cAMP Analogs Induce p53-independent p21 Expression through Sp1-responsive Promoter Activation and p300- associated Epigenetic Regulation in PC-3 Human Prostate Cancer Cells

cAMP signaling regulates cell proliferation and survival in various cancers, but the molecular mechanisms underlying its antiproliferative effects in androgen-independent prostate cancer are not fully understood.This study investigated the growth-inhibitory effects of cAMP signaling and its regulatory mechanisms in p53-deficient PC-3 human prostate cancer cells using cAMP analogs, 8-bromo-cAMP and dibutyryl-cAMP.Both cAMP analogs significantly suppressed PC-3 cell proliferation in a time-dependent manner, inducing G1 cell cycle arrest and apoptotic cell death.cAMP treatment reduced cyclin-dependent kinase (Cdk) activity and retinoblastoma protein (pRB) phosphorylation, which enhanced pRB-E2F complex formation and repressed E2F-dependent signaling.cAMP analogs significantly upregulated p21 expression at both transcriptional and translational levels, independently of p53.The increased binding of p21 to Cdk2 and Cdk4 supported its role in inhibiting cyclin-Cdk.Promoter analyses showed that cAMP transcriptionally activated the p21 promoter via Sp1-responsive elements.While the transcription co-activator p300, possessing histone acetyltransferase activity, enhanced cAMP-induced p21 promoter activity, dominant-negative p300 attenuated this effect.Furthermore, trichostatin A, a histone deacetylase inhibitor, potentiated p21 promoter activation, indicating that histone acetylation is crucial for cAMP-mediated p21 transcription.In conclusion, cAMP analogs suppress PC-3 cell proliferation through a mechanism involving p53-independent p21 induction, together with Sp1-responsive promoter activation and p300-associated epigenetic regulation.