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1 Endocrinology, Diabetes and Hypertension, Brigham and Womens Hospital, Boston, MA, USA; Department of Clinical Biochemistry and Endocrinology, Copenhagen University Hospital Hvidovre, Copenhagen, Denmark; Department of Cardiology, Copenhagen University Hospital Rigshospitalet, Copenhagen, Denmark
2 Endocrinology, Diabetes and Hypertension, Brigham and Womens Hospital, Boston, MA, USA
* To whom correspondence should be addressed. E-mail: tfelt{at}dadlnet.dk.
Nitric oxide (NO) is a versatile second messenger. NO is produced by Leydig cells, where NO is a negative regulator of steroidogenesis. In cancer cells, NO is thought to have mutagenic as well as proliferative effects. We have previously shown that the calcium-sensing receptor (CaR) has promalignant effects in rat H-500 Leydig cancer cells, a model for humoral hypercalcemia of malignancy. Calcium, the major physiological ligand of the CaR, is a recognized intracellular cofactor in the process of NO production, by virtue of its positive modulation of neuronal and endothelial nitric oxide synthase (NOS), but importantly, not of inducible (i)NOS activity. iNOS activity is regulated by changes in its expression level. Therefore, we investigated whether CaR activation changes iNOS expression. We found that high extracellular calcium (Ca2+°) upregulates the level of mRNA for iNOS, whereas no change was seen in neuronal or endothelial NOS, as assessed by microarray and real time PCR, respectively. The high Ca2+°-induced iNOS upregulation was also detected by Northern and western blotting. By quantitative real-time PCR, we showed that calcium maximally upregulates iNOS at 18h. The effect of calcium was abolished by overexpression of a dominant-negative CaR (R185Q), confirming that the effect of Ca2+° was mediated by the CaR. Cells treated with high calcium had higher NO production than those treated with low calcium, as detected with the NO-specific DAF2AM dye. This was confirmed in single-cell fluorescence determinations using confocal microscopy. In conclusion, high calcium upregulates the levels of iNOS mRNA and protein as well as NO production in H-500 cells, and the effect of Ca2+° on iNOS expression is mediated by the CaR.
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