Discovery, synthesis, and cytotoxic evaluation of isoquinolinequinones produced by Streptomyces albidoflavus derived from lichen

Four isoquinolinequinones (1–4) were isolated from the fermentation broth of Streptomyces albidoflavus which were derived from lichens. Among them, mansouramycin H (1) was identified as a new isoquinolinequinone by comprehensive spectroscopic data analysis. The mansouramycins from S. albidoflavus presented broad cytotoxic activities, especially against MDA-MB-231, but the SAR and mechanism were still unclear. The total synthesis of mansouramycin H (1) and its twenty-three derivatives were completed and their cytotoxic activities against MDA-MB-231 were evaluated in vitro. Primary SAR revealed that the piperazine moieties introduced into the amino group at C-7 could improve the activities of mansouramycins. Benzoyl and phenylacetyl groups on piperazine fragments had better activities than those of benzyl substitution; the alkyl substituent on piperazine exhibited optimal activity. Among them, compound 1g showed the strongest cytotoxicity against MBA-MB-231 cells with an IC(50) value of 5.12 ± 0.11 μM. Mechanistic studies revealed that 1g induced apoptosis in MBA-MB-231 cells through down-regulating the protein expression of Bcl-2, up-regulating the protein expression of bax, and, meanwhile, activating the cleavage of caspase-3 and caspase-9. 1g caused S phase cell cycle arrest in MBA-MB-231 cells by reducing the protein expression of CDK2 and cyclin A2 and increasing the protein levels of p21.