Identification of key mechanism in the cytotoxic effect of two novel anti-cancer compounds on breast cancer cells

Abstract

Organometallic chemotherapeutic agents, many of which target DNA, have been shown to be effective in the treatment of cancer. With that said though, these compounds have a number of side affects such as nephrotoxicity. Two novel compounds, Ferrocene [ferrocenoyltrichloroacetone] and Rhodium-ferrocene [(1.5 cyclooctadiene)(1-ferrocenyl- 4,4,4-trichloro-1,3-butanedionate], synthesised by the research group of J Swarts (University of the Free State) were evaluated to determine their mechanism of action and their potential use as novel therapeutic agents. It is hypothesized, by merit of their chemical structures, that these compounds’ anti-cancer activity is due to their interaction with DNA. Both drugs were evaluated from a cellular to a molecular level, in vitro, to validate this hypothesis. Linearised DNA was exposed to both drugs and digested with a variety of restriction enzymes. It was found that the compounds bind to the PstI restriction site; thereby inhibiting the enzyme’s restriction activity. From this point it was necessary to show that the compounds are able to interact with DNA in a cellular system. By exposing a transformed breast epithelial cell line (MCF-12A) and a cancerous breast epithelial cell line (MCF-7) to the compounds, for various times, followed by flow cytometric analyses, it was found that both affect progression through the cell cycle. Cells accumulated at various phases of the cell cycle, as a result of checkpoint gene activation. Further flow cytometric analyses showed that both drugs induce necrosis in MCF-7 cells. The “normal” cell line however did not show this response as it is believed that cell cycle arrest and repair mechanisms were initiated, which would delay cell death. Gene expression analyses were performed by reverse transcriptase real-time PCR in which panels of cell cycle related genes as well as DNA damage associated genes were probed in two separate array formats. These studies revealed that a number of DNA damage and repair genes are activated; specifically those associated with excision repair and free-radical induced DNA damage. Members of the RAD family as well as the genes GADD45A, XPC and OGG1 were found to be upregulated as a result of Ferrocene treatment. This could be expected as it was shown that ferrocene binds to DNA, and it logically then follows that this would lead to excision repair being attempted by the cell. Similar gene expression patterns were found following Rhodium-ferrocene treatment with the up-regulation of genes such as OGG1, ATM and GADD45G, albeit to a lesser extent. It is hypothesised that the larger molecule may not interact as effectively with DNA, due to steric hinderance. Arrest mechanisms, for both drugs, were more pronounced in the “normal” cell line and it is believed that this is due to the fact that many of these genes have been inactivated in the cancerous cell line. We have shown, on multiple levels, that both compounds’ therapeutic action is as a result of their interaction with the cell’s DNA. This interaction leads to cell death in both the transformed and the cancerous cell line. In order to clarify these mechanisms it is suggested that proteomic and metabolomic studies should be performed.