Turnover of Glutathione S-Transferase alpha mRNAs is Accelerated by 12-O-Tetradecanoyl Phorbol-13-Acetate in Human Hepatoma and Colon Carcinoma Cell Lines

Abstract

The phorbol ester, 12-O -tetradecanoyl phorbol-13-acetate (TPA), known to induce murine glutathione S -transferase (GST) Ya, was examined for its effect on the expression of human GST α. Unexpectedly, 24-h treatment of the human hepatoma cell line HepG2 with 100 nmol/1 TPA caused a decrease of the GST α mRNA level to below 5% of controls, i.e. opposite to the known response in the mouse. The level of mRNA for GST Mu was also decreased, but the mRNAs of c-jun and jun-B were elevated after 2 h. The decrease of GST α mRNAs was inhibited by staurosporine, suggesting an involvement of protein kinase C. Inhibition of transcription and translation by actinomycin D and cycloheximide also partially inhibited the effect of TPA on the expression of GST α. In the presence of Actinomycin D, GST α mRNA halflife was 14.5 h, compared to 3.5 h in the presence of TPA. The calcium ionophore A23187 caused a loss of GST α mRNAs to levels almost as low as those obtained with TPA. The effects of TPA and the calcium ionophore were also observed in CaCo2 colon carcinoma cells. As a consequence of the decrease of mRNA levels, GST α protein levels and total GST enzyme activity were also diminished. Also, the morphology of the cells was changed after 3 h exposure to TPA. These data suggest that human GST α expression can be regulated at the level of mRNA stability by a pathway involving protein kinase C.