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

NAD(P)H:quinone oxidoreductase (NMO1; EC 1.6.99.2), also called DT-diaphorase, is involved in the reduction of coenzyme Q, an important cellular lipophilic antioxidant that can function as an intermediate electron carrier in plasma membrane-associated electron transport regulating cell growth. We examined the ability of normal human trabecular osteoblastic cells to express NAD(P)H:quinone oxidoreductase (NMO1) and studied its modulation during cell proliferation and growth arrest. We found that confluent primary human trabecular osteoblastic cells derived from healthy individuals constitutively express NMO1 activity, measured using 2-6 dichlorophenol indophenol (DCI) or menadione (vitamin K3) as substrate, and NADPH or NADH as electron donor. We also found that NMO1 activity was related to osteoblastic cell growth. NMO1 activity increased with osteoblastic cell density. Confluent growth-arrested cells expressed an eightfold higher NMO1 specific activity than proliferative human osteoblastic cells. Reverse-transcription polymerase chain reaction analysis showed that NMO1 mRNA levels did not differ in growth-arrested confluent cell and growing cells, suggesting that the increased NMO1 activity with cell density was due to posttranslational events. Harvesting 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 cells resulted in a 48.5% decrease in NMO1 activity. NMO1 activity does not appear to be related to induction of osteoblast differentiation because treatment 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 constitutively and that NMO1 activity increases with density-dependent growth inhibition suggest a role for NAD(P)H:quinone oxidoreductase in the control of growth arrest in normal human osteoblastic cells.