We report here that cells preferentially release and retain miRNA subpopulations. These results are important when considering circulating miRNAs for diagnosis, and in assessing the biological significance of released miRNAs. For example, the finding that transformed and malignant mammary epithelial cells release most of their miR-451 into the environment has several implications. Interestingly, miR-451 abundance has been reported to correlate with breast cancer in tissue sections. However, miR-451 was low in mammary epithelia, but enriched interstitially 
. Therefore, miR-451 abundance was attributed to alternate origins, and hence the usefulness of miR-451 in diagnosis was rejected. Yet, miR-451 may have roles in breast- and other cancers 
, and maps to an amplicon that includes HER2 and BRCA1, which are commonly amplified in breast- and other cancers 
. Therefore, our finding that miR-451 is selectively released from malignant mammary epithelial cells in culture and in the body may be of importance for diagnostics.
In addition, we and others have described that viral RNAs and proteins transferred in exosomes have biological functions relevant to cancer (
and reviewed in 
). In addition vesicular miRNAs have been found to have signaling capacity 
. Therefore, the finding that some miRNAs are released more abundantly than retained may suggest a role of these miRNAs in signaling. Perhaps selectively released miRNAs, including miR-451 may be involved in paracrine signaling of the cancer and the stroma 
, and in field cancerisation 
Alternatively, the release of miRNAs in exosomes might fit with a trash disposal mechanism 
, in which cells release damaged and other cellular components into the environment akin to a house-cleaning mechanism. In agreement with this idea, we have found the accumulation of specific rRNA and snRNA fragments, and improperly processed miRNAs in the extracellular space. However, because specific tRNA fragments have been found to have distinct biological roles 
, RNA fragments may possibly be more than degradation products destined for disposal.
The mechanism of selective miRNA release remains to be determined. For example, in addition to an MVB-mediated miRNAs release mechanism, cell death may be a mechanism of miRNA release 
. We considered this possibility, and found that IMR90 cells and MDA-MB-231 cells in culture had similar proportions of dead cells (11% and 9% respectively), yet MDA-MB-231 cells released about 2.3 times more of their miR-16 molecules than IMR90 cells did (, and data not shown). Therefore, cell death alone cannot explain the accumulation of extracellular miRNAs. To identify the selection mechanism, it may be informative to determine if the selective release and retention of miRNA subpopulations is regulated by environmental cues. In addition, the recent findings that miR-451 biogenesis is dicer-independent 
, raise the possibility that miRNAs processed in noncanonical ways may be specifically targeted for release through mechanisms yet to be determined.
Our finding that the extracellular miRNA profile of ductal fluids and HMECs is similar has several consequences. For example, it opens the possibility of using the extracellular miRNA population of ductal lavages for diagnosis, as is considered for proteins (
and reviewed in 
). This may be especially useful for women at high-risk of breast cancer, where measurements of the extracellular miRNA composition might supplement other approaches, and overcome some of the diagnostic limitations of analyzing the cellular composition of lavages 
. In addition, the finding of differences in the extracellular miRNA composition of ductal fluids raises an interesting possibility that specific miRNAs contribute to the multifunctional roles milk may have during nursing 
or consuming animal milk 
, and the function ductal fluid miRNAs may have in the resting gland