Transgenic over-expression of plasminogen activator inhibitor-1 results in age-dependent and gender-specific increases in bone strength and mineralization.
The plasminogen activation system (PAS) and its principal inhibitor, plasminogen activator inhibitor-1 (PAI-1), are recognized modulators of matrix. In addition, the PAS has previously been implicated in the regulation of bone homeostasis. Our objective was to study the influence of active PAI-1 on geometric, biomechanical, and mineral characteristics of bone using transgenic mice that over-express a variant of human PAI-1 that exhibits enhanced functional stability. Femora were isolated from male and female, wildtype (WT) and transgenic (PAI-1.stab) mice at 16 and 32 weeks of age (n=10). Femora were imaged via DEXA for BMD and muCT for cortical mid-slice geometry. Torsional testing was employed for biomechanical properties. Mineral composition was analyzed via instrumental neutron activation analysis. Female femora were further analyzed for trabecular bone histomorphometry (n=11). Whole animal DEXA scans were performed on PAI-1.stab females and additional transgenic lines in which the functional domains of the PAI-1 protein were specifically disrupted. Thirty-two week female PAI-1.stab femora exhibited decreased mid-slice diameters and reduced polar moment of area compared to WT, while maintaining similar cortical bone width. Greater biomechanical strength and stiffness were demonstrated by 32 week PAI-1.stab female femora in addition to a 52% increase in BMD. PAI-1.stab trabecular bone architecture was comparable to WT. Osteoid area was decreased in PAI-1.stab mice while mineral apposition rate increased by 78% over WT. Transgenic mice expressing a reactive-site mutant form of PAI-1 showed an increase in BMD similar to PAI-1.stab, whereas transgenic mice expressing a PAI-1 with reduced affinity for vitronectin were comparable to WT. Over-expression of PAI-1 resulted in increased mineralization and biomechanical properties of mouse femora in an age-dependent and gender-specific manner. Changes in mineral preceded increases in strength/stiffness and deterred normal cross-sectional expansion of cortical bone in females. Trabecular bone was not altered in PAI-1.stab mice whereas MAR increased significantly, further supporting mineral changes as the underlying factor in strength differences. The primary influence of PAI-1 occurred during a period of basal bone remodeling, attributing a role for this system in remodeling as opposed to development. Comparison of transgenic lines indicates that PAI-1's influence on bone is dependent on its ability to bind vitronectin, and not on its proteolytic activity. The impact of PAI-1 on mouse femora supports a regulatory role of the plasminogen activation system in bone homeostasis, potentially elucidating novel targets for the treatment of bone disease.
Nordstrom
S M
SM
Department of Medicine, Vanderbilt University, Nashville, Tennessee, USA.
Carleton
S M
SM
Carson
W L
WL
Eren
M
M
Phillips
C L
CL
Vaughan
D E
DE
D000375:Aging / Q000502:physiology*; D000818:Animals; D015519:Bone Density / Q000502:physiology*; D001842:Bone and Bones / Q000502:physiology*; D005260:Female; D005786:Gene Expression Regulation; D016678:Genome / Q000235:genetics; D008297:Male; D051379:Mice; D008810:Mice, Inbred C57BL; D008822:Mice, Transgenic; D017395:Plasminogen Activator Inhibitor 1 / Q000235:genetics / Q000378:metabolism*; D012727:Sex Characteristics; D013314:Stress, Mechanical; D013718:Tensile Strength
D017395:Plasminogen Activator Inhibitor 1