Prolactin receptor expression in the gastrointestinal tract: characterization of the prolactin receptor of gastric mucosa

Abstract

There is evidence that prolactin (PRL) influences gastrointestinal function. However, the sites at which prolactin exerts these effects are not known. A monoclonal antibody was therefore generated against the rabbit mammary gland prolactin receptor (MAb 218) and used to study the distribution of the prolactin receptor in the rabbit gastrointestinal tract (GIT) by immunohistochemistry. MAb 218 is an IgG1 kappa-precipitating antibody which precipitates major affinity cross-linked mammary gland prolactin receptor subunits of molecular masses 45 and 80 kDa. It has an affinity of 0.8 x 10(9) mol/l for the prolactin receptor and does not react with GH or insulin receptors in precipitation assays. MAb 218 immunoreactivity was observed in classical prolactin target cells such as mammary gland epithelium, and this immunoreactivity was abolished by preincubation of MAb 218 with purified prolactin receptor but not by preincubation with purified GH receptor. In the GIT, the most intense immunoreactivity was associated with the oesophageal epithelium, chief (zymogenic) cells of the fundic mucosa, pancreatic islets of Langerhans and surface epithelial cells of the duodenum and jejunum. Other specific elements of the GIT were immunoreactive at lower levels or were immunonegative. No immunoreactivity was observed in these locations with a control monoclonal antibody (MAb 50.8) of identical isotype to 218. To support the immunohistochemical findings, rabbit gastric mucosal membranes were used to show the presence of lactogen-specific binding. Scatchard analysis of 125I-labelled human GH binding to the gastric mucosal membranes with rat prolactin as displacing ligand yielded an affinity constant (Ka) of 1.0 +/- 0.2 x 10(9) mol/l with a capacity of 3.5 +/- 0.4 fmol/mg protein. Affinity cross-linking and sodium dodecyl sulphate-polyacrylamide gel electrophoresis of the gastric receptor revealed lactogenic hormone-binding subunits of molecular masses 43, 68 and 83 kDa. The 68 kDa subunit was not seen in rabbit mammary gland or ovarian tissue, and may be unique to gastric mucosa. In conclusion, we have demonstrated the presence of a high affinity lactogenic receptor in specific epithelial cell subpopulations of the GIT. This localization of the prolactin receptor in the GIT will assist in further functional assignment of prolactin to gastrointestinal physiology.