Therapeutic Application of MicroRNAs in Cancer - RNA Interference from Biology to Therapeutics - page 303

through inward budding of endosomal membranes, giving rise to intracellular [ 46 ]
multivesicular bodies (MVBs) that later fuse with the plasma membrane, releasing
the exosomes to the extracellular space [ 46 ]. Exosomes are released by many types
of cells and are able to mediate communication between cells. The work by Lotvall
demonstrated that mouse and human mast cell-derived exosomes contain RNA and
miRNA and that RNA from mast cell exosomes is transferable to other mouse and
human mast cells. After the transfer of mouse exosomal RNA to human mast cells,
new mouse proteins were found in the recipient human cells, indicating that trans-
ferred exosomal mRNA can be translated after entering another cell. Observations
from this report indicated the important fact that miRNA could exist outside of the
cell and remain functional in an RNase-abundant environment.
Following the discovery of miRNA in exosomes, three reports showed transfer
and functionality of miRNAs contained within secretory exosomes. Pegtel et al.
demonstrated that mature EBV-encoded miRNAs are secreted in exosomes by EBV-
infected B cells and that these miRNAs repress the EBV target immunoregulatory
genes in primary EBV-associated lymphomas by in vitro study [ 47 ] . Zhang et al.
reported that miR-150 expressed in a human monocyte/macrophage cell line is con-
tained within exosomes and that such exosomes have the ability to deliver miR-150
into human microvascular endothelial cells, thereby inducing the downregulation of
c-Myb to enhance cell migration [ 48 ]. Our group has demonstrated that secreted
tumor-suppressive miR-146a, downregulated in prostate cancer, can be transported
to prostate cancer cells and exert gene silencing in the recipient prostate cancer cells
through the suppression of its target gene, ROCK1 protein, resulting in cell growth
inhibition [ 49 ]. In this study, we employed the approach to overexpress miR-146a
vector in donor cells, enabling us to obtain miR-146a highly enriched exosome. We
avoided the use of synthetic analogs of mature miRNAs for the overexpression
experiment because they might persist in the medium and interfere with accurate
quantification of extracellular miRNAs. Our study raises the possibility that secreted
miRNA could function as a cell-cell communication tool between the cancer cells
and cells in their microenvironment, such as endothelial cells, immune cells, and
fibroblast. These three reports demonstrated important biological functions of exo-
somal miRNAs in various physiological and pathological conditions, including
virus infection, vascular disease, and cancer.
Recently, the application of exosomes for targeted siRNA delivery to the brain in
mice was performed using exosomes [ 50 ]. In this paper, self-derived dendritic cells
for exosome production were used to reduce the immunogenicity and loaded with
GAPDH siRNA by an electroporation method. Targeting was achieved by engineer-
ing the dendritic cells to express Lamp2b, an exosomal membrane protein, fused to
the neuron-specific RVG peptide3. Intravenously injected RVG-targeted exosomes
showed specific gene knockdown in neurons, microglia, and oligodendrocytes in
the brain. The application of this system for delivery of ASO or miRNA could be
used for miRNA-based therapies.
Exploitation of natural carriers of miRNAs is not limited to exosomes. Vickers
et al. revealed that high-density lipoprotein (HDL) transports endogenous miRNAs
and delivers them to recipient cells with functional targeting capabilities [ 51 ] .