Lamellarin D (
LAM-D) is a marine
alkaloid endowed with potent cytotoxic activities against various
tumor cells, in particular human
prostate cancer cells and
leukemia cells. Its cytotoxic action is dependent, at least in part, to its capacity to inhibit
topoisomerase I. P388CPT5 murine
leukemia cells resistant to the reference
topoisomerase I poison camptothecin (
CPT) are cross-resistant to
LAM-D but the relative resistance index (RRI) is significantly reduced with
LAM-D (RRI=21) compared to
CPT (RRI=103). To comprehend further the mechanism of action of this novel marine
antitumor agent, we have investigated the influence of the
P glycoprotein (Pgp) on the cytotoxicity of
LAM-D and the proapoptotic effects induced by the
alkaloid. P388CPT5 cells, expressing a mutated top1 gene, display a functional Pgp, as judged from cytometry experiments performed with cells treated with
rhodamine 123 or
calcein-
ester whereas no Pgp activity was detected with the parental P388 cells. P388CPT5 cells are also cross-resistant to the
topoisomerase II poisons doxorubicin and
etoposide but the resistance is abolished in the presence of
verapamil or
quinine (at non toxic concentrations) which reverse the multidrug resistance (MDR) phenotype. In contrast, the RRI measured with
LAM-D and
CPT remain unchanged in the presence of the two MDR reversal agents. The effects of
LAM-D on the cell cycle progression were different in the parental P388 cells compared with the
CPT-resistant which were blocked in the S and subsequently G2-M phases of the cell cycle. Cytometry experiments with the
JC-1 fluorescent marker revealed that
LAM-D and
CPT promoted apoptosis in parental P388 cells via an activation of the mitochondrial pathway. In contrast, a massive depolarisation of the mitochondrial membrane potential and a nuclear fragmentation were detected only with
LAM-D on P388CPT5 cells. This in vitro work identifies
LAM-D as a potent pro-apoptotic agent and its cytotoxic action is fully maintained in multidrug-resistant cells compared to the sensitive parental cell line.