Mouse insulinoma beta TC3 cells express prodynorphin messenger ribonucleic acid and derived peptides: a unique cellular model for the study of prodynorphin biosynthesis and processing.

The tumor cell line beta TC3 has been established from insulinomas derived from transgenic mice carrying a hybrid insulin promoter-simian virus-40 tumor antigen gene. The beta TC3 cells express high steady state levels of proinsulin messenger RNA (mRNA). In this same cell line, we describe in the present study high expression levels of prodynorphin (pro-Dyn) mRNA and its derived peptides. By Northern blot analysis, the screening of 23 cell lines of endocrine (n = 10) and of nonendocrine (n = 13) origin revealed the presence of high levels of the 2.6-kilobase pro-Dyn transcript only in beta TC3 cells. The beta TC3 cells expressed levels of pro-Dyn mRNA comparable to those in rat tissues expressing pro-Dyn. Chromatographic and radioimmunological studies showed that pro-Dyn mRNA was translated and fully processed into opioid peptides with leucine-enkephalin (Leu-Enk)-extended sequences [dynorphin-A-(1-8), dynorphin-B-(1-13), and alpha-neo-endorphin]. The expression of the prohormone convertases was also examined in beta TC3 cells by Northern blot analysis. In addition to the ubiquitously expressed furin, beta TC3 cells have abundant levels of prohormone convertase-1 (PC1) and PC2 mRNAs, but undetectable levels of PACE4 or PC5 mRNAs. Incubation of beta TC3 cells with 8-bromo-cAMP for 24 h stimulated 3-fold both the pro-Dyn mRNA levels and the secretion of opioid peptides. In contrast to pro-Dyn mRNA, furin, PC1, and PC2 mRNA levels were not affected by 8-bromo-cAMP. The beta TC3 cells constitute a unique model to elucidate the biosynthetic pathway of pro-Dyn processing, to identify the proteolytic enzymes responsible for the production of pro-Dyn end products, and to assess the potential role of opioid peptides in the regulation of pancreatic function.