Biochemical Normal Rat Analysis of Secretory Proteins Synthesized by Pancreas and by Pancreatic Acinar Tumor Cells

We have examined the secretogogue responsiveness and the pattern of secretory proteins produced by a transplantable rat pancreatic acinar cell tumor. Dispersed tumor cells were found to discharge secretory proteins in vitro when incubated with hormones that act on four different classes of receptors: carbamylcholine, caerulein, secretin-vasoactive intestinal peptide, and bombesin. With all hormones tested, maximal discharge from tumor cells was only about one-half that of control pancreatic Iobules, but occurred at the same dose optima except for secretin, whose dose optimum was 10-fold higher. Biochemical analysis of secretory proteins discharged by the tumor cells was carried out by crossed immunoelectrophoresis and by two-dimensional isoelectric focusing-SDS polyacrylamide gel electrophoresis. To establish a baseline for comparison, secretory proteins from normal rat pancreas were identified according to enzymatic activity and correlated with migration position on two-dimensional gels. Our results indicate that a group of basic polypeptides including proelastase, basic trypsinogen, basic chymotrypsinogen, and ribonuclease, two out of three forms of procarboxypeptidase B, and the major lipase species were greatly reduced or absent in tumor cell secretion. In contrast, the amount of acidic chymotrypsinogen was notably increased compared with normal acinar cells. Although the acinar tumor cells are highly differentiated cytologically and express functional receptors for several classes of pancreatic secretagogues, they show quantitative and qualitative differences when compared with normal pancreas with regard to their production of secretory proteins. The acinar cell of the mammalian pancreas is highly differentiated and adapted for the synthesis, packaging, and discharge of secretory proteins. The anatomic organization of organdies within the cell expedites the secretory process in that the protein synthetic apparatus is located in the basal pole of the cell while secretory granules accumulate in the apical pole. Under physiologic conditions, hormones interact with receptors located on the basal plasmalemma, which in turn leads to granule release at the apical membrane. Specialized regions of the plasma membrane may play a role in directing vectorial release of secretory proteins. Evidence for such specialization includes differences in intramembrane particle distribution between basolateral and apical membranes as shown by freezefracture electron microscopy (1) as well as by marked differences in distribution of lectin binding sites (2). As we have shown in previous papers (3, 4), rat pancreatic acinar tumor cells lack polarized organization of the plasmalemma as deter734 mined by either freeze-fracture techniques or lectin binding. Though the acinar tumor ceils contain the cellular machinery for the production and packaging of secretory proteins, it is not known whether the lack of polar organization of the plasmalemma compromises their response to peptide secretagogues. In this investigation we sought to determine whether rat acinar tumor cells are capable of responding to hormones representative of four known classes of acinar cell secretagogues (5) and whether or not secretory proteins synthesized and discharged from the tumor ceils differ from those of the normal pancreas. Our results show that the acinar tumor cells discharge secretory proteins in response to several secretagogues, extending the report by Warren and Reddy (6) that acinar tumor cells respond to carbachol and that qualitative and quantitative differences can be detected among the tumor cell secretory proteins when compared with those of the normal THE JOURNAL Ol CELL BIOLOGY VOLUME 95 DECEMBER 1982 734-741 © The Rockefel ler Univers i ty Press • 0021-9525/82/12/0734/08 $1.00 on Jne 8, 2017 D ow nladed fom Published December 1, 1982