Malignant melanomas are generally
drug resistant and have a very poor prognosis. We have studied the effects of a chemical conjugate of pseudomonas
exotoxin A (PE) and the antibody 9.2.27, which recognizes the
high molecular weight melanoma associated antigen (
HMW-MAA) expressed in most
malignant melanomas and
melanoma cell lines. We demonstrate that the 9.2.27PE
immunotoxin (IT) induces cell death in
malignant melanoma cells through
protein synthesis inhibition followed by some morphological and biochemical features of apoptosis, like rounding up of cells,
chromatin condensation and inactivation of PARP. Unlike previous results with the 425.3PE IT in
breast cancer cells, we detected no depolarization of the mitochondrial membrane after 9.2.27PE IT treatment. This is likely due to the lack of strong activation of
caspase-8 and
caspase-3. The lack of depolarization suggests that
cytochrome c, a molecule that triggers activation of
caspase-3, was retained within the mitochondria. In addition, the
protein level of the antiapoptotic Bcl-2 did not decrease in contrast to other antiapoptotic molecules belonging to the inhibitor of apoptosis and the Bcl-2 family. This suggests that Bcl-2 may play a role in maintaining the mitochondrial membrane integrity in the 9.2.27PE-treated cells. Nevertheless, 9.2.27PE IT efficiently killed
malignant melanoma cells that can be ascribed to inhibition of
protein synthesis followed by some morphological and biochemical features of apoptosis.