Idiopathic pulmonary fibrosis (IPF) is a chronic, deathly disease with no recognized effective cure as yet. Furthermore, its diagnosis and differentiation from other diffuse interstitial diseases remain a challenge. Circulating miRNAs have been measured in IPF and have proven to be an adequate option as biomarkers for this disease. These miRNAs, released into the circulation outside the cell through exosomes and proteins, play a crucial role in the pathogenic pathways and mechanisms involved in IPF development. This review focuses on the serum/plasma miRNAs reported in IPF that have been validated by real-time PCR and the published evidence regarding the fibrotic process. First, we describe the mechanisms by which miRNAs travel through the circulation (contained in exosomes and bound to proteins), as well as the mechanism by which miRNAs perform their function within the cell. Subsequently, we summarize the evidence concerning miRNAs reported in serum/plasma, where we find contradictory functions in some miRNAs (dual functions in IPF) when comparing the findings in vitro vs. in vivo. The most relevant finding, for instance, the levels of miRNAs let-7d and miR-21 reported in the serum/plasma in IPF, correspond to those found in studies in lung fibroblasts and the murine bleomycin model, reinforcing the usefulness of these miRNAs as future biomarkers in IPF.

Idiopathic pulmonary fibrosis, a chronic and fatal condition with no effective cure, is a challenging disease within the group of interstitial lung diseases. The pathogenic mechanisms of this disease are still not fully understood. In this context, the research on miRNAs, which are small non-coding RNAs that regulate messenger RNAs, is of utmost importance. These miRNAs, released into the circulation outside the cell through exosomes, play a crucial role in the pathogenic pathways and mechanisms involved in IPF development. The analysis of miRNAs in the serum/plasma of IPF patients has opened up new possibilities in the search for biomarkers. This review focuses on circulating miRNAs validated by real-time PCR in IPF and the evidence reported about the pulmonary fibrotic process.