Bone resorption and response to calcium-regulating hormones in the absence of tissue or urokinase plasminogen activator or of their type 1 inhibitor

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Abstract
Plasminogen activators (PA) are implicated in cell migration and tissue remodeling, two componets of the bone resorption processes. Uning mice with inactivated tissue PA (tPA), urokinase PA (uPA), or type 1 PA inhibitor (PAI-1) genes, we evaluated whether these processes, or their stimulation by parathyroid hormone (PTH) or 1,25-dihydroxyvitamin (1,25{oH}2D3) are dependent on these genes. Two culture models were used, one involving 19-day fetal calvariae, to evaluate the direct resorptive activity of osteoclasts, and the other involving 45Ca-labeled 17-day fetal metatarsals, in which this activity depends on preliminary (pre)osteoclast migration. PTH similarly increased (about 10-fold) PA activity in calvariae from wild-type tPA+/+ and uPA+/+ or deficient uPA−/− and PAI−/− mice; it affected only tPA, not uPA. In tPA−/− bones, the low PA levels, due to uPA, were not influenced by PTH. Calcitonin did not affect PA responses to PTH. No differences were observed between tPA+/+, tPA−/−, uPA+/+, and uPA−/− calvariae for any parameter related to bone resorption (development of lacunae, release of calcium and lysosomal enzymes, accumulation of collagenase, loss of hydroxyproline), indicating similar responses to PTH or calcitonin. The progressive 45Ca release was largely similar in cultures of tPA+/+, tPA−/−, uPA+/+ uPA−/−, PAI+/+, or PAI−/− metatarsals and it was similarly enhanced by PTH or 1,25(OH)2D3. However, uPA−/− metatarsals released 45Ca at a slower rate at the beginning of the cultures, suggesting an impaired recruitment of the (pre)osteoclasts, which migrate at that time from the periosteum into the calcified cartilage. Thus, it appears that the direct resorptive activity of the osteoclasts does not necessitate the presence of either tPA or uPA, but uPA is likely to facilitate the migration of the (pre)osteoclasts toward the mineralized surfaces. Although considerably enhanced by PTH, tPA does not mediate the actions of PTH (nor of 1,25{OH}2D3) evaluated in these models.

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