Treatment of metastatic breast cancer, which is often
incurable, faces the challenge of the tumor microenviron-
ment. A number of studies examining tumor microenvi-
ronments have made it clear that ﬁbroblasts, which
comprise a major component of cancerous stroma, play
an important role in cancer metastasis . The cross- talk
that occurs between stromal ﬁbroblasts and cancer cells
leads to morphological changes in normal ﬁbroblastic cells.
Fibroblasts that are activated by cancer cells are called
carcinoma- associated ﬁbroblasts (CAFs), and they are
known to guide cancer cell migration by mechanically
modifying tumor stroma . Recent studies have described
how the interaction of CAFs with the cancer cells can
lead to chemotherapy resistance [3, 4]. Thus, a
combination of conventional therapy, along with CAF-
directed therapy, might lead to improved treatment of
breast cancer metastasis.
The major challenge in targeting CAFs is their hetero-
geneity. Although numerous studies have been performed
on CAFs, a speciﬁc marker recognizing these cells is lacking.
It has long been known that ﬁbroblasts become activated
during wound healing, and cancer has been described as
a wound that does not heal or as an over healing wound
[5, 6]. The link between ﬁbrosis and cancer suggests a
similar phenomenon in activated ﬁbroblasts migrating faster
than normal ﬁbroblasts. Ishii et al.  identiﬁed enhanced
migration as one of the biological properties of CAFs.
Studies on the role of CAFs in cancer cell migration and
invasion are already documented ; however, the mecha-
nism of ﬁbroblast activation by cancer cells is still unclear.
Identiﬁcation of a small- molecule ligand of β- arrestin1 as an
inhibitor of stromal ﬁbroblast cell migration accelerated by
, Kaori Honda
, Yasumitsu Kondoh
, Hiroyuki Osada
& Nobumoto Watanabe
Bio-Active Compounds Discovery Research Unit, RIKEN Center for Sustainable Resource Science, Saitama, Japan
Tokyo Medical and Dental University, Tokyo, Japan
Chemical Biology Research Group, RIKEN Center for Sustainable Resource Science, Saitama, Japan
© 2018 The Authors. Cancer Medicine published by John Wiley & Sons Ltd.
This is an open access article under the terms of the Creative Commons Attribution License, which permits use,
distribution and reproduction in any medium, provided the original work is properly cited.
Fibroblasts, ligands, signal transduction,
tumor microenvironment, wound healing
Nobumoto Watanabe, Bio-Active Compounds
Discovery Research Unit, RIKEN Center for
Sustainable Resource Science, 2-1, Hirosawa,
Wako, Saitama 351-0198, Japan. Tel:
+81-48-467-9542; Fax: +81-48-462-4669;
RIKEN International Program Associate.
Received: 4 July 2017; Revised: 28 November
2017; Accepted: 15 December 2017
Cancer Medicine 2018; 7(3):883–893
Stromal ﬁbroblasts, which occupy a major portion of the tumor microenviron-
ment, play an important role in cancer metastasis. Thus, targeting of these
ﬁbroblasts activated by cancer cells (carcinoma- associated ﬁbroblasts; CAFs)
might aid in the improved treatment of cancer metastasis. NIH3T3 ﬁbroblasts
cocultured with MCF7 cells displayed enhanced migration compared to NIH3T3
ﬁbroblasts cultured alone. We used this system to identify the small- molecule
inhibitors responsible for their enhanced migration, a characteristic of CAFs.
We selected β- arrestin1, which showed high expression in cocultured cells, as
a molecular target for such inhibitors. Coﬁlin, a protein downstream of
β- arrestin1, is activated/dephosphorylated in this condition. The small- molecule
ligands of β- arrestin1 obtained by chemical array were then examined using a
wound healing coculture assay. RKN5755 was identiﬁed as a selective inhibitor
of activated ﬁbroblasts. RKN5755 inhibited the enhanced migration of ﬁbroblasts
cocultured with cancer cells by binding to β- arrestin1 and interfering with
β- arrestin1- mediated coﬁlin signaling pathways. Therefore, these results dem-
onstrate the role of β- arrestin1 in the activation of ﬁbroblasts and inhibiting
this protein by small molecule inhibitor might be a potential therapeutic target
for the stromal ﬁbroblast activation (cancer–stroma interaction).