TGF-β signal transduction
Transforming growth factor- β (TGF- β ) is a secreted cytokine that regulates numerous responses, such as proliferation, differentiation, migration and apoptosis. TGF- β is the prototypic member of a family of 33 structurally related pleiotropic proteins. Members of the TGF- β superfamily have crucial roles in development and tissue homeostasis. We are interested in unraveling the mechanisms by which transforming growth factor- β (TGF- β ) family members elicit their multifunctional cellular effects and how perturbation in their signal transduction pathways contribute to human diseases. Previous research from us and other laboratories have now firmly established the intracellular signaling cascade of TGF- β via serine/threonine kinase receptors and SMAD transcriptional effectors. However, how this pathway is (mis)regulated in cancer and other diseases, remains not well understood. The present focus within our group is on 1) the identification of novel critical regulators of TGF- β family signaling pathways using functional genetic screens, 2) how TGF- β specificity and bioavailability is regulated via known (co-)receptors and ligand binding proteins and 3) interplay with other signaling pathways. In particular, we study the involvement of TGF- β family members in the metastasis of cancer cells, tumor angiogenesis and cardiovascular diseases. We use 3-dimensional cell culture systems that allow for a functional analysis of homo- and heterotypic cell-cell interactions in structures that resemble those found in vivo. Moreover, mouse and zebra fish models are used that mimic diseases with misregulated TGF- β signaling. Our long term aim is to translate our findings towards the development of novel treatment modalities.
T TGFβ signaling in colorectal cancer cells with microsatellite instability despite biallelic mutations in TGFBR2.
Most colorectal cancer (CRC) cells with high levels of microsatellite instability (MSI-H) accumulate mutations at a microsatellite sequence in the gene encoding TGFBR2. TGFβ signaling therefore is believed to be defective in these tumors, although CRC cells with TGFBR2 mutations have been reported to remain sensitive to TGFβ. We investigated how TGFβ signaling might continue in MSI-H CRC cells. SMAD2 was phosphorylated in most MSI-H CRC tissues (strong detection in 44% and weak detection in 34% of MSI-H tumors). Phosphorylation of SMAD2 in MSI-H cells required TGFBR2—even the form encoding a frameshift mutation. Transcription and translation of TGFBR2 with a 1-nucleotide deletion at its microsatellite sequence still produced a full-length TGFBR2 protein. However, protein expression required preservation of the TGFBR2 microsatellite sequence; cells in which this sequence was replaced with a synonymous nonmicrosatellite sequence did not produce functional TGFBR2 protein. Thus, TGFβ signaling remains active in some MSI-H CRC cells despite the presence of frameshift mutations in the TGFBR2 gene because the mutated gene still expresses a functional protein.
Targeting of αv integrin in breast cancer cells impairs metastasis.
Increased expression of αv integrins is frequently associated with tumor cell adhesion, migration, invasion and metastasis, and correlates with poor prognosis in breast cancer. Genetic interference and pharmacological targeting of αv integrin with small molecule inhibitor GLPG0187 in different breast cancer cell lines inhibited invasion and metastasis in the zebrafish or mouse xenograft model. Depletion of αv integrin in MDA-MB-231 cells inhibited the expression of mesenchymal markers and the TGF-β/Smad response. TGF-β induced αv integrin mRNA expression and αv integrin was required for TGF-β-induced breast cancer cell migration. Moreover, treatment of MDA-MB-231 cells with non-peptide RGD antagonist GLPG0187 decreased TGF-β signaling. In the mouse xenografts GLPG0187 inhibited the progression of bone metastasis. Maximum efficacy of inhibition of bone metastasis was achieved when GLPG0187 was combined with the standard-of-care metastatic breast cancer treatments. Our results also provide evidence that targeting αv integrin could be an effective therapeutic approach for treatment of breast cancer tumors and/or metastases that overexpress αv integrin.
c-Myb Enhances Breast Cancer Invasion and Metastasis through the Wnt/β-Catenin/Axin2 Pathway.
The molecular underpinnings of aggressive breast cancers remain mainly obscure. We found that activation of the transcription factor c-Myb is required for the prometastatic character of basal breast cancers. An analysis of breast cancer patients led us to identify c-Myb as an activator of Wnt/β-catenin signaling. c-Myb interacted with the intracellular Wnt effector β-catenin and coactivated the Wnt/β-catenin target genes Cyclin D1 and Axin2 Moreover, c-Myb controlled metastasis in an Axin2-dependent manner. Expression microarray analyses revealed a positive association between Axin2 and c-Myb, a target of the proinflammatory cytokine IL1β that was found to be required for IL1β-induced breast cancer cell invasion. Overall, our results identified c-Myb as a promoter of breast cancer invasion and metastasis through its ability to activate Wnt/β-catenin/Axin2 signaling.