Ectoprotein kinase-mediated phosphorylation of FAT/CD36 regulates palmitate uptake by human platelets.

Glycoprotein IV (FAT/CD36) has been shown to be phosphorylated by a cAMP-dependent, platelet membrane-bound ectokinase. In this study, we demonstrate that ectophosphorylation of FAT/CD36 regulates initial palmitate uptake. This is the first time that short-term regulation of the activity of a long-chain fatty acid carrier could be shown. Phosphorylation of FAT/CD36 was paralleled by a significant decrease in initial palmitate uptake by morphologically and functionally intact platelets. Maximum inhibition of palmitate uptake was achieved at 0.5 nM extracellular ATP, being significantly decreased to 72% compared to the control. Inhibition of palmitate uptake was abolished by co-incubation with the specific protein kinase A inhibitor peptide PKI or with beta,gamma-methylene-ATP, and was reversible upon addition of alkaline phosphatase. An extracellular ATP concentration above 5 microM completely prevented the ectophosphorylation-mediated inhibition of palmitate uptake. We conclude that FAT/CD36-mediated palmitate uptake by human platelets is short-term regulated via cAMP-dependent ectophosphorylation of FAT/CD36.