Angewandte
Chemie
DOI: 10.1002/anie.200805666
Cell Signaling
The Imidazopyridine Derivative JK184 Reveals Dual Roles for
Microtubules in Hedgehog Signaling**
Tommaso Cupido, Paul G. Rack, Ari J. Firestone, Joel M. Hyman, Kyuho Han, Surajit Sinha,
Cory A. Ocasio, and James K. Chen*
The Hedgehog (Hh) signaling pathway is a critical regulator
of embryonic patterning, and aberrant Hh pathway activation
in children and adults has been implicated in several cancers,
including basal cell carcinomas, medulloblastomas, small-cell
lung cancer, pancreatic adenocarcinomas, and prostate
tumors.[1] Small molecules that block this developmental
pathway are therefore potential chemotherapeutic agents,
and their discovery and mechanistic characterization are
critical steps toward realizing effective anticancer therapies.
One molecule reported to potently inhibit the Hh pathway is
the imidazopyridine derivative JK184, which can bind and
inhibit alcohol dehydrogenase 7 (Adh7).[2] How JK184 and
Adh7 regulate Hh target gene expression is not fully under-
stood, although interference with retinoic acid biosynthesis
and signaling has been proposed as one possibility.[2] Herein
we report our studies of JK184 and its mechanism of action.
We demonstrate that JK184 is a potent inhibitor of micro-
tubule assembly and that microtubule-depolymerizing agents
can either negatively or positively regulate the Gli family of
transcription factors, dependending on the mechanism by
which the pathway is activated.
Hh pathway activation during embryogenesis involves the
Figure 1. Schematic representation of the Hh signaling pathway in its
activated state, with Gli transcription factors depicted in red, regulatory
proteins in blue, and subcellular compartments in green. Two JK184-
sensitive, microtubule (MT)-dependent steps indicated by our studies
are shown. In the absence of Hh ligand, Ptch1 is localized to the
primary cilium and suppresses Smo trafficking to this organelle, thus
leading to the cilia-dependent proteolysis of Gli proteins into transcrip-
tional repressors. Upon Hh pathway activation, Ptch1 exits and Smo
accumulates in the primary cilium, Gli processing is inhibited (black
cross), and full-length Gli proteins are converted into transcriptional
activators.
binding of secreted proteins, such as Sonic Hedgehog (Shh) to
the transmembrane receptor Patched1 (Ptch1), thereby
inhibiting its repression of a second membrane-localized
receptor called Smoothened (Smo; Figure 1).[1] Through
mechanisms that are not yet clear but involve the micro-
tubule-based primary cilium,[3–6] Smo activation then inhibits
the proteolytic processing of the Gli2 and Gli3 transcription
factors into C-terminally truncated repressors and promotes
their stabilization as full-length forms. The full-length pro-
teins are further modified to activate the expression of
Hh target genes such as Gli1, which is not subject to
proteolytic processing and reinforces Hh pathway activation
through a positive feedback loop. Smo may also modulate the
function of Suppressor of Fused (Su(fu)), which binds and
negatively regulates the Gli proteins.
[*] T. Cupido,[+] P. G. Rack,[+] A. J. Firestone, Dr. J. M. Hyman, K. Han,
Dr. S. Sinha, Dr. C. A. Ocasio, Prof. Dr. J. K. Chen
Department of Chemical and Systems Biology
Stanford University School of Medicine
In contrast to embryonic Hh signaling, oncogenic Hh tar-
get gene expression can be initiated at several points within
the pathway. Some tumors require Hh ligand through auto-
crine or paracrine mechanisms,[7,8] and others arise from
pathway-activating mutations in downstream effectors. For
example, medulloblastomas can result from inactivating
mutations in Ptch1,[9] activating mutations in Smo,[10] or loss
of Su(fu)[11] function. Compounds that directly inhibit Smo
and exhibit antitumor activities have been described, such as
the natural product cyclopamine and the synthetic inhibitor
HhAntag.[12,13] However, these molecules are most active
against tumors that require Hh ligands or lack Ptch1 function,
269 Campus Drive, Stanford, CA 94305-5174 (USA)
Fax: (+1)650-723-2253
E-mail: jameschen@stanford.edu
[+] These authors contributed equally to this work.
[**] We thank Dr. G. Heffner, B. Moree, and Prof. Dr. A. F. Straight for
technical assistance. This work was supported by the Stanford
Interdisciplinary Translational Research Program, the Sidney
Kimmel Foundation for Cancer Research, the Astellas USA Foun-
dation, and the Brain Tumor Society/Rachel Molly Markoff Foun-
dation.
Supporting information for this article is available on the WWW
Angew. Chem. Int. Ed. 2009, 48, 2321 –2324
ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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