Differential modulation of the glutamate transporters GLT1, GLAST and EAAC1 by docosahexaenoic acid

At present, the ability of polyunsaturated fatty acids (PUFAs) to regulate individual glutamate transporter subtypes is poorly understood and very little information exists on the mechanism(s) by which PUFAs achieve their effects on the transport process. Here we investigate the effect of cis-4,7,10,13,16,19-docosahexaenoic acid (DHA) on the activity of the mammalian glutamate transporter subtypes, GLT1, GLAST and EAAC1 individually expressed in human embryonic kidney (HEK) cells. Exposure of cells to 100 μM DHA increased the rate of d-[ 3H]aspartate uptake by over 72% of control in HEK GLT1 cells, and by 45% of control in HEK EAAC1 cells. In contrast, exposure of HEK GLAST cells to 200 μM DHA resulted in almost 40% inhibition of d-[ 3H]aspartate transport. Removal of extracellular calcium increased the inhibitory potential of DHA in HEK GLAST cells. In contrast, in the absence of extracellular calcium, the stimulatory effect of DHA on d-[ 3H]aspartate uptake in HEK GLT1 and HEK EAAC1 cells was abolished, and significant inhibition of the transport process by DHA was observed. Inhibition of CaM kinase II or PKC had no effect on the ability of DHA to inhibit transport into HEK GLAST cells but abolished the stimulatory effect of DHA on d-[ 3H]aspartate transport into HEK GLT1 and HEK EAAC1 cells. Inhibition of PKA had no effect on the modulation of d-[ 3H]aspartate transport by DHA in any of the cell lines. We conclude that DHA differentially modulates the GLT1, GLAST and EAAC1 glutamate transporter subtypes via different mechanisms. In the case of GLT1 and EAAC1, DHA appears to stimulate d-[ 3H]aspartate uptake via a mechanism requiring extracellular calcium and involving CaM kinase II and PKC, but not PKA. In contrast, the inhibitory effect of DHA on GLAST does not require extracellular calcium and does not involve CaM kinase II, PKC or PKA.