Regional differential effects of hormone-replacement treatments on mitochondrial DNA repair mechanism are not exerted through gene expression regulation in the brain

Abstract

The brain is highly susceptible to mitochondria dysfunction and oxidative stress due to its high demand of energy and low antioxidant capacity. Mitochondrial DNA (mtDNA) is specially vulnerable to oxidative damage and Base Excision Repair (BER) is the main mtDNA repair mechanism. Ovarian hormone loss during natural or induced reproductive senescence is associated with mitochondrial alterations, synaptic decline and increased risk of age-related diseases.The aim of this work was to assess whether hormone-replacement treatments affect the expression of BER enzymes to explain previous results regarding the differential activity of such enzymes in the hippocampus (Hp) and cerebral cortex (Cc) of hormone-treated ovariectomized (OVX) rats. To this aim, adult OVX or sham-operated (SHAM) rats were s.c. with empty or containing estradiol (E) and/or progesterone (P) silastic capsules. After 12 weeks, cDNA was obtained from total RNA extracted from the Hp and Cc and amplified by qPCR using specific primers for BER enzymes.The expression of DNA glycosylases was either lower o similar to SHAM group in both Hp and Cc of OVX rats (NEIL1 p<0,01; NEIL2 p<0,05; UNG1 p<0,01; OGG1 ns; Student´s t test). Similar results were obtained for the rest of the enzymes of the pathway (AP endonuclease1, γ-polymerase and lygase3 p<0,05, Student´s t test). Hormone treatments did not affect the expression of OGG1, NEIL2 or UNG1 in any brain region, but increased the expression of NEIL1 only in the Hp (E+P p<0,05; ANOVA). On the other hand, P alone or combined with E, increased the expression of the rest of the enzymes of the pathway in both brain regions (p<0,05; ANOVA).Our results show that OVX decreases the expression of BER enzymes in both brain regions and that there is no association between ARNm levels and the activity of such enzymes in hormone-treated rats. Thus, hormones exert their regional differential action on BER pathway through a mechanism not involving gene regulation.