Understanding crosstalk between mesenchymal stem cells andcancer cells metastasizing to bone – a whole secretome approach

Abstract

The effect of mesenchymal stem cells (MSC) on cancer progression is still a matter of debate. It is increasingly appreciated that MSC can migrate towards the site of the primary tumor and participate in tumor stroma formation, supporting tumor growth and priming cancer cells for dissemination. Less is known about MSC or other stromal cells at the site of metastasis. Some cancers show a strong tendency to metastasize to bone, a tissue of mesenchymal origin and a prominent site of MSC. Recent reports have suggested that bone-metastasizing cancers may mimic the process of homing of hematopoietic stem cells to their bone niche, in which MSC play a crucial role. With regard to the fact that MSC play an important role in cancer progression, I aimed to dissect the interactions and the dynamics between tumor cells and MSC in metastasis formation. In order to understand the role of MSC in metastasis formation, I investigated primary human bone marrow MSC with established cancer cell lines able to metastasize to bone in a transwell migration assay. Combining this experimental set up with impedance measurements allowed quantitative analysis of cancer cell migration towards MSC in a time-resolved fashion and high-throughput format. This enabled an unbiased approach taking the complete secretome of MSC into consideration. The results showed that MSC induced a rapid migration response of prostate and breast cancer cell lines. In contrast to this finding, fibroblast cell lines were not able to induce a comparable migration response indicating specific MSC – cancer cell crosstalk. In order to identify the factors stimulating cancer cell migration, MSC cell culture supernatant was then purified by size exclusion and ion exchange chromatography. This was followed by mass spectrometry as well as antibody array analysis of the chromatographic fractions inducing migration. With this approach I identified extracellular matrix proteins to be the main drivers of rapid cancer cell migration requiring as little as two hours for a full migration response. These factors included type I and III collagen, fibronectin and Laminin 421, which were confirmed using recombinant proteins. RNAi experiments showed that the response to these molecules required the extracellular matrix receptor β1 integrin in the migrating cancer cells. This study shows that MSC are very potent mediators of cancer cell migration, based on diffusible gradients of extracellular matrix proteins acting independently of chemokines.