The transport activity of the human cationic amino acid transporter hCAT‐1 is downregulated by activation of protein kinase C

The human cationic amino acid transporter hCAT‐1 contains several consensus sequences for phosphorylation by protein kinase C (PKC). This study investigates the effect of PKC activation on hCAT‐1‐mediated transport. When expressed in Xenopus laevis oocytes, hCAT‐1‐mediated L ‐arginine transport was reduced to 44±3% after a 30 min treatment of the oocytes with 100 n M phorbol‐12‐myristate‐13‐acetate (PMA). 4α‐phorbol‐12,13‐didecanoate (4α‐PDD, 100 n M ) had no effect. In EA.hy926 endothelial cells, maximal inhibition of hCAT‐1‐mediated L ‐arginine transport (to 3 – 11% of control) was observed after treatment of the cells with 100 n M PMA for 4 h. A 20 – 30 h exposure of the cells to 100 n M PMA led to the recovery of the L ‐arginine uptake rate that was now resistant to a second application of PMA. Phorbol‐12,13‐dibutyrate had similar effects as PMA, whereas 4α‐PDD had no effect. One μ M bisindolylmaleimide I reduced the PMA effect significantly. Interestingly, a 4 h treatment with 100 n M PMA increased the expression of hCAT‐1 mRNA 3 – 5 fold. hCAT‐1 protein levels were unchanged for up to 4 h after PMA treatment and then increased slightly between 8 – 28 h. It is concluded that PMA downregulates the intrinsic activity of hCAT‐1 by a pathway involving protein kinase C.British Journal of Pharmacology (2001) 132 , 1193–1200; doi: 10.1038/sj.bjp.0703921