Transforming growth factor‐α mediates estrogen‐induced upregulation of glutamate transporter GLT‐1 in rat primary astrocytes

Abstract

Glutamate transporter-1 (GLT-1) plays a central role in preventing excitotoxicity by removing excess glutamate from the synaptic clefts. 17β-estradiol (E2) and tamoxifen (TX), a selective estrogen receptor modulator (SERM), afford neuroprotection in a range of experimental models. However, the mechanisms that mediate E2 and TX neuroprotection have yet to be elucidated. We tested the hypothesis that E2 and TX enhance GLT-1 function by increasing transforming growth factor (TGF)-α expression and thus, attenuate manganese (Mn)-induced impairment in astrocytic GLT-1 expression and glutamate uptake in rat neonatal primary astrocytes. The results showed that E2 (10 nM) and TX (1 μM) increased GLT-1 expression and reversed the Mn-induced reduction in GLT-1, both at the mRNA and protein levels. E2/TX also concomitantly reversed the Mn-induced inhibition of astrocytic glutamate uptake. E2/TX activated the GLT-1 promoter and attenuated the Mn-induced repression of the GLT-1 promoter in astrocytes. TGF-α knock-down (siRNA) abolished the E2/TX effect on GLT-1 expression, and inhibition of epidermal growth factor receptor (TGF-α receptor) suppressed the effect of E2/TX on GLT-1 expression and GLT-1 promoter activity. E2/TX also increased TGF-α mRNA and protein levels with a concomitant increase in astrocytic glutamate uptake. All estrogen receptors (ERs: ER-α ER-β and GPR30) were involved in mediating E2 effects on the regulation of TGF-α, GLT-1, and glutamate uptake. These results indicate that E2/TX increase GLT-1 expression in astrocytes via TGF-α signaling, thus offering an important putative target for the development of novel therapeutics for neurological disorders.

cAMP Response Element-binding Protein (CREB) and Nuclear Factor κB Mediate the Tamoxifen-induced Up-regulation of Glutamate Transporter 1 (GLT-1) in Rat Astrocytes

“…1B) and TGF-␣ mRNA (Fig. 1C) levels, showing a significant increase within 4 h of incubation, in agreement with our previous findings (26). Analogous to the TX-induced GLT-1 protein increase, GLT-1 mRNA was also continuously increased up to 6 days post-treatment (data not shown).…”

“…We have shown previously that TX enhanced expression of GLT-1 mRNA and protein in astrocytes that was mediated by TGF-␣ (26). Given the mediating role of TGF-␣ in TX-induced GLT-1 expression, we observed that TX increased GLT-1 mRNA levels before the TGF-␣ mRNA increase (30 min versus 1 h).…”

“…Similarly, estrogen increases GLT-1 and glutamate aspartate transporter expression, thereby enhancing astrocytic glutamate uptake (31,32). Our previous study established that both estrogen-and TX-induced astrocytic GLT-1 expression was mediated by TGF-␣ signaling upon 24 h exposure (26). Given that EGF or neuron-dependent activation require a prolonged period of time to induce GLT-1 expression (33), this study assessed the effects of TX on astrocytic GLT-1 protein levels after 6-day treatment.…”

“…Both EGF and TGF-␣ induce GLT-1 expression via NF-B activation in astrocytes (25). In our previous study, we showed that estrogen and TX induced up-regulation of GLT-1 via TGF-␣ (26). To further characterize the signaling pathways and molecular mechanisms involved in TX-induced up-regulation of GLT-1, we investigated the role of cAMP response element-binding protein (CREB) and NF-B in TX-induced GLT-1 up-regulation.…”

Yin Yang 1 Is a Repressor of Glutamate Transporter EAAT2, and It Mediates Manganese-Induced Decrease of EAAT2 Expression in Astrocytes

“…1C 1 ). Our earlier studies established that Mn reduced glutamate uptake by decreasing GLAST and GLT-1 mRNA and protein levels (13,23,24). Herein, we corroborated the Mn-induced decrease in astrocytic EAAT2 promoter activity in a time-dependent manner (Fig.…”