COMBINATION THERAPY WITH CISPLATIN AND NIFEDIPINE INDUCES APOPTOSIS IN CISPLATIN-SENSITIVE AND CISPLATIN-RESISTANT HUMAN GLIOBLASTOMA CELLS

We attempted to determine whether calcium channel blockers (CCBs) enha nce the anti-tumour activity of cis-diamminedichloroplatinum (cisplati n) against both cisplatin-sensitive human glioblastoma U87-MG cells an d cisplatin-resistant U87-MG-CR cells, the latter of which we develope d for resistance to cisplatin. Nifedipine, a dihydropyridine class CCB , significantly enhanced the anti-tumour effect of cisplatin on these two cell types in vitro and in vivo. Our findings also indicated that, in the absence of normal extracellular Ca2+ nifedipine was capable of enhancing the cytotoxicity of cisplatin. In addition, this anti-tumou r activity was partially inhibited by actinomycin D and cycloheximide, suggesting that it is possibly dependent upon new RNA and protein syn thesis. Interestingly, ultrastructural analysis, DNA fragmentation ass ay and cell cycle analysis demonstrated that synergism between cisplat in and nifedipine results in apoptosis (programmed cell death) at a re latively low concentration of cisplatin, which when tested alone did n ot induce apoptosis. Furthermore, we demonstrated that nuclei from the se cells lack a Ca2+-dependent endonuclease that degrades chromatin in the linker region between nucleosomes. In conclusion, our studies sug gest that the non-cytotoxic agent nifedipine is able to synergisticall y enhance the anti-tumour effects of cisplatin on U87-MG and U87-MG-CR cells lacking a Ca2+-dependent endonuclease and subsequently to induc e apoptosis via interaction of nifedipine with an as yet uncharacteris ed functional site other than a calcium channel on target cells.