From epithelial to mesenchymal: regulation of invasive cancer cell motility

Metastasis is the main cause of death among cancer patients. In order to initiate the metastatic cascade cancer cells have to undergo epithelial-to-mesenchymal transition (EMT). In EMT epithelial cells lose their cell-cell and cell-extracellular matrix (ECM) contacts and become more motile. The expression of the transcription factor Slug and of the mesenchymal intermediate filament vimentin is induced during EMT. Vimentin is often overexpressed in malignant epithelial cancers but the functional role of vimentin remains incompletely understood. In addition, kinases such as AKT and ERK are known to be involved in the regulation of EMT and cancer cell motility but the mechanisms underlining their functions are often unclear. Integrins are heterodimeric receptors that attach cells to the surrounding tissue and participate in regulating cell migration and invasion. Changes in integrin activity are linked to increased cell motility and further cancer metastasis. The aim for my PhD studies was to investigate the role of cellular signalling pathways and vimentin in the regulation of cancer cell motility and EMT. Our results revealed that in prostate cancer the downregulation of AKT1 and AKT2, but not AKT3, induces activation of cell surface 1-integrins leading to enhanced cell adhesion, migration and invasion. In addition, our findings demonstrated a reciprocal regulatory interaction between vimentin and ERK2 facilitating ERK-mediated phosphorylation of Slug at serine-87 (S87) in breast cancer. Surprisingly, Slug S87 phosphorylation is dispensable for E-cadherin repression but essential for the induction of vimentin and Axl expression in early onset of EMT. Our findings reveal previously unknown mechanistic information of how prostate and breast cancer cell motility and disease progression is regulated

Unconventional integrin-ligand interactions

Integrins are cell surface adhesion and signaling receptors. Cells use integrins to attach to the extracellular matrix and to other cells, as well as for sensing their environment. In addition to adhesion and migration, integrins have been shown to be important for many biological processes including apoptosis, cell proliferation, and differentiation into specific tissues. Many important next generation biological drugs inhibit integrin functions. Thus, research into interactions between integrins and their ligands under different physiological and pathological conditions is not only of academic interest, but is also important for the field of drug discovery. In this Ph.D. project, the functions of integrin-ligand interactions were studied under different physiologically interesting conditions including 1) human echovirus 1 binding to integrin α2β1, 2) integrin α2β1 binding to collagen under flow conditions, 3) integrin α2β1 binding to a ligand in the presence of the angiogenesis inhibitor histidine rich glycoprotein (HRG) and 4) integrin binding to posttranslationally citrullinated ligands. As a result of the project, we could show that for each condition the integrin-ligand interaction is somewhat unconventional. 1) Echovirus 1 binds only to non-activated conformations of integrin α2β1. 2) Surprisingly, the non-activated conformation is also the primary conformation of integrin α2β1 when it binds to collagen under flow conditions, like when platelets adhere to subendothelial collagen in vascular injuries. In addition, the pre-activation of integrin α2β1 does not increase adhesion under flow. 3) HRG binds to integrin α2β1 through a low-affinity interaction that inhibits integrin binding to collagen. This shows that low affinity interactions could be biologically relevant and possibly regulate angiogenesis. 4) The citrullination of collagen, a posttranslational modification reported to occur in rheumatoid arthritis, specifically inhibits the binding of integrin α10β1 and α11β1, but does not affect the binding of α1β1 ja α2β1. On the other hand, the citrullination of isoDGR in fibronectin and RGD in pro-TGF- β:n inhibit integrin binding completely. Citrullination seems to be an inflammation related process and integrin ligands become citrullinated frequently in vivo. This Ph.D. thesis suggests that unconventional interaction mechanisms between integrins and their ligands, such as posttranslational modifications, low affinity interactions, and non-activated integrin conformations, can have an important role in pathological processes. The study of these kinds of integrin-ligand interactions is important for understanding biological phenomena more deeply. The research might also be beneficial for the development of integrin based therapies for treating diseases.