Despite recent advances in chemotherapy, aggressive and metastatic breast cancers remain refractory to targeted therapy and the development of novel drugs is urgently needed. Retinoids are crucial regulators of cellular proliferation, differentiation, and cell death, and have shown potent chemotherapeutic and chemopreventive properties. The major drawback of the use of all-trans retinoic acid (ATRA) in cancer therapy is disease relapse. Therefore, synthetic retinoids, specifically ST1926, have emerged as potent anticancer agents. Given the importance of the microenvironment in modulating the response of cancer cells to chemotherapeutic drugs, we investigated the antitumor activities of ST1926 in two-dimensional (2D) and different three-dimensional (3D) human breast cancer models and compared them with ATRA. We have shown that in 2D cell culture models, ATRA-resistant MCF-7 and MDA-MB-231 cells were sensitive to ST1926 at submicromolar concentrations that spared the 'normal-like' breast epithelial cells. ST1926 induced apoptosis and S-phase arrest, caused DNA damage, and downregulated the Wnt/β-catenin pathway in breast cancer cells in 2D and 3D cell culture models. ST1926-mediated growth inhibition was independent of the retinoid receptor-signaling pathway. Long-term treatments with low submicromolar ST1926 concentrations reduced the anchorage-independent growth and decreased the sphere-forming ability of breast cancer progenitor cells in the sphere formation assay. Furthermore, ST1926 potently induced cell death of breast cancer cells under 3D conditions and spared the lumen-forming ability of normal-like breast epithelial cells. In tested 3D models, ATRA had minimal effects on the growth of breast cancer cells compared with ST1926. In summary, our results highlight the therapeutic potential of ST1926 in breast cancer and warrant its further clinical development.

The promotion stage in the multistep process of epidermal tumorigenesis is NF-kB-dependent, epigenetically regulated, and reversible, thus, a suitable target for chemoprevention. We investigated whether the NF-kB inhibitor, parthenolide, currently in cancer clinical trials, attenuates tumor promotion by modulating the epigenetically regulated NF-kB target genes, p21 and cyclin D1.Parthenolide selectively inhibited the growth of neoplastic keratinocytes while sparing normal ones. Specifically, in JB6Pþ cells, a model of tumor promotion, noncytotoxic parthenolide concentrations abrogated tumor promoter-induced cell proliferation and anchorage-independent growth. Furthermore, parthenolide decreased tumor promoter-induced NF-kB activity, increased p21, and decreased cyclin D1 expression. In parthenolide-treated cells, p21 transcription correlated with relaxed chromatin and p65/NFkB binding at the p21 promoter. However, cyclin D1 transcription correlated more with p65/NF-kB binding than with chromatin structure at the cyclin D1 promoter. Epigenetic regulation by parthenolide seemed specific, as parthenolide did not alter global histone acetylation and methylation and histone deacetylase activity. Because p21 expression by parthenolide was sustained, we used p21-siRNA and p21À/À cancer cells and showed that the loss of p21 is cytoprotective against parthenolide.Low parthenolide concentrations (0.25 mg/kg) inhibited tumor growth of promoted JB6Pþ cells in xenograft immunocompromised mice using two different chemoprevention protocols. Tissue microarray of mouse tumors showed that parthenolide decreased scores of the cell proliferation marker Ki67 and p65/NFkB, whereas it increased p21 expression.These results show that low doses of parthenolide inhibit tumor promotion and epigenetically modulate p21 expression, highlighting the potential role of this drug as a chemopreventive agent and in epigenetic cancer therapy. Cancer Prev Res; 5(11); 1298-309. Ó2012 AACR.

BackgroundSesquiterpene lactones (SL) are plant secondary metabolites that are known for their anti-fungal, anti-bacterial, anti-inflammatory, and anti-tumor properties. Considering that several SL-derived drugs are currently in cancer clinical trials, we have tested two SL molecules, 3-β-methoxy-iso-seco-tanapartholide (β-tan) isolated from Achillea falcata and salograviolide A (Sal A) isolated from Centaurea ainetensis, for their anti-tumor properties. We used the mouse epidermal JB6P + cells as a model for tumor promotion and cellular transformation. Key players that are involved in cellular transformation and tumorigenesis are the AP-1 and NF-κB transcription factors; therefore, we assessed how β-tan and Sal A modulate their signaling pathways in JB6P + cells.MethodsThe effects of β-tan and Sal A on the growth of normal and neoplastic keratinocytes and on the tumor promotion-responsive JB6P + cells were determined using the MTT assay. Anchorage-independent cell growth transformation assays were used to evaluate the anti-tumor promoting properties of these SL molecules in JB6P + cells and dual luciferase reporter assays and western blot analysis were used to investigate their effects on tumor promoter-induced AP-1 and NF-κB activities and protein levels of key AP-1 and NF-кB target genes.Resultsβ-tan and Sal A selectively inhibited tumor promoter-induced cell growth and transformation of JB6P + cells at concentrations that do not affect JB6P + and primary keratinocytes basal cell growth. In addition, both molecules reduced basal and tumor promoter-induced NF-κB transcriptional activities, differentially regulated basal and tumor promoter-induced AP-1 transcriptional activities, and modulated key players of the AP-1 and NF-κB signaling pathways.ConclusionsThese results highlight the anti-tumor promoting properties of β-tan and Sal A. These SL molecules isolated from two plant species native to the Middle East may provide opportunities for complementary medicine practices.

<p>PDF file - 149K, Parthenolide inhibits NF-�B DNA binding affinity</p>

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