O-6-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
Y. Liu et al., O-6-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, Carcinogenesis, 18(10), 1997, pp. 1889-1895
Extracts prepared from tissue specimens of normal, non-tumourous human
buccal mucosa, and cultured buccal epithelial cells and fibroblasts,
exhibited O-6-methylguanine-DNA methyltransferase (MGMT) activity by c
atalysing the repair of the premutagenic O-6-methylguanine lesion in i
solated DNA with rates of 0.2 to 0.3 pmol/mg protein, An SV40 T antige
n-immortalized buccal epithelial cell line termed SVpgC2a and a buccal
squamous carcinoma line termed SqCC/Y1, both of which lack normal tum
our suppressor gene p53 function, exhibited about 50 and 10% of the MG
MT activity of normal cells, respectively, The normal, experimentally
transformed and tumourous buccal cell types showed MGMT mRNA levels wh
ich correlated with their respective levels of MGMT activity, Exposure
of buccal cell cultures to various organic or water-based extracts of
products related to the use of tobacco and betel quid, decreased both
cell survival (measured by reduction of tetrazolium dye) and MGMT act
ivity (measured subsequently to the exposures in cellular extracts), O
rganic extracts of bidi smoke condensate and betel leaf showed higher
potency than those of tobacco and snuff, An aqueous snuff extract also
decreased both parameters, whereas an aqueous areca nut extract was w
ithout effect. The well-established sulph-hydryl-reactive agent Hg2+,
a corrosion product of dental amalgam, served as a positive control an
d decreased MGMT activity following treatment of cells within a range
of 1-10 mu M. Taken together, significant MGMT activities were demonst
rated in buccal tissue specimens and in the major buccal mucosal cell
types in vitro, Lower than normal MGMT activity in two transformed buc
cal epithelial cell lines correlated with decreased MGMT mRNA and lack
of functional p53, Finally, in vitro experiments suggested the potent
ial inhibition of buccal mucosal MGMT activity by complex mixtures pre
sent in the saliva of tobacco and betel nut chewers.