Effects of Acyclic Retinoid on Growth, Cell Cycle Control, Epidermal Growth Factor Receptor Signaling, and Gene Expression in Human Squamous Cell Carcinoma Cells

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Abstract
We described recently the growth inhibitory effects of the novel compound acyclic retinoid (ACR) in human hepatoma cell lines (M. Suzui et al., Cancer Res., 62: 3997–4006, 2002). In this study we examined the cellular and molecular effects of ACR on human squamous cell carcinoma (SCC) cells. ACR inhibited growth of the esophageal SCC cell line HCE7, and the head and neck SCC cell lines YCU-N861 and YCU-H891, with IC50 values of ∼10, 25, and 40 μm, respectively. Detailed studies were then done with HCE7 cells. Treatment of these cells with 10 μm ACR caused an increase of cells in G0-G1 and induced apoptosis. This was associated with two phases of molecular events. During phase 1, which occurred within 6–12 h, there was an increase in the retinoic acid receptor β (RARβ) and p21CIP1 proteins, and their corresponding mRNAs, and a decrease in the hyperphosphorylated form of the retinoblastoma protein. During phase 2, which occurred at ∼24 h, there was a decrease in the cellular level of transforming growth factor α, and the phosphorylated (i.e., activated) forms of the epidermal growth factor receptor, Stat3, and extracellular signal-regulated kinase proteins, and a decrease in both cyclin D1 protein and mRNA. Reporter assays indicated that ACR inhibited the transcriptional activity of the cyclin D1, c-fos, and activator protein promoters. On the other hand, ACR markedly stimulated the activity of a retinoic acid response element-CAT reporter when the cells were cotransfected with a RARβ expression vector. A hypothetical model explaining these two phases is presented. The diverse effects that we obtained with ACR suggest that this agent might be useful in the chemoprevention and/or therapy of human SCCs.