Glutathione S-transferase (GST) play significant role in the metabolism and detoxification of drugs used in treatment of melanoma, resulting in a decrease in their efficacy against melanoma. In the present study, we investigated: i) the selective inhibition of GST by luteolin in the presence of tyrosinase, an abundant enzyme found in melanoma, and ii) the induction of apoptosis, change in mitochondrial permeability, cell viability, cell migration and GSH depletion by luteolin in human melanoma SK-MEL-28 cell culture. The inhibition of GST was investigated using CDNB method. DTNB method was used to measure GSH depletion. MTT assay, Annexin V apoptosis assay and TMRM fluorescence were used in the investigation of biochemical mechanism of luteolin toxicity in melanoma cells. Luteolin (40 µM) induced toxicity and death in 50% of cells after 48 h incubation. Cell treatment with luteolin (40 µM) resulted in apoptotic cell death in 70% of cells and caused 45% decrease in TMRM fluorescence indicating change in mitochondrial membrane permeability. At 1 h incubation, luteolin was bioactivated by tyrosinase to luteoline-quinone for 71% as measured by GSH depletion. The luteolin is effective in preventing metastasis of cancerous cells as evident by cell migration assay. At 48 h incubation, luteolin as low as 5 µM effectively prevented the cell migration. In the presence of tyrosinase, luteolin (10 µM) showed about 87% GST inhibition; whereas in the absence of tyrosinase, luteolin led to negligible GST inhibition. Both luteolin-SG conjugate and luteolin-quinone (10 µM) inhibited ≥90% of GST activity via reversible and irreversible competitive mechanisms with Ki of 0.9 and 6.4 µM with respect to GSH, respectively. The luteolin-SG conjugate inhibited GST activity reversibly and non-competitively with Ki of 1.8, whereas luteolin-quinone showed irreversible competitive inhibition of GST activity with Ki of 1.0 µM with respect to CDNB. While luteolin is not a substrate for GST, Luteolin (25 µM) non-competitively inhibited GST with Ki of 39 µM with respect to GSH and competitively with Ki of 61 µM with respect to CDNB. Luteolin, at the concentration range of 5-80 µM, exhibited 78-99% GST inhibition in human SK-MEL-28 cell homogenate. In summary, our results suggest that luteolin was bioactivated by tyrosinase to form a quinone and luteolin-glutathione conjugate, which played major role in the inhibition of GST. For the first time, we demonstrate that luteolin acts as a selective inhibitor of GST in the presence of tyrosinase.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 1345. doi:10.1158/1538-7445.AM2011-1345