INCOMPLETE PROCESSING OF PROGASTRIN EXPRESSED BY HUMAN COLON-CANCER CELLS - ROLE OF NONCARBOXYAMIDATED GASTRINS

Incomplete processing of progastrin expressed by human colon cancer ce lls: role of noncarboxyamidated gastrins. Am. J. Physiol. 266 (Gastroi ntest. Liver Physiol. 29): G459-G468, 1994. - Gastrin is mitogenic for several colon cancers. To assess a possible autocrine role of gastrin in colon cancers, we examined human colon cancer cell lines for expre ssion of gastrin mRNA and various forms of gastrin. Gastrin mRNA was n ot detected in the majority of colon cancer cell lines by Northern hyb ridization but was detected in all human colon cancer lines by the sen sitive method of reverse transcriptase-polymerase chain reaction (PCR) . Gastrin mRNA was quantitated by the competitive PCR method. The majo rity of cell lines expressed very low levels of gastrin mRNA (< 1-5 co pies/cell); only one cell line expressed > 20 copies/cell. The mature carboxyamidated form of gastrin was not detected in any of the cell li nes by radioimmunoassay or immunocytochemistry. Results suggested that either gastrin mRNA expressed by colon cancer cells was altered (muta ted) or posttranslational processing of progastrin was incomplete. Gas trin cDNA from all the colon cancer cell lines had an identical sequen ce to the published sequence of human gastrin cDNA. Specific antibodie s against precursor forms of gastrin were used, and significant concen trations of nonamidated (glycine-extended) and prepro forms of gastrin were measured in tumor extracts of representative colon cancer cell l ines. The presence of precursor forms of gastrin suggested a lack of o ne or more ofthe processing enzymes and/or cofactors. Significant conc entrations of the processing enzyme (peptidylglycine alpha-amidating m onooxygenase) were detected in colon cancer cells by immunocytochemist ry. Therefore, lack of other cofactors or enzymes may be contributing to incomplete processing of precursor forms of gastrin, which merits f urther investigation. Since low levels of gastrin mRNA were expressed by the majority of human colon cancer cell lines and progastrin was in completely processed, it seems unlikely that gastrin can function as a viable autocrine growth factor for colon cancer cells. High concentra tions of glycine-extended gastrin-17 (GG) (> 10(-6) M) were mitogenic for a gastrin-responsive human colon cancer (DLD-1) cell line in vitro . It remains to be seen if GG or other precursor forms of gastrin are similarly mitogenic in vivo, which may then lend credibility to a poss ible autocrine role of gastrinlike peptides in colon cancers.