Download Targeting a Novel ER/HOXB7 Signaling Loop in

VIEWS
IN THE SPOTLIGHT
Targeting a Novel ER/HOXB7 Signaling Loop in
Tamoxifen-Resistant Breast Cancer
Marinus R. Heideman1, Anna Frei1,2, and Nancy E. Hynes1,2
Summary: The majority of patients with breast cancer present with an estrogen receptor–positive (ER+) tumor,
and the endocrine agent tamoxifen is a mainstay for their treatment. Unfortunately, however, resistance remains
a major problem because most patients who respond eventually have a recurrence. Thus, an enduring challenge in
the breast cancer field is to identify mechanisms underlying tamoxifen resistance. Jin and colleagues describe a
novel ER/HOXB7 signaling loop in tamoxifen-resistant breast cancer models. Importantly, they reveal that targeting this signaling loop has great promise as an approach to treat patients with tamoxifen-resistant breast cancer.
Cancer Discov; 5(9); 909–11. ©2015 AACR.
See related article by Jin et al., p. 944 (9).
The approximately 70% of breast cancer patients whose
tumors are estrogen receptor–positive (ER+) are treated with
endocrine agents, including tamoxifen, the first clinically successful ER modulator (SERM), and the more recently introduced
agents fulvestrant, an ER degrader (SERD), and aromatase
inhibitors that block estradiol production. Endocrine agents
have increased the survival of hundreds of thousands of breast
cancer patients since the introduction of tamoxifen into the
clinic in the mid-1970s (1). Unfortunately, tumor recurrence
caused by acquired resistance often occurs (reviewed in refs. 2
and 3). Therefore, it is crucial to gain an understanding of the
mechanisms underlying endocrine therapy resistance.
ER biology is quite complex, and it is likely that multiple,
nonexclusive mechanisms contribute to endocrine resistance.
For example, loss of ER, which is observed in 15% to 20% of
recurrences, is an obvious resistance mechanism (4). Receptor
tyrosine kinase (RTK) overexpression has also been proposed
to contribute to endocrine resistance. Indeed, breast tumors
with high expression and activity of EGFR and ERBB2 are less
sensitive to tamoxifen (5). Moreover, the subgroup of patients
with breast cancer with ER+ tumors and the ERBB2 amplicon
generally do not respond to tamoxifen (6). These clinical
data suggest that ERBB RTK signaling can circumvent the
requirement for ER signaling. Recent findings demonstrate
that mutations of ER (encoded by ESR1) are also linked
to acquired resistance. Genomic sequencing efforts revealed
that metastatic tumors have a higher frequency of ESR1
mutations than primary tumors (∼20% vs 0.5%). The ESR1
missense mutations identified in metastatic disease generally lead to ligand-independent constitutive activation of the
receptor and are thought to be acquired during treatment
(2). Biochemical analyses of some of the mutant ERs suggest
1
Friedrich Miescher Institute for Biomedical Research, Basel, Switzerland.
University of Basel, Basel, Switzerland.
2
Corresponding Author: Nancy E. Hynes, Friedrich Miescher Institute for
Biomedical Research, Maulbeerstrasse 66, 4058 Basel, Switzerland.
Phone: 41-61-6978107; Fax: 41-61-6973976; E-mail: [email protected]
doi: 10.1158/2159-8290.CD-15-0871
©2015 American Association for Cancer Research.
that treatment of these patients with higher doses of current
endocrine agents or with newer, more potent agents might
provide clinical benefit (7).
In this issue of Cancer Discovery, Jin and colleagues follow up on their 2012 Proceedings of the National Academy of
Sciences article (8), in which they showed that the HOXB7
transcription factor renders breast cancer cells resistant to
tamoxifen through activation of the EGFR pathway. Their
new article (9) provides novel mechanistic insight into the
regulation of HOXB7 in tamoxifen-resistant breast cancer
models and proposes novel approaches to target tamoxifenresistant breast cancer.
Interestingly, their data indicate that a direct interaction
between HOXB7 and ER is crucial for the upregulation of
ER target genes in tamoxifen-resistant cells. By performing
coimmunoprecipitations, they revealed a physical interaction
between these two proteins. Moreover, chromatin immunoprecipitation (ChIP) experiments demonstrated increased
binding of both HOXB7 and ER at EREs in known ER target
genes, such as MYC, GREB1, and CCND1, suggesting that
HOXB7 binds to these sites in association with the ER. Of
note, binding between HOXB7 and ER was shown to be
enhanced upon E2 or tamoxifen treatment.
An important question that the authors answer, then, is
“Why do these tamoxifen-resistant cells have increased
HOXB7 levels?” To summarize the answer, the authors demonstrated that MYC is stabilized by phosphorylation mediated by ERBB2–EGFR signaling. Subsequently, stabilized MYC
represses the expression of miR-196a, a known repressor of
HOXB7 (10), resulting in increased HOXB7 levels (Fig. 1).
One interesting aspect of this work is the direct relation
between HOXB7 and ERBB2–EGFR expression and activation. The authors convincingly showed increased levels of
phosphorylated ERBB2 and EGFR in cells that overexpress
HOXB7. Furthermore, their data suggest that ERBB2 levels
are directly regulated by ER/HOXB7 in tamoxifen-resistant
cells. This is based on ChIPs revealing increased HOXB7 and
transcriptional cofactor binding at an estrogen response element (ERE) site in the ERBB2 locus of HOXB7-overexpressing
tamoxifen-resistant MCF7 cells. In addition, quantitative
SEPTEMBER 2015CANCER DISCOVERY | 909
Downloaded from cancerdiscovery.aacrjournals.org on June 14, 2017. © 2015 American Association for Cancer Research.
VIEWS
Trastuzumab
EGFR
ERBB2
Lapatinib
Fulvestrant
TAM
ER
10058-F4
10074-G5
P
MYC
HOXB7 EGFR
P
MYC
miR-196a
Coactivators
HOXB7
TAM
ER
ER target genes,
including MYC and ERBB2
HOXB7
miR-196a
Nanoparticles
miR-196a
Figure 1. Targeting the ER/HOXB7 signaling loop in tamoxifen-resistant breast cancer. HOXB7 interacts with tamoxifen (TAM)-bound ER and coactivators, inducing ER target gene expression including MYC and ERBB2. HOXB7 binds the EGFR promoter directly, increasing EGFR transcription. Increased
signaling through ERBB2 and EGFR leads to MYC phosphorylation and stability. MYC represses miR-196a transcription, a repressor of HOXB7. Red
boxes represent potential treatment strategies to target the ER/HOXB7 signaling loop: ERBB2 by trastuzumab, EGFR/ERBB2 by lapatinib, MYC by
10058-F4 or 10074-G5, miR-196a by nanoparticles containing miR-196, and ER by fulvestrant.
PCR data show an increase in ERBB2 mRNA upon tamoxifen
treatment. Data demonstrating increased HOXB7 and ER
binding upon E2 or tamoxifen treatment are lacking; therefore, it still remains a question whether or not ERBB2 is
regulated in a similar manner to the other ER/HOXB7 targets
described in the article. Moreover, it cannot be ruled out that
HOXB7 controls ERBB2 and EGFR level and phosphorylation by other direct or indirect mechanisms.
Another important finding in the HOXB7-overexpressing
tamoxifen-resistant models is the stabilization of MYC by
ERBB2/EGFR signaling, leading to repression of miR-196a,
which in turn represses HOXB7. Overexpression of miR196a was shown to reduce expression of ER targets and
could reverse resistance to tamoxifen. In agreement, in vivo
910 | CANCER DISCOVERYSEPTEMBER 2015
xenograft studies, using miR-196a–overexpressing tamoxifenresistant BT474 cells, revealed a highly significant decrease in
primary tumor growth.
Importantly, uncovering this new ER/HOXB7 signaling
loop implies that targeting the ER-associated HOXB7, either
directly, or indirectly by ERBB2, MYC, or miR-196a, might
have potential for treating tamoxifen-resistant breast cancer
(Fig. 1). To test targeting different nodes of this loop, the
authors used several tamoxifen-resistant xenograft models.
In BT474 xenografts, a model for ER+/ERBB2 amplicon–
positive breast cancer, HOXB7 knockdown almost totally
blocked tumor outgrowth. This block was associated with a
strong decrease in ERBB2 and EGFR levels and a subsequent
decrease in AKT signaling. As there are currently no drugs
www.aacrjournals.org
Downloaded from cancerdiscovery.aacrjournals.org on June 14, 2017. © 2015 American Association for Cancer Research.
VIEWS
directly targeting HOXB7, the authors targeted MYC using
the inhibitor 10058-F4 and targeted ERBB2 using trastuzumab. As anticipated, treatment of BT474 xenografts with
trastuzumab caused a strong reduction in tumor growth.
MYC inhibition reduced primary tumor growth only slightly,
but, interestingly, the combination of 10058-F4 and trastuzumab revealed synergy and tumor stasis. In another model,
HOXB7-overexpressing MCF7 cells, treatment with fulvestrant resulted in complete remission. These data suggest
that patients with breast cancer with high HOXB7 levels
might be the target population for fulvestrant treatment
after tamoxifen resistance emerges. It would be interesting
to examine how the BT474 xenograft model responds to fulvestrant combined with the other inhibitors. Overall, these
preclinical findings reveal that tamoxifen-resistant tumors
expressing high levels of HOXB7 can be targeted at several
nodes of the ER/HOXB7 signaling loop.
Finally, the authors show analyses using several independent databases harboring information on endocrine
therapy–treated patients with ER+ breast cancer. These analyses revealed that patients expressing high HOXB7 have a
worse probability of overall survival. Interestingly, elevated
HOXB7 in combination with high ERBB2 and MYC showed
an even stronger decrease in the probability of overall survival. In summary, their findings imply that it could be beneficial to select tamoxifen-resistant patients based on these
three markers, for rationalized targeting of the ER/HOXB7
signaling loop.
Disclosure of Potential Conflicts of Interest
No potential conflicts of interest were disclosed.
Published online September 2, 2015.
REFERENCES
1. Jordan VC. Tamoxifen: catalyst for the change to targeted therapy.
Eur J Cancer 2008;44:30–8.
2. Jeselsohn R, Buchwalter G, Angelis C, Brown M, Schiff R. ESR1
mutations—a mechanism for acquired endocrine resistance in breast
cancer. Nat Rev Clin Oncol 2015 Jun 30. [Epub ahead of print].
3. Musgrove EA, Sutherland RL. Biological determinants of endocrine
resistance in breast cancer. Nat Rev Cancer 2009;9:631–43.
4. Hoefnagel LD, Moelans CB, Meijer SL, van Slooten HJ, Wesseling P,
Wesseling J, et al. Prognostic value of estrogen receptor alpha and
progesterone receptor conversion in distant breast cancer metastases.
Cancer 2012;118:4929–35.
5. Newby JC, Johnston SR, Smith IE, Dowsett M. Expression of epidermal growth factor receptor and c-erbB2 during the development
of tamoxifen resistance in human breast cancer. Clin Cancer Res 1997;
3:1643–51.
6. Borg A, Baldetorp B, Ferno M, Killander D, Olsson H, Ryden S, et al.
ERBB2 amplification is associated with tamoxifen resistance in steroidreceptor positive breast cancer. Cancer Lett 1994;81:137–44.
7. Toy W, Shen Y, Won H, Green B, Sakr RA, Will M, et al. ESR1 ligandbinding domain mutations in hormone-resistant breast cancer. Nat
Genet 2013;45:1439–45.
8. Jin K, Kong X, Shah T, Penet MF, Wildes F, Sgroi DC, et al. The
HOXB7 protein renders breast cancer cells resistant to tamoxifen
through activation of the EGFR pathway. Proc Natl Acad Sci U S A
2012;109:2736–41.
9. Jin K, Park S, Teo WW, Korangath P, Cho SS, Yoshida T, et al. HOXB7
is an ERα cofactor in the activation of HER2 and multiple ER target
genes leading to endocrine resistance. Cancer Discov 2015;5:944–59.
10. Braig S, Mueller DW, Rothhammer T, Bosserhoff AK. MicroRNA
miR-196a is a central regulator of HOX-B7 and BMP4 expression in
malignant melanoma. Cell Mol Life Sci 2010;67:3535–48.
SEPTEMBER 2015CANCER DISCOVERY | 911
Downloaded from cancerdiscovery.aacrjournals.org on June 14, 2017. © 2015 American Association for Cancer Research.
Targeting a Novel ER/HOXB7 Signaling Loop in
Tamoxifen-Resistant Breast Cancer
Marinus R. Heideman, Anna Frei and Nancy E. Hynes
Cancer Discovery 2015;5:909-911.
Updated version
Cited articles
E-mail alerts
Reprints and
Subscriptions
Permissions
Access the most recent version of this article at:
http://cancerdiscovery.aacrjournals.org/content/5/9/909
This article cites 9 articles, 3 of which you can access for free at:
http://cancerdiscovery.aacrjournals.org/content/5/9/909.full#ref-list-1
Sign up to receive free email-alerts related to this article or journal.
To order reprints of this article or to subscribe to the journal, contact the AACR Publications Department at
[email protected].
To request permission to re-use all or part of this article, contact the AACR Publications Department at
[email protected].
Downloaded from cancerdiscovery.aacrjournals.org on June 14, 2017. © 2015 American Association for Cancer Research.