Differential Regulation of Plasminogen Activator and Plasminogen Activator Inhibitor by Osteotropic Factors in Primary Cultures of Mature Osteoblasts and Osteoblast Precursors*

Abstract

Plasminogen activators (PA) and plasminogen activator inhibitors (PAI) have been implicated in the process of extracellular matrix degradation. To study their role in bone matrix turnover, we examined the activity and regulation of PA and PAI in cultures of periosteal osteoblast-like precursor cells and mature osteoblast-like cells from fetal rat calvariae. Both cell populations released PA activity of the tissue type and a 50K PAI species into the culture medium. However, mature osteoblasts had a strikingly lower PA activity and higher PAI activity than periosteal precursor cells, indicating that osteoblast differentiation is associated with a marked decrease in the PA/PAI ratio. PTH and prostaglandin E2 transiently increased PA activity and decreased PAI activity. In contrast, transforming growth factor-beta decreased PA activity and increased PAI activity. Differential effects of these factors on PA and PAI activity may be involved in the regulation of extracellular matrix deposition by osteoblasts.