Modulation of inducible nitric oxide synthase mRNA stability by tetrahydrobiopterin in vascular smooth muscle cells.

Tetrahydrobiopterin (BH4) regulates inducible nitric oxide synthase (iNOS) as cofactor and allosteric effector. The present paper describes a novel function of BH4 in vascular smooth muscle cells (SMC). By varying BH4 levels with dicumarol (an inhibitor of BH4 synthesis) and sepiapterin (an exogenous source of co-factor), we investigated iNOS expression in activated rat aortic SMC. In sepiapterin-supplemented cells, iNOS protein levels were increased while in dicumarol-treated cells, iNOS levels were diminished. Time-kinetic experiments revealed that inhibition or supplementation of BH4 synthesis had no effects on iNOS induction or transcription rate. However, iNOS mRNA was present over a prolonged time in sepiapterin-supplemented SMC. Analysis of iNOS mRNA levels showed stable iNOS mRNA in sepiapterin-treated cells 8 hours after transcription inhibition, while in dicumarol-treated cells iNOS mRNA disappeared. The decrease of iNOS mRNA by dicumarol was abolished by sepiapterin. These data indicate that BH4 post-transcriptionally stabilizes iNOS mRNA in SMC. By this way BH4 modulates iNOS expression in the vascular system.