We have previously shown that lung tumors arising in MMTV-IGF-II transgenic mice displayed elevated levels of phosphorylated cAMP response element binding protein (CREB). To investigate the role that insulin-like growth factor II (IGF-II) and CREB play in human lung tumorigenesis, A549 and NCI-H358 cells were examined. In these cell lines, IGF-II administration enhances human tumor cell survival and CREB phosphorylation. Further, the effects of IGF-II on cell survival and CREB phosphorylation appeared to be mediated, at least in part, by activation of the Erk pathways, as inhibition of these signaling pathways reduced tumor cell survival and CREB phosphorylation. Specifically, Erk5 appeared as the predominant mediator of CREB phosporylation. To further verify the importance of CREB in human lung tumorigenesis, A549 and NCI-H358 cells were stably transfected with a vector containing a dominant negative CREB construct (KCREB). KCREB transfection significantly inhibited the soft agar growth of both human tumor cell lines. In contrast, overexpression of wild-type CREB in the normal human bronchial epithelial cell line, HBE135, enhanced soft agar growth. Therefore, our results indicate that CREB and its associated proteins play a significant role in lung adenocarcinoma and IGF-II induces CREB phosphorylation, at least in part, via the Erk5 signaling pathway.