Smurf1 regulates lung cancer cell growth and migration through interaction with and ubiquitination of PIPKIγ.

Type Iγ phosphatidylinositol phosphate kinase (PIPKIγ), a phospholipid kinase generating PIP2, is positively expressed in breast cancer tissues, which correlates intimately with the progression of patients. However, little is known about the expression level of PIPKIγ in patients with other cancer types as well as their underlying regulation mechanisms. Here, we report that PIPKIγ is highly expressed in lung cancer tissues and its expression level is critical for lung cancer cell proliferation, which may serve as a prognostic marker for lung cancer patients. Meanwhile, we show that E3 ubiquitin ligase Smurf1 directly interacts with PIPKIγ and targets PIPKIγ for ubiquitination and degradation in lung cancer cells. Also, we discover that Smurf1 directly binds to the kinase domain of PIPKIγ via its C2 domain while Lysine 255 in PIPKIγ acts as the major ubiquitin acceptor site for Smurf1. In addition, we demonstrate that the phosphorylation mimicking mutant of Smurf1, Smurf1 T306D, prevents PIPKIγi2 from ubiquitination and subsequent degradation similar to the effect of forskolin-potentiated cAMP formation, suggesting that Thr306 in Smurf1 is critical for its phosphorylation by PKA. Moreover, PKA-Smurf1-PIPKIγ signal transduction takes a significant part in lung cancer cell growth and in vivo tumorigenesis. Thus, we propose that the PKA-Smurf1-PIPKIγ pathway has an important role in pulmonary tumorigenesis and imposes substantial clinical impact on development of novel diagnostic markers and therapeutic targets for lung cancer treatment.