Protein kinase activities in x-radiation-induced adenocarcinoma of rat small bowel

The activities of cyclic adenosine-3',5'-monophosphate (cAMP)-dependent and -independent protein kinases were measured in cellular homogenates prepared from an x-radiation-induced mucoid adenocarcinoma of Holtzman rat small bowel. Comparisons of these activities were made with those observed in similar cell preparations of normal unirradiated small bowel tissues and those irradiated intestines which did not have visible lesions. A significantly lower degree of cAMP-dependent endogenous and exogenous protein (histone) phosphorylation was observed in the cancer cells compared to that in other tissues. In addition, the activities of enzymes in tumor tissue appeared independent of the homogenate concentrations and were not stimulated by cAMP. The results suggest that either the radiation-induced cancer consists of cells which no longer respond normally to extracellular signals that are transmitted intracellularly by the cAMP system or a more rapid phosphoprotein phosphatase(s) exists in these cells which may be reversing the process of protein phosphorylation