Regulation of system x_c^-activity and expression in astrocytes by interleukin-1β: Implications for hypoxic neuronal injury

We recently demonstrated that interleukin-1β (IL-1β) increases system x_c^-(cystine/glutamate antiporter) activity in mixed cortical cell cultures, resulting in an increase in hypoxic neuronal injury when glutamate clearance is impaired. Herein, we demonstrate that neurons, astrocytes, and microglia all express system x_c^-subunits (xCT, 4F2hc, RBAT) and are capable of cystine import. However, IL-1β stimulation increases mRNA for Xct-the light chain that confers substrate specificity-in astrocytes only; an effect blocked by the transcriptional inhibitor actinomycin D. Additionally, only astrocytes show an increase in cystine uptake following IL-1β exposure; an effect associated with a change in xCT protein. The increase in cystine uptake that follows IL-1β is lacking in astrocytes derived from mice harboring a mutation in Slc7a11 (sut gene), which encodes for xCT, and in wild-type astrocytes treated with the protein synthesis inhibitor cycloheximide. IL-1β does not regulate the light chain of the amino acid transporter, LAT2, or the expression and function of astrocytic excitatory amino acid transporters (EAATs), demonstrating some target selectivity. Finally, the enhanced neuronal vulnerability to hypoxia that followed IL-1β treatment in our mixed culture system was not observed in chimeric cultures consisting of wild-type neurons plated on top of sut astrocytes. Nor was it observed in wild-type cultures treated with a system x_c^- inhibitor or an NMDA receptor antagonist. Overall, our data demonstrate that IL-1β selectively regulates system x_c^- activity in astrocytes and that this change is specifically responsible for the deleterious, excitotoxic effects of IL-1β found under hypoxic conditions.