5'-aminoimidazole-4-carboxamide riboside induces apoptosis in human neuroblastoma cells

5'-Aminoimidazole-4-carboxamide riboside (AICA riboside) has been previously shown to be toxic to two neuronal cell models [Neuroreport 11 (2000) 1827]. In this paper we demonstrate that AICA riboside promotes apoptosis in undifferentiated human neuroblastoma cells (SH-SY5Y), inducing a raise in caspase-3 activity. In order to exert its effect on viability, AICA riboside must enter the cells and be phosphorylated to the ribotide, since both a nucleoside transport inhibitor, and an inhibitor of adenosine kinase produce an enhancement of the viability of AICA riboside-treated cells. Short-term incubations (2 h) with AICA riboside result in five-fold increase in the activity of AMP-dependent protein kinase (AMPK). However, the activity of AMPK is not significantly affected at prolonged incubations (48 h), when the apoptotic effect of AICA riboside is evident. The results demonstrate that when the cell line is induced to differentiate both toward a cholinergic phenotype (with retinoic acid) or a noradrenergic phenotype (with phorbol esters), the toxic effect is significantly reduced, and in the case of the noradrenergic phenotype differentiation, the riboside is completely ineffective in promoting apoptosis. This reduction of effect correlates with an overexpression of Bcl-2 during differentiation. AICA riboside, derived from the hydrolysis of the ribotide, an intermediate of purine de novo synthesis, is absent in normal healthy cells; however it may accumulate in those individuals in which an inborn error of purine metabolism causes an increase in the rate of de novo synthesis and/or an overexpression of cytosolic 5'-nucleotidase, that appears to be the enzyme responsible for AICA ribotide hydrolysis. In fact, 5'-nucleotidase activity has been shown to increase in patients affected by Lesch-Nyhan syndrome in which both acceleration of de novo synthesis and accumulation of AICA ribotide has been described, and also in other neurological disorders of unknown etiology. Our results raise the intriguing clue that the neurotoxic effect of AICA riboside on the developing brain might contribute to the neurological manifestations of syndromes related to purine dismetabolisms.