Expression and activity of NAD(P)H : Quinone oxidoreductase (NMO1) in human osteoblastic cells

NAD(P)H:quinone oxidoreductase (NMO1; IEC 1.6,99.2), also called DT-diaphor ase, is involved in the reduction of coenzyme Q, an important cellular lipo philic antioxidant that can function as an intermediate electron carrier in plasma membrane-associated electron transport regulating cell growth. We e xamined the ability of normal human trabecular osteoblastic cells to expres s NAD(P)H:quinone oxidoreductase (NMO1) and studied its modulation during c ell proliferation and growth arrest, We found that confluent primary human trabecular osteoblastic cells derived from healthy individuals constitutive ly express NMO1 activity, measured using 2-6 dichlorophenol indophenol (DCI ) or menadione (vitamin K3) as substrate, and NADPH or NADH as electron don or, We also found that NMO1 activity was related to osteoblastic cell growt h. NMO1 activity increased with osteoblastic cell density, Confluent growth -arrested cells expressed an eightfold higher NMO1 specific activity than p roliferative human osteoblastic cells. Reverse-transcription polymerase cha in reaction analysis showed that NMO1 mRNA levels did not differ in growth- arrested confluent cell and growing cells, suggesting that the increased NM O1 activity with cell density was due to posttranslational events. Harvesti ng and replating the cells at low density resulted in a 93.4% loss of NMO1 enzymatic activity. Removal of serum from high-density growth-arrested cell s resulted in a 48.5% decrease in NMO1 activity. NMO1 activity does not app ear to be related to induction of osteoblast differentiation because treatm ent with the differentiating agent 1,25(OH)(2) vitamin D-3 had no effect on NMO1 activity, The finding that human osteoblastic cells express NMO1 cons titutively and that NMO1 activity increases with density-dependent growth i nhibition suggest a role for NAD(P)H:quinone oxidoreductase in the control of growth arrest in normal human osteoblastic cells. (C) 2001 by Elsevier S cience Inc. All rights reserved.