The bioflavonoid galangin blocks aryl hydrocarbon receptor activation and polycyclic aromatic hydrocarbon-induced pre-B cell apoptosis.

Bioflavonoids are plant compounds touted for their potential to treat or prevent several diseases including cancers induced by common environmental chemicals. Much of the biologic activity of one such class of pollutants, polycyclic aromatic hydrocarbons (PAH), is mediated by the aryl hydrocarbon receptor/transcription factor (AhR). For example, the AhR regulates PAH immunotoxicity that manifests as pre-B cell apoptosis in models of B cell development. Because bioflavonoids block PAH-induced cell transformation and are structurally similar to AhR ligands, it was postulated that some of them would suppress PAH-induced, AhR-dependent immunotoxicity, possibly through a direct AhR blockade. This hypothesis was tested using a model of B cell development in which pre-B cells are cultured with and are dependent on bone marrow stromal or hepatic parenchymal cell monolayers. Of seven bioflavonoids screened, galangin (3,5,7-trihydroxyflavone) blocked PAH-induced but not C(2)-ceramide- or H(2)O(2)-induced pre-B cell apoptosis. Because galangin blocked AhR-dependent reporter gene expression, AhR complex-DNA binding, and AhR nuclear translocation, inhibition of a relatively early step in AhR signaling was implicated. This hypothesis was supported by the ability of galangin to bind the AhR and stabilize AhR-90-kDa heat shock protein complexes in the presence of AhR agonists. These studies demonstrate the utility of pre-B cell culture systems in identifying compounds capable of blocking PAH immunotoxicity, define at least one mechanism of galangin activity (i.e., repression of AhR activation), and motivate the use of this and similar dietary bioflavonoids as relatively nontoxic inhibitors of AhR agonist activity and as pharmacologic agents with which to dissect AhR signaling pathways.

Aziridinyl steroid-induced lesions in DNA and apoptosis in promyelocytic leukemia cells.

Variety of synthetic steroids are reported to be mutagenic as well as carcinogenic. The mutagenic and carcinogenic nature of these compounds have been related to their potential of being reactive to genetic material and production of reactive oxygen species. Here we have analyzed the action of aziridinyl steroid on calf thymus DNA and human promyelocytic leukemia cell line HL-60 under in vitro conditions. Calf thymus DNA when treated with various doses of aziridinyl steroid induced a high degree of stand separation and sensitivity/susceptibility to S1 nuclease hydrolysis. The treatment also induced an increasing number of strand breaks per molecule of DNA as determined by alkaline unwinding assay. Relatively higher doses of steroid, however, displayed a reduced susceptibility to S1 nuclease hydrolysis and did not increase the number of strand breaks in DNA. Moreover, the high dose treatments result increased melting temperature and an enhanced rate of reanealing after thermal denaturation, indicating that interstrand crosslinks are induced at higher doses of steroid treatment. Moreover, steroid treatment caused cell death in human promyelocytic leukemia cell line HL-60 and induced DNA degradation, characteristic of apoptosis. The test steroid has the ability to produce reactive oxygen intermediates (ROI) as determined by chemical methods. Incorporation of oxygen radical scavengers into the system blocked the damaging effect of steroid in calf thymus DNA and HL-60 cells. These observations strongly suggest that aziridinyl steroid, a pharmaceutical, damages mammalian DNA and induces apoptosis by the production of ROI in the test system.