Abstracts (complete list) - Wissenschaft Online

Marion Leick, Tanja Hartmann, Susann Ewers, Andrea Diefenbacher, Robert Nibbs,
Meike Burger
CCL5 upregulates surface expression of the orphan atypical
chemokine receptor CRAM-A/B in a heparin sulphate
dependent manner in pre-B acute lymphoblastoid leukemia
cells
Chemokines work as cellular recruitment molecules. Specific combinations of
chemokines, chemokine receptors, and adhesion molecules determine which subgroups
of leukocytes migrate and what their destinations are. These migratory processes are
also regulated by the availability and binding of chemokines on endothelial layers by
interactions with glycosaminoglycans and atypical chemokine receptors that are
described as silent or decoy receptors. Here, we report that pre-B acute lymphoblastoid
leukemia cells Nalm6 and G2 express transcripts of a rare distributed splice variant of
the atypical orphan receptor CRAM-A. These pre-B cells show high surface expression of
CRAM in connection with CCL5-mediated functional responses. We investigated these
responses in detail in Nalm6 cells that do not express any of the known CCL5 receptors
but still specifically phosphorylate ERK1/2 in response to CCL5 stimulation. CCL5 did not
induce calcium responses or migration in Nalm6 cells. CRAM-A/B transfected cell lines
also did not show any migratory responses but a change in actin polymerization
together with ERK1/2 phosphorylation. In addition, CCL5-induced p44/42
phosphorylation was not inhibited by pertussis toxin suggesting that CRAM-A/B does not
signal via Gi proteins. Intriguingly, CRAM surface expression on Nalm6 cells is
specifically upregulated in response to CCL5 but not to other chemokines. The
upregulation of surface expression follows distinct time and concentration kinetics and is
mediated by heparin sulfate binding of CCL5 together with CCL5 binding to CRAM.
Radioactive ligand binding assays also indicate low affinity CCL5 binding directly to
CRAM. Our results characterize CRAM-A/B as belonging to the group of atypical
chemokine receptors that rather play a role in modulating chemokine induced immune
responses than in triggering direct motility.