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Vasoactive intestinal peptide and forskolin regulate proliferation of the HT29 human colon adenocarcinoma cell line

Although several lines of evidence implicate cAMP in the regulation of intestinal cell proliferation, the precise role of this second messenger in the control of the human colon cancer cell cycle is still unclear. In order to investigate the role of cAMP in HT29 cell proliferation, we have tested the effect of vasoactive intestinal peptide (VIP) and forskolin on DNA synthesis and cell number, focusing on the time‐dependent efficacy of the treatment. The cells were arrested in GO/G1 phase by incubation for 24 h in serum‐free medium and proliferation was re‐initiated by addition of either 85 nM insulin or 0.5% fetal calf serum. In the presence of fetal calf serum, G1/S transition was found to occur earlier than with insulin. Exposure of the HT29 cells to 10−5 M forskolin in the early stages of growth induction (within 12 h from FCS addition or within 14 h from insulin treatment) resulted in a significant inhibition of DNA synthesis and a delayed entry in the S phase. By contrast, VIP (10−7 M) was inhibitory only when added within a narrow window (10 to 12 h or 12 to 14 h following FCS or insulin addition, respectively). The difference in efficiency of forskolin and VIP to inhibit cell proliferation may be correlated with their own potency to promote long‐lasting cAMP accumulation. The combination of VIP plus forskolin had synergistic effects on both cAMP accumulation and cell‐growth inhibition. Taken together, our data indicate that cAMP may act at a step in the late G1 or G1/S transition.