Br-cAMP induction of apoptosis in synchronized CHO cells.

The proliferation of suspension cultures of malignant CHO cells was inhibited by 0.5 mM Br-cAMP treatment and restored by its removal. This treatment also inhibited histone H1 phosphorylation completely, reduced histones H2A and H4 phosphorylations, induced DNA degradation, and produced cells containing micronuclei. Agarose gel electrophoresis of the degraded DNA fragments produced a "ladder" pattern confirming these cells were undergoing apoptosis. Cell cycle synchrony experiments demonstrated culture growth inhibition was the result of two different cell cycle-specific processes: [1] arrested cell cycle traverse at a restriction point in mid-G1, and [2] rapid apoptosis following cell division. Br-cAMP did not stop cells in late-G1, S, G2, or M from traversing the cell cycle and dividing, but rather, induced apoptosis following mitosis. The restriction point of Br-cAMP arrest was located in the middle of a wider band of G1 arrest induced by isoleucine deprivation. The cells synchronized in G1 before the restriction point were held in G1-arrest by Br-cAMP and spared apoptotic death. These studies support the further study of cAMP derivatives as agents to induce tumor regression by apoptosis and reverse transformation.