Data from <i>Gprc5a</i> Deletion Enhances the Transformed Phenotype in Normal and Malignant Lung Epithelial Cells by Eliciting Persistent Stat3 Signaling Induced by Autocrine Leukemia Inhibitory Factor

Signal transducers and activators of transcription 3 (Stat3) is activated by cytokines and growth factors in lung cancers and regulates expression of genes implicated in cell growth, survival, and transformation. Previously, we found that mice with a deletion of the G protein–coupled receptor, family C, group 5, member a (Gprc5a) gene develop lung tumors, indicating that Gprc5a is a tumor suppressor. Herein, we show that epithelial cells from Gprc5a knockout mouse lung (Gprc5a^−/− cells) survive better in vitro in medium deprived of exogenous growth factors and form more colonies in semisolid medium than their counterparts from wild-type mice (Gprc5a^+/+ cells). Stat3 tyrosine 705 phosphorylation and expression of several Stat3-regulated antiapoptotic genes were higher in Gprc5a^−/− than in Gprc5a^+/+ cells. Both cell types secreted leukemia inhibitory factor (Lif); however, whereas Stat3 activation was persistent in Gprc5a^−/− cells, it was transient in Gprc5a^+/+ cells. Lung adenocarcinoma cells isolated from Gprc5a^−/− mice also exhibited autocrine Lif-mediated Stat3 activation. The level of Socs3, the endogenous Stat3 inhibitory protein, was higher in Gprc5a^+/+ than in Gprc5a^−/− cells, and expression of the tumor suppressor stabilized Socs3. Inhibition of Stat3 signaling in Gprc5a^−/− normal and cancer cells by the Janus-activated kinase 2 inhibitor AG490 or by a dominant negative Stat3(Y705F) increased starvation-induced apoptosis and inhibited colony formation. These results show that persistent Stat3 activation is important for the survival and transformation of Gprc5a^−/− lung cells and suggest that the tumor suppressive effects of Gprc5a are mediated, at least in part, by inhibition of Stat3 signaling through Socs3 stabilization. Cancer Res; 70(21); 8917–26. ©2010 AACR.