Abstract
The production of proteolytic emzymes by osteoblasts is considered important for initiating osteoclastic bone resorption. Using the established cell line NY as an example of osteoblast-like cells, the effect of intracellular cyclic AMP (cAMP) and protein kinase C (PKC) on plasminogen activator secretion and its specific binding to the cells were investigated. HT-1080 cells were used as the control. NY cells predominantly secrete single-chain urokinase-type plasminogen activator (scu-PA) and some two-chain u-PA. Both scu-PA and u-PA were present in the cell surface and cell lysate of NY cells, and their distribution in HT-1080 cells was quite similar to that of NY cells. Exposing cells to phorboi myristate acetate (PMA) or dibutyryl cyclic AMP (db cAMP) enhanced the secretion of scu-PA and two-chain u-PA, whereas 1-(5-isoquinolinylsulfonyl)-2-methylpiperazine (H7) decreased scu-PA secretion, indicating that it is enhanced by protein kinase C (PKC) as well as by cAMP in NY cells. On the other hand, in HT-1080 cells, PMA decreased the level of two-chain u-PA secretion into the conditioned medium. The binding assay of 125I-DFP-u-PA to NY cells revealed the presence of a single class of binding sites with a Kd of 2.23 nM and Bmax of 0.82×106 binding sites/cell. PMA however, altered neither the Kd nor the Bmax. Dibutyryl cAMP increased the Bmax 1.9 fold. Thus, NY cells secrete u-PA and express specific binding sites on the cell surface, which are modulated by cAMP and PKC. The u-PA/u-PA receptor system may contribute to osteoblastic bone resorption.
