Page 96 - Read Online

Dixit et al. J Cancer Metastasis Treat 2022;8:47 https://dx.doi.org/10.20517/2394-4722.2022.70 Page 5 of 11

carry homozygous deletions of CDKN2A, which encodes for two cell cycle regulatory proteins in the p53
and RB pathways i.e., p16 (INK4a) and p14 (ARF) [16,17] . Thus, wild-type p53 represents a long sought-after
[18]
therapeutic target in MPM . However, leveraging the biologic effects of miRNA could be a more practical
approach. Importantly, there are MPM-specific miRNA studies that have described reactivation of p53 and
its physiologic effects. In fact, p53 regulates the transcriptional expression and biogenesis of a group of
miRNAs, concurrently with its gene regulatory network being modulated by miRNAs at multiple levels, i.e.,
a feedback loop .
[19]

[19]
miR-145-5p indirectly regulates p53 by directly inhibiting its key negative regulator MDM2 . In addition to
a potential diagnostic role, miR-145-5p is downregulated in MPM tumors compared with normal pleura
[20]
due to being silenced by promoter hyper-methylation . Furthermore, re-expression of miR-145-5p
negatively modulates cell growth, clonogenicity, and migration, while inducing drug resistance and
senescence in MPM cells . miR-145-5p exerts these antitumor effects in MPM cells by downregulating
[21]
genes such as c-MYC, OCT4, and ZEB1, all of which mediate core cellular processes. To complement
in vitro studies, MSTO-211H cells transfected with control or miR-145-5p mimic were subcutaneously
implanted into SCID mice to demonstrate differential tumorigenicity. What remains unclear, and a major
limitation of this type of in vivo model, is whether therapeutic administration of miR-145-5p would result in
tumor shrinkage.

miR-215-5p is a unique MPM-associated miRNA that inhibits the malignant features of MPM cells in vitro,
largely via activating the MDM2/p53 positive feedback loop, which leads to caspase-dependent apoptosis
and/or G1/S cell cycle arrest . miR-215-5p is downregulated in MPM tumor cells/tissues and is associated
[22]
with poor prognosis. Ectopic overexpression of miR-215-5p inhibited cell growth and anchorage-dependent
and anchorage-independent colony formation in MPM cell lines. The basis for these physiologic effects,
aside from p53 activation, was also partially due to several novel gene targets of miR-215-5p, i.e., CDC7,
MAD2L1, BIRC5, LMNB2, and RUNX1 (and several not previously associated with MPM). Furthermore,
this study verified that in MPM cells, activation of the miR-215/MDM2/p53 positive feedback loop also
enhanced the activity of miR-145-5p, another p53-inducible miRNA. A single application of miR-215-5p
mimic via peri-tumoral injection around subcutaneous MPM xenografts in NSG mice was effective at
shrinking tumors. Finally, improved survival was observed in mice harboring intra-pleural xenografts
(orthotopic) that were treated with miR-215-5p complexed with atelocollagen (in vivo delivery vehicle).

The miR-34 family members are p53-inducible miRNAs that have anticancer effects in a variety of cancers
including MPM . The expression of miR-34a-5p, miR-34b-5p, and miR-34c-5p were measured in
[19]
formalin-fixed, paraffin-embedded (FFPE) MPM tumor samples obtained from patients undergoing extra-
pleural pneumonectomy (n = 60) and related normal pleura samples (n = 23), and the results showed that all
were under-expressed (see supplemental data of ref.) . In vitro, miR-34a-5p was downregulated in MPM
[23]
[24]
subclones that acquired drug resistance with phenotypic rescue upon expression of miR-34a-5p mimic .
Additionally, transfecting miR-34a-5p mimic into MPM cells reduced cell proliferation, induced cell cycle
arrest in G0/G1 phase, and modestly potentiated the anticancer effect of cisplatin . MPM tumor cells from
[25]
Nf2 /Cdkn2a mice exhibited reduced expression of miR-34a-5p via a p53-dependent pathway, which in
+/−
+/−
turn caused elevated c-MET expression .
[26]
Similarly, miR-34b-5p and miR-34c-5p (i.e., miR-34b/c) are epigenetically silenced by promoter
hypermethylation, accounting for their lowered expression in MPM cells and tissues . Ectopic
[27]
overexpression of miR-34b/c in MPM cells inhibited proliferation and migration/invasion as well as
inducing G1/S cell cycle arrest and apoptosis, an effect further enhanced by radiation synergizing with