Gene Expression of Enzymes Generating Tetrahydrobiopterin, an Essential Cofactor for NO Synthase, in Bone and Cultured Bone Cells.

Abstract

Tetrahydrobiopterin (BH4) is an unstable natural cofactor for nitric oxide synthase (NOS). Since nitric oxide (NO) is an important mediator of the effects of cytokines and mechanical strain in bone, the expression and cellular localization of mRNA and protein of the BH4-generating enzymes in tissue sections of bone and bone derived cell cultures were investigated. The expression of the mRNA of sepiapterin reductase (SPR), which is the terminal enzyme of the BH4 biosynthetic pathway, was confirmed in osteoblasts on the surface of the trabecular bone and in bone marrow of rat femora. SPR protein was detected by immunohistochemical analysis in similar regions of the femora where SPR mRNA was expressed. Cultured osteoblastic UMR106 cells expressed the mRNAs of all 3 enzymes involved in the BH4 biosynthetic pathway from guanosine 5'-triphosphate (GTP), i.e., GTP cyclohydrolase I, pyruvoyl tetrahydropterin synthase, and SPR, as determined by the reverse transcription-polymerase chain reaction (RT-PCR) technique. Moreover, a significant amount of biopterin, a stable oxide of BH4, was detected in the cell extract by high-performance liquid chromatography (HPLC) analysis. These results indicate that osteoblasts can biosynthesize BH4 by themselves to supply this cofactor to NOS to control NO formation in the bone tissue.