Lelio Orci

This study was performed to assess the relationships between prohormone transport and processing in separate cell types in pancreatic islet tissue. Anglerfish islets were subjected to pulse-chase incubation with [3H]tryptophan and/or [35S]cysteine. Tissue and media were removed at specific time points during the incubation and prepared for electron microscopic examination or biochemical analysis. Specific islet cell types were identified ultrastructurally using protein A gold immunocytochemistry. Transport of newly synthesized peptides through specific subcellular compartments was monitored using electron microscopic autoradiography. Prohormone-product ratios were established by gel filtration and high-performance liquid chromatography analyses of tissue extracts. Complete analyses were performed on A-cells (source of proglucagon-II, glucagon-II, and glucagon-like peptide-II), B-cells (proinsulin and insulin), D-cells (prosomatostatin-II and somatostatin-28), and S-cells (prosomatostatin-I and somatostatin-14). Transport of newly synthesized peptides proceeded from rough endoplasmic reticulum (RER) to Golgi complex and then to mature secretory granules in all cell types. The transport rate was most rapid in A- and B-cells, slower in S-cells, and slowest in D-cells. The T1/2 for conversion of prohormone to product(s) was shortest in S-cells (150 min), slightly longer in B-cells (155 min), much longer in D-cells (259 min), and greater than 300 min in A-cells. These results demonstrate that the transport/prohormone conversion relationships are unique in each of the islet cell types monitored.

We report here that heptanol (3.5 mM) induces in vitro a rapid formation of smooth endoplasmic reticulum aggregates (SERA) within isolated islets of Langerhans. SERA appeared after only 15 min of exposure to the alkanol and increased in number during the first 30 min of incubation. At that time, SERA represented 2% and 6% of the volume of B- and non-B-cells, respectively. Removal of heptanol resulted in the rapid disappearance of SERA, whereas reintroduction of the alkanol rapidly induced these structures again. SERA formation was seen in different types of endocrine and nonendocrine islet cells. In the insulin-producing B-cells, SERA formation was not modified by conditions known to alter the secretory activity and the microtubular-microfilament network or to inhibit protein synthesis. By contrast, SERA formation was inhibited by low temperature and by conditions depleting the energy sources of the cells. Similar observations were made in the presence of either octanol (1 mM) or nonanol (1 mM) but not of shorter chain alkanols, alkanes, oxidative derivates of either heptanol or octanol, and of other unrelated lipid-soluble compounds. Incubations in the presence of long-chain alkanols provide, therefore, a unique model to study in vitro the formation and disposal of smooth endoplasmic reticulum, as well as a system in which rapid membrane biogenesis is amenable to direct experimental testing.