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Tulotta et al. J Cancer Metastasis Treat 2019;5:74 I http://dx.doi.org/10.20517/2394-4722.2019.022 Page 5 of 11

ZEBRAFISH XENOGRAFT AS A MODEL TO STUDY CANCER
Research performed in pre-clinical in vivo models is constantly under development to provide further
insights into the communication between tumour and the surrounding microenvironment. Zebrafish
[43]
(Danio rerio) is a tropical freshwater teleost, increasingly used to study a range of disease processes as
well as being an excellent tool for the study of development. Several important advances in understanding
of cancer and inflammation have arisen from studies in zebrafish [44-46] . The rapid and external development
[47]
of transparent embryos , availability of reporter lines with traceable fluorescent cells [48-50] , ease of genetic
[53]
manipulation [51,52] and pharmacological approaches make the zebrafish an excellent in vivo model to
visualise single cell interactions in real time and to uncover the signalling mechanisms involved, on a
[54]
whole organism level. Zebrafish is increasingly used as a model organism to study cancer . There is
high conservation of oncogenes and tumour-suppressor genes between zebrafish and human therefore
[55]
data collected in zebrafish are relevant for humans . The histology of zebrafish tumours has been shown
[56]
to be highly similar to tumours found in human cancers . Moreover, zebrafish is a valuable tool to
study drug discovery in the context of cancer research [57,58] . Zebrafish larvae can absorb small molecular
weight compounds from water, which is advantageous when screening for anti-cancer compounds . The
[59]
experimental costs are low and procedure are simple and fast. This accounts for the experimental increase
in the use of zebrafish in drug discovery during the last two decades in a time- and cost- effective manner.
For melanoma, a presently on-going phaseI/II clinical trial of Leflunomide combined with vemurafenib is
the first to arise from initial screen in zebrafish. To study human cancer metastasis, our group generated
a xenotransplantation model of experimental micrometastasis [60,61] . Human tumour cells engrafted into
the blood circulation of 2-day-old zebrafish embryos induce angiogenesis and form micrometastasis
[60]
sustained by neutrophils and macrophages, nearby hematopoietic sites . In particular, tumour-induced
angiogenesis, metastasis formation and relative chemical approaches to inhibit these processes have been
studied using zebrafish as a xenotransplantation model, complementing current knowledge developed
[62]
through the use of in vitro and other in vivo models . Upon localised or haematogenous engraftment
of cancer cells, zebrafish xenografts allow qualitative and quantitative assessment of tumour burden and
tumour-microenvironment interaction, representing a powerful pre-clinical model to unravel cancer
[61]
mechanisms and to develop new therapeutic strategies . In particular, alongside murine models, the
use of PDXs in zebrafish has the potential to be used in personalised medicine [63-66] , with the advantage
[57]
of requiring less tumour material and shorter times for the monitoring of tumour development .
Several studies have shown that the combined use of zebrafish and murine models paves the way
towards important insights to elucidate the biology of metastatic cancers and the development of new
treatments [67-71] . Therefore, the zebrafish xenograft model bears the potential to elucidate crucial kinetics
and key mechanisms that regulate tumour-microenvironment interaction and ultimately support tumour
spreading.

CELL-AUTONOMOUS CXCR4 SIGNALLING: THE CXCR4 ANTAGONIST IT1T IMPAIRS EARLY
HUMAN METASTATIC EVENTS, IN A ZEBRAFISH XENOGRAFT MODEL WHERE THE
INTERSPECIES CROSS-TALK TAKES PLACE
[72]
Chemokines direct tumour and stromal cell bidirectional migration . CXCR4 plays a physiological
[78]
role in hematopoiesis [73,74] , leukocyte trafficking [75-77] , cell migration and embryo development , as well
[80]
[79]
as a pathological function in HIV pathogenesis , WHIM syndrome and cancer [81,82] . In addition to
[83]
its cognate ligand CXCL12, CXCR4 can bind ubiquitin , macrophage migration inhibitory factor [84-86]
[87]
and CXCL14 . The CXCR4-CXCL12 signalling axis is known to play a critical function in cancer cell
spreading, when tumour cells expressing high levels of CXCR4 communicate with CXCL12-secreting
[88]
stromal cells of distant organs that function as metastatic and secondary growth “soils” .