The effect of SM on apoptosis in pgp+ve and pgp−ve cell lines.

(A) U937 cells were cultured in 10% FCS for 3 hours in the presence of increasing doses of daunorubicin. The cells were then rinsed, incubated for 30 minutes with or without C₆-NBD-SM in 1% FCS, pelleted, and cultured for a further 18 hours. In the absence of daunorubicin, C₆-NBD-SM made no difference to U937 viability, demonstrating that SM was not inherently toxic at this dose. However, daunorubicin-induced apoptosis was augmented in a dose-dependent manner by C₆-NBD-SM. Error bars equal the standard error of the mean (SEM) of 3 experiments. (B) The lack of effect of C₆-NBD-SM on apoptosis induced by growth factor withdrawal in pgp+ve TF1 cells is demonstrated. TF1 cells were loaded with C₆-NBD-SM and cultured with and without GM-CSF. With growth factor withdrawal, 16% of cells were nonviable by 30 hours, but C₆-NBD-SM did not enhance apoptosis. Error bars equal SEM of 3 experiments.