Page 92 - Read Online

Ottewell et al. J Cancer Metastasis Treat 2021;7:11 https://dx.doi.org/10.20517/2394-4722.2021.14 Page 11 of 20

several factors should be taken into account: simultaneous to injection of PDXs and ER-positive cell lines,
mice were supplemented with oestradiol, and, although these cell types predominantly metastasised to
human bone implants, metastasis to mouse bone was also detected. Oestradiol supplementation is not
required for metastasis of ER-negative, MDA-MB-231 or SUM1315 cells, and these specifically spread to the
human bone implants . As oestradiol has been shown to stimulate ER-positive tumour metastasis to
[23]
mouse bone [23,40] , it is possible that this apparent tropism to mouse bone, seen in PDXs or ER-positive cell
lines in this model, is influenced by this hormone, and this hypothesis is supported by work carried out by
[54]
Yang et al. . Using a similar model, in which non-tumour bearing human bone and human bone
impregnated with MDA-MB-231-GFP breast cancer cells were implanted into opposite flanks of NOD SCID
mice, tumour cells were found to spontaneously metastasise from the original bone metastases, specifically,
to the contralateral non-tumour bearing human bone implant . Similarly, the same research group showed
[54]
that two out of five human primary tumour samples co-implanted with human bone were able to grow in
this bone (HuP2, a metaplastic carcinoma, and HuP4, a ductal carcinoma); despite both of these tumours
growing to 2-3 cm in diameter in the human bone grafts, metastasis to the contralateral non-tumour
bearing human bone implants was only detected from HuP4. Interestingly, mice used in this study were not
supplemented with oestradiol and metastasis to mouse bone was not observed. It must be noted, however,
that metastases in the contralateral human bones were small (only detectable microscopically), and whether
lack of metastasis to mouse bone is due to no tumours cells being disseminated in this site or metastases
being too small to detect remains to be established. In this manuscript, the authors hypothesise that this
apparent lack of metastasis to mouse bone is indicative of species specificity and that this model can be used
to decipher between metastatic and non-metastatic breast cancer; however, as only one patient sample was
[52]
metastatic, more data are required for these types of conclusion to be drawn . These human bone models
necessitate the use of immune-compromised mice; despite implanted bone demonstrating active
haematopoiesis and production of human B cells, the lack of autoimmunity observed in these models
suggests that human immune cells are not very active, and the presence of human T cells has not yet been
investigated [23,51] . It is apparent that, although data from mouse models of human breast cancer metastasis to
human bone implants are amassing, further research is required before we can explore the full potential of
these models.

For experiments designed to determine the latter stages of bone metastasis in a human-specific
environment, tumour cells can be seeded directly into subchondral bone discs of various sizes. These co-
cultures can subsequently be used for in vitro assays or can be implanted into NOD SCID mice for
investigating parameters associated with tumour cell-bone cell interactions . Importantly, molecular
[53]
profiling performed in both the spontaneous metastasis and co-culture implant models has identified novel
signaling pathways that have subsequently been confirmed to be clinically relevant in the bone metastatic
process .
[6]

It should be noted that the use of human-specific models for research purposes are not a simple option.
These models require appropriate ethical agreements to be in place as well as access to freshly resected
femoral heads that need to be sectioned and implanted into mice/placed in culture within 2-3 h following
surgical resection. Furthermore, to achieve consistent data, bone samples should be cut to a standard size;
this accuracy necessitates the use of specialised equipment (as described in [23,53] ). For researchers wishing to
model the entire metastatic process from tumour growth in the mammary site to overt metastases in bone,
this is lengthy and expensive. Tumour cells are injected four weeks following implantation of human bone,
and, dependent on the tumour cells being tested, it can take a further 6-15 weeks before metastases are
detected in this metastatic site [Table 1]. Injecting PDXs requires additional expansion of the tumour cells
in NSG mice prior to implantation to ensure sufficient numbers of cells from the same patient are available