DNA-PROTEIN CROSS-LINK FORMATION IN RAT NASAL EPITHELIAL-CELLS BY HEXAMETHYLPHOSPHORAMIDE AND ITS CORRELATION WITH FORMALDEHYDE PRODUCTION

Hexamethylphosphoramide (HMPA) is an aprotic polar solvent and nasal c arcinogen in rats. The metabolism of HMPA to formaldehyde, another nas al carcinogen in rats, was found to be approximately 6 times greater i n microsomes from olfactory tissues than from respiratory tissues (iso lated from both male and female rats). HMPA was shown to induce format ion of DNA-protein crosslinks (DPXLs) in isolated rat nasal epithelial cells. Using a filter binding assay, we demonstrated that microsomal activation is necessary for HMPA-induced crosslink formation between p lasmid DNA and calf thymus histones, presumably through metabolic N-de methylation of HMPA and the formation of formaldehyde. Both formaldehy de production and DPXL formation were inhibited by pre-incubation of n asal mucosal extracts with metyrapone, an inhibitor of cytochrome P-45 0. Significant dose-dependent increases in DPXL formation were observe d in respiratory and olfactory epithelial cells exposed to greater tha n or equal to 0.5 and 1 mM HMPA, respectively, for 3 h at 37 degrees C . This resulted in DPXL accumulation at 18-20% higher levels than untr eated cells. Increases in DPXL formation in rat nasal epithelial cells cultured with 1 mM HMPA were inhibited by over 70% by co-administrati on of metyrapone. These data suggest that metabolic liberation of form aldehyde from HMPA is involved in the mechanism of HMPA-induced nasal carcinogenesis. Comparative studies showed formaldehyde to be more pot ent than HMPA in the induction of DPXL in nasal epithelium. However, i nduction of tumor formation after two years at 50 ppb HMPA and 6 ppm f ormaldehyde show the former to be active at several-fold lower concent rations. Therefore, other mechanisms are likely to be involved in HMPA nasal carcinogenesis,