RNA-protein cross-links induced by sensitization with a pyrroloquinolinone derivative, a furocoumarin analogue.

The capacity of 2,6-dimethyl-9-methoxy-4H-pyrrolo [3,2,1-ij] quinolin-4-one (PQ), a furocoumarin analogue, of inhibiting protein synthesis in Ehrlich cells upon UVA irradiation was investigated. Using 8-methoxypsoralen (8-MOP) as a reference, we observed that in our short-term test the block of RNA synthesis do not affect protein synthesis, which is driven by pre-synthesised molecules of m-RNA; actually 8-MOP, studied at 100 microM concentration, practically abolished RNA synthesis without affecting significantly protein synthesis. Studying PQ sensitization in HL60 cells by alkaline elution and protein precipitation, the formation of covalent RNA-protein cross-links was observed. 8-MOP, assayed in severe experimental conditions, induced only moderate amounts of such lesion. On the basis of the data obtained in experiments carried out using various scavengers or exposing cells to UVA light in a nitrogen atmosphere, this damage appeared to be due to singlet oxygen formation, which is generated by PQ to a large extent. These results are consistent with the data obtained by H. Singh and J.A. Vadasz (Singlet oxygen: a major reactive species in the furocoumarin photosensitized inactivation of E.coli ribosomes, Photochem. Photobiol., 28 (1978) 539-545) on E.coli ribosomes. The lower activity we observed with 8-MOP might be attributed to a different sensitivity of whole mammalian cells in comparison with isolated ribosomes.