Chemotactic signals released during Burkitt’s lymphoma cell death
Tumour-associated macrophages (TAMs) have been shown to play an important role in tumour survival and progression. Thus, high numbers of macrophages in the tumour tissue are often associated with a poor prognosis. Identification of factors responsible for recruiting macrophages to the sites of different types of tumours might help to develop more effective cancer treatment. Burkitt's lymphoma (BL) is characterised by uncontrolled cell proliferation, high rate of spontaneous apoptosis and significant macrophage infiltration. Although BL cells undergo extensive apoptosis, in situ their corpses are cleared very effectively by macrophages infiltrating the tumour. It is now widely believed that dying cells are themselves able to release chemotactic molecules to ensure macrophage chemotaxis and subsequent clearance of their site of death. Previous work carried out in this laboratory identified fractalkine/CX3CL1 (FKN) released from dying BL cells to be an important player in macrophage chemotaxis to BL. Yet, these results have also indicated that FKN may not be the only chemokine involved in this process. Following from those observations, the first part of this work focused on examination of the potential role of monocyte chemoattractant protein-1 (MCP-1) in macrophage recruitment to BL. Despite the initial promising results, careful analysis of the data obtained by various techniques led to the conclusion that MCP-1 is, probably, not expressed by BL cells. Subsequently, effort was concentrated on understanding mechanisms regulating FKN processing during cell death. The studies performed before in this laboratory identified a new form of FKN to be present in apoptotic BL cells and showed that this is the form that is, most likely, responsible for mediating macrophage migration. Here, this apoptosis-related 60 kDa FKN was found to be a likely caspase-3 cleavage product. Moreover, it was demonstrated that FKN and active caspase-3 are released together in apoptotic BL cell-derived microparticles, suggesting that the proteolytic events could take place also extracellularly. In the final results chapter the differences between BL cell lines in the way they process FKN during cell death were revealed and a new cell death-associated 55 kDa FKN was observed. Through several lines of evidence, this new form was identified to be a possible product of calpain-mediated proteolysis. To conclude, this work provides the first evidence for a possible direct participation of the two major cell death executioner proteases – caspases and calpains, in production of ‘find me’ signals for macrophages and thus, ensuring effective clearance of dying cells. These results indicate that FKN cleavage and release might be of key importance during cell death. Moreover, the studies presented here contribute to better understanding of the process of FKN secretion.