Secretion granules of the rat transplantable pancreatic acinar carcinoma and of normal rat pancreas were isolated by differential centrifugation. Analysis of the granule content by sodium dodecyl sulphate/polyacrylamide-gel electrophoresis and isoelectric focusing procedures combined with specific enzyme assays indicated essential qualitative similarities between normal and neoplastic secretory proteins, suggesting retention of enzymic differentiation in this pancreatic acinar carcinoma. Accordingly, this epithelial tumor should serve as an important model for examination of regulatory mechanisms in cell differentiation and neoplasia.

Extracts from unfertilized sea urchin eggs contain an inhibitor of translation that inhibits protein synthesis in cell-free translation systems from sea urchin embryos or rabbit reticulocytes. The inhibitory effects of egg extracts can be reversed by the addition of mammalian eukaryotic initiation factor 4F (eIF-4F) in both sea urchin embryo and reticulocyte systems, suggesting that the inhibitor inactivates this initiation factor. The accumulated data suggest that the ability of eIF-4F to recycle may be compromised. The addition of eIF-4F to cell-free translation systems from unfertilized sea urchin eggs also stimulates protein synthesis. However, the stimulation does not increase protein synthetic activity in the egg cell-free translation system to the levels observed in those produced from 2-hr embryos. This suggests that, although the unfertilized egg contains an inhibitor of eIF-4F and reduced levels of eIF-4F activity, inactivation of this component is only one of the factors involved in the low rate of maternal mRNA utilization found prior to fertilization.

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The diversity of cytodifferentiation in a transplantable rat pancreatic acinar carcinoma provides a biological model system for the study of regulatory molecular events that differ from those in normal acinar cells. The well-formed secretory (zymogen) granules present in adult pancreatic acinar cells represent the final product of translational and post-translational events of a highly differentiated program of gene expression (1-3). Evidence indicates that all of the exocrine acinar cells of adult pancreas are similar in cytodifferentiation, each containing a relatively homogeneous population of secretory granules (1-4). In contrast, a transplantable pancreatic acinar carcinoma of rat established in our laboratory (5) is characterized by a continuum of cytodifferentiation ranging from cells totally lacking secretory granules to those with abundant well-formed ones (6). This diversity of cytodifferentiation in a neoplasm provides a eukaryotic cell model for the identification and study of regulatory molecular events that differ from those in normal acinar cells. Such studies are relevant in view of the notion that neoplasia reflects disordered cell differentiation and gene expression (7). In order to search for alterations that could be correlated with cytodifferentiation, it becomes necessary to define the patterns of gene expression in the granule-rich and granule-deficient subpopulations of neoplastic acinar cells. With this goal in mind, secretory proteins derived from highly purified secretory granules of pancreatic acinar carcinoma were analyzed by two-dimensional gel electrophoresis to determine whether gene expression in the apparently well-differentiated subpopulations of neoplastic acinar cells is similar to that of normal acinar cells. Polypeptide and phospholipid composition of the membranes derived from secretory granules of normal and neoplastic pancreatic acinar cells were also compared. MATERIALS AND METHODSPurification of Secretory Granules. Secretory granules from pooled normal adult male F344 rat pancreata and transplanted pancreatic acinar tumors (5) were isolated by a modification of the procedure described for the isolation of chromaffin granules from adrenal medulla (8). The tissues were minced in icecold sucrose buffer (0.27 M sucrose/8 mM sodium cacodylate/ 0.1 mM EDTA, pH 6.5) and homogenized (10% wt/vol). The homogenate was centrifuged at 710 x gavg first for 5 min and then for 3 min in a Beckman J-21C centrifuge to pellet nuclei and large cytoplasmic fragments. The supernatant was then centrifuged at 2,600x gavg for 10 min. The grey-white secretory granule pellet was washed with ice-cold sucrose buffer to remove the overlying mitochondria and was resuspended in a small volume of sucrose buffer. About 0.6 ml of granule suspension (10-15 mg of protein) was layered on a 10-ml density gradient consisting of 35% Percoll in 0.27 M sucrose/8 mM Na cacodylate/0.05 mM EDTA/1 mM phenylmethylsulfonyl fluoride/1 mM benzamidine hydrochloride in 15-ml Corex tubes and was centrifuged at 9,5...

The transplantable pancreatic acinar carcinomas of rat established recently provide useful model systems to examine the composition of secretory proteins as well as the secretory process in transformed pancreatic exocrine epithelium. The neoplastic acinar cells exhibit considerable variation in the extent of cytodifferentiation. In the present study the enzymatic profile of this heterogeneous tumor cell population has been investigated by the indirect immunofluorescent technique using antibodies against six pancreatic enzymes. By immunofluorescence, all neoplastic cells stained positively for the six enzymes tested: amylase, lipase, carboxypeptidase A, chymotrypsinogen, trypsinogen, and ribonuclease. Some variability in the intensity of immunofluorescence was noted, suggesting possible quantitative differences in the content of a given enzyme among tumor cells. These observations suggest that neoplastic acinar cells with or without secretory granules contain secretory proteins, but to a variable extent.