Lymphoma cells selected for resistance against the cytotoxic effect of oxygenated sterols are also resistant to nonsteroidal antiestrogens

Abstract

Oxygenated derivatives of cholesterol and related compounds (oxysterols) have long been known to be cytotoxic to many different cell types. The mechanism of this cytotoxic effect is not fully understood. Our laboratory has earlier reported that oxysterol cytotoxicity resembles that of nonsteroidal antiestrogens in some aspects: (i) the cytotoxic action of both types of compounds is blocked by inhibitors of protein or RNA synthesis, and (ii) both classes of compounds bind with high affinity to the microsomal antiestrogen binding site, a protein which may mediate the cytotoxicity of its ligands. We have now extended these studies by developing cell lines which are resistant to the cytotoxic action of oxysterols. Oxysterol-resistant cells were isolated by exposing two murine lymphoma cell lines, K36 and EL4, to incremental concentrations of 7-ketocholestanol. Intriguingly, the resistant cells thus obtained also exhibited considerable resistance to the cytotoxic effects of nonsteroidal antiestrogens such as tamoxifen and clomiphene, having LD 50 values which were 10–100-times higher than that of the parental cells. The resistance appeared to be selective for oxysterols and antiestrogens and did not extend to non-specific toxic agents such as azide, ethanol, Triton-X100, or heat. The biochemical basis of the resistance is not clear but is not due to diminished cellular uptake or increased metabolism of the cytotoxic agents or to changes in the antiestrogen binding protein. The availability of the resistant cell lines should facilitate further studies on the mechanism of oxysterol- and antiestrogen-induced cell death.