Maitotoxin and P2Z/P2X7purinergic receptor stimulation activate a common cytolytic pore.

The effects of maitotoxin (MTX) on plasmalemma permeability are similar to those caused by stimulation of P2Z/P2X7ionotropic receptors, suggesting that 1) MTX directly activates P2Z/P2X7 receptors or 2) MTX and P2Z/P2X7 receptor stimulation activate a common cytolytic pore. To distinguish between these two possibilities, the effect of MTX was examined in 1) THP-1 monocytic cells before and after treatment with lipopolysaccharide and interferon-γ, a maneuver known to upregulate P2Z/P2X7receptor, 2) wild-type HEK cells and HEK cells stably expressing the P2Z/P2X7 receptor, and 3) BW5147.3 lymphoma cells, a cell line that expresses functional P2Z/P2X7 channels that are poorly linked to pore formation. In control THP-1 monocytes, addition of MTX produced a biphasic increase in the cytosolic free Ca2+ concentration ([Ca2+]i); the initial increase reflects MTX-induced Ca2+ influx, whereas the second phase correlates in time with the appearance of large pores and the uptake of ethidium. MTX produced comparable increases in [Ca2+]iand ethidium uptake in THP-1 monocytes overexpressing the P2Z/P2X7 receptor. In both wild-type HEK and HEK cells stably expressing the P2Z/P2X7 receptor, MTX-induced increases in [Ca2+]iand ethidium uptake were virtually identical. The response of BW5147.3 cells to concentrations of MTX that produced large increases in [Ca2+]ihad no effect on ethidium uptake. In both THP-1 and HEK cells, MTX- and Bz-ATP-induced pores activate with similar kinetics and exhibit similar size exclusion. Last, MTX-induced pore formation, but not channel activation, is greatly attenuated by reducing the temperature to 22°C, a characteristic shared by the P2Z/P2X7-induced pore. Together, the results demonstrate that, although MTX activates channels that are distinct from those activated by P2Z/P2X7 receptor stimulation, the cytolytic/oncotic pores activated by MTX- and Bz-ATP are indistinguishable.