S19 ribosomal protein dimer augments metal-induced apoptosis in a mouse fibroblastic cell line by ligation of the C5a receptor

To analyze the role of S19 ribosomal protein (RP S19) in apoptosis, murine NIH3T3 were transfected with either hemagglutinin peptide-tagged (HA) wild-type human RP S19 or a mutant (Gln137Asn) that is resistant to transglutaminase-catalyzed cross-linked-dimerization. Transfection with the mutant HA-RP S19 inhibited manganese (II) (Mn II)-induced apoptosis whereas the wild-type HA-RP S19 augmented apoptosis and a mock transfection had no effect. Release of the wild-type HA-RP S19 dimer but not the mutant HA-RP S19 was observed during the apoptosis. The reduced rate of apoptosis of the cells transfected with the mutant HA-RP S19 was overcome by addition of extracellular wild-type RP S19 dimer. The apoptosis rates in cells transfected with either form of human HA-RP S19 and in mock transfectants were reduced to about 40% by the presence of anti-RP S19 antibody in the culture medium. Immunofluorescence staining and fluorescence-activated cell sorting (FACS) analysis showed that the cell surface expression of the receptor for cross-linked RP S19 dimer, C5a receptor, increased during apoptosis, concomitant with phosphatidylserine exposure. The expression of the C5a receptor gene also increased twofold. Apoptosis rates in the transfected and control cell lines were also reduced by the presence of an anti-mouse C5a receptor monoclonal antibody or of a peptide C5a receptor antagonist. These results indicated the presence of an RP S19 dimer- and C5a receptor-mediated autocrine-type augmentation mechanism during Mn II-induced apoptosis in the mouse fibroblastic cell line. In contrast to the RP S19 dimer, C5a actually inhibited apoptosis, suggesting that signaling through the C5a receptor varies depending on the ligand bound.