Zn 2+ and Cd 2+ increase the cyclic GMP level in PC12 cells by inhibition of the cyclic nucleotide phosphodiesterase

Abstract

In the present study, the influence of the heavy metal ions Cd 2+ and Zn 2+ on cGMP metabolism in the neurosecretory rat pheochromocytoma (PC12) cell line has been investigated. Cadmium and zinc ions showed a concentration-dependent increase of intracellular cGMP levels as determined by radioimmunoassay: a 20-fold increase in cGMP concentration was found after 15 min of incubation with 20 μM Cd 2+, and a 7-fold increase in cGMP was found after incubation with 50 μM Zn 2+ (control: 6.05±2.1 pmol cGMP/mg protein). To obtain further mechanistic informations, the effects of Cd 2+ and Zn 2+ on intracellular 3′,5′-cyclic nucleotide phosphodiesterase have been studied by a high performance liquid chromatography-based phosphodiesterase-assay. The cellular cGMP hydrolysis was found to be inhibited by these ions with an IC 50 value of 6±0.7 μM for Cd 2+ and 13±2.5 μM for Zn 2+. Hence, dose-dependent increase in cellular cGMP content is due to an inhibition of cGMP hydrolysis and not due to an increase in cGMP synthesis. Cd 2+ and Zn 2+ were taken up by PC12 cells as determined by atomic absorption spectroscopy, all measurements were performed in a subtoxic concentration range. Our data illustrate that zinc and cadmium ions are efficient inhibitors of the cGMP-stimulated cyclic nucleotide PDEII in PC12 cells resulting in elevated cellular cGMP concentrations. Therefore, subtoxic doses of these metals may disturb intracellular cGMP/cAMP-signalling pathways leading to an impaired or altered gene expression.