Therapeutic Application of MicroRNAs in Cancer - RNA Interference from Biology to Therapeutics - page 304

Several other groups found that circulating miRNA complexes in human plasma
and serum are associated with RNA-binding proteins, such as Argonaute 2 or
nucleophosmin 1 [ 52- 54 ]. It is tempting to postulate that natural processes can be
used for delivery of miRNA to reduce the side effects or increase target specificity.
However, it is essential to understand the molecular mechanisms of the uptake of
these carriers in recipient cells. Nevertheless, these natural carriers might represent
a breakthrough in the complex field of drug delivery systems.

14.3 Future Perspectives

A particular miRNA regulates several mRNAs and might influence multiple types
of signaling pathways. Therefore, undesirable side effects may occur upon admin-
istration of miRNA mimics or antagomirs (Fig. 14.2 ). It has been reported that many
of the mice administered with the miRNA treatment showed liver impairment and
that some of them ultimately died after intravenous infusion of shRNA using AAV
[ 55 ]. Morbidity was associated with downregulation of liver-derived miRNAs, indi-
cating possible competition of the latter with shRNAs for limiting cellular factors
required for the processing of various small RNAs. This report raised the possibility
that the introduction of small RNA itself induced the unwanted side effect. Indeed,
Khan et al. showed that targets of endogenous miRNAs are expressed at significantly
higher levels after transfection, consistently with impaired effectiveness of endog-
enous miRNA repression [ 56 ]. This effect exhibited concentration and temporal
dependence. Notably, the profile of endogenous miRNAs can be largely inferred by
correlating miRNA sites with gene expression changes after transfection. These
results suggest that this upregulation results from a saturation effect, which is a
competition among the transfected small RNAs and the endogenous pool of miR-
NAs for the intracellular machinery that processes small RNAs.
It is also essential to consider the degradation mechanism of miRNA. Recently,
Suzuki et al. identified mammalian immune regulator MCPIP1 ribonuclease as a
broad suppressor of miRNA activity and biogenesis [ 57 ] . MCPIP1 suppresses
miRNA biosynthesis via cleavage of the terminal loops of pre-miRNAs. Furthermore,
they clearly showed that elevated MCPIP1 expression is accompanied with poor
survival in lung adenocarcinoma patients and lung squamous cell carcinoma
patients. On the contrary, it has been reported that low expression of Dicer is associ-
ated with poor prognosis of lung cancer. Suzuki et al. also found a regulatory role of
MCPIP1 in the signaling axis comprising miR-155 and its target c-Maf. In addition
to MCPIP1, degradation of mature miRNAs in C. elegans , mediated by the 5¢ → 3 ¢
exoribonuclease XRN-2, affects functional miRNA homeostasis in vivo [ 58 ] .
Although release and degradation can both be blocked by the addition of miRNA
target RNA, after the release of the miRNA, exposing it to degradation by XRN-2.
These reports demonstrate the need to address the type and dosage of miRNAs.
Overdose of shRNA leads to the saturation of small RNA in the cells and then to
unwanted effects in nontargeting cells. Moreover, if the expression of MCPIP1 is