The exocrine pancreas consists of acinar cells that secrete digestive enzymes, and duct cells that produce bicarbonate-rich fluid. Pancreatic malignancies may arise from either cell type. The molecular signals that lead to malignant transformation and subsequent growth advantage of these cells are not known. However, it is established that cultured human pancreatic carcinoma cells overexpress the epidermal growth factor (EGF) receptor and exhibit increased EGF receptor mRNA levels. Some of these cell lines have clonal structural changes on the short arm of chromosome 7 in a region that contains the EGF receptor gene. Because of its homology with the v erbB oncogene, the EGF receptor gene is taken to be a protooncogene. Further, its over-expression has been associated with the malignant phenotype. The EGF receptor protein is autophosphorylated on tyrosine residues following activation by either EGF or transforming growth factor-alpha (TGF-alpha). TGF- alpha is produced by a variety of cancers, including human pancreatic cancer cells. In the pancreatic cells, EGF is recycled between the intracellular and extracellular compartments, whereas TGF-alpha is degraded extensively. Further, TGF-alpha is less efficient at inducing EGF receptor degradation than EGF, but more potent at enhancing anchorage-independent growth. Pancreatic cancer cells also produce transforming growth factor beta (TGF- beta). Although TGF-beta does not bind to the EGF receptor, it is known to modulate the actions of EGF and TGF-alpha. The molecular mechanisms that allow pancreatic cancer cells to concomitantly overexpress the EGF receptor, cycle EGF, and produce TGF-alpha are not known. Further, the role of TGF-beta in regulating the growth of these cells has yet to be elucidated. In the present proposal we will test the hypothesis that the growth advantage of human pancreatic cancer cells derives from autocrine activation of the EGF receptor by TGF-alpha, determine whether the recycling of EGF is the result of the presence in these cells of a variant of the EGF receptor, and evaluate the potential role of TGF-beta in modulating the EGF/TGF-alpha autocrine cycle. The mechanisms that lead to an excess of EGF receptors in these cells will be examined by looking for EGF receptor gene rearrangement using pulse-field gel electrophoresis. The characteristics of the EGF receptor will be examined in a number of pancreatic cancer cells by comparing the effects of EGF and TGF-alpha on EGF receptor phosphorylation, and by cloning and sequencing the EGF receptor gene from the T3M4 cell line. The roles of the EGF receptor and TGF-alpha in the regulation of the growth of pancreatic cancer cells will be assessed by the use of antibodies against the receptor and TGF- alpha, by studying the effects of TGF-beta on EGF/TGF-alpha actions and by using antisense RNA's for the EGF receptor. The distribution of the EGF receptor, TGF-alpha, and TGF-beta will be determined in normal and malignant human pancreatic tissues by in situ hybridization with the respective riboprobes, as well as within the pancreatic cancerous mass that develops after the injection of tumor cells into nude mice.
Showing the most recent 10 out of 94 publications
