that correlates to hyperactive secretion of cytokines, pro-fibrotic growth factors, and exosomes (EXOs).

Integrative pathway analysis has highlighted that radiation-induced senescence significantly enriched cell-cycle, extracellular matrix, transforming growth factor-β (TGF-β) signaling, and vesicle-mediated transport genes in MSCs. EXOs are cell-secreted nanovesicles (a subclass of small extracellular vesicles) that contain biomaterials—proteins, RNAs, microRNAs (miRNAs)—that are critical in cell-cell communication. miRNA content analysis of secreted EXOs further revealed that radiation-induced senescence uniquely altered miRNA profiles.

“In fact, several of the standout miRNAs directly targeted TGF-β or downstream genes.”

In this new study, researchers from Brown University further treated normal MSCs with senescence-associated EXOs (SA-EXOs) to examine bystander effects of radiation-induced senescence.

The researchers found that these modulated genes were related to TGF-β pathway and elevated both alpha smooth muscle actin (protein increased in senescent, activated cells) and Ki-67 (proliferative marker) expression in SA-EXO treated MSCs compared to untreated MSCs. They revealed that SA-EXOs possess unique miRNA content that influence myofibroblast phenotypes via TGF-β pathway activation. This highlights that SA-EXOs are potent SASP factors that play a large role in cancer-related fibrosis.

Senescent MSCs secrete unique EXO miRNA profile

MiRNA content from pre- and senescent MSC secreted EXOs were characterized and examined using immunoblots and microarrays. Pre-senescent and senescent harvested EXOs showed positive expression of EXO surface markers (EEA1, CAV-1, clathrin, β-actin) using immunoblots (A). Many miRNAs that were significantly regulated were positively enriched in senescent MSC EXOs compared to pre-senescent MSC EXOs (senescent vs. pre-senescent) (B). Microarray analysis showed overlapping miRNAs were involved in similar transcriptome pathways. Significant miRNAs that were involved in TGF-β (C), cell cycle regulation (D), and cell adhesion and focal adhesion (E) pathways are listed. DIANA miRPath analysis further showed miRNAs that regulated genes reported in transcriptome analysis (F). Venn diagrams indicate that several miRNAs regulate and target multiple genes in different pathways. miRNA microarray samples were processed (n = 3) and miRNAs were considered significant if p < 0.05 and fold changes are reported as log2 differences.

“Our integrated omics and EXO microarray analyses show that senescent MSCs possess differential transcriptional genes and secrete vesicles that contain unique post-transcriptional cargo. We subsequently demonstrated that these EXO miRNAs can play important roles in cell-cell communication during disease progression.”

“This shows that SA-EXOs can serve as potent SASP mediators that activate invasive characteristics in neighboring cells.”

Source – Eurekalert