Cancer-associated fibroblasts (CAF) are key contributors to malignant 
progression, but their critical regulators remain largely unknown. In this 
study, we examined the role of Twist1, a central regulator of 
epithelial-mesenchymal transition in carcinoma cells, in the 
transdifferentiation of normal quiescent fibroblasts to CAF and we defined its 
upstream controls and downstream effectors. Primary human gastric fibroblast and 
CAF cultures were established from gastrectomy specimens and validated as 
nontumor cells by somatic mutation analyses. In these cultures, exposure to the 
proinflammatory cytokine IL6 commonly expressed in tumors was sufficient to 
induce Twist1 expression in normal fibroblasts and transdifferentiate them into 
CAFs via STAT3 phosphorylation. In xenograft models, tumor infiltration of 
Twist1-expressing CAFs was enhanced strongly by ectopic IL6 expression in 
gastric or breast cancer cells. We found that Twist1 expression was necessary 
and sufficient for CAF transdifferentiation. Enforced expression of Twist1 in 
normal fibroblasts was also sufficient to drive CAF marker expression and 
malignant character in gastric cancer cells both in vitro and in vivo. 
Conversely, silencing the expression of Twist1 in CAFs abrogated their 
tumor-promoting properties. Downstream of Twist1, we defined the chemokine 
CXCL12 as a transcriptional target. Clinically, CXCL12 and Twist1 expression 
were correlated in CAFs present in gastric tumor specimens. Finally, ectopic 
expression of Twist1 in normal fibroblasts suppressed premature senescence, 
whereas Twist1 attenuation accelerated senescence in CAFs. Our findings define 
Twist1 as a compelling target to deprogram the tumor-supporting features of the 
cancer microenvironment.