17β-Estradiol – A New Modulator of Neuroglobin Levels in Neurons: Role in Neuroprotection against H₂O₂-Induced Toxicity

Abstract

Although discovered in 2000, neuroglobin (Ngb) functions are still uncertain. A contribution to the role played by Ngb in neurons could certainly derive from the identification of Ngb endogenous modulators. Here, we evaluate the possibility that Ngb could be regulated by 17β-estradiol (E₂) signaling in both SK-N-BE human neuroblastoma cell line and mouse hippocampal neurons. 1 nM E₂ rapidly induced a 300% increase in Ngb levels in both models. The E₂ effect was specific, being not induced by testosterone or dihydrotestosterone. The E₂-induced Ngb increase requires estrogen receptor (ER) β, but not ERα, as evaluated by the mimetic effect of ERβ-specific agonist DPN and by the blockage of E₂ effect in ERβ-silenced SK-N-BE cells. Furthermore, both rapid (15 min) ERβ-dependent activation of p38/MAPK and transcriptional ERβ activity were required for the estrogenic regulation of Ngb. Finally, E₂ exerted a protective effect against H₂O₂-induced neuroblastoma cell death which was completely prevented in Ngb-silenced cells. Overall, these data suggest that Ngb is part of the E₂ signaling mechanism that is activated to exert protective effects against H₂O₂-induced neurotoxicity.

Neuroglobin, estrogens, and neuroprotection

“…These results represent the first evidence for steroid hormone modulation of globin levels in cells (104). Recently, it has been reported that Hb is specifically expressed in neurons, its expression being upregulated by erythropoietin and accompanied by enhanced brain oxygenation under physiologic and hypoxic conditions (33).…”

“…Although it is possible that the experimental models we used (i.e., neuroblastoma cell line and embryonic mouse hippocampal neurons) do not express P450-aromatase causing the inability of testosterone to modulate Ngb levels, the reported results firmly indicate Ngb as a new E2 target that should be added to the variety of E2-specific actions in the brain (104).…”

“…Thus, although Ngb promoter does not contain any canonical ERE, it is not surprising that the transcription inhibitor actinomycin completely prevents the increase of the E2-induced Ngb levels. However, the sequence analysis of Ngb promoter indicates that several noncanonical half ERE sites are present along with responsive element for other transcription factors (104). These results suggest that the E2-induced Ngb transcription could be mediated by tethered interactions of ER with other transcription factors to activate gene expression (i.e., indirect genomic mechanism) (96).…”

“…The cell pretreatment with physiological E2 concentration decreases cell death and reduced caspase-3 activation triggers by exposure to H 2 O 2 (104). This E2 effect against H 2 O 2 toxicity requires ERb and Ngb being prevented by the ERb inhibitor (R,R)-5,11-diethyl-5,6,11,12-tetrahydro-2,8-chrysenediol and by knocking out Ngb using small interfering RNA (104).…”

ERβ-dependent neuroglobin up-regulation impairs 17β-estradiol-induced apoptosis in DLD-1 colon cancer cells upon oxidative stress injury

“…These somewhat 13 contrasting and tissue specific estrogen actions are guaranteed by 14 the differential expression of the two estrogen receptor subtypes 15 (i.e., ERa and ERb) which define the different biological cellular 16 response to hormones [2,43]. In particular, the proliferative and 17 anti-apoptotic effect of 17b-estradiol (E2) in cancer cells is 18 primarily mediated by the ERa activation, whereas ERb seems 19 to oppose ERa actions activating pro-apoptotic cascade 20 [1,26,41,54].…”