Molecular switches in the TGF-β signaling pathway – the role of inositol pyrophosphate messengers
Transforming growth factor-beta (TGF-β)-induced signal transduction mediated by complex formation of the latent transcription factors SMAD2, SMAD3 and SMAD4 orchestrates а variety of processes in embryonic development and adult tissue homeostasis by regulating cell proliferation, differentiation, apoptosis, migration and invasion in a context-dependent and tissue-specific manner. Dysregulation of this pathway is involved in many patho- physiological conditions, including cancer. Interestingly, in cancer TGF-β signaling plays a dual role: in pre- malignant cells TGF-β primarily functions as a tumor suppressor, while in the later stages of cancer TGF-β acts as a tumor promoter fostering epithelial-to-mesenchymal transition (EMT). TGF-β is one of the most potent inducers of ЕMT, facilitating tumor progression as well as metastatic colonization of secondary organ sites. Therefore, the long-term overarching goal of this project is to understand how this switch-like behavior from tumor suppressive to pro-oncogenic is orchestrated at the molecular and cellular level. Given the high energy demands of migrating cells, TGF-β-mediated signal transduction that triggers migration and invasion must be coordinated with cellular metabolism. Indeed, TGF-β-induced EMT is accompanied by a significant increase in cytosolic ATP. A group of cellular metabolic messengers immediately tied to ATP availability are the inositol pyrophosphates (PP-InsPs). The biosynthesis of these high-energy metabolites by the inositol hexakisphosphate kinases (IP6Ks) is critically dependent on local ATP concentration. Intriguingly, PP-InsP metabolites themselves appear to be required for tumor progression and metastasis in cell and mouse models. The goal of the current proposal is to decipher how PP-InsP messengers – metabolites that are thought to play a general energy sensing role in cells – communicate with the TGF-β signaling network to orchestrate EMT. Preliminary data from the Klingmüller and Fiedler groups point to two distinct intersections between PP-InsP messengers and the TGF-β signaling network. It is hypothesized that (i) the interaction of PP-InsP with SMAD complexes stabilizes the formation of the transcriptionally active trimeric SMAD-complexes altering the dynamics of target gene expression and (ii) the pyrophosphorylation (a modification that has thus far remained invisible using standard mass spectrometric approaches) of the non-muscle myosin MYH9 stabilizes the protein to prolong migratory responses. It is proposed that these intersections of metabolism and signaling constitute two critical molecular switches in the TGF-β signaling pathway, coordinating events that lead to EMT (Fig. 1). Therefore, we use a highly interdisciplinary approach – combining chemical biology and biochemistry approaches with systems biology, including mathematical modelling in collaboration with Thomas Höfer (A21N) – to obtain a detailed and comprehensive view of these molecular switches. Determining how and to which extent these switches are activated and characterizing the impact of these molecular switches on cellular behavior will be essential for tailoring therapeutic approaches targeting this pathway.