Activation of Rac1 promotes hedgehog-mediated acquisition of the myofibroblastic phenotype in rat and human hepatic stellate cells.
Hepatic accumulation of myofibroblastic hepatic stellate cells (MF-HSCs) is pivotal in the pathogenesis of cirrhosis. Two events are necessary for MF-HSCs to accumulate in damaged livers: transition of resident, quiescent hepatic stellate cells (Q-HSCs) to MF-HSCs and expansion of MF-HSC numbers through increased proliferation and/or reduced apoptosis. In this study, we identified two novel mediators of MF-HSC accumulation: Ras-related C3 botulinum toxin substrate 1 (Rac1) and Hedgehog (Hh). It is unclear whether Rac1 and Hh interact to regulate the accumulation of MF-HSCs. We evaluated the hypothesis that Rac1 promotes activation of the Hh pathway, thereby stimulating signals that promote transition of Q-HSCs into MF-HSCs and enhance the viability of MF-HSCs. Using both in vitro and in vivo model systems, Rac1 activity was manipulated through adenoviral vector-mediated delivery of constitutively active or dominant-negative rac1. Rac1-transgenic mice with targeted myofibroblast expression of a mutated human rac1 transgene that produces constitutively active Rac1 were also examined. Results in all models demonstrated that activating Rac1 in HSC enhanced Hh signaling, promoted acquisition/maintenance of the MF-HSC phenotype, increased MF-HSC viability, and exacerbated fibrogenesis. Conversely, inhibiting Rac1 with dominant-negative rac1 reversed these effects in all systems examined. Pharmacologic manipulation of Hh signaling demonstrated that profibrogenic actions of Rac1 were mediated by its ability to activate Hh pathway-dependent mechanisms that stimulated myofibroblastic transition of HSCs and enhanced MF-HSC viability.


CONCLUSIONS
These findings demonstrate that interactions between Rac1 and the Hh pathway control the size of MF-HSC populations and have important implications for the pathogenesis of cirrhosis.
Choi
Steve S
SS
Division of Gastroenterology, Duke University, Durham, NC, USA.
Witek
Rafal P
RP
Yang
Liu
L
Omenetti
Alessia
A
Syn
Wing-Kin
WK
Moylan
Cynthia A
CA
Jung
Youngmi
Y
Karaca
Gamze F
GF
Teaberry
Vanessa S
VS
Pereira
Thiago A
TA
Wang
Jiangbo
J
Ren
Xiu-Rong
XR
Diehl
Anna Mae
AM
D000818:Animals; D005347:Fibroblasts / Q000378:metabolism / Q000473:pathology*; D005786:Gene Expression Regulation; D053823:Hedgehog Proteins / Q000378:metabolism*; D055166:Hepatic Stellate Cells / Q000378:metabolism / Q000473:pathology*; D006801:Humans; D008103:Liver Cirrhosis / Q000235:genetics / Q000378:metabolism / Q000473:pathology*; D008297:Male; D010641:Phenotype; D051381:Rats; D017207:Rats, Sprague-Dawley; D020830:rac1 GTP-Binding Protein / Q000819:agonists / Q000235:genetics / Q000378:metabolism*
D053823:Hedgehog Proteins; D020830:rac1 GTP-Binding Protein