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)2 Protein 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 microM) 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.