Cyclic AMP and theophylline enhance DNA synthesis in L cells stimulated with anti-actin IgG and [(IgG)2 protein A]2 complex by recruiting cells into S-phase.

Surface binding of anti-actin IgG alone or in a Mr = 716 000 [(IgG)2Protein A]2 complex results in a stimulation of DNA synthesis and cell growth in L cells. Cyclic-AMP (0.01–1.0 mM) added to such cell cultures augmented DNA synthesis as measured by incorporation of [3H]thymidine into DNA. Theophylline (0.1–1.0 mM), a phosphodiesterase inhibitor which prevents enzymatic breakdown of cAMP, had similar effects, but cGMP (0.01–1.0 µM) reversed the effects of cAMP and theophylline upon DNA synthesis. Analysis of the cell cycle by flow cytometry revealed that antibody produced a shift (7%) of cells from the G1-phase to the S-phase (DNA-synthetic) of the cell cycle at 72 hr of incubation. Addition of cAMP (0.5 mM) to cell cultures, however, produced significant shifts of antibody stimulated cells from G1-phase to S-phase at all time points measured, i.e., 24 (12%),48 (22%),72 hr (23%). Thus, antibody recruited cells into S-phase of the cell cycle and cAMP greatly augmented the effect. These observations suggest that the mechanism of activation of L cell growth by antibody to surface antigens involves a recruitment of cells into the DNA-synthetic phase and that the effect may be mediated by cAMP.