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Maisel et al. J Cancer Metastasis Treat 2019;5:7 I http://dx.doi.org/10.20517/2394-4722.2018.82 Page 9 of 14

of EGFR trafficking to the lysosome for degradation. Given this, a potential therapeutic option may be to
inhibit retrograde trafficking of EGFR, possibly through the use of Retro-2, though no studies have yet
examined the effects in humans.

Alternative to inhibiting retrograde trafficking, current research has demonstrated an effort to capture
the retrotranslocation mechanisms of toxins and repurpose them for drug delivery. Using the non-toxic
subunit B of Shiga-like toxin, various attempts at conjugating therapeutics, incorporating nanoparticles, or
[134]
developing fusion proteins have been attempted (reviewed in Luginbuehl et al. ).

Should inhibition of retrograde trafficking prove untenable in patients, a secondary approach to inhibition of
the oncogenic activity driven by nuclear receptor tyrosine kinases would target receptors within the nucleus.
Nuclear EGFR has been shown to associate with promoter regions of a variety of proteins, including cyclin
D1, activated STAT3, E2F1, DNA-dependent protein kinase, and other nuclear targets, while nuclear ErbB2
is capable of interacting with COX-2, all resulting in increases in tumorigenesis, proliferation, metastasis,
chemoresistance, and radioresistance [106,135] . ErbB2-EEA1 complexes are capable of nuclear transport, and
given the high affinity of EGFR-MUC1 complexes in EEA1 positive vesicles in the perinuclear space of cells
after extended exposure to EGF ligand, it is feasible that EGFR endosomal machinery is promoting nuclear
localization of EGFR in a similar mechanism [41,87,136] .

Radiation treatment of cells is known to drive EGFR to the nucleus as part of the DNA-repair pathway
mechanism, and studies have shown that treatment of irradiated cells with cetuximab will inhibit EGFR
trafficking to the nucleus in both lung carcinoma and breast cancer cell lines [137] . Radiation treatment, along
with neuregulin stimulation or trastuzumab will also promote retrograde trafficking of ErbB2-ErbB3 dimers
to the nucleus, visible by super-resolution confocal microscopy as demonstrated by Pilarczyk et al. [138] .
Nuclear import might also be inhibited by targeting the importin-β1 molecule through treatment with small
molecule inhibitors such as Karyostatin1A to disrupt importin interactions with the GTPase Ran [139] .

Treatment of cells with 1,25-dihydroxyvitamin D was found to prevent EGFR from entering the nucleus by
promoting intracellular localization of inactive, unphosphorylated EGFR, even if bound to a ligand, to early
endosomes. By doing so, 1,25(OH) D downregulated the oncogenic activity associated with nuclear EGFR
2
3
without targeting vesicles to the lysosome [140] . This treatment could feasibly be used to target two processes
associated with cancer progression - inhibit nuclear targeting of EGFR and convert endosomally localized
EGFR to an inert state, essentially nullifying the need for lysosomal degradation.

CONCLUSION
Retrograde trafficking of transmembrane proteins, with an emphasis on receptor tyrosine kinases, results in
alternatively trafficked activated receptors accumulating in the perinuclear space of cells. The primary effect
of these undegraded, endosomally-retained, actively signaling receptors is the unabrogated transduction
of the PI3K/AKT pathway - resulting in upregulation of cancer metastasis. Not only that, but for those
receptors that transit through the Golgi or come in sufficient proximity to a nuclear pore, active receptors
can now act as transcriptional activators to a host of oncogenic activities. Seen in cancers ranging from
Ewing sarcoma to breast to prostate and more, it is clear this ubiquitous mechanism of translocation is
responsible for trafficking receptors to the wrong place at the wrong time, driving cancer metastasis. Future
therapeutics may choose to inhibit retrograde trafficking to prevent mislocalization or focus instead on
targeting receptors after their localization has already been altered. Given current treatment plans for many
cancers now involve adjuvant therapies, the most efficient method for overcoming metastasis-promoting
retrograde trafficking may involve some combination of both.