Exosomes of invasive urothelial carcinoma cells are characterized by a specific miRNA expression signature

Muscle-invasive bladder cancer (MIBC) represents a highly aggressive tumor type compared to non-muscle-invasive tumors. MIBC is characterized by specific molecular alterations, which may also modulate extracellular tumorigenic effects. Tumor-associated exosomes, especially exosomal miRNAs, are important regulators in the interaction between tumor cells and tumor microenvironment by affecting tumor-promoting processes in target cells. It is important to analyze whether their exosomal patterns also reflect the specific molecular characteristics of MIBC. Researchers at Saarland University Medical Center aimed to compare the miRNA expression in secreted exosomes from urinary bladder cancer cells (UBC) with different degrees of invasiveness. By electron microscopy, nanotracking analysis and western blot they proofed a high quality of isolated exosomes. Microarray analysis identified an invasion-associated signature of 15 miRNAs, which is significantly altered in exosomes of invasive UBC compared to non-invasive counterparts. Therefrom, 9 miRNAs are consistent differently expressed in both, invasive cells and their secreted exosomes. The remaining 6 exosome-specific miRNAs are only deregulated in exosomes but not in their parental cells. MiRNA alterations were verified by qPCR in cell culture and urinary exosomes.

Invasive UBC cells are characterized by a specific miRNA pattern

(A) The miRNA expression levels in invasive UBC cells (blue) compared to non-invasive UBC cells (yellow). Unsupervised hierarchical cluster analysis of differentially expressed miRNAs. P-value was determined using Mann–Whitney U test. (Biological replicates n = 2 per cell type; p-value < 0.05; Fold Change > 1.5; σ = 0.2); (B) Relative expression of 5 deregulated miRNAs in invasive UBC cells (grey) compared to non-invasive cells (dark blue) quantified by qPCR (biological replicates n = 3 per cell line; invasive cells n = 9; non-invasive cells n = 6). The expression level was normalized by RNU48. P-value was determined using Mann–Whitney U test. P = p-value; FC = Fold Change.

The researchers showed here that exosomes from invasive UBC cells are characterized by a specific miRNA signature. Further analyses have to clarify the functional relevance of exosomal miRNAs secreted by invasive bladder cancer cells for modification of the tumor microenvironment and their putative role as molecular markers in liquid biopsies.