Pre- and Post-translational Regulation of Lysyl Oxidase by Transforming Growth Factor-1 in Osteoblastic MC3T3-E1 Cells

The final enzymatic step required for collagen cross-linking is the extracellular oxidative deamination of peptidyl-lysine and -hydroxylysine residues by lysyl oxidase. A cross-linked collagenous extracellular matrix is required for bone formation. The goals of this study were to compare the transforming growth factor (TGF)-β1 regulation of lysyl oxidase enzyme activity and steady state mRNA levels to changes in COL1A1 mRNA levels in MC3T3-E1 osteoblastic cells. TGF-β1 increased steady state lysyl oxidase and COL1A1 mRNA levels in a dose- and time-dependent manner. The increase in lysyl oxidase mRNA levels was transient, peaking at 12 h and 8.8 times controls in cells treated with 400 pM TGF-β1. COL1A1 steady state mRNA levels increased maximally to 3.5-fold of controls. Development of increased lysyl oxidase enzyme activity was delayed and was of slightly lower magnitude than the increase in its mRNA levels. This suggested limiting post-translational processing of lysyl oxidase proenzyme. Pulse-labeling/immunoprecipitation studies demonstrated slow proenzyme secretion and proteolytic processing. Development and application of an independent assay for lysyl oxidase proenzyme proteolytic processing activity verified its proportionately lower stimulation by 400 pM TGF-β1. Thus, lysyl oxidase regulation by TGF-β1 in osteoblastic cell cultures occurs at both pre- and post-translational levels. This regulation is consistent with increased production of a collagenous extracellular matrix.