The effect and mode of action of the protein kinase C (PKC) inhibitor PKC412 on human multiple myeloma (MM) cell lines (HMCLs) and primary MM cells was explored. We found that PKC412 induced apoptosis of HMCLs and primary MM cells with variable efficacy; however, some activity was seen against all HMCLs and primary MM cells with at least 0.5 microM PKC412. PARP cleavage and decreased PKC activity was observed in all HMCLs tested. Furthermore, PKC412 inhibited C-FOS transcription and nuclear protein expression, induced reactive oxygen species (ROS) production, and induced both sustained C-JUN expression and phosphorylation. The latter was inhibited by cotreatment with the JNK inhibitor SP600125, which similarly abrogated PKC412-induced apoptosis, suggesting that PKC412-induced apoptosis is a JNK-dependent event. PKC412 treatment secondarily induced prosurvival stress responses as evidenced by activation of NFkappaB and increased expression of the heat shock proteins HSP70 and HSP90. Consistent with the former, sequential inhibition of NFkappaB activation with bortezomib or SN50 synergistically enhanced cell killing. Our results demonstrate that PKC412 induces JNK-dependent apoptosis of HMCLs and primary MM cells and that this effect is enhanced by NFkappaB inhibition. The further evaluation of PKC412 in the treatment of MM is justified.