Protein kinase C (PKC) activation stimulates transport system X-(AG) for anionic amino acids in cultured human fibroblasts (Franchi-Gazzola, R., Visigalli, R., Bussolati, O., and Gazzola, G. C. (1994) FEBS Lett. 352, 109- 112). To identify which PKC isoform is responsible for this effect, aspartate transport through system X-(AG), PKC activity, and the subcellular distribution of PKC isoforms have been studied before and after treatment with phorbol 12,13-dibutyrate (PDBu) in fibroblasts maintained at low serum for 1 (control cells) or 7 days (quiescent cells). In control cells aspartate transport and PKC activity in the particulate fraction were stimulated by short term PDBu treatment; both stimulatory effects were down-regulated by a prolonged exposure to the phorbol. In contrast, in quiescent cells aspartate transport and particulate PKC activity were higher than control under basal conditions, unaffected by a short term PDBu treatment, and lowered by a prolonged incubation with the phorbol. In both control and quiescent cells a short term PDBu treatment modified PKCα distribution, increasing its membrane-associated fraction. PKCδ was mostly in the soluble fraction and scarcely sensitive to PDBu. A brief exposure to PDBu increased membrane- associated PKCε in control but not in quiescent cells. In these cells ε isoform was found exclusively in the particulate fraction even in PDBu- untreated cells. A prolonged PDBu treatment caused a partial down-regulation of membrane-associated PKCε in control cells and its marked decrease in quiescent cells. It is concluded that PKC-dependent changes in system X(AG)- activity parallel the behavior of PKCε, thus suggesting a specific role for this isoform in system X(AG)- regulation.
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