Abstract
Estrogen deficiency causes bone loss as a result of microdamage, oxidative stress, and osteocyte apoptosis. A relationship between oxidative stress‐induced apoptosis, c‐Jun N‐terminal kinase (JNK) activation, and expression of factors involved in bone remodeling has been demonstrated in osteocytes. However, the molecular regulation of these events in osteocytes treated with 17β‐estradiol (17β‐E2) remains unexplored. The 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. In addition, the GSTP1‐1/JNK association was assessed by immunoprecipitation, and 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 in sclerostin expression and the RANKL/OPG ratio, which are all related to JNK activation due to oxidative stress in osteocytes. This occurs through GSTP1‐1 overexpression, which can inhibit JNK activation by formation of a GSTP1‐1/JNK complex. No early antioxidant action of 17β‐E2 has been found but the estrogen effect is similar to N‐acetylcysteine which, by increasing the intracellular redox state, maintains JNK bound to GSTP1‐1. Thus, the antiapoptotic and osteogenic effect of 17β‐E2 in MLO‐Y4 occurs by a redox‐independent process involving GSTP1‐1/JNK association. This study clarifies at molecular level the effect of 17β‐E2 on osteocyte activity and identifies a possible role of GSTP1‐1 and JNK activity in bone remodeling and repair mechanisms.
Highlights
Estrogen deficiency causes bone loss as a result of microdamage, oxidative stress, and osteocyte apoptosis
The decline in estrogen levels, associated with menopause, causes bone loss in women related to a high rate of bone remodeling [1,2] which results from the close balance between osteoblast and Abbreviations 17b-E2, 17b-Estradiol; AEBSF, 4-(2-aminoethyl)-benzenesulfonylfluoride; DPI, diphenyleneiodonium; ERK1/2, extracellular signal-regulated kinase; GSTP1-1, glutathione S-transferase P1-1; JNK, c-Jun N-terminal kinase; Mito TEMPO, 2-(2,2,6,6-Tetramethylpiperidin-1-oxyl-4ylamino)-2-oxoethyl-triphenylphosphonium chloride monohydrate; mitogen-activated protein kinase phosphatase-1 (MKP-1), protein kinase phosphatase-1; NAC, N-acetylcysteine; OPG, osteoprotegerin; RANKL, receptor activator kB ligand; ROS, reactive oxygen species; scr siRNA, scrambled siRNA
A novel and important direct involvement of GSTP1-1 in the 17bE2 effect on JNK activation and apoptosis in osteocytes has been demonstrated. 17b-E2 up-regulates GSTP1-1 expression, contributing to maintain JNK bound to GSTP1-1 and to inhibit JNK activity
Summary
Estrogen deficiency causes bone loss as a result of microdamage, oxidative stress, and osteocyte apoptosis. 17b-E2 prevents starvation-induced apoptosis (DNA fragmentation and caspase activation), the increase in sclerostin expression and the RANKL/OPG ratio, which are all related to JNK activation due to oxidative stress in osteocytes. The decline in estrogen levels, associated with menopause, causes bone loss in women related to a high rate of bone remodeling [1,2] which results from the close balance between osteoblast and Abbreviations 17b-E2, 17b-Estradiol; AEBSF, 4-(2-aminoethyl)-benzenesulfonylfluoride; DPI, diphenyleneiodonium; ERK1/2, extracellular signal-regulated kinase; GSTP1-1, glutathione S-transferase P1-1; JNK, c-Jun N-terminal kinase; Mito TEMPO, 2-(2,2,6,6-Tetramethylpiperidin-1-oxyl-4ylamino)-2-oxoethyl-triphenylphosphonium chloride monohydrate; MKP-1, protein kinase phosphatase-1; NAC, N-acetylcysteine; OPG, osteoprotegerin; RANKL, receptor activator kB ligand; ROS, reactive oxygen species; scr siRNA, scrambled siRNA. To our knowledge in osteocytes, no data are reported on the molecular processes by which estrogens modulate the expression of cytokines involved in bone remodeling in the presence of oxidative stress-induced apoptosis
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