Page 90 - Read Online

Ottewell et al. J Cancer Metastasis Treat 2021;7:11 https://dx.doi.org/10.20517/2394-4722.2021.14 Page 9 of 20

There are currently no published data showing that this method of modelling metastatic dormancy/tumour
outgrowth works for other breast cancer cell types. However, unpublished observations from the Ottewell
lab suggest that ovariectomy can stimulate metastatic outgrowth tumour cells disseminated in bones of
adult (14-16 weeks old) C57BL/6 mice following intra-ductal injection of E0771 cells (previously
unpublished observations from the Ottewell laboratory; Figure 2), suggesting that ovariectomy may be a
suitable method for modelling dormancy/tumour outgrowth in bone form a wider repertoire of cell lines.

PDX models
Breast cancer cell lines, which are commonly used to model bone metastasis including MDA-MB-231,
MCF7 and T47D, have been in continuous culture, on plastic, in various laboratories for approximately 40
years [44,45] . As a result, these cell lines have lost heterogeneity and no longer accurately represent the original
tumour from which they were isolated. PDXs are tumours that have been cultured in immune-
compromised whole body model systems immediately following surgical removal from the patient. These
have not been cultured on plastic and for at least a minimal number of passages retain their original
molecular sub-types and heterogeneity . Thus, there is increasing interest in the use of PDXs to make
[46]
more clinically relevant models of bone metastasis. Surgical implantation of dissociated PDXs into the
cleared mammary fat pad or surgical engraftment of PDX fragments onto the existing mouse mammary fat
pads results in tumour growth at the primary site and disseminated tumour cells in the bone marrow from
both ER-negative breast cancers and a small number of ER-positive breast cancers [47-49] . However, outgrowth
into overt metastases is very low (15%) compared with development of metastasis in soft tissues such as
ovaries, lung and liver which have been reported to be up to 100%, 40% and 20%, respectively, from the
same model . Interestingly, metastatic outgrowth in bone has only been reported in one ER-negative PDX
[47]
[47]
model without the use of oestradiol supplementation . Injection of dissociated breast cancer PDXs into
mice supplemented with oestradiol significantly increases occurrences of overt metastatic lesions. Injecting
-
-
-
-
ER PR HER2 PDXs (BB3RC32 and BB2RC08) or ER PR HER2 PDXs (BB6RC37) into fourth mammary
+
+
ducts of NSG mice, supplemented with oestradiol, resulted in spontaneous metastasis to bone in 75%, 20%
and 20% of mice and metastasis to lung in 70%, 60% and 100% of mice, respectively . Intracardiac injection
[23]
-
of the same PDXs into NSG mice further increases overt bone metastases to 80% from ER PR HER2
+
+
-
-
BB3RC32 and BB2RC08 cells and 30% from ER PR HER2 BB6RC37 cells with lung metastases being
-
detected in 40%, 80% and 100% of mice, respectively . Taken together, these data support the idea that
[23]
oestrogen stimulates development of overt metastases in bone, possibly through its anabolic actions,
stimulating the bone metastatic niches, and both the seed (tumour cells) and soil (bone microenvironment)
need to be optimal for metastases to develop in this organ.
(1) Models of human breast cancer metastasis to human bone
As discussed above, spontaneous metastasis of human breast cancer cells to mouse bone is rarely observed,
especially in the absence of interventions that stimulate bone remodelling. Furthermore, strong evidence
suggests a role for the metastatic niche in homing and colonisation of bone as well as regulating metastatic
dormancy and growth (reviewed in [39,50] ). It is possible that species differences may influence the ability of
primary tumours to prime the metastatic niche for future arrival of tumour cells and/or regulate tumour cell
dormancy/outgrowth in this metastatic site. To develop more clinically relevant models, in which human
breast tumour cells metastasise to a tissue of human origin, researchers have developed mouse models in
which pieces of human femoral bone are subcutaneously implanted into immunocompromised mice before
injection of tumour cells [51,52] . Initial publications reported low rates of metastasis to the human bone
implants from a number of cell lines injected via tail vein or orthotopic implantation, and osteotropism was
only reported from the SUM1315 breast cancer cell line .
[51]