Discussion
Moderate and advanced stages of ESCC patients lack effective systemic
treatment to eradicating tumor cells, and a new treatment alternatives
are urgently needed to improve patient prognosis. Currently, a series of
studies have supported the accumulation of ubiquitinated proteins
causing tumor cell death (Soldatenkovet al. , 1998; Mimnaugh et
al. , 2004; Itoh et al. , 2019).
Therefore, inducing ubiquitinated protein accumulation has become an
increasingly important treatment strategy in cancer therapy, proteasome
inhibitors have been reported to have significant antitumor activity
against various cancer cells through this
mechanism(Sato et al. , 2012;
Sato et al. , 2014;
Sato et al. , 2017). The novel
orally proteasome inhibitor ixazomib is currently used for treatment of
multiple myeloma patients, and it has been confirmed to have killing
efficiency to tumor cells (Sato et
al. , 2017; Yue et al. , 2019;
Harris et al. , 2020). In this
study, we explore whether proteasome inhibitor ixazomib have therapeutic
effects on ESCC. Our results proved that ixazomib effectively inhibits
cell proliferation in ESCC cell lines and xenograft tumor model mice. In
addition, we elucidate that ixazomib induces the apoptosis and
suppresses proliferation in esophageal squamous cell carcinoma through
activation of the c-Myc/NOXA pathway. Moreover, our results showed that
combined treatment with ixazomib and cisplatin induces synergistic
anti-ESCC activity. These results clearly suggest that ixazomib have
potential to be used as anti-ESCC therapeutics.
The pro-apoptotic NOXA is a Bcl-2 family protein, and it is a crucial
role that interacts with several apoptosis-related proteins such as Bad,
Mcl-1, Bim and Puma (Morsi et al. ,
2018). In addition, endoplasmic reticulum stress plays an important
role in mediating tumor cell apoptosis by inducing the upregulation of
NOXA (Cano-Gonzalez et al. , 2018).
The c-Myc oncogene is well known and acts as a driving force to activate
a variety of oncogenic signaling pathways
(Yoshida, 2018). c-Myc is frequently
dysregulated or overexpressed in multiple tumor cells, and its abnormal
expression contributes to reprogramming cell metabolism and to
maintaining the high rate of proliferation rate in cancer cells
(Dejure et al. , 2017). Therefore,
apoptosis in response to increased expression of c-Myc is an important
endogenous protection mechanism against failure that inhibits the
oncogenic properties and tumorigenesis of c-Myc
(Pelengaris et al. , 2002;
Nilsson et al. , 2003). Wirth M et
al. using quantitative promoter-scanning chromatin immunoprecipitation
revealed the binding of c-Myc to the promoters of NOXA upon proteasome
inhibition (Wirth et al. , 2014).
Other scholars also reported that c-Myc was a prime transcription factor
to transactivate NOXA (Nikiforov et
al. , 2007). A series of studies have shown that proteasome inhibitor
bortezomib can induce c-Myc-dependent up-regulation of NOXA, leading to
apoptosis in colorectal and pancreatic cancer cells
(Yue et al. , 2019;
Lankes et al. , 2020). Here, this
study suggested that ixazomib-mediated upregulation the expression of
NOXA and c-Myc in a time- and concentration-dependent manner, on the
other hand, NOXA and c-Myc silencing using siRNA partially rescued the
proliferation inhibitory effect of ixazomib in ESCC cells. These results
confirm that the ixazomib inhibits the proliferation of ESCC cells by
target on the c-Myc-NOXA pathway.
Bortezomib is the first proteasome inhibitor for used as a monotherapy
for the treatment of multiple myeloma and mantle cell lymphoma approved
by the FDA (Richardson, 2003). A series
of clinical studies found that bortezomib combined with other drugs,
including corticosteroids, lenalidomide and thalidomide can
significantly improve the therapeutic effect
(Kapoor et al. , 2012). In recent
years, research has explored the potential value of bortezomib in
combination with cytotoxic drugs in in solid malignancies. Bortezomib
combined with platinum-based agents has achieved encouraging results in
Phase I/II study in ovarian cancer and non-small-cell lung carcinomas
(Ramirez et al. , 2008;
Piperdi et al. , 2012;
Zhao et al. , 2015). There is a
pervasive belief that the synergism between two drugs was due to
platinum agents can block bortezomib-induced STAT1 activation which
suppresses apoptosis.(Fribley et
al. , 2006; Kao et al. , 2013)
Given that ixazomib, like bortezomib, is a boronate-based molecules, we
explored whether ixazomib similarly enhances the anti-ESCC activity of
the nonspecific cytotoxic agent (cisplatin and fluorouracil).