Gprc5a Deletion Enhances the Transformed Phenotype in Normal and Malignant Lung Epithelial Cells by Eliciting Persistent Stat3 Signaling Induced by Autocrine Leukemia Inhibitory Factor

Abstract

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.