Protein kinase C restricts transport of carnitine by amino acid transporter ATB0,+ apically localized in the blood–brain barrier

Carnitine (3-hydroxy-4-trimethylammoniobutyrate) is necessary for transfer of fatty acids through the inner mitochondrial membrane. Carnitine, not synthesized in the brain, is delivered there through the strongly polarized blood–brain barrier (BBB). Expression and presence of two carnitine transporters – organic cation/carnitine transporter (OCTN2) and amino acid transporter B0,+ (ATB0,+) have been demonstrated previously in an in vitro model of the BBB. Due to potential protein kinase C (PKC) phosphorylation sites within ATB0,+ sequence, the present study verified effects of this kinase on transporter function and localization in the BBB. ATB0,+ can be regulated by estrogen receptor α and up-regulated in vitro, therefore its presence in vivo was verified with the transmission electron microscopy. The analyses of brain slices demonstrated ATB0,+ luminal localization in brain capillaries, confirmed by biotinylation experiments in an in vitro model of the BBB. Brain capillary endothelial cells were shown to control carnitine gradient. ATB0,+ was phosphorylated by PKC, what correlated with inhibition of carnitine transport. PKC activation did not change the amount of ATB0,+ present in the apical membrane of brain endothelial cells, but resulted in transporter exclusion from raft microdomains. ATB0,+ inactivation by a lateral movement in plasma membrane after transporter phosphorylation has been postulated.