When apoptosis is initiated by manganese (II) loading, hyperthermia or thapsigargin treatment, human HL-60 and AsPC-1 cells initiate de novo synthesis of the C5a receptor (C5aR) and generation of its ligand, the ribosomal protein S19 (RP S19) homodimer. The ligand-receptor interaction, in an autocrine/paracrine fashion, promotes apoptosis, which can be bypassed by exogenous administration of C5a, another ligand. The proapoptotic function of the RP S19 dimer is reproduced by a C5a/RPS19 chimera that contains the body of C5a and the C-terminal region (Ile134-His145) of RP S19. The RP S19 dimer or C5a/RPS19 and C5a inversely regulate the expression of Regulator of G protein Signaling 3 (RGS3) gene in the apoptosis-initiated cells. Namely, the RP S19-type proteins upregulate RGS3 expression, whereas the C5a reduce it. Transformation of HL-60 cells to overexpress RGS3 promotes apoptosis in association with the downregulation of the Extracellular signal-Regulated Kinase (ERK) signal, and vice versa in the RGS3 knocked-down cells. Consistent with this result, an inhibitor of ERK phosphorylation effectively enhances the apoptotic rate in wild-type HL-60 cells. Moreover, a dominant negative effect on the RP S19 dimer production encourages apoptosis-initiated HL-60 cells with a longer lifespan in mouse than the natural effect. Our data indicate that, in apoptosis-initiated cells, the ligand-dependent C5aR-mediated dual signal affects the fate of cells, either apoptosis execution or survival, through regulation of RGS3 gene expression and subsequent modulation of ERK signal.

C-terminus of S19 ribosomal protein (RP S19) endows the cross-linked homodimer with a dual effect on the C5a receptor in leucocyte chemoattraction; agonistic effect on the monocyte receptor, and antagonistic effect on the neutrophil receptor. C5a exhibits the uniform agonistic effect on this receptor of both cell types. We have currently prepared a recombinant C5a-chimeric protein bearing the C-terminus of RP S19 (C5a/RP S19 chimera) to be used as a substitute of the RP S19 dimer. In vitro, this chimera similarly inhibited the intracellular Ca(2+) mobilization of neutrophils induced by C5a to the RP S19 dimer did. In the guinea pig skin, 10(-7) M C5a/RP S19 chimera exhibited an inhibitory capacity to the neutrophil infiltration induced by 3 x 10(-7) M C5a without enhancing monocyte infiltration. In reverse passive Arthus reaction, the neutrophil infiltration associated with plasma extravasation was significantly reduced by the simultaneous administration of 10(-7) M C5a/RP S19 chimera with antibodies. The C5a/RP S19 chimera is a useful tool not only to examine the molecular mechanism that underlies the functional difference of the C5a receptor between monocytes and neutrophils, but also to prevent C5a-mediated hyper-response of neutrophils in acute inflammation.