RESEARCH SUMMARY

Since the first reports on chemokine function, much information has been generated on the implications of these molecules in numerous physiological and pathological processes, as well as on the signaling events activated through their binding to receptors.

Despite these extensive studies, no chemokine-related drugs have yet been approved for use in patients with inflammatory or autoimmune diseases. This discrepancy between efforts and results has forced a re-evaluation of the chemokine field.

Using classic biochemical techniques and new methodologies based on energy transfer between fluorophores (FRET), we have explored chemokine receptor conformations at the cell surface and found that, as is the case for other G protein-coupled receptors, chemokine receptors are not isolated entities that are activated following ligand binding; rather, they are found as dimers and/or higher order oligomers at the cell surface, even in the absence of ligands. These complexes form organized arrays that can be modified by receptor expression and ligand levels, indicating that they are dynamic structures.

Clusters of chemokine receptors are expressed at the cell surface. It is thus plausible that receptor dimers organize in such clusters, like bundles of cigars. Ligands then modulate and stabilize specific receptor conformation to trigger functional responses without disrupting cell surface receptor arrays. Ligand-mediated internalization of a given receptor pair does not necessarily alter the levels of other receptors in the ‘bundle’. It is nonetheless possible that the conformation of resting receptors in an array might be affected by ligand binding to a responding receptor. Such ‘allosteric’ conformational changes might not be restricted to neighboring dimers, but might extend through the array in a domino effect.

The chemokines also activate a tyrosine kinase pathway that shares many components with the biochemical pathway activated by the cytokine receptors. We have reported that chemokines activate the JANUS kinases (JAK), which associate to the chemokine receptor and promote its rapid tyrosine phosphorylation. Through the JAK/STAT pathway, the chemokines trigger suppressor of cytokine signaling (SOCS) expression. The SOCS intracellular proteins are thus key physiological regulators of cytokine and chemokine responses, SOCS proteins regulate signal transduction by binding directly to JAK or by competing with STAT for the phosphorylated receptor; in addition, they target ubiquitinated signaling intermediates for degradation by the proteasome pathway through ECS (elongin-Cullin-SOCS) E3 ligase formation. As consequence of this last effect, SOCS1 function as a tumor suppressor blocking cell cycling in human melanoma by affecting G1/S and mitosis.