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Page 58 Hunt et al. Extracell Vesicles Circ Nucleic Acids 2020;1:57-62 I http://dx.doi.org/10.20517/evcna.2020.04

Keywords: MicroRNA, microenvironment, adrenergic neurons, solid tumors, neurotrophic growth, neuron-tumor
crosstalk

Cancers are predominantly characterized by their loss of proliferative control. Genetic changes drive
this increased proliferation, resulting in the formation, growth and spread of tumor cells throughout
the body. Mutations that commonly drive tumor growth, however, also drive molecular changes within
the tumor and the surrounding tissues. Encircling the solid tumors are collections of healthy cells that
typically act to support cell types and tissue functions in the local region. These cells include structural
fibroblasts, immune cells, neurons, blood vessels, and other cell types that in combination are known
as the tumor microenvironment. In the context of cancer, these supporting cells can be manipulated to
support the growth and spread of the tumor. These manipulative relationships become essential for the
[6]
[1,2]
[7,8]
survival of the cancer and are found in prostate , gastric [3-5] , pancreatic , skin , glioma [9-11] , and a
variety of other tumor types [12–15] . Subsequently, efforts to interrupt the relationships between solid tumors
[16]
[17]
and immune cells , as well as between tumors and blood vessels have shown efficacy in stymying
tumor growth, demonstrating the widespread therapeutic potential borne from understanding these
tumor-microenvironment relationships. To date, the relationships between tumors and other members
of the tumor microenvironment are not well understood. Additionally, the mechanisms by which these
[18]
relationships are formed and sustained are not well documented. Recent work from Amit et al. has
uncovered that tumors use extracellular vesicular signaling to drive nearby neuron survival, growth, spread,
and subtype switching, which, in turn, drives tumor growth.

This recent work focused on oral cavity squamous cell carcinoma (OCSCC), an aggressive tumor arising
from mouth epithelial cells. To study these tumors in greater depth, the group leveraged laboratory mouse
models of OCSCC. These include mice in which the tumor suppressor p53 was conditionally knocked
cre
out of epithelial cells (Krt5 ; Tp53 flox/flox ). The control group for these mice lacked the Krt5 allele, leaving
cre
Tp53 intact in all cells. In these models, tumor initiation was induced via introduction of the carcinogen,
4-nitroquinoline 1-oxide (4NQO) into the drinking water. Additionally, the group used patient-derived
WT
null
xenograft models in which p53 or p53 OCSCC cells were injected into the tongues of mice and allowed
to grow. Finally, in cell culture dishes, dorsal root ganglia (DRG) were co-cultured with oral keratinocytes
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and p53 or p53 OCSCC cells to model neural interactions with OCSCC tumors in vitro.
Initially, the group found that in tumor samples derived from both their conditional p53 knockout
mice, as well as their patient-derived xenograft models, loss of p53 coincided with increased adrenergic
nerve density within the tumor. These findings were recapitulated in human OSCSS samples, indicating
that tumor cells lacking p53 were driving increased adrenergic neuritogenesis. When testing this
[18]
same hypothesis using in vitro OCSCC-DRG co-cultures, Amit et al. concordantly found increased
null
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DRG innervation of p53 OCSCC cells. When p53 OCSCC-DRG co-cultures were incubated
null
with conditioned medium containing extracellular vesicles (EVs) from p53 OCSCC cells, the same
null
neuritogenic effect was observed. Additionally, when the p53 OCSCC cells co-cultured with DRGs were
null
inhibited from releasing EVs, the effect was lost, thus demonstrating that the p53 -derived EVs and their
contents were the driving force behind the described neuritogenic effect.
Extracellular vesicles serve as important vehicles of small regulatory RNA species, which are known to be
essential for proper neuronal development and function. By comparing the small RNAs found within EVs
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derived from p53 and p53 cell lines, Amit et al. found that 17 microRNAs (miRNAs) were down-
[18]
null
regulated in p53 cells. After narrowing this collection of miRNAs to the most down-regulated species,
null
they found that several of these miRNAs, including miR-34a, were also down-regulated in the p53