We have identified a pro-tumorigenic cooperation between TGFbeta-SMAD signaling and epidermal growth factor (EGF) signaling in breast cancer cells, involving activator protein (AP)-1 transcription factors and the deltaN isoform of the transcriptional regulator p63 (deltaNp63). AP-1 and deltaNp63 promote the recruitment of SMAD2/3 to several genes encoding proteins involved in cell migration and invasion, including ITGA2, LAMB3 and WNT7A/B. These findings are of importance for breast cancer patients with activated EGF signaling.
deltaNp63 has been shown to be an oncogene. We have shown that the pro-tumorigenic effect of Np63 in TGFbeta signaling is related to its ability to act as a pioneering transcription factor, facilitating the binding of other transcription factors to certain gene promoters.
The TGFbeta family of cytokines also contains members of the bone morphogenetic protein (BMP) subfamily. We found that overactivity of BMPs drives the progression of ovarian and endometrial cancers via induction of EMT and stemness. The mechanism involves induction of a receptor in the TNF receptor family, designated FN14 (encoded by the TNFRSF12A gene). The commonly used chemotherapeutical agent carboplatin induced BMP expression, suggesting that a combination of carboplatin and BMP receptor kinase inhibitors may be useful in the treatment of patients with gynecological cancers.
In collaboration with Maréne Landström at Umeå University, we have found that the type I receptor for TGFbeta (TbetaRI) upon ligand binding undergoes dual proteolytic cleavage, leading to release of the intracellular domain (TbetaRI-ICD), which is then translocated to the nucleus where it drives an invasiveness program. We found that TbetaRI-ICD also interacts with the kinase AURKB and regulates mitosis and cytokinesis. This finding suggests another possible mechanism by which TbetaRI-ICD promotes tumorigenesis, in addition to inducing several genes driving tumor cell invasion. We have found that inhibition of TbetaRI cleavage inhibits metastasis in a mouse model. This finding is of potential clinical importance.
Src is a well-known oncoprotein, which we and others have shown to be activated by TGF stimulation. We have elucidated the molecular mechanism for TGFbeta activation of Src. In short, before TGFbeta binding to its receptors, the type II TGFbeta receptor (TbetaRII) binds Src in an inactive form. After TGFbeta binding, TbetaRII phosphorylates TbetaRI on Tyr182 making it possible for the SH2 domain of Src to bind to this epitope, thereby opening up the conformation of Src which activates its kinase. Interactions between the SH3 domain of Src with a proline-rich region in TbetaRI contributes to the activation of Src. Thereafter, Src phosphorylates several tyrosine residues in TbetaRI and other substrates. We are now elucidating the role of Src in TGF receptor signaling and trafficking.
In order to investigate the mechanism by which TGFbeta induces EMT, we have, in collaboration with Aristidis Moustakas at our department, established a method to genetically mark tumor cells of epithelial vs mesenchymal character; using this imaging method, we have shown that TGFbeta selects for pro-stemness over pro-invasive phenotypes during EMT.
Looking for posttranslational modifications involved in the regulation of TGFbeta signaling, we have found that the function of the inhibitory Smad7 is enhanced by palmitoylation. In addition, we have noticed that also TbetaRI is palmitoylated, the functional effect of which now is investigated.
