Page 39 - Read Online

ALHulais et al. J Cancer Metastasis Treat 2019;5:3 I http://dx.doi.org/10.20517/2394-4722.2018.71 Page 9 of 23

findings indicate that LGR5 can function as a tumor suppressor promoting maintenance of stemness during
CRC progression and that LGR5 is a suitable marker of CSCs.

The most definitive studies for LGR5 as a CR-CSC marker were obtained by using a tamoxifen inducible
[2,4]
suicide gene inserted into the LGR5 gene of transgenic mice (reviewed in [102] ). The suicide gene could
be triggered upon expression of LGR5 and activated by the injection of tamoxifen, resulting in death of
the colon CSCs by tamoxifen. The more differentiated tumor cells do not transcribe LGR5 and hence,
do not express the suicide gene, such that they survived. When the suicide mechanism was activated in
growing tumors in mice, the LGR5-positive CSCs were eliminated. As long as the suicide mechanism was
maintained then tumors regressed but when their suicide was no longer induced (by tamoxifen withdrawal),
then differentiated tumor cells filled the void and reacquired CSC qualities with rapid tumor regrowth.
Furthermore, it was shown that metastasis and growth of preestablished metastases was halted. These
findings showed that unlike cancer at the primary tumor site, metastases were dependent on preexisting
+
LGR5 CSCs, so that once these CSCs were eliminated, reacquiring the CSC phenotype in the metastases
[4]
was not possible . Hence, LGR5 is expressed in CR-CSC and these studies support the Dandelion hypothesis
[Figure 1B], such that advanced metastatic CRC should be amenable to elimination by anticancer drugs that
target the CSCs.

CD326 (EPCAM)
The surface marker, EPCAM (also referred to as 17-1A, TROP-1, CD326, GA733-2, KS1/4 or gp40) is a type
1 transmembrane glycoprotein and a calcium-independent homophilic cell adhesion receptor of 30-40 kDa,
encoded by the TACSTD1 gene [103] . EPCAM is expressed by several human epithelial tissues, progenitor cells,
cancer cells, stem and germ cells [104] . This protein marker was discovered initially as an antigen expressed on
colon cancers that was involved in cell-cell adhesion and the structure of EPCAM comprises an extracellular
domain, thyroglobulin domains, epidermal growth factor domains, transmembrane domain and a short
intracellular domain (ICD) consisting of 26 amino acids known as EpICD [105,106] . EpCAM (TACSTD1) may
be an oncogene as the encoded EpICD can be proteolytically released to signal into the cell nucleus by
engagement with elements of the WNT signalling pathway including activation of the b‐catenin/MYC
pathway resulting in tumor cell proliferation (reviewed in [106] ). Expression of the EPCAM marker is elevated
in malignant neoplasia and most cancers abundantly express EPCAM [104] .

In normal human cells, EPCAM is located in the intercellular spaces at the tight junctions formed between
adjacent epithelial cells [107] . Hence, it is inferred that EPCAM is sequestered in normal epithelial, whereas
it becomes uniformly spread around the surfaces of cancer cells, where it is readily accessible to antibodies
targeting it. The role of EPCAM in tumor progression and development is controversial, as is its relationship
to another cell adhesion molecule, E-cadherin [105] . EPCAM has been proposed to act as a molecule for
homophilic cell to cell adhesion, which would prevent the occurrence of metastasis [103] . It has a protective
function in CRC since the deletion of EpCAM leads to an increased risk of developing CRC [108] and when
overexpressed on CRC cells, inhibits their metastasis [109] .

In contrast, EPCAM expression inhibits the extent of E-cadherin mediated cell to cell adhesion, enhancing
metastasis [110] , although EPCAM has been proposed to bind to E-cadherin, both of which are important to
the stability of the tight junctions (reviewed in [107] ). In addition, overexpressing EPCAM in tumor cells can
enhance WNT signaling to promote the proliferation of cancer cells [111] . In breast carcinoma, high levels
of EPCAM expression are associated with poorly differentiated tumors [112] , development of larger cancers,
nodal metastasis and poor survival of the patients [113] . Marked EPCAM expression on breast carcinomas
has been linked to poorer prognosis for both node positive and negative diseases [114] . In 2009, the European
Union approved the EPCAM antibody, Catumaxomab, a bivalent monoclonal antibody which cross-links
between T cells (via CD3 receptor binding) and EpCAM on tumor cells in order to promote the elimination