Cytotoxic effect of Laxaphycins A and B on human lymphoblastic cells (CCRF-CEM) using digitised videomicrofluorometry.

Laxaphycin A (laxa A) and Laxaphycin B (laxa B), cyclic peptides isolated from the terrestrial blue-green alga Anabaena laxa or the marine cyanobacterium Lyngbya majuscula have antifungal and cytotoxic activities. We used numerical videomicrofluorometry and a protocol of multiple labelling with Hoescht 33342 (nuclear DNA), Rhodamine 123 (mitochondria) and Nile Red (plasma membrane) to study the cytotoxicity of these substances in human lymphoblastic cells sensitive (CEM-WT) or resistant (CEM-VLB and CEM-VM1) to anticancer agents. The results indicate a low resistance index of 2 for CEM-VLB cells treated with laxa B or laxa A + lava B. For the three cell strains, following laxa B treatment, we observed an increase of a polyploid cell subpopulation that could result from the alteration of topoisomerase-II activity. On the contrary, the simultaneous treatment by laxa A and laxa B led to a decrease of that subpopulation with increasing laxa A doses. However, the effect of laxa A was less pronounced in the CEM-VM1 cells, which present a low intrinsic topoisomerase-II activity. For CEM-VLB cells, the higher doses needed can be attributed to their MDR resistance. Though we observed a synergistic effect between laxa B and laxa A (the latter is inactive by itself), these results indicate a different mode of action for laxa B and laxa A + laxa B. A more precise study of the mode of action of these compounds is warranted.