Differentiation and DNA synthesis in pancreatic acinar carcinoma of rat.

Morphological analysis of transplantable pancreatic acinar carcinoma of rat has revealed a continuum of cells from those which totally lack mature secretory granules to cells with abun dant well-formed secretory granules. This tumor is easily dis sociable into single cells which incorporate [3H]thymidine and [3H]leucine into DNA and protein at a linear rate for 3 to 4 hr. The number of cells synthesizing DNA, as assessed by light microscope autoradiography, was about 17%. Dissociated neoplastic pancreatic acinar cells are classified into four cell types (types I to IV) on the basis of nuclear morphology, degree of polarization, and the extent of specialization of cytoplasmic organelles. Type IV cells possess all the characteristics of mature pancreatic acinar cells, such as abundant rough endoplasmic reticulum, prominent Golgi apparatus, and numer ous secretory granules. Cell types III, II, and I are progressively less differentiated. An analysis of the cytological features of 285 dissociated tumor cells revealed the following distribution: type I cells, 21%; type II cells, 23%; type III cells, 22%; and type IV cells, 34%. Analysis of the cytological features of 233 [3H]thymidine-labeled cells in high-resolution autoradiograms indicated that all four types of neoplastic pancreatic acinar cells are capable of DNA synthesis and revealed the following distribution: type I, 14%; type II, 28%; type III, 21%; and type IV, 37%. The observation that all four types of neoplastic acinar cells are capable of DNA synthesis indicates that cell prolifer ation and differentiation in this neoplasm are not mutually exclusive. An analysis of 135 mitotic cells for the presence or absence of secretory granules revealed the presence of secretory gran ules in about 65% of mitotic tumor cells. This observation indicates that neoplastic pancreatic acinar cells with differen tiated phenotype continue to divide even in a fully mature state. Disproportionate partitioning of secretory granules observed in some cells during mitosis suggests that such a process could yield both mature (type IV) and immature (type III or lower) daughter cells. These studies demonstrate that the mature neoplastic pancreatic acinar cell is capable of synthesizing DNA and that it can apparently acquire immature phenotypic characteristics (i.e., retrodifferentiate) as a result of continued mitosis without adequate interphase time for cytodifferentiation. 1Supported by NIH Research Grants CA 23055 and CA 27443 from the National Cancer Institute. 2 To whom requests for reprints should be addressed, at the Department of Pathology, Northwestern University, 303 East Chicago Avenue, Chicago, III. 60611. 3 Recipient of International Research Fellowship FO5TWO2793. Present ad dress: Department of Cell Biology, Institute of Anatomy, University of Aarhus, 8000 Aarhus C, Denmark. Received February 12, 1980; accepted June 19, 1980. INTRODUCTION The hypothesis that neoplasia results from disordered cell differentiation is an attractive one for the important reason that such a process might be reversed (5, 10, 15, 20). However, the controversy of whether neoplastic transformation ensues from the abnormal differentiation of stem cells (5, 10, 20) or from mature differentiated cells which retrodifferentiate toward immature phenotypes (35) remains to be clarified. The elegant work of Pierce and coworkers (21 -23,38) on the differentiation of teratocarcinoma and the common epithelial tumors carci noma of the breast and squamous cell carcinoma of skin led to the concept that abnormal differentiation of stem cells is re sponsible for neoplastic transformation. Furthermore, these and other studies suggest that only undifferentiated neoplastic cells synthesize DNA and divide actively, some of which may subsequently differentiate toward a mature, nonneoplastic phe notype (9, 22, 23, 38). The alternative concept that neoplastic transformation occurs in mature differentiated cell types which subsequently acquire immature phenotypic features has, how ever, received little attention (35). The term "retrodifferentiation" is used to signify progressive cell division of a mature phenotype with acquisition of less mature or undifferentiated phenotypic features. The demonstration of fetal-type morpho logical and functional transition during liver regeneration and hepatocarcinogenesis provides support for retrodifferentiation of mature hepatic parenchymal cells (for review, see Ref. 35). Whether the process of retrodifferentiation accounts for the neoplastic transformation of other terminally differentiated cell types with no obvious proliferating stem cell pool (11 ) remains to be ascertained. The difficulty in establishing unequivocally the direction of events in neoplastic transformation, i.e., differ entiation from stem cells versus retrodifferentiation from mature cells, has recently been emphasized by Uriel (35). He pointed out the need for additional tumor models which may help to distinguish the possible mode of differentiation relevant to neoplastic transformation (35). The establishment recently of a transplantable rat pancreatic acinar carcinoma in our laboratory (26) provides a suitable model system to analyze morphogenetic, enzymic, and func tional differentiation in transformed secretory epithelial cells (28, 30). Many of the neoplastic pancreatic acinar cells contain secretory (zymogen) granules indicative of morphological dif ferentiation (24, 25, 30, 36). Thus, we decided to test the concept that differentiated cells containing mature secretory granules do not synthesize DNA and divide, whereas immature neoplastic cells lacking secretory granules are capable of DNA synthesis and cell division (22, 23, 38). The results presented in this paper indicate that neoplastic pancreatic acinar cells irrespective of the presence or absence of secretory granules are capable of synthesizing DNA and cell division. The ability