Increase in plasma membrane phosphodiesterase activity in contact-inhibited 3T3 cells and in phenotypically reverted SV-3T3 cells

Contact-inhibited 3T3 mouse fibroblast cells, in contrast to logarithmically growing 3T3 cells and SV-3T3 transformed cells, have increased levels of plasma membrane-bound phosphodiesterase (oligonucleotidase, E.C.3.1.4.19; nucleotide pyrophosphatase, E.C. 3.6.1.9) activity. The increase in enzyme, recorded as increased specific activity, is reversible, as evidenced by the return to normal values following dilution of confluent 3T2 cells and re-initiation of growth. Increased enzyme activity is induced again when the cells regain the confluent state. Transformed SV-3T3 cells can be induced to mimic the contact inhibited state, including increased plasma membrane phosphodiesterase activity, by exposure to a combination of: (i) agents that are known to induce increased intracellular cAMP levels and (ii) additions of purified 3T3 or SV-3T3 plasma membranes. Additions of either alone fails to induce the increase in membrane phosphodiesterase activity, although each alone can significantly suppress cell growth, as measured by incorporation of 3H amino acids. We suggest that the elevation of plasma membrane phosphodiesterase activity may serve as a measure of conversion to the contact-inhibited state in both normal cells and phenotypically reverted transformed cells.

Uncapping of viral messenger RNA by phosphodiesterase of fibroblast plasma membranes

Isolated plasma membranes from mouse fibroblast lines 3T3 and its tranformant SV-3T3 contain a phosphodiesterase (oligonucleotidase, E.C. 3.1.4.19; nucleotide pyrophosphatase, E.C. 3.6.1.9) that splits capped and methylated messenger RNA obtained from both reovirus and vesicular stomatitis virus. The isolated membranes are free of demonstrable ribonuclease activity and split the mRNA to produce 7-methyl guanosine diphosphate as a product. With ATP as substrate for the phosphodiesterase enzyme, the product is AMP. Synthetic caps, AMP, ADP and ATP, but not cyclic AMP, can compete with the substrate p-nitrophenyl thymidilic acid. A possible regulatory role on messenger translation is proposed.