Received for publication February 5, 2008.
Revised April 28, 2008.
Accepted for publication May 7, 2008.

T cell receptor/CD28-mediated activation of human T lymphocytes induces expression of functional µ-opioid receptors

Abstract

Opiates function as immunomodulators, partly by effects on T cells. Opioids act via µ-, - and -opioid receptors, among which the µ-type is of particular interest, since morphine-like opioids preferentially bind to it. Here we report that µ-opioid receptor mRNA was induced after CD3/28-mediated activation of primary human T lymphocytes and Jurkat T cells, which both do not express the gene constitutively. Moreover, a reporter gene construct containing 2624 bp of the µ-opioid receptor promoter was trans-activated by CD3/28-stimulation. Transcriptional induction of the µ-opioid receptor gene was mediated by AP-1, NF-B and NFAT. NFAT was found to bind to three sequences of the µ-opioid receptor promoter, located at nucleotides -1064, -785 and -486. Although the -486 element is in close proximity to a putative AP-1 site, there was no evidence for a combined AP-1/NFAT site. Furthermore, we demonstrated that the induction of interleukin-2 mRNA and protein in activated T cells was inhibited by morphine in cells, in which µ-opioid receptors had been induced by CD3/28 mAbs and that this effect was blocked by the µ-opioid receptor-specific antagonist D-Phe-Cys-Tyr-D-Trp-Arg-Thr-Pen-Thr-NH₂ (CTAP). CD3/28 mAbs-induced interleukin-2 transcription was also inhibited by the opioids fentanyl and loperamide. This indicates that the induced µ-opioid receptor mRNA is translated into functional receptor protein. Furthermore, a µ-opioid receptor-enhanced green fluorescent protein-fusion protein was localized in membranes of Jurkat cells and internalized in response to [D-Ala², N-Me-Phe⁴, Gly⁵-ol]-Enkephalin (DAMGO), but not morphine. In conclusion, these data emphazise the role of opioids in the modulation of T lymphocyte signaling.

Key words: Opioid, AP-1, NFAT, NFkappaB, DNA binding sites, Promoter analysis, Regulation of gene expression, Regulation - transcriptional