Raf-1 is dually down regulated by p,p′-DDE via reduced thyroid hormone and activated ERK

Abstract

Raf-1 is the best characterized member of the mammalian raf family. The raf-1 kinase has many important physiological functions but its regulation is complex. 1,1-Dichloro-2,2 bis(p-chlorophenyl) ethylene (p,p′-DDE), a major and important metabolite of organochlorine pesticide DDT, is a widespread environmental pollutant. Previous studies have reported that p,p′-DDE influences the MAPK pathway, but little details are known regarding how the raf-1 activity is influenced and regulated by p,p′-DDE in those events. And here, we used 20 male rats that received different doses of p,p′-DDE (0, 20, 60, 100mg/kg b.wt) every other day by intraperitoneal injection for 10days. Results showed that the raf-1 mRNA and protein expressions showed significant down-regulation. The thyroxine (T4) level and the protein kinase C alpha (PKCα) mRNA expression declined, but the raf kinase inhibitory protein (RKIP) up regulated. In addition, the p,p′-DDE-induced oxidative stress resulted in declines of glutathione peroxidase (GSH-PX) and superoxide dismutase (SOD) activities. The elevations of ERK mRNA level and phosphorylated ERK protein expression were also observed. Taken together, these results suggest that the raf-1 is dually down regulated by p,p′-DDE via two pathways. One pathway is that the p,p′-DDE exposure elevates RKIP level by decreasing T4 and PKCα level, and the increased RKIP eventually down regulates the raf-1. The other is that the p,p′-DDE exposure activates the ERK by oxidative stress, and the activated ERK down regulates the raf-1 through the negative feedback phosphorylation.