LOX/COX inhibitors enhance the antineoplastic effects of all-trans retinoic acid in osteosarcoma cell lines

Thiosemicarbazones Can Act Synergistically with Anthracyclines to Downregulate CHEK1 Expression and Induce DNA Damage in Cell Lines Derived from Pediatric Solid Tumors

Anticancer therapy by anthracyclines often leads to the development of multidrug resistance (MDR), with subsequent treatment failure. Thiosemicarbazones have been previously suggested as suitable anthracycline partners due to their ability to overcome drug resistance through dual Pgp-dependent cytotoxicity-inducing effects. Here, we focused on combining anthracyclines (doxorubicin, daunorubicin, and mitoxantrone) and two thiosemicarbazones (DpC and Dp44mT) for treating cell types derived from the most frequent pediatric solid tumors. Our results showed synergistic effects for all combinations of treatments in all tested cell types. Nevertheless, further experiments revealed that this synergism was independent of Pgp expression but rather resulted from impaired DNA repair control leading to cell death via mitotic catastrophe. The downregulation of checkpoint kinase 1 (CHEK1) expression by thiosemicarbazones and the ability of both types of agents to induce double-strand breaks in DNA may explain the Pgp-independent synergism between anthracyclines and thiosemicarbazones. Moreover, the concomitant application of these agents was found to be the most efficient approach, achieving the strongest synergistic effect with lower concentrations of these drugs. Overall, our study identified a new mechanism that offers an avenue for combining thiosemicarbazones with anthracyclines to treat tumors regardless the Pgp status.

Novel Thiosemicarbazones Sensitize Pediatric Solid Tumor Cell-Types to Conventional Chemotherapeutics through Multiple Molecular Mechanisms

Simple Summary

Combination of chemotherapeutics for the treatment of childhood cancer can lead to the use of lower cytotoxic drug doses and better therapeutic tolerability (i.e., lower side effects) for patients. We discovered novel molecular targets of two lead thiosemicarbazone agents of the di-2-pyridylketone thiosemicarbazone class. These molecular targets include: cyclooxygenase, the DNA repair protein, O6-methylguanine DNA methyltransferase, mismatch repair proteins, and topoisomerase 2α. This research also identifies promising synergistic interactions of these thiosemicarbazones particularly with the standard chemotherapeutic, celecoxib.

Abstract

Combining low-dose chemotherapies is a strategy for designing less toxic and more potent childhood cancer treatments. We examined the effects of combining the novel thiosemicarbazones, di-2-pyridylketone 4-cyclohexyl-4-methyl-3-thiosemicarbazone (DpC), or its analog, di-2-pyridylketone-4,4-dimethyl-3-thiosemicarbazone (Dp44mT), with the standard chemotherapies, celecoxib (CX), etoposide (ETO), or temozolomide (TMZ). These combinations were analyzed for synergism to inhibit proliferation of three pediatric tumor cell-types, namely osteosarcoma (Saos-2), medulloblastoma (Daoy) and neuroblastoma (SH-SY5Y). In terms of mechanistic dissection, this study discovered novel thiosemicarbazone targets not previously identified and which are important for considering possible drug combinations. In this case, DpC and Dp44mT caused: (1) up-regulation of a major protein target of CX, namely cyclooxygenase-2 (COX-2); (2) down-regulation of the DNA repair protein, O⁶-methylguanine DNA methyltransferase (MGMT), which is known to affect TMZ resistance; (3) down-regulation of mismatch repair (MMR) proteins, MSH2 and MSH6, in Daoy and SH-SY5Y cells; and (4) down-regulation in all three cell-types of the MMR repair protein, MLH1, and also topoisomerase 2α (Topo2α), the latter of which is an ETO target. While thiosemicarbazones up-regulate the metastasis suppressor, NDRG1, in adult cancers, it is demonstrated herein for the first time that they induce NDRG1 in all three pediatric tumor cell-types, validating its role as a potential target. In fact, siRNA studies indicated that NDRG1 was responsible for MGMT down-regulation that may prevent TMZ resistance. Examining the effects of combining thiosemicarbazones with CX, ETO, or TMZ, the most promising synergism was obtained using CX. Of interest, a positive relationship was observed between NDRG1 expression of the cell-type and the synergistic activity observed in the combination of thiosemicarbazones and CX. These studies identify novel thiosemicarbazone targets relevant to childhood cancer combination chemotherapy.

Enhanced Antiproliferative Effect of Combined Treatment with Calcitriol and All-Trans Retinoic Acid in Relation to Vitamin D Receptor and Retinoic Acid Receptor α Expression in Osteosarcoma Cell Lines

The main objective of this study was to analyze changes in the antiproliferative effect of vitamin D3, in the form of calcitriol and calcidiol, via its combined application with all-trans retinoic acid (ATRA) in osteosarcoma cell lines. The response to treatment with calcitriol and calcidiol alone was specific for each cell line. Nevertheless, we observed an enhanced effect of combined treatment with ATRA and calcitriol in the majority of the cell lines. Although the levels of respective nuclear receptors did not correlate with the sensitivity of cells to these drugs, vitamin D receptor (VDR) upregulation induced by ATRA was found in cell lines that were the most sensitive to the combined treatment. In addition, all these cell lines showed high endogenous levels of retinoic acid receptor α (RARα). Our study confirmed that the combination of calcitriol and ATRA can achieve enhanced antiproliferative effects in human osteosarcoma cell lines in vitro. Moreover, we provide the first evidence that ATRA is able to upregulate VDR expression in human osteosarcoma cells. According to our results, the endogenous levels of RARα and VDR could be used as a predictor of possible synergy between ATRA and calcitriol in osteosarcoma cells.

Oridonin synergizes with Nutlin-3 in osteosarcoma cells by modulating the levels of multiple Bcl-2 family proteins

The small-molecule inhibitors of p53-murine double minute 2 interaction, such as Nutlin-3, are effective against cancers bearing wild-type p53. However, murine double minute 2 inhibitors often are unable to completely eliminate solid tumor cells. To address this issue, we investigated the anticancer effects of Nutlin-3 in combination with Oridonin in osteosarcoma cells. We found that Oridonin at sub-toxic concentrations synergistically enhanced Nutlin-3-mediated cell viability inhibition in wild-type p53 U2OS and SJSA-1, but not in p53-mutant MNNG/HOS and in null-p53 Saos-2 osteosarcoma cell lines. Importantly, in the presence of Oridonin, Nutlin-3 could completely abolish cell viability in the wild-type p53 osteosarcoma cell lines. Western blotting analysis showed that Oridonin treatment rapidly and distinctly increased the levels of all three forms of Bim and also markedly reduced the levels of Bcl-2 and Bcl-xl in osteosarcoma cells. Western blotting analysis further showed that Oridonin considerably enhanced Nutlin-3-triggered activation of caspases-9 and -3 and poly(ADP-ribose) polymerase cleavage. Flow cytometry assay showed that Oridonin significantly enhanced Nutlin-3-mediated apoptosis in wild-type p53 osteosarcoma cells. Overall, our results suggest that the combined treatment of Nutlin-3 plus Oridonin may offer a novel therapeutic strategy for osteosarcoma.

Non-DHFR-mediated effects of methotrexate in osteosarcoma cell lines: epigenetic alterations and enhanced cell differentiation

Background

Methotrexate is an important chemotherapeutic drug widely known as an inhibitor of dihydrofolate reductase (DHFR) which inhibits the reduction of folic acid. DHFR-mediated effects are apparently responsible for its primary antineoplastic action. However, other non-DHFR-mediated effects of methotrexate have been recently discovered, which might be very useful in the development of new strategies for the treatment of pediatric malignancies. The principal goal of this study was to analyze the possible impact of clinically achievable methotrexate levels on cell proliferation, mechanisms of epigenetic regulation (DNA methylation and histone acetylation), induced differentiation and the expression of differentiation-related genes in six osteosarcoma cell lines.

Methods

The Saos-2 reference cell line and five other patient-derived osteosarcoma cell lines were chosen for this study. The MTT assay was used to assess cell proliferation, DNA methylation and histone acetylation were detected using ELISA, and western blotting was used for a detailed analysis of histone acetylation. The expression of differentiation-related genes was quantified using RT-qPCR and the course of cell differentiation was evaluated using Alizarin Red S staining, which detects the level of extracellular matrix mineralization.

Results

Methotrexate significantly decreased the proliferation of Saos-2 cells exclusively, suggesting that this reference cell line was sensitive to the DHFR-mediated effects of methotrexate. In contrast, other results indicated non-DHFR-mediated effects in patient-derived cell lines. Methotrexate-induced DNA demethylation was detected in almost all of them; methotrexate was able to lower the level of 5-methylcytosine in treated cells, and this effect was similar to the effect of 5-aza-2′-deoxycytidine. Furthermore, methotrexate increased the level of acetylated histone H3 in the OSA-06 cell line. Methotrexate also enhanced all-trans retinoic acid-induced cell differentiation in three patient-derived osteosarcoma cell lines, and the modulation of expression of the differentiation-related genes was also shown.

Conclusions

Overall non-DHFR-mediated effects of methotrexate were detected in the patient-derived osteosarcoma cell lines. Methotrexate acts as an epigenetic modifier and has a potential impact on cell differentiation and the expression of related genes. Furthermore, the combination of methotrexate and all-trans retinoic acid can be effective as a differentiation therapy for osteosarcoma.