Proteomic and pharmacological analyses of the mechanism of actions of anticancer gold(I) complexes

Abstract

Gold complexes have a long history of being used as therapeutic agents, especially in applications against immune diseases such as rheumatoid arthritis. In 1979, an oral gold(I) drug – auranofin (AuRF, brand name as Ridaura®) – was demonstrated to exhibit anticancer properties. Since then, a considerable number of gold(I) complexes have been reported to show remarkable anticancer activities, but the understanding of their mechanism of actions is limited.

In the present study, AuRF and several other anticancer gold(I)-phosphine complexes including AuPEt ([Au(triethylphosphine)Cl]) were demonstrated to induce autophagy – a cellular catabolic process of macromolecules and organelles through lysosomal degradation. The induced autophagy involved the accumulation of autophagosomes, which was mediated by the enhancement of autophagy initiation rather than by the blockage of autophagosomes maturation. Moreover, the AuRF and AuPEt induced autophagy was demonstrated to have a pro-survival effect for the cancer cells.

To better explore the mechanism of actions of AuRF and other anticancer gold(I) complexes, a subcellular fractionation-based proteomic approach has been developed and optimized. This approach combined the use of subcellular fractionation, protein extraction, HPLC-LTQ-Orbitrap mass spectrometry, and bottom-up protein identification and quantification. By using this approach, the proteome coverage was increased, the complexities of the sub-proteomes were reduced, and the low-abundant organelle proteins were enriched.

The nuclear sub-proteomes of AuRF-treated or AuPEt-treated cells were analyzed to identify the significantly regulated transcription regulators and the signaling pathways involved. The analysis delineates the possible AuRF-activated anticancer pathways involving up-regulation of the tumor suppressor cyclin-dependent kinase inhibitor 2A (〖p14〗^ARF), inhibition of the E2F transcription activity, blocking of the translocation of E3 ubiquitin-protein ligase (MDM2) from nucleus to cytoplasm and induction of the tumor suppressor p53. Furthermore, the KeyNode-based pathway analysis was applied to analyze the whole proteomes obtained from merging the sub-proteomes. Alongside the p53 pathway and E2F network, the regulation of 3-hydroxy-3-methylglutaryl-CoA reductase (HMGCR, the rate-limiting enzyme of cholesterol biosynthesis) is one of the most up-regulated pathways of AuRF treatment. AuRF also showed significant inhibition to HMGCR activity in vitro with an IC50 value at the micromolar level.

The effects of AuRF and AuPEt on the high mobility group box-1 protein (HMGB1), which exhibits distinct functions dependent on its cellular locations, were investigated. Treatment of cells with AuRF or AuPEt resulted in down-regulation of nuclear HMGB1, which is associated with p53-dependent cytotoxicities. The cytoplasmic HMGB1, which can induce autophagy, was found to be up-regulated. The levels of secreted HMGB1, which exhibits pro-inflammatory properties, were reduced, possibly contributing to anti-rheumatoid arthritis actions of AuRF.

Collectively, the pharmacological and proteomic analyses in this research of AuRF and other anticancer gold(I) complexes supplement the current knowledge of their mechanism of actions.