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Page 8 of 14 Maisel et al. J Cancer Metastasis Treat 2019;5:7 I http://dx.doi.org/10.20517/2394-4722.2018.82

demonstrated an ability to undergo endocytosis in the absence of clathrin or AP-2 at relatively uninhibited
rates, likely due to the nature of the receptor to promote T-cell proliferation - a system that would benefit
from redundancies [122] .

When undergoing non-CME, ILR2 is internalized through detergent-resistant membrane domains, similar
to the mechanism employed by cholera toxin [124,125] . Upon entry into the cell, vesicles containing ILR2 are
found in the glycolipid rich areas of the cell, particularly in the perinuclear spaces and near the organelle
responsible for glycolipid modification, the Golgi [124] . Retention of the receptor in the perinuclear space is
common in cancer, as ILR2 expression can be a predictor of patient survival and is significantly reduced at
cell surface levels in advanced ovarian cancers [126,127] .

TGF-β RECEPTOR
TGF-β is a secreted ligand involved in modulating cellular growth and arrest, differentiation, and immune
responses, capable of binding to type I or type II single-pass transmembrane receptors [47,128-130] . Unlike other
receptors presented in this review, TGF-β receptor can be internalized with or without ligand binding,
[47]
resulting in either CME-assisted recycling or non-CME degradation .
When subject to CME, TGF-β receptor is sorted into EEA1-positive vesicles where it acts to activate SMAD2
and potentiate TGF-β signal transduction. Activated receptors localize to the perinuclear space and remain
in endosomes, a phenotype also seen with the transferrin receptor and which we have previously shown
to occur with EGFR in breast cancer [41,47] . Alternatively, TGF-β receptor can also be sorted via lipid rafts to
interact with SMAD7. In either scenario, SMAD signaling nuclear translocation require internalization of
TGF-β receptor [131,132] .

Taken together, a range of transmembrane proteins are subject to retrotranslocation. While some localize
to the perinuclear space and can be internalized into the nucleus, such as InsR, FGFR, TGF-β receptor, and
ILR2, others can enter the Golgi, such as ErbB family members, and remain active and promote metastasis.
ErbB receptors also localize to the nucleus, resulting in transcriptional activity of cancer genes and pathways.

THERAPEUTIC TARGETING OF RETROTRANSLOCATION
Given the metastatic and oncogenic activity associated with nuclear localization of activated receptor
tyrosine kinases driven by retrograde trafficking, it is important to explore potential mechanisms by which
retrotranslocation could be inhibited as a therapeutic target.

It has been demonstrated that introduction of retrograde inhibitors can drastically reduce the cytotoxic
effects of toxins like ricin, cholera, or shiga, which rely on retrograde trafficking to be distributed throughout
cells. A compound designated Retro-2 works to inhibit toxin trafficking from the early endosome to the
trans-Golgi interface, actively protecting cells from ricin toxicity 2-3 fold in vitro and almost 50 percent of
mice from airborne exposure to the ricin toxin (in comparison to the 11 percent that survived in the absence
of retrograde inhibition) [133] . Retro-2 does not inhibit plasma membrane budding or endosome formation;
rather it inhibits transport of endosomes to the TGN without affecting Golgi morphology (or that of EEA1
or Rab11). Endocytic degradation and recycling pathways also remain unaltered. However, SNARE proteins
syntaxin 5 and 6 were subject to alterations in localization, indicating Retro-2 may work to inhibit retromer
trafficking of endosomes or inhibit interactions of endosomes with the Golgi [133] .

We have previously demonstrated the importance of retrograde trafficking in promoting a migratory
[41]
phenotype in association with the presence of MUC1 . Treatment of breast cancer cells in vitro with the
retrograde trafficking inhibitor Retro-2 led to the inhibition of MUC1-driven migration and re-introduction