Page 90 - Read Online

Squarzanti et al. J Cancer Metastasis Treat 2019;5:73 I http://dx.doi.org/10.20517/2394-4722.2019.19 Page 5 of 11
[35]
therapeutic approach improvement. Panebianco et al. also confirmed this link. Moreover, the same
authors underlined that not only gut bacteria, but also the intratumor ones, may influence patients’
[36]
response toward chemo- and immuno-therapeutic drugs and vice versa .
More recently, it has emerged that EMT can also be promoted by Porphyromonas gingivalis, an anaerobe
frequently found in chronic periodontitis. In fact, it has been observed that this bacterium causes an
early EMT of chronically infected human oral epithelial cells (OECs) that, through PI3K/Akt activation,
ultimately lead to the loss of E-cadherin and to the accumulation of β-catenin, together with an increased
expression of Zeb1, vimentin, MMP-2, -7, and -9. Moreover, upon stimulation with this bacterium, OECs
migratory capacity increases proportionally [37-39] .

Both F. nucleatum and P. gingivalis determine an enhanced transcription of mesenchymal markers, an
increase of TGF-β1, TNF-α and EGF and a downregulation of those associated to epithelial layer integrity,
[40]
as evidenced by transepithelial electrical resistance measures .

Recently, a new tumor metastasis model has been proposed. Till now it was supposed that epithelial-
like cancer cells acquire mesenchymal features in order to produce metastasis by exploiting the vascular
system, and then switch back to an epithelial phenotype to establish the new tumor. The existence of
a stable population of hybrid epithelial/mesenchymal cells, possessing epithelial and mesenchymal
characteristics with both tumorigenic and metastatic properties, has been hypothesized [41,42] . Moreover,
in predisposed individuals, with defects in the innate immunity response or with a specific epigenetic
background, bacterial or viral infections could lead to EMT, causing a chronic inflammatory state, through
[43]
the activation NF-kB and MAPK . Considering this and the emerging role of the microbiota on EMT and
cancer progression, more rational and shared laboratory models should be used to better investigate the
[44]
microbiota composition and its relationship with the human host.
IN VITRO epITHeLIaL mODeLs FOR sTUDYING THe HOsT-paTHOGeNs INTeRaCTIONs

[45]
The first studies focused on host-pathogens connections started in the 1970s with Todaro and Taylor-
[46]
Robinson , who described the interactions between oncogenic viruses and human cells, and mycoplasma
pneumonia and ciliated tracheal epithelium, respectively. Since then, in vitro models acquired popularity
in the microbiology field, due to their reproducibility, higher versatility and high-throughput data
[47]
acquisition respect to in vivo models [Table 1] [5,16,38,45-62] . Undeniably, due to the improvement in imaging
and screening techniques and to the in vitro models features, nowadays it is possible to directly analyse
characteristics such as: specific surface-adhesion processes (i.e., biofilm formation), spreading capabilities,
immune system escapes aptitudes, migration in 3D matrices, and host’s cell-type-specific interactions in
general.

The development of functional co-culture systems mostly depends on their ability to allow the physiological
growth of cellular components in optimized culture media inside trans-well devices or microfluidic
[50]
chambers. In this regard, Di Giulio et al. developed a model in which Streptococcus mitis was grown with
human gingival fibroblasts in saliva sterilized stocks enriched with 1% sucrose as culture medium. With
this system, the authors demonstrated that S. mitis can penetrate within fibroblasts, via a FAK-integrin β1-
vinculin-actin mediated process, which is modulated by the saliva and the microenvironment themselves.

Sterilized artificial saliva has also been used in more complex models such as the multi-culture trans-well
[51]
system developed by Millhouse et al. , in which a complex multispecies biofilm, composed by S. mitis, F.
nucleatum, P. gingivalis and Aggregatibacter actinomycetemcomitans, was grown on a glass slide and co-
cultivated in a trans-well together with immortalized oral keratinocytes (OKF6-TERT2). With this system,
these authors validated the efficacy of commercially available oral hygiene products.