Dibutyryl-cAMP (dbcAMP) up-regulates astrocytic chloride-dependent l-[3H]glutamate transport and expression of both system xc− subunits

Recent studies have shown that N(6),2'-O-dibutyryladenosine 3':5' cyclic monophosphate (dbcAMP) increases the expression of specific subtypes of Na(+)-dependent glutamate transporters in cultured astrocytes. Our group also found that treatment of astrocytes with dbcAMP for several days increases the Na(+)-independent accumulation of L-3H]glutamate. In this study, the properties of this Na(+)-independent accumulation were characterized, and the mechanism by which dbcAMP up-regulates this process was investigated. This accumulation was markedly reduced in the absence of Cl(-) and was also inhibited by several anion-exchange inhibitors, including 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid, 4,4'-dinitrostilbene-2,2'-disulfonic acid and 4-acetamido-4'-isothiocyanatostilbene-2,2'-disulfonic acid, suggesting that this activity is mediated by a Cl(-)-dependent transporter. In addition, this activity was inhibited by micromolar concentrations of several inhibitors of another Cl(-)-dependent (Na(+)-independent) transport activity frequently referred to as system xc(-) (L-cystine, L-alpha-aminoadipate, L-homocysteate, quisqualate, beta-N-oxalyl-l-alpha,beta-diaminopropionate, ibotenate). This activity was competitively inhibited by several phenylglycine derivatives previously characterized as inhibitors of metabotropic glutamate receptor activation. The concentration-dependence for Na(+)-independent, Cl(-)-dependent L-3H]glutamate uptake activity was compared for dbcAMP-treated and untreated astrocytes. Treatment with dbcAMP increased the V(max) of this Cl(-)-dependent transport activity by sixfold but had no effect on the K(m) value. System xc(-) requires two subunits, xCT and 4F2hc/CD98, to reconstitute functional activity. We found that dbcAMP caused a twofold increase in the levels of xCT mRNA and a sevenfold increase in the levels of 4F2hc/CD98 protein. This study indicates that dbcAMP up-regulates Cl(-)-dependent L-3H]glutamate transport activity in astrocytes and suggests that this effect is related to increased expression of both subunits of system xc(-). Because this activity is thought to be important for the synthesis of glutathione and protection from oxidant injury, understanding the regulation of system xc(-) may provide alternate approaches to limit this form of injury.