Inhibition of HSV-transformed murine cells by nucleoside analogs, 2′-NDG and 2′-nor-cGMP: Mechanisms of inhibition and reversal by exogenous Nucleosides

Abstract

A murine cell line transformed with HSV TK (LH-1) exhibits a greatly enhanced cytotoxicity to the nucleoside analog 9-[(2-hydroxy-1-(hydroxymethyl)ethoxy) methyl]guanine (2′-NDG) as compared to the parental LM cell line ( I 50 LH-1 = 0.4 μ M, I 50 LM = 44.4 μ M). Toxicity of 2′-NDG for LH-1 and LM is reversed only by the addition of 100 μ M thymidine (dThd), indicating that 2′-NDG is a substrate for the viral and cellular TK. In LM(TK −) cells-murine cells expressing no TK activity, 2′-NDG cytotoxicity is partially reversed only with dGuo. A cyclic phosphate derivative of 2′-NDG, 2′-norcGMP, contains a phosphodiester bond, is also taken up by cells, and does not depend on viral TK for activation. LH-1 cells and LM(TK −) cells are inhibited by similar concentrations of this analog (5.1 and 4.1 μ M, respectively). In all three cell lines (LM, LH-1, LM(TK-)), the toxicity of 2′-nor-cGMP is significantly reversed with dGuo or cyclic dGMP. This pattern of reversal differs significantly from that observed with 2′-NDG, suggesting that 2′-nor-cGMP is metabolized as a guanosine analog, similar to acyclovir, in LM and LM(TK −) cells. These results indicate that a cyclic monophosphate analog of 2′-NDG can be activated independently of viral TK expression and that cellular metabolic pathways resulting in elevated dGTP concentrations are important for reversal of toxicity induced by guanosine-like nucleoside analogs.