IDENTIFICATION OF A METHYLCHOLANTHRENE-INDUCED ALDEHYDE DEHYDROGENASEIN A HUMAN BREAST ADENOCARCINOMA CELL-LINE EXHIBITING OXAZAPHOSPHORINE-SPECIFIC ACQUIRED-RESISTANCE

The class-3 aldehyde dehydrogenase that is overexpressed (>100-fold) i n human breast adenocarcinoma MCF-7/0 cells made resistant (>30-fold a s judged by LC(90)s) to oxazaphosphorines, such as mafosfamide, by gro wing them in the presence of polycyclic aromatic hydrocarbons, e.g., m ethylcholanthrene (3 mu M for 5 days), was isolated and characterized. Its physical and catalytic properties were identical to those of the prototypical human stomach mucosa cytosolic class-3 aldehyde dehydroge nase, type-1 ALDH-3, except that it catalyzed, though not very rapidly , the oxidation of aldophosphamide, whereas the stomach mucosa enzyme essentially did not; hence, it was judged to be a slight variant of th e prototypical enzyme. Carcinogens that are not ligands for the Ah rec eptor, barbiturates known to induce hepatic cytochrome P450s, steroid hormones, an antiestrogen, and oxazaphosphorines did not induce the en zyme or the largely oxazaphosphorine-specific acquired resistance. Whe reas methylcholanthrene induced (a) resistance to mafosfamide and (b) class-3 aldehyde dehydrogenase activity, as well as glutathione S-tran sferase and DT-diaphorase activities, in the estrogen receptor-positiv e MCF-7/0 cells, it did not do so in two other human breast adenocarci noma cell lines, MDA-MB-231 and SK-BR-3, each of which is estrogen rec eptor negative. Expression of the class-3 aldehyde dehydrogenase and t he loss of sensitivity to mafosfamide by polycyclic aromatic hydrocarb on-treated MCF-7/0 cells were transient: each returned to essentially basal levels within 15 days when the polycyclic aromatic hydrocarbon w as removed from the culture medium. Insensitivity to the oxazaphosphor ines on the part of polycyclic aromatic hydrocarbon-treated MCF-7/0 ce lls was not observed when exposure to mafosfamide (30 min) was in the presence of benzaldehyde or octanal, each a relatively good substrate for cytosolic class-3 aldehyde dehydrogenases, whereas it was retained when exposure to mafosfamide was in the presence of acetaldehyde, a r elatively poor substrate for these enzymes. These observations demonst rate that ligands for the Ah receptor can induce a transient, largely oxazaphosphorine-specific, acquired cellular resistance, and they are consistent with the notion that elevated levels of a cytosolic class-3 aldehyde dehydrogenase nearly identical to the prototypical type-1 cl ass-3 aldehyde dehydrogenase expressed by human stomach mucosa account for the Ah receptor ligand-induced oxazaphosphorine-specific acquired resistance, most probably by catalyzing the detoxification of aldopho sphamide.