Diphyllin and its derivatives are well known cytotoxic natural products structurally related to the anti-
cancer drug podophyllotoxin. We here study their structure-activity relationship upon human
melanoma cells for first time. To this end, human
melanoma A375 cells were incubated with
Justicidin B and its 4-methoxylated or 4-glycosylated derivatives to evaluate their selective cytotoxicity and study their effects on cell cycle distribution,
caspase activation, apoptosis induction using
Annexin V-FITC/PI staining, cell morphology and western blot analysis.
Diphyllin methyl
ether (GI50 = 3.66 μM) and
Justicidin B (GI50 = 1.70 μM) caused an elevation of both early and late apoptosis populations whereas
Diphyllin apioside (GI50 = 0.84 μM) and its
acetate (GI50= 0.39 μM) enhanced late apoptosis population only (
Annexin V-positive/PI-positive). All induced cell cycle arrest at S phase and classic morphological indicators of apoptosis (blebbing, apoptotic bodies, and nuclear fragmentation) accompanied with an elevation of cells with low
DNA content (sub-G1). All compounds increased the Bax/Bcl-2 ratio by enhancing Bax expression which evidences the involvement of the mitochondria (intrinsic pathway) in the apoptotic process. These
caspase-3/7 results evidence that 4-methoxylation or 4-O-glycosylation of
Justicidin B -a
caspase independent mitochondrial apoptosis-inducer- triggers
caspase-3/7 activation at different times (24h vs. 48h, respectively). Interestingly, the methoxylation causes attenuation of Bcl-2
protein expression contrarily to
Diphyllin methyl
ether or the O-glycosylated derivatives. Finally, the compounds exhibited significantly less toxicity when tested in adult human dermal fibroblasts and their GI50 in
melanoma Sk-Mel-5 cells was not influenced by MDR1/Pgp inhibitors. This study may inform the synthesis of future
Diphyllin derivatives with different apoptosis mechanism of action towards human
melanoma cells.