Increased immunoreactivity and protein tyrosine kinase activity of the protooncogene pp60c-src in preneoplastic lesions in rat pancreas

This study investigates the possibility that the c-Src protein tyrosine kinase is involved in experimental exocrine pancreatic carcinogenesis. Expression and activity of the protooncogene pp60c-src (c-Src) are investigated in acinar pancreatic (pre-) neoplastic lesions induced in rats by azaserine and compared with normal rat pancreas. Low or absent c-Src immunoreactivity and c-Src tyrosine kinase activity were found in the pancreas of untreated control rats. Compared with these controls, c-Src protein immunoreactivity was increased in "normal" acinar cells and even more in putative preneoplastic atypical acinar cell nodules (AACN) after azaserine treatment. In contrast, more advanced (secondary transformed) acinar cell lesions demonstrated no c-Src immunoreactivity. Rats treated with azaserine showed a 7-fold-higher c-Src tyrosine kinase activity in their pancreas. The level of c-Src tyrosine kinase activity correlated positively with the number of lesions in the pancreas, inasmuch as promotion of azaserine-initiated carcinogenesis by cerulein resulted in a more than 10-fold increase in the number of AACN, which was accompanied by a 6-fold increase in c-Src activity when compared with azaserine treatment alone. c-Src tyrosine kinase activity was responsible, on average, for 40% of the total tyrosine kinase activity in the pancreatic homogenates and was predominantly found in the cytoskeletal subcellular fraction. Furthermore, the transformation from normal to preneoplastic pancreatic tissue in azaserine-treated rats was accompanied by a change in the localization of the c-Src protein. With the use of immunohistochemistry and confocal laser scanning microscopy, the protein was detected in the cytoplasm in morphologically normal pancreatic acini, whereas in AACN it was detected both in the cytoplasm and in the nuclei. It is concluded that the protooncogene c-Src might be involved early in experimental pancreatic carcinogenesis: c-Src probably plays a minor role in pancreatic acinar cells after transformation to malignancy.