Table 1.

Distribution and cellular localization of enzymes responsible for nitrosamine metabolism in the smokeless tobacco-associated SCC tumor tissuea

Metabolic enzymePotential carcinogenic or protective mechanismsEnzyme distribution profile by IHC-tumor tissue
P450 1A2P450-mediated oxidation of NNK and NNN generates reactive nucleophiles that can mutate and initiate miscoding of DNA. Higher oxidation rates for NNK.Full thickness surface of dysplastic epithelium, entire tumor islands (100% tumor staining-cytosol staining)
P450 2E1Inducible by alcohol, oxidative bioactivation of NDMA and NNN to genotoxic products. Highest P450-mediated bioactivation profile for NNN.Faint staining of dysplastic epithelium, tumor islands (no staining in keratin pearls).
P450 2A6Enzyme responsible for mutagenic bioactivation of all tobacco-related N-nitrosamines. Genetic polymorphisms observed. Reduced function variants are associated with decreased lung cancer.Strong cytosolic staining at basilar and suprabasilar cell layers in overlying dysplastic epithelia. Many OSCC tumor islands very intense staining, absence of staining in keratin pearls. Dysplastic and tumor staining intensity is greater relative to normal oral epithelia.
P450 2A13Strong oxidative bioactivation of tobacco-specific NNK to genotoxic products.Portions of dysplastic epithelium, tumor nests, 30% shows staining of basal and spinous epithelial layers, central tumor nests.
P450 3A4Oxidative bioactivation of NNN to genotoxic productsBasal extending into superficial spinous layers of dysplastic epithelium, tumor nests (no staining in central keratin pearls).
UGT 1A FamilyGlucuronide attachment enhances water solubility, which facilitates urinary excretionTransition in staining from negligible at dysplastic epithelium to intense at site of invasion, tumor islands (no staining in keratin pearls).
UDP-Glu-DHGenerates UGTs' cofactor, glucuronide, via oxidation of UDP-glucoseFaint staining in basal and spinous layers of dysplastic epithelium that intensifies at the site of tumor invasion, tumor islands (no staining in keratin pearls).
MRP1ATP-dependent efflux transporter that participates in the efflux of glucuronidated compoundsFaint staining in dysplastic epithelium that disappears at the site of invasion. Faint-to-moderate staining in tumor islands (no staining in keratin pearls).
BCRPATP-dependent efflux transporter that participates in the efflux of phase II enzyme conjugatesFaint staining in basal and spinous layers of dysplastic epithelium that intensifies at the site of tumor invasion, tumor islands (no staining in keratin pearls).
  • aIHC stains were conducted to determine the presence and delineate intracellular localization of these smokeless tobacco relevant metabolic enzymes in OSCC tumor tissue and overlying dysplastic surface epithelia.