Pterocarpans, the second group of natural isoflavonoids, have received considerable interest on account of their medicinal properties. These drugs are employed as
antitoxins, but display antifungal,
antiviral and antibacterial properties as well.
Erybraedin C and
bitucarpin A are two new structurally related
pterocarpans recently purified and characterized.
Bitucarpin A differs from
erybraedin C for the absence of a
prenyl group in 5' position and the presence of a methoxylate
hydroxyl group in 7, 4' positions. These compounds proved not to be
clastogens in human lymphocytes per se but displayed anticlastogenic activity against mytomicin C and
bleomycin C. Here we extended the study of their antiproliferative and apoptosis-inducing mechanism on human cell lines. Two human
adenocarcinoma cell lines, LoVo and HT29, as examples of slow-growing solid
tumors, proficient and deficient in mismatch repair system (MMR), p53 and Bcl-2, were used to evaluate the cytotoxicity of the drugs and their effects on the cell cycle, measured by flow cytometry.
Erybraedin C similarly affects the survival of HT29 (MMR +/+, p53 -/- and Bcl-2 +/+) and LoVo (MMR -/-, p53 +/+ and Bcl-2 -/-) cells (LD(50): 1.94 and 1.73 microg/ml, respectively). By contrast,
bitucarpin A exhibits a differential cytotoxicity in the cell lines (LD(50): 6.00 microg/ml, HT29, and 1.84 microg/ml, LoVo). The cell cycle distributions of the LoVo and HT29 cells treated with
erybraedin C lacked a specific checkpoint arrest, whereas they underwent a characteristic sub-G(1) peak, time- and
drug-concentration dependent. So that apoptotic process induced by
erybraedin C in both
adenocarcinoma cell lines is independent of cell cycle arrest and of phenotypic status of the cells as well. By contrast,
bitucarpin A affects cell cycle progression on both cell lines, inducing a transient block in G(0)/G(1) along 24-96 h, and induces apoptosis with a cell density and treatment time dependency. Similar results were obtained with the positive control drug
etoposide. The programmed cellular death on human
adenocarcinoma cell lines may be efficiently activated, via a
topoisomerase II poison pattern, by
erybraedin C, the
drug containing regio-specific
hydroxyl and
prenyl groups. The apoptotic effect induced by the methoxylated
bitucarpin A proved to be conditioned by cell density and required higher dose (5-fold-LD(50)) and longer treatment time. The present study provides evidences that
erybraedin C may act as a potent growth inhibitory compound, at low and high cell density, comparable to other clinically important
antineoplastic natural drugs including
etoposide, on human
colon adenocarcinoma cells.
Bitucarpin A proved less active because it was conditioned by cell density effect, but this finding may represent a clinical advantage against early micrometastatic diseases.