Estrogen (17β-Estradiol) Enhances Glutamine Synthetase Activity in C6-Glioma Cells

Abstract

Glutamine synthetase (GS) is the major glutamine-forming enzyme of vertebrates and is accepted to be a marker of astroglial cells. Maturation of astroglial cells is characterized by an increase of GS activity, and the regulation of this enzyme is the topic of many publications. Because of the fundamental role of the GS in controlling brain glutamate and glutamine level, it is essential to understand the mechanism of expression of this enzyme. To our knowledge, the effect of estrogen (17beta-estradiol) on GS activity in glial cells has not been reported. We examined the effect of treatment with estrogen on glutamine synthetase enzyme activity in glial cells. C6-glioma cells in later passage have many astrocytic characteristics and provided a convenient and well-established model system. We adapted a colorimetric method to measure GS-catalyzed gamma-glutamyltransferase (GT) activity in C6-glioma cells. The assay monitors GT activity of glutamine synthetase by following the absorbance of the product gamma-glutamyl hydroxamate at 540 nm. We observed that, the absorbance of gamma-glutamyl hydroxamate significantly increased in estrogen treated cells (0.13 +/- 0.03), as compared to untreated cells (0.058 +/- 0.015). Estrogen also significantly increased concentration of glutamine in C6-glioma cells as measured by fluorometric assay. In addition, western blot analysis showed that estrogen significantly increased the amount of glutamine synthetase compared to control. This estrogen effect could have important physiological implications on cerebral glutamate and glutamine metabolism.