EXPRESSION OF NITRIC-OXIDE SYNTHASE ISOFORMS IN BONE AND BONE CELL-CULTURES

Recent work has shown that nitric oxide (NO) acts as an important medi ator of the effects of proinflammatory cytokines and mechanical strain in bone. Although several bone-derived cells have been shown to produ ce NO in vitro, less is known about the isoforms of NO synthase (NOS), which are expressed in bone or their cellular distribution, Here we i nvestigated the expression, cellular localization, and regulation of N OS mRNA and protein in cultured bone-derived cells and in bone tissue sections, We failed to detect inducible NOS (iNOS) protein in normal b one using immunohistochemical techniques, even though lo tr levels of iNOS mRNA were detected by sensitive reverse transcribed polymerase ch ain reaction (RT-PCR) assays in RNA extracted from whole bone samples, Cytokine stimulation of bone-derived cells and bone explant cultures caused dramatic induction of iNOS mRNA and protein in osteoblasts and bone marrow macrophages, but no evidence of iNOS expression was seen i n osteoclasts by immunohistochemistry or in situ hybridization, Endoth elial NOS (ecNOS) mRNA was also detected by RT-PCR in whole bone, and immunohistochemical studies showed widespread ecNOS expression in bone marrow cells and trabecular lining cells in vivo, Related studies in vitro confirmed that ecNOS was expressed in cultured osteoblasts, stro mal cells, and osteoclasts, Neuronal NOS mRNA was detected by RT-PCR i n whole bone, but we were unable to detect nNOS protein in bone cells in vivo or in studies of cultured bone-derived cells in vitro. In summ ary, our data show that mRNAs for all three NOS isoforms are expressed in bone and pro, ide evidence for differential expression and regulat ion of the enzymes in different cell types. These findings confirm the likely importance of the L-arginine-NO pathway as a physiological med iator of bone cell function and demonstrate that it may be possible to exert differential effects on osteoblast and osteoclast activity in v ivo by differential targeting of constitutive and inducible NOS isofor ms by selective NOS inhibitors.