We have earlier shown that the 9.2.27 Pseudomonas
Exotoxin A (PE)
immunotoxin (IT) efficiently kills
melanoma cells through inhibition of
protein synthesis followed by some morphologic and biochemical features of apoptosis, a different cell killing mechanism than the one caused by
Dacarbazine (
DTIC), a chemotherapeutic
drug used to treat
malignant melanoma. To examine whether induced
DTIC resistance also is a determining factor for the effectiveness of 9.2.27PE IT, we developed a
DTIC resistant subline, FEMX-200DR, from the
DTIC sensitive cell line FEMX. The cell variants were treated with 9.2.27PE, an IT binding to the
high molecular weight-melanoma associated antigen (
HMW-MAA) expressed on most
malignant melanoma cells. The IT was equally effective in killing the FEMX-200DR and the FEMX cells, and the cell death was primarily caused by inhibition of
protein synthesis. The
DNA repair enzyme and apoptotic marker PARP, a substrate of
caspase-3, was inactivated, although we observed only a minor activation of
caspase-3 and
caspase-8, intracellular
proteases involved in apoptosis. In addition to being
DTIC resistant, the FEMX-200DR cells were also more resistant to apoptosis than the parent cells as a 3 times higher concentration of the apoptotic inducer
Staurosporine was needed to obtain IC50. Furthermore, in early passage
malignant melanoma cell lines established from
lymph node metastases, the 9.2.27PE caused a time-dependent and dose-dependent decrease in cell viability independent of their
DTIC sensitivity. These findings show that the 9.2.27PE IT efficiently can cause cell death in
malignant melanoma cells independent of their level of resistance to apoptosis and
DTIC.