Regulatory Mechanisms Involved in Activator‐Protein‐1 (AP‐1)‐Mediated Activation of Glutathione‐S‐Transferase Gene Expression by Chemical Agents

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Abstract
Induction of murine glutathione-S-transferase (GST) Ya gene expression by a variety of chemical agents is mediated by a regulatory element, EpRE, composed of an Ets and two adjacent activator protein-1 (AP-1)-like sites and activated by the Fos/Jun heterodimeric complex (AP-1). The mechanism of this induction was examined in the present study. We find that the regulation of EpRE-mediated GST Ya gene expression by 3-methylcholanthrene, tert-butylhydroquinone and beta-naphthoflavone is associated with an induction of AP-1 DNA-binding activity and that the AP-1 complex induced in hepatoma cells by these chemicals contains members of the Fos and Jun protein families. We show that tert-butylhydroquinone induces c-fos gene expression and indicate the formation of a transcriptionally active AP-1 complex that contains Fos/Jun heterodimer. In F9 cells, which are considered to lack AP-1 complex, a careful examination reveals that tert-butylhydroquinone induces a low level of an AP-1-related activity responsible for the enhanced expression of EpRE as well as of AP-1 reporter constructs. We find that protein phosphorylations mediate the activation of the GST Ya gene by chemical agents since okadaic acid, an inhibitor of protein phosphatases, can mimic this activation while protein kinase inhibitors abolish it. Evidence is presented that 3-methylcholanthrene, tert-butylhydroquinone and beta-naphthoflavone use a signal transduction pathway to Fos/Jun-dependent GST Ya gene expression via Ras and protein-tyrosine kinase activity. Furthermore, we find that activation by phorbol 12-myristate 13-acetate, which uses both protein kinase C and protein-tyrosine kinase activities, may share a common pathway with these chemicals downstream of Ras.