The anaphylatoxin C3a is a proinflammatory mediator generated during complement activation. The tight control of C3a receptor (C3aR) expression is crucial for the regulation of anaphylatoxinmediated effects. Key factors regulating constitutive expression of the C3aR in the mast cell line HMC-1 and receptor induction by dibutyryl-cAMP in monomyeloblastic U937 cells were determined by functional characterization of the C3aR promoter. Nucleotides ؊18 to ؊285 upstream of the translational start site proved to be critical for promoter activity in HMC-1 cells. Binding sites for the transcription factors AP-1 and Ets could be located. Overexpressed c-Jun/c-Fos (AP-1) and Ets-1 led synergistically to increased promoter activity that was substantially reduced by site-directed mutagenesis of the corresponding elements within the C3aR promoter. In HMC-1 cells, Ets interacted directly with the predicted binding motif of the C3aR promoter as determined by electromobility shift assays. AP-1 binding to the C3aR promoter was augmented during C3aR induction in U937 cells. A retroviral gene transfer system was used to express a dominant negative mutant of Ets-1 in these cells. The resulting cells failed to up-regulate the C3aR after stimulation with dibutyryl-cAMP and showed decreased AP-1 binding, suggesting that Ets acts here indirectly. Thus, it was established that Ets and the AP-1 element mediates dibutyrylcAMP induction of C3aR promoter activity, hence providing a mechanistic explanation of dibutyryl-cAMP-dependent up-regulation of C3aR expression. In conclusion, this study demonstrates an important role of AP-1 and a member of the Ets family in the transcriptional regulation of C3aR expression, a prerequisite for the ability of C3a to participate in immunomodulation and inflammation.The complement system is activated in a variety of diseases (1-3) and leads to the generation of the anaphylatoxic peptide C3a.2 This proinflammatory mediator binds specifically to its heptahelical receptor (C3aR) (4, 5) which is expressed on various cells such as leukocytes, endothelial and broncho-epithelial cells, smooth muscle cells of the lung, and even neurons (4, 6, 7).The best experimental evidence for the involvement of C3a in diseases has been obtained in animal models using C3aR knock-out mice, guinea pigs with a natural C3aR defect, or specific C3aR inhibitors (8 -12). Intriguingly, C3a and its receptor act partially protective, partially destructive. In endotoxin shock, C3a seems to be protective; the mortality rate of C3aR Ϫ/Ϫ mice was much higher than that of their wild type littermates, investigated by a cecal ligation puncture sepsis model (12). Moreover, C3a participates in liver regeneration after partial hepatectomy or toxic injury (13,14).As demonstrated in an adjuvant-induced arthritis rat model, C3a aggravates the inflammatory reaction (8). In asthma, C3a is deleterious because C3aRϪ/Ϫ -mice are protected from ovalbumin-driven airway hyper-responsiveness and early phase bronchoconstriction (15). In man C3a might play a si...
