Magalí Sáez-Ayala

Reactivation of the Tumour Suppressor RASSF1A in Breast Cancer by Simultaneous Targeting of DNA and E2F1 Methylation

Background Tumour suppressor genes are often transcriptionally silenced by promoter hypermethylation, and recent research has implicated alterations in chromatin structure as the mechanistic basis for this repression. In addition to DNA methylation, other epigenetic post-translational modifications that modulate the stability and binding of specific transcription factors to gene promoters have emerged as important mechanisms for controlling gene expression. The aim of this study was to analyse the implications of these mechanisms and their molecular connections in the reactivation of RASSF1A in breast cancer. Methods Compounds that modulate the intracellular concentration of adenosine, such as dipyridamole (DIPY), greatly increase the antiproliferative effects of 3-O-(3,4,5-trimethoxybenzoyl)-(−)-catechin (TMCG), a synthetic antifolate derived from the structure of tea catechins. Quantitative real-time PCR arrays and MALDI-TOF mass spectrometry indicated that this combination (TMCG/DIPY) induced apoptosis in breast cancer cells by modulating the methylation levels of DNA and proteins (such as E2F1), respectively. Chromatin immunoprecipitation (ChIP) assays were employed to confirm that this combination induced chromatin remodelling of the RASSF1A promoter and increased the occupancy of E2F1 at the promoter of this tumour suppressor gene. Results The TMCG/DIPY combination acted as an epigenetic treatment that reactivated RASSF1A expression and induced apoptosis in breast cancer cells. In addition to modulating DNA methylation and chromatin remodelling, this combination also induced demethylation of the E2F1 transcription factor. The ChIP assay showed enhancement of E2F1 occupancy at the unmethylated RASSF1A promoter after TMCG/DIPY treatment. Interestingly, inhibition of E2F1 demethylation using an irreversible inhibitor of lysine-specific demethylase 1 reduced both TMCG/DIPY-mediated RASSF1A expression and apoptosis in MDA-MB-231 cells, suggesting that DNA and protein demethylation may act together to control these molecular and cellular processes. Conclusions/Significance This study demonstrates that simultaneous targeting of DNA and E2F1 methylation is an effective epigenetic treatment that reactivates RASSF1A expression and induces apoptosis in breast cancer cells.

Binding of natural and synthetic polyphenols to human dihydrofolate reductase.

Dihydrofolate reductase (DHFR) is the subject of intensive investigation since it appears to be the primary target enzyme for antifolate drugs. Fluorescence quenching experiments show that the ester bond-containing tea polyphenols (-)-epigallocatechin gallate (EGCG) and (-)-epicatechin gallate (ECG) are potent inhibitors of DHFR with dissociation constants (KD)of 0.9 and 1.8 μM, respectively, while polyphenols lacking the ester bound gallate moiety [e.g., (-)-epigallocatechin (EGC) and (-)-epicatechin (EC)] did not bind to this enzyme. To avoid stability and bioavailability problems associated with tea catechins we synthesized a methylated derivative of ECG (3-O-(3,4,5-trimethoxybenzoyl)-(-)-epicatechin; TMECG), which effectively binds to DHFR (KD = 2.1 μM). In alkaline solution, TMECG generates a stable quinone methide product that strongly binds to the enzyme with a KD of 8.2 nM. Quercetin glucuronides also bind to DHFR but its effective binding was highly dependent of the sugar residue, with quercetin-3-xyloside being the stronger inhibitor of the enzyme with a KD of 0.6 μM. The finding that natural polyphenols are good inhibitors of human DHFR could explain the epidemiological data on their prophylactic effects for certain forms of cancer and open a possibility for the use of natural and synthetic polyphenols in cancer chemotherapy.

Melanoma coordinates general and cell-specific mechanisms to promote methotrexate resistance.

Melanoma, the most aggressive form of skin cancer, is notoriously resistant to all current modalities of cancer therapy, including to the drug methotrexate. Melanosomal sequestration and cellular exportation of methotrexate have been proposed to be important melanoma-specific mechanisms that contribute to the resistance of melanoma to methotrexate. In addition, other mechanisms of resistance that are present in most epithelial cancer cells are also operative in melanoma. This report elucidates how melanoma orchestrates these mechanisms to become extremely resistant to methotrexate, where both E2F1 and checkpoint kinase 1 (Chk1), two molecules with dual roles in survival/apoptosis, play prominent roles. The results indicated that MTX induced the depletion of dihydrofolate in melanoma cells, which stimulated the transcriptional activity of E2F1. The elevate expression of dihydrofolate reductase and thymidylate synthase, two E2F1-target genes involved in folate metabolism and required for G(1) progression, favored dTTP accumulation, which promoted DNA single strand breaks and the subsequent activation of Chk1. Under these conditions, melanoma cells are protected from apoptosis by arresting their cell cycle in S phase. Excess of dTTP could also inhibit E2F1-mediated apoptosis in melanoma cells.

Comparison of a Pair of Synthetic Tea-Catechin-Derived Epimers: Synthesis, Antifolate Activity, and Tyrosinase-Mediated Activation in Melanoma

Despite bioavailability issues, tea catechins have emerged as promising chemopreventive agents because of their efficacy in various animal models. We synthesized two catechin‐derived compounds, 3‐O‐(3,4,5‐trimethoxybenzoyl)‐(−)‐catechin (TMCG) and 3‐O‐(3,4,5‐trimethoxybenzoyl)‐(−)‐epicatechin (TMECG), in an attempt to improve the stability and cellular absorption of tea polyphenols. The antiproliferative and pro‐apoptotic activities of both compounds were analyzed with various cancer cell systems, and TMCG, which was easily synthesized in excellent yield, was more active than TMECG in both melanoma and non‐melanoma cell lines. TMCG was also a better inhibitor of dihydrofolate reductase and was more efficiently oxidized by tyrosinase, potentially explaining the difference in activity between these epimers.

Cellular and Molecular Mechanisms of Methotrexate Resistance in Melanoma

Melanoma is a cancer that develops in melanocytes, the pigment cells present in the skin. It can be more serious than the other forms of skin cancer because it may spread to other parts of the body (metastasize) and cause serious illness and death. For malignant melanomas standard treatment options have remained remarkably static over the past 30 years [1,2]. At present, the incidence of melanoma continues to increase despite public health initiatives that have promoted protection against the sun. Thus, during the past ten years, the incidence and annual mortality of melanoma has increased more rapidly than any other cancer and according to the American Cancer Society estimate, there will have been approximately 76,250 new cases of invasive melanoma diagnosed in 2012 in the United States, which resulted in approximately 9,180 deaths [3].

Factors influencing the antifolate activity of synthetic tea-derived catechins.

Novel tea catechin derivatives have been synthesized, and a structure-activity study, related to the capacity of these and other polyphenols to bind dihydrofolate reductase (DHFR), has been performed. The data showed an effective binding between all molecules and the free enzyme, and the dissociation constants of the synthetic compounds and of the natural analogues were on the same order. Polyphenols with a catechin configuration were better DHFR inhibitors than those with an epicatechin configuration. Antiproliferative activity was also studied in cultured tumour cells, and the data showed that the activity of the novel derivatives was higher in catechin isomers. Derivatives with a hydroxyl group para on the ester-bonded gallate moiety presented a high in vitro binding to DHFR, but exhibited transport problems in cell culture due to ionization at physiologic pHs. The impact of the binding of catechins to serum albumin on their biological activity was also evaluated. The information provided in this study could be important for the design of novel medicinal active compounds derived from tea catechins. The data suggest that changes in their structure to avoid serum albumin interactions and to facilitate plasmatic membrane transport are essential for the intracellular functions of catechins.

Novel Antifolates as Produgs for the Treatment of Melanoma

Malignant melanoma is a deadly disease in which standard treatment options have remained remarkably static over the past 30 years (Sullivan & Atkins, 2009). At present, the incidence of melanoma continues to increase despite public health initiatives that have promoted protection against the sun. Thus, during the past ten years, the incidence and annual mortality of melanoma has increased more rapidly than any other cancer and according to an American Cancer Society estimate, there will have been approximately 68,720 new cases of invasive melanoma diagnosed in 2009 in the United States, which resulted in approximately 8,650 deaths (American Cancer Society, 2009). Unfortunately, the increase in incidence has not been paralleled by the development of new therapeutic agents with a significant impact on survival. Although many patients with melanoma localized to the skin are cured by surgical excision, increased time to diagnosis is associated with higher stage of disease, and those with regional lymphatic or metastatic disease respond poorly to conventional radiation and chemotherapy with 5-year survival rates ranging from 10 to 50% (Tawbi & Buch, 2010). Currently, limited therapeutic options exist for patients with metastatic melanomas, and all standard combinations currently used in metastasis therapy have low efficacy and poor response rates. One example of the complications involved in melanoma chemotherapy is the limited effectiveness of antifolates. Although methotrexate (MTX), the most frequently used antifolate, is an efficient drug for several types of cancer, it is not active against melanoma (Kufe et al., 1980). Undoubtedly, unravelling the mechanism of the resistance of melanomas to this drug could help to improve current therapeutic approaches. Moreover, it could help to develop a novel generation of antifolate drugs that overcome resistance problems and present low toxicity for the prophylaxis and treatment of melanoma.