Estrogen inhibits starvation-induced apoptosis in osteocytes by a redox-independent process involving association of JNK and glutathione S-transferase P1-1

Estrogen deficiency causes bone loss as a result of microdamage, oxidative stress and osteocyte apoptosis. A relationship among oxidative stress-induced apoptosis, c-Jun N-terminal kinase (JNK) activation, and expression of factors involved in bone remodeling has been demonstrated in osteocytes while the molecular regulation processes of these events in osteocytes, by treatment with 17β-Estradiol (17β-E2), remain unexplored. MLO-Y4 murine osteocyte-like cell line was used as a model to study starvation-induced apoptosis and ROS production during 17β-E2 treatment; expression of glutathione S-transferase P1-1 (GSTP1-1), receptor activator kB ligand (RANKL), osteoprotegerin (OPG), sclerostin and kinases activation were measured by Western blot; GSTP1-1/JNK association was assessed by immunoprecipitation; GSTP1-1 involvement in the osteocyte response to 17β-E2 was detected by specific siRNA transfection. 17β-E2 prevents starvation-induced apoptosis (DNA fragmentation and caspase activation), the increase of sclerostin expression and RANKL/OPG ratio which are related to JNK activation due to oxidative stress in osteocytes. This occurs through GSTP1-1 overexpression that can inhibit JNK activation by formation of GSTP1-1/JNK complex. No early antioxidant action of 17β-E2 has been found, however, estrogen effect is similar to N-acetylcysteine which, increasing the intracellular redox state, maintains JNK bound to GSTP1-1. Conclusions: It has been demonstrated that the antiapoptotic and osteogenic effect of 17β-E2 in MLO-Y4 occurs by a redox-independent process involving GSTP1-1/JNK association through GSTP1-1 overexpression and inhibition of JNK activation. General significance: This study clarifies at molecular level 17β-E2 effect on osteocyte activity and identifies a possible role of GSTP1-1 and JNK activity in bone remodeling and repair mechanisms.