2,2-Diphenethyl Isothiocyanate Enhances Topoisomerase Inhibitor-Induced Cell Death and Suppresses Multi-Drug Resistance 1 in Breast Cancer Cells

Simple Summary Mutations in the p53 gene frequently occur in human cancers. The mutations are localized to "hotspot" residues and are classified as structural or contact mutants. p53 mutations disable wild-type (WT) p53 activity; they exert a "dominant negative" effect on WT p53 activity or a "gain-of-function" activity rendering the p53 mutants oncogenic. Mutant p53 rescue by small molecules is a promising chemotherapeutic strategy. Previously, we discovered that a dietary-related phenethyl isothiocyanate could reactivate p53(R175H) structural mutants in HER2 + SK-BR-3 breast cancer (BC) cells. The current study revealed that 2,2-diphenethyl isothiocyanate (DPEITC) is a more potent synthetic analog that can inhibit the growth of different subtypes of BC cells, irrespective of p53 mutant-type, via mutant p53 rescue. DPEITC acts synergistically with the topoisomerase inhibitors doxorubicin and camptothecin. Furthermore, for the first time, we report that DPEITC suppresses multi-drug resistance 1 and ETS1, which have been shown to play a role in chemoresistance. We previously reported that phenethyl isothiocyanate (PEITC), a dietary-related compound, can rescue mutant p53. A structure-activity relationships study showed that the synthetic analog 2,2-diphenylethyl isothiocyanate (DPEITC) is a more potent inducer of apoptosis than natural or synthetic ITCs. Here, we showed that DPEITC inhibited the growth of triple-negative breast cancer cells (MDA-MB-231, MDA-MB-468, and Hs578T) expressing "hotspot" p53 mutants, structural (p53(R280K), p53(R273H)) or contact (p53(V157F)), at IC50 values significantly lower than PEITC. DPEITC inhibited the growth of HER2+ (p53(R175H) SK-BR-3, p53(R175H) AU565) and Luminal A (p53(L194F) T47D) breast cancer (BC) cells harboring a p53 structural mutant. DPEITC induced apoptosis, irrespective of BC subtypes, by rescuing p53 mutants. Accordingly, the rescued p53 mutants induced apoptosis by activating canonical WT p53 targets and delaying the cell cycle. DPEITC acted synergistically with doxorubicin and camptothecin to inhibit proliferation and induce apoptosis. Under these conditions, DPEITC delayed BC cells in the G1 phase, activated p53 canonical targets, and enhanced pS1981-ATM. DPEITC reduced the expression of MDR1 and ETS1. These findings are the first report of synergism between a synthetic ITC and a chemotherapy drug via mutant p53 rescue. Furthermore, our data demonstrate that ITCs suppress the expression of cellular proteins that play a role in chemoresistance.