Figure 2.

SAPK and MEKK1 activity correlate with the onset of caspase 3 activation after anti-Fas-induced apoptosis. (A) MEKK1 (Upper) and SAPK (Lower) activity in anti-Fas-treated Jurkat cells gradually increased over a time course of 4 hr (Upper). (B) The activation of SAPK during 6-hr anti-Fas-treatment (lane 2) was blocked by z-VAD-fmk beginning at 15 min before addition of the antibody (lane 3). (C) Cleavage of endogenous MEKK1 in anti-Fas-treated Jurkat cells (Upper) was evidenced by a 100-kDa MEKK1 form detected after 2 hr that was absent in the untreated cells (lane 1). MEKK was detected by immunoprecipitation followed by immunoblotting against MEKK1. Caspase 3 (CPP32) proteolytic activation (Lower) from the same cell lysates correlate with appearance of the proteolysed MEKK1 fragment. Caspase 3 was detected by immunoblotting.