Synthesis of the third component of complement (C3) by human gastric cancer‐derived cell lines

SUMMARY This is a study of complement components secreted by gastric cancer‐derived cell lines (MKN28. MKN45, MKN74 and KATO‐III), each of which has a different histological origin. Haemolytic activity of complement component was detected only in the culture supernatant of KATO‐III (C2 activity) and in that of MKN45 (C5 activity). However, the third component of complement, C3, was detected by an ELISA assay in the supernatants of all cell lines. In our studies focusing on C3 production by these cell lines, we have found that: (i) tumour necrosis factor (TNF) induced an increase in the amount of secreted C3 in a dose‐ and time‐dependent fashion; (ii) TNF (10 U/ml) stimulated C3 secretion by these cell lines to levels of 25.4–62.9 ng C3/10 6 cells per 24 hours; (iii) C3 haemolytic activity was detected in supernatants of TNF‐stimulated cell lines. The mean specific activities of C3 by TNF (10 U/ml)‐stimulated cell lines were 1.2–5.6 × 10‐ effective molecules/ng (e.m./ng), when that of C3 in normal human serum (NHS) was 1.7 × 10 6 e.m./ng; (iv) de novo synthesis of C3 by these cell lines was demonstrated by the effect of cycloheximide and by the incorporation of 35 S‐methionine into secreted C3; (v) immunoblot analysis of culture supernatants indicated that secreted C3 was mainly composed of C3α and C3β chains, but pro‐C3 was also present. These results, which show the de novo synthesis and secretion of C3 by all the tested gastric cancer‐derived cell lines in response to TNF, suggest the possibility that C3 may be secreted in the gastric wall as part of its normal physiology, or as a result of tumour pathology, and thereby participate in local immune or inflammatory responses.