Nuclear targets of photodynamic tridentate ruthenium complexes

Abstract

Octahedral ruthenium complexes, capable of photodynamic singlet oxygen production at near 100% efficiency, were shown to cause light-dependent covalent crosslinking of p53 and PCNA subunits in mammalian cells and cell lysates. Azide, a singlet oxygen quencher, greatly reduced the p53 photocrosslinking, consistent with the idea that singlet oxygen is the reactive oxygen species involved in p53 photocrosslinking. A photodynamically inactive ruthenium complex, [Ru(tpy)₂]²⁺ (tpy = [2,2′;6′,2′′]-terpyridine), had no effect on p53 or PCNA photocrosslinking. Photodynamic damage to p53 has particular relevance since p53 status is an important determinant of phototoxicity and the effectiveness of photodynamic cancer therapy. The two photodynamic complexes studied, [Ru(tpy)(pydppn)]²⁺, where pydppn = (3-(pyrid-2′-yl)-4,5,9,16-tetraaza-dibenzo[a,c]naphthacene, and [Ru(pydppn)₂]²⁺, differed in their efficiency of p53 and PCNA photocrosslinking in cells, but showed similar efficiency of photocrosslinking in cell lysates, suggesting that they differ in their ability to enter cells. Photocrosslinking of PCNA by [Ru(tpy)(pydppn)]²⁺ increased linearly with concentration, time of uptake, or light exposure. Both [Ru(tpy)(pydppn)]²⁺ and [Ru(pydppn)₂]²⁺ caused photodynamic protein-DNA crosslinking in cells, but [Ru(tpy)(pydppn)]²⁺ was more efficient. The efficiency of photodynamic protein-DNA crosslinking by [Ru(tpy)(pydppn)]²⁺ in cells increased with increasing levels of photodynamic damage. Photodynamic damage by [Ru(tpy)(pydppn)]²⁺ caused inhibition of DNA replication in a classical biphasic response, suggesting that DNA damage signaling and cell cycle checkpoint pathways were still operative after significant damage to nuclear proteins.