Tumor-specific Cytotoxicity and Type of Cell Death Induced by Naphtho[2,3-b]furan-4,9-diones and Related Compounds in Human Tumor Cell Lines: Relationship to Electronic Structure

A total of thirty-nine naphtho(2,3-b)furan-4,9- diones and related compounds were tested for their cytotoxicity against three human normal oral cells (gingival fibroblast, HGF, pulp cell, HPC, periodontal ligament fibroblast, HPLF) and four human tumor cell lines (oral squamous cell carcinoma HSC-2, HSC-3, HSC-4, promyelocytic leukemia HL-60). 2-Acetylnaphtho(2,3- b)furan-4,9-dione (1) was highly cytotoxic to both normal and tumor cells, yielding low tumor-specificity. 2-Acetyl-4,9- dimethoxynaphtho(2,3-b)furan (4), the 2-(3-furanoyl) benzoic acids (5, 6) and the 1,4-naphthoquinones (7, 8) showed much reduced cytototoxicity and low tumor- specificity. The introduction of phenoxy (18), isopropylamino (23) or 2-methylpiperidino (33) groups to the 2-position of naphtho(2,3-b)furan-4,9-dione yielded compounds that showed the greatest tumor-specificity. These compounds, at twice or four times higher concentrations than CC 50 , induced the activation of caspase-3, caspase-8 and caspase-9 in the HSC-2 and HL-60 cells, but not so apparently in the HSC-4 cells. However, they did not induce internucleosomal DNA fragmentation in the HSC-2 and HSC-4 cells even after 24 hours incubation and only slightly induced DNA fragmentation in the HL-60 cells. Compound (18) induced the production of annexin-positive cells, but did not induce microtubule-associated protein light chain 3 (LC3) accumulation in autophagosomes in LC3-green fluorescent protein (GFP)-transfected HSC-2 cells. These data suggested that naphtho(2,3-b)furan- 4,9-diones may induce the early apoptotic marker, without induction of caspase activation and DNA fragmentation in oral squamous cell carcinoma cell lines. Quantitative structure-activity relationship (QSAR) analysis suggests the applicability of the theoretical calculations such as frontier molecular orbital, dipole moments and hydrophobicity in predicting their cytotoxic activity. Several naphtho(2,3-b)furan-4,9-diones isolated from plants have shown biological activity (1). It has been reported that 2- acetylnaphtho(2,3-b)furan-4,9 -dione (1) isolated from Tabebuia cassinoides (Lam.) DC (Bignoniaceae) was highly cytotoxic (2), and 2-methylnaphtho(2,3-b)furan-4,9-dione showed three times higher cytotoxicity than (1) (3). The activity of 2-substituted naphtho(2,3-b)furan-4,9-diones varies with the type of substituents on the parent naphtho(2,3-b)furan-4,9-dione