Age affects ERK1/2 and NRF2 signaling in the regulation of GCLC expression

We previously reported that activator protein-1 (AP-1) DNA binding activity was increased in vascular smooth muscle cells (VSMC) from old rats when exposed to high glucose or tumor necrosis factor (TNF-alpha) (Li et al., 2003. J Cell Physiol 197:418-425). We have now examined the relationship between the age-dependent activation of the ERK1/2-AP-1 pathway and modulation of constitutive gene expression of the catalytic subunit of glutamate-cysteine ligase (GCLC) in response to high glucose and TNF-alpha. GCLC mRNA levels were higher in VSMC from old rats compared to young, a pattern consistent with its protein levels. To determine whether age-related activation of ERK1/2-AP-1 signaling is responsible for the up-regulation of GCLC, the MEK inhibitors, PD98059 and U0126, were used to block ERK1/2 in VSMC from old rats. An increase in GCLC with inhibitors was observed, diminishing the likelihood of ERK1/2-AP-1 activation as the up-regulating signal for GCLC. However, the transcription factor Nrf2 was higher in nuclei and accompanied by increased Nrf2-ARE binding in VSMC from old rats. Furthermore, MEK inhibitors increased nuclear Nrf2 and Nrf2/ARE binding. These data suggest opposing effects of Nrf2 and ERK1/2 signaling in the modulation of GCLC expression in old animals.