Σηματοδοτικοί μηχανισμοί κατά την απόπτωση και την κυτταροφαγία στα αιμοκύτταρα της μύγας της Μεσογείου

Abstract

Programmed cell death (apoptosis) is an evolutionary conserved process, crucial for the development and maintenance of multicellular organisms. Medfly hemocytes undergo apoptosis during larval-pupal transformation. Hemocytes also undergo apoptosis in response to external chemical or physical stimuli (cycloheximide, staurosporine and heat shock). Expression and phosphorylation of FAK, Src, ERK, PI-3K and Akt appear to be dependent upon developmental stages. The maximum expression and phosphorylation of the above signalling molecules appeared in hemocytes of the white pupal stage, which is also the peak of apoptosis levels. FAK/Src, Ras/ERK and PI-3K/Akt signalling pathways are involved in the regulation of apoptosis. Furthermore, FAK silencing experiments show that FAK, as well as, its downstream targets (Src, ERK, PI-3K, Akt) are involved in the developmentally regulated apoptosis. Phagocytosis is an important innate immune response against pathogens hemocytes, responsible for cellular defense responses, recognize a variety of foreign biotic and abiotic targets and respond by activating several intracellular signalling pathways via hemocyte surface receptors, leading to phagocytosis. Integrin beta-subunit silencing experiments demonstrated that the uptake of bacteria by insect hemocytes is regulated by integrins and results to the activation of FAK/MAPKs signalling pathways. Focal adhesion kinase and its downstream targets (MAPKs and Src) are also implicated in the process of phagocytosis. In addition, experimental results strongly support a physical association between FAK, Src and ERK. The above signalling cellular innate immune responses result to the phosphorylation of Elk-1-like transcription factor in E. coli/latex beads-challenged hemocytes. Elk-1-like protein is a downstream target for activation of MAPKs. Osmotic loading experiments using Elk-1 antibodies demonstrated the dependence of phagocytosis from Elk-1-like protein. In light of the functional association of FAK with Elk-1-like protein, confocal microscopy analysis shows that these two signalling molecules are localized in the same subcellular compartments, where co-localization also occurs. In light of the functional and physical association of FAK with Elk-1-like protein in medfly hemocytes, we explored whether an analogous functional and physical association between these two essential molecules also exists in mammalian cell systems, in particular, in HK-2 human cells. Indeed, FAK and Elk-1 are co-localized in the periphery of the nuclear membrane in glucose activated cells. Elk-1 silencing in HK- whereas the levels of FAK and MAPKs were significantly decreased. The above results suggest that Elk-1 appears to affect apoptosis via regulating the expression of FAK and MAPKs.