Transactivation of phase II detoxification enzymes and antioxidant proteins is mediated by the Cap'NCollar transcription factor, Nrf2, which is sequestered in the cytoplasm by the actin-binding protein Keap1. Mutation of a conserved serine (S104A) within the Keap1 BTB/POZ domain disrupts Keap1 dimerization and eliminates the ability of Keap1 to sequester Nrf2 in the cytoplasm and repress Nrf2 transactivation. Disruption of endogenous Keap1 dimerization using BTB/POZ dominant negative proteins also inhibits the ability of Keap1 to retain Nrf2 in the cytoplasm. Exposure to an electrophilic agent that induces Nrf2 release and nuclear translocation disrupts formation of a Keap1 complex in vivo. Collectively, these data support the conclusion that Keap1 dimerization is required for Nrf2 sequestration and transcriptional repression. Furthermore, exposure to inducing agents disrupts the Keap1 dimerization function and results in Nrf2 release.

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Pyrrolidine dithiocarbamate (PDTC) induction of the human glutamate cysteine ligase modulatory (GCLM) gene is dependent on activation of the mitogen-activated protein kinases (MAPKs) extracellular regulated kinase (Erk) and p38, and is not affected by protein kinase C (PKC) or PI3K inhibitors. Nrf2 binding to the electrophile response element (EpRE) located within the GCLM promoter is decreased after MAPK inhibition, suggesting that Nrf2 could be a downstream target of activated MAPK. To evaluate this hypothesis, a series of Nrf2 proteins harboring mutations in conserved consensus MAPK phosphorylation sites were developed and used in multiple functional assays. All mutated Nrf2 proteins tested interacted with the cytoplasmic repressor Keap1 in a manner indistinguishable from wild-type Nrf2. Furthermore, the mutant and wild-type Nrf2 proteins were similarly capable of transactivating an EpRE-containing GCLM/luciferase reporter transgene. Collectively these functional assays suggest that Nrf2 is not likely to be a direct downstream target of activated MAPK in vivo. However, treatment of HepG2 cells with MAPK inhibitors PD98059 and/or SB202190 prior to exposure to PDTC, reduced Nrf2 translocation to the nucleus, suggesting that MAPK-directed phosphorylation is a requirement for nuclear localization during PDTC induction of GCLM gene expression.

Variants of the Escherichia coli 1-lip pyruvate dehydrogenase multienzyme complex (1-lip PDHc) with the C259N and C259S substitutions in the putative thiamin diphosphate-(ThDP-) binding motif of the pyruvate dehydrogenase component (E1, EC 1.2.4.1) were characterized. Single substitutions were made at the five remaining cysteines of the E1 component, creating the C120A, C575A, C610A, C654A, and C770S variants to test the hypothesis that the activity loss that accompanies exposure of the enzyme to fluoropyruvate, bromopyruvate, and 2-oxo-3-butynoic acid is the result of the modification of approximately one cysteine residue per E1 monomer. Surprisingly, all single cysteine E1 variants could be reconstituted with E2-E3 subcomplex and showed PDHc activity ranging from 74% to 96% that of the parental enzyme. The specific activities of C259N and C259S variants of 1-lip PDHc were 58% and 27% relative to that of the parental 1-lip PDHc. All five single cysteine E1 variants, along with the C259N and C259S variants of 1-lip PDHc, could also (1) be inactivated with fluoropyruvate and 2-oxo-3-butynoic acid, (2) were subject to inactivation by the monoclonal antibody 18A9 reported from one of our laboratories, and (3) were subject to regulation by pyruvate and acetyl-CoA. It was therefore concluded that none of the six cysteine residues is essential for the activity of the E1 component or of the complex. When tested with the putative transition-state analogue, thiamin 2-thiothiazolone diphosphate, all but the C259S and C259N variants were very potently inhibited, the stoichiometry for parental E1 being about 1.6 mol of inhibitor/mol of E1 subunit. The C259S and C259N E1 variants required at least 25-fold greater inhibitor concentration to achieve the same level of inhibition. C259 is located in the putative thiamin diphosphate-binding motif of the enzyme [more exactly, it is adjacent to a ligand to the Mg(II) ion]. It is therefore concluded that thiamin 2-thiothiazolone diphosphate is not a transition-state analogue; rather, it is a potent inhibitor of the complex because of a specific interaction with the C259 residue.