Received for publication June 23, 2005.
Revised October 24, 2005.
Accepted for publication October 24, 2005.

Delta Opioid Receptors Stimulate Akt-Dependent Phosphorylation of c-jun in T-cells

Abstract

Activation of naïve T-cells markedly up-regulates the expression of delta opioid receptors (DORs). These receptors are bound by DOR peptides released by T-cells, modulating T-cell functions such as interleukin-2 production, cellular proliferation and chemotaxis. Previous studies have shown that DOR agonists (e.g., [D-Ala₂-D-Leu₅]-enkephalin; DADLE) modulate T-cell antigen receptor signaling through mitogen-activated protein kinases (MAPKs; i.e., ERKs 1,2), and that DORs directly induce phosphorylation of activating transcription factor-2 (implicated in cytokine gene transcription) and its association with the MAPK, c-jun NH2-terminal kinase (JNK). Such observations suggest that DORs may induce the phosphorylation of c-jun. These experiments were performed to test this hypothesis and determine the potential roles of phosphoinositide 3-kinase (PI3K) and Akt (protein kinase B). DADLE (10^-10-10^-6M) dose-dependently induced c-jun phosphorylation. This was blocked by pertussin toxin and the DOR-specific antagonist, naltindole. Fluorescence flow cytometry showed that DADLE significantly stimulated c-jun phosphorylation by T-cells. DADLE stimulated phosphorylation of membrane-associated Akt; wortmannin and LY294002, specific inhibitors of PI3K, abolished the DADLE-induced phosphorylation of c-jun. Finally, inhibitors of Akt and JNK blocked DADLE-induced phosphorylation of c-jun. Thus, activated DORs directly stimulate c-jun phosphorylation through a PI3K-dependent pathway in T-cells, apparently involving Akt. This implies that DORs activate JNK through a novel pathway dependent on PI3K and Akt, thereby regulating the function of AP-1 transcription complexes containing c-jun and other transcription partners.

Key words: Akt, JNK, T-cell, c-jun, delta opioid receptor, phosphorylation