Identification of Pirin as a Molecular Target of the CCG-1423/CCG-203971 Series of Antifibrotic and Antimetastatic Compounds

A series
of compounds (including CCG-1423 and CCG-203971) discovered
through an MRTF/SRF-dependent luciferase screen has shown remarkable
efficacy in a variety of in vitro and in
vivo models, including significant reduction of melanoma
metastasis and bleomycin-induced fibrosis. Although these compounds
are efficacious in these disease models, the molecular target is unknown.
Here, we describe affinity isolation-based target identification efforts
which yielded pirin, an iron-dependent cotranscription factor, as
a target of this series of compounds. Using biophysical techniques
including isothermal titration calorimetry and X-ray crystallography,
we verify that pirin binds these compounds in vitro. We also show with genetic approaches that pirin modulates MRTF-dependent
luciferase reporter activity. Finally, using both siRNA and a previously
validated pirin inhibitor, we show a role for pirin in TGF-β-induced
gene expression in primary dermal fibroblasts. A recently developed
analog, CCG-257081, which cocrystallizes with pirin, is also effective
in the prevention of bleomycin-induced dermal fibrosis.