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Abstract: Data on the biologic activity of protocatechuic acid are contradictory; some studies have shown that it acts as an antioxidant and suppresses chemical-induced carcinogenesis and others that it induces oxidative stress and promotes tumour formation. The anticarcinogenicity of protocatechuic acid was postulated to be related, in part, to its specific suppression of neoplastic hyperproliferation. To determine whether protocatechuic acid was preferentially antiproliferative to malignant cells, non-malignant and carcinoma cells were exposed for 24 hr to protocatechuic acid (2.5 to 25 mM) and viability was assessed with the neutral red assay. The cell lines were derived from tissues of the human oral cavity, the initial site of exposure upon ingestion of dietary protocatechuic acid, and included normal GN61 gingival fibroblasts, immortalized, non-tumorigenic S-G gingival epithelial cells, and malignant HSG1 cells derived from the salivary gland, HSC-2 cells from the floor of the oral cavity, and CAL27 cells from the tongue. Selective toxicity of protocatechuic acid to malignant cells was not observed. Furthermore, using a total cellular protein determination to quantitate cell growth, no differences in comparative sensitivities of S-G epithelial cells and HSG1 carcinoma cells were noted in a 3 day continuous exposure to 2.5 to 12.5 mM protocatechuic acid and in recovery from a 24 hr exposure to 3 to 15 mM protocatechuic acid. The S-G and HSG1 cells were then used to study the effects of elevated concentrations of protocatechuic acid on oxidative stress. For both cell types, protocatechuic acid induced oxidative stress, presumably through its bioactivation by a tyrosinase pathway. A brief exposure to 25 mM protocatechuic acid lowered the levels of intracellular glutathione and potentiated Fe2+-induced lipid peroxidation of the cells. As determined with the neutral red assay, S-G and HSG1 cells exposed briefly to a non-toxic level (0.5 mM) of the glutathione depleter, 1,3-bis(2-chloroethyl)-N-nitrosourea, were hypersensitive to a subsequent challenge with 10 mM protocatechuic acid and preexposure of the S-G and HSG1 cells to a nontoxic level of protocatechuic acid (2.5 mM) enhanced their sensitivity to a subsequent exposure to tert-butyl hydroperoxide. These findings were consistent with protocatechuic acid, at high levels (≥10 mM), acting as an inducer of oxidative stress.