Synthesis of plasminogen activator inhibitor 1 by bovine mammary epithelial and myoepithelial cell lines

Conversion of plasminogen to plasmin provides an important source of proteolytic activity in the bovine mammary gland. Plasminogen activator inhibitor 1 (PAI-1) plays a key role in limiting plasminogen activation. The PAI-1 biosynthetic capabilities of various bovine mammary cells were determined. The immortalized epithelial cell lines MAC-T and BME-UV and the myoepithelial cell line BMM-UV were used as model systems. Northern blot analysis indicated that both epithelial and myoepithelial cells contained PAI-1 mRNA. Bovine PAI-1 was encoded by a single mRNA species approximately 3.0 kb long. BME-UV cells contained 2.0-fold (P < 0.01) the PAI-1 mRNA of MAC-T or BMM-UV cells. Reverse zymography indicated that both epithelial and myoepithelial cells synthesized PAI-1 protein with a molecular mass of approximately 50 kDa.