The Role of Surface Protein Kinase in the ATP‐Induced Growth Inhibition in Transformed Mouse Fibroblasts a

Addition of ATP to cultures of transformed mouse fibroblasts (Swiss mouse 3T6 cells) results in cell growth inhibition, whereas the growth of the nontransformed counterparts of 3T6 cells, namely 3T3 cells, is only slightly affected.',' The mechanisms underlying the growth inhibition are only partly understood. The uptake of adenine nucleotides' and the alteration of ion fluxed were suggested to mediate the inhibition, in adenocarcinoma and erythroleukemia cells, respectively. These effects, however, were not detected in mouse fibroblasts. We have shown that hydrolysis of ATP, and uptake of the adenosine generated, has a role in the inhibition process? Because ATP and its products are metabolized within one day: the continuation of the inhibition is probably mediated by additional mechanisms. We found that conditioned medium from ATP-treated cells inhibits cell proliferation, indicating that extracellular factors are involved in the inhibition.' The selectivity for transformed cells and the specificity for ATP (and to a lesser extent to other adenine nucleotides) indicate that the effects exerted by ATP are mediated by cell surface enzymes or receptors. In this study we examined the possible role of cell surface protein kinase (SPK, casein type I1 protein kinase"') in the ATP-induced growth inhibition, using mouse fibroblasts, Balb/c 3T3 cells, and their transformed derivatives, Balb/c SV40-3T3 cells. The SPK activity found in these cells could be removed by washings with an assay