Modulation of Cellular Interactions between C3H/10T½ Cells and Their Transformed Counterparts by Phosphodiesterase Inhibitors

Abstract

It has been demonstrated previously that nontransformed C3H/10T½CL8 mouse embryo fibroblasts (10T½) can induce a state of reversible growth inhibition in cocultured malignantly transformed mouse fibroblasts and that this inhibition is modulated by serum concentration. The present study suggests that cyclic nucleotides may be implicated in this intercellular communication.

The phosphodiesterase inhibitors theophylline, caffeine, and 3-isobutyl-1-methylxanthine (IBX) at concentrations of 10^-3 m, maintained continuously, were all found to inhibit the expression of 3-methylcholanthrene-induced malignant transformation when added 7 days after removal of carcinogen. IBX was the most potent, causing 100% inhibition at 10^-4 m and 70% inhibition at 10^-5 m. This inhibition was partially reversible in the former case and completely reversible in the latter case by removal of drug. Complete inhibition by 10^-4 m IBX was still observed when treatment was delayed 21 days postcarcinogen. In reconstruction experiments, utilizing confluent monolayers of 10T½ cells overlaid with transformed cells, IBX caused a dose-dependent inhibition of colony size of the transformed cells. Adenosine cyclic 2′:3′-monophosphoric acid (cAMP) and N⁶,O²-dibutyryladenosine cyclic 3′:5′-monophosphoric acid potentiated this response. The presence of nontransformed 10T½ cells was required for this effect, since a concentration of IBX (10^-4 m) inhibitory for the growth of transformed cells in mixed cultures was without effect on the growth rate, plating efficiency, or saturation density of pure cultures of 10T½ cells or of their transformed counterparts. Conditioned medium removed from IBX-treated 10T½ cells was not growth inhibitory for transformed cells, indicating a requirement for cell-cell contact. IBX caused a dose-dependent increase in intracellular cAMP in confluent 10T½ cells and a more pronounced increase in cAMP concentration in the culture medium of these cells. The dose-response effects of IBX on growth inhibition of malignant cells in mixed cultures appear to correlate well with its ability to elevate cAMP levels. Thus, IBX increased the capacity of 10T½ cells to cause reversible growth arrest of transformed cells and appears to act in a manner analogous to the previously reported effects of serum.