Evidence by interaction : a new role of caspase-2

Sammanfattning: Caspase-2 is the best conserved member of the caspase family. Although being known to mediate cell death following DNA damage, many studies have implicated that this protease regulates a variety of cellular processes. Despite this, few well-defined and mechanistically explained functions of caspase-2 has been described. In an attempt to shed light on this enigma, we performed a yeast two-hybrid screen, searching for interaction partners of caspase-2 which could explain many of the reported observations. From the screen two hits stood out, relating to the proteins RFXANK and FAN. In the first study we investigated the relationship between caspase-2 and RFXANK, a protein known for regulating expression of MHC class II genes. The interaction between the two proteins was confirmed to take place primarily in the cytoplasm of cells. Caspase-2 was able to bind to a construct resembling the four ankyrin repeats of RFXANK, indicating that this is the region important for the interaction. Cells lacking caspase-2 contained higher total levels of MHC II, thereby suggesting that caspase-2 suppresses normal expression of the complex. Surprisingly, antigen-presenting cells from caspase-2-/- mice did not display any differences in surface distribution of the MHC II, indicating that the transport of MHC II from the cell interior to the exterior was somehow impaired. In the second study, we were interested in the understanding of the relationship between caspase-2 and FAN. Like in the first study, the interaction between the two proteins was confirmed by methodologies separated from yeast two-hybrid. FAN is a protein which has been reported to regulate a wide range of processes. We, therefore, systematically evaluated how a lack of caspase-2 would affect cells, while comparing to what is known about FAN-deficiency. Interestingly, we found that loss of caspase-2 caused the same outcomes as has been described for cells which have lost FAN. Notably, the ability to secrete IL-6 was greatly impaired in caspase-2-deficient cells, when comparing with relevant controls. Furthermore, enzyme-deficient cells took longer time to repopulate cell-free areas, indicating hampered cell motility. Although FAN is known to also regulate ceramide production, we did not observe any differences in sphingolipid contents when removing caspase-2. We did, however, observe that caspase-2-deficient cells contained abnormally enlarged vesicular/lysosomal structures, similar to what has been described following loss of FAN. In the third study we investigated the potential involvement of caspase-2 in the cell death process induced by Gemtuzumab ozogamicin (GO; Mylotarg®). This was based on the fact that GO causes DNA damage, a cause known to activate the proapoptotic function of caspase-2. We found that inhibition or removal of caspase-2 protected AML cells from GO-induced apoptosis. Strikingly, caspase-2 appeared to be involved in the processing of BID, but not the activation of BAX following treatment with GO. This may imply that the protease acts in parallel with GO, mainly by increasing the effect of the drug rather than being vital for the drug toxicity. Taken together we describe novel interaction partners to caspase-2 and highlight how the protease may regulate processes which are not necessarily tied with apoptosis.