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Page 2 of 13 Gambari et al. J Cancer Metastasis Treat 2019;5:55 I http://dx.doi.org/10.20517/2394-4722.2019.18

functionally controlled by several miRNAs (eventually differentially expressed in cells of different histotype);
following these considerations, it is calculated that more than 60% of human mRNAs can be considered
molecular targets of miRNAs . It has been reported in different studies and reviews that the miRNA/
[5,6]
mRNA interaction occurs at the level of the RNA-induced silencing complex (RISC), a ribonucleoprotein
which incorporates one strand of a single-stranded RNA (in our case a microRNA), acting as a template to
recognize complementary mRNA transcripts . This molecular interaction is associated to (1) repression of
[1-4]
translation or (2) mRNA degradation, depending on the level of miRNA complementarity with nucleotide
sequences of the target mRNA . Since their discovery and first characterization, the number of human
[3,4]
microRNAs identified and deposited in the miRBase database (miRBase v.22, www.mirbase.org) is much
more than 2600 and the research studies on microRNAs have confirmed the very high complexity of the
[7-9]
networks constituted by miRNAs and RNA targets .
[7]
Changes of microRNA expression have been demonstrated to be associated with different human pathologies,
and guided alterations of specific miRNAs have been suggested as novel approaches to develop innovative
therapeutic protocols [10-13] . Several reports conclusively demonstrated that microRNAs are deeply involved in
tumor onset and progression, behaving as tumor promoting miRNAs (oncomiRNA and metastamiRNAs) as
well as tumor suppressor miRNAs [14-19] . In general, a miRNA able to promote cancer targets mRNA coding
for tumor-suppressor proteins, while microRNAs exhibiting tumor-suppression properties usually target
mRNAs coding oncoproteins .
[15]

Targeting oncomiRNAs and mimicking tumor-suppressor miRNAs: overcoming drug resistance
With respect to targeting oncogenic RNAs and mimicking tumor-suppressor miRNAs in translational
medicine, it should be underlined that these non-coding RNAs are suitable targets for therapeutic
interventions, as summarized in Figure 1 [10-13] . The use of modified miRNA mimetics, either synthetic
or produced by plasmid or lentiviral vectors, might lead to potentiation of miRNA functions (miRNA
replacement therapy) [20-24] . In this case the miRNA replacement molecules (mimicking the miRNA functions
to be up-regulated) are transfected to target cells [Figure 1A, step “a”] where interact with the mRNA to be
modulated [Figure 1A, step “b”]. This interaction leads to down-regulation of this mRNA and associated
suppression of protein production [Figure 1A, step “c”, dotted arrows]. According to this procedure, it is
possible to mimic the activity of tumor-suppressor miRNAs (down-regulated in tumors) to achieve down-
regulation of miRNA-regulated oncogenes [23,24] .

On the contrary, forced down-regulation of miRNA biological functions can be obtained using a large
variety of well-characterized miRNA-inhibitor oligomers (such as in the case of direct miRNA antisense
therapy based on RNA, DNA, LNA and other DNA analogues) [25-31] , miRNA sponges [32-38] , mowers or
[39]
through miRNA masking strategy that interferes with miRNA function by masking the miRNA binding
site of target mRNA through hybridization with complementary molecules [40-42] [see Table 1]. In this case the
miRNA inhibitors [“a”, “b” and “c” of Figure 1B, suppressing the miRNA functions to be down-regulated]
are transfected to target cells where they interact with the microRNA target [Figure 1B, step “d”]. This
interaction prevents the target miRNA (for instance an oncomiRNA in protocols designed to develop anti-
tumor therapy) to bind the specific 3’UTR sequence of the regulated mRNA, causing up-regulation of this
mRNA and the associated increase of protein production [Figure 1B, step “e”]. According to this procedure,
it is possible to inhibit the activity of oncomiRNAs (up-regulated in tumors) to achieve up-regulation of
miRNA-regulated tumor suppressor genes.

Interestingly, in the case of the development of anti-cancer protocols, targeting miRNAs in drug-resistant
tumor cell lines has been associated with partial or total reversion of the drug-resistance phenotype, as
reported in different studies [43-48] .