O6-methylguanine-DNA methyltransferase activity in human buccal mucosal tissue and cell cultures. Complex mixtures related to habitual use of tobacco and betel quid inhibit the activity in vitro

Extracts prepared from tissue specimens of normal, non-tumourous humanbuccal mucosa, and cultured buccal epithelial cells and fibroblasts,exhibited O6-methylguanine-DNA methyltransferase (MGMT) activity bycatalysing the repair of the premutagenic O6-methylguanine lesion inisolated DNA with rates of 0.2 to 0.3 pmol/mg protein. An SV40 Tantigen-immortalized buccal epithelial cell line termed SVpgC2a and abuccal squamous carcinoma line termed SqCC/Y1, both of which lack normaltumour suppressor gene p53 function, exhibited about 50 and 10% of the MGMTactivity of normal cells, respectively. The normal, experimentallytransformed and tumourous buccal cell types showed MGMT mRNA levels whichcorrelated with their respective levels of MGMT activity. Exposure ofbuccal cell cultures to various organic or water- based extracts ofproducts related to the use of tobacco and betel quid, decreased both cellsurvival (measured by reduction of tetrazolium dye) and MGMT activity(measured subsequently to the exposures in cellular extracts). Organicextracts of bidi smoke condensate and betel leaf showed higher potency thanthose of tobacco and snuff. An aqueous snuff extract also decreased bothparameters, whereas an aqueous areca nut extract was without effect. Thewell- established sulph-hydryl-reactive agent Hg2+, a corrosion product ofdental amalgam, served as a positive control and decreased MGMT activityfollowing treatment of cells within a range of 1-10 microM. Taken together,significant MGMT activities were demonstrated in buccal tissue specimensand in the major buccal mucosal cell types in vitro. Lower than normal MGMTactivity in two transformed buccal epithelial cell lines correlated withdecreased MGMT mRNA and lack of functional p53. Finally, in vitroexperiments suggested the potential inhibition of buccal mucosal MGMTactivity by complex mixtures present in the saliva of tobacco and betel nutchewers.