The CXCR4 chemokine receptor and the delta opioid receptor (DOR) are pertussis toxinsensitive G protein-coupled receptors (GPCR). Both are widely distributed in brain tissues and immune cells, and have key roles in inflammation processes and in pain sensation on proximal nerve endings. We show that in immune cells expressing CXCR4 and DOR, simultaneous addition of their ligands CXCL12 and [D-Pen2, DPen5]enkephalin does not trigger receptor function. This treatment does not affect ligand binding or receptor expression, nor does it promote heterologous desensitization. Our data indicate that CXCR4 and DOR form heterodimeric complexes that are dynamically regulated by the ligands. This is compatible with a model in which GPCR oligomerization leads to suppression of signaling, promoting a dominant negative effect. Knockdown of CXCR4 and DOR signaling by heterodimerization might have repercussions on physiological and pathological processes such as inflammation, pain sensation and HIV-1 infection.Supporting Information for this article is available at http://www.wiley-vch.de/contents/jc_2040/2008/37630_s.pdf See accompanying article: http://dx.doi.org/10.1002/eji200738101 IntroductionThe G protein-coupled receptors (GPCR) represent the largest, most diverse family of transmembrane receptors expressed in the body. They act through G proteins to regulate intracellular processes including cell adhesion, migration and proliferation [1]. Studies show that the GPCR can function as oligomers [2,3]. Perhaps their most striking feature is that GPCR form not only functional homo-oligomers, but can also associate with other GPCR to form hetero-oligomers. Homo-and hetero-oligomers differ in their pharmacological pro- files, ligand binding affinity, and/or internalization pathways [4][5][6]. GPCR hetero-oligomerization would thus enable generation of new types of signaling units. Opioid and chemokine receptors are members of the Ga i protein-linked GPCR. These receptors and their ligands are expressed in neurons and glial cells, and in immune system cells such as lymphocytes, monocytes and neutrophils [7]. They share the same microenvironment in many physiological situations and are essential for inflammation processes. Chemokine receptors promote immune cell migration to and adhesion at the inflammation site, whereas opioid receptors reduce pain sensation on proximal nerve endings. Opioid receptors can also regulate the immune response, as they alter antibody responses, cell-mediated immunity, phagocytic activity, adhesion and chemotaxis [8][9][10][11]. Opioids also prevent leukocyte movement toward chemokine gradients [12], and cross-desensitization is described between opioid and chemokine receptors [8,10]. The CCR5 chemokine receptor can form heterodimers with each of the three opioid receptor subtypes (l, d, and j) [8,10].CXCR4, the most widely expressed chemokine receptor [13], is crucial for correct lymphocyte trafficking [14], hematopoiesis, and development [15][16][17]. It is also a coreceptor for T-tropic HIV strains [1...

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kip1 . These correlated events are necessary for FRTL-5 cell proliferation after TSH stimulation. Moreover, TSH through PKA pathway increases cyclin-dependent kinase 2 levels, whereas PI 3-kinase signaling increases cyclin E levels. Together, both pathways finally converge, increasing the formation and activation of cyclin E⅐cyclin-dependent kinase 2 complexes and the phosphorylation of the retinoblastoma protein, two important steps in the transition from G 1 to S phase in growth-stimulated cells. Somatostatin exerts its antiproliferative effect inhibiting more upstream the TSH stimulation of PKA and PI 3-kinase, interfering with the TSH-mediated increases of intracellular cAMP levels by inactivation of adenylyl cyclase activity. Together, these results suggest the existence of a PKA-dependent pathway and a new PKA-independent PI 3-kinase pathway in the TSH/cAMP-mediated proliferation of FRTL-5 thyroid cells.