The team then made use of the fact that the scramblase can be activated in some cells in the laboratory by increasing the concentration of calcium ions. This activation mechanism works best in red blood cells, platelets, lymphocytes, and macrophages and does not work at measurable levels in fibroblasts or epithelial cells. In cells activated by calcium ions, neither the presence nor the absence of ABCA1 changes phospholipids transport. In cells that cannot be activated by adding calcium ions, adding ABCA1 as well does not induce activation. The experiments have the same outcome whether the team used cells from Tangier individuals or knock-out mice. These results suggest strongly that ABCA1 does not code for scramblase and is not required for its activation.
The team then turned to the function of ABC1 in apoptosis. Did ABCA1 cause or activate phospholipid transport across the cell membrane only during apoptosis? "We could not demonstrate that the addition or deletion of ABCA1 had any effect on the movement of PS to the cell surface in apoptotic cells," says Schlegel. "We conclude that the ABCA1 enzyme doesn't influence the transport of PS either under normal conditions or in apoptotic cells."
A third possibility was that ABCA1 functioned only in macrophages to cause the expression of PS on the cell surface. Additional experiments refuted this suggestion too. The absence of ABCA1 did not prevent macrophages from expressing PS on their surfaces. Further, the rate at which macrophages recognized and engulfed their target apoptotic cells remained constant whether ABCA1 was present in the macrophages or not.
"This work really changes our understanding of the mechanics of
cholesterol metabolism," remarks Schlegel. "We can't find any
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