Increased intracellular Ca2+ selectively suppresses IL-1-induced NO production by reducing iNOS mRNA stability

This study addresses the role of intracellular calcium (Ca2+) in the expression of iNOS, an IL-1 inducible gene in human articular chondrocytes. The calcium ionophore A23187 and ionomycin did not induce NO release or iNOS expression but inhibited dose dependently IL-1-induced NO release with IC50 of 200 nM and 100 nM, respectively. Increased intracellular Ca2+ induced by thapsigargin or cyclopiazonic acid, inhibitors of the endoplasmic reticulum Ca2+ ATPase, had similar inhibitory effects with IC50 of 1 nM and 3 microM, respectively. LPS and TNF alpha induced NO production were also suppressed by these Ca2+ elevating drugs. Levels of IL-1-induced iNOS protein were reduced by A23187, thapsigargin, and cyclopiazonic acid. These drugs as well as Bay K 8644 and KCl inhibited IL-1-induced iNOS mRNA expression. To analyze the role of Ca2+ in the expression of other IL-1 responsive genes in chondrocytes, these Ca2+ modulating drugs were tested for effects on COXII. In contrast to the inhibitory effects on iNOS mRNA, these drugs induced COXII mRNA expression and in combination with IL-1, enhanced COXII mRNA levels. Ca2+ mediated increases in COXII mRNA expression were associated with an increase in COXII protein. The kinetics of Ca2+ effects on IL-1-induced iNOS mRNA levels suggested a posttranscriptional mechanism. Analysis of iNOS mRNA half life showed that it was 6-7 h in IL-1-stimulated cells and decreased by A23187 to 2-3 h. In conclusion, these results show that Ca2+ inhibits IL-1-induced NO release, iNOS protein, and mRNA expression in human articular chondrocytes by reducing iNOS mRNA stability. Under identical conditions increased Ca2+ enhances IL-1-induced COXII gene and protein expression.