Rm. Richardson et al., REGULATION OF HUMAN INTERLEUKIN-8 RECEPTOR-A - IDENTIFICATION OF A PHOSPHORYLATION SITE INVOLVED IN MODULATING RECEPTOR FUNCTIONS, Biochemistry, 34(43), 1995, pp. 14193-14201
The human type A interleukin-8 receptor (IL-8RA) was modified to expre
ss an amino-terminal epitope tag and stably overexpressed in a rat bas
ophilic leukemia cell line (RBL-2H3). This receptor (ET-IL-8RA) displa
yed functional properties similar to those of the native receptor in n
eutrophils in that exposure to IL-8 stimulated GTPase activity, phosph
oinositide (PI) hydrolysis, intracellular calcium mobilization, and de
granulation in a pertussis toxin (PTx) susceptible fashion. IL-8 induc
ed dose- and time-dependent phosphorylation of ET-IL-8RA. Phorbol 12-m
yristate 13-acetate (PMA) treatment also resulted in phosphorylation o
f the receptor although to a lesser extent. Staurosporine totally bloc
ked PMA-induced phosphorylation but only partially inhibited IL-8-medi
ated phosphorylation. Phosphorylation of ET-IL-8RA correlated with its
desensitization as measured by GTPase activation and calcium mobiliza
tion. To determine the role of phosphorylation in IL-8RA signal transd
uction, three mutants lacking specific serine and threonine residues l
ocated at the C-terminal of this receptor were constructed by site-dir
ected mutagenesis (M1, M2, and M3). The mutated receptors expressed in
RBL-2H3 cells displayed pharmacological properties (K-d similar to 2-
2.8 nM and B(max)similar to 3-3.5 pmol/mg of protein) similar to those
of the wild-type ET-IL-8RA. M2 and M3, but not M1, showed a marked de
crease in IL-8-induced phosphorylation compared to the wild-type recep
tor. M2 and M3 but not M1 were resistant to PMA-mediated phosphorylati
on and desensitization and were also more resistant to homologous dese
nsitization than M1 or ET-IL-8RA. Following exposure to IL-8, M1 and M
3 stimulated PI hydrolysis and secretion to the same extent as wild-ty
pe IL-8RA. M2, however, showed an similar to 4- and similar to 12-fold
increase in IL-8-induced PI hydrolysis and secretion, respectively. T
hese data suggest that the IL-8RA is susceptible to phosphorylation an
d desensitization by a receptor kinase (GRK) and protein kinase C (PKC
), respectively. Moreover, the residues modified in M2 and M3 are requ
ired for PKC-mediated phosphorylation and desensitization. Interesting
ly, the M2 cluster appears to participate in the downregulation of IL-
8RA-mediated responses.