Nuclear localization of brain-type glycogen phosphorylase in some gastrointestinal carcinoma

Our previous reports have demonstrated frequent and strong expression of glycogen phosphorylase (EC 2.4.1.1) activity mainly in the cytoplasm of gastric carcinoma. Although previous studies have suggested the phosphorylase glycosyltransferase system to be in the nucleus from enzyme histochemical analyses, intranuclear localization of the phosphorylase has not been fully established. The aims of the present study are to investigate the nuclear localization of glycogen phosphorylase and to identify the isoform of phosphorylase in the nucleus of gastrointestinal carcinoma. The activity of glycogen phosphorylase in carcinoma cells corresponding to the nucleus was demonstrated using enzyme cytochemical analysis. The phosphorylase activity coincided with localization revealed by immunocytochemistry using affinity-purified specific anti-human brain-type glycogen phosphorylase antibody. The isoform expressed in the nuclei of carcinoma cells was identified as being only the brain type according to a polymerase chain reaction-based assay using RNA obtained from gastric carcinoma cells and primers specific to muscle, liver and brain types of glycogen phosphorylase. The intranuclear localization of the brain-type isoform was confirmed by immunoelectron microscopical analyses. Further investigation to examine the nuclear localization in human carcinoma tissue (145 and 25 specimens with gastric and colonic carcinoma respectively) was carried out by immunohistochemistry using specific anti-brain-type antibody. Nuclear immunostaining was observed in seven cases out of 145 gastric carcinoma. The present study is the first to clarify the nuclear localization of glycogen phosphorylase with enzymatic activity in gastrointestinal carcinoma. The isoform of the enzyme expressed in the carcinoma was identified as the brain type. These results warrant further studies on the mechanisms for transporting the large molecule of brain-type glycogen phosphorylase to nuclei and its function in the nucleus of carcinoma cells.