ACE2 (angiotensin converting enzyme 2) plays a critical role in the local tissue 
RAS (renin-angiotensin system) by hydrolysing the potent hypertensive and 
mitogenic peptide AngII (angiotensin II). Changes in the levels of ACE2 have 
been observed in a number of pathologies, including cardiovascular disease, but 
little is known of the mechanisms regulating its expression. In the present 
study, therefore, the potential role of miRNAs in the regulation of ACE2 
expression in primary human cardiac myofibroblasts was examined. Putative 
miRNA-binding sites were identified in the 3'-UTR of the ACE2 transcript using 
online prediction algorithms. Two of these, miR-200b and miR-421, were selected 
for further analysis. A reporter system using the 3'-UTR of ACE2 fused to the 
coding region of firefly luciferase was used to determine the functionality of 
the identified binding sites in vitro. This identified miR-421, but not 
miR-200b, as a potential regulator of ACE2. The ability of miR-421, an miRNA 
implicated in the development of thrombosis, to down-regulate ACE2 expression 
was subsequently confirmed by Western blot analysis of both primary cardiac 
myofibroblasts and transformed cells transfected with a synthetic miR-421 
precursor. Real-time PCR analysis of miR-421 revealed widespread expression in 
human tissues. miR-421 levels in cardiac myofibroblasts showed significant 
inter-patient variability, in keeping with the variability of ACE2 expression we 
have observed previously. In conclusion, the present study is the first to 
demonstrate that ACE2 may be subject to post-transcriptional regulation and 
reveals a novel potential therapeutic target, miR-421, which could be exploited 
to modulate ACE2 expression in disease.