NONMUTAGENIC MECHANISMS IN CARCINOGENESIS - ROLE OF PROTEIN-KINASE-C IN SIGNAL TRANSDUCTION AND GROWTH-CONTROL

There is accumulating evidence that the multistage carcinogenic process is associated with the progressive acquisition of mutations in cellular proto-oncogenes and in growth-suppressor genes. At the same time, several types of evidence indicate that nongenotoxic agents and epigenetic events also play an important role in the evolution of tumors. One of the most intensively studied nongenotoxic agents is the phorbol ester tumor promoter 12-O-tetradecanoyl-phorbol-13-acetate (TPA) and related compounds. Since TPA appears to exert its biologic effects through protein kinase C (PKC), a key enzyme in signal transduction, we have studied this enzyme in considerable detail. Our strategy has been to perturb signal transduction by developing cell lines that overexpress the beta-1 isoform of PKC. Such derivatives of rat fibroblasts display alterations in morphology and growth factors, altered expression of c-myc, ornithine decarboxylase, and phorbin, and increased susceptibility to transformation by certain oncogenes, H-ras, myc, and fos. These findings provide direct genetic evidence that PKC plays a critical role in growth control and the action of certain growth factors, tumor promoters, and oncogenes. In related studies, we have characterized the beta-1 isoform that is overproduced in the above cell systems in terms of its biochemical, kinetic, and immunologic properties. The enzyme has several properties characteristic of native PKCs. A surprising finding is that c-H-ras-transformed derivatives of the cells that overexpress PKC-beta-1 display a several-fold increase in the expression of the endogenous alpha-1 isoform of PKC and a decrease in the expression of the endogenous epsilon-isoform. Thus, cell transformation can lead to altered expression of specific endogenous isoforms of PKC. Colon cancer is the second most common cause of cancer deaths in the U.S., but its precise etiology is not known. Several types of evidence will be presented suggesting that PKC plays a role in the pathogenesis of this disease. We have also found that bacteria in the human gastrointestinal flora can convert lipids to diacylglycerol (DAG). a known activator of PKC. We hypothesize that the DAG thus produced influences the growth of the colonic epithelium and thus plays a tumor-promoting role. The above findings coupled with findings from other laboratories are discussed in terms of a unifying concept of multistage carcinogenesis, which we have termed "a progressive disorder in signal transduction." This concept emphasizes the fact that signal transduction pathways are normally linked to each other via complex and overlapping networks, and the evolution of tumor cells involves both genetic and epigenetic disruptions of such net works.