Absence of Activation of DNA Repair Genes and Excellent Efficacy of Phosphaplatins against Human Ovarian Cancers: Implications To Treat Resistant Cancers

Abstract

Phosphaplatins, platinum(II) and platinum(IV) complexes coordinated to a pyrophosphate moiety, exhibit excellent antitumor activities against a variety of cancers. To determine whether phosphaplatins trigger resistance to treatment by engaging DNA damage repair genes, a yeast genome-wide fitness assay was used. Treatment of yeast cells with pyrodach-2 (D2) or pyrodach-4 (D4) revealed no particular sensitivity to nucleotide excision repair, homologous recombination repair, or postreplication repair when compared with platin control compounds. Also, TNF receptor superfamily member 6 (FAS) protein was overexpressed in phosphaplatin-treated ovarian tumor cells, and platinum colocalized with FAS protein in lipid rafts. An overactivation of sphingomyelinase (ASMase) was noted in the treated cells, indicating participation of an extrinsic apoptotic mechanism due to increased ceramide release. Our results indicate that DNA is not the target of phosphaplatins and accordingly, that phosphaplatins might not cause resistance to treatment. Activation of ASMase and FAS along with the colocalization of platinum with FAS in lipid rafts support an extrinsic apoptotic signaling mechanism that is mediated by phosphaplatins.

Phosphaplatin Anti-tumor Effect Enhanced by Liposomes Partly via an Up-regulation of PEDF in Breast Cancer

“…Previous work from our laboratory revealed that decreased cell viability was accompanied by FAS overexpression in cells treated with RRD2 (6). Similarly, we found that decreased cell viability accompanied by increased cell apoptosis in 4T1 cells treated with either free RRD4 or LD was paralleled by an overexpression of FAS protein.…”

“…Neither lysates nor supernatants from 4T1 cultures treated with EL produced significant amounts of activated caspase-3 relative to cultures treated with PBS (Table V). Previous data from our laboratory showed that TNF receptor superfamily member 6 (FAS) protein was overexpressed in phosphaplatin-treated ovarian tumor cells (6). Thus, we sought to determine whether FAS protein was also present in 4T1 cells treated with and without RRD4 using Western blot analysis.…”

“…Both free and liposome formulated can efficiently induce apoptotic death of 4T1 tumor cells. Our early studies showed that one of the key antitumor mechanisms of phosphaplatins is by inducing apoptotic death of tumor cells of ovarian origin (6). We first sought to determine the viability of 4T1 cells in the presence and absence of RRD4 using the CellTiter-Glo™ luminescence assay.…”

“…aquation and phosphate hydrolysis of phosphaplatins, together with the second-order reaction of this complex with cysteine, might have significant implications in exhibiting reduced toxicities versus other platinum compounds (4). Furthermore, previous work from our laboratory showed that there were no platinum-DNA adducts, no nucleotide excision repair, homologous recombination repair, or post-replication repair in cancer cells treated with phosphaplatins (6). Instead, phosphaplatins promoted overexpression of the death receptor FAS and sphingomyelinase proteins, implying protein targeting and stimulation of an extrinsic apoptotic signaling mechanism.…”

“…We evaluated the ability of phosphaplatins to reduce regional tumor growth and metastasis. We specifically selected the RRD4 compound, since it exhibits extremely low in vitro cytotoxic activity, with IC 50 values as low 0.2 μM when tested on a panel of human cancer cells (6). The compound was given either as the free form or encapsulated in liposomes (LD).…”