© 2015 Guo et al. A key feature of lung fibrosis is the accumulation of myofibroblasts. Interleukin 13 (IL-13) is a pro-fibrotic mediator that directly and indirectly influences the activation of myofibroblasts. Transforming growth factor beta (TGF-β) promotes the differentiation of fibroblasts into myofibroblasts, and can be regulated by IL-13. However, IL-13's downstream signaling pathways are not completely understood.We previously reported that the transcription factor Yin Yang 1 (YY1) is upregulated in fibroblasts treated with TGF-β and in the lungs of mice and patients with pulmonary fibrosis. Moreover, YY1 directly regulates collagen and alpha smooth muscle actin (a-SMA) expression in fibroblasts. However, it is not known if IL-13 regulates fibroblast activation through YY1 expression. We hypothesize that IL-13 upregulates YY1 expression through regulation of AKT activation, leading to fibroblast activation. In this study we found that YY1 was upregulated by IL-13 in lung fibroblasts in a doseand time-dependent manner, resulting in increased a-SMA. Conversely, knockdown of YY1 blocked IL-13-induced a-SMA expression in fibroblasts. Furthermore, AKT phosphorylation was increased in fibroblasts treated with IL-13, and AKT overexpression upregulated YY1, whereas blockade of AKT phosphorylation suppressed the induction of YY1 by IL-13 in vitro. In vivo YY1 was upregulated in fibrotic lungs from CC10-IL-13 transgenic mice compared to that from wild-type littermates, which was associated with increased AKT phosphorylation. Taken together, these findings demonstrate that IL-13 is a potent stimulator and activator of fibroblasts, at least in part, through AKT-mediated YY1 activation.