Politis I
Department of Animal and Food Sciences, University of Vermont, Burlington 05405, USA.
J Dairy Sci. 1996 Jun;79(6):1097-107. doi: 10.3168/jds.S0022-0302(96)76463-9.
Several tissue remodeling events that require extracellular proteolysis are thought to be mediated by plasminogen activators that convert the inactive proenzyme plasminogen to active plasmin. The involvement of plasminogen activator in many biological phenomena reflects the ubiquitous presence of plasminogen and the ability of numerous cell types to synthesize plasminogen activator in a highly regulated manner. Increased plasmin and plasminogen activator in bovine milk are correlated with gradual involution (the declining phase of lactation). Treatment with bST prevented the increase in plasmin during gradual involution, indicating that bST interferes with conversion of plasminogen to plasmin. Concentrations of plasminogen activator in mammary tissue are high after cessation of milking. These results reinforce the association of the plasmin-plasminogen system with gradual involution postlactation. Recently, a role has been proposed for plasminogen activator in cell proliferation in several cellular systems. Insulin and IGF-I increased synthesis of urokinase plasminogen activator and enhanced proliferation of cultured bovine mammary epithelial cells. In contrast, phorbol myristate acetate, which increased expression of urokinase plasminogen activator mRNA by mammary epithelial and myoepithelial cells, stimulated proliferation of myoepithelial cells, but not epithelial cells. Thus, expression of plasminogen activator is not simply related to mitogenesis but is likely to serve multiple functions in bovine mammary epithelial cells.
几种需要细胞外蛋白水解作用的组织重塑事件被认为是由纤溶酶原激活剂介导的，这些激活剂可将无活性的酶原纤溶酶原转化为活性纤溶酶。纤溶酶原激活剂参与多种生物学现象，这反映了纤溶酶原的普遍存在以及众多细胞类型以高度调控的方式合成纤溶酶原激活剂的能力。牛乳中纤溶酶和纤溶酶原激活剂的增加与逐渐退化（泌乳的下降阶段）相关。用牛生长激素（bST）处理可防止在逐渐退化过程中纤溶酶的增加，这表明bST会干扰纤溶酶原向纤溶酶的转化。挤奶停止后，乳腺组织中纤溶酶原激活剂的浓度很高。这些结果进一步证实了纤溶酶 - 纤溶酶原系统与泌乳后逐渐退化之间的关联。最近，有人提出纤溶酶原激活剂在几种细胞系统的细胞增殖中发挥作用。胰岛素和胰岛素样生长因子 -I（IGF -I）增加了尿激酶型纤溶酶原激活剂的合成，并增强了培养的牛乳腺上皮细胞的增殖。相比之下，佛波酯肉豆蔻酸酯可增加乳腺上皮细胞和肌上皮细胞尿激酶型纤溶酶原激活剂mRNA的表达，刺激肌上皮细胞的增殖，但不刺激上皮细胞。因此，纤溶酶原激活剂的表达不仅仅与有丝分裂相关，而且可能在牛乳腺上皮细胞中发挥多种功能。
