Received for publication November 22, 2004.
Revised March 14, 2005.
Accepted for publication March 17, 2005.

Evidence for Biphasic Effects of Protein Kinase C on Serotonin Transporter Function, Endocytosis, and Phosphorylation

Abstract

The serotonin transporter (SERT) regulates serotonin (5-HT) neurotransmission and is a high affinity target for antidepressants and psychostimulants. In the present study, we investigated the mechanisms that contribute to a previously unidentified biphasic regulation of endogenous SERTs expressed in the platelets. Treatment of rat platelets with -PMA for 5 min or less resulted in a rapid inhibition of SERT involving changes in intrinsic activity of the transporter (increased Km and decreased Vmax). -PMA-treatment for 30 min or more produced a sustained inhibition of SERT with a decrease only in the Vmax. While inhibition of SERT activity was detected from 1 to 45 min following phorbol ester addition, the decrease in surface SERT required at least 30 min of phorbol ester incubation. Increased endocytosis of SERT accounted for the decrease in surface SERT at the later point. PKC-mediated phosphorylation of SERT occurs on the plasma membrane during the initial phase of rapid transporter inhibition, and later on, the phosphorylated SERT enters the intracellular pool. -PMA-induced phosphorylation of SERT occurs initially on serine residues(s) and then, on threonine residue(s). The initial serine phosphorylation corresponded to the first phase of rapid inhibition mediated by changes in intrinsic activity and/or silencing of SERT. The later phosphorylation on threonine residue(s) corresponded to the later phase of sustained inhibition mediated by an enhanced endocytosis of SERT. Together, these data reveal that in platelets, SERT function is regulated by PKC in a biphasic manner involving both trafficking-dependent and independent mechanisms, and that these two events occur at distinct phases of transporter phosphorylation.

Key words: Serotonin, Protein Kinase C, Protein Phosphatases (other), Biogenic Amine, Regulation - post-transcriptional, Amphetamines, Anti-depressants, Cocaine, Platelets

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