Regulation of Mastomys ECL Cell Function by Transforming Growth Factor Alpha

Abstract

Little progress has been made in the understanding of the pathobiology of gastric neoplasia over the past 4 decades. This reflects the paucity of information available regarding the biology of gastric mucosal cell proliferation. More recently it has become apparent that growth factor regulation of cell proliferation is of considerable relevance in initiating mucosal mitogenesis. We have recently identified the histamine secreting enterochromaffin-like (ECL) cell as a pivotal cellular regulator of gastric acid secretion. In addition to its critical role in initiating acid secretion, we have proposed that the ECL cell may produce agents responsible for the regulation of mucosal cell proliferation. We have therefore hypothesized that such a function may be subserved by production of transforming growth factor alpha (TGFα). TGFα is known to play a significant role both in normal physiology and in the transformation of naive cells into a neoplastic form. We therefore proposed that increased levels of gastrin induced by low acid states might stimulate TGFα secretion and that this agent might be capable of regulating ECL cell DNA synthesis and cell proliferation. We used the mastomys rodent to generate anin vivohy- pergastrinemia model using long-term histamine-2 receptor blockade (loxtidine 1 mg/kg/day). In order to evaluate the cell-specific effects, we developed a pure isolated ECL cell system from the mastomys stomach. This utilized pronase digestion (1.0 mg/ml) and EDTA exposure (1 mM) of the mucosa followed by particle size separation with countercurrent elutriation and density purification on a Nycodenz step gradient. ECL cells were obtained with a purity of 90–95%. Histamine secretion from ECL cells was measured by radioimmunoassay (RIA). TGFα content was measured by RIA, and TGFα expression was measured by RNAse probe protection assay. DNA synthesis was quantified by measuring bromo-deoxyuridine (BrdU) incorporation into cultured cells. TGFα levels were increased in fundic mucosa after 16 weeks of hypergastrinemia (from 4.3 ± 0.6 to 32.6 ± 2.6 fmole/mg protein,P< 0.05). TGFα message was identified in the ECL cells by RNAse probe protection assay, and was fourfold amplified in ECL cell tumors after 16 weeks of exposure to hypergastrinemia. Gastrin stimulated (10 nM) histamine secretion in isolated naive ECL cells was inhibited by TGFα (IC502 × 10−10M). DNA synthesis was stimulated by gastrin (EC502 × 10−11M) and TGFα (EC505 × 10−9M). These data are consistent with the proposal that elevated gastrin levels are associated with ECL cell TGFα production and that TGFα stimulates ECL cell DNA synthesis.